title page review: enhancing the reactivity of
TRANSCRIPT
1
Title page
Review: Enhancing the Reactivity of Aluminosilicate Materials towards Geopolymer Synthesis
L.N. Tchadjie and S.O. Ekolu
Department of Civil Engineering Science, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa.
L.N. Tchadjie (Corresponding author)
Department of Civil Engineering Science, University of Johannesburg, PO Box 524, Auckland
Park 2006, South Africa.
Email address: [email protected], [email protected]
Tel: +27 063 047 2317
ORCID ID: 0000-0002-3778-3040
S.O. Ekolu
Department of Civil Engineering Science, University of Johannesburg, PO Box 524, Auckland
Park 2006, South Africa.
Email address: [email protected]
ORCID ID: 0000-0002-8742-2083
2
Abstract Geopolymers are alternative materials to portland cement, obtained by alkaline activation of
aluminosilicates. They exhibit excellent properties and a wide range of potential applications in
the field of civil engineering. Several natural aluminosilicates and industrial by-products can be
used for geopolymer synthesis, but a lot of starting materials have the disadvantage of poor
reactivity and low strength development. This paper presents a comprehensive review of the main
methods used to alter the reactivity of aluminosilicate materials for geopolymer synthesis, as
reported recently in the literature. The methods consist of mechanical, thermal, physical separation
and chemical activation, of which mechanical activation is the most commonly employed
technique. The reactivity of the activated aluminosilicate materials is mainly related to the
activation method and the treatment parameters. Chemical activation by alkaline fusion is a
promising method allowing preparation of one-part geopolymer materials, an alternative class of
geopolymeric binders. However, the resulting alkaline-fused geopolymer products are vulnerable
to attack by excessive alkalis.
Keywords: Geopolymer; Activation; Aluminosilicate; Reactivity.
3
1. Introduction
Geopolymers are alternative cementitious materials first proposed in 1972 by J. Davidovits
and have attracted major research interest worldwide over the past decade [1]. These materials
have a high potential of minimising CO2 generation relative to portland cement. In fact, optimal
design of geopolymer cement requires less energy and could produce 80 % less CO2 than portland
cement [2]. Moreover, geopolymers show some attractive properties for specific applications such
as fast hardening, high early strength, stability under chemical attack, high-temperature resistance
and low permeability [2], [3]. Geopolymers are inorganic polymeric materials which can be
synthesized from various aluminosilicate sources in alkaline or acidic medium condition [1], [4].
Indeed, a great number of minerals and industrial by-products have been investigated as raw
materials for geopolymer synthesis including, kaolin, metakaolin, fly ash, metahalloysite, volcanic
scoria, granite and slag [5]–[10]. However, some geopolymeric systems have shown weak
performance characteristics such as long setting time, low mechanical and durability properties
[11]–[13]. Such poor results arise from weak reactivity of the starting materials and/or
inappropriate design of the geopolymer recipe. It is worth noting that geopolymer synthesis via
the conventional route, involves two constituents comprising the solid part (aluminosilicate) and
liquid part (alkaline solution). It is well known that the properties of geopolymers are affected by
many factors such as the nature of starting materials which includes mineralogical and chemical
composition, particle size, glass content and composition; alkaline solution composition, liquid to
solid ratio, mixing procedure, and curing conditions [5], [11], [12], [14]–[16]. Therefore,
geopolymer properties can be manipulated by adjusting these parameters.
Given the physical and chemical properties of geopolymers, some geopolymeric systems have
been successfully used in several applications including those of industrial scale. In civil
engineering, the development of PYRAMENT cement, an ultra-fast and high-performance binder
is notable. Marketed in the United States since 1988, it is used for repair and construction of air
strips [17]. Recently, 40,000 m3 of geopolymer concretes were used for construction of the
Brisbane West Wellcamp Airport in Australia, making it the largest single application of this new
type of concrete worldwide. Heavy duty geopolymer concrete was used for the turning node, apron
and taxiway pavements in the same project [18]. The Global Change Institute at the University of
Queensland in Australia has been built using precast geopolymer panels, representing the first use
of suspended geopolymer concrete worldwide [19]. Hermann et al. [20] developed a new method
4
for solidification of sludges containing radio nuclides, heavy metals and organics, using
geopolymeric matrix. Bai et al. [21] reported that under specific conditions, highly porous
materials with a homogenous structure can be made with geopolymer materials. Their results
showed that the foamed geopolymers can be used as adsorbents for removal of copper and
ammonium ions from wastewater. The foams could also be used for thermal insulation due to their
low thermal conductivity. They also designed geopolymer mixtures that could be printed using
additive manufacturing technology. Geopolymers can also be used in the field of biomaterials, for
bone restoration. Best results in terms of biological compatibility were obtained with a geopolymer
matrix resulting from a mole ratio of K2O/ SiO2 = 0.54, heat-treated at 500 °C. In the same
biomedical field, Jämstorp et al. [22] showed that a sustained drug release delivery system can be
made using metakaolin-based geopolymers. Based on their results, the opioid Fentanyl and its
structurally similar sedative Zolpidem, were embedded into metakaolin-based geopolymer pellets
to provide prolonged release dosage forms, with mechanical strengths of the same order of
magnitude as that of human teeth. As reported in several studies on thermal properties of
geopolymers, these materials are highly fire and heat resistant [23]–[25]. For instance, fire resistant
geopolymer/carbon composites have been developed and used for racing car manufacture [17].
These advances in research have provided confidence that geopolymers may offer similar or better
performance to conventional ordinary portland cement[25].
In the literature, several attempts have been made to improve the geopolymeric reactivity of
starting materials. The purpose of this review was to assess the different techniques used for
increasing reactivity of aluminosilicate raw materials and in turn improve engineering properties
of resulting geopolymers. An overview of the main parameters and effects of different activation
methods is presented. This study also sought to identify and highlight new or emerging
developments on the subject so as to promote future research. The subject is discussed on the basis
of geopolymer chemistry, reactivity of aluminosilicate raw materials and mechanical properties of
the resulting geopolymer cement products.
2. Geopolymer chemistry and product properties
2.1 Synthesis
5
The traditional process of synthesizing geopolymers involves mixing a reactive aluminosilicate
precursor powder with highly soluble alkaline usually Na/K- hydroxide solution or soluble Na/K-
silicate. The hardening occurs at room temperature or at moderate temperatures below 100 °C,
depending on reactivity of starting materials [17]. The reaction is a polymerization process,
yielding nanometric macromolecules with a three-dimensional matrix made of tetrahedral AlO4
and SiO4 units linked randomly by sharing all the oxygen atoms. Alkali metal cations including
Na+, K+, Li+, Cs+
, Ca2+ provide stabilization of the negative charge of the AlO4 tetrahedron, where
the trivalent ion Al3+ is four-coordinated. The term poly (sialate) proposed by Davidovits is used
to describe building units of geopolymer structures with ‘‘sialate’’ standing for alkali silicon-oxo-
aluminate. The inorganic polymer structure is X-ray amorphous at room temperature when
hardened and crystallised at high temperatures above 500 °C. Poly(sialate) has the empirical
formula Mn(-(SiO2)z-AlO2)n,wH2O, where M is a cation such as K+, Na+, Li+ or Ca2+; z is 1,2,3 or
higher and n is a polymerization degree. Thus, four (4) basic units are derived from this formula
according to the Si/Al atomic ratio in the molecular structure Si/Al = 1 (sialate); Si/Al = 2 (sialate-
siloxo); Si/Al = 3 (sialate-disiloxo) and Si/Al > 3 (sialate link) [17]. Recent studies on
geopolymerization of iron-rich aluminosilicate materials demonstrated that the iron atom (Fe) can
be incorporated to the geopolymer network, resulting in ferro-poly(sialate) molecular units
[26][27].
2.2 Aluminosilicate raw materials
Generally, materials containing mostly silica (SiO2) and alumina (Al2O3) are possible sources
for geopolymer synthesis. From this point of view, a wide range of geological rocks and mineral
by-products can be used, since silicon and aluminium oxides constitute around 75 wt. % of earth’s
crust [28]. So far, a great number of raw materials have been investigated for geopolymer synthesis
around the world. It is worth noting that the choice of materials for investigations is mainly based
on local availability. Also, reactivity of starting materials through geopolymerization in alkaline
medium is an important parameter, and will be discussed further in the present paper. All
aluminosilicate materials may be classified in two groups comprising natural pozzolanic materials
and industrial waste by-products. Table 1 presents the relative abundance of common oxides in
the earth’s crust, their potential source materials, and their effects on the geopolymer network.
6
Table 1 Relative abundance of common oxides in the earth’s crust and their involvement in
geopolymer chemistry [27]–[31].
Oxides Abundance (wt. %) Effects on geopolymer structure
Potential sources
SiO2 60 Network forming Clay, igneous rock, feldspar,
volcanic ash, silica fume, fly
ash, rice husk ash, palm oil fuel
ash
Al2O3 15 Network forming Alumina clays, bauxite
Fe2O3 7 Network forming Slag-iron blast furnace, laterite
CaO 5 Network modifier Slag, limestone
Kaolinite is the major clay mineral often investigated for geopolymer synthesis. It shows low
reactivity through geopolymerization in its natural state and consequently, presents low strength
development [12], [32]. Most of the time, kaolinite is transformed to metakaolin by calcination
between 700-900 °C before being used for geopolymer synthesis [7], [33]–[35]. The calcination
induces the dehydroxylation of kaolinite and formation of aluminosilicate amorphous phase that
is highly reactive in alkaline medium [7], [34], [35]. It is worth noting that besides kaolinite, other
clay minerals such as illite, smectite, halloysite and calcined laterite may be suitable for use as raw
material for preparation of geopolymer binders [8], [36], [37].
Volcanic rocks are some of the oldest construction materials used since the Romans age, due
to their pozzolanic properties. In fact, volcanic materials have been successfully used in modern
construction technology as supplementary cementitious materials (SCMs) characterized to ASTM
C 618, and as lightweight aggregate as per to ASTM C 330 [38], [39]. The most abundant chemical
component in most volcano-related materials is SiO2, which can vary from 35 % to nearly 80 wt%,
followed by Al2O3 usually in the range of 15-20 wt%. Volcanic rocks also may contain other
components such as CaO, Na2O, K2O and Fe2O3 [39]–[41]. Several studies have investigated
volcanic ash pozzolans as aluminosilicate sources for geopolymer synthesis [13], [24], [42]–[44].
Volcanic ash materials generally have fairly low reactivity and are highly variable in physical
properties, chemical and the mineralogical compositions, depending on its geological source [44],
[45]. It should be noted that several other factors influence the engineering properties of volcanic
7
ash-based geopolymers including the reactivity of volcanic ash, curing conditions, mix design, and
type of alkaline solutions. Thus, these parameters can be used to optimize the characteristics of its
geopolymer products [27]. Generally, volcanic ash materials are suitable and sustainable raw
materials for geopolymer synthesis. They are also available in huge amounts in certain
geographical regions, are easily accessible which ensures low cost, and have low environmental
impact [43].
Use of feldspars and igneous rocks for geopolymer syntheses have been reported [5], [10],
[46]. Feldspars are a major constituent of igneous rocks and the most abundant mineral group in
the earth’s crust [28]. Feldspar-based geopolymers generally achieve low compressive strengths
of paste and low degree of reaction due to their mineralogical composition which generally
consists of high crystalline minerals that are weakly reactive [5], [10].
Several industrial by-products have been used to synthesize geopolymer products. They
include fly ash, slags, palm oil fuel ash, rice husk ash and red mud [6], [31], [47]–[49]. A lot of
research has been done on fly ash due to its availability in abundance. The major chemical
components in fly ash are SiO2, Al2O3, Fe2O3, and CaO [50][51]. Much of the published literature
has shown major interest in class F fly ash-based geopolymers. In fact, low calcium content is
preferable for geopolymers production [2], [6], [11]. Furthermore, the presence of free lime CaO
is not recommended as it generates flash set. Also, a high amount of unburnt carbon exceeding 10
% inhibits geopolymerization [17]. Generally, fly ash geopolymerization exhibits low reactivity,
which leads to slow setting and low strength development. In fact, the reactivity of fly ash in an
alkaline environment is controlled by various factors such as the natural pH of the fly ash, particle
size distribution, reactive SiO2 content, glass content and glass composition [11], [52], [53].
However, use of these materials bears the advantage of being economic, with enormous benefits
of low cost and limited negative environmental impact [47], [48].
2.3 Reaction mechanism and mechanical properties
In geopolymer synthesis, the alkaline solution usually consists of a mixture of MOH (M = Na,
K) and alkali-metal (M) silicate solution. Soluble silicates or water glass are commonly made in
industry by alkali-fusion of purified sand (SiO2) with soda ash (Na2CO3) or potash (K2CO3) at
temperatures around 1300 °C, followed by dissolution of the resulting alkali-silicate glass in water
before or after cooling [54]. Another method of producing silicate solutions is by hydrothermal
8
dissolution of a reactive silica source in an alkaline solution [55]. Generally, the chemical
composition of soluble silicates is described by the SiO2/M2O weight ratio (M = Na, K). For
sodium silicates, this SiO2/Na2O weight ratio is quite close to the molar ratio, but both values are
quite different in potassium silicates [54].
Although sodium silicate and potassium silicate solutions appear to have similar properties,
the former is less expensive and most commonly used [56]. However, potassium silicate solutions
present some advantages in their characteristics. For a given SiO2/M2O ratio, the viscosity of
potassium silicate solutions is ten times lower than that of sodium silicate solutions, which is of
benefit in terms of obtaining good workability with less solution [17]. Also, potassium silicate
solutions are less likely to develop efflorescence, i.e. formation of alkali carbonate deposits which
is one of the major issues in sodium-based geopolymers [56], [57]. Finally, materials made from
potassium silicates present better refractory properties [58], [59]. The Geopolymers Institute
recommends the use of low alkaline solution for geopolymerization as the so called ‘‘user-
friendly’’ solution, which typically consists of any soluble silicates with SiO2/M2O ratio greater
than 1.40 [17]. Such use favours the implementation of resulting geopolymer recipes in mass
production which can be easily handled without strong specific safety measures.
Although the geopolymerization mechanism is not fully understood, it generally involves three
main stages [17], [56].
(a) The dissolution of aluminosilicate materials in the alkaline medium through the severing
of covalent Si–O–Si and Al–O–Al bonds and formation of silicate and aluminate species.
This process is initiated by the hydroxyl OH- groups which after bonding to the silicon
atom make the Si-O-Si bond more susceptible to breaking. Dissolution of reactive
aluminosilicates is rapid at high pH [60]. In metakaolin specifically, the dissolution leads
to formation of ortho-sialiate molecule ((OH)3Si-O-Al(OH)-3Na+), the primary unit in
geopolymerization [17].
(b) Condensation of silicate and aluminate monomers to form a gel, until the solution reaches
saturation with liberation of NaOH which reacts again [17]. Duxson et al. [60] claimed
that there is coexistence of two types of gel consisting of an aluminium-rich gel yielding
to the geopolymer matrix, and another gel yielding to crystallized zeolite phases. Also,
this process releases the water that was nominally consumed during the first step [60].
9
(c) Polycondensation and structural re-organization of the gel, yielding to the three-
dimensional aluminosilicate network of geopolymers [17], [60].
Geopolymerization yields products with different physical and/or chemical properties which
depend on the aluminosilicate source material and synthesis conditions. Geopolymers present
some attractive properties of interest, including [3], [17]:
high flexural and compressive strength, such as compressive strength of paste > 90 MPa
at 28 days;
fast hardening with some geopolymer cements achieving 70 % of their final compressive
strength within 4 h;
high thermal stability with mass loss < 5 % and strength loss < 60 % at 1000 °C;
effective passivation of reinforcing steel;
strong adhesion to metallic and non-metallic surfaces;
low permeability;
Dimensional stability in service with shrinkage as low as 0.2 to 0.4 %;
long-term durability properties;
low CO2 footprint.
2.4 Starting materials
It is well known that the properties of geopolymers are affected by the nature of raw materials
such as their mineralogical and chemical composition, particle size, and glass content [5], [11],
[52], [53]. The release rate of silica and alumina from source materials plays a significant role in
the development of geopolymer network and its final properties [61], [62]. High silica availability
leads to more contribution from Al in the geopolymer matrix and promotes more homogenous
geopolymer binder gel [61]. At early stages of the reaction, rapid release of alumina is shown to
impede the dissolution of silica particles. It is known that a more homogeneous gel is observed
with slower alumina release [63]. Hajimohammadi et al. investigated the dissolution rate of some
geopolymer precursor materials using a kinetic approach. They observed that dissolution rates of
silica-rich materials is much higher than that of aluminosilicates. Also, among the aluminosilicates
studied, metakaolin showed a distinctively higher release of Si species from the very early hours
of dissolution while release rate of Si and Al species was almost similar in fly ash and slag. They
10
also demonstrated that milling of fly ash increases the average dissolution rate of Si species much
more than for Al species. But the opposite trend is observed with slag where milling rapidly
increases the release rate of Al while the release rate of Si is increased only slowly [62]. Generally,
highly crystallized minerals present weak dissolution in alkaline medium [5], [10], [12]. Heal et
al. [12] observed that kaolin-based geopolymers do not undergo complete geopolymerization and
show slow strength development, being the limitation by the structure of kaolin, which has
kaolinite stacks and plates of low surface area.
Fernández-Jiménez et al. [52] investigated the effect of chemical composition of fly ash on
mechanical properties of resulting geopolymers. They found that fly ash with a high content of
reactive SiO2 and Al2O3, and lower SiO2/Al2O3 ratio showed the best mechanical strengths.
Tennakoon et al.[64] found that fly ash materials with reactive SiO2/Al2O3 molar ratio > 3.3
induced formation of zeolitic phases, resulting in low strength development. They also reported
that there was a poor correlation between the total SiO2/Al2O3 ratio and compressive strength
development. De Silva et al. [65] stated that for metakaolin-based geopolymer, the SiO2/Al2O3
ratio in the range of 3.4 - 3.8 showed high strength development at later ages. Chindaprasirt et al.
[66] reported that during the geopolymerization of calcium-rich fly ash (ASTM Class C), the
setting process is controlled by initial calcium-aluminate-silicate-hydrate (C-A-S-H) formation
while sodium-aluminate-silicate-hydrate gel (N-A-S-H) formation, contributes to strength
development. Also, SiO2/Al2O3 ratio in the range of 3.2–3.7 showed products with best mechanical
properties. Zibouche et al. [67] studied the influence of secondary minerals on the
geopolymerization reaction of metakaolin. They showed that kaolin deposits containing up to 30
% secondary minerals are still suitable for geopolymer synthesis, under moderate curing
temperatures. Temuujin et al. [68] reported that the reduction of particle size and change in
morphology after milling, led to a higher dissolution rate of fly ash particles, yielding 80 %
increase in compressive strength when compared with the geopolymer mixtures made from non-
milled fly ash.
3. Reactivity of aluminosilicate raw materials
Properties of geopolymers can be significantly affected by minor changes in the amount of Si
and Al available for the reaction [62], [64], [65]. Therefore, the reactivity of aluminosilicate
11
materials through geopolymerization can be defined by its ability to suitably release alumina and
silica species in alkaline medium for further development of rigid 3D geopolymer network. In
geopolymer synthesis, compressive strength of the resulting product is commonly used as a
quantitative indicator of geopolymeric reactivity of raw materials [69], [70]. Several attempts have
been made to assess reactivity of aluminosilicate materials and determine their potential for
geopolymerization [29], [64], [71]. Of the common oxides in geopolymer chemistry i.e. SiO2,
Al2O3, Fe2O3 and CaO; alumina and silica are the two major oxides which control the
geopolymeric network formation and strength development [29]. Studies have attempted to
evaluate source materials based on their bulk chemical composition but it has been demonstrated
that there is poor correlation between the total SiO2 or Al2O3 content of the raw materials, and
compressive strength development [12], [29], [44], [64]. This implies that prediction of
geopolymeric reactivity based on the bulk chemical composition of the starting materials, may not
be of much value.
Usually, the inter-relationship between chemical composition and reactivity of individual
minerals is extremely complex [5]. The mineral content, glassy phase and composition of the
starting materials all play significant roles in geopolymeric reactivity [44], [71]–[74]. Tchakoute
et al. [75] reported the effects of mineralogical composition of two volcanic ash types on their
geopolymeric reactivity. They found that the presence of structural water from muscovite which
is a clayey mineral, in one type of volcanic ash was responsible for its higher reactivity. Similarly,
Zhan et al. [76] investigated the effects of halloysite in kaolin on the formation and properties of
geopolymers. The isothermal conduction calorimetry (ICC) results showed that the presence of
halloysite in kaolin led to a higher geopolymerization rate of metakaolin.
It is widely accepted that amorphous phases are the reactive components in the
geopolymerization reaction [71], [77], [78]. Usually, most crystalline phases hardly dissolve
during geopolymer reactions [5], [10], [12], [71]. The bulk amorphous composition can be
determined using a combination of X-ray fluorescence spectroscopy (XRF) and quantitative X-
ray diffraction (QXRD) [78]. Also, quantification of amorphous content in raw materials can be
made using alkaline or acidic dissolution techniques [71], [73]. Williams and Van Riessen [78]
reported that the use of amorphous composition for geopolymer mix design gave products with
better compressive strength than those formulated using the chemical bulk composition. Djon Li
12
Ndjock et al. [44] investigated the effects of amorphous composition of five volcanic ash types on
properties of resulting geopolymers. They observed that best compressive strengths were obtained
from samples containing a high amount of amorphous phase and low SiO2/Al2O3 molar ratio of
amorphous phase. These findings agree with results of similar work done on fly ash by Fernández-
Jiménez et al. [52]. Fig. 1 suggests potential uses of volcanic ash, according to composition and
the amount of amorphous phase.
Fig. 1. Utilization of volcanic ash based on amount of amorphous phase and composition [44]
(reproduced with permission from Elsevier).
It has also been reported that particle size distribution of aluminosilicate materials is a key
factor in the process of geopolymer synthesis. Generally, the improvement of geopolymeric
reactivity is observed with reduction in particle size [68], [79]–[81]. Temuujin et al. [68]
investigated the effects of mechanical activation of fly ash on physical properties of geopolymers.
They observed that a reduction of the median size (d50) from 14.4 µm for the raw fly ash to 6.8
13
µm for the milled fly ash, led to 80 % increase in compressive strength of the resulting geopolymer
paste. This increase in compressive strength is attributed to reduction of particle size and change
in morphology, allowing a higher dissolution rate of the fly ash particles [68]. It is important to
note that the aforementioned factors affecting the geopolymeric reactivity of aluminosilicate
materials are inter-related. Each factor plays a significant role but may not singularly be sufficient
for good strength development. It is suggested that the effects of these factors may not be
considered individually when a specific raw material is studied.
4. Techniques for increasing reactivity of aluminosilicate raw materials
The geopolymeric reactivity of source materials is a major parameter for its utilization. A great
number of natural aluminosilicates and industrial by-products, can be used for geopolymer
synthesis, as discussed in Section 2.2. Nevertheless, a large number of potential starting materials
have poor reactivity and consequently yield products with low mechanical strength. Improving
reactivity of geopolymers is essential for their potential structural use in concrete, while promoting
utilization of locally available source materials to mitigate environmental issues of landfill and
disposal of some industrial wastes. In the literature, several techniques have been investigated to
alter reactivity of aluminosilicate raw materials. These activation methods may be divided into
four categories consisting of mechanical, thermal, physical separation and chemical activation, as
given in Fig. 2. A combination of these treatments can be performed.
14
Fig. 2. Summary of activation methods.
4.1 Mechanical activation
Mechanical activation can be defined as a process that increases the reactivity of a solid by
imparting mechanical energy, without altering chemical composition [82]. If the activation
simultaneously leads to change in composition or structure, it is a mechanochemical reaction. The
increase in reactivity during mechanical treatment is usually a result of disordering of the crystal
and generation of defects or other metastable forms that cause decrease of activation energy barrier
for the process [83]. The primary effect of mechanical activation is the reduction of particle sizes,
causing changes in physical properties such as particle size distribution, specific surface area,
surface energy, and phase composition [84][85]. The mechanism of transformation occurring
during mechanical activation of materials can be divided into three main stages which depend on
the degree of dispersion and grinding time, as illustrated in Fig. 3 [86].
15
Fig. 3. Theoretical diagram of the grinding process [86] (reproduced with permission from
Elsevier).
(a) At stage 1, grinding initially leads to relatively rapid particle size reduction and increase in
specific surface area. The energy consumed during grinding is proportional to the particle
surface area produced. This stage is referred to as Rittinger’s stage.
(b) At stage 2, grinding continues and as particles get finer, they begin to adhere on surfaces of
grinding media and mill, as well as on each other. The energy used for size reduction is no
longer proportional to the increase of surface area. Despite this non-linearity, the increase in
dispersion can still be remarkable. This is considered to be the aggregation stage.
(c) At stage 3, more grinding does not lead to further size reduction but may cause decrease in
the degree of dispersion in certain materials. In this stage of agglomeration, the crystal
structure or even the chemical composition of the material changes.
During comminution of solid matter, adhesion phenomena become more and more important
with decreasing particle size causing aggregation, agglomeration, coating, caking, and build-up
[87]. Aggregation is a reversible process of particle adhesion due to van der Waals forces.
Agglomeration, on the other hand, takes place following long durations of grinding. It is caused
16
by strong, irreversible interparticle bonds [84]. Adhesion during grinding is always undesirable as
it diminishes the effect of grinding, lengthens the grinding time, and increases the energy
requirement [87]. While aggregation mostly influences the progression and effectiveness of the
grinding process, agglomeration has a detrimental effect on certain properties of the ground
material [86]. However, adhesion can be avoided by using dispersion agents, also called grinding
aids [84], [87].
Mechanical activation is usually carried out using devices called mills, which may be of
different characteristics. Several types of mills have been designed for specific purposes related to
particle size reduction of products and to milling efficiency. For instance, mills used for fine
grinding operations to particle sizes below 100 µm, include roller mills, impact mills, ball mills,
agitation mills, jet mills, shear-type mills, and colloid mills [88]. Depending on the type of mill,
the stresses occurring during milling may include compression, shear (attrition), stroke impact and
collision impact [82]. Ball mills are commonly used in the laboratory and industry for particle size
reduction of most aluminosilicate materials such as rocks, ores, cement, fly ash, etc. It has been
reported that vibratory mills, characterized by high energy density, provide a higher rate and extent
of grinding efficiency compared to the normal rotary ball mills [89].
The efficiency of milling can be affected by many factors such as type of mill, types of grinding
media such as balls, rods or other shapes, material of milling media e.g. stainless steel, tungsten
carbide, zirconium oxide, aluminium oxide, silicon nitride; grinding atmosphere such as air, inert
gas, reductive gas; wet or dry milling atmosphere, fill level of the milling chamber, ball to powder
ratio (BPR), grinding temperature, speed of the mill and grinding time [82].
Mechanical activation has been found to improve the reactivity of solids in many materials
processing operations such as extractive metallurgy, building materials, food, and chemicals [83],
[90]–[92]. Several literatures have reported the effects of mechanical activation of raw materials
on properties of geopolymers [11], [80], [81], [85], [93]–[97]. From reported data, the
geopolymeric reactivity of mechanically activated materials depends on factors such as type of
mill, milled material, milling time and milling atmosphere [80], [95], [96], [98]. Kumar et al. [99]
prepared geopolymer materials from mechanically activated fly ash. The alkaline activator was a
solution with 20 % NaOH concentration and the liquid/solid ratio was 0.5. They found that
geopolymerization of mechanically activated fly ash occurred at a lower temperature and setting
17
time, as compared to the reaction of raw fly ash. The effect of mechanical activation was influenced
by the mill type. Moreover, ground materials from vibratory mill achieved better compressive
strength than those obtained using attrition mill. The improvement in compressive strength was
found to be related to increased reactivity and the resulting formation of compact microstructure.
Compressive strength of up 120 MPa was obtained with 50 mm cube geopolymer pastes made
using mechanically activated fly ash. These results were in agreement with findings of Temuujin
et al. [68], who also observed increased reactivity in mechanically activated fly ash during the
synthesis of geopolymers cured at ambient temperature. In their study, a mixture of 14 M NaOH
and sodium silicate solution was used as activator. Their results indicated that mechanical
activation led to reduction of particle size and change in particle shape, allowing a higher
dissolution rate of the fly ash particles. Mechanical activation led to 80 % increase in compressive
strength of 25 x 50 mm paste cylinders over strengths of geopolymers obtained from raw fly ash.
On the other hand, mechanical activation of the fly ash destroyed some of its spherical morphology
and reduced the “ball bearing effect” which reduces workability. Fig. 4 (a) and (b) show the
microstructure of raw fly ash and milled fly ash, respectively. Also, faster setting was observed
with mechanically activated fly ash. This was mitigated by addition of extra water in the starting
mixture.
Fig. 4. Morphology of raw (a) and milled fly ash particles (b) [68] (reproduced with permission
from Elsevier).
18
Mucsi et al. [96] investigated the correlation between grinding process, ground materials, and
geopolymer properties. A 6 M solution of NaOH was used as activator with a mix of 0.65
liquid/solid ratio. Mechanical activation was carried out using three different types of laboratory
scale mills comprising the conventional tumbling ball mill, vibratory mill, and stirred media mill.
The finest ground product produced with lowest energy input was obtained using the stirred media
mill. It was reported that the compressive strength of the geopolymer samples strongly depended
on the type of mill and milling time. The compressive strengths of paste obtained from the fly ash
that was activated in stirred media mill, vibratory milled, and ball milled were 22, 16 and 15 MPa,
respectively. The superiority in strength of the geopolymer product from activated fly ash that was
milled using stirred media mill, was attributed to its particle size distribution which reportedly
varied from superfine particles of <1 μm to coarser particles of size 200–300 μm. Its median
particle size was 5.2 μm. Finer particles exhibit better solubility resulting in more geopolymer gel
while coarser particles behave as “aggregate” in the geopolymer matrix. Also, the filler effect of
the superfine unreacted size fraction <1 μm leads to a more compact microstructure.
Kumar and Kumar [11] investigated the effects of milling duration conducted at 5, 10, 20, 30,
45, 60 and 90 mins, on the reaction kinetics of geopolymerization as well as on the characteristics
of resulting products. A solution of 6 M NaOH concentration was used as the alkaline activator.
The reactivity of fly ash increased with reduction in median particle size and with increase in
milling time. Mechanical activation enhanced the rate of geopolymeric reactivity and decreased
the setting time when the median particle size was reduced to less than 5–7 µm. Results of
isothermal conduction calorimetry studies conducted at 27 °C, showed evidence of
geopolymerization of mechanically activated fly ash, characterized by a broad exothermic peak.
The improvement in physical properties of the geopolymer products was correlated with median
particle size reduction and change in reactivity of fly ash. Similarly, Djobo et al. [80] studied the
effects of extended milling of volcanic ash done using vibratory mill, on geopolymeric reactivity.
The alkaline activator used was sodium silicate solution with SiO2/Na2O molar ratio of 1.45. X-
ray diffraction (XRD) results showed that mechanical activation considerably reduced the degree
of crystallinity for samples that were milled for 90 min. This was followed by change in the
mineralogy of samples milled for 120 min. The rate and extent of geopolymerization increased
with milling time. A 100 % increase in compressive strength was observed after the mechanical
treatment. Thus, mechanical activation may be considered as a suitable method for improving the
19
reactivity of raw volcanic ash. Wei et al. [81] showed that the silicon and aluminium species from
mechanically activated materials, increasingly dissolved in alkaline medium the longer the milling
time, as seen in Fig. 5. It was reported that the development of strength for milled material-based
geopolymers was related to the amount of reactive species i.e. Si and Al, in the ground materials.
Higher dissolution of Si and Al species substantially improved the compressive strength.
Fig. 5. Content of active Si and Al in tailings subjected to different milling times [81]
(reproduced with permission from Elsevier).
Other studies have reported the negative effect of prolonged grinding. Mucsi et al. [100]
claimed that during mechanical activation of lignite fly ash in stirred media mill, the compressive
strength of the geopolymer samples increased with milling time up to a certain fineness. After that,
further grinding led to decrease in compressive strength. Earlier work by Mucsi et al. [96] gave
similar results. Fig. 6 shows that indeed the strength enhancing effect of grinding is only effective
to a certain level of material fineness achieved. Extreme fineness, due to a prolonged duration of
grinding, may not improve mechanical properties any further. Prior to using a starting material as
a geopolymer, it seems essential that this optimum fineness should be determined to avoid potential
adverse effects of over-grinding.
20
Fig. 6. Variation of compression strength as a function of grinding time (constructed with data
from Ref. [96]).
Mechanical activation was also found to be effective for less reactive kaolin, achieving
products with good compressive strengths [32], [101]. Hounsi et al. [32] observed the partial
amorphization of kaolin during mechanical activation, as indicated by intensity reduction of main
characteristic peaks of XRD patterns for the milled samples. Thus, milling changed the
crystallinity degree of the raw kaolin and promoted geopolymerization reaction, leading to increase
in compressive strength. Results showed that without mechanical activation, the optimal curing
condition was 24 h at 70 °C for which the compressive strength of 50 mm cube pastes after 28
days of curing, was 15 MPa. Mechanical activation led to 35 % strength increase for the same
curing and testing conditions. The alkaline activator used was a mixture of sodium silicate solution
and 8 M NaOH solution at the Na2SiO3/NaOH mass ratio of 0.32. Heah et al. [101] conducted
scanning electron microscopy (SEM) study and reported that the plate-like structure of kaolin
reduced after mechanical treatment. In their study, the milled kaolin showed a smoother surface
and exhibited edge distortion of particles. Mechanical activation improved the geopolymerization
process. Extended milling time increased compressive strength while the microstructure of the
21
geopolymer products became more compact and denser. Fig. 7 shows the micrographs of milled
kaolin geopolymers [101].
Fig. 7. SEM micrograph of mechanically-activated kaolin geopolymers for different milling
times (a) 1 hour, (b) 3 hours, (c) 5 hours [101] (reproduced with permission from Ref. [101]).
Milling atmosphere is also an important parameter that affects the geopolymeric reactivity of
milled materials. Preliminary studies by Kalinkin et al. [102] have shown that mechanical
activation of Cu–Ni slag in CO2 gives higher compressive strength for the geopolymer samples,
compared to its mechanical activation in air. Sodium silicate solution with a SiO2/Na2O ratio of
1.5 was used as alkaline activator. The higher geopolymeric reactivity of samples milled in CO2
was attributed to chemisorption of CO2 molecules in the form of distorted carbonate ions during
mechanical activation. This phenomenon promoted more intensive interaction of alkaline solution
with activated slag particles. After 360 days, compressive strength was 89.5 and 119.0 MPa for
14.1 mm cube geopolymer mortars prepared using slag that was activated in air and in CO2,
respectively [102]. A similar trend in which greater compressive strength of geopolymer mortars
was obtained using CO2-milled slag relative to air-milled slag, was reported for zinc slag [98].
A recent study investigated the influence of mechanical activation of fly ash on the micro- and
nano-structure of geopolymers using transmission electron microscopy (TEM-EDS). They found
that milling mostly decreased the amount of the amorphous aluminosilicate rather than crystalline
phases present in fly ash, as shown in Fig. 8 [103]. Thermodynamically, glass has higher free
energy than crystalline phases, thus the introduction of additional energy by milling results in more
particle breakage and alteration. A minor effect was observed on mullite that is at nanometric scale
and is embedded in the amorphous phase. It was demonstrated that mechanical activation
promoted the formation of geopolymer gel, mainly N-A-S-H gel with an increase in its nano-pore
size. Also, isothermal conduction calorimetric study showed that the early reaction corresponding
22
to dissolution/precipitation had a linear relationship with milling time, but not with median particle
size. A conceptual mechanism of the reaction was also proposed, as illustrated in the schematic
diagram of Fig. 9 [103].
Fig. 8. Quantitative phase analysis showing variation of crystalline and amorphous phases with
milling time [103] (reproduced with permission from Elsevier).
23
Fig. 9. Conceptual mechanism of the effect of milling on fly ash particle characteristics and
geopolymerization [103] Q = Quartz, M= Mullite, G = Glass (reproduced with permission from
Elsevier).
Generally, mechanical activation of the raw material leads to a reduction in particle size,
increase in specific surface area, decrease in crystallinity, change in the mineralogical composition
and increase of the amorphous phase. These effects promote greater dissolution of the milled
materials in alkaline solution, consequently enhancing the geopolymerization reaction [11], [80],
[94], [104]. As a result, there is improvement in properties of products made from mechanically
activated geopolymer materials [93], [95], [97]. Therefore, mechanical activation can be
considered as an effective technique for improving the properties of geopolymer products. The
technique may be employed to beneficially utilize less reactive waste materials that are typically
disposed-off as landfills [81], [85], [96].
4.2 Activation by physical separation
This process which gives unmodified particles is based on physical properties of the materials
such as density, particle size, shape, surface properties, electrical and magnetic properties [105],
24
[106]. Activation by physical separation of particle fractions includes use of techniques such as air
classifier, flotation, sieving, sorting, clarification, magnetic separation, etc. [87], [106], [107] The
method is widely used in mineral processing for beneficiation purposes [108], [109]. It has been
shown that fly ash behavior can be tailored by sieving and magnetic separation [110]. For instance,
blended portland cement from magnetic and non-magnetic fly ash fractions display different
characteristics [107], [111]. The beneficial effects of employing the separation method, have been
reported in relation to use of fine fractions of fly ash in preparing blended portland cements [112],
[113]. Several attempts have been made in recent decades to study the effects of separation method
of activation, on the properties of geopolymers. Kumar et al. [114] argued that the geopolymeric
reactivity of fly ash was altered by air classification using a laboratory classifier operated at 10000
rev/min. An increase in the glass content of finer fractions obtained by air classification was
observed relative to raw fly ash, resulting in increase of compressive strength of its geopolymer
pastes. Chindaprasirt et al.[115], also observed that after separation of high-calcium fly ash using
air classifier, the amorphous phase content in the fine fraction was higher than in the coarser
fraction. A mixture of Na2SiO3 solution and 10 M NaOH solution was used as alkali activator. It
was demonstrated that the setting time of geopolymer pastes decreased with an increase in fly ash
fineness. There was improvement in workability, strength, and drying shrinkage properties of
mortars made using the fine fly ash. Geopolymer mortars giving 28-day compressive strength of
up to 86.0 MPa were obtained using 50 mm size cube specimens.
Nugteren et al.[116] prepared geopolymer pastes using six different size fractions of fly ash
obtained by air classification. The alkaline solution used was potassium silicate solution with
SiO2/K2O molar ratio of 1.25. They found that physical separation affected the pH, bulk chemical
and mineralogical composition of the fractions, as well as their particle size distributions. The pH
obtained for the various size fractions varied from 10.2 for the finest fraction to 12.5 for the raw
sample. The workability and setting time of the paste were also influenced by size fractions.
However, the relationship between particle size and strength was not well established, although
the variations in chemistry and pH of the different fractions could play a role in strength
development. Compressive strengths of over 100 MPa for cylinder specimen of sizes 29 mm
diameter x 25 mm height, were obtained in this study. It is worth noting that when the different
size fractions of the fly ash were obtained by grinding, their pH values, bulk chemical and
mineralogical compositions showed no notable differences. Kumar et al. [117] investigated the
25
geopolymerization activity for different fractions of fly ash, separated into various sizes then
drawn from hoppers of an electrostatic precipitator. They found that the different size fractions of
fly ash collected, showed marked variations in chemistry, mineralogy, particle size distribution
and glass content. From chemical analyses, the SiO2/Al2O3 ratio increased with increase in
fineness, whereas Fe2O3, CaO and loss on ignition decreased in finer fractions. A linear correlation
between geopolymeric reactivity and glass content was observed. Strength development was
attributed to the combined effects of SiO2/Al2O3 ratio, particle size, and glass content.
Microstructural studies done using SEM-EDS on hardened geopolymer pastes, revealed the
presence of more reaction products (i.e. N-A-S-H gel) in finer fractions while unreacted particles
were prevalent in coarser fractions.
4.3 Chemical activation
Chemical activation is a method of improving reactivity of a material system by addition of
admixtures. Concrete admixtures are materials other than hydraulic cement, water, or aggregates
that are added immediately before or during mixing [118]. These admixtures enhance the
properties of portland cement concrete in the fresh and/or hardened state [119]. There are several
types of admixtures used in concrete technology, broadly categorized as chemical or mineral
admixtures [118]. Chemical admixtures are generally water soluble, and are added in small
amounts to control various properties of fresh or hardened concrete. The different types of
chemical admixtures include accelerators, water reducers, superplasticizers, and retarders [118].
Mineral admixtures are fine ground solid materials usually categorized as filler materials,
pozzolans or SCMs, latent hydraulic materials [120]. The most common SCMs are fly ash, ground
granulated blast furnace slag, natural pozzolans and silica fume [119]. Incorporation of mineral
admixtures as SCMs in concrete may reduce or increase strength, improve durability, decreased
the heat of hydration in mass concretes, reduced cost [119], [120]. Numerous studies have reported
the use of chemical and mineral admixtures in the synthesis of geopolymers and their effects on
properties [121]–[124]. The present literature review is limited to ingredients that may be used to
improve the geopolymeric reactivity of the raw aluminosilicate materials. The activation of
geopolymer materials using chemical admixtures is usually carried out using two methods namely,
blending and alkaline fusion.
4.3.1 Blending technique
26
The conventional blending technique involves combining the raw material with a chemical
additive by mixing the two. This technique adjusts the bulk chemical composition of the raw
material, causing a change of its geopolymeric reactivity. The compositional changes typically
involve modification in the amount of mainly SiO2, Al2O3, and CaO oxides. Olalekan et al. [125]
investigated the effects of Al(OH)3 addition on the properties of slag/ultrafine palm oil fuel
ash geopolymer system. A combination of 10 M NaOH and Na2SiO3 solution were used as
alkaline activator. The mass ratio of sodium silicate to NaOH of 2.5, was employed. They found
that the inclusion of Al(OH)3 in the range of 2–6 wt.%, led to change in SiO2/Al2O3, H2O/Na2O
ratios, and compressive strength of the geopolymer products. Microstructural analysis revealed
that there was increase in Si–Al substitution, amorphous gels, and carbonation with the inclusion
of Al(OH)3 in the mixture. Compressive strength increased with addition of up to 4 wt.% Al(OH)3.
A three-day compressive-strength of 42 to 49.5 MPa was achieved with addition of 3–4 wt.%
Al(OH)3. Similar response of volcanic ash geopolymer to addition of alumina (Al2O3) has been
reported [126]. In the study, a mixture Na2SiO3 solution and 12 M NaOH solution at
Na2SiO3/NaOH mass ratio of 2.0, was used as alkaline activator. It was found that adding Al2O3
at an optimal value of 40 wt.% improved the extent of geopolymerization and led to a 32.4 %
increase in the 28-day compressive strength of 31 mm diameter x 62 mm height cylindrical paste
specimens [126].
Recently, it has been shown that incorporation of nano-particles can significantly enhance the
properties of geopolymer materials [127], [128]. For example, adding 1–2 % nano-SiO2 and nano-
Al2O3 as additive to fly ash-based geopolymers produced additional calcium-silicate-hydrate (C-
S-H) or C-A-S-H and N-A-S-H phases, which shortened the setting time and improved mechanical
properties [129]. Also, Adak et al. [130] reported that the addition of nano-silica to fly ash-based
geopolymer concrete enhanced the geopolymerization process at ambient temperature, by
increasing the dissolution rate of Si and Si–Al phases. A 12 M NaOH solution mixed with Na2SiO3
solution at Na2SiO3/NaOH mass ratio of 1.75 was used to prepare the alkaline activator. The nano-
silica modified geopolymer concrete showed better structural performance than heat-cured
geopolymer concrete containing conventional cement concrete samples, as shown in Fig. 10. It has
been suggested that heat curing of fly ash-based geopolymers to achieve desired strength, could
be avoided by addition of appropriate amount of nano-silica in the mixture [130].
27
Fig. 10. Compressive strength of nano-silica modified geopolymer (12GC6), heat cured
geopolymer concrete (12GC0H) and control cement concrete (CC) at varied ages [130]
(reproduced with permission from Elsevier).
The potential use of calcium-based compounds to enhance geopolymerization has also been
investigated [131]–[133]. It has been suggested that adding a small proportion of CaO or Ca(OH)2
to class F fly ash may improve the dissolution of the fly ash in the alkaline medium, which in turn
increases the rate and extent of geopolymerization reaction but this effect is dependent on curing
temperature. Addition of 3 wt.% CaO or 3 wt.% Ca(OH)2 increased the 7-day compressive
strengths from 11.8 MPa to 22.8 or 29.2 MPa, respectively at ambient temperature but gave very
low strengths at elevated curing temperature of 70 °C. Calcium hydroxide seemed to be a more
beneficial additive than calcium oxide [134].
Phummiphan et al [135] showed that recycled calcium carbide (CaC2) residue can be used to
develop geopolymer binders. It was found that CaC2 promoted geopolymerization and improved
early strength. To obtain the highest 90-day compressive strength of fly ash-based geopolymers,
optimum CaC2 replacement limited to 20 % was recommended. Phetchuay et al.[136] also
investigated the effects of CaC2 addition into fly-based geopolymers. Use of 12 % CaC2 addition
improved the strength of the geopolymer products by up to 1.5 times.
28
As mentioned previously, geopolymer reactivity of raw materials can also be modified by
mineral additive. Generally, it is a reactive material such as metakaolin or slag, which is added to
a less reactive geopolymeric system. This process also influences the bulk chemical composition
of the geopolymer precursor. Several successful attempts have been reported on the beneficial
effects of mineral admixtures to geopolymerization enhancement [6]. It has been found that adding
ground granulated blast-furnace slag (GGBS) to fly ash altered the reactivity of the latter, resulting
in reduction of the setting time and increase in compressive strength of the resulting geopolymer
composite [137]–[139]. The improvement in compressive strength and setting time with slag
addition was attributed to the formation of C-A-S-H and N-A-S-H gel phases, which co-exist.
Also, development of a compact microstructure was observed [6][140][141][139]. Xu et al. [137]
investigated the effect of blast furnace slag grades on fly ash based geopolymer waste. Their results
showed that at a higher grade, GGBS generated more hydration heat, suggesting higher reactivity
and geopolymerization.
Davidovits et al. [142] developed slag-fly ash-based geopolymer cements that hardened at
room temperature. Potassium silicate solution with the SiO2/K2O molar ratio of 1.25 was used as
alkaline reagent. They showed that the main contributors to the geopolymer matrix, which are
consequently responsible of the compressive strength development, were the glassy fly ash
particles supplying Si and Al, the high Ca slag supplying Ca, and the alkaline solution providing
K-silicate. The geopolymer cements produced in their study [142] showed compressive strengths
of 29 mm diameter x 25 mm height cylinder pastes of up to 95 MPa after 28 days of curing. It also
exhibited long-term durability properties. Similarly, Salih et al. [143] demonstrated that high
strength geopolymer binders made using palm oil fuel ash (POFA) and GGBS could be
synthesized at ambient temperature. In the study, a combination of Na2SiO3 and NaOH, was used
as activator. Addition of GGBS as partial replacement of POFA increased the compressive strength
at all ages. Up to 65 % increase in compressive strength of 50 mm cube paste samples was achieved
when 50 % of POFA was replaced with GGBS. Ye et al. [144] investigated the properties of
geopolymers synthesized by blending of 70 % calcined tailings and 30 % slag. The alkaline
solution consisted of Na2SiO3 solution modified using NaOH to SiO2/Na2O molar ratio of 1.8. The
microstructure and strength development for the geopolymer mortars cured at ambient temperature
were monitored over a period of 6 years. Results indicated that hardened geopolymer mortars
became more compact with advancing age, due to progressive geopolymerization attributed to
29
formation of coexisting C–A–S–H and N–A–S–H gels. Compressive strength of 40×40×160 mm
prismatic mortars significantly increased from 50.0 MPa at 28 days to 75.0 MPa at 6 years.
Carbonation was observed in both the early-age and long-term geopolymers, but deterioration of
the system was inhibited by the stable N–A–S–H gels which were the predominant geopolymeric
products.
The influence of highly reactive metakaolin as substitute in less reactive geopolymeric system
has been widely investigated. Robayo-salazar et al. [145] observed that the partial replacement of
natural pozzolans by metakaolin of up to 20 wt.% significantly improved the compressive strength
of the mixtures. This enhancement of mechanical properties was attributed to an increase in the
amount of amorphous SiO2 and Al2O3. The addition of metakaolin promoted the formation of a
more stable geopolymeric gel with lower porosity. A combination of NaOH and sodium silicate
solution was employed as activator with different SiO2/Na2O molar ratio in the range of 0.55 to
2.78. A 28-day compressive strength of up to 68 MPa was obtained under room temperature curing
on 20 mm cube pastes. These results agree with those obtained by Djobo et al. [9] who claimed
that the reactivity of volcanic scoria could be altered by its partial replacement by metakaolin to
compensate for the deficiency of amorphous SiO2 and Al2O3 in the volcanic scoria. The alkaline
activator used in the study was a mixture of 12 M NaOH solution and Na2SiO3 solution with two
SiO2/Na2O molar ratios of 1.1 and 1.4. In their study, 5 to 25 % metakaolin was added, resulting
in higher dissolution of Al2O3 and SiO2 species, polycondensation, increase in compressive
strength and decrease in setting time were reported. The compressive strengths of 31 mm diameter
x 62 mm height cylinder pastes at 28 days were up to 68.8 MPa.
Similarly, Ogundiran and Kumar [146] investigated the effects of calcined clay addition to fly
ash reactivity, physical and mechanical properties of geopolymer composite. A mixture of 8 M
NaOH solution and Na2SiO3 solution at NaOH/Na2SiO3 mass ratio of 1, was used as activator. It
was found that calcined clay addition accelerated dissolution/hydrolysis of fly ash, while fly ash
controlled the exothermic reaction that accompanied alkaline dissolution and hydrolysis of
calcined clay. Addition of 25 wt.% calcined clay improved the early and late-age strengths of the
geopolymer pastes. They concluded that the rate of geopolymer formation and the compressive
strength of the less reactive silica rich geopolymer source materials, can be improved by the
addition of calcined kaolin clay.
30
4.3.2 Alkaline fusion
Geopolymeric reactivity can be also improved by alkaline fusion of the raw materials. In this
method, the mixture of raw material and sodium hydroxide is calcined at a temperature higher
than the melting point of NaOH prior to the geopolymerization [10], [75]. This method involves
decomposing of raw materials during the synthesis of zeolites [147], [148]. Xu et al. found that
alkali fusion process promoted the dissolution of Si and Al species from the low reactive fly ash
materials, and thus improved their reactivity. Alkali fusion of fly ash was achieved by thoroughly
mixing the raw material with NaOH pellets at a low alkali/ash mass ratio of 0.5 in a ball mill for
10 min, followed by fusing the resulting mixture in a muffle furnace at 550 °C for 2 h at a heating
rate of 15 °C/min. The fused fly ash materials were air cooled to ambient temperature and then
ground in the ball mill for another 10 min to obtain a homogeneous sodium silicate-rich mixture
[149]. Studies indicate that alkali fusion modifies the mineralogical composition of the raw
material and induced the formation of an amorphous phase and/or new crystalline phases [10],
[150].
Tchadjie et al. [10] demonstrated that the amount of reactive phase in the fused geopolymer
mixture significantly increases with increase in the amount of Na2O added during the fusion
process. Na2SiO3 solution with silica modulus of 3.2 was used as alkaline activator. The results
showed that there is an increase in the geopolymeric reactivity of the aluminosilicate raw material
after the treatment. Tests on 20 mm cube specimens showed that the compressive strengths of
resulting geopolymer mortars varied between 6.25 and 40.5 MPa, depending on the amount of
Na2O used during the alkali fusion process, as shown in Fig. 11. It is worthwhile noting that excess
amount of Na2O is detrimental to strength development and durability properties of geopolymers
made from fused materials [150], [151]. Generally, a reactive Al2O3-rich material is used as
additional aluminosilicate source to consume the excess alkali from the fusion process [149],
[152]. Tchakoute et al. [152] investigated the effect of metakaolin addition on geopolymeric
reactivity of fused volcanic ash. Na2SiO3 solution of silica modulus of 3.2 was used as alkaline
activator. It was found that addition of metakaolin in the range of 30-60 % increased the amount
of reactive phase formed, resulting in the dissolution of more silicon and alumina species. In turn,
geopolymerization was enhanced and compressive strength of the geopolymer product increased.
31
A linear correlation between compressive strength and metakaolin content was observed.
Replacement of fused materials with up to 60 % metakaolin showed the best performance.
Fig. 11. Increase in 28-day compressive strength of geopolymer mortars with addition of various
amounts of Na2O used: MGf1-10%, MGf2-20%, MGf3-30%, MGf4-40%, MGf5-50%, MGf6-
60% [10] (reproduced with permission from Elsevier).
It is worthwhile mentioning that the development of geopolymers as a one-part ‘‘just add
water’’ mixture, as done in portland cement mixtures, has been reported to be possible based on
alkali fusion activation of the aluminosilicate materials as precursor. Usually, in this method, the
alkali-thermal treatment is carried out at high temperatures of about 900 °C. The resulting
geopolymer systems exhibit similar performance as conventional geopolymers [153]–[155].
However, there is limited research on this alternative and promising approach of one-part
geopolymeric materials [151], [156], [157].
4.4 Thermal activation
The thermal activation process modifies physicochemical properties of a material through
heating [158]. The heat treatment is conducted in three major stages consisting of: heating to a
32
specific predetermined temperature (stage 1), maintaining this temperature for required time (stage
2) and finally, cooling (stage 3) [159]. During heat treatment, phase transformations occur in the
material, resulting from either loss of volatile components or change of entropy and the
reorganization of atomic structures. These changes can be monitored using thermoanalytical
techniques such as differential thermal analysis (DTA) and thermogravimetric analysis (TGA)
[160]. For effective thermal activation, thermoanalytical methods can be used to select the
appropriate holding temperature necessary to achieve desired properties of the heated material
[151], [161]. Heat treatment can be affected by several factors such as heating rate, holding
temperature, holding time, heating atmosphere, and the rate of cooling [158]–[160]. The heat
treatment process is a significant operation used in manufacturing of various engineering materials
such as steel, ceramics, cement, etc. For instance, portland cement is made by thermal activation
of a mixture of limestone and clay or other materials of similar bulk composition and sufficient
reactivity, at a temperature of about 1450 °C. Partial fusion of the raw mix occurs, and nodules of
clinker are produced [119].
Various studies have investigated the effect of thermal activation of raw geopolymer materials
on geopolymerization process, of which thermal activation of kaolin has been of interest as
reported in the literature [8], [47], [162]–[164]. Generally, heating of clay minerals leads to change
in its structure which varies for different clay mineral groups [165]. Kaolinite is transformed to
metakaolin through calcination. Due to the high reactivity and purity of metakaolins, it is one of
the most explored aluminosilicates for synthesis of geopolymers [166], [167]. Often, kaolinite is
converted to metakaolin by calcination at temperatures between 700-900 °C, before being used
for geopolymer synthesis [7], [33]–[35]. Calcination causes the dehydroxylation of kaolinite and
formation of an amorphous phase which is highly reactive in alkaline medium [7], [34], [35].
Similar effects of calcination are observed with other aluminosilicate materials as well. Ye et al.
[161] reported that thermal activation improved the solubility of red mud in alkaline solution and
promoted geopolymerization. The activator was alkaline solution containing sodium silicate
prepared by dissolving sodium hydroxide in sodium silicate solution to reach different SiO2/Na2O
molar ratios. Mineral phases in the red mud transformed successively with an increase in
temperature, forming new phases with different solubility. The dissolution efficiencies of alumina
and silica reached a maximum when red mud was calcined at 800 °C, resulting in the highest
compressive strength of binders.
33
Also, Bondar et al. [168] investigated the effect of thermal activation at 700, 800 and 900 °C
on the geopolymeric reactivity of five natural pozzolans. A mixture of Na2SiO3 solution with
SiO2/Na2O molar ratio of 2.1 and KOH solution, was used as the activators. It was found that
calcination significantly increased the dissolution of Si species in alkaline medium, but Al species
solubility was less influenced. In one case, compressive strength of 20 mm cube pastes increased
from 5 MPa for the raw pozzolan to 19.3 MPa for the same pozzolan calcined at 800 °C. Generally,
the geopolymeric reactivity of calcined natural pozzolans was correlated to their mineralogical
composition. Rieger et al. [169] studied geopolymerization of shale–slag calcined at different
temperatures. An aqueous solution of Na2SiO3 with silica modulus of 1.71 and content of water
soluble solids 32.28 %, was used as activator. The XRD and Fourier transform infrared
spectroscopy (FTIR) analysis revealed that the raw material subjected to thermal treatment at 600
°C provided the most reactive state, giving the highest compressive strength of geopolymer mortar
at 82.5 MPa. However, further firing at temperatures greater than 800 °C was detrimental, due to
an increase in the crystalline content and a significant reduction in the alkaline hardening reaction.
Also, other studies have similarly reported the negative effect of very high temperature calcination
of the raw material on geopolymerization [170].
Temuujin et al. [170] found that preliminary calcination of fly ash at 500 and 800 °C induced
de-carbonation and also led to some decrease in the amorphous content of the fly ash from 60 to
57 %. The alkaline activator used was 14 M NaOH and Na2SiO3 solution. In this case, compressive
strength of 25 mm diameter x 50 mm height cylinder pastes decreased from 55.7 MPa for raw fly
ash-based geopolymer to 44.3 MPa after calcination at 800 °C. The drop in compressive strength
of the geopolymer pastes after calcination, was attributed to the partial crystallization of the
aluminosilicates and hematite on the surface of the fly ash particles. It was suggested that such
crystallized aluminosilicate (mullite) and hematite prevent dissolution of the aluminate and silicate
species in the alkaline solution. Similar findings were also reported by Wan et al. [171] who
highlighted that recrystallization taking place during calcination of kaolinite at higher temperatures
is significantly detrimental to the dissolution of resulting metakaolin during geopolymerization.
Fig. 12. shows the compressive strength of the 25 mm diameter x 50 mm height cylinder
geopolymer pastes for metakaolin heated to various high temperatures.
34
Fig. 12. Compressive strength of geopolymer synthesized with metakaolin samples calcined at
temperatures 550 to 950 °C [171] (reproduced with permission from Elsevier).
It is worth noting that calcination temperature is not the only important parameter that controls
the geopolymeric reactivity of aluminosilicates. The method of calcination also plays a role in the
phase transformations induced by thermal treatment and, in turn affects reactivity of the calcined
geopolymer material. Kenne et al. [172] studied the effect of calcination rate on the properties of
metakaolin-based geopolymers. In their experiment, kaolinite was calcinated at 700 °C for 30 min
using different rates of calcination: 1 °C/min, 2.5 °C/min, 5 °C/min, 10 °C/min, 15 °C/min and 20
°C/min. The alkaline activator used was prepared by mixing sodium silicate solution and 12 M
NaOH to obtain Na2O/SiO2 molar ratio of 0.7. They reported that geopolymer pastes with low
setting time and high compressive strength were obtained from metakaolin calcined at a low rate.
Compressive strength of 20 mm cube mortars decreased from 49.4 MPa to 20.8 MPa, when the
rate of heating increased from 1 °C/min to 20 °C/min. Recently, some studies have reported on the
use of flash calcination to produce metakaolin [167], [173], [174]. In this method, a pulverized
material is heated quickly, held at temperature for a short time, and cooled down at a rate of about
103–105 °C per second [173].
35
Autef et al. [35] studied geopolymeric reactivity of three sources of metakaolins produced
using three different processes consisting of a rotary-kiln, flash calcination, and oven-heating. The
dehydroxylation temperature was carried out at approximately 750 °C for all metakaolins. It was
found that flash calcined metakaolin was more reactive than metakaolins produced through the
rotary kiln or oven-heating. Similarly, Nicolas et al. [173] compared the geopolymeric reactivity
of metakaolin obtained using two different methods of flash calcination and rotary-kilning. For
flash calcination, the material was subjected to a temperature of about 1200 °C for few fractions
of a second and rapidly cooled down to 100 °C. In the rotary kiln, the material was calcined
between 650 and 700 °C for about 3 to 5 h. Their results showed that traditional rotary-calcined
metakaolins tend to be angular layered particles, whereas flash-calcined metakaolins contained
spherical particles. However, compressive strength test results showed that the two methods of
calcination produced synthesized geopolymer materials of comparable properties.
5. Conclusions
The geopolymeric reactivity of aluminosilicate materials can be significantly improved by
using several techniques. These activation methods are classified into four categories of
mechanical, thermal, physical separation and chemical activation which may be used singly or in
combination. These activation processes enhance reactivity by increasing the rate and extent to
which silicon and aluminium species from the activated materials dissolve in alkaline medium.
Consequently, more geopolymeric gel is formed resulting in the formation of a more compact
microstructure and improved mechanical properties of the geopolymer products.
While mechanical activation is an effective predominantly used method, chemical activation
by alkali-thermal fusion has been much less studied than the other activation methods. Alkali-
thermal fusion presents interesting advantages such as the production of highly reactive phases
with a wide range of non-reactive raw materials, even with those where other activation methods
are not efficient. This method allows the synthesis of one-part geopolymer materials, an alternative
and promising approach for making geopolymeric binders. However, the problem of mitigating
excess alkali in the fused materials, requires incorporation of reactive Al2O3-rich materials as
additives, for which further studies are required.
36
To alter the reactivity of a given raw material, an activation method and optimal treatment
parameters need to be suitably selected, considering that the two factors majorly determine the
final properties of the resulting geopolymers. In addition, energy and cost efficiency of the
activation process must be considered for purposes of industrial application.
Acknowledgement This paper is part of the PhD study of Leonel Noumbissie Tchadjie conducted under the NRF-
TWAS Doctoral Scholarship, grant no. 99993. The candidate thanks the National Research
Foundation (NRF) of South Africa for offering him this grant and study opportunity.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
References [1] J. Davidovits, “Geopolymers - Inorganic polymeric new materials,” J. Therm. Anal., vol.
37, no. 8, pp. 1633–1656, 1991.
[2] J. S. J. Van Van Deventer, J. L. Provis, and P. Duxson, “Technical and commercial progress
in the adoption of geopolymer cement,” Miner. Eng., vol. 29, pp. 89–104, 2012.
[3] J. L. Provis and S. A. Bernal, “Geopolymers and related alkali-activated materials,” Annu.
Rev. Mater. Res., vol. 44, pp. 299–327, 2014.
[4] Y. Wang, J. Dai, Z. Ding, and W. Xu, “Phosphate-based geopolymer: formation mechanism
and thermal stability,” Mater. Lett., vol. 190, no. 1, pp. 209–212, 2017.
[5] H. Xu and J. S. J. Van Deventer, “The geopolymerisation of alumino-silicate minerals,” Int.
J. Miner. Process., vol. 59, no. 3, pp. 247–266, 2000.
[6] S. Kumar, R. Kumar, and S. P. Mehrotra, “Influence of granulated blast furnace slag on the
reaction , structure and properties of fly ash based geopolymer,” J. Mater. Sci., vol. 45, pp.
607–615, 2010.
37
[7] A. Elimbi, H. K. Tchakoute, and D. Njopwouo, “Effects of calcination temperature of
kaolinite clays on the properties of geopolymer cements,” Constr. Build. Mater., vol. 25,
no. 6, pp. 2805–2812, 2011.
[8] V. Zivica, M. T. Palou, and T. I. Ľ. Bágeľ, “High strength metahalloysite based
geopolymer,” Compos. Part B Eng., vol. 57, pp. 155–165, Feb. 2014.
[9] J. N. Y. Djobo, L. N. Tchadjié, H. K. Tchakoute, B. B. D. Kenne, A. Elimbi, and D.
Njopwouo, “Synthesis of geopolymer composites from a mixture of volcanic scoria and
metakaolin,” J. Asian Ceram. Soc., vol. 2, no. 4, pp. 387–398, 2014.
[10] L. N. Tchadjié, J. N. Y. Djobo, N. Ranjbar, H. K. Tchakouté, B. B. D. Kenne, and A. Elimbi,
“Potential of using granite waste as raw material for geopolymer synthesis,” Ceram. Int.,
vol. 42, no. 2, pp. 3046–3055, 2016.
[11] S. Kumar and R. Kumar, “Mechanical activation of fly ash: Effect on reaction, structure
and properties of resulting geopolymer,” Ceram. Int., vol. 37, no. 2, pp. 533–541, 2011.
[12] C. Y. Heah et al., “Study on solids-to-liquid and alkaline activator ratios on kaolin-based
geopolymers,” Constr. Build. Mater., vol. 35, pp. 912–922, 2012.
[13] H. K. Tchakoute, A. Elimbi, E. Yanne, and C. N. Djangang, “Utilization of volcanic ashes
for the production of geopolymers cured at ambient temperature,” Cem. Concr. Compos.,
vol. 38, pp. 75–81, 2013.
[14] K. Komnitsas and D. Zaharaki, “Geopolymerisation: A review and prospects for the
minerals industry,” Miner. Eng., vol. 20, pp. 1261–1277, 2007.
[15] U. Rattanasak and P. Chindaprasirt, “Influence of NaOH solution on the synthesis of fly ash
geopolymer,” Miner. Eng., vol. 22, pp. 1073–1078, 2009.
[16] A. Gharzouni, E. Joussein, B. Samet, S. Baklouti, and S. Rossignol, “Effect of the reactivity
of alkaline solution and metakaolin on geopolymer formation,” J. Non. Cryst. Solids, vol.
410, pp. 127–134, 2015.
[17] J. Davidovits, Geopolymer Chemistry and Applications, 3rd ed. France: Institut
Géopolymère, 2011.
38
[18] T. Glasby et al., “EFC geopolymer concrete aircraft pavements at Brisbane West Wellcamp
Airport,” in Concrete 2015, 2015, pp. 1–9.
[19] R. Bligh and T. Glasby, “Development of geopolymer precast floor panels for the Global
Change Institute at University of Queensland,” in Concrete 2013, 2013, pp. 1–8.
[20] E. Hermann, C. Kunze, R. Gatzweiler, G. Kießig, and J. Davidovits, “Solidification of
various radioactive residues by geopolymer with special emphasis on long term stability,”
Géopolymère ’99 Proc., pp. 1–15, 1999.
[21] C. Bai et al., “High-porosity geopolymer foams with tailored porosity for thermal insulation
and wastewater treatment,” J. Mater. Res., pp. 1–9, 2017.
[22] E. Jämstorp, J. Forsgren, S. Bredenberg, H. Engqvist, and M. Strømme, “mechanically
strong geopolymers offer new possibilities in treatment of chronic pain,” J. Control.
Release, vol. 146, no. 3, pp. 370–377, 2010.
[23] D. L. Y. Kong and J. G. Sanjayan, “Effect of elevated temperatures on geopolymer paste,
mortar and concrete,” Cem. Concr. Res., vol. 40, no. 2, pp. 334–339, 2010.
[24] P. N. Lemougna, K. J. D. Mackenzie, and U. F. C. Melo, “Synthesis and thermal properties
of inorganic polymers (geopolymers) for structural and refractory applications from
volcanic ash,” Ceram. Int., vol. 37, pp. 3011–3018, 2011.
[25] A. Z. Mohd Ali, J. Sanjayan, and M. Guerrieri, “Performance of geopolymer high strength
concrete wall panels and cylinders when exposed to a hydrocarbon fire,” Constr. Build.
Mater., vol. 137, pp. 195–207, 2017.
[26] P. N. Lemougna, J. D. Mackenzie, G. N. L. Jameson, H. Rahier, and U. F. C. Melo, “The
role of iron in the formation of inorganic polymers (geopolymers) from volcanic ash: a 57
Fe Mössbauer spectroscopy study,” J. Mater. Sci., vol. 48, no. 15, pp. 5280–5286, 2013.
[27] J. N. Y. Djobo, A. Elimbi, H. K. Tchakoute, and S. Kumar, “Reactivity of volcanic ash in
alkaline medium, microstructural and strength characteristics of resulting geopolymers
under different synthesis conditions,” J. Mater. Sci., vol. 51, no. 22, pp. 10301–10317, 2016.
[28] R. A. Bailey, H. M. Clark, J. P. Ferris, S. Krause, and R. L. Strong, “The earth’s crust,” in
39
Chemistry of the Environment, Elsevier, 2002, pp. 443–482.
[29] M. S. Reddy, P. Dinakar, and B. H. Rao, “A review of the influence of source material’s
oxide composition on the compressive strength of geopolymer concrete,” Microporous
Mesoporous Mater., vol. 234, pp. 12–23, 2016.
[30] J. He, Y. Jie, J. Zhang, Y. Yu, and G. Zhang, “Synthesis and characterization of red mud
and rice husk ash-based geopolymer composites,” Cem. Concr. Compos., vol. 37, pp. 108–
118, 2013.
[31] N. Ranjbar, M. Mehrali, A. Behnia, U. J. Alengaram, and M. Z. Jumaat, “Compressive
strength and microstructural analysis of fly ash / palm oil fuel ash based geopolymer
mortar,” Mater. Des., vol. 59, pp. 532–539, 2014.
[32] A. D. Hounsi, G. L. Lecomte-nana, G. Djétéli, and P. Blanchart, “Kaolin-based
geopolymers: Effect of mechanical activation and curing process,” Constr. Build. Mater.,
vol. 42, pp. 105–113, 2013.
[33] J. Davidovits and J. L. Sawyer, “Early high-strength mineral polymer,” 4,509,985, 1985.
[34] Z. Zuhua, Y. Xiao, Z. Huajun, and C. Yue, “Role of water in the synthesis of calcined
kaolin-based geopolymer,” Appl. Clay Sci., vol. 43, no. 2, pp. 218–223, 2009.
[35] A. Autef et al., “Role of metakaolin dehydroxylation in geopolymer synthesis,” Powder
Technol., vol. 250, pp. 33–39, 2013.
[36] A. Buchwald, M. Hohmann, K. Posern, and E. Brendler, “The suitability of thermally
activated illite / smectite clay as raw material for geopolymer binders,” Appl. Clay Sci., vol.
46, no. 3, pp. 300–304, 2009.
[37] E. A. Obonyo, E. Kamseu, P. N. Lemougna, A. B. Tchamba, U. C. Melo, and C. Leonelli,
“A sustainable approach for the geopolymerization of natural iron-rich aluminosilicate
materials,” sustainability, vol. 6, pp. 5535–5553, 2014.
[38] K. M. A. Hossain, “Properties of volcanic pumice based cement and lightweight concrete,”
Cem. Concr. Res., vol. 34, no. 2, pp. 283–291, 2004.
[39] G. Cai, T. Noguchi, H. Degée, and R. Kitagaki, “Volcano-related materials in concretes: a
40
comprehensive review,” Environ. Sci. Pollut. Res., vol. 23, pp. 7220–7243, 2016.
[40] R. A. F. Cas and J. V. Wright, Volcanic Successions Modern and Ancient, 5th ed. London:
Chapman & Hall, 1996.
[41] S. O. Ekolu, M. D. A. Thomas, and R. D. Hooton, “Studies on Ugandan volcanic ash and
tuff,” in Proceedings of the First International Conference on Advances in Engineering and
Technology, 2006, pp. 75–83.
[42] D. Bondar, C. J. Lynsdale, N. B. Milestone, N. Hassani, and A. A. Ramezanianpour, “Effect
of type, form, and dosage of activators on strength of alkali-activated natural pozzolans,”
Cem. Concr. Compos., vol. 33, no. 2, pp. 251–260, 2011.
[43] J. N. Y. Djobo, A. Elimbi, and H. K. Tchakouté, “Volcanic ash-based geopolymer cements
/ concretes: the current state of the art and perspectives,” Environ. Sci. Pollut. Res., vol. 24,
no. 5, pp. 4433–4446, 2016.
[44] B. I. Djon Li Ndjock, A. Elimbi, and M. Cyr, “Rational utilization of volcanic ashes based
on factors affecting their alkaline activation,” J. Non. Cryst. Solids, vol. 463, pp. 31–39,
2017.
[45] K. L. Scrivener, “Options for the future of cement,” Indian Concr. J., vol. 88, no. 7, pp. 11–
21, 2014.
[46] N. Eroshkina and M. Korovkin, “The effect of the mixture composition and curing
conditions on the properties of the geopolymer binder based on dust crushing of the granite,”
Procedia Eng., vol. 150, pp. 1605–1609, 2016.
[47] S. Noor, H. Guy, N. L. J. Joanne, and K. J. D. Mackenzie, “Synthesis and properties of
inorganic polymers (geopolymers) derived from Bayer process residue (red mud) and
bauxite,” J. Mater. Sci., vol. 50, no. 23, pp. 7713–7724, 2015.
[48] F. Matalkah, P. Soroushian, S. Ul, and A. Peyvandi, “Use of non-wood biomass combustion
ash in development of alkali-activated concrete,” Constr. Build. Mater., vol. 121, pp. 491–
500, 2016.
[49] D. Ziegler, A. Formia, J. Tulliani, and P. Palmero, “Geopolymers using fly ash and rice
41
husk ash as raw materials,” Materials (Basel)., vol. 466, no. 9, pp. 1–21, 2016.
[50] R. C. Joshi and R. P. Lohita, Fly ash in concrete: production, properties and uses.
Amsterdam: Gordon and Breach Science Publishers, 1997.
[51] A. Naghizadeh and S. O. Ekolu, “Mixture factors influencing alkali-silica reaction in fly
ash geopolymer mortars,” in International Conference on Advances in Construction
Materials and Systems, 2017, pp. 395–400.
[52] A. Fernández-Jiménez and A. Palomo, “Characterisation of fly ashes. Potential reactivity
as alkaline cements,” Fuel, vol. 82, no. 18, pp. 2259–2265, 2003.
[53] H. W. Nugteren, Secondary Industrial Minerals Fronm Coal Fly Ash and Aliminium
Anodising Waste Solutions. Ridderprint BV, 2010.
[54] H. H. Weldes and K. R. Lange, “Properties of soluble silicates,” Ind. Eng. Chem., vol. 61,
no. 4, pp. 29–44, 1969.
[55] H. K. Tchakoute, C. H. Ruscher, S. Kong, E. Kamseu, and C. Leonelli, “Comparison of
metakaolin-based geopolymer cements from commercial sodium waterglass and sodium
waterglass from rice husk ash,” J. Sol-Gel Sci. Technol., vol. 78, pp. 492–506, 2016.
[56] J. L. Provis and J. S. J. Van Van Deventer, Geopolymers: structures, processing, properties
and industrial applications. Woodhead Publishing Limited, 2009.
[57] D. E. Veinot, K. B. Langille, D. T. Nguyen, and J. O. Bernt, “Efflorescence of soluble
silicate coatings,” J. Non. Cryst. Solids, vol. 127, no. 2, pp. 221–226, 1991.
[58] PQ Europe, “Sodium and Potassium Silicates,” PQ Corporation, pp. 1–16, 2004.
[59] T. Bakharev, “Thermal behaviour of geopolymers prepared using class F fly ash and
elevated temperature curing,” Cem. Concr. Res., vol. 36, pp. 1134–1147, 2006.
[60] P. Duxson, A. Fernández-Jiménez, J. L. Provis, G. C. Lukey, A. Palomo, and J. S. J. Van
Deventer, “Geopolymer technology: the current state of the art,” J. Mater. Sci., no. 4, pp.
2917–2933, 2007.
[61] A. Hajimohammadi, J. L. Provis, and J. S. J. Van Deventer, “The effect of silica availability
on the mechanism of geopolymerisation,” Cem. Concr. Res., vol. 41, no. 3, pp. 210–216,
42
2011.
[62] A. Hajimohammadi and J. S. J. Van Deventer, “Dissolution behaviour of source materials
for synthesis of geopolymer binders: A kinetic approach,” Int. J. Miner. Process., vol. 153,
pp. 80–86, 2015.
[63] A. Hajimohammadi, J. L. Provis, and J. S. J. Van Deventer, “Effect of alumina release rate
on the mechanism of geopolymer gel formation,” Chem. Mater., vol. 22, no. 18, pp. 5199–
5208, 2010.
[64] C. Tennakoon, P. De Silva, K. Sagoe-Crentsil, and J. G. Sanjayan, “Influence and role of
feedstock Si and Al content in Geopolymer,” J. Sustain. Cem. Mater., vol. 4, no. 2, pp. 129–
139, 2015.
[65] P. De Silva, K. Sagoe-crentsil, and V. Sirivivatnanon, “Kinetics of geopolymerization: Role
of Al2O3 and SiO2,” Cem. Concr. Res., vol. 37, pp. 512–518, 2007.
[66] P. Chindaprasirt, P. De Silva, K. Sagoe-crentsil, and S. Hanjitsuwan, “Effect of SiO2 and
Al2O3 on the setting and hardening of high calcium fly ash-based geopolymer systems,” J.
Mater. Sci., vol. 47, pp. 4876–4883, 2012.
[67] F. Zibouche, H. Kerdjoudj, J.-B. D. De Lacaillerie, and H. Van Damme, “Geopolymers
from Algerian metakaolin. Influence of secondary minerals,” Appl. Clay Sci., vol. 43, pp.
453–458, 2009.
[68] J. Temuujin, R. P. Williams, and A. van Riessen, “Effect of mechanical activation of fly ash
on the properties of geopolymer cured at ambient temperature,” J. Mater. Process. Technol.,
vol. 209, no. 12–13, pp. 5276–5280, 2009.
[69] V. Nikolic, M. Komljenovic, Z. Bašcarevic, N. Marjanovic, Z. Miladinović, and R.
Petrović, “The influence of fly ash characteristics and reaction conditions on strength and
structure of geopolymers,” Constr. Build. Mater., vol. 94, pp. 361–370, 2015.
[70] B. A. Fillenwarth and S. M. L. Sastry, “Development of a predictive optimization model for
the compressive strength of sodium activated fly ash based geopolymer pastes,” FUEL, vol.
147, pp. 141–146, 2015.
43
[71] N. W. Chen-Tan, A. van Riessem, C. V. LY, and D. C. Southam, “Determining the
reactivity of a fly ash for production of geopolymer,” J. Am. Ceram. Soc., vol. 887, pp. 881–
887, 2009.
[72] F. Winnefeld, A. Leemann, M. Lucuk, P. Svoboda, and M. Neuroth, “Assessment of phase
formation in alkali activated low and high calcium fly ashes in building materials,” Constr.
Build. Mater., vol. 24, no. 6, pp. 1086–1093, 2010.
[73] C. Li, Y. Li, H. Sun, and L. Li, “The composition of fly ash glass phase and its dissolution
properties applying to geopolymeric materials,” J. Am. Ceram. Soc., vol. 94, no. 6, pp.
1773–1778, 2011.
[74] A. Autef, E. Joussein, G. Gasgnier, and S. Rossignol, “Feasibility of aluminosilicate
compounds from various raw materials: Chemical reactivity and mechanical properties,”
Powder Technol., vol. 301, pp. 169–178, 2016.
[75] H. K. Tchakoute, S. Kong, J. Noël, Y. Djobo, L. N. Tchadjie, and D. Njopwouo, “A
comparative study of two methods to produce geopolymer composites from volcanic scoria
and the role of structural water contained in the volcanic scoria on its reactivity,” Ceram.
Int., vol. 41, no. 10, pp. 12568–12577, 2015.
[76] Z. Zhang, H. Wang, X. Yao, and Y. Zhu, “Effects of halloysite in kaolin on the formation
and properties of geopolymers,” Cem. Concr. Compos., vol. 34, no. 5, pp. 709–715, 2012.
[77] P. Duxson and J. L. Provis, “Designing precursors for geopolymer cements,” J. Am. Ceram.
Soc., vol. 91, no. 12, pp. 3864–3869, 2008.
[78] R. P. Williams and A. Van Riessen, “Determination of the reactive component of fly ashes
for geopolymer production using XRF and XRD,” Fuel, vol. 89, no. 12, pp. 3683–3692,
2010.
[79] C. Tennakoon, A. Nazari, J. G. Sanjayan, and K. Sagoe-crentsil, “Distribution of oxides in
fly ash controls strength evolution of geopolymers,” Constr. Build. Mater., vol. 71, pp. 72–
82, 2014.
[80] J. N. Y. Djobo, A. Elimbi, H. K. Tchakouté, and S. Kumar, “Mechanical activation of
volcanic ash for geopolymer synthesis: effect on reaction kinetics, gel characteristics,
44
physical and mechanical properties,” RSC Adv., vol. 6, no. 45, pp. 39106–39117, 2016.
[81] B. Wei, Y. Zhang, and S. Bao, “Preparation of geopolymers from vanadium tailings by
mechanical activation,” Constr. Build. Mater., vol. 145, pp. 236–242, 2017.
[82] P. Baláž, Mechanochemistry in nanoscience and minerals engineering. Berlin, 2008.
[83] V. V Boldyrev and K. Tkáčová, “Mechanochemistry of solids: past, present, and prospects,”
J. Mater. Synth. Process., vol. 8, no. 3–4, pp. 121–132, 2000.
[84] Z. A. Juhász, “Colloid-chemical aspects of mechanical activation,” Part. Sci. Technol., vol.
16, no. 2, pp. 145–161, 1998.
[85] G. Mucsi, “Mechanical activation of power station fly ash by grinding – A review,” J. Silic.
Based Compos. Mater., vol. 68, no. 2, pp. 56–61, 2016.
[86] L. Opoczky, “Fine grinding and agglomeration of silicates,” Powder Technol., vol. 17, no.
1, pp. 1–7, 1977.
[87] M. E. Fayed and L. Otten, Handbook of powder science & technology, 2nd ed. Springer
Science & Business Media, 1997.
[88] Q. Zhang, J. Kano, and F. Saito, “Fine grinding of materials in dry systems and
mechanochemistry,” in Handbook of PowderTechnology, vol. 12, 2007, pp. 510–528.
[89] I. Krycer and J. A. Hersey, “A comparative study of comminution in rotary and vibratory,”
Powder Technol., vol. 27, pp. 137–141, 1980.
[90] P. Baláž, “Mechanical activation in hydrometallurgy,” Int. J. Miner. Process., vol. 72, pp.
341–354, 2003.
[91] G. Mucsi, Á. Rácz, and V. Mádai, “Mechanical activation of cement in stirred media mill,”
Powder Technol., vol. 235, pp. 163–172, 2013.
[92] S. P. Mehrotra, T. C. Alex, G. Greifzu, and R. Kumar, “Mechanical activation of gibbsite
and boehmite: new findings and their implications,” Trans. Indian Inst. Met., vol. 69, no. 1,
pp. 51–59, 2016.
[93] N. Marjanović, M. Komljenović, Z. Baščarević, and V. Nikolić, “Improving reactivity of
45
fly ash and properties of ensuing geopolymers through mechanical activation,” Constr.
Build. Mater. J., vol. 57, pp. 151–162, 2014.
[94] N. I. Kozhukhova, I. V. Zhemovsky, V. V. Strokova, and V. A. Kalashnikova, “Influence
of mechanical and chemoactivation processes on operational characteristics of geopolymer
binder,” Res. J. Appl. Sci., vol. 10, no. 10, pp. 620–623, 2015.
[95] M. Kanuchova, M. Drabova, M. Sisol, J. Mosej, L. Kozakova, and J. Skvarla, “Influenc of
mechanical activation of fly ash on the properties of geopolymers investigated by XPS
method,” Environ. Prog. Sustain. Energy, vol. 35, no. 5, 2016.
[96] G. Mucsi et al., “Control of geopolymer properties by grinding of land filled fly ash,” Int.
J. Miner. Process., vol. 143, pp. 50–58, 2015.
[97] V. Nikolic, M. Komljenović, N. Džunuzović, T. Ivanović, and Z. Miladinović,
“Immobilization of hexavalent chromium by fly ash-based geopolymers,” Compos. Part B
Eng., vol. 112, pp. 213–223, 2017.
[98] T. C. Alex et al., “Utilization of zinc slag through geopolymerization: Influence of milling
atmosphere,” Int. J. Miner. Process., vol. 123, pp. 102–107, 2013.
[99] S. Kumar, R. Kumar, T. C. Alex, A. Bandopadhyay, and S. P. Mehrotra, “Effect of
mechanically activated fly ash on the properties of geopolymer cement,” in Geopolymer:
green chemistry and sustainable development solutions, 2005.
[100] G. Mucsi, A. Szenczi, Z. Molnár, and J. Lakatos, “Structural formation and leaching
behavior of mechanically activated lignite fly ash based geopolymer,” J. Environ. Eng.
Landsc. Manag., vol. 24, no. 1, pp. 48–59, 2016.
[101] C. Y. Heah et al., “Strength and microstructural properties of mechanically-activated kaolin
geopolymers,” Adv. Mater. Res., vol. 626, pp. 926–930, 2013.
[102] A. M. Kalinkin et al., “Geopolymerization behavior of Cu – Ni slag mechanically activated
in air and in CO 2 atmosphere,” Int. J. Miner. Process., vol. 112–113, pp. 101–106, 2012.
[103] S. Kumar, G. Mucsi, F. Kristály, and P. Pekker, “Mechanical activation of fly ash and its
influence on micro and nano-structural behaviour of resulting geopolymers,” Adv. Powder
46
Technol., vol. 28, no. 3, pp. 805–813, 2017.
[104] F. Mádai, F. Kristály, and G. Mucsi, “Microstructure, mineralogy and physical properties
of ground fly ash based geopolymers,” Ceram. - Silikaty, vol. 59, no. 1, pp. 70–79, 2015.
[105] J. Y. Hwang, X. Sun, and Z. Li, “Unburned carbon from fly ash for mercury adsorption: I.
separation and characterization of unburned carbon,” J. Miner. Mater. Charact. Eng., vol.
1, no. 1, pp. 39–60, 2002.
[106] R. Hela and D. Orsáková, “The mechanical activation of fly ash,” Procedia Eng., vol. 65,
pp. 87–93, 2013.
[107] J. Payá, J. Monzó, M. V. Borrachero, and E. Peris-Mora, “Comparisons among magnetic
and non-magnetic fly ash fractions: Strength development of cement-fly ash mortars,”
Waste Manag., vol. 16, pp. 119–124, 1996.
[108] N. E. Altun, C. Xiao, and J. Hwang, “Separation of unburned carbon from fl y ash using a
concurrent fl otation column,” Fuel Process. Technol., vol. 90, no. 12, pp. 1464–1470, 2009.
[109] D. S. Rao and B. Das, “Characterization and beneficiation studies of a low-grade bauxite
ore,” J. Inst. Eng. Ser. D, vol. 95, no. 2, pp. 81–93, 2014.
[110] J. Payá, M. V Borrachero, J. Monzo, E. Peris-Mora, and M. Bonilla, “Long term mechanical
strength behaviour in fly ash / Portland cement mortars prepared using processed ashes,” J.
Chem. Technol. Biotechnol., vol. 77, no. 3, pp. 336–344, 2002.
[111] P. Garcés, L. G. Andión, E. Zornoza, M. Bonilla, and J. Payá, “The effect of processed fly
ashes on the durability and the corrosion of steel rebars embedded in cement – modified fly
ash mortars,” Cem. Concr. Compos., vol. 32, no. 3, pp. 204–210, 2010.
[112] P. Chindaprasirt, C. Jaturapitakkul, and T. Sinsiri, “Effect of fly ash fineness on
microstructure of blended cement paste,” Constr. Build. Mater., vol. 21, pp. 1534–1541,
2007.
[113] T. Sinsiri, P. Chindaprasirt, and C. Jaturapitakkul, “Influence of fly ash fineness and shape
on the porosity and permeability of blended cement pastes,” Int. J. Miner. Metall. Mater.,
vol. 17, no. 6, 2010.
47
[114] S. Kumar, R. Kumar, T. C. Alex, A. Bandopadhyay, and S. P. Mehrotra, “Influence of
reactivity of fly ash on geopolymerisation,” Adv. Appl. Ceram., vol. 106, no. 3, pp. 120–
127, 2007.
[115] P. Chindaprasirt, T. Chareerat, S. Hatanaka, and T. Cao, “High-strength geopolymer using
fine,” J. Mater. Civ. Eng., no. March, pp. 264–270, 2011.
[116] H. W. Nugteren, V. C. L. Butselaar-orthlieb, M. Izquierdo, G.-J. Witkamp, and M. T.
Kreutzer, “High strength geopolymers from fractionated and pulverized fly ash,” in 3rd
World of Coal Ash, 2009.
[117] S. Kumar, F. Kristály, and G. Mucsi, “Geopolymerisation behaviour of size fractioned fly
ash,” Adv. Powder Technol., vol. 26, no. 1, pp. 24–30, 2015.
[118] V. S. Ramachandran, R. M. Paroli, J. J. Beaudoin, and A. H. Delgado, “Introduction to
concrete admixtures,” in Handbook of Thermal Analysis of Construction Materials,
Elsevier, 2002, pp. 143–188.
[119] H. F. W. Taylor, Cement chemistry, 2nd editio. Thomas Telford, 1997.
[120] A. Zayed, N. Shanahan, V. Tran, A. Markandeya, A. Williams, and A. Elnihum, “Effects
of Chemical and Mineral Admixtures on Performance of Florida Structural Concrete,”
Bartow, 2016.
[121] P. Rovnaník, “Influence of C12A7 admixture on setting properties of fly ash geopolymer,”
Ceram. – Silikáty, vol. 54, no. 4, pp. 362–367, 2010.
[122] U. Rattanasak, K. Pankhet, and P. Chindaprasirt, “Effect of chemical admixtures on
properties of high-calcium fly ash geopolymer,” Int. J. Miner. Metall. Mater., vol. 18, no.
3, pp. 364–369, 2011.
[123] A. Kusbiantoro, M. S. Ibrahim, K. Muthusamy, and A. Alias, “Development of sucrose and
citric acid as the natural based admixture for fly ash based geopolymer,” Procedia Environ.
Sci., vol. 17, pp. 596–602, 2013.
[124] T. Revathi, R. Jeyalakshmi, N. P. Rajamane, and M. Sivasakthi, “Evaluation of the role of
Cetyltrimethylammoniumbromide (CTAB) and Acetylenicglycol (AG) admixture on fly
48
ash based geopolymer,” Orient. J. Chem., vol. 33, no. 2, pp. 783–792, 2017.
[125] M. Olalekan, M. Azmi, M. Johari, Z. Arifin, and M. Maslehuddin, “Effects of addition of
Al (OH)3 on the strength of alkaline activated ground blast furnace slag-ultrafine palm oil
fuel ash (AAGU) based binder,” Constr. Build. Mater., vol. 50, pp. 361–367, 2014.
[126] H. K. Tchakoute, A. Elimbi, J. A. Mbey, C. J. N. Sabouang, and D. Njopwouo, “The effect
of adding alumina-oxide to metakaolin and volcanic ash on geopolymer products: A
comparative study,” Constr. Build. Mater., vol. 35, pp. 960–969, 2012.
[127] D. Adak, M. Sarkar, and S. Mandal, “Effect of nano-silica on strength and durability of fly
ash based geopolymer mortar,” Constr. Build. Mater., vol. 70, pp. 453–459, 2014.
[128] M. Sumesh, U. J. Alengaram, M. Z. Jumaat, K. H. Mo, and M. F. Alnahhal, “Incorporation
of nano-materials in cement composite and geopolymer based paste and mortar – A review,”
Constr. Build. Mater., vol. 148, pp. 62–84, 2017.
[129] T. Phoo-ngernkham, P. Chindaprasirt, V. Sata, and S. Hanjitsuwan, “The effect of adding
nano-SiO2 and nano-Al2O3 on properties of high calcium fly ash geopolymer cured at
ambient temperature,” Mater. Des., vol. 55, pp. 58–65, 2014.
[130] D. Adak, M. Sarkar, and S. Mandal, “Structural performance of nano-silica modified fly-
ash based geopolymer concrete,” Constr. Build. Mater., vol. 135, pp. 430–439, 2017.
[131] C. K. Yip, G. C. Lukey, J. L. Provis, and J. S. J. Van Deventer, “Effect of calcium silicate
sources on geopolymerisation,” Cem. Concr. Res., vol. 38, pp. 554–564, 2008.
[132] H. M. Khater, “Effect of calcium on geopolymerization of aluminosilicate wastes,” J.
Mater. Civ. Eng., vol. 24, no. 1, pp. 92–101, 2012.
[133] K. T. Nguyen, A. T. L, J. Lee, D. Lee, and K. Lee, “Investigation on properties of
geopolymer mortar using preheated materials and thermogenetic admixtures,” Constr.
Build. Mater., vol. 130, pp. 146–155, 2017.
[134] J. Temuujin, A. van Riessen, and R. Williams, “Influence of calcium compounds on the
mechanical properties of fly ash geopolymer pastes,” J. Hazard. Mater., vol. 167, pp. 82–
88, 2009.
49
[135] I. Phummiphan, S. Horpibulsuk, T. Phoo-ngernkham, A. Arulrajah, and S. Shen, “Marginal
lateritic soil stabilized with calcium carbide residue and fly Ash geopolymers as a
sustainable pavement base material,” J. Mater. Civ. Eng., vol. 29, no. 2, pp. 1–10, 2017.
[136] C. Phetchuay, S. Horpibulsuk, A. Arulrajah, C. Suksiripattanapong, and A. Udomchai,
“Strength development in soft marine clay stabilized by fly ash and calcium carbide residue
based geopolymer,” Appl. Clay Sci., vol. 127–128, pp. 134–142, 2016.
[137] H. Xu, W. Gong, L. Syltebo, K. Izzo, W. Lutze, and I. L. Pegg, “Effect of blast furnace slag
grades on fly ash based geopolymer waste,” Fuel, vol. 133, pp. 332–340, 2014.
[138] S. Saha and C. Rajasekaran, “Enhancement of the properties of fly ash based geopolymer
paste by incorporating ground granulated blast furnace slag,” Constr. Build. Mater., vol.
146, pp. 615–620, 2017.
[139] S. K. Nath and S. Kumar, “Reaction kinetics, microstructure and strength behavior of alkali
activated silico-manganese (SiMn) slag – Fly ash blends,” Constr. Build. Mater., vol. 147,
pp. 371–379, 2017.
[140] S. M. Laskar and S. Talukdar, “Development of ultrafine slag-based geopolymer mortar for
use as repairing mortar,” J. Mater. Civ. Eng., no. 1990, pp. 1–11, 2013.
[141] S. K. Nath and S. Kumar, “Influence of iron making slags on strength and microstructure
of fly ash geopolymer,” Constr. Build. Mater., vol. 38, pp. 924–930, 2013.
[142] J. Davidovits et al., “Geopolymer cement based on European coal fly ashes,” 2014.
[143] M. A. Salih, N. Farzadnia, A. Abdullah, A. Ali, and R. Demirboga, “Development of high
strength alkali activated binder using palm oil fuel ash and GGBS at ambient temperature,”
Constr. Build. Mater., vol. 93, pp. 289–300, 2015.
[144] J. Ye, W. Zhang, and D. Shi, “Properties of an aged geopolymer synthesized from calcined
ore-dressing tailing of bauxite and slag,” Cem. Concr. Res., vol. 100, no. December 2016,
pp. 23–31, 2017.
[145] R. A. Robayo-salazar, R. Mejía, D. Gutiérrez, and F. Puertas, “Effect of metakaolin on
natural volcanic pozzolan-based geopolymer cement,” Appl. Clay Sci., vol. 132–133, pp.
50
491–497, 2016.
[146] M. Ogundiran and S. Kumar, “Synthesis of fly ash-calcined clay geopolymers: Reactivity,
mechanical strength, structural and microstructural characteristics,” Constr. Build. Mater.,
vol. 125, pp. 450–457, 2016.
[147] H. Chang and W. Shih, “A General method for the conversion of fly ash into zeolites as ion
exchangers for cesium,” Ind. Eng. Chem. Res., vol. 37, pp. 71–78, 1998.
[148] C. Wang, J. Zhou, Y. Wang, M. Yang, and C. Meng, “Synthesis of zeolite X from low-
grade bauxite,” J. Chem. Technol. Biotechnol., vol. 88, pp. 1350–1357, 2012.
[149] H. Xu, Q. Li, L. Shen, M. Zhang, and J. Zhai, “Low-reactive circulating fluidized bed
combustion (CFBC) fly ashes as source material for geopolymer synthesis,” Waste Manag.,
vol. 30, no. 1, pp. 57–62, 2010.
[150] H. K. Tchakoute, A. Elimbi, B. B. D. Kenne, J. A. Mbey, and D. Njopwouo, “Synthesis of
geopolymers from volcanic ash via the alkaline fusion method: Effect of Al 2 O 3 / Na 2 O
molar ratio of soda – volcanic ash,” Ceram. Int., vol. 39, no. 1, pp. 269–276, 2013.
[151] X. Ke, S. A. Bernal, N. Ye, J. L. Provis, and J. Yang, “One-Part geopolymers based on
thermally treated red mud/NaOH blends,” J. Am. Ceram. Soc., vol. 7, no. 34896, pp. 1–7,
2014.
[152] H. K. Tchakoute, J. A. Mbey, A. Elimbi, B. B. K. Diffo, and D. Njopwouo, “Synthesis of
volcanic ash-based geopolymer mortars by fusion method: Effects of adding metakaolin to
fused volcanic ash,” Ceram. Int., vol. 39, no. 2, pp. 1613–1621, 2013.
[153] D. Feng, J. L. Provis, and J. S. J. Van Deventer, “Thermal activation of albite for the
synthesis of one-part mix geopolymers,” J. Am. Ceram. Soc., vol. 572, no. 29905, pp. 565–
572, 2012.
[154] N. Ye et al., “Synthesis and strength optimization of one-part geopolymer based on red
mud,” Constr. Build. Mater., vol. 111, pp. 317–325, 2016.
[155] P. M. Xun et al., “Alkali fusion of bentonite to synthesize one-part geopolymeric cements
cured at elevated temperature by comparison with two-part ones,” Constr. Build. Mater.,
51
vol. 130, pp. 103–112, 2017.
[156] D. Koloušek, J. Brus, M. Urbanova, J. Andertova, V. Hulinsky, and J. Vorel, “Preparation,
structure and hydrothermal stability of alternative (sodium silicate-free) geopolymers,” J.
Mater. Sci., vol. 42, no. 22, pp. 9267–9275, 2007.
[157] Z. Abdollahnejad, J. B. Aguiar, and C. Jesus, “Durability performance of fly ash based one-
part geopolymer mortars,” Key Eng. Mater., vol. 634, pp. 113–120, 2015.
[158] G. J. Bonami, Ed., Heat treatment: theory, techniques and applications. New York: Nova
Science Publishers, Inc. 2011, 2011.
[159] T. V Rajan, C. P. Sharma, and A. Sharma, Heat treatment: principles and techniques. PHI
Learning Pvt. Ltd., 2011.
[160] P. J. Haines, Ed., Principles of thermal analysis and calorimetry. Royal Society of
Chemistry, 2002.
[161] N. Ye et al., “Synthesis and characterization of geopolymer from Bayer red mud with
thermal,” J. Am. Ceram. Soc., vol. 95, no. 5, pp. 1652–1660, 2014.
[162] N. Belmokhtar, M. Ammari, J. Brigui, and L. Ben, “Comparison of the microstructure and
the compressive strength of two geopolymers derived from Metakaolin and an industrial
sludge,” Constr. Build. Mater., vol. 146, pp. 621–629, 2017.
[163] S. Selmani, A. Sdiri, S. Bouaziz, E. Joussein, and S. Rossignol, “Effects of metakaolin
addition on geopolymer prepared from natural kaolinitic clay,” Appl. Clay Sci., vol. 146,
no. July, pp. 457–467, 2017.
[164] N. Ranjbar and C. Kuenzel, “Influence of preheating of fly ash precursors to produce
geopolymers,” J. Am. Ceram. Soc., pp. 1–10, 2017.
[165] L. Heller-Kallai, “Chapter 7.2 Thermally modified clay minerals,” in Handbook of Clay
Science, 1st ed., F. Bergaya, B. K. G. Theng, and G. Lagaly, Eds. Elsevier Science, 2006,
pp. 289–308.
[166] Y. M. Liew, C. Y. Heah, A. B. Mohd Mustafa, and H. Kamarudin, “Structure and properties
of clay-based geopolymer cements: A review,” Prog. Mater. Sci., vol. 83, pp. 595–629,
52
2016.
[167] A. Gharzouni, C. Dupuy, I. Sobrados, E. Joussein, N. Texier-mandoki, and X. Bourbon,
“The effect of furnace and fly ash heating on COx argillite for the synthesis of alkali-
activated binders,” J. Clean. Prod., vol. 156, pp. 670–678, 2017.
[168] D. Bondar, C. J. Lynsdale, N. B. Milestone, N. Hassani, and A. A. Ramezanianpour, “Effect
of heat treatment on reactivity-strength of alkali-activated natural pozzolans,” Constr.
Build. Mater., vol. 25, no. 10, pp. 4065–4071, 2011.
[169] D. Rieger et al., “Effect of thermal treatment on reactivity and mechanical properties of
alkali activated shale – slag binder,” Constr. Build. Mater., vol. 83, pp. 26–33, 2015.
[170] J. Temuujin and A. Van Riessen, “Effect of fly ash preliminary calcination on the properties
of geopolymer,” J. Hazard. Mater., vol. 164, pp. 634–639, 2009.
[171] Q. Wan, F. Rao, and S. Song, “Reexamining calcination of kaolinite for the synthesis of
metakaolin geopolymers - roles of dehydroxylation and recrystallization,” J. Non. Cryst.
Solids, vol. 460, pp. 74–80, 2017.
[172] B. B. D. Kenne, A. Elimbi, M. Cyr, J. D. Manga, and H. T. Kouamo, “Effect of the rate of
calcination of kaolin on the properties of metakaolin-based geopolymers,” J. Asian Ceram.
Soc., vol. 3, no. 1, pp. 130–138, 2015.
[173] R. S. Nicolas, M. Cyr, and G. Escadeillas, “Characteristics and applications of flash
metakaolins,” Appl. Clay Sci., vol. 83–84, pp. 253–262, 2013.
[174] G. Samson, M. Cyr, and X. Xiao, “Formulation and characterization of blended alkali-
activated materials based on flash-calcined metakaolin, fly ash and GGBS,” Constr. Build.
Mater., vol. 144, pp. 50–64, 2017.