State of the art of geopolymers: A review

1.1 Definition

The concept of geopolymer is a composition of the prefix “geo,” which corresponds to “earth” in Greek, referring to the content of Al and Si both of which are highly present in the earth’s crust (2). The “polymer” reference corresponds to its structure made up of various Al and Si monomers.

Geopolymers are formed by the reaction of an alkaline reagent with a source of aluminosilicates (2). This process can occur at room temperature and at high temperatures (e.g., 90°C). The material with aluminosilicates (fly ash, metakaolin, calcined clay, among others) reacts with an alkaline activator that contains alkali hydroxides, silicates, aluminates, carbonates, and/or sulfates (Figure 1).

Figure 1

Diagram of the process of creating a geopolymer.

Sometimes, geopolymers are related as zeolitic precursors. They are so called because they have a three-dimensional (3D) tetrahedron structure formed by aluminates and silicates. However, they differ in that zeolites have an ordered crystalline structure and geopolymers have an amorphous one. In addition, higher pressures and temperatures are used to form zeolites (over 100°C and around 200 kPa, respectively) to achieve the formation of the crystalline structure. The reaction process of geopolymers oriented to the formation of cementitious materials can be considered as a zeolitization where the crystallization stage is not achieved (3).

1.2 History

The chemical studies of Joseph Davidovits focused on the chemistry of organic polymers. After several catastrophic fires in France between 1970 and 1972, where organic plastic was involved, the study of flammable plastic materials became paramount. In 1972, Davidovits founded a private research company “Cordi S.A.,” later renamed “Cordi-Géopolymère.” Davidovits was intrigued that simple hydrothermal conditions controlled the synthesis of some organic plastics in alkaline media, such as feldspars and zeolites.

On the other hand, kaolinite aluminosilicate reacts with sodium hydroxide (NaOH) at 100–150°C and polycondenses into hydrated sodalite.

From the studies covering the synthesis of zeolites and molecular sieves (essentially in the form of powders) it was determined that this geochemistry had not been investigated to produce mineral binders and mineral polymers. As a result, Davidovits proceeded to develop a 3D material of aluminosilicates between amorphous and semi-crystalline which he baptized as a geopolymer in 1988.

2.1 Aluminosilicate sources

According to previous research, there are a variety of sources of aluminosilicates for the manufacture of geopolymers, these are classified as follows:

1. By-products from other industries, such as fly ash, low calcium slags (4), and mining wastes (5,6,7) (e.g., copper tailings).

2. Natural reagents: volcanic glass, silica gel diagnosed from acidic environments (8), and clays (9,10).

3. Heat-treated aluminosilicates: heat-treated clays or metakaolin.

These materials share the characteristic of having large amounts of silica and Al. Some of the materials that have been most researched will be described in the following paragraphs.

2.2 Alkaline activators

As mentioned above, an alkaline reagent is also needed, which is responsible for the dissolution of the aluminosilicates present in the raw material. There are different reagents for the activation of geopolymers, the most used are silicate solutions and alkaline hydroxides. The use of different reagents produces geopolymers of different characteristics, so the choice of reagents depends on the properties sought (17). There are currently studies that analyze the performance of calcium hydroxide as an activator.

3.1 Composition

A geopolymer is created from AlO 4 − and SiO 4 − tetrahedra, each of which is attached at its four (or less) corners to another tetrahedron by bonding of atoms of oxygen, forming a 3D structure. The structure of the geopolymer is mostly amorphous, although it may also have some amounts of zeolitic phases. The amorphous component of the geopolymer is called N–A–S–H gel, due to the final composition of the geopolymerization product ( Na 2 O − Al 2 O 3 − SiO 2 − H 2 O ).

Geopolymers are inorganic polymeric materials obtained by mixing a dry solid (aluminosilicate) with an alkaline solution and other reactives. The most relevant component is the source material, which must be rich in Si and Al (25).

For the solidification of the geopolymer to occur, Al is essential. Mixtures with high concentrations of alkali silicates are commonly metastable. This is because the silica tetrahedra are attacked by water, which generates an adjacent pair of silanol (Si–OH) (Eq. 1) until reaching an equilibrium and forming Si ( OH ) 4 , but being in an alkaline environment, the reaction shown in Eq. 2 occurs, which causes the remaining oxygen bonds to weaken and continue being attacked by water until the silicate is completely dissolved (26). Therefore, soluble silica alone is not sufficient to produce a chemically hardened material.

Silicon–oxygen–silicon bonds and silicon–oxygen–aluminum bonds are formed due to the bonding of aluminate and silicate tetrahedra. On the other hand, because they are not energetically stable, no aluminum–oxygen–aluminum bonds are formed, which implies that the Si/Al ratio can reach a minimum value of 1. On the other hand, the amount of Al must be sufficient so that the dissolution of silica does not occur (26).

Due to the alkalinity caused by the activating solution, which raises the pH values, dissolution of the aluminosilicates present in the raw material occurs. Subsequently, during molecular organization, the Si tetrahedra can be replaced by Al tetrahedra, which results in the Al tetrahedron being negatively charged. This negative charge is counteracted by the positive charge of the alkaline cations, so that these cations from the activating solution become part of the network (27).

Regarding the elements present in the N–A–S–H gel, Fernández-Jiménez and Palomo (28) provide the following information in this regard (the study focuses on fly ash):

1. Si mainly participates in the gel based on the zeolitic nuclei. It has a fundamental role in the control and kinetics of the formation of this gel in the early stages, especially if the alkaline activator to be used is water glass (SS), since it undergoes a dissolution in the initial stages of polymerization, due to which the first monomers are delivered to form the silica-rich gel. If this silicate has many dimers in its composition, the gel formation will be faster, but it will also imply that it is more metastable, so the Si/Al ratio cannot be increased indefinitely.

2. Al has an active role in the beginning of chemical reactions, if the source has a large amount of alumina, large amounts of Al are released into the solution, which increases the reactivity of the source, otherwise, the content of Al that is released will be consumed quickly and will reduce the reactivity of the source. It is speculated that the Al content in the source should correspond to at least 20% of this. An excess of Al can lead to the reaction of the crystalline products. Dissolved Al enters the structure of the Si-rich gel, which gives it better stability.

3. Sodium plays a role as a load balancer, either to balance the Al monomers present within the gel, which accordingly ends up giving greater stability to the gel, or to the Al present in the solution at low Si/Al ratios, filling the pores of mixtures with zeolitic products. The Na + coordination tends to be the same in all systems and binds oxygen and water molecules.

When activating the sources of aluminosilicates, the Al species undergo transformations, the majority being transformed to Al(iv) (or Al ( OH ) 4 − ); therefore, if there is still the presence of Al(vi) (or Al ( OH ) 6 3 − ), one can have a notion of the amount of aluminosilicate that did not react (16).

Davidovits (29) and Rowles and O’Connor (30) have observed the formation of phases described as semi-crystalline or polycrystalline. These crystalline phases are generally zeolites and their content increases when the synthesis is carried out under hydrothermal conditions in alkaline media with a lesser degree of soluble silica content (31). The hydrothermal reaction in alkaline media of kaolin, metakaolin, fly ash, and other sources of aluminosilicates leads to the formation of zeolites with different structures depending on the reaction conditions (temperature, nature of the alkali cations, Si/Al ratio, etc.).

A high water content allows the species in the mixture to be fully hydrated, with small interactions between ion pairs. Although the silicate and aluminate species depend on the concentration and the Si/Al ratio of the mixture, this dilution of the medium improves the transport of these species and their subsequent reorganization in the gel. Under these conditions, the growth of the gel precipitates takes place without hindrance.

María (31) presented in his study a diffractogram based on a gel with NaAlSiO 4 composition, synthesized at 40°C from metakaolin. Although the material initially appears amorphous, the formation of crystalline zeolites is observed after 7 days, which indicates that their formation is favored by the reaction time.

Duxson (16) showed an X-ray diffractogram of a metakaolin-derived polymer with nominal composition KAlSiO 4 , which was cured at temperatures of 70°C, 90°C, and 120°C for 24 h. A new crystalline phase could be observed at a temperature of 120°C, showing that even mild increase in the synthesis temperature leads to visible increase in the crystallinity.

3.2 Synthesis

The synthesis mechanisms that occur from ingredient mixing to hardened geopolymer are very complex and remain poorly understood at this moment in time.

Murayama et al. (32) described zeolitization starting with a solution of Si and Al monomers from the reactant aluminosilicate, followed by a condensation of silicate and aluminate ions where the aluminosilicate gel precipitates and ends with the crystallization of zeolite. The geopolymerization process is similar to that of zeolitization, except that the former does not complete the crystallization stage (33). In geopolymers, crystallization is mainly attributed to the low water content and high alkalinity of the systems.

Davidovits (29) proposed a nomenclature to describe the connections of the structure through polysialates, based on the following empirical formula: M p { ( SiO 2 ) z AlO 2 } p w H 2 O , where p is the degree of polycondensation; z is between the values 1, 2, and 3; w is the water contained; and M corresponds to the alkali ion.

Despite giving clarity on how the geopolymer structures are formed based on the monomers from the aluminosilicate source, it assumes that the construction is two-dimensional, with the geopolymer structure being 3D.

Subsequently, a more suitable annotation emerged to characterize the 3D structure of aluminosilicate systems, including glasses, gels, zeolites, and minerals (16). Tetrahedral structures are designated Q n , in which n is the link to other tetrahedra. In the case of aluminosilicates, a component is added to indicate how many of the bonds are attached to Al tetrahedra ( Q n ( m Al ) ) (20).

Polymerization can be represented schematically as follows (34):

Glukhovsky (35) proposed a polymerization model consisting of three stages: (1) destruction–coagulation, (2) coagulation–condensation, and (3) condensation–crystallization.

3.2.3 Third stage: condensation–crystallization

For the condensation–crystallization stage, in addition to the microparticles formed from the condensation, the particles of the solid phases from the source indicate the precipitation of products dependent on the mineralogy and chemistry of the initial phase, as well as the nature of alkaline component and curing conditions.

Duxson et al. (36) made a model where he considered the formation of zeolite: (1) the stage of formation of the zeolite precursors (the first two Glukhovsky stages) and (2) a stage where the zeolite nuclei reach a critical size and begin to crystallize.

The process, albeit though it seems linear, occurs simultaneously. The initial contact between the solids and the alkaline solution causes the dissolution of the amorphous components, releasing silicates and aluminates generally as monomers. Monomers interact to form dimers, which interact with other monomers to create trimers, then tetramers and so on (monomers should be understood as low molecular weight compounds that can bind to other compounds, so a dimer would be two monomers joined and so on with the other cases). When the solution is saturated, the first gel rich in Al is formed because this element is concentrated in the initial stages of dissolution (first 4–5 h) (31). As polymerization occurs, more Si reaches the solution, increasing the concentration of Si in the solution and in the gel phase of the zeolite precursor. The system continues to reorganize itself and the connectivity of the gel network increases accordingly. The result is the formation of a 3D network of aluminosilicates. The final stage of polymerization and hardening determines the microstructure and pore distribution of the hardened geopolymer, which is crucial for the physical properties of the resulting mixture (28).

The geopolymerization process is often approximated by the following highly simplified conceptual reactions: (i) the raw materials are dissolved in alkali solutions, such as NaOH and KOH, to release the reactive aluminate and silicate monomers and (ii) the aluminosilicate oligomers polymerize in the alkali environment to form geopolymer gels. Because of the charge deficiency in Al (which has a 3+ charge compared to Si having 4+), cations of Na or K are needed to balance the presence of Al. Water is consumed during the dissolution of raw materials and released in the polymerization processes (37) which are schematically represented by Eqs. 4 and 5 (Figure 2).

(4) n ( Si 2 O 5 , Al 2 O 2 ) + 2 n SiO 2 + 4 n H 2 O ⟶ NaOH,KOH n ((OH) 3 − Si − O − Al ( − ) − O − Si − ( OH ) 3 ) | ( OH ) 2

(5) n ( ( OH ) 3 − Si − O − Al ( − ) | ( OH ) 2 − O − Si − ( OH ) 3 ) ⟶ NaOH , KOH ( Na , K ) 3 ( + ) − | Si   | O | − O −       | Al ( − ) | O | − O −       | Si | O | − O −       + ⋯ + 4 n H 2 O

Figure 2

Conceptual design of geopolymerization process.

It is hypothesized that the polymerization process includes a series of reactions which are intertwined with each other: (i) aluminate and silicate monomers are first polymerized to form oligomers with different sizes, (ii) large clusters and ring structures are subsequently formed by the gelation of the oligomers, and (iii) cross-linked structures are finally formed via the condensation of the clusters and rings (37).

Provis and Rees (38) conducted a study on the kinetics of the synthesis of geopolymers based on the use of fly ash and caustic soda, where it was observed that at low concentrations of the activator (3 M NaOH), the induction time (period in which the reagents come into contact, hence a gel has not yet formed) increases, then by increasing the concentration to 9 M it was possible to reduce the induction time, then the time increases again due to the excess of NaOH that can lead to precipitation. After this time has elapsed, all concentrations present a similar gel formation ratio.

Additionally, it was suggested that the presence of zeolite crystals as nuclei allows it to reduce the induction time. Provis and Rees (38) showed that this hypothesis is correct, where the gel formation in the presence of crystals is immediate, and after some time a similar formation ratio is maintained in both cases.

Later it was proven that the presence of Al also reduces this induction time in cases where crystals are not present (39).

4.1 Mechanical strength

The early and final strength depends on the composition of the geopolymer, more specifically on the ratios that define the geopolymer mixtures, and the reactivity of the aluminosilicate source.

Lahoti et al. (40) analyzed four design variables for metakaolin-based geopolymers (water/solid ratio, Si/Al ratio, Al/Na ratio, and water/Na ratio). Their results revealed a trend, where the highest compressive strengths obtained were with Si/Al ratios close to 2. It also showed the dependence of compressive strength on water/solid ratio, while a general trend showed that the strength decreases with increasing water/solid ratio, strong variation in compressive strength at the same water/solids is also observed. The results of the variation in the Al/Na ratio indicated that with a value close to 1, the highest compressive strengths are obtained. Finally, the variation in the water/Na ratio showed no trend.

Subsequently, the same author made an analysis of the relevance of the reasons previously studied. From this it was obtained that the most important variable for the formulation of geopolymers is the Si/Al ratio.

The general improvement in compressive strength is mainly due to the improvement in the microstructure of the polymer, which becomes denser and less porous (27,41,42).

A compilation of information on compressive strength obtained with different Si/Al ratios collected from previous research on geopolymers from various sources is presented in Figure 3. From this it was observed that most authors used Si/Al ratios between 1.5 and 2 to obtain the highest compressive strengths. In spite of this, the compressive strength of geopolymers also depends on other variables, such as the raw material used, the type of alkaline agent, among others.

Figure 3

Graph compiling compressive strength vs Si/Al ratio data obtained from previous investigations (16,40,71,72,73,74,75).

The curing temperature of geopolymers has an important effect on the occurrence of geopolymerization reactions, because it increases the kinetics of the reactions that occur, as well as increases the reaction between the alkaline agent and the raw material. On the other hand, it has been observed that longer curing times produce higher compressive strengths.

According to previous studies, elevated curing temperatures produce increase in the compressive strength of the geopolymers (43), due to the increase in the speed of the reactions that occur, in addition to the increase in the interaction between the components of the geopolymer, which favors geopolymerization. This effect can be observed in Figure 4, where the variation in the compressive strength of the geopolymers with the curing temperature is shown. In most cases, it was observed that the optimum curing temperature varied between 80°C and 90°C.

Figure 4

Compressive strength as a function of curing temperature data obtained from previous investigations (11,76,77,78,79,80).

An important development in the compressive strength of geopolymers is due to the concentration of the alkaline activator, within this topic, the SS/NaOH ratio is one of the most influential parameters in the development of good compressive strength of geopolymers. Therefore, Figure 5 shows a compilation of data obtained from previous studies, which analyzed the effect of the SS/NaOH ratio on the compressive strength of geopolymers. No trends were found when varying the SS/NaOH ratio, because the authors had very different results. It should be considered that the type of raw material used for the manufacture of geopolymers is an important parameter, so the variation in the compressive strength of geopolymers also depends on this factor, within this, it has been observed that geopolymers created based on fly ash have higher compressive strengths than those made based on copper tailings. It should be noted that in most cases it is preferred to use both alkaline reagents together to obtain geopolymers with denser matrices.

Figure 5

Graph compiling compressive strength data vs Na₂SiO₃/NaOH ratio obtained from previous investigations (80,81,82).

Because the initial water content in geopolymer blends has an important effect on the compressive strength of geopolymers, data were collected on previous studies of geopolymers that analyzed the effect of water content, more precisely, the water/solid ratio.

From the information collected on the variation in the water/solid ratio, it was observed that higher values of this ratio led to geopolymers with lower compressive strengths (Figure 6). However, very low values of this ratio, i.e., low water content can lead to lower compressive strengths. On the other hand, it should be mentioned that the necessary water content depends on the type of compounds used for the formation of the geopolymer. Optimizing the water content is an important parameter for the generation of geopolymers, because the concentration of the alkaline reagents depends on the water content in the mixture.

Figure 6

Graph compiling compressive strength data vs water/solids ratio obtained from previous investigations (80,83,84).

4.2 Permeability

In order to assess the durability and enhance the useful life of alkali-activated geopolymers as a construction material, the understanding of water transport and its correlation with microstructure parameters is a crucial step.

Water transport, pore structure characteristics, and relevant ion transport are closely linked to permeability, especially for reinforced geopolymer concrete structures. It is well known that water can be a carrier of Cl − , SO 4 2 − , or CO₂ that can penetrate into the geopolymer material and interact with the reaction products. The result is an altered matrix microstructure and corrosion initiation which will further lead to degradation of the reinforcement (44).

The most important parameter that influences the water and ion permeability is the material pore structure, particularly the pore volume, pore size distribution, connectivity, and shape of the pores. It was observed that the increase in the total porosity, effective porosity, and pore diameter produces an increase in the geopolymer permeability (44).

Lower porosity leads to a more compact microstructure, and therefore lower permeability (45). In Figure 7, the hydraulic conductivity test by flexible wall is presented for a test piece compacted with a tamper for a sample of a copper tailing-based geopolymer dried in an oven at 90°C and cured for a period of 7 days. The mixture presented a behavior like soil, since in the measurement of the Skempton variable, which measures the degree of saturation of the sample that must reach a minimum of 95% to carry out the test, it very quickly reached 93% but the remaining 2% took more than a week. The test result indicated that the copper flotation tailings have a permeability of 7.0 × 10⁻⁷ cm·s⁻¹. Compared to a tailings dam with permeability values of 1.0 × 10⁻⁵ cm·s⁻¹, it can be indicated that the copper tailings geopolymers is more impermeable than other tailings.

Figure 7

Viscosity–hydraulic gradient for a copper tailing-based geopolymer.

4.3 Rheology

The rheology of geopolymer pastes depends mainly on the raw material used, the amount of initial water, and the alkaline solution used. SS solutions are generally more viscous than NaOH solutions (46,47,48).

Riffai et al. (48) conducted an investigation on the influence of NaOH concentration on rheology during the formation of geopolymers from fly ash. From this it wasobtained that the yield strength of the geopolymeric pastesincreased with the concentration of the activator, where itwas maximal at a concentration of 8 M NaOH (49), then decreases to below 4 Pa and increases gradually. The viscosity increases to a concentration of 3.5 M. After decreasing, the viscosity increases again from a concentration of 8 M NaOH, as a result of the increase in the viscosity of the NaOH solution at high concentrations (48).

In an analysis carried out in which the composition of the alkaline activator used for the production of geopolymers based on copper tailings was varied, it was found that an increase in the SS content produced a decrease in the yield strength of the geopolymer pastes. It should be noted that the 100% SS sample showed a different behavior from the others. Figure 8 shows the yield stress of a copper tailing-based geopolymer paste at different times after mixing.

Figure 8

Yield stress for a copper tailing-based geopolymer paste at different times after mixing.

From Figure 9, it can be seen that the longer the time after shaking, the higher the yield strength of the geopolymeric paste, increasing from 1,140 Pa at time 0 to 1,370 Pa at 4 h after mixing. The sample was dried in an oven at 90°C and cured for a period of 7 days.

Figure 9

Rheological curves of a geopolymer paste based on copper glues with different compositions of alkaline activators.

4.4 Acid resistance

Acidic media cause damage to the geopolymers, which is due to the proton exchange with the alkaline cations, in addition to producing a loss of Al in the initial gel (18).

The resistance of geopolymers under acid condition depends on multiple factors: the acid solution concentration, exposure period, and environmental condition, such as pressure and temperature. The acid resistance also depends on the type of the alkaline activator and the mineralogical composition of the raw materials (50).

It has been shown that ordinary Portland cement (OPC) has a worse resistance to acidic media than geopolymers (51). This was also observed in a study carried out by Neven et al. (52), where it was estimated that the degradation rate of geopolymers in acidic environments is at least half of that of OPC.

In order to obtain greater durability, studies of geopolymers in recent years have been based on studying the addition of different natural aggregates in order to improve resistance to acids, forming more compact matrices, which prevents these acids from penetrating the structure of the geopolymer.

4.5 Resistance to alkali–aggregate reaction (AAR)

Previous studies have shown that the basic reaction of silica is due to the calcium present in the aluminosilicate sources, for this reason, in fly ash-based geopolymers this effect is rarely seen.

In one of the studies (53), the authors compared the expansion of Portland cement mortar prisms (OPC) with fly ash cement activated with NaOH and soluble silica. Soluble silica-activated fly ash and Portland cement prisms exceed the 16-day expansion limit (this limit indicated by ASTM C1260-94 indicates that if it exceeds 0.1% expansion within the first 16 days of the mixture there is a potential detrimental effect caused by the AAR). In contrast, the NaOH-activated fly ash geopolymer exceeds the limit at around 4 months. The expansion of both the geopolymers can be attributed to an “alkali–silica reaction” gel formation (a gel that is formed by the hydration of sodium silicate [NSH] in the activator solution and that expands when more water is absorbed) and the crystallization of zeolite (51).

4.6 High temperature resistance

Geopolymer is a novel material that has a wide range of applications, including its use in structures. Protection of structures from fire is of extreme importance, this is why many studies have been carried out lately to analyze the properties of geopolymers after being exposed to high temperatures.

Different authors have found several ways to improve the strength of geopolymers after exposure to high temperatures. Geopolymers activated with KOH showed improvements in compressive strength (30–40%) in contrast to those activated with NaOH (54). Geopolymers created with 50% metakaolin and 50% fly ash have shown that this composition is optimal for obtaining better bending and compressive strengths both at room temperature and after exposure to high temperatures (55). Other authors have added natural aggregates to obtain geopolymers with better properties after exposure to high temperatures, such as sand plus glass powder (56) and dolomite (57).

4.7 Retention of hazardous and radioactive waste

Because polymers are compounds of amorphous aluminosilicates similar to zeolites and thus possess similar cation-exchange properties, geopolymers have been successfully used to remove toxic metals and organic dyes from aqueous solutions.

Geopolymers have been shown to be capable of retaining hazardous wastes such as lead, boron, copper, arsenic, and ammonium (58,59). The first two cannot be stabilized in Portland cements as they tend to inhibit the hydration process of the cement. However, chromium cannot be retained in fly ash-based geopolymeric matrices due to the formation of soluble salts.

In addition, geopolymers can retain radioactive elements, such as cesium, molybdenum, and strontium (60,61,62).

Geopolymers appear to act like zeolites by retaining certain cations (60).

4.8 Carbonation and corrosion

Geopolymers can passivate the steel structure of concrete. The duration of this passivating layer depends on the activating solution used (51).

Carbonation at early age is expected to reduce the extent of reaction by limiting the availability of alkalis, which are needed for the reaction process to continue, and so this may be damaging to the performance of the material. The influence of carbonation on the mechanical integrity of the binder phases also needs to be examined, as there are indications that a loss of strength may take place during carbonation (21,22,63).

Carbonation can also induce a loss of strength and an increase in pore volume in alkali-activated concretes (60).

4.9 Volume stability

Kuenzel et al. (64) investigated the amount of water in moles necessary to avoid a chemical contraction by altering the Si/Al ratio, therein observing that the mixture became more sensitive on raising the Si/Al ratio. There are possible reasons to explain this increase and the greater gel formation, and again a possible increase in porosity and/or changes in the ion pairs Na + :AlO 4 − due to changes in Al content in the gel (64).

Increasing the Na content dramatically increases the amount of water needed to prevent shrinkage. This implies that the hydration of Na + cations has an important role in the stability of the mixture (64).

On the other hand, when geopolymers are used as construction materials, they are expected to maintain their volumetric stability under different circumstances, such as high temperatures, high humidity, aggressive chemical environments, etc.

A study by Lahoti et al. (65) showed that the variation in the Si/Al ratio of geopolymers based on metakaolin presents different properties in geopolymers affected by high temperatures. It was observed that a Si/Al ratio of 2 is the optimal one for obtaining good resistance and volumetric stability after heat treatment.

Luhar et al. (66) also obtained improvements in the volumetric stability of geopolymers by varying the composition of the raw material. In this case, the addition of 20% of glass wastes produced a significant reduction in the weight loss of the geopolymer when it was subjected to sulfate attack, compared to the case without the addition of this aggregate.

4.10 Ecological advantages

The action of reusing waste from other industries for the generation of geopolymers and thus avoid their deposition in landfills is an ecological advantage.

Duxson et al. (67) and Komnitsas and Zahraki (68) contrasted the generation of CO 2 for the production of geopolymers with the generation of CO 2 for the generation of cement, where it was obtained that the emissions of this greenhouse gas are reduced to 20–80%.

However, in order to reach a verdict on the ecological advantages of geopolymers, more in-depth studies must be carried out to analyze variables that have not yet been taken into account.

4.11 Applications

Geopolymers have been manufactured for the production of different materials and uses, among which the following stand out (2) (69):

1. Geopolymer cement molds.

2. Thermal insulation materials.

3. Coatings to strengthen structures.

4. Cement-like construction material.

Despite the wide range of uses in which they can be used, they have only been applied on a large scale as construction materials, such as:

1. Geopolymer cement was produced on a large scale using the name “Pyrament.” The production lasted only 4 years due to financial problems of the company (2).

2. Train sleepers based on geopolymers were produced with good results, mainly due to the physical characteristics of the geopolymers (70).

3. In Australia, geopolymer cements are produced, where they sell prefabricated geopolymer parts under the name “E-crete,” where they are used for different structural purposes. In addition, this company has reported that its carbon dioxide emissions are up to 80% lower than those of cement.

From the examples, geopolymers are made for certain market sectors, but not for the entire CPO market. More pilot plants are needed to develop more applications for geopolymers, in order to make maximum use of this environmentally friendly material.