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The Alkali-Activated Materials (AAM) are defined as materials obtained through the reaction between precursors and activators, and are separated into two classes depending on the products formed in the reaction, those rich in calcium, as the blast furnace slag, whose Ca/(Si+Al) ratio is higher than 1; and poor in calcium, which is the geopolymers subclass. In this review article, some bibliographical aspects were discussed regarding the discovery of these materials, through research conducted by Victor Glukhovsky and through the characterization of historical monuments by Davidovits, which began in the 50s and 60s and persist to the present day. The main products obtained in the alkaline activation reaction were also addressed, using the definition of polysialates and zeolites, in the case of geopolymers, and the tobermorite structure, in the case of materials rich in calcium. The main steps of the alkali-activated reaction, such as dissolution, condensation, polycondensation, crystallization, and hardening, were discussed. Some techniques for characterizing the AA reaction products were also examined, such as X-ray diffraction (XRD), nuclear magnetic resonance spectrometry (NMR), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Finally, the main factors that interfere in the kinetics of AA reactions were explored, in which the type of cure and the activating solution used in the alkali-activated materials production stands out.
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Rev. IBRACON Estrut. Mater., vol. 14, no. 3, e14309, 2021
REVIEW
Corresponding author: Markssuel Teixeira Marvila. E-mail: markssuel@hotmail.com
Financial support: CNPq, proc. nº 301634/2018.1; FAPERJ, proc. nº E-26/202.773/2017.
Conflict of interest: Nothing to declare.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution,
and reproduction in any medium, provided the original work is properly cited.
Rev. IBRACON Estrut. Mater., vol. 14, no. 3, e14309, 2021| https://doi.org/10.1590/S1983-41952021000300009 1/26
Received 17 April 2020
Accepted 28 October 2020
Reaction mechanisms of alkali-activated materials
Mecanismos de reação de materiais álcali ativados
Markssuel Teixeira Marvilaa
Afonso Rangel Garcez de Azevedoa
Carlos Maurício Fontes Vieiraa
aUniversidade Estadual do Norte Fluminense – UENF, Laboratório de Materiais Avançados – LAMAV, Campos dos Goytacazes, RJ, Brasil
Abstract: The Alkali-Activated Materials (AAM) are defined as materials obtained through the reaction
between precursors and activators, and are separated into two classes depending on the products formed in
the reaction, those rich in calcium, as the blast furnace slag, whose Ca/(Si+Al) ratio is higher than 1; and poor
in calcium, which is the geopolymers subclass. In this review article, some bibliographical aspects were
discussed regarding the discovery of these materials, through research conducted by Victor Glukhovsky and
through the characterization of historical monuments by Davidovits, which began in the 50s and 60s and
persist to the present day. The main products obtained in the alkaline activation reaction were also addressed,
using the definition of polysialates and zeolites, in the case of geopolymers, and the tobermorite structure, in
the case of materials rich in calcium. The main steps of the alkali-activated reaction, such as dissolution,
condensation, polycondensation, crystallization, and hardening, were discussed. Some techniques for
characterizing the AA reaction products were also examined, such as X-ray diffraction (XRD), nuclear
magnetic resonance spectrometry (NMR), Fourier transform infrared spectroscopy (FTIR), and scanning
electron microscopy (SEM). Finally, the main factors that interfere in the kinetics of AA reactions were
explored, in which the type of cure and the activating solution used in the alkali-activated materials production
stands out.
Keywords: alkali-activated materials, geopolymers, precursors.
Resumo: Os materiais álcali ativados (MAA) são definidos como materiais obtidos por intermédio da reação
entre precursores e ativadores, e são separados em duas classes dependendo dos produtos formados na reação,
os ricos em cálcio, como é o caso da escória de alto forno, cuja relação Ca/(Si+Al) é maior do que 1; e pobres
em cálcio, que constitui a subclasse dos geopolímeros. Nesse artigo de revisão foram discutidos alguns
aspectos bibliográficos a respeito da descoberta desses materiais, por meio de pesquisas realizadas por Victor
Glukhovsky e por meio da caracterização de monumentos históricos por Davidovits, que se iniciaram nas
décadas de 50 e 60 e persistem até os dias atuais. Também foram abordados os principais produtos obtidos na
reação de ativação alcalina, utilizando a definição de polissialatos e zeólitas, no caso dos geopolímeros, e da
estrutura da tobermorita, no caso dos materiais ricos em cálcio. As principais etapas da reação álcali ativadas,
como dissolução, condensação, policondensação, cristalização e endurecimento, foram debatidas. Algumas
técnicas para caracterização dos produtos da reação AA também foram examinadas, tais como difração de
raios-X (DRX), espectrometria de ressonância magnética nuclear (RMN), espectroscopia no infravermelho
com transformada de Fourier (FTIR) e microscopia eletrônica de varredura (MEV). Por fim, foram explorados
os principais fatores que interferem na cinética das reações AA, onde destaca-se o tipo de cura e de solução
ativadora utilizada na produção dos MAA.
Palavras-chave: materiais álcali ativados, geopolímeros, precursores.
How to cite: M. T. Marvila, A. R. G. Azevedo, and C. M. F. Vieira, “Reaction mechanisms of alkali-activated materials,” Rev. IBRACON Estrut.
Mater., vol. 14, no. 3, e14309, 2021, https://doi.org/10.1590/S1983-41952021000300009
M. T. Marvila, A. R. G. Azevedo, and C. M. F. Vieira
Rev. IBRACON Estrut. Mater., vol. 14, no. 3, e14309, 2021 2/26
1 INTRODUCTION
1.1 Alkali -activated materials definition
Alkali-activated materials (AAM) are obtained from two basic components, the activator, and the precursor. The
material used as a precursor is usually in powdered and mineralogically amorphous form. It can be composed of
aluminum silicates, and they are named geopolymers, or present in its composition a predominance of calcium oxide [1],
as occurs with blast furnace slag (BFS). The activating materials, in turn, are composed of alkali metals, in the form of
hydroxide or silicates, dissolved in an aqueous solution. They are responsible for causing the hardening reactions due
to high alkalinity, manifested by the high pH, above 14 [2]
1.2 Historical aspects
Historically alkali-activated materials were developed by Victor Glukhovsky in the 1950s and 1960s in the Soviet
Union. The researcher developed systems activated in an alkaline manner by mixing materials from volcanic processes
(rocks and ash) with activating solutions based on sodium hydroxide. In response, he obtained materials with a
composition similar to Portland cement after hardening, that is, hydrated calcium silicate phases (C-S-H) [3]. These
materials were named as soil silicates.
Around 1979, Davidovits, a French researcher and chemist, developed an Alkali-activated material using natural
origin materials rich in silicon and aluminum, such as kaolin clay, activated by the solution of alkaline liquids [4]. The
researcher patented his discovery by naming the materials obtained as geopolymers since they are obtained by a
polymerization reaction similar to the one that gives rise to polymeric materials. However, unlike polymers,
geopolymers have an inorganic composition. Because of this, another appropriate definition for these materials is that
they are called inorganic polymers [5].
Although only the contributions of Victor Glukhovsky and Davidovits were highlighted, other researchers
contributed considerably to the knowledge of AAM. Table 1 presents the main contributions of several researchers to
the development of alkali activation reaction science. It is observed that initially, the researchers were concerned with
characterizing building materials extracted from ancient monuments, such as the Roman aqueducts studied by
Malinowski in 1979 and the Egyptian pyramids studied by Davidovits, and Sawyer in 1985 [6]. The authors proved
that the materials used in these important monuments were obtained through alkaline activation processes. This fact
motivated more researches in this area, and other alkaline activation studies carried out by Roy et al. (1991), Palomo
and Glasser (1992) and Krivenko (1994), who, despite obtaining important results, still could not fully explain the
mechanisms of alkali-activated reactions [6].
In 1994, Davidovits [7] presented a complex analysis on the microstructure of geopolymers through polysialate
networks, and in 1995 Wang and Scrivener presented microstructural analyzes of alkali-activated materials rich in
calcium [6]. These surveys improved the researchers' understanding of alkali-activated materials. In 2007, Provis
and Van Deventer [8] presented a kinetic model of geopolymerization reactions using X-ray diffraction techniques
that were able to explain the stages of this reaction by correlating them with the morphological structure of the gels
formed.
In 2011, Habert et al. [9] carried out the environmental evaluation of the geopolymer concrete production by
analyzing the life cycle, comparing it with the production cycle of Portland cement. The authors proved the
environmental importance of the study of the alkali-activated material, which was shown as an ecological alternative
to conventional Portland cement concrete.
After the consolidation of the main concepts related to the alkali-activated materials science, the material
started to be used in larger constructions. As an example, we can mention the Australian constructions of the
Victoria bridge in 2013 and the expansion of Brisbane West Wellcamp airport in 2014 with E-Creta, an alkali-
activated concrete [10].
Recently, Davidovits developed research characterizing the material present in ancient monuments of pre-
Columbian civilizations in South America, concluding that they were alkali-activated materials [11]. The studies
carried out between 2016 and 2019 in the Tiahuanaco monuments (Tiwanaku/Pumapunku), Bolivia, and other
monuments in Peru complement the studies carried out in the 70s and 80s and prove the durability of alkali-activated
materials [12].
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Table 1. Bibliographic history of some important discoveries about activated alkali materials. Source [6]–[12]:
Author
Year
Research
Feret
1939
Use of slag in cement
Purdon
1940
Combination of slag and alkaline solutions
Glukhovsky
1959
Theoretical basis and development of alkaline solutions in cement
Glukhovsky
1965
First material called alkaline cement
Davidovits
1979
Use of the term geopolymer
Malinowski
1979
Characterization of ancient Roman aqueducts
Forss
1983
Cement with alkaline slag and superplasticizers
Langton e Roy
1984
Characterization of old building materials
Davidovits e Sawyer 1985
Patent of a cement with chemical composition similar to the material
that it believes to have been used in Egyptian pyramids
Krivenko
1986
Definition of M2O-MO-SiO2-H2O
Malolepsy e Petri
1986
Activation of synthetic slags
Malek. et al.
1986
Application of radioactive waste in activated slag cements
Davidovits
1987
Comparison of old and modern cements
Deja e Malolepsy
1989
Proof of chloride resistance of AA materials
Kaushal et al.
1989
Incorporation of nuclear waste in AA materials
Roy e Langton
1989
Concrete production similar to the old ones
Majundar et al.
1989
C12A7 - slag activation
Talling e Brandstetr
1989
Alkaline slag activation
Wu et al.
1990
Alkaline activation of slag-based cement
Roy et al.
1991
Production of quick-setting alkaline cements
Roy e Silsbee
1992
Publication of the article “Alkaline activated cements: an overview”
Palomo e Glasser
1992
Activation of metakaolin
Krivenko
1994
Properties of alkaline cements
Davidovits
1994
Definition of the structure of geopolymers through polysialates
Wang e Scrivener
1995
Discovery of the microstructure of AA materials
Palomo 1999
Publication of the article “Alkaline activation of fly ash: the cement of
the future”.
Krivenko, Roy e Shi
2006
Publication of the first book on alkaline activation
Provis e Deventer
2007
Definition of the kinetic modeling of the alkaline activation reaction
Habert et. al 2011
Environmental assessment of geopolymeric concrete production by
life cycle
Australia
2013
Construction of the bridge in Victoria with E-Crete.
Australia
2014
Brisbane West Wellcamp airport expansion with E-Crete.
Davidovits 2016
Studies of deposits and monuments used by pre-Columbian
civilizations in Bolivia and Peru
Davidovits 2019
Characterization of pre-Columbian monuments in Pumapunku-
Tiwanaku, Bolivia
1.3 Advantages and disadvantages
As noted by the main invocations highlighted in the text and Table 1, research involving alkali-activated materials
has gained considerable prominence in the construction materials scenario. Provis [2] highlights that this interest in the
development of alkali-activated binders is motivated by economic advantages since the process allows the use of
industrial by-products, such as slag and ash, to produce a material with considerable added value, which rivals with
Portland cement. Another issue pointed out by Provis to explain why the increase in research is linked to sustainable
development. It is known that the Portland cement production is very exploratory, besides emitting several tons of CO2
into the environment. This fact is not repeated in the cycle for the production of alkali-activated materials due to the
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possibility of using by-products from other processes, which would be generated regardless of their application as a
precursor to AAM.
In addition to those mentioned, different motivations for the development of more research on alkali-activated
binders are related to the properties that these materials have. High chemical resistance to acids and high-temperature
resistance is mentioned, due to the presence of alkaline aluminosilicate gels, with a highly reticulated nature, in addition
to the low presence of water in their structure, when compared to hydrated Portland cement, which presents hydrated
calcium silicate gels [13], [14]. AAM can also have good durability and high initial resistance, mainly in slag-based
compounds [6]. They also have low shrinkage, low thermal conductivity, strong adhesion to metallic and non-metallic
substrates [15], low permeability to ions and chlorides, low cost, a possibility to recover industrial waste, low CO2
emissions, effective passivation of reinforcing steel, making it possible to be armed, if necessary [14].
Although the researchers do not deal much with the subject, the main disadvantages to be overcome for the
application of AAM are related to the activating solution, due to two aspects: the difficulty of handling this material,
which because it has a very high pH and it is very alkaline it can cause burns or respiratory problems when inhaling the
hydroxides dust used in the solution; and the cost and availability of the silicates and hydroxides used in the solution.
There are currently few studies on the use of waste as activators in AAM. The vast majority of researchers focus their
studies on the development of new precursors. One of the major disadvantages of alkali-activated materials is linked to
activators, which still use commercial materials, causing the exploitation of natural resources and making the price of
the materials go up. It is noteworthy that the use of alternative activators tends to reduce these disadvantages, although
in some cases there is a need for additional treatments, besides those carried out in commercial products, to make them
suitable for use. This fact can cause an increase in the price of alternative activators, as highlighted by Azevedo et al. [16],
who studied the application of glass waste in place of sodium silicate. The authors concluded that it is necessary to
grind the residue for a long time, consuming a considerable amount of energy in the process.
1.4 Alkali-activated materials classification
To organize the structure of this study, the explanation of the alkaline activation mechanisms, the principal objective
of this article, will be divided into two groups, as suggested by some authors [17], [18]:
a) Materials with precursors rich in aluminosilicates and without calcium oxide in their composition, such as
metakaolin and ashes. These materials were the focus of Davidovits' study, and when they suffer the reaction, it
becomes a mineralogical structure very similar to zeolites, as will be discussed below. They are known as
geopolymers, geocements or polysialates [6].
b) Materials with precursors rich in calcium oxide, which may have aluminum oxide or not. They are precursors that
have a Ca/(Si+Al) ratio higher than 1 [1]. They were the focus of study by Victor Glukhovsky, and when they
harden, they present a composition similar to ordinary Portland cement, that is, they form C-S-H gels [19], [20].
2 ALKALI ACTIVATION REACTION OF LOW-CALCIUM PRECURSORS
2.1 Main low-calcium precursors
The main precursors used in alkaline activation for the geopolymers production, that is, the low-calcium precursors
content are metakaolin and fly ash. Regarding metakaolin, it is worth noting that this compound has several studies
carried out and it is considered by researchers as the principal material used as a precursor in AA reactions [21], [22].
The significant use of metakaolin can be attributed to its reactivity, which is quite high [23], and the way of obtaining
the material, which is very simple.
This material is generated by calcining kaolin at temperatures ranging from 500 to 800ºC, depending on the degree
of crystallization and the purity of the material [24]. It is known that kaolinite, which is the main compound present in
kaolin, undergoes a dehydroxylation reaction at around 550ºC, transforming into metakaolinite. Figure 1 shows a
scheme for the production of metakaolin from traditional kaolin [1], [20], [25]. It is observed that the burning at
temperatures below 400ºC is not suitable for the precursor production, as well as the burning of temperatures above
950ºC, due to the mullite production that cannot be activated alkali due to its high crystallinity [15], [19].
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Figure 1. Calcination of kaolin for percussion production. Source: White et al. [20].
On environmental issues, it is interesting to note that there are some advantages in using metakaolin because its
synthesis emits 5 to 6 times less CO2 than the Portland cement production process [26]. Besides, kaolin can be extracted
not only from mineral sources but depending on the composition, it can be obtained through mine tailings or the paper
industry [1]. The different sources will influence the reactivity of the obtained metakaolin, depending on, for example,
the chemical and mineralogical composition of the clay used as raw material, but they are more ecologically viable
solutions.
As for the disadvantages of the metakaolin use, we can mention a higher tendency of drying retraction presented by
these materials when compared to fly ash [27]. This characteristic can be related to the chemical composition of these
two precursors, which it is observed that metakaolin has a higher amount of aluminum oxide (Al2O3) than fly ash.
Overall, some research shows that metakaolin has about 55 to 62% SiO2 and between 35 to 42% Al2O3, while fly ash
has between 50 to 55% SiO2 and approximately 20 to 25% Al2O3 [24], [28]–[30]. The retraction is higher because the
aluminum oxide dissolves faster than the silicon oxide, related to the two principal network makers in geopolymers [24].
That also explains the reactivity of metakaolin being higher than fly ash.
Another disadvantage is the price of metakaolin, which, because it is a commercial material, is usually higher
than when using blast furnace slag, fly ash, or other industrial waste [15]. However, it is noteworthy that the price
of metakaolin is extremely variable, depending on the location and specifications of the product, for example.
Thus, even though there is a tendency for the price of metakaolin to be higher than in the other components used
as precursors, commonly residues, this statement cannot be generalized. However, due to the high purity and
reactivity, the products obtained by the alkaline reaction of metakaolin, in general, present a more well-defined
gel microstructure [1].
On fly ash, it is worth noting that this material is a powdered by-product produced in thermoelectric plants during
the coal burning, containing mainly aluminosilicate. They present fine spherical particles with the main chemical
components of aluminum, silicon, calcium, iron, magnesium, and carbon wastes [31]. The particle size in the fly ash
ranges from <1 μm to more than 100 μm, and this by-product has an annual worldwide production of more than 900
million tons, according to the updated data from 2019 [15]. Therefore, the storage and fly ash disposal as industrial
waste has become a serious environmental problem and a worldwide technical challenge. In Brazil, however, the energy
matrix uses predominantly hydroelectric plants, which do not generate fly ash. For this reason, the study of this type of
waste in Brazilian research is less practical, different from what occurs in other countries, such as China and the USA,
which present easy availability of the waste [32].
Given its low price, good spherical structure, and a large amount of highly active amorphous aluminates and silicates,
fly ash class F is the most recommended raw material for geopolymers synthesis [33]. The activity and solubility of Al
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and Si can transform precursors into important geopolymeric products in an alkaline activator solution [34], [35].
According to the American Society of Test Materials classification [34], fly ash is defined as Class C (CaO> 20%) and
Class F fly ash (CaO <20%), respectively. Class C fly ash can be hydrated to form hydrated calcium silicate (CSH),
has been widely used to partially replace cement in concrete, or as a calcium-rich precursor [31], [35], which will be
discussed next in the explanation about AA of BFS. Class F fly ash is more suitable for geopolymers synthesis [36] because
its amorphous content is higher than that one of class C fly ash [37].
The geopolymer products obtained with the use of Class F fly ash have excellent mechanical properties, as reported
by the literature [35], [38]–[40]. The authors, however, report difficulty in the geopolymers synthesis with fly ash due
to the curing of the material that must be done at high temperatures to increase the reactivity of the material, which is
very low at ambient temperatures. This characteristic can be associated with the fly ash chemical composition, which,
due to its higher levels of Al2O3,, as reported [24], [28]–[30], and, in some specific cases, may have a high CaO content,
around 12% [41], [42], justifying the need for thermal curing.
Besides metakaolin and fly ash, other types of precursors that have low amounts of calcium have reaction
mechanisms typical of geopolymers. For example, other types of burnt clay can be cited as illite-smectites [43] or
feldspars [44], [45]. There are also other precursors such as volcanic ash, natural pozzolans, and metallurgical slags
with low amounts of calcium [46]–[49]. Recent research has proposed the application of industrial wastes, such as
chamotte [50], from ceramics industries, and magnesium phosphate, from the chemical industry through ammonium
production [51]. In both cases, there was a potential for the application of waste, but not in isolation. In the case of
chamotte, the best results were obtained using 50% metakaolin and 50% waste [50], while in the case of magnesium
phosphate, the most satisfactory results were obtained using 30% metakaolin and 70% waste, in which the authors
obtained compressive strength of 30 MPa with only 7 hours of curing [51].
2.2 Stages of the alkaline activation reaction of geopolymers
According to Duxson et al. [52], in geopolymers, the alkaline activation reaction is divided, in general, into the
following steps: dissolution, condensation, polycondensation, and crystallization of the gels. These steps constitute
the Davidovits model, outlined by Duxson, and are briefly explained below, according to the scheme represented by
Figure 2.
The first reaction process is the dissolution of the aluminosilicate materials and the release of the reactive
monomers silicate and aluminate, represented respectively by [Si(OH)4]- and [Al(OH)4]-. Dissolution occurs by
dropping the covalent bonds Si-O-Si and Al-O-Al that characterize aluminosilicates and is only possible in the
presence of a strongly alkaline medium with a pH higher than 14, provided by the activating solution. More
simply, it can be said that the alkaline solution breaks the bonds that hold aluminosilicates together, creating a
colloidal phase [15], [52], [53].
The colloidal phase initiates a water elimination process, due to a nucleophilic substitution reaction, where
the specimens [Si(OH)4]- and [Al(OH)4]-, which present an electric charge -1, are connected to the others due
to the attraction among the OH groups of the silicate with the Al ions of the aluminates. The colloidal phases
initiate the chemical equilibrium process, which is known as condensation, giving rise to intermediate
compounds. In this stage, the formation of an unstable aluminosilicate species occurs, releasing water
molecules in the process [14], [53].
This procedure continues, with more water release and the first gels formation. The search for balance continues,
however, load balancing is not possible, because both aluminates and silicates have negative charges. Because of this,
the presence of alkali metal ions, such as Na+ ou K+, in the alkaline solution is so important. The positive charge of
these ions provides a balance in the charges of the unstable gels that had been formed, causing a reorganization in the
structure of the intermediate compounds, which initiate the formation of a more resistant final compound.
The polycondensation of the gels then occurs, which may or may not undergo crystallization and give rise to the
stable gels present in the geopolymers final structure. Amorphous gels are called by some authors N-A-S-H (hydrated
sodium aluminosilicate), while the crystalline or semi-crystalline phases are just called zeolites [54]. The material
starts the hardening process, acquiring mechanical resistance and the other known properties of these activated alkali
materials [15], [53]. Figure 3 shows the final structure of the 3D network formed in the composition of the
geopolymers.
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Figure 2. Duxson model of alkaline activation reaction of geopolymers (low in calcium). Source: Duxson et al. [53].
Figure 3. Illustration of the geopolymeric network in formation, amorphous N-A-S-H gels. Source: Lahoti et al. [14].
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2.3 Compounds formed in the alkaline activation reaction of geopolymers
As mentioned above, some authors point out that the geopolymers formation follows a logic very similar to the
zeolites formation, and this characteristic is addressed in the sequence of this text [14], [15], [52], [53]. What is worth
noting is that the geopolymerization reactions give rise to polysialate compounds, which can have different
configurations, depending on how the alkaline activation reaction is processed [7], [55].
The polysialate compounds correspond to three-dimensional networks of SiO4 and AlO4 tetrahedrons sharing the
oxygen atoms, as shown in Figure 3. To maintain balance, the existence of positive Na+, K+ e Ca2+ ions is necessary,
which must be present in the structure cavities to balance the negative charges of geopolymeric gels. Polysialates can
also be defined as chain and ring polymers with Si4+ and Al3+ in coordination 4 times with oxygen, and their empirical
formula is given by the equation below [3].
( )
.
n2 2
zn
M Si O Al O wH O−− −


(1)
Where z=1, 2 or 3, represents the atomic relationship between Si/Al; M= monovalent cation as K+ ou Na+;
n= geopolymerization degree.
The types of polysialates distinguished by Davidovits are illustrated in Figure 4, being differentiated according to
the atomic relationship between Si/Al [6] or between the molar ratio of SiO2/Al2O3 [5], depending on the studied author.
It is worth noting that the atomic relationship between Si/Al is half the molar ratio between SiO2/Al2O3. For example,
when the atomic ratio is 1, the molar ratio is 2, and so on.
Figure 4. Geopolymer networks of Polysialates. Source: Majidi [5].
Taking the molar ratio as a base, we have the following classification: common polysialates (PS), where the
SiO2/Al2O3 ratio is 2, forming a type structure (Si - O - Al - O); polysialates-siloxo (PSS), with a SiO2/Al2O3ratio of
approximately 4 and (Si - O - Al - O - Si - O) structure; and polysialates-disiloxo (PSDS), with a molar ratio of
SiO2/Al2O3 around 6 and chains of the type (Si - O - Al - O - Si - O - Si - O). This information can be seen in Figure 4,
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where the networks of PS and PSS formed with the alkali metals sodium and potassium are visualized, such as the
networks of kalsilite, leucilite, analcime, sodalite. The formation of these networks is directly related to the dosage of
geopolymers in their production phase, either through molar relationships, solution molarity, or in the relationship
between the activator and the precursor.
It is worth noting that, besides the model proposed by Davidovits for the geopolymerization reaction, there is another
model that is very widespread and accepted in the study of this material, which is the Provis model [56]. In this model,
detailed in Figure 5, it is defined that the precursor reaction rich in aluminosilicates and poor in calcium oxide, as in
the case of metakaolin and fly ash, occurs initially with the formation of silicate and aluminate monomers, which
grouped forming combined aluminosilicate oligomers. Thereafter, the reaction “separates” into two stages, in which
part of the oligomers polymerizes in an amorphous and random manner, giving rise to geopolymeric gels. The other
part undergoes the process of crystalline nucleation, forming nanocrystalline aluminosilicates nuclei, which
subsequently originates crystalline zeolite phases. According to the Provis model, two distinct and simultaneous
geopolymer formation reactions occur to a higher or lesser degree depending on different factors (such as temperature,
cure time, solution’s molarity, and molar relationships of the precursors), originating an amorphous phase and another
crystalline [8], [57].
Figure 5. Provis model for the formation of geopolymers. Source: Wu [15].
The mechanism of polysialates formation, and also of the zeolite phases, is much more complex than was mentioned
above. It is known, for example, that in the alkaline activation reaction of geopolymers, the polysialates formation
formed mainly by iron oxide can occur, named as ferrosialates [58]. Iron can be present in three different forms in the
ferrosalate chain: in Fe+3 species trapped in the main chain, in Fe+3 species acting for load balancing in interstitial sites
or in the form of Fe3O4 oxide linked to the chain through bonds secondary [59]. In general, iron forms a [Fe(OH)]2+
specimen that replaces [Al(OH)4]- specimens. In some cases it can also partially replace specimens [Si(OH)4]- in the
geopolymer chain [58].
These compounds promote tensile strength in the geopolymer, which in general is low, but make the material
conducive to the occurrence of corrosion, which in the case of use in conjunction with reinforced elements, may make
the application of the material unfeasible [60]. Another property presented by ferrosialates is the of the geopolymer
density reduction, due to its lamellar, tubular and spongy structure. This characteristic, however, increases the porosity
of geopolymers with a high content of ferrosialates [59].
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Although the formation of ferrosialates is benefited in precursors that have a high amount of iron oxide and whose
cure is carried out at temperatures above 80ºC, practically all geopolymers have low amounts of ferrosialates in their
composition [61]. This fact illustrates and proves the complexity of the polysialate networks formation, which although
simplified by several authors, presents mechanisms that are still little explored.
Figure 5 presents the information for Qn, where n can take values from 1 to 4. This notation represents the silicon
tetrahedral atom coordination, connected via oxygen to other silicon tetrahedral atoms [25]. It is possible to notice that
as the geopolymerization reaction becomes more intense, larger amounts of chains are formed, the silicon tetrahedrons
coordination increases, which bind and form gels. This analysis is performed by specific techniques such as nuclear
magnetic resonance spectrometry, where it is possible to obtain the Qn which means the silicon atoms quantity
covalently linked.
It is noticed that the zeolites formation is a common characteristic and highlighted by different authors. That is why
it will be explained what zeolites are and what their similarities are with geopolymers. Zeolites are hydrated, crystalline,
or nanocrystalline aluminosilicates with a specific structure, composed of silica and alumina tetrahedra connected by
shared oxygen atoms and containing well-defined channels and chambers, filled with ions and water molecules [54].
This structure makes its physical and chemical properties unique, which results in a wide range of practical applications [25].
The zeolites are based on the tetrahedron TO4, where T is an aluminum or silicon atom, which can be considered as a
basic building block. The connections between several TO4 tetrahedrons are called secondary units of construction
(USF) [40].
The main difference between conventional zeolites and those formed in geopolymerization is that, in
conventional production the molar ratios are much higher. For example, while in conventional production the molar
ratio between H2O/SiO2 is between 10 and 100 and the molar ratio between OH-/SiO2 is between 2 and 20, in the
case of geopolymers these relationships change to between 2 and 10 and between 0.1 and 0.5, respectively [62]. As
an impact on the standard zeolites formation, a high crystallinity content is achieved, while in the formation of
geopolymers zeolites are not formed separately, but they are mixed with amorphous gels of the N-A-S-H type,
leading to more amorphous compounds [40]. In the case of geopolymers, a kind of composite material is formed and
it has two distinct phases: a crystalline one formed by zeolites and an amorphous one, formed mainly by N-A-S-H
gels.
Even if the geopolymerization reaction forms zeolites are not as pure as in the conventional process, which occurs
by autoclave requiring a specific hydrothermal reactor, there is a great energy advantage in the geopolymerization
process. The geopolymerization reaction, in addition to saving energy, provides time spent, contributing to
sustainability in obtaining zeolites [54]. However, it is worth noting that zeolites are formed only in very specific
situations within geopolymers, under predetermined temperatures (25 to 300ºC) and pressure [63], [64].
The most commonly known zeolite species are analcime, zeolite A, zeolite X, sodalite, natrolite, among others [54], [65],
highlighted in Table 2. As can be seen in Figure 4, some zeolite minerals are the same detailed in this figure as
geopolymerization reaction products. The polysialates highlighted above are nothing more than zeolites with a lower
degree of crystallinity due to the presence of amorphous N-A-S-H gels. Figure 6 presents an illustration of the sodalite-
type zeolites (SOD), zeolite A (LTA), and zeolite X (FAU), which are formed by the arrangement of sodalite cages in
different configurations. This fact illustrates that the most common zeolites obtained by the autoclave process are
formed in geopolymers, but the simplest crystallographic arrangements, which favor the formation of larger amounts
of sodalite as a result of zeolite A or X [65].
Table 2. Parameters of the main zeolites. Source: Rożek et al. [54]:
Abbreviation
Name
Chemical formula
ANA
analcime
Na[AlSi2O6].H2O
CAN
hydroxycancrinite
Na8[AlSiO4]6(OH)2.2H2O
CHA
chabazite (herschelite)
Na[AlSi2O6].3H2O
FAU
Faujasite (Zeolite X)
Na2[Al2Si2.4O8,8].6,7H2O
FAU
Faujasite (Zeolite Y)
Na1.88[Al2Si4,8O13,54].9H2O
GIS
Zeolite Na-P1
Na3,6[Al3,6Si12,4O32].12 H2O
LTA
Zeolite A
Na2[Al2Si1,85O7,7].5 H2O
NAT
Natrolite
Na2[Al2Si3O8].2 H2O
SOD
Sodalita
Na6[AlSiO4]6.8 H2O
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Figure 6. Illustration of the most common zeolites obtained by autoclave. Source: Rożek et al. [54].
3 ALKALI ACTIVATION REACTION OF OF CALCIUM-RICH PRECURSORS
After discussing the reaction of low-calcium precursors, it is valid to present the reaction mechanisms of AA
materials formed from blast furnace slag, for example. It is known that in these cases, the precursor material is rich in
calcium, and the products formed are different, being partly similar to the hydration products of Portland cement. As
highlighted in the introduction, the calcium-rich precursor is defined as one with a Ca/(Si+Al) ratio higher than 1 [1].
The first distinguishing feature, which deserves to be highlighted, of the systems formed by calcium-rich precursors
concerning the geopolymers previously defined is that blast furnace slag, for example, is much more reactive at
moderately alkaline pH than geopolymer materials [2]. This allows the use of several other materials as an activating
solution, besides sodium and potassium hydroxides and silicates, such as alkali metal carbonate or sulfate solutions.
That is because BFS reacts very slowly with water and the presence of alkaline compounds only accelerates the
material's hardening reaction [53].
The alkali-activated reaction products of blast furnace slag in alkali metal silicate and hydroxide solutions are
generally predominantly hydrated calcium silicate gels, similar to those obtained in the hydration of Portland
cement [8], [66]. However, there is an important difference, because the gels have lower amounts of Ca and more
amounts of Al in tetrahedral locations. That leads to a higher degree of polymerization and also to a significant degree
of crosslinking between formed gel chains. While C-S-H compounds are formed in cement hydration, in BFS alkali-
activated reaction, C-A-S-H gels are formed, giving rise to minerals known as tobermorites [8], [57].
Figure 7 shows an illustration of the tobermorite. As with Portland cement systems, the C-A-S-H gel includes layers
of silicate chains coordinated tetrahedrically with a Dreierketten structure. The region between the coverslips contains
Ca2+ cations, alkalis, and hydration water chemically incorporated into the gel structure. Some alkaline cations also
balance the net negative charge generated when Al3+ replaces Si4+ at the tetrahedral chain locations [1].
Figure 7. Tobermorite Structure. Source: Provis and Bernal [1].
However, as the amount of Al increases in the precursor’s composition, Al precipitation begins to occur in the C-
A-S-H chains, making it difficult for the chains to cross-link and become saturated. Thus, the precipitation of another
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phase, rich in Al, occurs from the chains and impairing the formation of gels and sometimes changing the mechanical
properties of the material [67].
The gels formation described is not as simple as mentioned in the previous paragraph. There is a wide formation of
secondary phases that can be set up depending on the chemical composition of the studied precursors, as highlighted
by Figure 8. That is because several factors, such as alkalinity, water/binder ratio, curing environment (duration,
humidity, and temperature), besides the relationship among the components, the main ones being Ca, Mg, Si, Al, and
Na, they affect the phases and compounds formed in the alkali-activated relationship [68], [69]. It is worth noting that
the secondary phases formed are rich in Al since, as highlighted in the previous paragraph, the excess of Al causes the
precipitation of other phases out of the principal C-A-S-H gels [2], [8], [57].
Figure 8. Formation of gels in the hydration of precursors with different calcium levels. Source: Provis [2].
Al may be substituted for Mg in C-A-S-H gels, but in very limited levels. If there is an excess of Al, and there is
Mg in the material composition, another secondary compound called hydrotalcite is formed, which is often mixed with
C-A-S-H gels [70]. The use of precursors with low Mg content, such as slag without this element, favors the formation
of zeolites instead of hydrotalcite. Another important characteristic of secondary gels is the presence of alkali metals,
such as Na, present in the activating solution. In general, alkali metals are located at the interstitial sites of the gels,
producing a balance of electrical charges. However, an N-A-S-H phase can coexist as side products along with the
predominant C-A-S-H phase in the alkaline activation of BFS [71]. It is known that N-A-S-H gels have a higher degree
of amorphism than C-A-S-H gels and are favored in alkali activation with silicates. In the reaction with hydroxides, the
formation of C-A-S-H gels is favored, presenting a high degree of crystallinity, driven by the presence of Na in the
interstices of the gels [52].
The alkali activation mechanism of calcium-rich precursors is even more complex than the one of precursors low
in this compound (geopolymers) and needs many studies. Besides the exemplary blast furnace slag, other materials
such as steel, nickel, titanium, and phosphorous slag fit together in this group [39], [72], [73].
It is interesting to note that some researchers carried out the mixture of two different types of precursors to study
the formation of alkali-activated reaction gels. The idea that reaction mechanisms occur separately for low and calcium-
rich precursors is not always true in experimental research. Some studies, for example, carried out the mixture of blast
furnace slag and fly ash in different proportions to study the alkaline activation of these materials together and how the
gels are formed in this reaction [74], [75]. Figure 9, for example, presents a ternary diagram with the elements CaO,
SiO2 and Al2O3, and the gels formed in the AA reaction. The formation of gels types C-A-S-H and N-A-S-H is verified
depending on the curing age studied or depending on the material composition evaluated.
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Figure 9. Ternary diagram of formation of alkaline activation gels. Source: Ismail et al. [74].
4 CHARACTERIZAION OF GELS FORMED IN THE AA REACTION
A way to identify the formation of the gels in the alkali activation reaction and to prove the mechanisms presented
in the previous paragraphs is through characterization tests. Some examples of this type of assay are X-ray diffraction
(XRD), transmission electron microscopy (TEM), and/or scanning electron microscopy (SEM), nuclear magnetic
resonance spectrometry (NMR), and Fourier transform infrared spectroscopy (FTIR).
For the determination of the crystalline structure, one of the main techniques employed is X-ray diffraction, and it
is even possible to perform the quantitative determination of zeolite since a reliable database is available [76]. This
technique is very widespread in the control of zeolites industrial production [77], for example, and some authors have
sought to apply it to the study of alkali-activated materials [78], [79].
The creation of the database for quantitative analysis of the zeolite phases present in the alkali-activated materials
is complex, and it is necessary to use information known from the international bibliography. For example, it is possible
to associate the formation of zeolite X with peaks in the 2θ values of 32º, 43.5º and 50.5º, obtained through the file of
the International Center for Diffraction Data (ICDD) # 39-0218 [80]. Zeolite A, in turn, can be related to peaks in the
2θ values of 7.5º, 10.5º, 30º and 34.9º by ICDD file # 351009 [81], while zeolite K is detected at the peak with 2θ
value of 10º by ICDD file # 220793 [82]. It is observed that the use of XRD for quantitative analysis of the crystalline
phases formed by the alkali-activated mechanisms is not so simple, which is why several authors use a qualitative
characterization, relating the appearance of the zeolitic phase peaks with the efficiency of the AA reaction.
More satisfactory results are obtained using XRD techniques in conjunction with others, such as FTIR [83], [84],
or even thermogravimetric analysis (TGA) techniques [85]. In the case of the use of FTIR, it is possible to identify
materials with a short-range structural order through the relation of the spectrometry obtained with standards known in
the bibliography [86], [87]. That technique allows the analysis of amorphous and semi-crystalline bands within the
material structure, complementing the XRD technique.
Figure 10, for example, shows the results of FTIR obtained in the geopolymers analysis based on metakaolin with
partial replacement by palm oil in 5, 10, and 15% [86]. The authors found that the best resistance results were obtained
with geopolymers containing 5% palm oil, attributed to the efficiency in geopolymerization. That fact was checked by
the authors using FTIR, as illustrated by Figure 10, whose presence of bands in 1659, 1408, 1000, 723, 589, and 446 cm-1 is
verified. The bands of 1659 and 1408 cm-1 were attributed by the authors to the presence of NaOH and free water due
to the activating solution, while the bands between 800 and 400 cm-1 are related to the Si-O-Si bond. The most
noticeable band in the figure is found at 1000 cm-1, related to the stretching of the Si-O-Al bond in the reaction products.
The bands observed by the authors are compatible with other published works [18], [79], [80], [84]. The authors
attributed the higher resistance obtained by geopolymers with 5% palm oil to the most noticeable bands observed by
this composition, which proves the efficiency of the alkaline activation reaction [86].
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Figure 10. FTIR analysis for metakaolin geopolymers with partial incorporation of palm oil. Source: Hawa et al. [86].
Another way of characterizing the products of the AA reaction is by nuclear magnetic resonance (NMR)
spectrometry, which in the case of the geopolymers study, for example, the resonance of two types of isotopes is carried
out: Al27 e Si29 [39], [88], [89].
In the isotope Al27 resonance, analyzes are performed based on the frequency of 55 ppm, related to aluminum
hydroxide, and the frequency of 76 ppm, related to sodium aluminate (Al4), where aluminum presents coordination
4, that is, it presents 4 covalent bonds [39], [84]. The analysis of the formation of geopolymeric networks is carried
out by verifying the number of covalent bonds made by Al27. For example, in the case of Al4Q0, aluminum has 4
covalent bonds, but none of them are made with silicon, that is, it is still sodium aluminate whose frequency is 76
ppm. In the case of do Al4Q1, aluminum has 4 covalent bonds, but one of them is with silicon, represented by a
change in the resonance frequency for an interval between 71 to 75 ppm. The same pattern occurs in Al4Q2, change
in the resonance frequency to a value between 65 to 70 ppm, and Al4Q3, between 60 to 65 ppm. When aluminum
makes four covalent bonds with silicon, the symbol used is Al4Q4, forming a three-dimensional network with a
frequency between 52 to 58 ppm [90], [91]. In this way, it is possible to check the progress or the alkaline activation
reaction efficiency.
In the isotope Si29 resonance, the reference is made using tetramethylsilane, whose frequency is -94 ppm [88]. The
analysis is carried out in the same way as for the Al27 isotope, with the difference that the connections between silicon
and aluminum, or between silicon and silicon can be studied [89]. Unlike aluminum, which can be tetravalent,
pentavalent, or hexavalent, silicon has tetracoordination, which is why the Si4Qn symbology is not widely used. In
general, only Qn is used to represent the bonds of this type of isotope [18], [92], [93]. Knowing all the frequency patterns
for the isotopes of Al27 and Si29, it is possible to create a database and correlate these values with, for example, what is
observed in the main zeolites. It is known that zeolite A has a value of -88.9 ppm and sodalite of -88.4 ppm, with Si4Al4
coordination [18], [94]. Thus, it is possible to identify the types of products obtained by the alkaline activation process
using resonance techniques.
The morphological analysis of the alkali-activated reaction products can be observed by TEM or SEM, for example.
Although some studies use the TEM technique [95], the vast majority of articles published in the AA materials area use
SEM analysis, which will be presented in Figures 11-13. Figure 11 shows the SEM results obtained by the alkali-
activated reaction of fly ash [10]. It is possible to identify N-A-S-H gels of amorphous and random nature, as well as
crystalline gels, which correspond to zeolite P and analcime, two distinct types of zeolites.
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Figure 11. SEM of the activated alkali reaction of fly ash: (a) fly ash before the reaction; (b) N-A-S-H gels; (c) P zeolite gels; (d)
analcime gels. Source: Palomo et al. [10].
Figure 12. SEM of zeolites formed in the production of geopolymers: (a) faujasite; (b) zeolite A; (c) sodalite. Source:
Rożek et al. [54].
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Figure 13. Morphology of tobermorite obtained in the alkaline activation of BFS. Source: Aziz et al. [99].
Figure 12 shows different zeolite gels formed in the alkali activation reaction of fly ash and metakaolin [54]. It is
possible to identify faujasite, zeolite A, and sodalite gels, through the microstructural analysis of the products and the
performance of additional characterization techniques mentioned above, such as XRD, FTIR and NMR [54]. These
gels are very common in the formation of geopolymers. They are responsible for important properties in this material.
Sodalite, for example, contributes to the crystalline structure stability and has hydrophilic properties, allowing cation
exchange and the formation of stable and resistant products [96], [97].
It is considered the geopolymers' basic forming unit, and it is present even in other zeolite gels [97]. Faujasite and
zeolite A gels have a more porous structure as their principal property due to their microscopy that is derived from the
overlap of several sodalite cages [54], [98], as seen in Figure 6. The overlap of several sodalite cages groups makes it
possible to obtain more complex and more crystalline zeolites, which have better mechanical properties, further
reducing the density of the material obtained [96], [98].
On the morphological aspects formed in calcium-rich precursors, such as blast furnace slag, Figure 13 is analyzed,
which shows the SEM of an BFS activated by sodium hydroxide, and silicate. The figure shows the presence of
tobermorite gels (C-A-S-H), which do not have an aspect as visible as that of zeolites formed in geopolymers [99]. It
is possible to prove the similarities of the formed material with those found in cementitious materials, where the C-S-
H phase is verified. In addition, the Figure illustrates the presence of calcite particles mixed with tobermorite particles,
making the difference between the two almost undetectable.
5 REACTION KINETICS OF ALKALI-ACTIVED
5.1 Influence of activators on the reaction kinetics of AA.
After explaining in detail the alkali-activated reactions mechanisms, it is interesting to present the factors that
interfere with the reaction kinetics, that is, the factors that make these reactions faster or more efficient. One of the most
important factors is related to the types of activators used in the process, as well as their viscosity and pH. Other factors
are the influence of the cure type and the precursor granulometry, which modifies the reactivity of this material.
Regarding the activators, these materials are defined as hydroxides, silicates, sulfates, or carbonates of alkali metals,
which, when diluted in water in a determined proportion can make the precursor harden [6]. Initially, it is necessary to
highlight that the types of activators most used in research are hydroxides and silicates and that the two cause different
kinetics in the AA reaction. Besides, it is usually common to use only sodium or potassium hydroxide, or even a
combined solution of sodium hydroxide and silicate [13].
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When using only hydroxides, there is no increase in the amount of silica in the system, while when using silicates
the amount of silica in the system reaches higher levels [58]. It is known that in the AA reaction, the precursor's alumina
is more reactive than silica, it is released first and available for the geopolymerization reaction in a shorter time than
the silica present in the precursor [52]. Thus, the use of silicate-based activators promotes an acceleration in the reaction
of geopolymerization or alkaline activation, due to the fact that the silica present in the silicate reacts more quickly with
the alumina released by the precursors. The use of silicate favors the AA reaction process, leading to more resistant
products than with the use of hydroxide alone [100].
It is necessary to highlight that the precursors’ properties are unique and individual within each research due to
issues related to granulometry and oxide composition. That makes it impossible to define an optimal amount of silica.
Even so, some researchers report that the use of mixed actives containing hydroxide and sodium silicate allowed to
obtain a resistance of 60 MPa at 7 days. The same authors had obtained resistance of 30 MPa at 28 days using only
sodium hydroxide in alkaline activation [101]. In this research, a more resistant product was obtained in a much shorter
curing time using a mixed solution composed of sodium hydroxide and silicate. The same pattern was found in other
studies [30], [50], [102]. That indicates the benefits of applying silicate on the mechanical properties of alkali-activated
materials.
Also, other negative factors can be cited by using only hydroxide in the solutions. When used alone, these materials
are used in high molar concentrations to increase the precursor reactivity. It is noteworthy that each precursor presents
exclusive parameters of reactivity, making it difficult to compare different studies. However, some authors highlight
and prove this fact with experimental results [21], [51].
That high concentration can lead to significant occupational health and safety considerations in a large production
facility, as these solutions are classified as corrosive under the workplace legislation in force in almost all countries in
the world [1]. Also, the need for thermal curing is common when only hydroxides are used, as occurs when fly ash is
used as a precursor, at temperatures of about 60ºC [55], [103]. This fact is acceptable for pre-molded parts, but it makes
the molded parts on site unfeasible.
Hydroxide-activated binders, whether based on fly ash or BFS, also tend to show higher permeability than their
silicate-activated equivalents and tendency to efflorescence [104]. That is because the reaction extent reached by the
binder before curing is generally low, which leads to an open microstructure and higher material porosity [105].
Efflorescence and other visible effects of alkali mobility are undesirable. However, they can be overcome to some
extent by appropriate control of curing conditions or by the addition of secondary aluminum sources. That ensures that
a sufficient extent of reaction is achieved before the material is put into service [106], [107].
One of the disadvantages of using mixed solutions is the silicate cost, which is higher than that of hydroxides,
making the alkali-activated materials production more expensive, especially when compared to Portland cement-based
materials [2]. Also, the use of silicate solutions in alkaline activation impairs the viscosity of the activating solution,
making it difficult to work with the paste, mortar or alkali-activated concrete used [108]. Thus, although it presents a
potential to increase the mechanical resistance, problems during the structures molding due to the low workability can
cause defects or pathologies in the parts and reduce its resistance and durability parameters. Another important factor
in the reactions kinetics is the of the solution’s molarity, in the case of the hydroxide type activator using, or of the
silica modulus (Ms or s) in the case of the use together of silicates and hydroxides. These parameters are defined by
Equations 2 and 3, respectively, and have simple definitions. The molarity represents the amount of of alkali metal
hydroxide moles contained in a volume of solvent, usually water. The silica modulus represents the molar relationship
between SiO2 and M2O of a system formed by silicates and hydroxides, where M represents an alkali metal, such as
sodium or potassium [1], [2], [57].
[ ]
NaOH
MV
=
(2)
where M= molarity (in moles/l); [NaOH]= number of moles of sodium, calculated by dividing the mass used in the
solution by the molar mass worth 40g /mol, in moles; V= volume of the solution, in l.
[ ]
[ ]
2
s
2
SiO
MNa O
=
(3)
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where Ms= silica module, dimensionless; [SiO2]= number of moles of silicon oxide, calculated by dividing the mass
used in the solution by the molar mass that is worth 60g / mol, in moles; [Na2O]= number of moles of sodium oxide,
calculated by dividing the mass used in the solution by the molar mass worth 62g / mol, in moles.
On molarity, Figure 14 presents a graph that illustrates the rate of alkali-activated reaction as a function of different
molarity values obtained by the calorimetry technique (5M, 10M, 12M, 15M, and 18M). It appears that, for the
precursor used by the authors, the higher the molarity, the longer the reaction takes to reach its peak. The 10 M molarity
has practically the same reactivity as 12 M, while the same thing happens for 15 M and 18 M. This study’s authors
concluded that the use of molarity of 5M is inefficient in the alkali-activated reaction and that the most recommended
values of molarity are between 10 to 18M [109].
Figure 14. Calorimetry test as a function of molarity. Source: Alonso and Palomo [109].
On the silica modulus, Figure 15 shows the rates of alkali-activated reaction as a function of different values of Ms
obtained by the calorimetry technique (0.6, 0.9, and 1.2) using blast furnace slag as a precursor. It appears that the
higher the silica module, the higher the rate of energy released, and the higher the reactions kinetics, proving that the
increase in the silica modulus accelerates and increases the reaction rates [110].
Figure 15. Calorimetry test as a function of Ms. Source: Krizan and Zivanovic [110].
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There are few studies using sodium sulphate or sodium carbonate as alkaline activators, primarily because these
compounds have low efficiency when compared to the hydroxides and silicates mentioned above, due to the influence
of pH [111], [112]. Most research uses sulfates and carbonates together with sodium or potassium hydroxide, for
example [113], in an attempt to correct the pH to values close to 14. Besides, there is a problem with using sulfate or
carbonate solutions in reinforced structures because these materials can cause attack or degradation of armor [114] and
can cause pathologies.
Regarding the types of alkali metals used in research, those with the highest applications are sodium and potassium.
Comparing the efficiency of these two types of metals, it appears that the size of the cation is directly linked to the
kinetics of the geopolymerization reactions. As Na+ is smaller than K+, a smaller amount of silicate oligomers is formed [115].
Therefore, it is observed that the larger the cation, the higher the reaction kinetics of the AA because the more favored
is the formation of larger silicate oligomers in which Al(OH)-4 prefers to bind. Thus, precursors activated with KOH
have higher compressive strength compared to geopolymers synthesized from NaOH solutions [116]. Also, since K+ is
more basic, higher silicate dissolution rates are possible, allowing for denser and more efficient polycondensation
reactions, which increase the final mechanical strength of the matrix [115].
Although there is research with other alkali metals, such as lithium and cesium, the availability of these materials is much
less than that of sodium and potassium and makes research related to these materials much less frequent [117], [118]. In any
case, the cation size scale is as follows: Cs> K> Na> Li. That is why it is believed that that is also the efficiency scale
of the reactions kinetics of alkaline activation, that is, that cesium is the most reactive and lithium the least, comparing
these four alkali metals. The results found in the bibliography confirm this pattern of reaction kinetics [118], [119].
5.2 Cure type Influence
Another very important factor in the reactions kinetics of AA is the type of cure performed in the research.
Even though each precursor has its peculiarities, due to the difference in chemical composition, reactivity, and
particle size, there are reports of researchers studying the alkaline activation of fly ash, metakaolin, and blast
furnace slag at room temperatures (25 to 30ºC) and in a greenhouse environment (60 to 90ºC). It was found that
curing performed at a temperature of 60ºC considerably increases the compressive strength of the materials [28],
[120], [121]. This increase in the cure temperature favors resistance because it increases the dissolution of reactive
species, such as silica and alumina, increasing the reactions kinetics [122]. However, oven curing deserves special
attention since prolonged curing times distort the reactions, causing partial water evaporation with the formation
of microcavities that lead to cracking of the samples, weakening the structure of the formed gels, suggesting that
small amounts of structural water they need to be maintained in order to reduce cracks and maintain the integrity
of the material [123], [124].
Curing at very high temperatures, above 100ºC, also impairs the alkaline activation reaction because when curing
occurs at very high temperatures, the samples do not have sufficient moisture [124]. The loss of water in an accelerated
and precocious way accelerates carbonation, lowers pH levels, and results in a delay in the precursors activation,
resulting in a high aluminum content in the formed gels. Under these conditions, the final product is granular, porous,
and characterized by low mechanical resistance [125], [126]. It appears that the most appropriate is to perform thermal
curing in milder temperatures and use a not too long curing period. To illustrate this information, Table 3 presents data
from several studies with different precursors cured at different temperatures, also showing the compression strength
obtained.
Table 3. Interference of the curing temperature in the kinetics of the AA reaction.
Precursor Type Temperature (ºC) Time (days)
Compressive strength
(MPa)
Fly ash
60
7
5.12
Source [120].
80
7
7.54
120
7
7.90
60
28
12.53
80
28
15.21
120
28
11.63
Blast furnace slag
60
7
35.11
Source [120].
80
7
30.78
M. T. Marvila, A. R. G. Azevedo, and C. M. F. Vieira
Rev. IBRACON Estrut. Mater., vol. 14, no. 3, e14309, 2021 20/26
Table 3. Continue...
Precursor Type
Temperature (ºC) Time (days)
Compressive strength
(MPa)
120
7
40.20
60
28
35.44
80
28
35.80
120
28
36.19
Fly ash
30
7
21.20
Source [123].
90
7
40.53
30
28
24.98
90
28
42.84
Blast furnace slag
25
7
25.16
Source [121].
60
7
40.99
95
7
29.98
25
28
43.52
60
28
45.23
95
28
31.06
Metakaolin
25
7
34.78
Source [28].
50
7
59.28
80
7
55.72
25
28
50.41
50
28
60.14
80
28
58.42
Metakaolin
30
7
7.03
Source [122].
60
7
17.87
90
7
13.13
6 FINAL CONSIDERATIONS
This study aims to present an extensive and detailed review on the reaction mechanisms of alkali-activated materials,
dividing them into two large groups, depending on the amount of calcium oxide contained in its chemical composition.
Materials with reduced amounts of calcium (Ca / (Si + Al) <1) are defined as precursors of low calcium content, also
named as geopolymers, and are therefore predominantly aluminum silicates. Upon undergoing geopolymeric reaction
it forms compounds similar to zeolites, which are highly crystalline or nanocrystalline, but also have amorphous gels
named N-A-S-H, both of which are based on polysialates. The main characteristics of these gels were discussed in the
text, being approached the most studied types of precursors that fit in this class of materials, such as fly ash and
metakaolin. The steps of the typical alkaline activation reaction of these materials were established, through the phases
of dissolution, condensation, reorganization, polycondensation, with or without crystallization, and finally hardening.
Materials with significant amounts of calcium (Ca/(Si+Al)>1) present an alkali-activated reaction similar to
Portland cement, however forming gels called C-A-S-H (tobermorite). Also, some secondary products are formed in
this reaction, as is the case with N-A-S-H and hydrotalcite. The main example of a precursor that follows this reaction
pattern is blast furnace slag. The characteristics of mixed gels and techniques for identifying formed gels were
addressed, using X-ray diffraction (XRD), nuclear magnetic resonance spectrometry (NMR), Fourier transform infrared
spectroscopy (FTIR), and scanning electronic microstructure (SEM).
Finally, a study of the reaction kinetics of alkali-activated was carried out and it was found that the type of activator
solution used, such as the hydroxide-only base or the silicate and hydroxide base, modify the efficiency and speed of
these reactions. Other factors, such as the type of alkali metal used (sodium, potassium), the molarity, or the silicon
modulus of the solution, also affect the kinetics of the reactions. Curing factors, such as age and temperature conditions,
can act as catalysts for alkali-activated reactions, and cures performed during excessive exposure times and at elevated
temperatures (above 100ºC) in general impair the mechanical properties obtained from alkali-activated materials.
As suggestions for future work to improve the understanding of the reaction mechanisms of alkali-activated
materials, the development of standards and procedures with exclusive application to this class of materials stands out
since currently technical standards of other construction components are adopted. Other important contributions are the
M. T. Marvila, A. R. G. Azevedo, and C. M. F. Vieira
Rev. IBRACON Estrut. Mater., vol. 14, no. 3, e14309, 2021 21/26
development of activators with less impact than conventional silicates and hydroxides and the understanding of how
these activators modify the steps and kinetics of the alkaline activation reaction.
On precursors, additional studies are needed on the interference of other oxides in the mechanisms of alkali-
activated reaction. It is known that some types of industrial waste contain iron oxide, magnesium, nickel, chromium,
among others. The technology of alkali-activated materials is a great contribution to the evaluation of the interferences
that these compounds cause in the formation of gels and the kinetics of the alkali-activated reaction, although there is
already recent research that deals with this subject in an appropriate way. However, there are still relevant issues that
need further understanding and discussion regarding the influence of these metal oxides on the alkali-activated reaction.
It also stands out as a suggestion the need for the development of tools that can accurately measure the degree of
activation of alkali-activated binders, as well as the rheological parameters of the material, enabling the development
of even more efficient materials.
Another relevant contribution is the evaluation of the production of alkali-activated materials in large scale and real
size, not only through simulations in laboratories with specimens, but using beams in reduced size, for example. This
type of study allows the assessment of logistics and the understanding of how the reaction mechanisms of alkali-
activated materials in a proportion closer to the real one is.
ACKNOWLEDGEMENTS
The authors would like to thank FAPERJ and CNPq for their support.
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Author contributions: MTM: conceptualization, writing, formal analysis, revision. ARGA, CMFV: funding acquisition, supervision, formal analysis.
Editors: José Marcio Calixto, Guilherme Aris Parsekian.
... The mixed proportions of the GPC consist of aluminosilicate source binder materials, fine and coarse aggregates, alkaline solutions, and water [12]. The polymerization process consists of four main steps: dissolution, condensation, polycondensation, and crystallization of the gels, between the alkaline solutions and source binder materials, produced solid concrete, like traditional concrete composites [13,14]. Sodium hydroxide and sodium silicate are commonly used alkaline activators to create geopolymer composites. ...
... Regarding the SI parameter, it can be said that a model has a (poor performance) when SI > 0.3, a (fair performance) when 0.2 < SI < 0.3, a (good performance) when 0.1 < SI < 0.2, and an (excellent performance) when SI < 0.1 [32]. Furthermore, the OBJ parameter was employed as a performance measurement parameter in Eq. (13) to measure the efficiency of the suggested models. Fig. 21. ...
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Since nanotechnology can enhance the performance of materials, significant effort has been expended in recent years to incorporate nanoparticles (NPs) into geopolymer concrete (GPC) to improve performance and produce GPC with improved characteristics. Recent efforts have been made to incorporate various nanomaterials, most notably nano-silica (nS), into GPC to improve the composite's properties. Compressive strength (CS) is an important property of all concrete composites, including geopolymer concrete. Several mix proportion parameters and curing temperature and ages influence the CS of geopolymer concrete. As a result, developing a credible model for forecasting concrete CS is critical for saving time, energy, and money while also providing guidance for scheduling the construction process and removing formworks. This paper consists of three phases; in the first phase, a detailed review on the effect of adding nS on the CS of GPC was provided; then, in the second phase, a large amount of mixed design data were extracted from literature studies to create five different models including artificial neural network, M5P-tree, linear regression, nonlinear regression, and multi logistic regression models for forecasting the CS of GPC incorporated nS. Finally, the developed models were validated in the last phase by carrying out experimental laboratory works. Results revealed that the addition of nS improves the CS of GPC, and the ANN model estimated the CS of GPC incorporated nS more accurately than the other models. On the other hand, the alkaline solution to binder ratio, molarity, NaOH content, curing temperature, and ages were those parameters that significantly influenced the CS of GPC incorporated nS.
... The results of blast furnace steel slag activated by hydroxide solution and sodium silicate were compared to those of ordinary concrete. The scientists demonstrated that the mechanical strength of geopolymers did not decline much at temperatures up to 600 • C, but the strength of concrete decreased dramatically [16][17][18]. ...
... The NaOH base is the most often employed solution, which significantly changes the viscosity of mortars in their fresh form. This is because the salt concentration in the material, even when dissolved, creates an extra barrier and increases friction between the grains of the fresh mortar, impairing the material's flow and, as a result, its workability [11,17,18]. ...
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Geopolymer is a long-lasting substance that helps to safeguard the environment and provides an alternative to Portland Cement. By using by-products and using less OPC in this manner, CO2 emissions are also decreased. This field might benefit from a study into the use of various binders in geopolymer manufacturing. Bentonite and zeolite, both naturally occurring and inexpensive, were calcined at 750°C and employed as binders in this study. Calcined bentonite and zeolite were substituted in five different percentages for each other (20%, 40%, 60%, 80%, and 100% by weight). In addition, the investigation was carried out by substituting river sand for metakaolin (MK) in four different percentages (25%, 50%, 75%, and 90% by weight). At 7, 28, and 56 days, ten geopolymer series were tested for flexural-compressive strength and ultrasonic pulse velocity (UPV). High-temperature test were also applied to geopolymer mortars to examine the impacts following the durability conditions. Variable UPV, weight, and strength findings were discovered in addition to the durability testing. SEM analyses were also used to look at the results of the durability testing. Due to its more excellent Si/Al ratio and more stable zeolitic structure, metazeolite produced better results. The primary products of the alkaline activation process were also studied, with polysialates and zeolites defined in the case of geopolymers and the tobermorite structure defined in the case of calcium-rich materials. Dissolution, condensation, polycondensation, crystallization, and hardness were considered as the key processes of the alkali-activated reaction. The usage of metakaolin enhanced the pozzolanic characteristic by up to 40%, which had a beneficial impact on the findings. When all of the impacts were considered combined, it was discovered that the produced geopolymer samples were resistant to the effects of durability and the sample 50MZ50MB25MK yielded the higher result.
... Alkali-activated materials are made when precursors and activators react. These were split into two groups according to the calcium level of the objects created during the chemical reaction: these are rich in calcium, having a Ca/(Si + Al) ratio of more than one, and others that have low calcium levels are called geopolymers [18]. ...
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The depletion of natural resources and greenhouse gas emissions related to the manufacture and use of ordinary Portland cement (OPC) pose serious concerns to the environment and human life. The present research focuses on using alternative binders to replace OPC. Geopolymer might be the best option because it requires waste materials enriched in aluminosilicate for its production. The research on geopolymer concrete (GPC) is growing rapidly. However, substantial effort and expenses are required to cast specimens, cures, and tests. Applying novel techniques for the said purpose is the key requirement for rapid and cost-effective research. In this research, supervised machine learning (SML) techniques, including two individual (decision tree (DT) and gene expression programming (GEP)) and two ensembled (bagging regressor (BR) and random forest (RF)) algorithms were employed to estimate the compressive strength (CS) of GPC. The validity and comparison of all the models were made using the coefficient of determination (R2), k-fold, and statistical assessments. It was noticed that the ensembled SML techniques performed better than the individual SML techniques in forecasting the CS of GPC. However, individual SML model results were also in the reasonable range. The R2 value for BR, RF, GEP, and DT models was 0.96, 0.95, 0.93, and 0.88, respectively. The models’ lower error values such as mean absolute error (MAE) and root mean square errors (RMSE) also verified the higher precision of ensemble SML methods. The RF (MAE = 2.585 MPa, RMSE = 3.702 MPa) and BR (MAE = 2.044 MPa, RMSE = 3.180) results are better than the DT (MAE = 4.136 MPa, RMSE = 6.256 MPa) and GEP (MAE = 3.102 MPa, RMSE = 4.049 MPa). The application of SML techniques will benefit the construction sector with fast and cost-effective methods for estimating the properties of materials.
... They have been categorized into two kinds based react, alkali-activated compounds are formed. They have been categorized into two kinds based on the calcium proportion of the products formed during the reaction: those that are calcium-rich, with a Ca/(Si+Al) fraction above 1, and those that are calcium-deficient, i.e., geopolymers [17][18][19]. ...
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Geopolymers may be the best alternative to ordinary Portland cement because they are manufactured using waste materials enriched in aluminosilicate. Research on geopolymer composites is accelerating. However, considerable work, expense, and time are needed to cast, cure, and test specimens. The application of computational methods to the stated objective is critical for speedy and cost-effective research. In this study, supervised machine learning approaches were employed to predict the compressive strength of geopolymer composites. One individual machine learning approach, decision tree, and two ensembled machine learning approaches, AdaBoost and random forest, were used. The coefficient correlation (R2), statistical tests, and k-fold analysis were used to determine the validity and comparison of all models. It was discovered that ensembled machine learning techniques outperformed individual machine learning techniques in forecasting the compressive strength of geopolymer composites. However, the outcomes of the individual machine learning model were also within the acceptable limit. R2 values of 0.90, 0.90, and 0.83 were obtained for AdaBoost, random forest, and decision models, respectively. The models’ decreased error values, such as mean absolute error, mean absolute percentage error, and root-mean-square errors, further confirmed the ensembled machine learning techniques’ increased precision. Machine learning approaches will aid the building industry by providing quick and cost-effective methods for evaluating material properties.
... In alkaline activation of geopolymers, it is quite common to obtain zeolite species such as analcime, sodalite, natrolite, nepheline, among others [59]. However, the type and quantity of these phases depend on the nature of precursor material, calcination temperature, and alkaline sources [60]. ...
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... The appearance of humps at values of 10-30 • to 20-45 • (2 • ) indicates the formation of an alkali-aluminosilicate hydrate gel N-A-S-H that was identified as the primary reaction product of the polymerization reaction in the diffraction patterns of geopolymer materials [76]. During the multi-polymerization process, the stable amorphous N-A-S-H gels are formed, while the crystalline or semi-crystalline phases are formed with zeolite [77]. Alkaline reaction processes of FA with an alkali activator produce hardened pastes composed of Na 2 O-Al 2 O 3 -SiO 2 (N-A-S-H) bonds, as well as small amounts of CaO, MgO, and FeO 2 . ...
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... Lately, there has been a noticeable surge in awareness among a varied set of experts in the management of tailings in standard practices↱. Over a dozen publications have been published highlighting the efforts undertaken to improve our understanding of the geopolymerisation processes of tailings in order to regulate the characteristics of GPs for applications like pollutant removal [90][91][92], sustainable building [63,64,[93][94][95], and other particular usage [63,94,[96][97][98][99][100][101][102][103][104][105]. ...
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The mining sector generates a considerable amount of waste stone and tailings, which constitute a ‎‎substantial hazard to the ecology. The most prevalent method of disposing of these industrial ‎‎wastes is by dumping, which adds to soil deterioration and water contamination and also takes up ‎‎valuable land. Fortunately, it can be recycled in a variety of ways, including the promising ‎‎geopolymerisation technique, which converts waste into value. This article discusses recent ‎‎advances in the production of mine tailings-based geopolymers composites (MT-GPC) from waste and ‎‎industrial by-products as a potentially sustainable construction material. Besides, focusing on the ‎‎following ‎‎aspects: economic and production perspective; environmental consequences and waste ‎‎disposal; physical and chemical properties; mechanical properties; durability properties; ‎‎microstructure properties‎; thermal properties; and potential applications of MT-GPC.
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In this study, waste bottom ash obtained from fluidized bed combustion of low-grade South African coal in a bubbling fluidized bed reactor was used to produce geopolymers. The geopolymers were cured using both a conventional oven and a household microwave. An alkaline solution of Na2SiO3/NaOH (1:0.5) was mixed on a 1:1 ratio with a mixture of bottom ash/kaolin (1:1) mixture. Thereafter the resulting mixture was mixed with sand on a 1:0.5 ratio. Characterization of the geopolymers carried out using the following techniques, scanning electron microscope (SEM-EDX) analysis, compressive strength test, Thermogravimetric analysis (TGA), X-ray diffraction (XRD) and X-ray fluorescence (XRF) analysis. A household microwave and a conventional oven were used to enhance the geopolymerisation process and strength of the geopolymers. The results showed that the microwave curing enhanced the reactivity and compressive strength of the geopolymers. The microwave and oven cured geopolymer had the highest Si/Al ratio of 4.42 and reached a reasonable high compressive strength test of 31 MPa in 7 days compared to geopolymers cured with a conventional oven only or with a microwave only. The microwave radiation followed by conventional oven curing reduced the heat curing time and energy but improved the reactivity and strength of the geopolymer.
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Rare earth tailing (RET) is a specific tailing containing high content of heavy metals. In this work, RET-based geopolymer was successfully prepared, which not only possesses good mechanical performance but also can immobilize the heavy metals well. The compressive strength of the optimized RET-based geopolymer after 7 d can reach over 35 MPa. XRD results showed that the RET-based geopolymer had a typical broadening diffraction peak corresponding to the amorphous geopolymer structure, which was also supported by SEM micromorphology. The incorporated RET played a role similar to aggregate and has a reinforcement effect on the overall mechanical strength of RET-based geopolymer. The immobilizing mechanism of heavy metals in the RET-based geopolymer was investigated by XRD, FTIR, XPS, etc. The introduced heavy metal cations (Pb²⁺ and Ba²⁺) were immobilized in the geopolymer by combining with unbridged oxygen or with the Si/Al chain-like framework thereby generating corresponding new phase of PbO/BaSiO3. This work provides a feasible strategy to utilize the RET solid waste through the preparation of geopolymer, which possesses extraordinary immobilizing ability on heavy metals and is conducive to the environmental-friendly waste management.
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Nanomaterials, owing to their extraordinary properties are known to improve the microstructure of concrete, enhances the fresh and hardened properties of cement concrete, are widely used in cementitious materials. Many studies have been conducted so far to understand the effects of inclusion of nanomaterials on the geopolymerisation reaction, fresh and hardened state properties, microstructure, and durability of geopolymer composites. The current paper summarizes these studies mainly focusing on the effects of various nanomaterials such as nano-SiO2, nano-Al2O3, nano-TiO2, carbon nanotubes and nano-clay on geopolymer paste, mortar and concrete derived from various industrial by-products as sources of aluminosilicates. Most of the geopolymer products revealed that nanomaterials enhance the fresh and hardened state properties if used in a controlled quantity. Nano-silica and nano clay inclusion up to 2% by weight significantly enhances the rate of geopolymerisation reaction, reduces the setting times and improves the hardened state properties. Carbon nanotubes and nano-TiO2 enhances geopolymerisation by offering additional nucleation sites. Nano-alumina more prominently reduces the porosity but lesser effective in geopolymerisation. X-ray diffraction studies report the increase in XRD peaks indicating the formation of additional hydration products that comply with SEM studies. Investigation of SEM and FTIR reveals that the inclusion of nanomaterials densify the microstructure of geopolymer composites and produce high mechanical strength. Durability studies reveal that enhanced geopolymerisation with nanomaterials also prevents interconnectivity of micropores due to the formation of a denser matrix of geopolymer gel. The possible health-related issues associated with the use of nanomaterials has also been identified. The cost effectiveness of using nanomaterials is also discussed.
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The present work synthesized geopolymer with quartz mechanical activated at different time, and the strengthening mechanism of geopolymer by activated quartz was investigated. The changes of quartz before and after activation and the microstructure of geopolymer was explained by the determinations of particle size, specific surface area and the concentration of reactive Si, as well as XRD, XPS and NMR measurement. With the activation time of quartz increasing from 0 to 60 min, the compressive strength of geopolymer increased from 31.5 to 55.2 MPa. The results showed that increase of quartz reactivity is mainly due to the formation of the Si-centered radicals and non-bridging oxygen on the surface rather than the changes in physical property or crystal structure of quartz. The high activity tetrahedra Si with non-bridge oxygen on the surface of quartz replace the tetrahedra Al in geopolymer gel, which lead to a Si-rich gel formed around quartz and the improvement of compressive strength.
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This study sought to analyze the effect of curing temperature on mechanical strength and microstructure of a copper tailing-based geopolymer via scanning electron microscopy (SEM), HCl extraction, nuclear magnetic resonance (NMR), and X-ray diffraction (XRD). The distribution of gel formed in geopolymers tended to be uniform with increasing curing temperature from 25 to 80 °C. Moreover, the percentage of Si sites in C–S–H and N-A-S-H gels increased from 62.08% to 78.94% and more tetrahedral [AlO4] was incorporated into the tetrahedron [SiO4] backbone, leading to an increase of compressive strength from 10.2 to 39.6 MPa. When the curing temperature was increased to 120 °C, the percentage of Si sites in C–S–H and N-A-S-H gel decreased to 69.52%, and the compressive strength decreased to 27.5 MPa. Moderately elevated curing temperature promoted the dissolution of aluminosilicate while curing temperatures above 80 °C hindered it. Excessive curing temperature led to a decrease in the geopolymer alkaline medium.