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The current widespread use of calcium silicate or aluminate hydrate binder systems in the construction industry finds its roots in the Antique world where mixtures of calcined lime and finely ground reactive (alumino-)silicate materials were pioneered and developed as competent inorganic binders. Architectural remains of the Minoan civilization (2000-1500 BC) on Crete have shown evidence of the combined use of slaked lime and additions of finely ground potsherds to produce stronger and more durable lime mortars suitable for water-proof renderings in baths, cisterns and aqueducts (Spence and Cook 1983). It is not clear when and where mortar technology evolved to incorporate volcanic pumice and ashes as a functional supplement. A plausible site would be the Akrotiri settlement at Santorin (Greece), where archeological indications of strong ties with the Minoan culture were found and large quantities of suitable highly siliceous volcanic ash were present. This so-called Santorin earth has been used as a pozzolan in the Eastern Mediterranean until recently (Kitsopoulos and Dunham 1996). Evidence of the deliberate use of this and other volcanic materials by the ancient Greeks dates back to at least 500-400 BC, as uncovered at the ancient city of Kamiros, Rhodes (Efstathiadis 1978; Idorn 1997). In the subsequent centuries the technological knowledge was spread to the mainland (Papayianni and Stefanidou 2007) and was eventually adopted and improved by the Romans (Mehta 1987). The Roman alternatives for Santorin earth were volcanic pumices or tuff found in neighboring territories, the most famous ones found in Pozzuoli (Naples), hence the name pozzolan, and in Segni (Latium). Preference was given to natural pozzolan sources, but crushed ceramic waste was frequently used when natural deposits were not locally available. The exceptional lifetime and preservation condition of some of the most famous Roman buildings such as the Pantheon or the Pont du …
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6
Reviews in Mineralogy & Geochemistry
Vol. 74 pp. 211-278, 2012
Copyright © Mineralogical Society of America
1529-6466/12/0074-0006$10.00 DOI: 10.2138/rmg.2012.74.6
Supplementary Cementitious Materials
Ruben Snellings, Gilles Mertens and Jan Elsen
Department of Earth and Environmental Sciences
Katholieke Universiteit Leuven
B-3001 Leuven, Belgium
e-mail: ruben.snellings@ep.ch
INTRODUCTION
The current widespread use of calcium silicate or aluminate hydrate binder systems in
the construction industry nds its roots in the Antique world where mixtures of calcined
lime and nely ground reactive (alumino-)silicate materials were pioneered and developed
as competent inorganic binders. Architectural remains of the Minoan civilization (2000-1500
BC) on Crete have shown evidence of the combined use of slaked lime and additions of nely
ground potsherds to produce stronger and more durable lime mortars suitable for water-proof
renderings in baths, cisterns and aqueducts (Spence and Cook 1983). It is not clear when and
where mortar technology evolved to incorporate volcanic pumice and ashes as a functional
supplement. A plausible site would be the Akrotiri settlement at Santorin (Greece), where
archeological indications of strong ties with the Minoan culture were found and large quantities
of suitable highly siliceous volcanic ash were present. This so-called Santorin earth has been
used as a pozzolan in the Eastern Mediterranean until recently (Kitsopoulos and Dunham 1996).
Evidence of the deliberate use of this and other volcanic materials by the ancient Greeks dates
back to at least 500-400 BC, as uncovered at the ancient city of Kamiros, Rhodes (Efstathiadis
1978; Idorn 1997). In the subsequent centuries the technological knowledge was spread to
the mainland (Papayianni and Stefanidou 2007) and was eventually adopted and improved by
the Romans (Mehta 1987). The Roman alternatives for Santorin earth were volcanic pumices
or tuff found in neighboring territories, the most famous ones found in Pozzuoli (Naples),
hence the name pozzolan, and in Segni (Latium). Preference was given to natural pozzolan
sources, but crushed ceramic waste was frequently used when natural deposits were not
locally available. The exceptional lifetime and preservation condition of some of the most
famous Roman buildings such as the Pantheon or the Pont du Gard constructed with the aid
of pozzolan-lime mortars and concrete testify to both the excellent workmanship reached by
Roman “engineers” and to the durable properties of the utilized binder materials.
In designing the binder mixes Romans seem to have paid much attention to a very
thorough grinding and mixing of components and to the granulometric gradation of the
aggregate with an increased proportion of nes (Day 1990). This principle of extending the
range of particle sizes to improve space lling is based on the so called Apollonian concept
and has more recently been applied to the binder phase in modern concrete to achieve ultra-
high strength concretes (Scrivener and Kirkpatrick 2008). The utilization of lime-pozzolan
binders recovered gradually after the decline of the Roman Empire, particularly due to their
hydraulic capability of hardening underwater. However, pozzolan-lime binders were gradually
replaced by Portland cement based binders over the course of the 19th and early 20th century
(Blezard 2001).
212 Snellings, Mertens, Elsen
More recently, especially since the 2nd half of the 20th century, the addition to Portland ce-
ment of natural or articial materials able to react with lime to produce a cementitious product
has received renewed attention. The production of Portland clinker being an energy-intensive
process, in which raw materials are typically burned at 1450 °C, renders the economic ad-
vantages of replacing a substantial part of the clinker by cheap naturally available pozzolans
or industrial by-products obvious. The replacement of a part of the cement clinker does not
need to have negative effects on the mechanical and durability performance of the so-called
blended cement. To the contrary, many studies have indicated that particularly the durability
of blended cements is substantially improved. An overview of the technological effects of us-
ing blended cements is included in the present chapter. A more recent important incentive to
increase and optimize the incorporation of supplementary cementitious materials (SCMs) in
blended cements is the problem of climate change associated with the anthropogenic emis-
sion of greenhouse gasses. Additional to the need of reaching high temperatures in the cement
kilns, the release of CO2 by decomposition of limestone results in an average ratio of 0.87 kg
CO2 emitted per kg of Portland cement produced (Damtoft et al. 2008). It is estimated that
with a current annual cement production of 2.8 billion tonnes the cement industry alone is re-
sponsible for 5-7% of the anthropogenic emissions. Future prospects foresee a drastic increase
in Portland cement production in developing countries. Therefore, the contemporary cement
industry is faced with the challenges of producing more sustainable, less energy intensive
products without sacricing the mechanical or durability performance of the end product.
The societal incentives and associated emission quotas in order to reduce global CO2 emis-
sions are rapidly evolving into an all-important issue for the cement industry (Damtoft et al.
2008; Habert et al. 2010). In response, the currently most common development with limited
interference in the conventional production process is the increased blending of supplementary
cementitious materials or pozzolans with Portland cement (Gartner 2004).
The utilization of industrial by-products available in large and regular quantities of suitable
consistent composition, i.e., ground granulated blast furnace slags from iron smelting and coal
or lignite y ashes from electricity production, has been rmly established in many countries.
However, the supply of high quality SCM by-products is limited and many local sources are
already fully exploited. In addition, a decline in production of blast furnace slags and y
ashes is expected due to future developments in steel and electricity production (Scrivener
and Kirkpatrick 2008). An illustration of the evolution of the clinker substitution in France is
given by Habert et al. (2010). Figure 1 shows that since the early 70’s the total replacement
percentage has remained stable at 20%, while the main cement replacement materials have
shifted from slag and y ash to limestone. Alternatives to the traditional industrial by-product
SCMs are to be found on the one hand in an increased usage of naturally occurring SCMs and
on the other hand in the expansion of the range of industrial by-products or societal waste to
substitute for clinker. The development of cement and concrete prescriptions and standards
towards more performance based conditions highlights the generally accepted view that the
utilization of a wider array of SCMs at higher replacement percentages should be allowed and
judged on the eventual performance of the end product (Hooton 2008; Kaid et al. 2009).
Natural SCMs or pozzolans are abundant in certain locations and are extensively used
as an addition to Portland cement in countries such as for example Italy, Germany, Greece
and China. Compared to traditional industrial SCMs they are characterized by a larger range
in composition and a larger variability in physical properties. The application of natural
pozzolans in Portland cement is mainly controlled by the local availability of suitable deposits
and the competition with the accessible traditional industrial by-product SCMs. In part due to
the exhaustion of the latter sources and the extensive reserves of natural pozzolans available,
partly because of the proven technical advantages of an intelligent use of natural pozzolans,
their use is expected to be strongly expanded in the future (Mehta 1987; Damtoft et al. 2008).
Supplementary Cementitious Materials 213
A substantial part of the by-products and waste materials generated by present-day society
have the potential of being utilized in construction materials. Immobilization of harmful
metals in waste materials by incorporation in the high pH environment of hydrated cement
offers interesting perspectives for application. Apart from avoiding costs of waste disposal and
environmental pollution, lowering cement costs and increasing the product sustainability, some
materials may also benecially affect the microstructure and the mechanical and durability
properties of mortars and concrete (Meyer 2009). Recently, a substantial amount of research
has been dedicated to improve the understanding of the behavior of a broadened range of
waste materials in cement and concrete products and to explore the potential applications.
This review will be limited to waste materials or by-products which are already extensively
employed as reactive binder components, e.g., y ashes and blast furnace slags, and have
successfully passed the development and testing stage. For a recent review of the usage of
by-product or waste materials as aggregate, the reader is referred to the comprehensive survey
of Siddique (2008).
The potential of clinker substitution by SCMs to decrease production costs and to increase
sustainability and durability of the end-products is reected in the large and steadily growing
numbers of peer-reviewed papers published on the subject. Excellent general literature reviews
which have been remarkable sources of information for the present paper have been published
earlier by Massazza (1974, 2001), Sersale (1980, 1993), Takemoto and Uchikawa (1980),
Swamy (1986), Malhotra and Mehta (1996) and Siddique (2008).
This literature review will mainly consider more recent insights and research ndings
and put them into perspective with the previously existing knowledge on supplementary
cementitious materials. Though the combined use of chemical admixtures and SCMs is
common, a thorough treatment on the effect of chemical admixtures on cement properties
would require a separate review. Also, no account is made of the different methodologies for
testing the reactivity of SCMs or pozzolans. Instead, here, a general denition and classication
of SCMs is presented rst, followed by sections treating the physico-chemical properties of
specic SCM groups. A detailed account of recent developments in the understanding of
the pozzolanic reaction mechanism and kinetics has been provided, together with a general
overview of the reaction products encountered. Finally, a concise outline of the properties of
SCM-blended cement binders is offered.
Figure 1. Evolution of clinker substitution materials in France from 1973 to 2007 (Habert et al. 2010).
214 Snellings, Mertens, Elsen
DEFINITION AND CLASSIFICATION OF
SUPPLEMENTARY CEMENTITIOUS MATERIALS
Denition
The group of supplementary cementitious materials comprises materials that show either
hydraulic or pozzolanic behavior. A hydraulic binder is a material that can set and harden sub-
merged in water by forming cementitious products in a hydration reaction. Iron blast furnace
slags commonly show a “latent hydraulicity,” i.e., their hydraulic activity is relatively low com-
pared to Portland cement and activation by chemical or physical means is needed to initiate and
accelerate the hydration reaction (Regourd 1986; Lang 2002). Blast furnace slags can be chemi-
cally activated by addition of alkali-hydroxides, sulfates in the form of gypsum or anhydrite or
more frequently by the addition of lime or lime-producing materials such as Portland cement. It
should be noted that hydraulic materials can replace Portland cement up to a much larger extent
than materials showing pozzolanic behavior.
A pozzolan is generally dened in ASTM C618 as a siliceous or siliceous and aluminous
material which, in itself, possesses little or no cementitious value but which will, in nely
divided form and in the presence of moisture, react chemically with calcium hydroxide (lime)
at ordinary temperature to form compounds possessing cementitious properties (Mehta 1987).
It should be remarked that the denition of a pozzolan imparts no bearing on the origin of the
material, only on its capability of reacting with lime and water. A quantication of this capabil-
ity is comprised in the term pozzolanic activity.
The pozzolanic activity is by convention described as a measure for the degree of reaction
over time between a pozzolan and Ca2+ or Ca(OH)2 in the presence of water. Physical surface
adsorption is not considered as being part of the pozzolanic activity, because no irreversible
molecular bonds are formed in the process (Takemoto and Uchikawa 1980). The driving force
underlying the pozzolanic activity is the difference in Gibbs free energy between the initial
and nal reaction stages, while the reaction kinetics are governed by the activation energy bar-
rier which needs to be surmounted to proceed in the reaction (Felipe et al. 2001). It should be
remarked that the bulk properties of the end product (i.e., mortar, concrete etc.) are not directly
related to the SCM inherent pozzolanic activity. Physical bulk properties such as permeabil-
ity and mechanical strength are more strongly dependent on the type, shape, dimensions and
distribution of reaction products and pores than on the extent of the lime-pozzolan reaction
(Takemoto and Uchikawa 1980; Massazza 2001). The former factors are mainly affected by the
mix design and curing conditions and can thus be controlled. However at equal binder prepara-
tion conditions, the pozzolanic activity remains a primary factor that controls the capability of
a material to engage in the formation of cementitious compounds and in consequence also the
contribution of the material to the binder performance.
Classication
The general denition of supplementary cementitious material embraces a large number
of materials which vary widely in terms of origin, chemical and mineralogical composition,
and typical particle characteristics. Although it is generally accepted that the hydraulic or poz-
zolanic activity of SCMs depends largely on their physico-chemical properties rather than their
origin (Sersale 1993), classications of SCMs according to their activity or their performance
in concrete (e.g., Mehta 1989) have known little success. Still more commonly accepted is the
classication based on the origin of the SCM (Massazza 2001) and this will be followed here.
Two broad categories can be distinguished, on the one hand materials of a natural origin
and on the other hand materials of man-made or articial origin. The former group consists of
materials that can be used as SCM in their naturally occurring form. In most cases they only
need conditioning of particle characteristics by sieving and grinding processes. Typical natu-
Supplementary Cementitious Materials 215
ral SCMs are pyroclastic rocks, either diagenetically altered or not, and highly-siliceous sedi-
mentary rocks such as diatomaceous earths. The group of articial SCMs includes materials
which have undergone structural modications as a consequence of manufacturing or produc-
tion processes. Articial SCMs can be produced deliberately, for instance by thermal activation
of kaolin-clays to obtain metakaolin, or can be obtained as waste or by-products from high-
temperature processes such as blast furnace slags, y ashes or silica fume. A general genetic
classication scheme is presented in Figure 2. Natural SCMs are subdivided into materials of
primary volcanic origin and materials of sedimentary origin. The volcanic materials utilized are
generally pyroclastics and can be altered by diagenetic processes to zeolite-rich tuffs. The sedi-
mentary rocks comprise chemical and detrital sediments. Both biochemically deposited SCMs
such as diatomaceous earths and deposits resulting from the circulation of hydrothermal uids
are included in the former category. Naturally burned clays, such as gliezh, are an example of
the use of detrital sediments. Some individual materials, e.g., Danish moler and French gaize,
cannot be distinctively categorized as either a natural or articial SCM because their natural
pozzolanic activity is commonly enhanced by thermal treatment. Here, they are included to-
gether with the articial SCMs. Other materials, such as Sacrofano earth, containing compo-
nents of mixed natural origins (volcanic, detritic and biogenic) are classied under the category
of materials of sedimentary origin. The classication of articial SCMs is conveniently based
either on the industrial processes producing the SCMs or on the original materials that are ther-
mally treated to intentionally manufacture SCMs. Some waste materials with the potential to
become more widely used as SCM in the future, but which need further experimental evaluation
are also mentioned in the classication scheme.
Figure 2. General classication scheme of supplementary cementitious materials. * denotes materials
which can present hydraulic activity, all other materials display pozzolanic behavior. A selection of
promising SCMs still largely under development are positioned below the dashed line (modied after
Massazza 2001).
216 Snellings, Mertens, Elsen
Within a group of SCMs of the same origin there can be considerable variability in the
physico-chemical properties. The extent of variability depends on the SCM origin. The large
compositional variability of volcanic extrusive rocks is a reection of the natural variability
in magmatic and diagenetic processes leading to their present condition. In contrast, the
characteristics of silica fume produced in the controlled manufacturing of silicon metal and
ferrosilicon alloys show much more consistency. Apart from differences in variability, separate
groups show characteristic ranges of chemical composition. Figure 3 illustrates the chemical
composition and typical variability for the most commonly used groups of SCMs in a CaO-SiO2-
Al2O3 ternary variation diagram. Alkalis, MgO and Fe2O3 content are ignored in this diagram.
The gure can also be instrumental in estimating the impact of SCM incorporation on the
blended cement chemistry and can eventually be used to predict reaction product assemblages
(cf. infra).
MINERALOGY AND CHEMISTRY OF SCMS
Natural SCMs
The great majority of natural pozzolans in use today is of volcanic origin, mainly owing
to the widespread availability of volcanic rocks in many countries. In Figure 4 the global
distribution of volcanic rocks can be compared with the reported occurrences of natural pozzolan
deposits. It is apparent that the overwhelming majority of these deposits are located in areas of
Cenozoic volcanic activity. However, not all volcanic rocks are suitable as pozzolanic material.
Pyroclastic materials resulting from explosive eruptions such as ashes or pumices show higher
pozzolanic activity owing to their higher glass content and highly porous or vesicular nature.
The eruption type largely depends on the magma viscosity which is related to the “acidity”
(i.e., SiO2 content) of the magma. In general, more siliceous magma produces more explosive
volcanism and products with better pozzolanic properties. Coarser highly vesicular pyroclastic
Figure 3. Ternary CaO-SiO2-Al2O3 diagram (wt% based) situating the chemical constitution of the major
SCM groups (modied after Glasser et al. 1987).
Supplementary Cementitious Materials 217
material forms pumice-type deposits. Finely divided materials are transported further away
from the volcanic source and are deposited as ash layers.
Subsequent to the deposition of the pyroclastic material, diagenetic alteration of the
vitreous material to crystalline zeolites can occur under certain circumstances. The resulting
zeolite-bearing rocks are often coherent tuffs and present, when ground to sufcient neness,
high pozzolan activities. As the likelihood of a material having undergone diagenetic alteration
increases with its age, zeolitized tuffs tend to become more abundant in rocks of increasing
geologic age throughout the Neogene (Gottardi and Obradovic 1978). The type of zeolites
formed and the quality of the zeolitized rocks is largely related to the composition of the original
vitreous material. Regional differences are therefore common and are ultimately related to the
type of volcanism and the corresponding geological situation.
The utilization of materials of sedimentary origin is scarcer, which is evidently related to
the general stability at ambient conditions of the mineral assemblages deposited as sediments.
However, in some specic conditions sediments rich in pozzolanically active components can
be formed during deposition, e.g., diatomaceous earths, or due to subsequent alteration, e.g.,
naturally burned clays.
Topical reviews on natural pozzolans and their applications can be found in Cook (1986a),
Day (1990), Malhotra and Mehta (1996), Colella et al. (2001) and Massazza (2001, 2002).
Unaltered pyroclastic materials. The major pozzolanically active component of unaltered
pyroclastic pumices and ashes is a highly porous glass (Ludwig and Schwiete 1963). The
easily alterable, or highly reactive, nature of these ashes and pumices limits their occurrence
largely to recently active volcanic areas. Most of the traditionally used natural pozzolans
belong to this group, i.e., volcanic pumice from Pozzuoli, Santorin earth and the incoherent
parts of the German trass. The international (IUGS) classication of glassy or aphanitic rock
types based on the chemical composition has been applied to natural pozzolans of volcanic
origin. The reported chemical data of 150 unaltered pyroclastic materials and 83 zeolitized
rocks used as natural pozzolanic material were plotted in a total alkali over SiO2 diagram on a
recalculated 100% volatile-free basis in Figure 5. A large spread of data indicates the variability
in composition. SiO2 being the major component, most natural unaltered pumices and ashes
fall in the intermediate (52-66 wt% SiO2) to acid (> 66 wt% SiO2) composition range. The
Figure 4. Global distribution of volcanic rocks (grey areas) and deposits of reported natural supplementary
cementitious materials (black dots).
218 Snellings, Mertens, Elsen
predominant rock types are dacite and rhyolite, representing respectively 29% and 21% of the
reported analyses. Basic (45-52 wt% SiO2) and ultrabasic pyroclastics (< 45 wt% SiO2) are
less commonly used as natural pozzolans, and represent only 15% of the reported analyses.
The total alkali content is variable and linked to the regional type of volcanism. It can reach
levels higher than 11 wt% on an anhydrous basis in Neapolitan pozzolans (e.g., Battaglino and
Schippa 1968), Moroccan “leucitite” (Hilali et al. 1981 in Day 1990) or pumicite from Idaho
(Asher 1965 in Day 1990).
A synthesis of the collected chemical data of the unaltered pyroclastics is shown as a
series of box plots in Figure 6. Apart from SiO2 as the main component, Al2O3 is present in
substantial amounts in most reported pozzolans. Most samples contain Fe2O3 and MgO in mi-
nor proportions, which is typical of more acid rock types. CaO and alkali concentrations are
usually modest, but can vary substantially depending on for instance the presence of calcite as
a secondary phase. Loss on ignition (LOI) is generally low but can reach values over 10 wt% in
some trasses and tuffs which probably contain substantial amounts of unreported zeolites and/
or clay minerals.
A summarizing representation of the collected chemical data of the unaltered pyroclastics
is shown as a series of box plots in Figure 6. Apart from SiO2 as the main component, Al2O3
is present in substantial amounts in most reported pozzolans. Most samples contain Fe2O3
and MgO in minor proportions, which is typical for more acid rock types. CaO and alkali
concentrations are usually modest, but can vary substantially depending on for instance the
presence of calcite as a secondary phase. Loss on ignition (LOI) is generally low but can reach
values over 10 wt% in some trasses and tuffs which probably contain substantial amounts of
unreported zeolites and/or clay minerals.
Figure 5. IUGS classication (Le Maître et al. 1989) of vitreous rock types based on chemical composition
on an anhydrous basis.
Supplementary Cementitious Materials 219
The mineralogical composition of unaltered pyroclastic rocks is mainly determined by
the presence of phenocrysts and the chemical composition of the parent magma. Additionally,
the pyroclastic material can become intermixed with the constituents of detrital or biochemical
sediments during deposition. Finally, some limited alteration can have occurred after
deposition. The major component is volcanic aluminosilicate glass typically present in
quantities over 50 wt%. Unaltered pyroclastics containing signicantly less volcanic glass, such
as the trachyandesite from Volvic (France) with only 25 wt% are reported to be less reactive
(Mortureux et al. 1980). Within a group of unaltered natural pozzolans with similar volcanic
origin and composition, a correlation between amorphous phase content and pozzolanic activity
has been observed (Mehta 1981). The glass SiO2 content ranges between 45 and 75 wt% and its
chemical composition can differ slightly from the bulk composition through the incorporation
of lithophile elements in high-temperature phenocrysts (e.g., Ca in anorthite). Mielenz et al.
(1950) reported that basaltic glass appeared to be inferior to more acid glasses in terms of
pozzolanic activity. Akman et al. (1992) conrmed the better performance of natural pozzolans
of rhyolitic and trachytic composition over more basaltic pozzolans. Apart from the glass
content and its morphology associated with the specic surface, also defects and the degree
of strain in the glass phase appear to be important for the pozzolanic activity (Mehta 1981).
The impact of the latter parameters remains unclear due to difculties in their experimental
quantication.
Typical associated minerals present as large phenocrysts are members of the plagioclase
feldspar solid solution series varying between albite and anorthite depending on the parent
magma composition. In pyroclastic materials in which alkalis predominate over Ca, K-feldspar
such as sanidine or albite Na-feldspar were identied (Neapolitan pozzolans). Leucite is present
as phenocrysts in the K-rich, silica-poor Latium pozzolans (Costa and Massazza 1974). Quartz
is usually present in minor quantities in acidic pozzolans, while pyroxenes and/or olivine
phenocrysts are often found in more basic materials. Xenocrysts or rock fragments incorporated
during the violent eruptional and depositional events are also encountered. Zeolite and clay
minerals are often present in minor quantities as alteration products of the volcanic glass. While
zeolitization in general is benecial for the pozzolanic activity, clay formation or argillitization
Figure 6. Box plots of the distribution of collected chemical data (N = 150) of unaltered pyroclastic
materials investigated as pozzolanic material. Crosses are outliers.
220 Snellings, Mertens, Elsen
has adverse effects on the performance of lime-pozzolan blends or blended cements (Massazza
1974; Sersale 1980; Akman et al. 1992; Türkmenoğlu and Tankut 2002). Other early diagenetic
products reported are opal CT (a more or less disordered interlayering of the SiO2 polymorphs
cristobalite and tridymite) and secondary calcite. The former shows elevated pozzolanic activity,
the latter is not a pozzolan but can contribute to the material performance of Portland cement
by reacting with calcium-aluminate-hydrate reaction products (cf. infra; Matschei et al. 2007a;
Matschei and Glasser 2010) or acting as a ller material.
Another type of unaltered volcanic pozzolanic material is perlite, a volcanic glass
containing relatively high amounts of water. Perlite is typically rhyolitic, containing around 75
wt% SiO2, 10-15% Al2O3 and additional alkalis. The pozzolanic activity of ground perlite from
deposits in Turkey (Erdem et al. 2007) and China (Yu et al. 2003) was evaluated to be high.
Altered pyroclastic materials. Diagenetic alteration of pyroclastic deposits by alkaline
uids results in the formation of micrometer size zeolite minerals (Fig. 7). Alteration by less
alkaline uids usually leads to the development of clay minerals. The recrystallization of the
volcanic glass often results in a more compact and coherent tuffaceous rock. Zeolites are
members of the tectosilicate group and possess open, porous framework structures of corner-
sharing AlO4 and SiO4 tetrahedra. The aluminosilicate tetrahedra are arranged in rings which
are three-dimensionally connected to form open cages and channels running through the
crystal. The substitution of Al3+ for Si4+ imposes a net negative framework charge, which is
compensated by exchangeable cations in cages and channels. Water molecules are sequestered
in the cages due to charge-dipole interactions. Zeolitization can occur in a range of geological
environments. The most important zeolitization processes in terms of deposit volume are due
to circulation of saline-alkaline lacustrine waters, of alkaline ground water or of hydrothermal
uids and due to low-grade burial diagenesis metamorphism (Hay and Sheppard 2001).
The range in chemical composition of the zeolitized pyroclastic materials used as pozzolans
broadly coincides with that of their unaltered counterparts (Fig. 8). The most prominent
difference is the higher loss on ignition due to the formation of zeolite and/or clay minerals
containing substantial amounts of water. On an anhydrous base, the altered pyroclastics used as
Figure 7. SEM picture of typical micrometer-size “cofn”-shaped clinoptilolite crystals formed as
alteration product of volcanic glass in a tuff sample from Karacaderbent, Turkey.
Supplementary Cementitious Materials 221
pozzolans tend to be more siliceous and contain somewhat more alkalis. A comparison of the
unaltered and altered pyroclastics in a ternary diagram in Figure 9 corroborated this observation
in that a larger proportion of the unaltered pumices and ashes are shifted towards the CaO
and Al2O3 apices. This can be linked with the fact that zeolitized rocks used as pozzolans
are mostly reported from regions with products of more siliceous or alkali-rich volcanism.
More siliceous zeolitized rocks are encountered in the Balkan region (e.g., Držaj et al. 1980;
Figure 8. Box plots of the distribution of collected chemical data (N = 83) of zeolitized pyroclastic
materials investigated as pozzolanic material.
Figure 9. Ternary CaO-SiO2-Al2O3 diagram (wt% based) comparing the chemical composition of unaltered
and zeolitized pyroclastics used as pozzolanic material.
222 Snellings, Mertens, Elsen
Naidenov 1991; Janotka et al. 2003), Greece (e.g., Kitsopoulos and Dunham 1996; Fragoulis et
al. 1997; Stamatakis et al. 1998; Perraki et al. 2003) and in the Turkey-Iran volcanic chain (e.g.,
Canpolat et al. 2004; Yilmaz et al. 2007; Uzal et al. 2010; Ahmadi and Shekarchi 2010). In the
Circumpacic siliceous zeolite deposits utilized or evaluated as pozzolans occur for instance
in New Zealand, the Philippines (Mertens et al. 2009), Japan (Takemoto and Uchikawa 1980),
the western United States (Mielenz et al. 1950), Mexico (Rodriguez-Camacho and Uribe-Af
2002) and Ecuador (Mertens et al. 2009). Zeolite deposits of trachytic to phonolitic composition
are situated on the Italian peninsula (e.g., Sersale and Frigione 1987; Liguori et al. 2003) and in
the Eifel region in Germany (e.g., Liebig and Althaus 1998).
The type of zeolite assemblage formed during alteration depends on a number of factors.
Most important are the temperature of formation, the chemical composition of the volcanic
glass and the zeolitizing uid (Chipera and Apps 2001). Zeolites are not the thermodynamically
most stable phases and form because of faster reaction kinetics. The most widespread siliceous
zeolites are clinoptilolite, mordenite and erionite. These zeolites are observed to form in the
alteration of siliceous glasses, whereas the common more aluminous zeolites, phillipsite,
chabazite, analcime and heulandite (alumina-rich polymorph of clinoptilolite) form from
the alteration of more basic glasses. Heulandite-clinoptilolite is by far the most frequently
(up to 60%) identied zeolite mineral in natural zeolite-rich pozzolans. The compositional
exibility of this zeolite in terms of Si-Al and exchangeable cation composition promotes its
formation. Mordenite was identied as a major constituent in about 10% of the altered natural
pozzolans and is a silica-rich zeolite with a predominance of Na over Ca and K as exchangeable
cation. Most of the identied chabazite and phillipsite pozzolans are derived from alkali-rich,
intermediate to basic pyroclastics occurring in Italy or Germany and together make out 25%
of the reported occurrences. Phillipsite is usually formed in K-rich volcaniclastics such as the
Neapolitan Yellow Tuff. Analcime is a Na-rich zeolite which can be formed by the reaction of
volcanic glass or zeolite precursors with Na-rich uids and commonly occurs together with
quartz. The eventual zeolite content of the tuff is a function of the original amount of volcanic
glass and the extent of the zeolitization process. Altered pyroclastic materials can consist of
several types of zeolite and zeolites are often associated with other alteration products such as
opal A (amorphous), opal CT, clay minerals and authigenic feldspar and the original pyrogenic
phenocrysts (cf. supra).
The main pozzolanically active phases are considered to be the zeolite and silica-
polymorphs together with relict volcanic glass. Zeolite is reported to be more reactive than
its unaltered vitreous counterpart (Sersale and Frigione 1987). The pozzolanic activity of a
zeolitized pozzolan seems to be a function of a number of variables. First of all, the content
of zeolite and other active phases is important (Liguori et al. 2003), next the reactivity of
the zeolite itself depends on its crystallinity (Snellings et al. 2010b), and its framework and
exchangeable cation composition (Caputo et al. 2008; Mertens et al. 2009; Snellings et al.
2009). Silica-rich zeolites have been found to render the blended cements more performant in
terms of strength and durability than aluminous zeolites (Huizhen 1992; Fragoulis et al. 1997;
Rodriguez-Camacho and Uribe-Af 2002; Caputo et al. 2008), which can be linked with larger
amounts of calcium-silicate-hydrate reaction products (C-S-H) being formed. Furthermore,
clinoptilolite tuffs exchanged to Na or K were more reactive than tuffs exchanged to Ca (Luke
2007; Mertens et al. 2009). A classication solely based on the presence of a particular zeolite
type does not seem to be meaningful due to the variability in zeolite content, composition and
crystallinity in tuffs.
(Bio)chemical sediments. This category comprises both biogenic sediments, resulting
from the deposition of (micro-) organism skeletons, as well as chemical precipitates resulting
from the circulation of hydrothermal waters. Diatomaceous earths are the principal biogenic
materials that show pozzolanic activity. They mainly consist of siliceous skeletons or frustrules
Supplementary Cementitious Materials 223
of diatom micro-organisms together with variable amounts of calcareous biogenic material and
detrital sediment such as clay minerals. The diatoms can be deposited in lacustrine and marine
waters. The depositional environment inuences the size of the diatoms, those of fresh-water
origin generally being smaller and more pozzolanically reactive (Stamatakis et al. 2003). The
distribution of chemical data of 22 diatomaceous earths is presented in Figure 10. The silica
content is usually high but can drop signicantly because of mixing of siliceous diatoms with
calcareous material. In reported impure diatomaceous earths calcite contents may reach 50
wt% or more. The presence of feldspars, clay minerals and nely divided iron(hydr)oxides is
evidenced by signicant amounts of Al2O3 and Fe2O3. The levels of alkalis and MgO are usually
very low.
Diatom frustrules are composed of opal A. The higher the opal A content of the diatomaceous
earth, the higher the pozzolanic activity and the better the performance. The pozzolanic activity
of diatomaceous earths containing large amounts of clay minerals, such as Danish “moler,” can
be improved by thermal decomposition of the clay minerals (Johansson and Andersen 1990).
In calcite-rich diatomites the reduced amount of opal A can be partially compensated by a ller
effect of the calcareous compounds (Stamatakis et al. 2003). The intricate surface morphology
and high porosity of the disk-shaped diatom frustrules (Fig. 11) results in typically elevated
specic surface areas and water demand, limiting the incorporation of diatomite earths to about
15% without usage of water-reducing agents such as superplasticizers (Stamatakis et al. 2003;
Degirmenci and Yilmaz 2009).
Interest in some hydrothermal siliceous sinters has been raised principally because of their
high silica content. Deposits considered for utilization as pozzolanic material are located in
New Zealand, Japan (Takemoto and Uchikawa 1980) and Turkey (Davraz and Gunduz 2005,
2008). Opal A is the main constituent together with some quartz and the material is described
as a pumiceous, porous rock demonstrating a high specic surface area.
Materials of detrital and mixed origin. Detrital sediments are usually largely composed
of stable mineral compounds derived from the erosion and weathering of other rocks. In these
Figure 10. Box plots of the distribution of collected chemical data of diatomaceous earths (N = 22)
materials investigated for application as pozzolanic material.
224 Snellings, Mertens, Elsen
sediments, it is rather uncommon to nd pozzolanically reactive compounds such as reworked
volcanic glass fragments or diatoms in sufcient quantities to be suitable for application. Some
exceptional materials of detrital or mixed origin have nevertheless been used as a pozzolan. One
is the Sacrofano earth of mixed origin which can be found near Viterbo (Italy) and shows a very
high SiO2 content around 85-90 wt%. Volcanic particles, diatoms and some crystalline minerals
present were severely altered by acid uids inltrating the uppermost layers of the deposit
and leaving essentially a dessicated pozzolanically reactive silica gel (Massazza 2001). Other
reactive materials deriving from detrital rocks are naturally burned clays such as porcellanite
from Trinidad (Day 1990) and gliezh from Central Asia (Kantsepolsky et al. 1969). The former
is thought to have formed by spontaneous combustion of bituminous or lignitic clays and the
latter are shales burned by natural subsurface coal res (Massazza 2001). Gaize is a sedimentary
rock occurring in the French Ardennes and Meuse valley. It has a high proportion of quartz and
biogenic siliceous material and a substantial amount of clay. Gaize is generally calcined before
use as a pozzolan (Cook 1986a). Table 1 shows an overview of the chemical composition of
several particular natural pozzolans. In Figure 12 natural pozzolans of a sedimentary origin can
be compared in a CaO-SiO2-Al2O3 ternary plot. The presence of calcite in some diatomaceous
earths is illustrated by the mixing trend occurring between the CaO and SiO2 apices. The
majority of other pozzolans are characterized by their high SiO2 content.
Thermally activated SCMs
The use of natural pozzolans was traditionally limited to the regions where they were
locally available. To produce water-proof lime-based binders in other regions, people
traditionally resorted to blends of thermally activated clays or soils and lime. Usually waste
streams of clay pottery, bricks or tiles were recycled as pozzolanic additive. The widespread use
of these thermally activated pozzolans before the advent of Portland cement can be illustrated
by their presence in e.g., the ooring fundaments of the San Marco basilica in Venice or the
Hagia Sophia in Istanbul (Zendri et al. 2004). Crushed brick pozzolans remains continued as a
traditional building material in some countries, e.g., under the name of surkhi in India, homra in
Egypt and sarooj in Oman (Cook 1986b; Al-Rawas et al. 1998). In modern construction, large
volumes of purposely burned clays have been used in the construction of large scale structures
such as dams (Jones 2002). The excellent pozzolanic properties of kaolinite-rich materials
Figure 11. SEM picture of disk-shaped diatom frustules from Giannota, Greece. [Used by permission of
Elsevier, from Fragoulis et al. (2005), Cement and Concrete Composites, Vol. 27, Fig. 1, p. 206.]
Supplementary Cementitious Materials 225
burned under controlled conditions, better known as metakaolin, has drawn renewed attention
towards deliberately thermally activated clays. The pozzolanic properties of burned clays and
shales have been reviewed by Cook (1986b), more specic reviews on metakaolin can be found
in Sabir et al. (2001), Jones (2002) and Siddique (2008).
Next to burned clays, shales and soils, burned agricultural residues consisting predominantly
of amorphous silica have received considerable attention as pozzolan for construction purposes
in rural areas deprived of other SCMs. General reviews are presented by Cook (1986c) and
Siddique (2008).
Burned clays and shales. Clays are dened as sediments which consist primarily of
particles smaller than 2 mm. The constituting particles are mostly phyllosilicates incorporating
a considerable amount of water. These minerals are commonly termed clay minerals and
are composed of sheets of tetrahedrally (T) coordinated SiO4 and AlO4 connected to sheets
of octahedrally (O) coordinated cations such as Al3+ or Mg2+ to form T-O or T-O-T layers.
Depending on the layer charge, exchangeable interlayer cations can be present between the
compound layers. Water is present as H2O in the interlayer and as (OH)- in the octahedral
Table 1. Overview of the chemical composition of natural pozzolans of sedimentary origin.
pozzolana origin Al2O3SiO2Fe2O3CaO Na2O K2O MgO SO3LOI SUM ref.
Sacrofano earth Italy 3.1 89.2 0.8 2.3 - - - - 4.7 100.1 1
Gliezh USSR 12.4 72.8 5.1 2.8 0.4 1.7 1.1 1.5 - 97.8 2
Gaize France 7.1 79.6 3.2 2.4 - - 1.0 0.9 5.9 100.1 3
Silica sinter Japan 2.4 87.7 0.5 0.2 0.1 0.1 0.2 0.1 4.1 95.4 4
Silica sinter Turkey 2.6 92.5 0.1 0.3 1.1 0 0 0.1 1.9 98.6 5
1: Battaglino and Schippa (1968); 2: Kantsepolsky et al. (1969); 3: Lea (1970);
4: Takemoto and Uchikawa 1980; 5: Davraz and Gunduz 2005
Figure 12. Ternary CaO-SiO2-Al2O3 diagram (wt% based) illustrating the distribution in chemical
composition of a selection of natural pozzolans from a sedimentary origin.
226 Snellings, Mertens, Elsen
layer. During burial, clays compact to coherent mud rock and eventually shale when cleavage is
developed. Clay minerals are the common product of chemical weathering of primary igneous
minerals such as feldspars or form during low-temperature diagenetic alteration. Depending on
the weathering conditions and the chemical composition of the altered material, various clay
minerals form. Commonly encountered clay minerals are kaolinite, smectites, illite, chlorite
and palygorskite-sepiolite. General chemical compositions of common clay minerals, lateritic
soils and red mud waste are given in Table 2. Untreated clays show unsatisfactory pozzolanic
activity and low technological performance. According to He et al. (2000), this is due to the
stability of their crystal structures, their high specic surface requiring a high water to binder
ratio for a desired workability, and the platy particle morphology and preferential cleavage.
Non-pozzolanic micro- and nanoscale clay particles were observed to change the cement paste
structure by serving as preferential substrates for C-S-H nucleation. Addition of smectite and
palygorskite resulted in a more open, interconnected cement pore structure compared to the
addition of silica fume (Lindgreen et al. 2008). Only kaolinite and smectite are reported to show
good pozzolanic activity when red at appropriate temperatures (Mielenz et al. 1950; Ambroise
et al. 1985; He et al. 1995; Liebig and Althaus 1997).
The pozzolanic activity of thermally activated clay minerals is closely linked with their
behavior during heating. Optimal activation is achieved through amorphization of the clay
mineral upon complete dehydroxylation of the octahedral sheet. Overheating results in particle
agglomeration and crystallization of inactive high-temperature phases. The temperatures at
which dehydration, dehydroxylation and recrystallization occur are determined by the clay
mineral structure and composition. Most important in the formation of a pozzolanically active
material is the temperature range of dehydroxylation which depends on the bonding of the
hydroxyl groups in the octahedral sheet. The thermal strength of these bonds increases from
Fe-OH over Al-OH to Mg-OH. The formation of recrystallization products depends mainly on
the chemical composition of the raw materials. The commonly formed minerals are mullite,
cordierite, enstatite and cristobalite (Emmerich 2010). The thermal behavior of the various
common clay minerals is illustrated in Figure 13.
The T-O clay mineral kaolinite, Al2Si2O5(OH)4, does not contain exchangeable cations
or interlayer water. The temperature of dehydroxylation (endothermal) of kaolinite depends
on the structural layer stacking order. Disordered kaolin dehydroxylates between 530 and
570 °C, ordered between 570 and 630 °C. Dehydroxylated disordered kaolinite shows higher
pozzolanic activity than ordered (Kakali et al. 2001; Bich et al. 2009). Upon dehydroxylation
kaolinite transforms into metakaolin, a complex amorphous structure which retains some long-
range order due to layer stacking (Bellotto et al. 1995). Much of the aluminum contained in the
octahedral layer becomes tetrahedrally and pentahedrally coordinated (Justnes et al. 1990; Rocha
and Klinowski 1990; Fernandez et al. 2011). Upon further heating a defect Al-Si spinel and
Table 2. Characteristic chemical compositions for a selection of
clay minerals, a laterite soil and red mud waste.
Material SiO2Al2O3Fe2O3TiO2CaO MgO Na2O K2O H2O SUM
Kaolinite 46.6 39.5 14.0 100.1
Illite 54.0 17.0 1.9 3.1 7.3 12.0 95.3
Montmorillonite 43.8 18.6 1.0 1.1 36.1 100.6
Palygorskite 58.4 6.2 14.7 19.7 99.0
Chlorite 30.3 17.1 15.1 25.4 12.1 100.0
Laterite 3 10 75 10 98.0
Red mud 5.0 15 26.6 15.8 22.2 1.0 1.0 0.0 12.1 98.7
Supplementary Cementitious Materials 227
eventually mullite are formed. Reported optimal activation temperatures vary between 550°C
(He et al. 1994) and 850 °C (Ayub et al. 1988) for varying durations, however most values
cluster around 650-750 °C (Murat 1983; Ambroise et al. 1987; De Silva and Glasser 1992;
Kakali et al. 2001; Chakchouk et al. 2009). In comparison with other clay minerals kaolinite
shows a broad temperature gap between dehydroxylation and recrystallization, this much
favors the formation of metakaolin. Also, because the Al-OH groups of the kaolinite octahedral
sheets are directly exposed to the interlayer, structural disorder can be attained more easily
upon dehydroxylation of kaolinite than in T-O-T clay minerals (Fernandez et al. 2011). If the
raw clay material is sufciently pure, metakaolin is a highly active SCM which can be used in
high-performance, high-strength binders to improve the compressive and exural strength and
increase durability and resistance to chemical attack (Siddique 2008). For low-cost applications
kaolinite deposits or tropical soils of lower purity can be used (e.g., Cara et al. 2006; Kakali et
al. 2001; Badogiannis et al. 2005). Highly active metakaolin can also be produced from paper
sludge waste or oil sand ne tailings. Paper sludge waste is mainly composed of cellulose bers,
kaolinite and calcite. Calcination at 700 °C for 2-5 hours decomposes and volatilizes the organic
material and activates the kaolinite while no calcite decarbonation occurs (Péra and Amrouz
1998; Frias et al. 2008). In some cases the clay fraction of oil and tar sands consists mainly of
kaolinite. The separated ne tailings can be valorized by controlled ring and production of
metakaolin (Wong et al. 2004). Alternatively, kaolinite can be activated by mechanochemical
(Vizcayno et al. 2009) or acid treatment. Aluminum extraction from calcined kaolin by sulfuric
acid results in a dealuminated kaolin with high pozzolanic activity (Mostafa et al. 2001).
Figure 13. Representative differential thermal analysis (DTA) curves for common clay minerals and
gibbsite (modied after Cook 1986b).
228 Snellings, Mertens, Elsen
Smectites are a group of T-O-T clay minerals containing exchangeable cations and show-
ing the remarkable capability to absorb water between the layers and swell. Isomorphous
substitutions in the tetrahedral and octahedral sheets as well as the interlayer cation type inu-
ence the dehydration and dehydroxylation behavior. Dehydration occurs below 300 °C and
is associated with the release of water bound to outer surfaces and coordinated to interlayer
cations. Dehydroxylation temperature depends on the type of cations contained in the octahe-
dral sheet (Fe < Al < Mg) and their arrangement. When the cations contained in the octahedral
sheet are mainly trivalent (Al3+, Fe3+) then only two out of three cation positions are lled and
the clay minerals are termed dioctahedral. The disposition of the hydroxyl groups with respect
to the resulting vacancy denes two kinds of congurations: cis-vacant when both hydrox-
yls are located on one side of the vacancy and trans-vacant when they are located on either
side (Tsipursky and Drits 1984). Dehydroxylation of smectites which show mainly cis-vacant
octahedral sheets occurs at temperatures around 700 °C, 150-200 °C above the dehydroxyl-
ation of trans-vacant octahedral sheets. Smectites contain predominantly cis-vacancies while
illite clay minerals are mostly trans-vacant (Drits et al. 1995). Recrystallization starts above
850 °C, depending on the chemical composition of the clay. The reported optimal activation
temperature ranges for montmorillonite-rich clays are typically higher and narrower dened
when compared to kaolinite: 800-830 °C for 1-5 hours (Mielenz et al. 1950; He et al. 1996;
Liebig and Althaus 1997; Habert et al. 2009). Overheating to recrystallization temperatures is
easily attained and is together with the variable chemical composition a complicating factor
in thermal activation. Activated montmorillonite has been observed to be a good pozzolanic
material, incorporation into lime or Portland cement based binders contributed signicantly to
the compressive strength (He et al. 1995; Liebig and Althaus 1997).
Other clays are reported to be less pozzolanically active upon thermal activation. The
dehydroxylation of illite, a T-O-T clay mineral with a composition intermediate between mica
and smectite, occurs at relatively low temperatures (580 °C). However dehydroxylation does
not result into a collapse of the structure into a largely amorphous state before recrystalliza-
tion into spinel and corundum occurs (He et al. 1995). In consequence, the resulting calcined
material shows much lower pozzolanic activity than that formed from less stable smectites or
kaolinite.
Mg-rich T-O-T clays of the palygorskite-sepiolite group contain both interlayer and zeo-
lite water and show complex dehydration reactions. The release of zeolite water and adsorbed
water proceeds simultaneously at low temperature (Fig. 13) and is succeeded by the release of
interlayer water. Dehydroxylation of palygorskite occurs below 500 °C, sepiolite dehydrox-
ylates at 820 °C, simultaneously with decomposition and recrystallization into enstatite and
cristobalite. The pozzolanic activity was observed to be low (He et al. 1996). The T-O-T octa-
hedral sheet in chlorite clay minerals tends to dehydroxylate at rather high temperatures due to
the elevated Mg content. Recrystallization into spinel and olivine occurs at low temperatures
leaving a narrow window for thermal activation.
Natural clay deposits often contain more than one type of clay mineral. Habert et al.
(2009) observed that ideal activation of all different clay minerals in a mixture is difcult, be-
cause recrystallization of previously activated clay can occur when a second one is activated.
Furthermore they found that optimal activation and recrystallization temperatures were low-
ered in the impure mixtures with respect to the puried materials.
In tropical climates, intense chemical weathering leaches out silica, alkaline earths and
alkalis from soils to leave a residue of aluminum and ferric (hydr)oxides. The resulting soils
are bauxites and laterites, respectively. Calcination of laterites at 750 °C has been reported to
produce a pozzolanically active material which improved the compressive strength of blended
cements (Péra et al. 1998). Red mud waste obtained from the extraction of aluminum from
bauxites can be transformed into a pozzolanically active material by calcination in the range
Supplementary Cementitious Materials 229
of 600-800 °C (Péra et al. 1997). Dehydroxylated aluminum and ferric hydroxides tend to
produce cementitious reaction products when contacted with lime.
Oil shales are ne-grained sedimentary rocks containing 5-65% of organic material and
detrital minerals such as clay minerals, quartz and calcite. If the level of organic material is
sufcient, oil shales can be combusted and utilized for electricity production or as alternative
raw material in the production of Portland cement. Depending on the type of organic material,
the oil can also be extracted by pyrolysis. The resulting ashes can show hydraulic or pozzolanic
activity. In ashes abundant in calcium, hydraulic calcium silicates (b-C2S) and aluminates were
observed to form (Smadi and Haddad 2003). The pozzolanic activity of the low calcium ashes is
strongly linked to the original mineralogy of the shale and the burning conditions (Ish-Shalom
et al. 1980). Inactive phases such as quartz and mica can represent an important fraction of the
ash and lower the pozzolanic activity (Feng et al. 1997).
Although waste ceramic material has been utilized intensively as a pozzolanic additive in
the past, contemporary ceramic waste does not nd the same application. Modern ceramic pro-
duction usually involves controlled ring at elevated temperatures between 900-1050 °C, which
are invariably above the optimal activation temperature of the original clay minerals (Baronio
and Binda 1997). In most cases signicant high-temperature recrystallization occurs and in
consequence the pozzolanic activity is relatively low, resulting in lowered compressive strength
values upon increasing replacement of Portland cement (Wild et al. 1997; Lavat et al. 2009).
Burned organic matter residues. Ashes of burned agricultural residues can be used as an
inexpensive alternative material for partial replacement of Portland cement or lime. While a
large proportion of agricultural residues are recycled for fertilization or stock feed purposes,
some is utilized as fuel or left as waste. Firing leads to the decomposition of the organic carbon
and a concentration of the silica in the residue. Under controlled ring conditions a highly
active pozzolan material can be produced (Mehta 1977). The elevated specic surface area and
considerable surface roughness are often associated with a high water demand of the ashes and
necessitate the use of superplasticizers when the replacement percentage of optimally red
ashes is more than 10-20%. The yield of production of a pozzolanic material from organic
residues is determined by two factors: the inorganic residue after ignition or ash content and the
silica content of the inorganic residue. Typical inorganic residue fractions and silica contents
are enlisted in Table 3 for common agricultural residues. The greatest yield can be expected for
the calcination of rice husks. Upon ring, a highly siliceous material with a very large surface
area can be obtained. Some of the enlisted materials such as corn or rice straw face strong
competition from the utilization as stock feed (Cook 1986c).
Table 3. An overview of ash content of plant parts and silica
content of the ash (modied after Cook 1986c).
Plant Part of plant Ash (%) Silica (%)
Sorghum Leaf sheath 12.55 88.7
Wheat Leaf sheath 10.48 90.56
Corn Leaf blade 12.15 64.32
Sugar cane Bagasse 14.71 73
Sugar cane Straw 62
Sunower Leaf and stem 11.53 25.32
Rice Husk 22.15 93
Rice Straw 14.65 82
Palm Fibers and shells 65
230 Snellings, Mertens, Elsen
A ternary plot of reported chemical compositions of various organic material ashes in Figure
14 demonstrates the highly siliceous nature of rice husk ash compared to other residues. The
level of soil derived impurities can often reach signicant levels and can become concentrated
in the inorganic residue upon ring. This can lead to a dilution of the active components in the
ash (Cordeiro et al. 2008). Also soil and climate conditions can inuence the ash and silica
contents of the residue (Biricik et al. 1999).
The pozzolanic activity of the inorganic residue is strongly affected by the ring temperature.
Optimal activation is achieved when the cellulose and other combustibles are removed and the
pore structure and associated large surface area (25-40 m²/g) of the silica-rich skeleton are
preserved (Jauberthie et al. 2000) (see Fig. 15). The dissolution of the silica is strongly linked
with the concentration of silanol groups at the surface (Nair et al. 2008). Over-heating leads to
reductions in specic surface and to transformation of the amorphous silica to crystalline high-
temperature silica polymorphs such as cristobalite and tridymite. Firing should therefore be
performed in an oxidizing atmosphere at temperatures above 400 °C to decompose the organic
material (James and Rao 1986) but, for rice husk ash, below 700 °C to avoid the formation of
cristobalite and tridymite (Hamdan et al. 1997). Prolonged exposure to elevated temperatures
leads to a collapse of the pore structure and to decreased recrystallization temperatures (Cook
1986c). The sensitivity of the activated material to the ring conditions and the utilization of
agricultural residues as fuel for domestic or industrial purposes have been primary obstacles
to the widespread application of organic material ashes as a pozzolanic material. Therefore,
recently, research focus shifted to the beneciation of ashes of organic residues used as fuel.
The sensitivity of the activated material to the ring conditions and the utilization of
agricultural residues as fuel for domestic or industrial purposes have been primary obstacles
to the widespread application of organic material ashes as a pozzolanic material. Therefore,
recently, research focus shifted to the beneciation of ashes of organic residues used as fuel.
Sugar cane bagasse and straw ashes resulting from uncontrolled combustion in boilers of
Figure 14. CaO-SiO2-Al2O3 ternary diagram (wt% based) of chemical composition of reported organic
residue ashes.
Supplementary Cementitious Materials 231
sugar and alcohol factories at 700-900 °C were found to behave pozzolanically when nely
ground (Cordeiro et al. 2008, 2009a; Chusilp et al. 2009; Morales et al. 2009). Also the ashes
of uncontrolled ring of rice husks (De Sensale 2006; Cordeiro et al. 2009b), rice husks and
eucalyptus bark (Tangchirapat et al. 2008) and palm oil residue (Tangchirapat et al. 2009) show
pozzolanic activity when nely ground.
By-product SCMs
The valorization of industrial and societal waste in construction materials is one of the
main routes for progress in increasing the sustainability of present-day society. A large diversity
of waste materials can be considered and applications can range from low-value products such
as aggregates to high-value products such as some SCMs. In this section, the properties of
the most widely used industrial by-product SCMs are reviewed. The widespread success of
blast furnace slags, coal and lignite y ash and silica fume can serve to corroborate the view
that construction materials will increasingly become a primary target for disposal of industrial
and societal waste. The extensive experience and knowledge built up over years of using these
established by-product SCMs should be considered as valuable when proceeding into a future
expansion of the range of waste materials suitable for construction purposes.
Blast furnace slags. The most widely utilized slags in cements are obtained as a by-
product in the extraction of pig iron in blast furnaces. Blast furnace slags commonly present
latent hydraulic behavior if they are quenched sufciently rapidly as a vitreous phase from
the melt at 1350-1550 °C to below 800 °C and nely ground. The rst hydraulic cements
incorporating blast furnace slags were pioneered in the 2nd half of the 19th century in Germany,
rst as Ca(OH)2-slag blends, later on as SCM in Portland cement. Nowadays, blast furnace slags
are mostly used in combination with Portland cement and, being latent hydraulic, can constitute
a much larger proportion of the blended cement than pozzolanic materials. In addition, blast
furnace slags present marked cementitious properties when activated by either lime, alkali
hydroxides, sodium carbonates or sodium silicates (water glass) and calcium or magnesium
sulfates (e.g., supersulfated slag cements). Over recent years the activation of various types
of slags has received much attention as potential alternative for Portland cement, a detailed
Figure 15. SEM picture of fragments of the highly-porous silica-rich skeletons of rice husks after
uncontrolled burning in a eld furnace.
232 Snellings, Mertens, Elsen
account of the progress made is however out of the scope of this chapter. Topical reviews on
blast furnace slags in cements are available (Smolczyk 1980; Regourd 1986, 2001; Lang 2002).
The chemical composition of a slag varies widely depending on the composition of the
raw materials in the iron production process. Silicate and aluminate impurities from the ore and
coke are combined in the blast furnace with a ux which lowers the viscosity of the slag. In
the case of pig iron production the ux consists mostly of a mixture of limestone and forsterite
or in some cases of dolomite, the latter for reasons of economy. In the blast furnace the slag
oats on top of the iron and is decanted for separation. Slow cooling of slag melts results in an
unreactive crystalline material consisting of an assemblage of Ca-Al-Mg silicates. To obtain the
slag hydraulicity, the slag melt needs to be rapidly cooled or quenched below 800 °C in order to
prevent the crystallization of merwinite and melilite (Regourd 1986). To cool and fragment the
slag a granulation process can be applied in which the molten slag is subjected to jet streams of
water or air under pressure. Alternatively, in the pelletization process the liquid slag is partially
cooled with water and subsequently projected into the air by a rotating drum. Pumiceous,
porous pellets are produced in which the mostly glassy fraction ner than 4 mm can be used as
a supplementary cementitious material. The coarser, typically more crystalline fraction can be
used as lightweight aggregate. The pelletization process is more economical in terms of water
consumption during the cooling process and energy needed to dry the slurry of quenched slag
fragments but is usually less effective in obtaining high glass contents (Taylor 1990) compared
to the granulation process. In order to obtain a suitable reactivity, the slag fragments are ground
to reach the same neness as the Portland cement.
The main components of blast furnace slags are CaO (30-50%), SiO2 (28-38%), Al2O3 (8-
24%) and MgO (1-18%) (Regourd 2001). In a CaO-SiO2-Al2O3 ternary diagram (Fig 16, Fig.
3) the blast furnace slags can be situated somewhere in between typically pozzolanic materials
such as natural SCMs or silica fume and Portland cements, indicating the “latent hydraulic-
ity” of the slags. In general, increasing the CaO content results in raised slag basicity and an
Figure 16. Ternary diagram (wt% based) of the compositional distribution of a selection of blast furnace
slags.
Supplementary Cementitious Materials 233
increase in compressive strength of the binder, the MgO and Al2O3 contents show the same
effect up to respectively 10-12% and 14% beyond which no further improvement can be ob-
served. Also higher levels of alkali lead to strength improvement, especially at early ages. Other
minor components such as MnO and TiO2 exert a negative effect on the hydraulic character
of the slag. Several compositional ratios or so-called hydraulic indices have been used to cor-
relate slag composition with hydraulic activity; the latter being mostly expressed as the binder
compressive strength. Normative compositional requirements have also been formulated based
on a number of hydraulic indices, for instance the EN-197-1 norm requires a (CaO+MgO)/
SiO2 larger than 1, the German and Japanese standard demand a (CaO+MgO+Al2O3)/SiO2
larger than respectively 1 and 1.4. However, the relationship between the hydraulic indices and
compressive strength are very roughly dened and do not allow an accurate prediction of com-
pressive strength solely based on the slag chemical composition for a series of slags originating
from different production sites (Lang 2002). Other parameters which have been often included
in multiple regression analyses for strength prediction are the slag neness and glass content
(e.g., Douglas et al. 1990; Escalante et al. 2001; Pal et al. 2003; Bougara et al. 2010).
The glass content of slags suitable for blending with Portland cement typically varies
between 90-100% and depends on the cooling method and the temperature at which cooling is
initiated. The glass structure of the quenched slag largely depends on the proportions of network-
forming elements such as Si and Al over network-modiers such as Ca, Mg and to a lesser
extent Al. The network-forming atoms are tetrahedrally coordinated by oxygen atoms and show
a varying degree of polymerization or connectivity depending on the ratio of network-forming
to network-modifying elements. Increased amounts of network modiers lead to higher degrees
of network depolymerization and reactivity (Goto et al. 2007). The rate of cooling inuences the
amount of structural defects in the glass phase, the higher the cooling rate, the more defects and
the higher the reactivity. The presence of small amounts of nely dispersed crystalline material
has been observed to improve the reactivity of the slag (Demoulian et al. 1980; Frearson and
Uren 1986), especially when dendritic crystallization of merwinite (Ca3Mg(SiO4)2) occurs.
The Al-enrichment of the glass near the merwinite crystallites and the mechanical stress and
presence of nucleation sites introduced by the phase separation are suggested to enhance the
slag reactivity.
Another common crystalline constituent of blast furnace slags is melilite, a solid solution
between gehlenite (Ca2Al2SiO7) and åkermanite (Ca2MgSi2O7). The CaO-SiO2-Al2O3 phase
diagram at 10% MgO (Fig. 17) shows that most blast furnace slags will initially crystallize
melilite, other minor components which might form during progressive crystallization are belite
(Ca2SiO4), monticellite (CaMgSiO4), rankinite (Ca3Si2O7), (pseudo-) wollastonite (CaSiO3)
and forsterite (Mg2SiO4). The a′ and b-polymorphs of belite and possibly also melilite show
hydraulic activity and can contribute to the compressive strength. Minor amounts of reduced
sulfur are commonly encountered as oldhamite (CaS) (Scott et al. 1986). The reducing
environment that blast furnace cements can provide are often advantageously used for waste
stabilization (Roy 2009). Deleterious free lime (CaO) or periclase (MgO) are usually not
present in blast furnace slags.
Fly ash. Coal y ash is a gigascale material (Scheetz and Earle 1998). Over one billion
tons of by-products are generated annually during the combustion of coal (Kutchko and Kim
2006). These by-products include mainly dry bottom ash, wet bottom boiler slag, economizer
ash, y ash and ue gas desulphurization or scrubber sludge. Of these by-products, the annual
production of y ash is estimated around 500 millions of tons (Joshi and Lothia 1997). Coal
is not exclusively composed of organic matter, which produces the energy during coal ring
in power plants. It also contains a variable amount of inorganic material. This intermixed
inorganic material may remain unaffected or will be transformed during combustion. It will
then be concentrated in the by-products. Besides coal y ash, other types of y ash; for instance
234 Snellings, Mertens, Elsen
Municipal Solid Waste Incineration (MSWI) y ash exist. However, coal y ash is by far the
most widely used and investigated type of y ash. Therefore most attention will go to coal y
ash in what follows. It will simply be designated as “y ash” hereafter. Much of the information
in the following paragraphs is taken from the valuable work of Joshi and Lothia (1997) and
Sear (2001) who made extensive reviews of the production, the use and the properties of y ash.
From all coal combustion products (CCP), coal y ash (CFA), also designated as y
ash (FA) or pulverized fuel ash (PFA) is most widely used as a supplementary cementitious
material. Fly ash is produced when C-rich sediments are burned at temperatures reaching
1450 °C or higher. Whereas the carbon-based materials are mainly transformed to gaseous
compounds as CO2, H2O, NOX and SO2, most of the minerals that are present in the burned
sediments will not be volatilized. Instead, they may undergo various chemical, physical and
mineralogical changes. Whereas some of the primary minerals remain unaffected during the
burning process, others will melt, become amorphous and/or recrystallize to form secondary
minerals. These materials are then recovered at the bottom of the furnace/boiler or from the ue
gases by electrostatic or mechanical precipitators or bag houses (Joshi and Lothia 1997). Fly
ash is the material recaptured from the exhaust gases of power plants using pulverized coal as
a combustion product. Fly ash is generally subdivided in categories following the national or
international standards. It is a versatile, heterogeneous material that is extensively researched
regarding all its intrinsic properties, its use as a supplementary cementitious material or for
multiple other applications.
Since the beginning of the 20th century, y ash has been recognized as a pozzolanic
constituent (Joshi and Lothia 1997). Initially, only the Ca-poor y ashes resulting from the
burning of bituminous coal were considered as supplementary cementitious materials. However,
from about the middle of the 20th century, also the more Ca-rich y ashes were considered as
a cement replacement material. The actual use of y ash dates back to more than 50 years
ago (Sear et al. 2003). Besides scientic research dealing with y ash characterization, many
studies were done on the various applications of y ash. A considerable portion of this research
focuses on its use as a supplementary cementitious material in cement or concrete, even though
other uses increasingly attract attention. Most of the scientic work focuses on the physical/
Figure 17. Phase diagram of the CaO-SiO2-Al2O3 system (wt% based) at 10 wt% MgO, the blast furnace
chemical composition distribution largely falls into the melilite eld (modied after Satarin 1974).
Supplementary Cementitious Materials 235
mechanical properties of cement/y ash mixtures (e.g.: Papadakis 1999; Papadakis 2000;
Maltais and Marchand 1997; Erdoğdu and Türker 1998; Grzeszczyk and Lipowski 1997; Payá
et al. 1997), investigates the durability of y ash use (e.g.: Bijen 1996; Lilkov et al. 1997;
Papadakis 2000; Roy et al. 2001), or studies the hydration reactions more in general (e.g.,
Sharma and Pandey 1999; Lilkov et al. 1997; Papadakis 1999; Maltais and Marchand 1997).
Although the suitability of some types of y ash may still be a matter of debate (Papadakis
2000; Sear et al. 2003), the use of y ash in concrete is generally considered as benecial from
an ecological, economical and technological point of view.
In addition to the use as a SCM in traditional calcium silicate or aluminate hydrate binders,
y ash has many other applications. Amounts of 5-15% are generally cited as the proportion of
y ash used in concrete compared to the global amount of y ash produced. Some European
countries consume up to 100% of their production (Queralt et al. 1997), whereas other countries
as Israel seem to have already reached a much lower maximum for the addition of y ash
to cement clinker or concrete (Nathan et al. 1999). Mainly in Europe, other applications
account for the use of increasingly large amounts of this secondary raw material. Due to its
bulk mineralogy and chemistry, it can serve as a source of raw materials for large-volume,
low-tech applications (Scheetz and Earle 1998). As y ash is a complicated heterogeneous
material (Vempatie et al. 1994) of variable quality (Sakai et al. 2005), a detailed knowledge of
the physical and chemical characteristics is generally required (Vempatie et al. 1994) before
use. The numerous cementitious applications of y ash include grouts, block manufacture and
road sub-base and base construction (Sear et al. 2003). High proportions of y ash can also be
incorporated in dams, walls, girders, roller-compacted concrete pavements and parking areas
(Manz 1998; Sakai et al. 2005). However, large amounts of y ash can also be used for land
reclamation (Mondragon et al. 1990; Sear et al. 2003). Moreover, increasing efforts are done to
use the material for technologically valuable applications as for the manufacture of monolithic
ceramics (Mondragon et al. 1990; Queralt et al. 1997; Ilic et al. 2003) or for alkali-activated
materials (Puertas and Fernández,-Jiménez 2003; Bakharev 2005) with a signicant compressive
strength. Other high-tech applications include the production of zeolites (Mondragon et al.
1990; Murayama et al. 2002) or its use for phosphate immobilization from agricultural activities
(Grubb et al. 2000). Instead of using y ash as a bulk resource, some authors (Cheriaf et al.
1999; Vassilev et al. 2003) suggest to separate several fractions for different uses. The ultra-
ne fraction (0.1-1 mm) could for instance be used in high performance concretes (Sear 2001),
cenospheres as a strong lightweight inert ller material (Sear 2001) or for specic aerospace
applications (Mondragon et al. 1990). The magnetic fraction for instance could be competitive
as ferro-pozzolan for the production of dense concretes (Vassilev et al. 2004). Bottom ash,
another secondary raw material formed during coal combustion is also used in concrete as a
low-cost replacement material for sand or as a base in road construction. Bottom ash is also used
as a fertilizer (Cheriaf et al. 1999). Future research will be required to ensure the continuous
proper application of coal combustion waste products as y ash, considering for instance a
changing y ash composition. The types of fuel used in the future will change and diversify
and blends of fuels will be used (Steenari and Lindqvist 1999; Koukouzas et al. 2007). Thereby,
the nature of the y ash will inevitably be affected, requiring an adapted utilization. Moreover,
other coal combustion by-products such as bottom ash will probably gain more interest in the
future, as they are potentially appropriate as secondary raw material for various applications.
Other types of y ash such as MSWI y ash could similarly be used more often in the future
(Kirby and Rimstidt 1993; Ferreira et al. 2003).
The quality of the y ash varies widely (Sakai et al. 2005). Depending on the burning
temperature, the coal type, the processing and many other factors, y ash exhibits different
physical, chemical and mineralogical properties (Erdoğdu and Türker 1998; Vassilev et al.
2003). These will in turn affect the properties of the concrete or other products in which they are
used. The rst standards were introduced to classify y ashes in order to reduce this variability
236 Snellings, Mertens, Elsen
for y ash users. In 1953-1954, the ASTM standard C350 for y ash as an admixture was
introduced (Malhotra 1993; Manz 1998). In 1960 it was extended to the use of y ash as a
pozzolan. The ASTM standard C618 was introduced in 1968 and covers both natural pozzolans
and y ashes for their use as mineral admixtures in Portland cement concrete. A rst British
standard BS 3892 was introduced in 1965 in which y ash was treated as a ne aggregate with
three classes of neness for use in concrete. It was revised afterwards. In 1995, a common
specication for EU countries, EN 450, was introduced for y ash used in concrete. Particular
for this common European norm compared to the BS 3892, is the denition of the Activity
Index, imposing a minimum strength for y ash/Portland Cement mixtures. Also the ASTM
standard, C618 requires both y ash/Portland cement and y ash/lime mixtures to achieve a
minimum strength after 28 or 7 days respectively.
The ASTM C618 denes three categories of mineral admixtures; Class N, Class F and
Class C. Class N includes mainly the group of natural pozzolans. Class F y ash is a siliceous
type of y ash mainly obtained from the combustion of bituminous or hard coals. The main part
of the y ash used in concrete is of this type. Class C y ash is richer in calcium and results pri-
marily from the combustion of lignite or brown coal. Whereas the classication in ASTM C618
of mineral admixtures in Portland cement concrete derives from a genetic differentiation, some
authors (Mehta 1983, 1989; Joshi and Lothia 1995, 1997; Manz 1998) repeatedly suggested
making a distinction based on the properties of the admixtures. Joshi and Lothia therefore pro-
posed to dene one class of “pozzolanic but non-self cementitious” materials and another class
of “pozzolanic and self cementitious” or “hydraulic” materials. The two classes would in that
case be distinguished by their difference in loss on ignition.
It is generally believed that there is a close relationship between the properties of sup-
plementary cementitious materials (as y ashes) and their mineralogy (Vassilev and Vassileva
1996; Manz 1998). The mineralogy of y ashes is in general not considered in the norms. This
is due to a lack of quantitative analytical data (Ward and French 2006) and/or knowledge of
mineralogical analysis techniques (Manz 1998). However, reliable methodologies for mineral
quantication have been developed (Winborn et al. 2000) and can be used to establish cor-
relations with the Activity Index and other relevant parameters. It is therefore reassuring that
an increasing number of studies systematically use quantitative mineralogical information for
interpreting data.
Pulverized Fuel Ash may also be classied as “low-lime” and “high-lime” y ashes (Dhir
1986). This classication roughly corresponds to the ASTM class F and C y ashes respec-
tively. An alternative classication was proposed by Roy et al. (1981) and has been adopted by
others (Goodarzi 2006). Three major groups of oxides are dened; SiO2 + Al2O3 + TiO2 (sialic);
CaO + MgO + Na2O + K2O + (BaO) (calcic) and Fe2O3 + MnO + P2O5 + SO3 (ferric). The y
ash compositions are plotted in a ternary diagram where the three major classes sialic, calcic
and ferric, are dened next to the intermediate classes ferrocalsialic, ferrosialic, calsialic and
ferrocalcic.
Fly ash is composed of mainly spherical particles ranging in size from less than 1 to about
300 mm (Fig. 18). These particles form upon rapid cooling of droplets of viscous, molten or
even vaporized mineral matter that was initially present in the combustion product. The major
consequence of the rapid cooling is that only few minerals will have time to crystallize and
that mainly amorphous, quenched glass remains. Nevertheless, some refractory minerals in
the pulverized coal will not melt (entirely) and remain crystalline. In the following sections,
the physical, chemical and mineralogical properties of y ashes in general and of their discrete
constituents in particular will be discussed.
The average specic gravity of y ash is estimated around 2.2 with a standard deviation of
about 0.3. The BET specic surface area may range from less than 0.5 to more than 10 m²/g.
Supplementary Cementitious Materials 237
Average Blaine surface areas are in the order of 0.35 m²/g and are relatively less divergent
compared to tabulated BET surface areas. When pulverized, the specic gravity of the y ash
increases to about 2.7 as hollow particles break up (Dhir 1986). There also seems to be a direct
relation between specic gravity of the bulk y ash and its mineralogy. Higher quartz and
mullite contents account for a lower specic gravity (Joshi and Lothia 1997). The particle size
distribution appears to be strongly dependent on the method of collection (Dhir 1986). Fly
ashes collected from electrostatic precipitators are two to ve times ner compared to y ashes
from mechanical separators. The former type is therefore more generally used as a SCM. The
neness of y ash is often expressed as the fraction passing a 45 mm wet sieve. It also serves as
a routine measure for quality control and to assure uniformity in y ash supply. A substantial
correlation exists between the neness of the y ash (% retained on a 45 mm sieve) and its
Activity Index (Dhir 1986).
However, y ash is a heterogeneous material. Physically distinct particles can be
distinguished. 1 to 2 wt% (Sear 2001) or 15 to 20 vol% (Vassilev and Vassileva 1996) of the y
ash consist of hollow spherical particles known as cenospheres. Cenospheres have diameters
ranging from 50 to 200 mm, with a wall thickness of approximately 10% of their radius (Sear
2001). Cenospheres form when trapped organics, carbonates, suldes, sulfates or hydrosilicates
decompose or water evaporates and induces an expansion while the particle is still in a viscous
state (Kolay and Singh 2001; Kutchko and Kim 2006). They appear to be more characteristic
for y ashes obtained from coals enriched in nely dispersed illite and quartz (Vassilev and
Vassileva 1996). Their density is very low and ranges from 250 to 800 kg/m³. If the y ash
is stored in a lagoon, the cenospheres will oat at the surface. The Blaine surface area of
cenospheres collected from such a lagoon in Dahuna, India, is about 0.05 m²/g (Kolay and
Singh 2001) and thereby much lower than the surface area of the bulk y ash. This is explained
by their nearly perfect spherical shape and their hollow structure. Because of their particular
properties, cenospheres are used for specic applications as in lightweight constructions. As
cenospheres are hollow, they have good isolating properties. Plerospheres are like cenospheres,
but instead of being empty, they contain smaller spherical or other particles. Dermaspheres
are dened as plerospheres that have crystal nuclei of mullite, hematite and other minerals,
Figure 18. SEM picture of a typical y ash with its mostly spherical and some angular particles.
238 Snellings, Mertens, Elsen
covered with amorphous aluminosilicate envelopes (Vassilev and Vassileva 1996). Ferrospheres
are spherical particles enriched in iron, be it amorphous or crystalline. Spheroïds are a specic
type of spheres that look more porous or vesicular particles, with sizes ranging from 10 to 80
mm (Vassilev and Vassileva 1996). Ramsden and Shibaoka (1982) made their own classication
of y ash particles and dened seven categories; (1) unfused detrital minerals, (2) irregular-
spongy particles, (3) vesicular colorless glass, (4) solid glass, (5) dendritic iron oxide particles,
(6) crystalline iron oxide particles and (7) unburned char particles.
Like in most other supplementary cementitious materials, SiO2, Al2O3, Fe2O3 and
occasionally CaO are the main components present in y ashes. A selection of y ash
compositions is plotted in Figure 19. Following the ASTM classication, a main distinction can
be made between Class F and Class C y ashes. The relatively more CaO-rich Class C y ashes
plot further away from the SiO2-Al2O3 border of the diagram compared to the Class F y ashes.
The ASTM standard requires the sum of SiO2, Al2O3 and Fe2O3 to be greater than 70% for Class
F and greater than 50% for Class C y ashes.
The mineralogy of y ashes is very diverse. The main phases encountered are a glass phase,
together with quartz, mullite and the iron oxides hematite, magnetite and/or maghemite. Other
phases often identied are among others; cristobalite, anhydrite, free lime, periclase, calcite,
sylvite, halite, portlandite, rutile and anatase. The Ca-bearing minerals anorthite (feldspar),
gehlenite, åkermanite and various calcium silicates and calcium aluminates identical to those
found in cement clinker can be identied in Ca-rich y ashes. These, mainly type C, y ashes
may have hydraulic properties, as they contain minerals that react with water to form calcium-
silicate/aluminate hydrates with binding properties. A similar mineralogical composition to that
of coal y ash is found in Municipal Solid Waste Incinerator ash (Kirby and Rimstidt 1993).
For y ashes in general, the Ca-minerals, free lime, periclase and sulfate minerals are probably
most critical towards their properties (Sear et al. 2003). There appears to be a strong relation
between the mineralogy of the y ashes and the mineralogy of the feed coals (Ward and French
Figure 19. The reported chemical compositions of y ashes plotted into a CaO-SiO2-Al2O3 ternary diagram
(wt% based).
Supplementary Cementitious Materials 239
2006; Koukouzas et al. 2007) as well as their combustion temperature (Koukouzas et al. 2007).
This is also true for alternative fuel sources. Wood chips, being rich in Ca, will generate Ca-rich
y ashes, whereas the combustion of biomass rich in alkalis will generate y ashes containing
minerals alkali-bearing minerals.
Three large groups of components are identied in y ashes; (1) an organic char fraction,
(2) an inorganic amorphous and (3) an inorganic crystalline fraction.
1) Char particles are concentrated in the larger grain size fractions (Kutchko and Kim
2006). The char fraction mainly consists of carbon and correlates with the Loss on
Ignition (LOI). Whereas the relative abundances of the main elements in the y ash are
chiey dependent on the source of the pulverized fuel, the LOI is strongly dependent
on the burning process. During the “boosting period,” i.e., at the start-up of the power
plant, an increase of the LOI is generally observed (Sear 2001). About 90% of this
LOI is due to unoxidized elemental carbon. Older power stations also tend to yield
high LOI y ashes. The same goes for modern low NOx burners. In general, lower
temperatures correspond to lower NOx values, but higher unburned carbon. LOI
values may range from less than 1 to more than 20%, although values of maximum 6
or 7% are accepted by most standards for y ash in Portland cement concrete.
2) The glass or inorganic amorphous phase in y ashes may represent up to 90% of the
total weight. An average value is probably in the order of about 60 to 80 wt%. Its
quantity can be accurately determined from Quantitative X-Ray Diffraction (QXRD)
measurements using an internal standard. Microscopic methods are often not suitable
for quantifying y ashes as the glass and crystalline phases are generally intimately
intermixed (Ward and French 2006). The glass phase is principally composed of silica
and alumina, although many other constituents are present. The silica is present as
cross-linked tetrahedra, thereby showing a short- but no long-range ordering (Bijen
1996). The basicity of the glass phase can be calculated through the same formula as
that used for blast furnace slag employed in German and Japanese concrete (Sakai et
al. 2005):
( )
23
2
CaOMgOAlO (1)
SiO
basicity ++
=
Sakai et al. (2005) found that when the glass content of the y ash is low, its basicity
tends to decrease. Moreover, there appears to be an inverse correlation between the
mullite content and the amount of amorphous material in y ashes (Sakai et al. 2005).
Obviously, higher mullite contents result in lower quantities of amorphous material
with a lower average basicity. Compared to granulated blast furnace slags, y ashes
have low basicities. Some authors (Bijen 1996; Manz 1998; Ward and French 2006)
consider the amorphous part as the “active part.” It is likely that the amorphous fraction
is not composed of a single glass phase, but that it consists of different phases with a
dissimilar composition (Nathan et al. 1999).
3) A tremendous work was done by Vassilev and Vassileva (1996) and Vassilev et al.
(2003), who discussed the origin and occurrence of all minerals and groups of minerals
found in 11 Bulgarian y ashes. Their data include an exhaustive list of minerals,
which is relevant for all studies on y ash mineralogy. Individual y ash particles, with
the exception of plerospheres, are chemically fairly homogeneous (Gieré et al. 2003).
Nevertheless, bulk y ash is heterogeneous and differences in chemistry are observed
between size fractions and between the categories of y ash particles dened earlier.
Cenospheres for instance are more silica-, alumina- and potassium-rich and poorer
in calcium compared to the bulk y ash (Vassilev and Vassileva 1996; Sear 2001).
Ferrospheres are obviously rich in Fe and Fe-bearing minerals. Element partitioning
in y ashes induces important differences in chemistry and mineralogy among the
240 Snellings, Mertens, Elsen
size fractions. Many comprehensive papers deal with this partitioning (Filipidis
and Georgakopulos 1992; Vassilev and Vassileva 1996; Erdoğdu and Türker 1998;
Hower et al. 1999; Gieré et al. 2003; Vassilev et al. 2004; Chen et al. 2005) and the
environmental/toxicological issues related with the nest y ash particles (Coles et al.
1979; Tazaki et al. 1989). Magnetic or other separable fractions have also been studied
separately (Hower et al. 1999; Vassilev et al. 2004).
The majority of trace elements in y ash is found in the nes (Vassilev and Vassileva 1996).
Moreover, there is a clear correlation with the mineralogy, as accessory minerals are mainly
identied in the smaller grain size fractions. In these smaller fractions, trace elements may form
discrete mineral phases (Vassilev and Vassileva 1996). Crystalline particles as small as 20 nm
have been observed by Transmission Electron Microscopy (TEM) (Chen et al. 2005). Toxic
elements that are volatilized during the combustion can also be absorbed on the surface of very
small particles (Gieré et al. 2003). More in particular, glass particles tend to attract many trace
elements, due to their reactive surface (Dudas and Warren 1987). It has been mentioned that the
composition of the glass fraction is probably not uniform throughout the y ash. The same is
true for the crystalline components. In a TEM-study dedicated to the composition of the mullite
phase, Gomes and François (2000) discovered that its composition is very heterogeneous.
However, from the determination of the lattice parameters by X-ray diffraction, an average
mullite composition can be obtained (Cameron 1977). Similarly, the composition of magnetite
can also vary between individual y ash particles (Gomes et al. 1999).
Silica fume. Silica fume is a by-product of the silicon metal and ferro-silicon alloy
industries, the terms “condensed silica fume” and “microsilica” are also used. It is an excellent
supplementary cementitious material with a high pozzolanic activity due to a high content of
SiO2 in amorphous form and a very ne particle size distribution (0.1-0.2 mm average diameter).
Major reviews on silica fume and its applications in concrete can be found in Mehta (1986),
Kjellsen et al. (1999), Fidjestol and Lewis (2001), Justnes (2002), Chung (2002) and in Justnes
(2007).
Silica fume is produced during the reduction of quartz at high temperatures in electric arc
furnaces. High purity quartz is heated to 2000 °C together with coal, coke or wooden chips to
remove the oxygen. One of the reactions involves the formation of SiO vapor which oxidizes
and condenses in the form of very small amorphous silica spheres. The rst experiments on
the use of silica fume in concrete were carried out at the Norwegian Institute of Technology in
Trondheim (1950) but extensive research started only in the 1970’s and widespread commercial
use of silica fume in concrete started in the 1980’s. Important production countries at this
moment are China, Norway, South Africa, USA, Canada, Spain, Russia and France.
Unlike other thermally activated supplementary cementitious materials such as for example
y ash, silica fume from one production source has nearly no variation in chemical composition
over time, because of the use of relatively pure raw materials. The SiO2 content varies with
the silica content of the alloy being produced and should be higher than 85% for silica fumes
suitable for use as pozzolan (ASTM C 1240). In general, the chemical composition of silica
fume is not complex and consists usually of more than 90% of SiO2 (85-99%). Other oxides
such as Fe2O3, Al2O3, CaO, MgO, Na2O and K2O are normally below 1.0%, the value for the
loss on ignition varies between 1.0% and 2.0%.
Silica fume consists essentially of an amorphous silica structure and a very large peak can
be observed centered at about 4.4 Å using X-ray powder diffraction. The silica fume particles
are spherical in shape and using electron microscopy it was shown that they have an average
diameter size between 0.1 and 0.2 mm (Fig. 20). Silica fume has an approximate value of 2.2 g/
cm³ for the specic gravity and a very high surface area value of about 20-22 m²/g as measured
with BET N2-adsorption.
Supplementary Cementitious Materials 241
THE POZZOLANIC REACTION
The recombination of an (alumino-)silicate or aluminate material and Ca2+ or Ca(OH)2
in the presence of water into hydrated reaction products with binding properties can be
schematically formulated in cement chemistry notation (A = Al2O3, C = CaO, H = H2O, S =
SiO2; hyphenation denotes variable stoichiometry) as:
AS + CH + H C-S-H + C-A-H (2)
The driving force behind this simplied reaction is the difference in Gibbs energy between the
reactants and the eventual products. The reaction rate is however, determined by the individual
elementary steps or processes in the reaction. The reaction step with the slowest rate of conver-
sion is the rate-controlling process and is typically the one with the highest activation energy
barrier. It is generally accepted that the initial rate-controlling process consists of the release or
dissolution of silica from the pozzolan. The increasing pore solution saturation degree eventu-
ally gives rise to heterogeneous nucleation and growth of the C-S-H reaction products at the
pozzolan surface. Subsequent to the formation of a layer of reaction products enveloping the
reactant grains, the reaction rate is commonly assumed to be limited by the diffusion of ions
through the growing and densifying layer of products (Kondo et al. 1976; Držaj et al. 1978;
Takemoto and Uchikawa 1980). Most published reviews on supplementary cementitious ma-
terials do not treat the subject of the mechanism of the pozzolanic reaction in detail, but tend
to focus on material properties dening the activity and performance of pozzolans. However,
difculties are met when trying to compare and relate the activity controlling properties among
SCMs of different origin (e.g., Mehta 1987; Sersale 1993). In this respect, empirically estab-
lished relationships between pozzolan properties and activity remain limited to materials of a
similar origin. To determine which material properties are of importance and when they become
essential in the pozzolanic reaction, a detailed knowledge of the pozzolanic reaction mechanism
is needed. In this particular area much work remains to be done. Unraveling the impact of the
heterogeneous group of SCMs on cement hydration will constitute one of the major challenges
in cement science for years to come. As a starting point, in the following sections, the traditional
views on the pozzolanic reaction are combined with recent fundamental insights into the dis-
solution of minerals developed in geochemistry (Lasaga 1998; Dove et al. 2005).
Figure 20. SEM picture of a typical silica fume with average particle diameter of 0.1 mm. [Used by
permission of Elsevier, from Jo et al. (2007), Construction and Building Materials, Vol. 21, Fig. 1, p. 1352.]
242 Snellings, Mertens, Elsen
The pozzolanic reaction mechanism
A general overview of the different stages in the pozzolanic reaction can be obtained by
monitoring the rate at which heat is evolved during the reaction. The rate of heat evolution is
closely related to the actual rate of reaction provided that the reactions are exothermal (Gartner
et al. 2002). In Figure 21 an idealized heat release curve for the pozzolanic reaction between
silica and Ca(OH)2 is presented. Three main stages can be recognized. The duration of each
of the stages and also the shape of the heat release curve vary signicantly depending on the
experimental conditions and the pozzolanic activity of the material. The initial sharp peak
occurring directly after mixing (stage I) lasts only for several minutes and is followed by a
period of low activity designated as the induction period (stage II). Renewed activity denes
the initiation of stage III, which can be divided tentatively in parts of an accelerating and a
decelerating reaction rate. The induction period usually lasts only for several hours to some
days. The duration of stage III is indenite. In some cases the reaction can proceed at very low
rates for extended periods of years to decades (e.g., Taylor et al. 2010).
I. Initial dissolution period and aluminosilicate dissolution. Both lime-based and Portland
cement based binders have in common that initial fast dissolution of Ca(OH)2 or clinker
minerals in water rapidly results in an alkaline solution saturated in Ca(OH)2. In Ca(OH)2-
based systems the solution pH is usually around 12.4, while in Portland cement based systems
the solution may reach values of 13.7 when abundant soluble alkalis were released from the
clinker minerals. At alkaline pH above 10.7 the solubility of silica and silicates (i.e., amount
of silica in solution) increases continuously with pH (Iler 1979; Knauss and Wolery 1988) and
pozzolans will be subjected to dissolution. The silicate dissolution rate at high pH is governed
by processes of hydration, deprotonation, ion-adsorption and hydrolysis at the mineral-water
interface. Greenberg (1961) concluded that the rate-controlling step in the pozzolanic reaction
was the hydrolysis of surface silica groups. To understand and predict the kinetics of dissolution,
the molecular details and energetics of the actual pathway that links reactants and products via
the activated complex need to be known. The activation energy is the energy difference between
the activated complex or transition state and the reactants. Ab initio quantum mechanical
calculations of nite molecular clusters claried the pathway to dissolution of silicates
and aluminosilicates at high pH in recent years (cf. Xiao and Lasaga 1994; Lasaga 1998).
The generally accepted reaction pathway for the hydrolysis of the bridging bond (Obr) in a
Figure 21. Schematic overview of the rate of heat release during the pozzolanic reaction of silica and
Ca(OH)2. Stage I, represents the initial dissolution period. Stage II corresponds to the induction period and
stage III is the phase in which the main reaction occurs.
Supplementary Cementitious Materials 243
(HO)3-Si-Obr-Si-(OH)3 (Q1) cluster is illustrated in Figure 22 and the corresponding energetics
are given in Figure 23 (Morrow et al. 2009). In alkaline conditions the silanol groups at the
silicate surface are partially deprotonated. Xiao and Lasaga (1996) have shown that nucleophilic
hydroxyl attack on a neutral Si-OH surface group is equivalent to hydrolysis of a deprotonated
Si-O- surface group. Both situations result in a H-bonded H2O adsorption onto the negatively
charged Si-O site (RC or reaction precursor complex). The key step in the reaction is the
formation of a negatively charged vefold coordinated trigonal bipyramidal Si species. To form
this reaction intermediate (INT) the reaction has to pass through a transition state (TS1) and
surmount the associated large energy barrier (calculated to be 110 kJ/mol in the gas phase;
Morrow et al. 2009). The Si-Obr bond in the formed intermediate state is signicantly weakened
and the energy barrier associated with the nal step of bond breakage is much smaller (22 kJ/
mol in gas phase; Morrow et al. 2009).
The activation energy of Si-Obr-Si hydrolysis in larger clusters where the considered Si
atoms are doubly (Q2) or triply (Q3) connected to neighbor Si atoms via Si-Obr-Si bonds (e.g.,
Fig. 24) was investigated by Pelmenschikov and co-workers for dissolution by H2O attack
(Pelmenschikov et al. 2000, 2001) and by Criscenti et al. (2006) for dissolution by H3O+ attack
on a Q3 cluster. The calculations indicated that the activation energy increased with connectivity
(up to 205 kJ/mol for a Q4 cluster), this was attributed to resistance of the remaining bridging
bonds against relaxation of the partially uncoupled Si species (Pelmenschikov et al. 2000). In
the neutral silanol state the breakage of the Si-Obr-Si bond is expected to be followed by a very
fast condensation or rehealing reaction. The experimentally determined activation energy of
silicate dissolution (67-92 kJ/mol; Knauss and Wolery 1988; Brady and Walther 1990; Dove
and Crerar 1990; Walther 1996) would then be associated with the hydrolysis of the last Si-Obr-
Si bonds (Q1 and/or Q2) (Criscenti et al. 2006). In an alkaline medium the rehealing reaction
is suggested to be partially prevented by the deprotonation of the formed Si-OH HO-Si defect
(Pelmenschikov et al. 2001).
Figure 22 (above). Reaction stages along
the reaction prole for the hydrolysis of
a deprotonated Si-O-Si species (Morrow
et al. 2009). RC is the reaction precursor
complex, TS1 and TS2 are transition states,
INT is an intermediary state and PC the
product complex.
Figure 23 (to the left). Energy prole for
the Si-O-Si hydrolysis reaction shown in
Figure 22 (modied after Morrow et al.
2009).
244 Snellings, Mertens, Elsen
Morrow et al. (2009) compared the pathways of dissolution of aluminosilicate Q1 clusters
with the earlier reported silicate dissolution mechanism. At high pH dissolved Al is fourfold
coordinated (Swaddle et al. 2005), only one transition state was found for the dissolution and
release of Al(OH)4. This step corresponded with the formation of a vefold coordinated almost
trigonal bipyramidal Al species with a much lengthened Al-Obr bond interaction (Fig. 25). At
acid and neutral pH the barrier heights of Si-Obr-Si hydrolysis (resp. 63 and 146 kJ/mol) were
considerably higher than that of Al-Obr-Si linkages (resp. 38 and 39 kJ/mol). This is supported
by the experimental observation of leaching of Al from the surface of feldspars at low pH
(Stillings and Brantley 1995). However, at high pH there was no signicant difference (79 kJ/
mol for both), which is in agreement with the observation that little or no Al depletion occurs
on aluminosilicate surfaces at high pH (Hamilton et al. 2001).
The dissolution rates of silicate minerals were observed to be affected by the solution
pH or, equivalently, by the distribution of protonated, neutral or deprotonated groups at the
mineral surface. Based on such a surface speciation model and transition state theory (cf.
Lasaga 1998) the pH dependence of the dissolution rate of quartz can be predicted (Dove 1994;
Nangia and Garrison 2008). However, for mixed oxides containing Si-Obr-Si linkages such as
aluminosilicate feldspars, the activities of the leached cations in solution should be included into
the model (Oelkers 2001). For orthosilicates and minerals without silicate linkages, dissolution
rates can be correlated with the solvation-water exchange constant of the non-silicate cations
(Westrich et al. 1993). Background cations in solution change the aluminosilicate surface
speciation by adsorption and proton exchange reactions. At high pH, alkali and alkaline-earth
cations are drawn to the negatively charged surface by electrostatic attraction (Iler 1979). Both
Figure 24. Reaction prole for the hydrolysis of a Q3 connected Si-Obr-Si bond at the (111) b-cristobalite
surface (Pelmenshikov et al. 2000). Obr stands for bridging oxygen. Ow was originally part of the attacking
water molecule.
Figure 25. Reaction stages along the reaction prole for hydrolysis of a deprotonated Al-Obr-Si (Morrow et
al. 2009). RC is the reactant complex, TS the transition state and PC the product complex.
Supplementary Cementitious Materials 245
experimental (Sjöberg 1989; Dove and Crerrar 1990) and computational (Strandh et al. 1997)
results show that the adsorption of cations generally leads to enhanced dissolution rates and
weakened bridging bonds of the Si or Al species with the lattice. In general, alkaline-earth
cations in solution bind more strongly to the mineral surface and remain only partially hydrated.
Alkali cations retain signicant water shielding upon interaction and would increase dissolution
rates by increasing the reaction frequency compared to alkaline-earth cations which show
slower exchange of solvation water (Dove 1999).
When water is added to dry blended cement or lime-SCM binders, dissolution of clinker
constituents, SCMs or lime occurs at rst in far-from-equilibrium, highly undersaturated
conditions. The initial fast dissolution rate subsequently slows down signicantly until a Ca(OH)2
saturated solution is reached. In terms of silicate dissolution theory, this parabolic dissolution
rate behavior has been attributed to the rapid initial dissolution of ne particles or sites with
high surface energy (Brantley 2008). Other models explain the parabolic rate behavior by the
formation of a leached surface layer due to non-stoichiometric dissolution or by the precipitation
of a protective membrane at the mineral surface through which dissolved ions must diffuse
(Schott and Petit 1987). The latter two models are very similar to the mechanisms invoked to
explain the occurrence of the induction period during alite hydration (see below; Gartner et
al. 2002; Bullard et al. 2011). More recently a mechanistic model for mineral dissolution was
formulated to enable the prediction of dissolution rates depending on the saturation state of the
solution (Dove et al. 2005; Lasaga and Lüttge 2005). This model is based on concepts borrowed
from crystal growth theory where dissolution can be regarded as the inverse of crystal growth.
Dissolution occurs as the result of horizontal step retreat at incomplete surface layers and of
vertical removal of atoms at plain surfaces or at the intersection of dislocation or point defects
with the surface (Fig. 26). Three different regimes of mineral dissolution can be distinguished
depending on the dominant mechanism of dissolution. At very high undersaturation, two-
dimensional pits or vacancy islands can nucleate at perfect surfaces without any dislocations
present. However, the activation energy barrier for this mechanism is high and is expected
to occur for only a very short time in cementitious systems (Juilland et al. 2010). Closer to
equilibrium, two-dimensional pitting on plain surfaces will end. However, step nucleation can
proceed at dislocation defects due to the associated strain eld. Eventually, when the saturation
degree reaches near-equilibrium, only step retreat is possible. No more steps can form at the
surface or near dislocation defects. Step nucleation only occurs at crystal edges and the crystal
becomes progressively smoother and edges more rounded (Brantley 2008). In this model the
dissolution rate depends on the density of steps present or nucleated at the surface and the
velocity of step retreat at the surface. Both parameters are a function of the degree of solution
undersaturation, thus the dissolution rate will decrease sharply when the undersaturation degree
Figure 26. The dominant dissolution mechanism depends on the level of undersaturation. At very high
undersaturations two-dimensional pits or vacancy islands can nucleate at perfect surfaces, at lower levels
of undersaturation step nucleation can still proceed at dislocation defects. Eventually near equilibrium
step nucleation ends and step retreat becomes the dominant dissolution mechanism. W identies with the
saturation degree (modied after Dove et al. 2005 and Juilland et al. 2010).
246 Snellings, Mertens, Elsen
approaches equilibrium. However, the step nucleation mechanism and rate depend also on the
nature of the crystal and the crystal face, especially in terms of the liquid-solid interface energy
(Dove et al. 2005). Furthermore, dissolution rates near equilibrium increase with increasing
defect density as exemplied for alite by Juilland et al. (2010) or for quartz by Blum et al.
(1990). The suggestion by ab initio cluster studies that the precursor molecules of silicate
dissolution should generally have a connectedness lower than 3 is consistent with a steady-state
silicate dissolution mechanism dominated by step retreat.
II. Induction period. In the pozzolanic reaction the reaction rate decreases rapidly during
period I and remains low during the ensuing induction period (Fig. 21). In most models this
evolution is linked to the formation of a protective barrier layer on the reacting pozzolan
particles (Takemoto and Uchikawa 1980; Glasser et al. 1987; Fraay et al. 1989). This barrier
layer is expected to shield the pozzolan from the surrounding basic solution and to hamper
its dissolution. Two distinct hypotheses are put forward regarding the nature of the protective
layer. 1) The leached layer hypothesis is based on the incongruent dissolution of the pozzolan
(cf. Livingston et al. 2001 for alite). As alkalis are leached from the surface an amorphous layer
consisting of Si and Al remains, meanwhile Ca2+ is adsorbed at the surface and a double layer
is created, inhibiting further dissolution. Eventually Si and Al dissolve and recombine with
the adsorbed Ca2+ to form C-S-H and C-A-H phases (Takemoto and Uchikawa 1980). 2) The
protective precipitate hypothesis is based on a dissolution-reprecipitation process, in which the
pozzolan rst releases Si and Al which then subsequently reprecipitate at the pozzolan surface
as a coating of stable or metastable C-S-H and C-A-H reaction products (Greenberg 1961;
Glasser et al. 1987). Alternatively, the reaction rate can also decrease sharply because of a
decrease in the degree of undersaturation of the solution as suggested by the mechanistic model
for mineral dissolution. If the degree of undersaturation would drop below the threshold for step
nucleation at dislocation edges, the dissolution rate would be considerably lowered. The latter
mechanism could of course also take place together with the formation of a barrier layer at the
pozzolan-liquid interface.
The end of the induction period is marked by the massive nucleation and subsequent growth
of reaction products. Several theories have been proposed to explain the sudden transition from
the induction to the main period of reaction. The suggested theories were mainly derived from
concepts developed to explain the hydration behavior of alite (cf. Gartner et al. 2002; Bullard et
al. 2011). Takemoto and Uchikawa (1980) have suggested that the protective membrane would
be semi-permeable, allowing osmosis of water from the outer solution to the concentrated inner
solution. The rising osmotic pressure due to the ongoing dissolution of the pozzolan would
eventually result in a rupture of the membrane, release of dissolved silica and alumina and
the start of the main period of C-S-H and C-A-H precipitation. This theory was based on the
osmotic pump model for the hydration of alite by Double et al. (1978) and the observation of
relict hollow shells of reaction products by electron microscopy. Also more recent Nuclear
Resonance Reaction Analysis results that indicate the development of a depleted silica gel layer
at the surface of alite grains during the induction period have been linked to the existence of a
semi-permeable layer (Livingston et al. 2001). A different, more widely accepted mechanism
was rst suggested by Stein and Stevels (1964) for the hydration of alite in the presence of
silica. They proposed that the conversion of the protective layer of metastable C-S-H(m) or
disordered gel type phases to a more stable C-S-H form would result in the acceleration of the
hydration reaction of alite. The same mechanism can be applied to the pozzolanic reaction, the
transformation would be triggered by the reaching of a certain degree of supersaturation with
respect to the stable C-S-H assemblage. However, detailed studies on the evolution of the uid
saturation degree during the rst stages of the pozzolanic reaction are still lacking.
III. Main reaction period. At the end of the induction period, typically only a very limited
amount of reaction products has been formed and the pozzolan has barely reacted (Snellings
Supplementary Cementitious Materials 247
et al. 2009). The initiation of the main reaction period can be observed by an exponentially
increasing heat release rate (Fig. 21) related to the large-scale nucleation and growth of the
reaction products as the rate-controlling steps. The exponential reaction rate increase is short-
lived, and eventually the reaction rate starts decreasing again. It has been widely considered
that the decrease in rate is due to the onset of a diffusion-controlled process. Many observations
of the formation of an enveloping layer of reaction products on the pozzolan grains supported
the interpretation that the reaction rate becomes limited by the diffusion of reactants through
the reaction product layer (Držaj et al. 1978; Türker and Yeginobali 2003; Mertens et al. 2009).
Other processes may also cause a deceleration of the reaction rate. Consumption of the smallest
particles will leave coarse particles that react more slowly. Also a lack of space or densication
of the C-S-H rim can hinder the growth of C-S-H particles and thus decrease the reaction rate
(Bishnoi and Scrivener 2009). Finally, the growing competition for a dwindling supply of water
can lead to the deceleration of the reaction (Bullard et al. 2011).
In the traditional view of reaction rate deceleration due to diffusion control, several
mathematical models have been used to t the evolution of the degree of reaction a. In Ca(OH)2-
pozzolan systems the degree of reaction is usually quantied by the direct determination by
X-ray diffraction or thermogravimetry of the amount of Ca(OH)2 reacted (Kondo et al. 1976;
Takemoto and Uchikawa 1980; Shi and Day 2000) or by the evolution of the overall heat
release curve measured by calorimetry. Most kinetic models are based on the Jander equation
developed to describe three-dimensional diffusion control during solid-state sintering of a
contracting reactant sphere (Jander 1927):
( )
2
1/3
2
2
1 1 (3)
kt Kt
r

− −a = =

Where a identies with the fractional reaction, k with the rate constant for the diffusion
process, t equals to the time since the onset of the diffusion controlled reaction process, r is
the initial radius of the spherical reactant and K represents a constant proportional to k. In
the original Jander equation the thickness of the interface layer or equivalently, its diffusion
coefcient is taken not to change over time. However, in cementitious systems the interface
layer is expected to grow or densify gradually with the proceeding reaction. Therefore, a
modied, more general form of this equation has been used more frequently (Kondo et al. 1976;
Shi and Day 2000; Mertens et al. 2009).
( )
1/3
1 1 (4)
N
Kt

− −a =

This equation allows the classication of the ongoing reaction based on the value of the
exponent of reaction N. If N = 1, then dissolution or nucleation/precipitation processes at the
surface of the grains are the rate-limiting step. With N > 1, three-dimensional diffusion through
a layer of reaction products is considered to be the rate-limiting step. A decreasing permeability
of the interface layer induced by thickening or densication by reaction product precipitation
would correspond with N < 2. In all studies a conspicuous increase in N was observed over the
course of hydration, implying a decreasing permeability of the reaction product interface layer
(Takemoto and Uchikawa 1980; Shi and Day 2000; Mertens et al. 2009). Cabrera and cowork-
ers have successfully applied both the original Jander equation on metakaolin-Ca(OH)2 systems
(Cabrera and Rojas 2001) as well as the modication developed by Ginstling and Brounshtein
(1950) to allow for the decreasing permeability of the interface layer on silica fume-Ca(OH)2
and trass-Ca(OH)2 systems. The Ginstling-Brounhstein equation is formulated as follows:
( ) ( )
23
1/3 1/3
2
11 11 (5)
3Kt

− −a − −a =

Villar-Cociña et al. (2003) have applied a decreasing nucleus model to model the decrease in
248 Snellings, Mertens, Elsen
electrical conductivity of a saturated Ca(OH)2 solution upon the addition of a pozzolan. The
model was used to determine diffusion coefcients of the interlayer and overall reaction rate
constants to indicate the activity of the added pozzolans.
It should be noted that the tting of the presented kinetic models can only give general
information of the reaction mechanism and that the results should be interpreted cautiously.
Variations in reaction product morphology, thickening or densication of the interface layer will
inuence the permeability and diffusion coefcient of the barrier layer. Additionally, obtained
reaction rate constants should be considered as overall apparent values because the apparent
rate constant will encompass a series of processes which are considered to be combined in a
pseudo-rst order reaction. Averaging occurs also over the reaction rates of inner and outer
product formation. Additionally, ner fractions of a pozzolan will react more rapidly than the
coarser ones and an overall value will be obtained.
Pozzolanic activity
The reaction rate during and the timing and duration of the described stages of the pozzo-
lanic reaction are highly dependent on the intrinsic activity and characteristics of the pozzolan.
Highly active pozzolans such as metakaolin will increase heat release during the initial disso-
lution period and signicantly shorten the duration of the induction period (Massazza 2001).
Additional to intrinsic pozzolan properties such as specic surface area, chemical composition
or active phase content, the consumption of Ca(OH)2 over time is also depending on external
factors such as mix design and curing conditions.
Intrinsic pozzolan properties. It is generally accepted that a rst-order relationship exists
between the rate of dissolution and the total or reactive mineral surface area under far-from-
equilibrium conditions (Brantley 2008). However, the determination of the specic surface area
is not entirely straightforward. It can be measured by geometric calculations departing form
the particle size distribution curve or by the BET N2 adsorption technique. Both techniques
have drawbacks. Geometric calculations demand the assumption of particle shapes and in the
BET technique the internal particle porosity is often included (White and Brantley 2003). Both
mineral dissolution and the initial stages of the pozzolanic reaction consist of processes taking
place at the liquid-solid interface. Therefore, at least initially, a linear relationship is expected
between the activity of a pozzolan and the available surface area for reaction. Correlations
between the Blaine neness or the BET specic surface of a specic pozzolan and its activity or
even the evolution of compressive strength have been reported frequently for the early reaction
period, varying in duration from 7 days up to 3 months (Ludwig and Schwiete 1963; Costa
and Massazza 1974; Takemoto and Uchikawa 1980; Day and Shi 1994, Mertens et al. 2009).
However, inconsistencies arise when the activity dependence on specic surface of materials
of different origins are compared. The absence of a single encompassing relationship points
to the fact that the pozzolanic activity is dependent on more factors. To illustrate the effect
of particle shape and internal porosity, for similar particle size distributions or neness the
complex structure and highly porous nature of rice husk ash or diatomite constituents may
result in very different activities compared to that of spherical non-porous y ash particles.
To enhance the specic surface area and thus the activity of a pozzolan, grinding is a com-
monly used technique. Even materials which are commonly not regarded to behave pozzolani-
cally, such as quartz, can become reactive when ground below a certain critical particle diame-
ter (Benezet and Benhassaine 1999). The particle diameter or the curvature of the surface of a
pozzolan particle also affects the surface energy or surface tension through the Gibbs-Thomson
effect and results in a higher solubility of smaller particles (cf. Ostwald ripening) (Iler 1979).
Additional to increasing the total surface area, grinding also results in the creation of surface
defects, i.e., sites experiencing lattice strain or sites partially disconnected from the underlying
undisturbed material (Alexander 1960). The lowered activation energy of hydrolysis of the Si-
Supplementary Cementitious Materials 249
Obr-Si bonds at these sites has a denite positive effect on the pozzolanic activity. Activation
of pozzolans by acid treatment results in an etched surface and the enhanced activity relies
on a similar creation of abundant surface pits and steps. More generally, the bulk density of
defects in a material exerts a signicant inuence on the activity of both minerals and vitreous
materials (Shi 2001; Nair et al. 2008). Highly-crystalline materials showing very few linear or
planar defects are generally observed to be much more stable in Ca(OH)2 saturated solutions
than amorphous materials with large concentrations of bulk and surface defects. This can be
illustrated by the higher pozzolanic activity of metakaolin produced from paper sludge ash
compared to regularly produced metakaolin of higher purity and larger specic surface area.
The increased reactivity of the former was related to an increased concentration of surface
defects (Péra and Amrouz 1998). Bich et al. (2009) characterized the presence of surface
defects in kaolinite based on the asymmetry of the DTA peak of dehydroxylation. Thermally
activated disordered kaolinite with many surface defects was shown to be more reactive than
burned ordered kaolinite showing few defects. Activation of kaolinite by prolonged grinding
was related to surface defect creation. This is illustrated by the disappearance of the DTA peak
of dehydroxylation and the remarkable increase in weight loss over the lower temperature range
of 100-500 °C in Figure 27, indicating the presence of disordered weakly bonded hydroxyl and
water groups (Vizcayno et al. 2009).
In determining the pozzolanic and cementitious properties of SCMs also the mineralogical
composition plays a prominent role. The proneness to reaction of the amorphous and crystalline
phases is linked to their structural stability in an alkaline Ca(OH)2 saturated solution. The
effect of crystal structural properties on the mineral stability can be illustrated by variations in
Figure 27. DTA (top) and TG (bottom) curves for natural kaolinite and kaolinite ground for 15, 30, and 60
min. in a Herzog-mill (modied after Vizcayno et al. 2009).
250 Snellings, Mertens, Elsen
enthalpy of formation of pure-silica polymorph structures. Besides the naturally occurring silica
polymorphs, a- and b-quartz, cristobalite, tridymite, coesite and stishovite, a large diversity of
synthesized all-silica molecular sieves or zeolites exists and is used in industrial processes.
Petrovic et al. (1993) and Piccione et al. (2000) experimentally determined the enthalpies of
transition from the stable a-quartz structure to the metastable polymorph structures. The range
of energies is quite narrow at only 6.8-14.4 kJ/mol SiO2 above quartz, and a strong linear
correlation between enthalpy and framework density (the number of tetrahedral framework
atoms per nm³) was observed (Fig. 28). Both experimental and theoretical (Henson et al.
1994; Sastre and Corma 2006; Zwijnenburg et al. 2007) results indicate that for the pure silica
polymorphs the quality of packing of the SiO4 tetrahedra is the most important parameter
controlling silica stability. Furthermore, the formation enthalpy of an amorphous silica glass
was only 7 kJ/mol higher than quartz and lower by 0-7 kJ/mol than the zeolitic structures
(Petrovic et al. 1993). On purely crystal structural grounds, this difference may serve as an
explanation of the observed higher activity of natural zeolites compared to volcanic glass of
similar composition and origin (Mortureux et al. 1980; Sersale 1980). The higher enthalpy
of formation of less dense structures is expected to increase the difference in Gibbs energy
between reactants and reaction products. Nevertheless, the effect of framework density on
dissolution activation energy and thus on reaction kinetics remains unclear.
The situation becomes more complicated when typical SCMs consisting of several mixed
oxide phases are considered. In general, the pozzolanic activity of minerals thermodynamically
stable at ambient conditions is low when compared on an equal specic surface basis to less
stable mineral assemblages. Volcanogenic deposits containing large amounts of volcanic glass or
zeolites are more reactive than quartz sands or detrital clays. In this respect, the thermodynamic
driving force behind the pozzolanic reaction may serve as a rough indicator of the potential
reactivity of a specic crystalline or non-crystalline phase (Takemoto and Uchikawa 1980).
Obviously, the content of active phases in a specic SCM is a factor of primary importance
(Millet and Hommey 1974; Sersale 1993).
Many studies have suggested that a correlation exists between the long term performance
in terms of compressive strength or durability and the bulk chemistry of the SCM without refer-
ence to the mineralogical composition (e.g., Watt and Thorne 1965; Costa and Massazza 1974;
Figure 28. Experimental transition enthalpy from quartz compared to framework density of pure-silica
polymorphs (Piccione et al. 2000). Qtz stands for quartz, Co for coesite, Cr for cristobalite, Tr for tridymite,
CHA for chabazite and FAU for faujasite.
Supplementary Cementitious Materials 251
Hanna and Afy 1974; Cavdar and Yetgin 2006). The pozzolanic activities and blended cement
or pozzolan-lime binder performance showed positive linear correlations of varying statistical
signicance with the sum of bulk SiO2 and Al2O3, in some cases also Fe2O3 was added. The
sum SiO2 + Al2O3 + Fe2O3 ≥ 70 wt% remains one of the fulllment criteria for pozzolans in
ASTM C618. In case of a one phase material the chemical composition can be considered as
a meaningful parameter. However most natural and articial SCMs consist of a heterogeneous
mixture of phases and then a direct relationship between overall chemical composition and
pozzolanic activity becomes less obvious. The fact that all correlations were reported for long
term pozzolanic activity and/or performance indicates that the characteristics and the distribu-
tion of the reaction products should relate SCM chemistry and long term performance. The
eventual, long term reaction product assemblage is controlled by the overall chemistry of the
active phases (Massazza 2001). Addition of blast furnace slag or metakaolin has been observed
to change the Ca/Si ratio, the silicate polymerization and the morphology of the main C-S-H re-
action products and can thus alter the permeability of the reaction product barrier layer and the
eventual performance of the binder (Richardson 1999, 2004). As the main reaction products are
calcium-silicate-hydrates (with some Al incorporation) and calcium-aluminate-hydrates (con-
taining additional Si and Fe), the total SiO2 + Al2O3 + Fe2O3 content of the active phases may be
considered as an indication of the Ca(OH)2 binding potential of an SCM.
In practice, it is very difcult to separate the contributions to the SCM activity of physical
particle characteristics and mineralogical properties. This is considered one of the primary
reasons of contradictory ndings in literature concerning the relative activities of SCM phases.
Furthermore, because SCMs of comparable origin often show broad similarities in physical
particle properties if not in mineralogical and chemical composition, the widespread adoption
of the genetic classication scheme of SCMs can be considered to remain sensible.
External factors. The rate of the pozzolanic reaction also depends on the mix design, larger
water/binder ratios will result in increased pozzolanic activity but will inevitably decrease the
performance of the binder due to the increased overall porosity.
The ratio of SCM over Ca(OH)2, or equivalently the ratio of SCM over Portland cement
obviously affects the pozzolanic activity in increasing or decreasing the frequency of fulllment
of the reaction conguration. The Ca(OH)2:pozzolan ratio for optimal performance and activity
is depending on the overall content, composition and activity of the constituent phases of the
SCM, but is usually situated in between 1:1 (Murat 1983; Bakolas et al. 2006) and 2:1 (Take-
moto and Uchikawa 1980; Costa and Massazza 1974). Pozzolans rich in Al2O3 generally need
higher Ca(OH)2:SCM ratios for optimal reactivity, and SCMs displaying hydraulic activity usu-
ally need much less Ca(OH)2 to activate the hydration reactions (Lang 2002). In terms of Port-
land cement over SCM ratio optimal replacement percentages are often dened based on the
desired properties of the hardened cement. In general, the optimal replacement ratio depends
on the water demand, i.e., surface roughness and specic surface, and the activity of the SCM.
The higher the water demand and activity, the lower the optimal replacement ratio is, typically
10-15 wt% for silica fume or metakaolin. Optimal replacement ratios dened for durability
properties tend to be somewhat higher than ratios for optimal strength performance. At exces-
sive replacement levels the pozzolanic activity is lowered because of the premature depletion of
the solution alkalinity by the reacting SCM. A signicant drop in solution pH below 10 may not
only effectuate a decrease in pozzolanic reaction rate, but can also lead to the destabilization of
AFm and AFt reaction products in the blended cement (Lothenbach et al. 2011).
To increase the pozzolanic reaction rate, curing at elevated temperatures can be applied.
The temperature dependence of the pozzolanic reaction on the short term can be described by
the Arrhenius equation, implying an exponential dependence of the reaction rate on temperature
(Snellings et al. 2009). The curing temperature should however not exceed the stability eld
of the C-S-H phase and result in the precipitation of more crystalline phases (typically above
252 Snellings, Mertens, Elsen
80-100 °C; Taylor 1990). The acceleration induced by elevated curing temperatures is much
more pronounced for the pozzolanic reaction compared to the hydration of Portland cement
constituents due to the higher activation energy of the pozzolanic reaction (Shi and Day 1993).
Hydration mechanism and kinetics of blended cements
In blended Portland cements the hydration reactions of the clinker phases are complemented
by the pozzolanic or hydraulic reactions of the added SCM. Although the hydration processes
of the clinker phases follow different mechanisms and rates than the pozzolanic or hydraulic
reactions of the SCM, the clinker hydration in blended cements is inuenced by the presence
of SCMs. Reaction kinetics, products and the properties of fresh and hardened pastes can
be manipulated by the replacement of a fraction of the Portland cement by SCMs. Both the
properties of the SCM as the mix design are determining factors with the potential to affect all
stages of the hydration and pozzolanic reactions in the blended cement.
Inuence of SCMs on the hydration of clinker phases. To eliminate the interference of
simultaneously occurring reactions in a blended cement, the effect of SCM addition on the
hydration kinetics of single clinker compounds has been investigated by numerous researchers.
The hydration mechanisms of the individual clinker components have been recently reviewed
by Gartner et al. (2002) and Bullard et al. (2011). Similar to the pozzolanic reaction mechanism,
the hydration of C3A in the presence of gypsum and the hydration of C3S experience both a
brief initial phase of high reactivity followed by a dormant period and an eventual main reaction
stage.
C3S. The main component of Portland cement is C3S, constituting 60-70 wt% of the
cement. In general, the addition of an SCM has an accelerating effect on the hydration of C3S
(Ogawa et al. 1980). The heat evolution rate during the main reaction period and the cumulative
amount of heat released over the complete reaction are increased, especially when recalculated
to the C3S content in the samples (Massazza 2001). The effect of the SCM on the early reaction
is mainly governed by its neness, as illustrated in Figure 29. The initial dissolution period is
Figure 29. The effect of the silica fume specic surface area on the heat evolution rate curves in hydrating
pastes of C3S with 20 wt% silica fume (Beedle et al. 1989).
Supplementary Cementitious Materials 253
lengthened and the induction stage shortened, when SCMs of increasing neness are introduced
(Stein and Stevels 1964; Kurdowski and Nocun-Wczelik 1983; Beedle et al. 1989; Korpa et
al. 2008). This phenomenon is usually termed the ller effect. The addition of extremely ne
particles results however in a decreased hydration rate because the high water demand of the
particles limits the amount of water that can participate in the hydration and pozzolanic reactions
(Beedle et al. 1989; Korpa et al. 2008). SCMs with low specic surface area such as certain
y ashes have been observed to lengthen the induction period, but increase the cumulative heat
released during the main reaction stage (Watt and Thorne 1965, 1966). This dependence on the
SCM specic surface has been attributed to Ca2+ adsorption and C-S-H nucleation at the SCM
surface. In consequence, the layer of reaction products on the C3S particles would be thinner
and C3S dissolution would thus last longer (Wu and Young 1984). In addition, the lowered Ca2+
and hydroxyl concentrations at the C3S surface may accelerate the conversion of an initially
precipitated impermeable metastable C-S-H(m) to a more stable and permeable C-S-H form
(Stein and Stevels 1964). Eventually Ca(OH)2 formed as a result of the C3S hydration, will be
partially or completely consumed by the pozzolanic reaction.
C3A. The hydration of C3A in the presence of gypsum is an important regulator of the
setting of the blended cement. Contrary to the effect of SCMs on C3S hydration, the heat
evolution rate of the main C3A hydration stage is lowered in the presence of SCMs (Collepardi
et al 1978; Uchikawa and Uchida 1980). The reasons underlying the decreased C3A hydration
rate remain unclear. A correlation seems to exist with the SCM specic surface; the higher the
specic surface, the lower the hydration rate (Collepardi et al. 1978). Other factors could be the
alteration of solution composition by SCM addition, the adsorption of sulfate at the pozzolan
surface or the altered hydration product assemblage.
Blended cements. The hydration behavior of blended cements is dominated by the
hydration of its main component. In the case of pozzolan containing blended cements, C3S
is the most prominent constituent. In cements consisting of a large fraction (50-90 wt%) of a
hydraulic SCM, e.g., granulated blast furnace slag, the long term behavior is governed by the
hydration of the SCM. In consequence, for pozzolanic SCMs, the early hydration behavior
is mainly affected by the specic surface of the SCM. On the condition that sufcient water
remains available for hydration, an increase in specic surface results in an accelerated cement
hydration, lengthened early dissolution stage and shortened induction period (Takemoto and
Uchikawa 1980). To the contrary, slag dominated cements are known to hydrate more slowly and
expel less heat than Portland cement and are therefore suitable for applications in mass concrete
structures such as dams, reservoir, quays etc. (Lang 2002). However, when recalculated to the
actual clinker content, the heat liberated by the slag-cement over a longer time period is higher
than that released by the Portland cement (Hooton and Emery 1983). Obviously, the added
SCMs consume Ca(OH)2 released by clinker hydration to form supplementary cementitious
reaction products. In addition the reaction product composition, structure, morphology and in
some cases also the assemblages are considerably changed.
REACTION PRODUCTS
Compared to the wide variability in supplementary cementitious materials, there exists
only a relatively small range of compounds formed in the pozzolanic, hydraulic or hydration
reactions. This is mainly due to the similarity in overall chemical composition of the hydrating
mixtures of pozzolans and lime or blended cements, regardless of the type of SCM. In
consequence, only a limited number of phases will be thermodynamically stable or metastable
at ambient conditions. This observation enabled the application of thermodynamic models to
calculate the ultimate hydrate mineralogy from chemistry by Lothenbach and coworkers (e.g.,
Lothenbach and Winnefeld 2006; Matschei et al. 2007b). Combined with quantitative kinetic
254 Snellings, Mertens, Elsen
data or models on reactant consumption or product formation, thermodynamic calculations
have also been used to study evolving hydration processes (e.g., Lothenbach et al. 2007, 2008a;
Winnefeld and Lothenbach 2010). Although kinetic barriers may impede the attainment of
ultimate thermodynamic equilibrium, or formation of products may not conform to the predicted
assemblage due to locally prevailing chemical conditions. The thermodynamic approach has
shown to be successful in predicting the behavior of many product assemblages in function of
variable pH, sulfate, carbonate or temperature conditions (e.g., Lothenbach and Gruskovnjak
2007; Pelletier et al. 2010; Matschei and Glasser 2010). Hence, reported observations on
product assemblages of the pozzolan-lime reaction or the hydration of blended cements may be
compared at least in a qualitative way with thermodynamic predictions.
Product assemblages
The chemical composition of the mix, more specically the amount and composition of
the pozzolanically or hydraulically active phases together with the pozzolan - lime or pozzo-
lan - Portland cement ratio are primary factors in the determination of the reaction product
assemblage. In addition, the presence of soluble sulfate, carbonate or chloride may provoke
the formation of AFt and AFm phases incorporating the corresponding anion groups (Matschei
et al. 2007c; Balonis et al. 2010). Curing conditions are equally important, partial CO2 pres-
sure and curing temperature may affect the product assemblage considerably (Lothenbach et al.
2008a). Evolving reaction conditions, e.g., the depletion of Ca(OH)2 by the pozzolanic reaction
or decreasing released heat of hydration, may result in changes in the product assemblage dur-
ing ageing.
Pozzolan-lime reaction products. In Ca(OH)2 saturated solutions silica released from the
pozzolan in combination with Ca2+ primarily forms C-S-H. Dissolved alumina can be incor-
porated into the C-S-H phase (Richardson et al. 1993) or can be precipitated in combination
with Ca2+ as calcium-aluminate-hydrates. The Ca/Si ratio of the C-S-H phase is variable both
in space as in time and also depends on the activity and composition of the pozzolan and the
mix design. Increasing polymerization of the silicate groups in the C-S-H is observed upon pro-
longed curing—the silicate monomer and dimer contents decrease while the polymer content
increases (Massazza and Testolin 1983; Brough et al. 1995).
In the absence of sulfate, carbonate or chlorides C4AH13-19 forms as main calcium aluminate
hydrate (Taylor 1990). If general or local deciency of Ca(OH)2 occurs, often encountered
when metakaolin is added as SCM, the recombination of alumina and silica with Ca2+ to form
strätlingite (hydrated gehlenite), C2ASH8, can be observed (Serry et al. 1984; Ambroise et al.
1994). Strätlingite allows structural substitution of Na+ and K+ for Ca2+ (Jones 2002). At later
stages of reaction, higher water/solid ratio or in the presence of abundant alkalis, hydrogarnet or
katoite, C3AH6, is stabilized (Takemoto and Uchikawa 1980; Serry et al. 1984; Sersale 1993).
Hydrogarnet can incorporate Si in a solid solution series between katoite Ca3Al2(OH)12 and
grossularite Ca3Al2Si3O12. In metakaolin-lime pastes the formation of hydrogarnet is mostly
observed at elevated curing temperatures above 40 °C (De Silva and Glasser 1992). Although
C4AH13 and strätlingite are considered to be thermodynamically unstable towards hydrogarnet
and Ca(OH)2, no evidence of the transformation of strätlingite and C4AH13 to hydrogarnet was
observed to occur at 60 °C (Rojas and Cabrera 2002; Rojas 2006). At temperatures below
40°C, typically only traces of hydrogarnet can be observed. Hydrogarnet forms very slowly at
low temperatures, probably due to kinetic reasons (Takemoto and Uchikawa 1980; Massazza
2001). C4AH13 can possibly be stabilized by the incorporation of small amounts of carbonate
or sulfate (Pöllmann 2006).
SCMs containing soluble sulfate such as y ashes or blast furnace slags will react with
Ca(OH)2 to form ettringite, C6AS3H32, or monosulfoaluminate (kuzelite), C4ASH12, or a
combination of both depending on the sulfate over alumina ratio (Mortureux et al. 1980). A
Supplementary Cementitious Materials 255
limited solid solution of sulfate into C4AH13 up to SO4/2OH of 0.5 was observed at 25 °C
(Matschei et al. 2007c). Addition of Na2SO4 accelerates the pozzolanic reaction. Removal
of dissolved sulfate by ettringite precipitation is compensated by an increase in hydroxide
concentration and pH to maintain electroneutrality (Shi and Day 2000). Ettringite initially
formed transforms partially or completely into kuzelite when all soluble sulfate is consumed.
If excess alumina is present also other hexagonal calcium aluminate hydrates or hydrogarnet
may be encountered (De Silva and Glasser 1992). The considerable amount of MgO present in
blast furnace slags can also give rise to hydrotalcite-type phases with an ideal composition of
Mg4Al2(OH)14∙3H2O but allowing extensive cationic and anionic substitution. Virtually no Mg2+
was observed to enter the C-S-H phase (Richardson et al. 1994; Richardson and Groves 1997).
Upon exposure to air or in binders with very low carbonate contents, carbonation of the C4AH13
to hemicarboaluminate, C4AC0.5H12, will occur. More extensive carbonation due to prolonged
exposure or due to substantial amounts of carbonates such as calcite present in the binder results
in the formation of monocarboaluminate, C4ACH11 (Matschei et al. 2007c). In Figure 30 the
phase assemblages of the AFm-type structure are presented in a ternary diagram with at the
apices OH-AFm (C4AH13), SO4-AFm (kuzelite) and CO3-AFm (monocarboaluminate).
Except from the limited solid solution between C4AH13 and kuzelite, the end members
behave as separate phases from a mineralogical point of view. The carbonation of C4AH13
prevents the conversion of ettringite into kuzelite (Kuzel 1996), therefore at high levels of
carbonation often ettringite can be found, while at low levels kuzelite is present (Massazza and
Daimon 1992; Atkins et al. 1993). In the presence of calcite and at temperatures below 20 °C,
also ettringite was observed to show substantial solid solution between the ideal SO4-ettringite
and the CO3-ettringite end members (Barnett et al. 2001).
Blended cement reaction products. The reaction product assemblages formed in the
hydration of blended cements are similar to the compounds formed in the reaction between SCMs
Figure 30. Calculated phase assemblage between different AFm phases at 25 °C. A possible range of
stoichiometry of hemicarboaluminate and the limited formation of ternary solid solutions are not shown
(Pöllmann 2006) (modied after Matschei et al. 2007c).
256 Snellings, Mertens, Elsen
and lime. The underlying reason is that in most cases the overall bulk chemical composition
of the binders are comparable, with the exception that Portland cement usually contains
calcium sulfates and commonly incorporates some carbonate from adsorption of atmospheric
CO2 or interblending with a limited amount of limestone. Compared to the hydration product
assemblage of ordinary Portland cement, SCM addition mostly results in a variation in the
relative proportions of the reaction products. At ambient conditions the hydration products in
blended cements commonly comprise C-(A)-S-H and AFm-type phases, ettringite and when
substantial MgO is available, hydrotalcite-type phases. The Ca(OH)2 content depends mainly
on the blending ratio of SCM over Portland cement and the composition and activity of the
SCM (Massazza 2001). When all Ca(OH)2 is consumed in the pozzolanic reaction, strätlingite
may develop.
The C-S-H phase in blended cements usually displays Ca/Si ratios in the range of lower
than 1 to 1.8, i.e., lower than the observed C-S-H Ca/Si range in ordinary Portland cement of
1.2 to 2.1 (Richardson 1999). This is generally accepted to be due to the larger availability
of Si and Al originating from the dissolving SCMs (Richardson 2008). Conjointly the mean
(alumino)silicate chain length is increased from values between 2 to 5 in respectively young
and mature PC pastes to 10 or more in some blended cements (Richardson 1999). The extent
to which mean chain lengths are affected depends on the activity and composition of the SCM
and the curing conditions. The C-S-H phase nanostructure in blended cements is considered to
be most compatible with a defect-tobermorite structure, rather than a jennite-based structure
(Cong and Kirkpatrick 1996a, 1996b; Richardson 2004). Tetrahedral substituent ions such as
Al3+ can be incorporated into C-S-H only in the bridging tetrahedron (Richardson and Groves
1993; Andersen et al. 2006). Sorption or incorporation of alkali and sulfate ions at the C-(A)-
S-H surface is related to its Ca/Si ratio and Al content. Alkali sorption increases with decreasing
Ca/Si ratio and increasing Al content (Hong and Glasser 1999), while sulfate adsorption
decreases with decreasing Ca/Si (Matschei et al. 2007b). In addition to sorption behavior, also
the morphology of the C-S-H phase is dependent on its composition (Richardson 2004). As Ca/
Si decreases and Al/Ca increases in C-A-S-H in GGBFS and MK blended cements a transition
occurs from brillar, thin particles to sheet-like two-dimensional foils (Richardson and Groves
1997; Richardson 1999).
Due to the presence of easily soluble calcium sulfates, ettringite is able to form in the
initial hydration stages. At more advanced stages of hydration ettringite can be partially or
completely transformed to kuzelite. This transformation is obviously controlled by the overall
bulk SO3/Al2O3 ratio, but also, as mentioned before, by the CO2/Al2O3 ratio. Calculated phase
assemblage variations in function of changing sulfate and carbonate levels as expected to occur
in Portland cement hydrated at 25 °C are given in Figure 31. At low carbonate and sulfate levels
hydrogarnet was calculated to be present in the product assemblage, at higher carbonate and
sulfate levels hydrogarnet is destabilized (Matschei and Glasser 2010). In blended cements,
strätlingite can be encountered when Ca(OH)2 is not present and SO3/Al2O3 is low (Grutzeck
et al. 1981).
Hydration thermodynamics
The recent development of a comprehensive and internally consistent thermodynamic da-
tabase for cement hydrate compounds allows clarifying and eventually predicting the response
of product assemblages on compositional changes of the binder (Matschei et al. 2007b). To
illustrate the ultimate binder mineralogy, the product assemblages in the ternary CaO-SiO2-
Al2O3 system at 25 °C and water over solid ratio of 1:1 were explored following the methodolo-
gy outlined by Lothenbach and Winnefeld (2006) based on a Gibbs energy minimization. For il-
lustration, two different cases were considered. In one case calcium sulfates were absent, in the
other 5 wt% of gypsum was added to the system. To visualize the phase relationships between
the hexagonal hydrates and the C-S-H, the precipitation of the hydrogarnet solid solution series
Supplementary Cementitious Materials 257
needed to be suppressed. Ideal solid solution between C-S-H of tobermorite type (Ca/Si = 0.83)
and jennite (Ca/Si = 1.6) was assumed. No incorporation of Al into C-S-H was accounted for.
Obviously, this model is a simplication and serves mainly to highlight the expected changes in
product assemblages. Incorporation of sulfate, carbonate, chloride, alkali or magnesium or iron
compounds would denitely alter the nature and extent of the product stability elds.
CaO-SiO2-Al2O3. Upon inspection of the ternary diagram in Figure 32, the phases containing
silica are C-S-H and strätlingite. Alumina is distributed over the C4AH13 and strätlingite phases.
C-S-H is the most stable phase at low Ca levels and excess silica and alumina can be considered
as unreacted SCM material. Increasing the Ca content from the eld where C-S-H is the sole
reaction product, a tieline is crossed connecting C-S-H of tobermorite composition (Ca/Si =
0.83) with the Al2O3 apex. In the corresponding eld the C/S ratio of the mix is sufciently
high to allow the precipitation of strätlingite in the presence of alumina. When augmenting the
Ca proportion eventually excess alumina will combine with Ca to form C4AH13, in addition
strätlingite will start decomposing into C-S-H and C4AH13. Finally, at high Ca levels strätlingite
disappears and the ultimate product assemblage is predicted to consist of C-S-H, C4AH13
and excess Ca(OH)2. In the rare case that alumina is prevalent over silica and sufcient Ca is
available, C-S-H is destabilized in favor of strätlingite, C4AH13 and excess alumina.
The presented ternary diagram conrms the observed incompatibility of Ca(OH)2 and
strätlingite. Takemoto and Uchikawa (1980) indicated that the Ca2+ concentration needed
for the precipitation of C-A-H phases is in general higher than for C-S-H phases, possibly
explaining why C-A-H phases can usually be found outside the C-S-H layer enveloping the
reacting SCM or clinker particles.
CaO-SiO2-Al2O3-SO4. In the presence of sulfate the topology of the ternary diagram
remains essentially unchanged (Fig. 33), only ettringite and kuzelite are stabilized over
C4AH13. In the presence of C4AH13, a solid-solution member of the kuzelite-C4AH13 series
Figure 31. Calculated phase assemblage of a hydrated mixture consisting of C3A, Ca(OH)2 and varying
initial sulfate and carbonate ratios at 25 °C (modied after Matschei and Glasser 2010).
258 Snellings, Mertens, Elsen
Figure 32. Reaction product assemblages in the CaO-SiO2-Al2O3 ternary system (wt% based) at 25 °C
and w/s ratio of 1:1. Str stands for strätlingite, Hc for C4AH13, CH and C-S-H are cement shorthand for
Ca(OH)2 and calcium-silicate-hydrates. In a) excess silica and alumina is present respectively as quartz and
gibbsite, in b) excess alumina is present as gibbsite.
Figure 33. Reaction product assemblages in the CaO-SiO2-Al2O3 ternary system (wt% based) at 25 °C
and w/s ratio of 1:1, 5 wt% gypsum was added to the system. Str stands for strätlingite, Hc for C4AH13
and Ett for ettringite. Kuz corresponds with kuzelite and Kuz-ss with a solid solution member of the
kuzelite-C4AH13 solid solution series. CH and C-S-H are cement shorthand for Ca(OH)2 and calcium-
silicate-hydrates. In a) excess silica, alumina and gypsum are present, in b) excess alumina and gypsum, in
c) excess alumina and d) excess gypsum.
Supplementary Cementitious Materials 259
is predicted to be present. Strätlingite is predicted to precipitate in the absence of Ca(OH)2.
Higher CaO/SO3 and Al2O3/SO3 favor the formation of kuzelite over ettringite as corroborated
by Figure 31.
In general, at higher curing temperatures denser and more heterogeneously distributed
hydrates are formed resulting in a coarser porosity (Kjellsen et al. 1991). Above 50 °C kuzelite
is increasingly stabilized over ettringite and monocarboaluminate (Thomas et al. 2003;
Christensen et al. 2004; Lothenbach et al. 2008b). The product assemblage formed in concrete
cured at elevated temperature may thus change when temperatures are lowered under service.
The stabilization of ettringite over kuzelite at lower temperatures may thus be a primary cause
of delayed ettringite formation and the associated concrete deterioration (Famy et al. 2002).
Matschei and Glasser (2010) reported that above 25 °C kuzelite formation is favored over
C4AH13 or a solid solution member of the kuzelite-C4AH13 series.
One of the benets of thermodynamic calculations of the hydration processes is the
prediction of the volume of solids present in the cement. Coupled with kinetic data on the
consumption of reactants an evolving picture of the hydrate assemblage in a binder can be
established. Assuming that the total volume of the binder paste remains constant, the increase in
volume of the solid phases due to the formation of hydrated compounds allows evaluation of the
total porosity of the system (Lothenbach et al. 2008a). Properties known to be roughly correlated
with total porosity such as compressive strength or permeability can as such be estimated. The
calculated solid phase evolution of a hydrating slag-silica fume cement as displayed in Figure
34 was shown to correspond well with experimental observations on the product assemblage
(Lothenbach et al. 2009).
Figure 34. Modeled volume changes (cm³/100 g of unhydrated binder) during the hydration of low alkali
blended cement containing 66.6% ground granulated blast furnace slag, 10% of silica fume and less than
23.3% of Portland cement (Lothenbach et al. 2009).
260 Snellings, Mertens, Elsen
PROPERTIES OF MORTAR AND CONCRETE CONTAINING
SUPPLEMENTARY CEMENTITIOUS MATERIALS
The physical bulk properties of SCM blended lime or cement binders depend both on mix
design and curing conditions and on the inherent characteristics of the SCM. Therefore, the
literature on performance shows a myriad of studies evaluating the effect of one or more of
these factors. Variations in the inherent SCM characteristics often preclude direct comparisons
between different studies, but some general trends can be observed. The emerging view is
that the utilization of SCMs is regarded as benecial in terms of performance, durability and
sustainability. Instead of going into detail on these aspects for all separate SCMs, this section
rather presents a brief generic overview of the effect of SCM addition on lime and Portland
cement binders illustrated by specic examples. More comprehensive reviews centered on the
effect of addition of different SCMs on the performance and durability of SCM-lime or SCM
blended cements binders can be found in Malhotra and Mehta (1996), Hewlett (2001), Bensted
and Barnes (2002) and Siddique (2008).
Properties of uncured mortar and concrete containing SCMs
The properties of the freshly mixed and early cured mortar and concrete are important
because they can exert a signicant control on the ultimate performance and durability of the
binder. In this respect the particle characteristics and the initial reactivity of SCMs can inuence
the water demand and setting time of the mortar or concrete. This is for instance of importance
when low water/binder ratios are needed in the preparation of high-performance concrete and
the use of superplasticizers becomes necessary.
Water demand. The effect of SCMs on the amount of water needed for the binder to reach a
specied workability or uidity is largely depending on the particle characteristics of the SCM
and its proportion in the mortar or concrete. Both the particle size distribution and the particle
shape and porosity determine the specic surface and neness of the SCM. In general, the
higher the neness or specic surface area and the more irregular the particle shape of SCM,
the higher the water requirement of the blended binder is. SCMs such as silica fume, metakaolin
or diatomite earths that show large specic surface areas typically increase the water demand
substantially. Obviously, the higher the proportion of ne SCM particles added to the mix, the
higher the water demand as illustrated in Table 4 for metakaolin blended cements (Badogiannis
et al. 2005). To the contrary, the spherical particles and relatively low specic surface area
encountered in y ashes may result in lowered water requirements. Since the water/binder ratio
is directly related to the binder porosity, increased binder water requirement commonly results
in lowered strength performance and vice versa. Especially at higher replacement levels it may
thus be essential to introduce superplasticizers to reduce the water/binder ratio.
Setting time. SCMs can both increase and decrease the setting in blended cements. The end
of setting is generally conceived to be related to the end of the induction period and the start
of hardening due to C-S-H reaction product formation. In this respect the acceleration of alite
hydration can decrease setting times in blended cements. However, this acceleration effect is
neutralized when at higher cement replacement ratios the overall content in clinker hydration
products is lowered due to the dilution effect of the SCM. Also increased water requirements at
higher replacement ratios may extend the setting time. In metakaolin blended cement the initial
and nal setting times at low replacement ratios and low water demand were observed to be
slightly reduced, while at higher replacement ratios and water demand the rate of setting was
slowed down (Table 4, Badogiannis et al. 2005).
Heat evolution. The heat evolved during hydration of blended cements strongly depends on
the early reactivity of SCMs. Highly reactive SCMs such as silica fume or metakaolin increase
the heat released during the initial dissolution period and the subsequent main hydration period
(cf. Fig. 21). This has been attributed in part to the acceleration of clinker hydration and in part
Supplementary Cementitious Materials 261
to the extensive formation of supplementary products of the pozzolanic reaction. However, most
SCMs of lower activity can be effectively used to reduce the hydration heat in large structures.
The combined effect of clinker dilution and a more sluggish pozzolanic reaction lowers the
maximum temperature reached and minimizes the risk of cracking as exemplied in Figure 35
for ground granulated blast furnace slag cement.
Properties of hardened mortar and concrete containing SCMs
The properties of hardened SCM blended binders are strongly related to the development
of the binder microstructure, i.e., to the distribution, type, shape and dimensions of both reaction
products and pores. The general benecial effects of SCM addition in terms of both strength
performance and durability are mostly attributed to the pozzolanic reaction in which Ca(OH)2
is consumed to produce additional C-S-H and C-A-H reaction products. The formation of
Table 4. Variation of water demand and initial and nal setting times for cements blended
with metakaolin from differing sources. MK identies with metakaolin in the sample index,
followed by the respective replacement percentages of 10 or 20 wt% (Badogiannis et al. 2005).
Sample Metakaolin
(wt%)
Water demand
(water/solid)
Setting time (min)
Initial Final
PC 27.5 105 140
MK1-10 10 29 75 130
MK2-10 10 29 85 130
MK3-10 10 32 105 160
MK4-10 10 32.5 155 180
MKC-10 10 31 95 130
MK1-20 20 32 105 160
MK2-20 20 31.5 110 165
MK3-20 20 38.5 120 160
MK4-20 20 41 205 230
MKC-20 20 37.5 140 170
Figure 35. Inuence of ground granulated blast furnace slag replacement ratio on adiabatic temperature
rise (modied after Wainwright and Tolloczko 1986). PC stands for Portland cement. PC stands for
Portland cement.
262 Snellings, Mertens, Elsen
pozzolanic reaction products results in inlling of interstitial porosity and a rening of the pore
size distribution or pore structure.
Pore structure. The pore structure is one of the most important factors governing the
durability in terms of attack by aggressive agents such as CO2, sulfates and chlorides (Luke
2002). It is generally observed that the addition of SCMs results in an increase of total porosity,
but a consistent decrease in mean pore sizes. Capillary porosity, larger than 30-40 nm, is
generally reduced and “gel porosity” increased, the latter being smaller than 10 nm and related
to the typical distances between C-S-H particles (Takemoto and Uchikawa 1980). This can be
explained by the inlling of coarse and capillary pores by C-S-H phases. In Table 5 the total pore
volume and volume of pores smaller than 20 mm are compared for ordinary Portland cement
and Portland cement blended with metakaolin. An overall increase of both pore volume and
proportion of ne pores can be observed. Also the threshold pore radius, the radius below which
the porosity sharply increases, is lowered considerably (Fig. 36) (Khatib and Wild 1996). The
renement of the pore structure generally results in lowered permeability and ionic diffusion
coefcients, thus effectively improving the durability of the binder.
Moreover, the coarse and capillary porosity have been observed to be closely correlated
with the compressive strength, increasing porosities leading to decreased binder strength
Table 5. The effect of blending Portland cement with metakaolin on the pore volume
and proportion of small pores in a blended cement (Khatib and Wild 1996).
Age (days)
Pore volume (mm/g) % of ne pores (radii < 20 mm)
Metakaolin (%) Metakaolin (%)
0 5 10 15 0 5 10 15
3 262 257.6 284.1 277.6 22.2 28.3 31 39.9
7 229.6 261.7 268.8 251.6 26.5 32.1 41 50.4
14 209.9 203.4 221 212.1 30.3 43 53.9 55.7
28 189.1 205.3 237.1 222.7 33.7 43.5 48.7 54.9
90 181.4 180.8 219.6 198.9 37.3 44.7 49.9 57.6
Figure 36. The introduction of
metakaolin reduces the thresh-
old pore radius signicantly. The
threshold radius is the pore radius
below which the porosity sharply
increases (Khatib and Wild 1996).
Supplementary Cementitious Materials 263
(Takemoto and Uchikawa 1980; Papayianni and Stefanidou 2006). The improvement of the
cement microstructure is also visible at the interface zone between aggregate and binder in
mortars and concrete. The interface zone in ordinary Portland cement is a region of low C-S-H
and high ettringite and Ca(OH)2 concentrations between 25 and 100 mm in thickness with
decreased microhardness. In blended cements both the amount of Ca(OH)2 and its preferential
orientation is reduced (Larbi and Bijen 1990). Instead additional C-S-H replaces Ca(OH)2, lls
porosity and reduces the thickness of the interface zone (Shannag 2000). The microhardness
of the interface zone is increased (Asbridge et al. 2002) and the adhesion between binder and
aggregate improved.
Strength. The water/binder ratio is a factor of primary importance in governing strength
development and ultimate strength of both SCM-lime and SCM-Portland cement based binders.
As mentioned earlier, a close relationship exists between water/binder ratio and binder porosity
on the one hand and binder porosity and strength on the other hand. The high water demand
of very ne pozzolans such as silica fume may necessitate the use of water-reducing agents to
lower the water requirement of the blend and improve the eventual binder strength.
Strength development in SCM-lime binders depends on the one hand on the mix design,
the SCM/lime and water/binder ratio, and the curing conditions, elevated temperature curing
increases the rate of strength development but often lowers nal strength, and on the other hand
on the SCM reactivity. The latter can widely differ from SCM-type to SCM-type as exemplied
in Figure 37 (Shi and Day 1995), depending on the particle characteristics, active phase content
and chemical and mineralogical composition of the SCM. The ultimate strength of SCM-lime
binders may exceed 20 MPa, which is high enough to serve for many common applications
(Massazza 2002).
In blended cement pastes, mortars and concrete the result of SCM incorporation on the
development of strength is conceived to be controlled by the dilution effect, the ller effect,
the hydration acceleration effect and the pozzolanic or hydraulic reaction. The rst three
Figure 37. Compressive strength development of blends of lime (20%) and a variety of SCMs (80%).
SCMs with hydraulic properties develop the highest strengths over time (Shi and Day 1995). NPB stands
for natural vitreous pozzolan from Bolivia, NPG for natural vitreous pozzolan from Guatemala, LFA for
low-lime y ash, HFA for high-lime y ash, the slag used was a typical ground granulated blast furnace
slag.
264 Snellings, Mertens, Elsen
factors have similar effects as an inert ller and dominate the initial strength development. The
contribution of the pozzolanic or hydraulic reactions to cement strength is usually developed in
a later curing stage, depending on the SCM reactivity. In the large majority of blended cements
initial lower strengths can be observed compared to the parent Portland cement. However,
especially in the case of SCMs of higher neness than the Portland cement, the decrease in
early strength is usually less than what can be expected based on the dilution factor. This can be
explained on the one hand by the contribution of the ller effect, in which small SCM grains ll
in the interstitial space between the cement particles, resulting in a much denser binder matrix.
On the other hand, the acceleration of the clinker hydration reactions (cf. Fig. 29) can also at
least partially accommodate the loss of early strength.
At later curing ages blended cements typically show a higher rate of strength development
due to the supplementary formation of products of the pozzolanic or hydraulic reactions.
Depending on the mix design and the SCM activity, in many cases the ultimate strength can
become higher than that of the parent Portland cement as illustrated in Figure 38 for ground
granulated slag cement. The rate of the pozzolanic and hydraulic reactions principally
determines the moment when the blended cement strength exceeds the parent Portland cement
strength. Highly reactive SCMs such as metakaolin or silica fume were observed to enhance
even early strengths within one day (Sabir et al. 2001), while slags of much lower reactivity
typically present positive strength contributions only after 14 to 28 days of curing (Lang 2002).
It should be noted that because hydrated ordinary Portland cement only contains about 20 wt%
of Ca(OH)2, at high cement replacement percentages of 40% or more by pozzolans, strength
development may be hampered because of a general lack of Ca(OH)2 (e.g., Yilmaz et al. 2007).
Durability of mortar and concrete containing SCMs
One of the main advantages of SCMs, and one of the early incentives to introduce SCMs
into blended cements, is the signicantly increased chemical resistance of the binder to the
ingress and deleterious action of aggressive solutions. The improved durability of SCM-
blended binders enables to lengthen the service life of structures and reduces the costly and
inconvenient need to replace deteriorated constructions. In general, one of the principal reasons
of increased durability in SCM-blended cements is the lowered Ca(OH)2 content available
to take part in deleterious reactions. Furthermore, the higher content in C-S-H binder phase
with a reduced Ca/Si ratio results in ner pore size distributions and lower permeabilities
Figure 38. Development of compressive strength in concrete blended with various amounts of ground
granulated blast furnace slag (Khatib and Hibbert 2005).
Supplementary Cementitious Materials 265
and in a thermodynamically more stable and chemically more resistant C-S-H phase with
increased bonding capability of chlorine and alkaline ions. It is apparent that the replacement
ratios of Portland cement by SCMs required for optimal durability are usually higher than the
replacement ratios needed for optimal strength.
Sulfate attack. The interaction of sulfate containing solutions with concrete structures
can result in swelling, cracking and eventual structural failure. Sulfate attack involves the
formation of expansive compounds in the reaction of sulfate with Ca(OH)2 to form gypsum
or in combination with aluminates to form ettringite. The intensity of sulfate attack depends
on the associated cation, increasing in the order Ca2+ < Na+ < Mg2+. In addition of ettringite
formation induced by interaction with Ca-sulfate solutions, Na-sulfate leads to the additional
formation of gypsum when reacted with Ca(OH)2. MgSO4 is very aggressive, not only leading
to the formation of gypsum and ettringite but also to the disintegration of the C-S-H phase into
brucite, gypsum and silica.
SCM addition can reduce or eliminate the deleterious formation of expansive compounds
by lowering the overall amount of Ca(OH)2, by reducing the diffusion rate of sulfate in the pore
solution and possibly by increasing the chemical resistance of the C-S-H phase. Replacement
percentages of 30-40% of Portland cement by vitreous natural pozzolans were observed to
be very effective in reducing expansion upon sulfate attack (Fig. 39) (Massazza and Costa
1979). Lower additions do not completely consume Ca(OH)2, leaving a potential for expansion.
Resistance to MgSO4 attack can only be improved at low (2%) concentrations of MgSO4.
Chloride attack. Exposure to de-icing salts, seawater or salt-bearing groundwater may
result in increased leaching of Ca(OH)2, higher binder porosity and lower strength. In addition,
crystallization of salts in pores may cause expansion. In reinforced concrete, increased Cl/
(OH) ratios can lead to steel passivation and rebar corrosion. The lowered permeability of
SCM-blended cements strongly reduces the diffusion rate of chloride ions into the binder
matrix. Moreover, relatively large amounts of Cl can be bound to C-S-H and AFm phases
giving blended cements that contain higher amounts of these reaction products a larger chloride
binder capacity (Balonis et al. 2010). Lowered Cl concentrations in the depth proles from a
surface exposed to a chlorine bearing solutions in Figure 40 illustrate the improved resistance
of blended concrete to chlorine attack (Chan and Ji 1999).
Figure 39. Reduction of mortar expansion due to attack of a 1% MgSO4 solution decreases with increasing
Portland cement replacement by a vitreous natural pozzolan (modied after Massazza and Costa 1979).
266 Snellings, Mertens, Elsen
Carbonation. Reaction of hydration products with carbonate bearing solutions results in
the formation of CaCO3 and silica and/or alumina gel. In porous binders, intense carbonation
can result in a decreased pore solution pH, possibly leading to steel passivation in reinforced
concrete. However, in relatively impermeable binders, carbonation is usually conned to the
upper surface and does not progress into the binder matrix. Although a depleted reserve of
Ca(OH)2 might render blended cements more susceptible to carbonation, in general the reduced
permeability effectively counteracts the loss of buffering Ca(OH)2.
Alkali-silica reaction. Alkali-silica reactions cause severe damage to concrete structures.
Expansion is generally caused by the formation of an alkali-calcium-silica gel due to the
interaction of alkalis present in the concrete pore solution with Ca(OH)2 and reactive silica
from the aggregate. The expansion risk is especially high when cements containing high levels
of alkali are used in combination with reactive aggregates. The utilization of SCMs can prevent
alkali-silica reactions by reducing the availability of alkalis, lowering the pH and depleting
Ca(OH)2. Although SCMs can contain relatively large amounts of alkali, in most cases the
effective soluble amount of alkalis is rather low. Furthermore, the increased alkali-binding
capability of the hydration products (i.e., C-A-S-H) enables to lower the alkalinity of the pore
solution. However, it should be noted that at low replacement ratios below 20% the addition
of SCMs can increase alkali-silica reactions with respect to the parent Portland cement. This
pessimum behavior can be related to the supplementary release of alkalis from SCMs combined
with an incomplete consumption of Ca(OH)2 (Hobbs 2002). More than 20% replacement is
usually sufcient to avoid expansion. Figure 41 illustrates the expansion abatement in zeolite
tuff blended cements, showing that expansion is strongly reduced when 20% or more zeolite
tuff is added to the high-alkali cement (Feng and Peng 2005). In general, low-alkali SCMs
containing both silica and alumina are observed to be most effective in minimizing expansion.
Alumina-poor SCMs have less potential to reduce the alkali-silica reaction.
CONCLUSIONS
The benets of supplementary cementitious materials utilization in the cement and
construction industry are threefold. First is the economic gain obtained by replacing a substantial
part of the Portland cement by cheap natural pozzolans or industrial by-products. Second is
the lowering of the blended cement environmental cost associated with the greenhouse gases
emitted during Portland cement production. A third advantage is the durability improvement
of the end product. Additionally, the increased blending of SCMs with Portland cement is of
limited interference in the conventional production process and offers the opportunity to valorize
and immobilize vast amounts of industrial and societal waste into construction materials.
Supplementary cementitious materials can be conveniently classied according to a
genetic classication scheme. A distinction is made between naturally occurring and articial
SCMs. The latter category is subdivided into intentionally thermally activated materials and by-
products of industrial processes. Detailed accounts on the physical, chemical and mineralogical
characteristics for the various SCM groups and subgroups corroborate the view that there
exists a close relationship between the material properties and their hydraulic and pozzolanic
reactivity that promises to be quantied and predicted.
To identify the material properties of importance and when they become essential in the
pozzolanic reaction, a detailed knowledge on the pozzolanic reaction mechanism is needed.
Recent developments regarding the dissolution kinetics and mechanisms of aluminosilicates in
high pH environments are illustrated to be instrumental in obtaining more fundamental insights
into the pozzolanic reaction. Improved knowledge on the kinetics and mechanism of the
pozzolanic reaction will allow making better thermodynamic predictions of evolving reaction
product assemblages over the course of hydration.
Supplementary Cementitious Materials 267
The reaction product assemblages are observed to be mainly a function of the chemical
composition of the reactive components in the reactant mixture. In contrast to the wide
variability in SCMs, there exists only a relatively small range of thermodynamically stable
or metastable compounds formed in the pozzolanic, hydraulic or hydration reactions. Recent
developments in thermodynamic modeling allow an improved, more quantitative understanding
of the parameters controlling the assemblage of reaction products to be obtained.
A synopsis of the technological effects of using blended cements is included in the present
paper. Properties of fresh and hardened mortar and concrete incorporating SCMs are compared.
The effects of SCM incorporation are obviously strongly linked to the specic physical
characteristics and can differ widely from slowly reacting blast furnace slag to highly reactive
metakaolin. The emerging image is that the appropriate use of SCMs should be regarded as
benecial in terms of performance, durability and sustainability of the end product.
ACKNOWLEDGMENTS
R. Snellings extends his gratitude towards the Research Foundation - Flanders (FWO) for
nancial support.
Figure 40. Chloride concentrations in
function of depth from the concrete
surface exposed to a Cl- rich solution
after 30 days of exposure. The con-
crete water to binder ratio was 0.33
(modied after Chan and Ji 1999).
Figure 41. Alkali-silica expansion abatement in natural zeolite blended concrete at various Portland
cement replacement percentages (Feng and Peng 2005).
268 Snellings, Mertens, Elsen
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... However, some of these modeling approaches involve assumptions that cannot be easily generalized to cover the diversity of materials, binder kinetics, and curing conditions, and others require input data that is not always available. The problem with these modeling approaches becomes more pronounced as the cementitious material mixture has become more complex due to the partial replacement of Portland cement (PC) with supplementary cementitious materials (SCM) [32][33][34][35][36], as demonstrated by Vollpracht et al. [37]. These blended systems have distinct reaction mechanisms that influence the pore solution composition [36,37]. ...
... Established knowledge about cement hydration, SCM reactions, and alkali binding [34,36] support the main features influencing the OH − , Na + , and K + concentration in the pore solution. Within this context, emphasis is given to the alkali oxides of PC ("PC_K2O", "PC_Na2O"), the amount of SiO 2 in the SCM ("SCM1_SiO2"), and the relative amount of PC and SCM ("PC" and "SCM") Fig. 9. ...
... It is also clear in Fig. 8 that SCM type and amount ("SCM", "SCM1_type_1", "SCM1_SiO2") affect the pore solution composition significantly. These materials modify the cementitious reactions by means of physical and chemical effects [33,34]. Fine particles can contribute to the filler effect and accelerate cementitious reactions by acting as nucleation sites [35]. ...
Article
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This study explores machine learning (ML) algorithms to predict the pore solution composition of hardened cementitious systems produced with Portland cement (PC) and supplementary cementitious materials (SCM). Literature data on pore solution compositions for different cementitious systems was collected and compiled in a comprehensive database containing >300 entries with >80 features. Improved decision tree regressors were applied to the database. It was found that the trained ML models were capable of predicting OH⁻, Na⁺, and K⁺ concentrations reliably (75–90 % of predicted systems within 25 % relative error). Ca²⁺ and sulfur species had lower prediction accuracy. The silica content of SCM, the alkalis content of PC, and the SCM replacement level were identified as important features in determining the ion concentrations. When applied to this database, ML performed better than conventional, theory-based prediction models. Thus, ML models are a promising, complementary technique to determine pore solution compositions.
... Pozzolans are a broad class of siliceous and aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically with Ca(OH)2 (calcium hydroxide) at ordinary temperature to form compounds possessing cementitious properties [18]. The quantification of the capacity of a pozzolan to react with calcium hydroxide and water is given by measuring its pozzolanic activity [19]. ...
Thesis
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Cement constitutes undoubtedly one of the most important well barrier elements, in terms of maintaining well integrity by preventing undesired flow of reservoir fluid into the wellbore. NORSOK D-010 standard dictates that well cement must be impermeable, non- shrinking, ductile and resistant to corrosion among other requirements to ensure long term integrity of the wells [1]. Although, various surveys have found that conventional well cement have serious shortcomings that are the reasons for compromising well integrity globally. It was discovered in 2003 that 8000 to 11000 wells in GOM experienced SCP due to compromised cement integrity as the ages of the wells increased [2]. In 2008, cement failure induced well integrity issues were detected in around 10.7% of 75 NCS wells from a total of 406 well that were included in this study [3]–[4]. Poor cement qualities affect the environment as well. Reported in 2013, authorities in Pennsylvania had issued notices of environmental violations to 1144 wells out of 3533 wells, where 8.7% violations originated from poor cement and casing quality that led to environmental pollution [5]. These investigations reveal that cement in its neat form fails to maintain permanent zonal isolation in wells. It goes without saying, the current cement properties do not meet the regulatory requirements. This is why investigations were conducted to improve the mechanical and rheological properties. Applications of nanotechnology have spread over numerous industries. Numbers of research publications concerning the implementations of nanotechnology are ever growing with time and some studies have shown to have obtained promising results using different metallic nanoparticles. Consequently, Al2O3 and TiO2 nanoparticles (NPs) were considered to be investigated in conjunction with G-class Ordinary Portland Cement (OPC), fly ash (FA), geopolymer cement, carbon fibers (CF) and white glass fibers (WF). Moreover, two different curing environments of room temperature of 20°C and oven temperature of 80°C were introduced to study the temperature effects on the nano-modified cementitious and non- cementitious materials. Because the conventional cement gets degraded when exposed to high temperature for a long time. In terms of experimental structures, total of five test designs were constructed. Four test designs were made with OPC had curing ages of 3, 7 and 28 days and they were cured in both room and oven. One design consisting of geopolymer cement were mainly cured in room temperature for a period of 10 days. Lower concentration of Al2O3 NPs seemed to improve the uniaxial compressive strength (UCS) of OPC by 100.1% in room and 24.8% in oven after 28 days of curing. OPC modified with solitary FA and binary hybrid of FA and intermediate Al2O3 had shown improvement in early strength after 3 and 7 days. However, after 28 days, solitary and binary blend of FA showed opposing results. Medium concentration of TiO2 NPs significantly improved the UCS of geopolymer cement by 41.8%. When applied in OPC, lowest dosage of TiO2 improved the strength of OPC plug by 19.2% in room temperature after 28 days, whereas the highest concentration performed the best in oven with 42.1% UCS raise. With an intention to potentially reducing cement’s brittleness, CF and WF were added with TiO2 as respective binary hybrids to OPC, which had presented with very optimistic results. Generally, TiO2 with highest dosage of CF had shown the most improvements in OPC strength developments in both curing environments. The stiffness, energy absorption capacity and modulus of elasticity of the samples were also analysed, however there were no clear trends of the developments of these properties. Rheological properties for the additives-modified OPC slurries were determined to have workability and slurries were comparatively pumpable, whilst nano-added geopolymer cement was highly viscous for the given LSR. The internal structures of nano-added OPC and geopolymer plugs were analysed with SEM. An empirical UCS vs compressional wave velocity (vp) model was developed from the experimental data from this thesis and it was subsequently tested with three other models to verify its competence and was found to be quite fitting.
... A plethora of pozzolanic materials was employed since ancient times as additions in hydraulic lime mortars, namely natural or synthetic silica glasses, volcanic ashes, diatomaceous earths, phytoliths, ceramics, clays, ashes from burnt organics and metallurgical slags [2], with several of these still used in modern binders as supplementary cementitious materials (SCM) [3,4]. ...
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... On the other hand, secondary CMs, while also limited by the overall workability and chemistry of the cement system, frequently have small environmental impacts relative to clinker production, and are often considered to be burden free in cement LCA studies 16,[21][22][23][24][25] . This is because they are usually reactive in their existing forms due to high temperature upstream processing conditions 26 and provide lower revenues than their corresponding main co-products (e.g., pig iron is the main co-product of blast furnace slag). Secondary CMs also span a range of suitable chemistries for cement systems 11 . ...
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