ASSESSMENT OF THE ALKALI-REACTIVITY POTENTIAL OF SEDIMENTARY ROCKS

Abstract

The reactive forms of silica present in an aggregate depend on the origin and geological history of the rocks. The detection of specific reactive silica must be focused on characteristics such as the identification of polymorphs, the quantification of microcrystalline to cryptocrystalline quartz, and/or on the deformation manifestations for each aggregate. In this paper the types of sedimentary rocks usually exploited as aggregates for concrete, such as sandstone, greywacke, chert, siliceous limestone and mudstone, are presented. In addition, the rocks exhibiting low metamorphic grade are included when the sedimentary structure is still preserved and the features of metamorphic conditions are slight. The main characteristics of the sedimentary rocks regarding alkali-aggregate reactions are discussed and the importance of complementary methods for the detection of reactive forms of silica explained.

Full text

ASSESSMENT OF THE ALKALI-REACTIVITY POTENTIAL OF
SEDIMENTARY ROCKS
Isabel Fernandes1,2*, Maarten Broekmans3, Maria dos Anjos Ribeiro2,4, Ian Sims5
1University of Lisbon, Faculty of Sciences, Department of Geology, Campo Grande, 1749-016
Lisbon, PORTUGAL
2ICT, Institute of Earth Sciences, FCUP, PORTUGAL
3Geological Survey of Norway - NGU, PO Box 6315 Sluppen, N-7491, Trondheim, NORWAY
4University of Porto, Faculty of Sciences, DGAOT, Rua do Campo Alegre, 4169-007 Porto,
PORTUGAL
5RSK Environment Ltd, 18 Frogmore Road, Hemel Hempstead HP3 9RT, UNITED KINGDOM
Abstract
The reactive forms of silica present in an aggregate depend on the origin and geological history
of the rocks. The detection of specific reactive silica must be focused on characteristics such as the
identification of polymorphs, the quantification of microcrystalline to cryptocrystalline quartz, and/or
on the deformation manifestations for each aggregate.
In this paper the types of sedimentary rocks usually exploited as aggregates for concrete, such
as sandstone, greywacke, chert, siliceous limestone and mudstone, are presented. In addition, the
rocks exhibiting low metamorphic grade are included when the sedimentary structure is still preserved
and the features of metamorphic conditions are slight. The main characteristics of the sedimentary
rocks regarding alkali-aggregate reactions are discussed and the importance of complementary
methods for the detection of reactive forms of silica explained.
Keywords: aggregates, reactive forms of silica, petrographic examination, sedimentary rocks
1 INTRODUCTION
Most supracrustal rocks have components identified as alkali-reactive somewhere, contributing
to alkali-silica reaction damaged concrete structures, whereas the same rock types seem to perform as
non-reactive elsewhere. Large volumes of sedimentary rocks from quarries as well as sand and gravel
from natural sedimentary deposits are exploited all over the world for the manufacture of concrete.
Petrographic examination is the first step of the study when assessing new aggregates for
concrete. RILEM AAR-0 gives guidance on the further testing that may be undertaken on the basis of
the findings of the RILEM AAR-1.1 [1], petrographic examination procedure, namely laboratory
expansion tests with boosted alkali contents and storage of concrete prisms in conditions of elevated
temperature and high relative humidity. RILEM AAR-1.1 aims at providing the methodology to be
followed in the petrographic analysis of new aggregates or periodically of quarries being exploited, as
heterogeneities are commonly found in relatively short distances in the same rock mass.
The recognition of textural features and minerals in very fine grained rocks is usually quite
challenging. Therefore, AAR-1.1 has been complemented with a petrographic atlas RILEM AAR-1.2
[2]. For the preparation of the atlas, a large number of samples were analysed and information collated
about alkali-reactive rocks all around the world. The atlas aims to assist petrographers with the correct
identification of reactivity potential of virgin aggregate materials from igneous, sedimentary, and
metamorphic rock types.
The atlas shows micrographs of potentially reactive forms of silica and textural features in
confirmed alkali-reactive lithologies, and describes how to identify these in thin-sections under the
optical microscope. Reliable identification of very fine-grained rocks benefits from supplementary
methods, most notably SEM-EDS in polished thin-sections, bulk whole-rock chemical analysis, or
powder X-ray diffraction – XRD.
* Correspondence to: mifernandes@fc.ul.pt

The present paper presents a summary of the main characteristics of sedimentary rocks. A list
of the most common aggregates is included as well as of the forms of silica that can be found in
different types of rocks. Overall, it appears that, on a global basis, cryptocrystalline quartz is the most
frequent reactive component. However, specific groups of rocks also contain silica polymorphs
thought to be involved in alkali-silica reactions.
2 MATERIALS AND METHODS
2.1 Materials
Different materials were studied, including rock fragments, gravel specimens, post-mortem
concrete prisms from completed expansion testing, and pieces of concrete from damaged structures.
The alkali-reactivity of all rock materials presented in the atlas had been prior confirmed by reliable
laboratory expansion testing, or observed in field structures.
Thin sections of 23.0×48.0×0.030 mm were manually prepared using standard routine
procedures, some using yellow fluorescent dye to facilitate observation of fractures and voids, and
some were polished for later analysis by SEM-EDS.
2.2 Methods for assessment and analysis
The thin sections were studied in a Nikon Eclipse E400POL petrographic microscope in plane
(PPL) and cross polarized light (XPL). Photomicrographs were acquired using a Zeiss MRc5 camera
and Axiovision 4.8 software.
For the study of the samples, the methodologies used in petrography were followed aiming at
the identification of the mineral composition and the texture of the rock. The reduced sizes of the
samples of rock may raise questions regarding the representativeness of the samples, most notably in
places where heterogeneous natural sedimentary deposits are common. The gravel contained
polymictic aggregate materials with particles from a variety of rock types and textures. Due to the
difficulty of identifying potentially alkali-reactive lithologies, individual particles of interest were
selected in concrete samples post-mortem after completed expansion testing.
The polished thin sections were sputter-coated with carbon under vacuum in a VG
MICROTECH E6700/T800 instrument for analysis in a JEOL JSM-6301F scanning electron
microscope instrument, equipped with a field-emission gun (FEG) and a NORAN VOYAGER
energy dispersive spectrometer (EDS). The presence of quartz in very fine grained rocks was
confirmed by element mapping using EDS.
3 RESULTS
For the atlas, a total of 56 samples of sedimentary rocks were characterised, listed in Table 1 in
decreasing order of abundance, revealing the predominance of sandstone.
Regarding the forms of silica identified as potentially reactive, microcrystalline quartz
(<100 µm) occurs in sandstone, greywacke and chert; cryptocrystalline quartz (<10 µm) is the main
constituent of chert and of the silica fraction in mudstone, but may also be abundant in the matrix of
sandstones and greywackes as well as finely dispersed in some siliceous limestones. Other forms of
silica, such as opal and chalcedony (and possibly moganite), are most common in chert/flint, and may
also occur in the cement of some sandstones consolidating the detrital/clastic grains into rock.
According to traditional nomenclature, sedimentary rocks can be divided into three main
groups: detritic or clastic sediments (sandstone, greywacke and mudstone from above list), of chemical
origin by precipitation from solution (chert and siliceous limestone), or of organic origin containing
plant or animal remains and/or secretions (chert and siliceous limestones). Compaction and
consolidation occur in the upper crust, and by definition exclude formation of metamorphic minerals.
However, in this study, the rocks which were submitted to very low metamorphic conditions and
where the sedimentary features were dominant, were kept in the sedimentary group.
Sandstone, as applied here, refers to arenite with less than 15vol% matrix [3]][[4]. The samples
analysed are mainly composed of rounded clasts of individual quartz grains, and minor feldspar.
Frequently, clasts also comprise quartzite, chert, and a variety of other lithic fragments, containing
deformed quartz with sutured boundaries and cryptocrystalline quartz, respectively. Structural/textural
features attributed to compaction/diagenesis are commonly observed, most notably pressure
dissolution and/or stylolite formation perpendicular to the main (geological) stress direction, and
formation of microcrystalline quartz along grain boundaries in stress-shadows. The texture showing
stretched grains of quartz is common to a number of well sorted sandstones with scarce or absent

rock cement. Alkali-reactivity of sandstones appear most frequently associated with the rock cement
(in rare cases containing opal and chalcedony), or scarce matrix filling interstitial space. Detailed study
of thin-sections of ASR damaged field concrete reveal that ASR cracks with reaction product tend to
concentrate along an inter-granular path, separating clasts rather than fracturing, though off-shoots
may enter large grains >100 µm (Figure 1). Matrix present in the interstitial spaces is normally
composed of clay minerals, fine-grained sericite and possibly cryptocrystalline quartz, usually both
discernible under optical microscope.
One peculiar example of sandstone illustrating the relevance of supplementary analytical
methods is the alkali-reactive glauconitic sandstone from northern Germany. Detailed assessment of
polished specimen using SEM reveals the presence of opaline cement and high rock porosity (in non-
fluorescent thin-sections). In addition to glauconite, ‘pompoms’ of very thin acicular silica crystals are
commonly found in interstices, which could be identified as radiolarian remains (Figure 2).
According to their definition, greywackes are immature sedimentary rocks with variable clast
composition (quartz, feldspar, lithic) and matrix content 15-75vol% [3][4]. The samples analysed for
the atlas exhibit a wide variability in granularity, from very fine grained to medium size grained. As
already mentioned for the sandstones, greywacke clasts may also comprise (deformed) metamorphic
rocks and chert. Some samples show signs of incipient prograde metamorphism, with noticeable
development of cleavage. The main alkali-reactive components are found in the greywacke rock
matrix, in most of the cases with abundant clay minerals and cryptocrystalline quartz (Figure 3).
Detailed study of field concrete samples confirms that ASR cracks tend to follow the matrix in
between coarser-grained clasts, sub-parallel to a developing foliation if present.
Chert/flint composed of cryptocrystalline quartz with chalcedony, opaline silica and fossil
remains [5], is widespread and locally abundant in regions with outcropping Cretaceous rocks, notably
NW Europe. Chert/flint particles commonly occur as minor constituent in limestone samples.
Reliable identification of opaline silica and moganite in such fine-grained rocks is difficult and
normally requires supplementary methods [6]. Texture, porosity and silica grain size is variable within
and among chert/flint particles (Figure 4), but is effectively visualized in thin-sections impregnated
with fluorescent dye.
Detrital quartz and other silicates, as well as neogenic clays in siliceous limestone, are micro- to
cryptocrystalline and well beyond optical resolution of a petrographic microscope. Example of a
siliceous limestone is shown in Figure 5. Element mapping by EDS reveals the presence of dolomite
in characteristic rhombs, and of silica and clay minerals finely interspersed throughout the carbonate
matrix. The combination of Si-Al-K maps suggest that some detrital feldspar may be present.
4 DISCUSSION
Applying the correct lithological name to a sample material of unknown or undisclosed
geological origin may pose a great challenge, even to an experienced petrographer. The atlas and this
paper apply the most recent nomenclature schemes and glossaries such as Hallsworth and Knox [7]
for sediments, in addition to local designations more common in the respective region of origin, as
applicable. Thus, “sandstone” was preferred over ‘arenite’ or ‘psammite’, due to its greater popularity
in alkali-silica reaction literature. The second source of ambiguity regarding application of correct
nomenclature relates to the in fact arbitrary distinction from metamorphic rocks. After all, the
transition of a well-compacted into a low-grade metamorphic quartzite is gradual, with a large overlap
between the two. In the present paper as for the atlas, the distinction is based on the predominant
characteristic. Thus, a sedimentary rock has a recognizable protolith and displays very subordinate
blastesis (=formation of metamorphic minerals from sedimentary/diagenetic precursors).
The reactivity potential of sedimentary rocks has been subject of extensive study (eg.
[8][9][10][11][12]). Some types of sedimentary rocks, which are used as coarse and fine aggregates for
concrete all over the world, seem consistently identified as potentially reactive. Examples such as the
crushed greywacke from the UK [13], the siliceous limestone from the Spratt quarry (Bobcaygeon
Formation) [14][15][16]) and from the Tournaisian Formation [11], and the chert and opaline
lithologies in aggregates from northwestern Europe ([17][18][19][20][21]) and the Appalachians
[22],[23] are well known. However, the reactivity of these rocks is variable depending on the mineral
composition, texture, and internal porosity.
The most common clastic sedimentary rocks used as aggregate for concrete are sandstone
and greywacke and, to a much lower extent, certain types of mudstone. These rocks are predominantly
composed of siliceous clasts consolidated by diagenetic compaction and cementation. Compaction
may induce pressure-solution, with dissolution of minerals and fluid transport generated by the
differential stress. The shapes and sizes of the detrital and diagenetic components define the fabric of

the rock. According to Renard et al [24], this type of deformation at a grain scale is commonly
attributed to the ‘water film diffusion’ mechanism, consisting of three successive steps: (i) dissolution
at the grain interface, (ii) diffusion of solutes along an adsorbed water film along grain interstices, and
(iii) precipitation on grain surfaces in open pore space. The slowest step controls the rate of the
overall progress. This process is observed in well sorted sandstones with clast indentation in the main
stress direction revealed by one quartz grain penetrating into another forming a curved grain contact,
and deposition of silica in the direction of the lower stress, commonly seen as epitaxial overgrowths
on detrital clasts with their original periphery decorated by impurities (e.g. [25][26]). Where pressure-
solution is more intense, grain contacts become serrate (as in Figure 1a), and ultimately stylolitic
(‘stylos’= Gr. pillar). Rock cement may be composed of neogenic silica, carbonate, limonite (ie. ‘iron
[hydr-]oxides) deposited in the interstices between detrital fragments, consolidating the clasts into
solid rock. Embedded in concrete as aggregate, relic interstitial space may still be open and connected,
facilitating access of concrete pore fluid to the rock particle’s interior. Matrix consists of detrital
particles <30 µm and is mainly composed of clay minerals with micro- to cryptocrystalline quartz.
Rocks subject to prograde metamorphism at low-grade conditions may contain chlorite, sometimes
intercalated with white mica.
Examples of alkali-reactive sandstones have been published (e.g. [18][19][27][28][29]),
identifying the neogenic siliceous cement as the cause of reactivity. A detailed study by Rivard et al
[28] on reaction rims in NE Canadian concrete made with porous Potsdam sandstone confirmed
alkali-reactivity and quartz dissolution along the periphery of the detrital fragments in the aggregate
particle interior. Advanced dissolution of the intergranular rock cement eventually leads to progressive
disintegration of the sandstone particle. Marfil et al [27] studied a sandstone mainly composed of well-
sorted, equidimensional, subrounded to irregular quartz particles cemented by chalcedony and
opal-CT. Post-mortem petrography on laboratory specimen after completed expansion testing showed
the rock’s siliceous cement was corroded, confirming the rock cement as the alkali-reactive
component. Sandstones, greywackes and argillites present in gravel in the Sudbury area of Canada are
also considered to be potentially reactive to alkalis (e.g. [30]). In the UK and Canada (Nova Scotia,
New Brunswick), greywacke was found to have given deleterious reactions in concrete structures
[13][31][32][33]. Some potentially reactive varieties of greywackes are also found in Germany, Sweden,
USA and Norway [34][35][36]. In South Africa, a number of structures were diagnosed with ASR in
South West Cape region due to the use of Malmesbury Group metasediments which contain
greywacke [8][9]. In their work on Dutch aggregates, Broekmans & Jansen 1998 [18], Broekmans [19]
and Nijland et al [29] found that alkali-reactive particles from damaged field structures are most
commonly porous chert with chalcedony and/or opaline silica, and ‘impure’ sandstones, in
approximately equal proportions.
Chert (francophone: ‘silex’) is a dense, very abrasion-resistant non-clastic siliceous sedimentary
rock containing more than 50vol% micro- to cryptocrystalline quartz with porosity <10vol% [7][37].
The term flint applies to the nodular variety of (bedded) chert [38], but as the distinction is obliterated
in particulate (aggregate) materials, chert applies as the common denominator [5]. Chert can be
composed of authigenic silica due to precipitation from hydrous solution, or alternatively may be of
secondary origin [6]. Primary chert forms from siliceous shells, sponge spiculae, radiolarians and
diatoms. Chert of secondary origin results from the replacement of carbonate rocks in nodules (flint),
or entire layers. Fibrous chalcedony typically fills voids, and clay minerals, pyrite, iron oxides and
organic matter may also be present. A number of papers have been written addressing the
performance of chert in concrete aggregate.
Arguably due to its very fine-grained nature and hard to distinguish ‘silica’ mineralogy, chert is
often characterized by supplementary methods, in particular powder X-ray diffraction – PXRD. To
facilitate comparison between different (sets of) samples, (partial) diffractograms are often recalculated
as Quartz Crystallinity Index – QCI values, following the method originally described by Murata and
Norman [39]. However, a recent critical assessment by Marinoni & Broekmans [40] revealed that this
method suffers from a number of fundamental shortcomings, including lacking descriptions of sample
and specimen preparation as well as of the reference material used for internal calibration (ie.
prerequisites any analytical standard). Most important, their proposed index is not attributed to any
specific crystal-structural feature, an explicit requirement elsewhere (see eg. [41]).
In general, a crystallinity index is a composite number representing contributions from
crystallite size and shape, domain size and shape, and microstrain (p222, [40]). Individual
contributions from these QCI-constituents will certainly vary per lithology, but variation within a
single lithology from one specific locality or stratigraphic unit might be less and could possibly allow
comparison within the sample set (eg. [42][43]). QCI values for chert of Dutch and Appalachian origin

(ie. the original samples of Kneller [22]) did not reveal any consistent correlation with expansion data
[19]. Furthermore, Murata & Norman [28] implicitly use non-monochromated bulk CuKα radiation
(of wavelength λ=1.54184Å), whose two components CuKα1 (λ=1.54056Å) and CuKα2 (λ=1.54439Å)
double (with fixed intensity ratio determined by quantum physics) the triplet centered at 67.74 °2θ into
a sextuplet, as applied in the original method. To eliminate this peak-doubling and achieve higher
accuracy, McNally et al [44] use monochromatic CuKα1 radiation. However, this also instantly
invalidates their QCI values (as CuKα2-peaks are essential in their determination cf. Murata &
Norman [28]), and consequently, their interpretations.
In summary, the highly incomplete description of the QCI-procedure, the omission of
attributing values to a specific crystal-structural feature, and the inconsistent relation to alkali-reactivity
potential makes the original method of Murata & Norman [39] unsuitable for routine ASR
assessment, or indeed ‘quartz crystallinity’ in general [40]. On the other hand, the reactivity of chert is
thought to be related with the porosity as explained by Barisone & Restivo [45] who concluded that
Italian cherts are less reactive than from NW Europe, namely from Great Britain, Netherlands,
Germany, and Denmark. The authors attribute the higher reactivity to a higher degree of crystal-
structural disorder, and/or to a higher porosity due to different exposure conditions during
weathering and transport. Their observation seems to coincide with the consistent presence of
moganite in Cretaceous cherts from NW Europe [26], a somewhat obscure yet officially
IMA-approved silica polymorph. Remarkably, sea-dredged porous chert that has resided on the
Northsea floor for ~10ka does not react in expansion testing, which is attributed to leaching of
soluble silica species [unpublished data, MATMB]. However, recent data on field concrete may
contradict this again. In summary, chert should always be regarded as potentially deleterious in
concrete [46] but should be studied in detail due to possible pessimum effect.
Regarding argillaceous dolomitic limestones, work has been developed to clarify the origin of
reactivity of Kingston limestone (Gull River Formation) and cryptocrystalline quartz present in the
carbonate mud has been identified as deleterious [47][48]. Previous work carried out in Canada on the
Neuville Formation (Quebec), the western part of Newfoundland and on the Bobcaygeon Formation
(Spratt quarry; Ontario) showed that micro- to cryptocrystalline quartz present in the carbonate mud
was the cause of reactivity [49][50][51][52][49]. These formations are similar to the one reported in
[11]. The identification of the reactive silica in such cases usually requires extraction through selective
dissolution in excess acid and XRD analysis of the insoluble residue, complemented by SEM/EDS to
study the texture and spatial distribution. In a detailed assessment of Spratt’s aggregate, Rogers &
McDonald [15] note that this Middle Ordovician rock is medium grey colored, fine to coarse
crystalline limestone with about 9wt% SiO2, also comprising chalcedony. However, little silica is
observed in thin-section because it is very fine grained and inconspicuous in the highly birefringent
calcite matrix. The micritic limestones containing phyllosilicates, minor chert, and cryptocrystalline
quartz from the Tournaisian Formation produced deleterious reactions in a number of concrete
structures. Guédon-Dubied et al [11] identified two sub-units in the limestone deposit. The lower unit
consists of micritic limestone, clayey in the basis, impregnated with diagenetic silica and with abundant
fossil fragments (bioclasts). The upper unit is composed of clayey micrites also impregnated with
diagenetic silica and containing pseudomorphs of gypsum, anhydrite and black siliceous nodules,
composed of chalcedony and opal. Both the nodular and diagenetic silica (microscopic) may be the
cause of the rock reactivity, as confirmed by expansion laboratory tests.
5 CONCLUSIONS
This paper presents a summary of the main reactive components observed in detrital/clastic
and chemical sedimentary rocks. A number of samples from different locations around the world were
studied to obtain information on the most common sedimentary rock types, and their potentially
reactive components.
 Optical thin-section petrography conforming to RILEM AAR 1.1 is considered the prime
assessment method for aggregate materials, being most cost- and time-effective.
 Supplementary methods are essential for the reliable identification of cryptocrystalline silica,
especially when dispersed throughout the lithology.
 Identification of reactive mineral constituents is complex, and operator skill and experience
remain quintessential for reliable assessment by thin-section petrography.
 Micro- to cryptocrystalline quartz are the most common reactive component is sedimentary
rocks.
 In coarser-grained rocks such as sandstones and greywackes, reactive mineral constituents
usually occur in the rock cement, or throughout the finer-grained matrix.

 Aggregate materials are to be classified according to local criteria, based on local experience
with ASR-damage in field structures and local geology.
 Rocks predominantly composed of cryptocrystalline quartz like eg. chert/flint are to be
considered potentially reactive, but performance should be confirmed by expansion testing as
eg. porosity may affect alkali-reactivity.
 Chert/flint may show pessimum effect which can be studied by performing expansion
laboratory tests.
ACKNOWLEDGEMENTS
The authors are grateful to all colleagues contributing to the atlas in AAR-1.2 by sending
samples and thin-sections, as well as to the constructive reviews. The SEM/EDS results were acquired
at CEMUP, financed through grants REEQ/1062/CTM/2005 and REDE/1512/RME/2005 da FCT
- Fundação para a Ciência e Tecnologia.
This work was developed in the scope of the activities of ICT – Earth Sciences Institute,
financed by FCT, under the scope of the activities of the G3G working group.
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TABLE 1: List of the sedimentary samples studied.
lithology number of samples
p
ercent
sandstone 26 45
chert/flint 12 21
greywacke 10 18
siliceous limestone 7 14
mudstone 1 2
SUM 56 100

F
IGURE
1: Example of sandstones: a) grain-supported sandstone with pressure-solution features such as
elongation, sutured boundaries and deposition of quartz in the direction of lower stress (PPL and XPL); b) crack
filled with alkali-silica gel; the crack develops mainly along the interstitial spaces between the clasts; some thinner
cracks develop inside the clasts, parallel to the main crack (both in XPL).
F
IGURE
2: Opaline sandstone composed of glauconite (greenish) and sponge spicules. Photomicrographs in PPL
(left). The rock is highly porous as can be observed in the SEM image (centre). The rosette-like crystals of pure
silica may correspond to radiolarian or to an inorganic structure formed by silica (Si and oxygen are the main
components). Image by SEM-BSE (BSE - back-scatter electrons) and spectrum obtained by EDS (right).
a
b
Si
Al
O
C
20 m

F
IGURE
3: Examples of ASR in greywacke. The clasts are dominantly composed of monocrystalline quartz and
feldspar. The abundant matrix is composed of cryptocrystalline quartz, sericite and chlorite (and possibly some
clay minerals). The images show the preferential path of the cracks along the porous interstitial space containing
matrix (left, in PPL; right, in XPL).
F
IGURE
4: Chert showing variable porosity (light yellow colour of the fluorescence dye) and limonite staining in
dendritic distribution (PPL) and fossil remains replaced by feather-like chalcedony (XPL).
F
IGURE
5: SEM-EDS element maps revealing porosity and presence of two different carbonate minerals, namely
calcite and dolomite (Ca and Mg) (b), and presence of very fine-grained silica (Si) as well as clay minerals (Si-Al)
and possibly some detrital feldspar (Si-Al-K). Image on the top/left, in BSE, with scale bar.
100

m C M
g
Si Al
K