geopolymers category

2. Introduction
 A geopolymer is a semi-crystalline amorphous material formed through the polymerization reaction
between an aluminosilicate source and an alkaline reagent. The polymerization process can be induced
and/or catalyzed by the application of heat.
 Geopolymers have been studied largely as building materials. It is a technology being developed in
order to find a more eco-friendly option compared to Portland cement that forms concretes, bricks, etc.
Their great advantage is that they can be generated through by-products from other industries. The best-
known example is fly ash from coal-fired power plants.
 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. The “polymer”
reference corresponds to its structure made up of various Al and Si monomers.

3. Geopolymers are formed by the reaction of an alkaline reagent with a source of aluminosilicates. 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.
Introduction, Cont.

4. Raw materials
For the formation of geopolymers, the following variables must be taken into account, which have repercussions on the
final properties of these materials:
1. Si/Al,
2. Si/ M+M+ ( M+M+ : alkaline ion such as Na+, K+, Na+, K+, etc.),
3. Water/solids.
 Aluminosilicate sources
1. By-products from other industries, such as fly ash, low calcium slags, and mining wastes (e.g., copper tailings).
2. Natural reagents: volcanic glass, silica gel diagnosed from acidic environments, and clays.
3. Heat-treated aluminosilicates: heat-treated clays or metakaolin.
 Alkaline activators
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.

5. Composition
 A geopolymer is created from 𝐴𝑙𝑂4− and 𝑆𝑖𝑂4− tetrahedra, each of which is attached at its four (or less) corners to
another tetrahedron by bonding atoms of oxygen, forming a 3D structure. The structure of the geopolymer is mostly
amorphous, The amorphous component of the geopolymer is called N–A–S–H gel, due to the final composition of the
geopolymerization product ( Na2O−Al2O3−SiO2−H2O ).
 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.
 For the solidification of the geopolymer to occur, the soluble silica alone is not sufficient to produce a chemically
hardened material and the Al is essential.
 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.

6. Synthesis
 First stage: destruction–coagulation
The high amount of OH ions cause the dissolution of the aluminosilicates in the raw material and, more
specifically, breaks the bonds that form these aluminosilicates.
The appearance of the ≡ Si–𝑂−–𝑁𝑎+ bonds prevent the Si–O–Si bonds from being reformed again. The
aforementioned alkali silicate bonds can contribute to the exchange of ions and types of complexes such as ≡Si–
O–Ca–OH. With the Al–O–Si bonds, the same type of reaction occurs and ends up generating complexes
predominantly of the type 𝐴𝑙(𝑂𝐻)4− . The –Si–O–Na+ complexes are stable in an alkaline medium, so they play a
transport role and allow the development of a coagulated structure based on the units mentioned above.
 Second stage: coagulation–condensation
In the coagulation–condensation stage, due to the high pH, the Si–O–Si bonds form a hydroxylated complex, 𝑆𝑖(𝑂
𝐻)3𝑂− , which condenses to form a new Si–O–Si bond to generate dimers. These particles grow in many
directions, generating colloidal particles.

7. Synthesis
 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.
 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 (Figure 2).

8. Synthesis
Figure 2 Conceptual design of
geopolymerization process.

9. Synthesis
 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.

10. Properties of geopolymers
 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. 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.
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.

11. Properties of geopolymers
Figure 3 Graph compiling
compressive strength vs
Si/Al ratio data obtained
from previous investigations

12. Properties of geopolymers
Effect of curing temperature on compressive strenth
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, 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.

13. Properties of geopolymers
Figure 4 Compressive
strength as a function of
curing temperature data
obtained from previous
investigations.

14. Properties of geopolymers
 Permeability
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−, 𝑆𝑂42− , 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.
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.

15. Properties of geopolymers
 Lower porosity leads to a more
compact microstructure, and
therefore lower permeability. In
Figure 5, the hydraulic conductivity
test by a 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 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.

16. Properties of geopolymers
 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.
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.

17. Properties of geopolymers
 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. 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. Other authors have added natural aggregates to obtain geopolymers with better properties after exposure
to high temperatures, such as sand plus glass powder and dolomite.

18. Properties of geopolymers
 Retention of hazardous and radioactive waste
Geopolymers have been shown to be capable of retaining hazardous wastes such as lead, boron, copper, arsenic, and
ammonium. 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.

19. Properties of geopolymers
 Carbonation and corrosion
Geopolymers can passivate the steel structure of concrete. The duration of this passivating layer depends on the
activating solution used.
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 .
Carbonation can also induce a loss of strength and an increase in pore volume in alkali-activated concretes.
 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. and Komnitsas and Zahraki contrasted the generation of CO2CO2 for the production of geopolymers
with the generation of CO2CO2 for the generation of cement, where it was obtained that the emissions of this
greenhouse gas are reduced to 20–80%.

20. Applications
 Geopolymers have been manufactured for the production of different materials and uses, among which the
following stand out:
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. Train sleepers based on geopolymers were produced with good results, mainly due to the physical
characteristics of the geopolymers.
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.

21. References
 (1) Provis JL , Van Deventer JSJ . Geopolymers – structure, processing, properties and
industrial applications. New Delhi: Woodhead Publishing Limited; 2009.
 (2) Davidovits J . Geopolymers – chemistry and applications. San Quintín: Institut
Géopolymere; 2008. p. 1–22.
 (3) Palomo A , Alonso S , Fernández-Jiménez A . Alkali activation of fly ashes: a NMR study of
the reaction products. J Am Ceram Soc. 2004;87(6):1141–5.
 (4) Komnitsas K , Zaharaki D . Utilisation of low-calcium slags to improve the strength and
durability of geopolymers. In Provis JL , Van Deventer JSJ , editors. Geopolymers – structure,
processing, properties and industrial applications. New Delhi: Woodhead Publishing Limited;
2009. p. 343–75.
 (5) Murayama N , Yamamoto H , Shibata S . Mechanism of zeolite synthesis from coal fly ash
by alkali hydrothermal reaction. Int J Miner Process. 2002;64(1):1–17.