Geopolymer is an alkali-activated cementitious material, an inorganic polymer, a binder that functions like ordinary Portland cement. It gains strength by activating aluminosilicate-rich material in an alkaline solution. It is cost-effective and eco-friendly, thermally stable, with high mechanical strength, which makes it a strong alternative to ordinary Portland cement. In the 1970s, Joseph Davidovits announced his idea of geopolymer for the first time.
Geopolymer source materials
An industrial waste, natural, or other materials, which are rich in aluminosilicates in increased temperature or ambient temperature react with alkali activator solution to form geopolymer. The aluminosilicate sources are called precursors. Precursors were obtained from a high range of materials including fly ash, metakaolin, silica fume, blast furnace slag. Alkali activator solution, typically obtained by mixing sodium hydroxide, and sodium silicate. Filler materials used to make geopolymer composites are the same aggregates used in cement-based composites, like, natural sand or gravel, recycled aggregates. Alkali-activated system components are schematically shown in Figure 2.1.
The reaction mechanism during the formation of geopolymer is described in detail in the following section.
Reaction mechanisms in geopolymerization
While curing geopolymer composite, geopolymerization process occurs, until now the process has not been completely described, however, it is just like zeolitization. The process illustrates the reaction of the precursor material (for example fly ash) within a high alkali environment, leading to the formation of three-dimensional networks of -Si-O and -Al-O units that is just like organic polymers structure. Curing can be done at ambient or elevated temperatures. The fundamental theory of reaction mechanism of geopolymerization process is provided by Glukhovsky in the 1950s–1970s, then the theory expanded by others. This breaks down the mechanism into dissolution, rearrangement, condensation, and resolidification stages.
The multiple processes are connected and occur concurrently, even though they are presented in a linear fashion. Alkaline hydrolysis causes the dissolution of solid aluminosilicate sources, which is induced by a high concentration of hydroxide anion. Destruction of Si-O-Si, Si-O-Al, and Al-O-Al bonds takes place, resulting in the formation of Si-OH and Al-OH units, that are often monomeric. The aqueous phase, which may already include silicate, is infused with silicate and aluminate, as well as aluminosilicate compounds. A complicated composite is produced when Si–O–(Si,Al), as well as aluminosilicate compounds, are integrated within the aqueous phase, which can also include silicate from the alkaline activation solution.
At elevated alkalinity, crystalline aluminosilicates decompose quickly, resulting in the production of an over-hydrated aluminosilicate solution. Aggregation improves links between the dissolved components, resulting in a clotted structure in which polycondensation occurs. Gel formation releases back the pre-consumed water throughout the dissolving process, to serve as reaction medium in the gel’s pores. The time it takes for the gel to develop is determined by the status of dissolved ions, as well as the presence or absence of the conditions required for gel precipitation.
As the gel network’s cohesion grows, the existence of components from the initial solid phase, as well as the emergence of microparticles caused by condensation, realign and reorganize to form a three-dimensional aluminosilicate network. The solidified geopolymer’s texture and voids dispersion are determined by the early phase’s mineralogical composition, the nature of the alkaline component, and the final stage’s polymerization and hardening process. Figure 2.3 shows the reaction mechanism of geopolymerization.
Mechanical properties of geopolymers
Deep knowledge of the mechanical properties of GPC is required in order to start designing GPC mixtures with predictive engineering properties. A considerable number of studies have been done on these properties in the past 20 years. Mechanical properties of GPC is affected by the following parameters; (a) ratio of alkali activator solutions to source (precursor) material (fly ash, metakaolin, etc.), (b) molarity of sodium hydroxide solution, (c) SS/SH ratio, this also relies on SS composition, (d) temperature of curing, (e) duration of curing, (f) water content.
Compressive strength has been the main interest of researchers studying GPC. Factors affecting this property will be discussed in short. (Shehab et al., 2016) concluded that maximum compressive strength, bond strength, flexural and splitting tensile strength is achievable when %50 of ordinary Portland cement (OPC) is replaced by fly ash (FA), whereas observed that when 10% of FA is replaced with OPC compressive strength, splitting tensile strength and flexural strength are increased.
A study by witnessed that the compressive strength of GPC increased by adding 24 h time prior to curing. There is a high advantage of using elevated curing temperature over ambient curing when it comes to the compressive strength of GPC. Increasing the curing period will increase compressive strength and benefits the geopolymerization process up to a limit, then further curing will negatively affect the compressive strength (Nurruddin et al., 2018). As a general rule increased early curing temperature and time enhance compressive strength.
(Kumar et al., 2017) studied the strength gain of a ternary blend GPC with age and found that the 7 days compressive strength was about 88% to 90% of 28 days compressive strength. Other investigations by (Nguyen et al., 2020) found that fly ash type, heat curing method, or fly ash (FA) replacement with GGBS has no effect on 7-day to 28-day compressive strength ratio and the ratio was 93%.
However, experiments by (Chi, 2017) showed that the ratio changes depending on curing temperature and it is 88% for mortar cured at 65 °C and its higher than 66% when normally cured. When metakaolin-based GPC blend cured at ambient temperature, 7 days compressive strength was 73% and 88% of the strength at 28 days.
A study by (Nguyen et al., 2020) showed that when the water to solid ratio increased from 0.2 to 0.3 compressive strength lowered in an FA-based GPC with alkaline to binder ratios of 0.3 and 0.4, whereas experiments by (A. A. Mohammed et al., 2021) showed that maximum compressive strength is achieved by using optimum water to binder ratio of 0.25 for GPC cured at 70 °C (Oven) for 24 h. (Abdullah et al., 2011; Mustafa Al Bakri et al., 2012) indicate that increasing the alkali/binder ratio has a positive effect on strength development up to 0.45, less than 0.5.
According to (Mustafa Al Bakri et al., 2012) highest compressive strength is achieved with a ratio of Na2SiO3/NaOH of 2.5 for a GPC mix with early oven curing at 70 °C for 24 hours. The same result was witnessed by (Abdullah et al., 2011; Aliabdo et al., 2016; Aziz et al., 2020; Hadi et al., 2017; Joseph & Mathew, 2012).
Maximum compressive strength of GPC is obtained by using NaOH solution molarity of 12, however, other studies found optimum molarity to be 14. In contrast, other researchers set optimum molarity as 16.
The highest compressive strength is achieved by using 100% GGBS with any curing regime, and strength is declined whenever GGBS is replaced by slag or fly ash. Similar observations obtained by claimed that the highest compressive strength is achieved when GGBS is used alone and any incorporation of other precursors (fly ash, silica fume, or metakaolin) as the partial replacement will decrease strength, or the strength of GPC improves when the source precursor is replaced with GGBC.
Adding superplasticizer up to 2% by mass of fly ash didn’t considerably affect the compressive strength of GPC. However, compressive strength decreased after increasing the content of superplasticizer from 2% to 4%. In contrast, increasing the content of superplasticizer had very little effect on compressive strength in a study by.
Splitting tensile strength and flexural strength
Any increase or decrease in compressive strength of GPC will affect indirect tensile strength and flexural strength in a similar way (Raijiwala & Patil, 2010; Rashad, 2013). It means that the factors affecting compressive strength will also affect these two properties. Experiments by (Rangan et al., 2005) indicated that the splitting tensile strength of GPS in just a fraction of compressive strength. Although, there are some differences from this general rule explained by a number of researchers.
A study by (Ryu et al., 2013) concluded that the rate of tensile strength development decreases while compressive strength increases. When fly ash is partially substituted by GGBS, the impact on splitting tensile strength and flexural strength is less than that on compressive strength.
Flexural strength decreased while compressive strength increased when the replacement of fly ash to slag increased from 15% to 20%. Initial curing of GPC at 75 °C for 26 hours duration unlike elastic modulus, both compressive strength and flexural strength improved. Further studies by (Saravanan & Elavenil, 2018) indicated that when 50% of fly ash is replaced to GGBS, unlike compressive strength, splitting tensile strength is considerably improved.
Applications of geopolymer
Geopolymer composites have a wide range of applications. The following are a few examples of which this technology is used for.
Immobilization of nuclear waste
The fast advancement of nuclear energy has necessitated the safe treatment of radioactive nuclear wastes produced in nuclear fuel cycles. Immobilization has recently been hailed as a viable option for storing radioactive nuclear waste. The standard immobilization approach for intermediate-level nuclear waste (ILW) and low-level nuclear waste (LLW) is typically cement immobilization.
Cement’s large porosity, poor temperature stability, and corrosion resistance, on the other hand, limit its use as an optimal long-term immobilization binder. Because of its superior mechanical strength, stability, and immobilization abilities, geopolymer has lately been proposed as a solution for capturing radioactive nuclear wastes.
According to (Li et al., 2013), the leaching rate of 133Cs+ immobilized in geopolymer made with fly ash is only 7% of that immobilized in cement, implying that geopolymer outperforms conventional cement composites.
Thermal-resistant and fire-retardant characteristics and uses
Offshore oil mining stations, aircraft claddings, military, aerospace, and transports are all potential usage for geopolymer composites. Geopolymers supplemented with high-performance fibers, for example, can just be made at ambient temperature and have a lower heat conductivity. This qualifies them as prospective coating systems for inner ignition engine exhaust tubes and other refractory compounds.
Future construction materials
Because of its great hardness and short curing time, geopolymer has been suggested as being one of the contenders for developing upcoming construction materials. The University of Queensland in Australia (Fig. 2.5) and the Wellcamp Airport in Brisbane have both benefited from the usage of geopolymer composite. Furthermore, inorganic geopolymer composite exhibits great resilience to high and low-temperature cycles, as well as superior vacuum stability, decreased water utilization, and good mechanical properties, satisfying all of the necessities for lunar building materials.
Fire resistant properties of GPC
On November 15th, 2010, a 28-story skyscraper in Shanghai, China, went up in flames, killing at least 42 persons and gravely injuring 90 more, causing widespread worry about building fire resistance. One other catastrophe of this age is the destruction of the two buildings in New York City during the world trade center attacks in 2001. During 2 hours, the steel frame had fallen caused by fire.
Furthermore, many organic matrixes rarely withstand temperatures beyond 200°C and will emit noxious gas when exposed to heat or fire. As a result, there is a pressing need to improve the fire/heat resistance of buildings. These issues could be solved with geopolymers, coatings, and matrix. The geopolymers developed lately are said to have great burning resistance due to their ceramic-like properties, and they are made via alkali treatment and polymerization. source ingredients made of alumino-silicate.
The mechanical characteristics of carbon compounds produced with an inorganic polymer were initially investigated by (Foden et al., 1996), comprising tension, flexure, and shear forces. Even at 1000°C, the carbon compound constructed using geopolymer composite has been shown to maintain significant load-carrying capacity.
Later works by (Bakharev, 2005, 2006; Deventer et al., 2012; Guerrieri & Sanjayan, 2010; Kong & Sanjayan, 2008, 2010; Liu et al., 2020; Pan & Sanjayan, 2010; Provis & Van Deventer, 2009; Zhao & Sanjayan, 2011) indicates that geopolymers made with fly ash are superior to those made with metakaolin when it comes to fire resistance. According to (Kong et al., 2007) findings, the strength of the fly ash-based geopolymer mortar increased by 6%, whereas the strength of metakaolin-synthesized geopolymer mortar equivalents decreased by 34% following heat exposure of 800°C.
It is due to the porous structure as well as correct pore structure of geopolymer matrixes made with fly ash which expedite the drying of liberated water and water within the chemically bonded substances while the temperature is increased, such reducing the matrix’s degradation. The substantial fraction of empty spheres as well as the sintering reactions of un-reacted fly ash particles are likewise partially responsible for the increased strength and considerable thermal resistance of the fly ash-derived geopolymers.
Fly ash is cheaper and pollutes the environment less thanmetakaolin. (Kong & Sanjayan, 2008; Xu & Deventer, 2000) found that KOH-based composites outperform NaOH-based mixtures at ambient temperature and at increased heating temperatures (following higher temperature disclosure until 850°C).
Furthermore, (Kong & Sanjayan, 2008) demonstrated that the sodium-silicate-based geopolymer had higher compressive strength over its potassium silicate-based counterpart, either prior to or following the high temperatures (850oC) exposure. Figure 2.6 shows residual compressive strength of geopolymer composite after exposure to elevated temperature.
Figure 2.7 shows compressive strength of geopolymer materials of w/b= 0.09 prepared using class F fly ash and sodium hydroxide before and after firing experiments. Specimens G1 was not pressure compacted, while all other specimens were pressure compacted.