**8. Properties of clay-based geopolymer**

Geopolymers exhibit excellent mechanical and physical properties, such as low density, good chemical, fire and thermal resistance, high mechanical strength, and so on. Therefore, they are widely applied in various fields as new materials with high tech application. Geopolymers harden rapidly. In general, metakaolin geopolymers set and harden within 24 h. Short set time of 4 h has been reported by De Silva et al. [77] cured at 40°C. Fpr fly ash based geopolymer paste, in another way, sets and hardens faster compared to metakaolin geopolymers. Accordance to Hardjito et al., fly ash geopolymer can be hardened up to 2 h when cured at 65 and 80°C [78]. However, setting time is significantly dependent on the curing temperature. The geopolymer will set faster when cured at higher temperature. At 50°C, geopolymerization process required 4 h. Furthermore, geopolymerization process needed 1.5 h and 0.5 h at 85 and 95°C, respectively [2]. If the geopolymer paste is cured at temperatures lower than ambient temperature, it might need more than 1 day to set. No degradation in the strength of geopolymers at 28 days even they set at a longer time, as reported by Rovnanik [74].

The bulk density of metakaolin geopolymers is reported in the range between 1.20 and 1.80 g/ cm3 . Thus, lightweight products can be made out of geopolymers. The bulk density reported is lower than ordinary Portland cement paste and almost or even lower than geopolymers based on slag and fly ash . For instance, ordinary Portland cement paste has density of more than 1.80 g/cm3 [35] while coal fly ash geopolymers have density in the range between 1.40 to 1.80 g/ cm3 [79, 80]. Bulk density is mainly affected by the curing condition as well as other synthesis parameters, such as the nature of alkali metal silicate, the type of geopolymers and alkali concentration. Bulk density decreases with increasing curing temperature [74]. Compressive strength increses with the increases of bulk density. Almost similar bulk density values were recorded for K-based (1.39–1.82 g/cm3 ) and Na-based (1.25–1.72 g/cm3 ) metakaolin geopolymers. Na-based geopolymers are generally lighter than K-based geopolymers. This is due to K-based geopolymers are denser and contain fewer pores as aforementioned [65].

From the result obtained by De Silva et al. [77] in **Figure 14**, high SiO2 /Al2 O3 ratio in the initial composition shows longer setting and hardening times. Strength development of metakaolin geopolymers with SiO2 /Al2 O3 of 3.81 became high and stabilized at a later age, even though the setting time was longer. Setting time is short providing that there is high Al<sup>2</sup> O3 content; however, it will deteriorate strength due to low SiO2 content. Besides, the calcium content in the precursor materials would definitely affect the setting time. This is due to the fact that the Ca content provides extra nucleation sites for precipitation of dissolved species and hence leads to setting and hardening at a faster rate [55].

stretching of Si-O-Si and Si-O-Al bonds [42, 74]. This band is shifted from the band at 1031 cm−1 in metakaolin. In addition, this FTIR band becomes more intense as the reaction proceeds indicating more geopolymer networks are formed. The band is usually shifted to lower wavenumber from raw materials and further shifted to higher wavenumber as a consequence of curing process. This is because of the changes in the silicate network with more substitution of non-bridging oxygen and increasing substitution of Al in the silicate sites. This is proved by the model by Duxson et al. [54] who proposed the transformation from Gel 1 to Gel 2 over time, aforementioned. This band shift is also observed in fly ash geopolymers (**Figure 13**) [75]. Another bands at 720 cm−1 (Si-O-Si/ Si-O-Al stretching), 560 cm−1 (tetrahedral aluminum stretching bands) and 690-440 cm−1 (Si-O-Si/ Si-O-Al bending vibrations) are also present in clay-based geopolymers [41, 42, 56]. High Si content in geopolymer structure produces stron-

Geopolymers exhibit excellent mechanical and physical properties, such as low density, good chemical, fire and thermal resistance, high mechanical strength, and so on. Therefore, they are widely applied in various fields as new materials with high tech application. Geopolymers harden rapidly. In general, metakaolin geopolymers set and harden within 24 h. Short set time of 4 h has been reported by De Silva et al. [77] cured at 40°C. Fpr fly ash based geopolymer paste, in another way, sets and hardens faster compared to metakaolin geopolymers. Accordance to Hardjito et al., fly ash geopolymer can be hardened up to 2 h when cured at 65 and 80°C [78]. However, setting time is significantly dependent on the curing temperature. The geopolymer will set faster when cured at higher temperature. At 50°C, geopolymerization process required 4 h. Furthermore, geopolymerization process needed 1.5 h and 0.5 h

ger geopolymers as the Si-O-Si bonds are stronger than Si-O-Al bonds [76].

**8. Properties of clay-based geopolymer**

**Figure 13.** Shifts of FTIR bands from Gel 1 (G1) to Gel 2 (G2) [75].

252 Cement Based Materials

**Figure 14.** Final setting times and compressive strength of metakaolin geopolymers with varying SiO<sup>2</sup> /Al2 O3 molar ratios at constant H2 O/Na2 O molar ratio of 13.6 [81].

Geopolymers achieve compressive strength of 20 MPa after only 4 h at 20°C. The 28-day compressive strength of geopolymers could be as high as 70–100 MPa [1]. High strength means the easier or higher dissolution of source materials, generating more aluminosilicate species, which are the most important ingredients for geopolymerization process. The reaction extent of source materials can be measured directly by the compressive strengths of prepared geopolymers. The strength of geopolymers is dependent on the strength of gel phase, the amount of gel phase formed and amorphous nature of the reaction products [73].

On the other hand, geopolymers have excellent thermal stability with low shrinkage (2%). Geopolymers are stable up to 1000–1200°C [4, 58, 82, 83] and have ceramic-like structure [3]. Geopolymers are dimensionally stable in the working range between 250 and 800°C, accordance to Subaer and van Riessen [84]. In order to improve the thermal properties of geopolymers, filler (e.g. granite or quartz) and foaming agents (e.g. Al powder, hydrogen peroxide) have been added during geopolymer mixing. Addition of quartz or granite reduced shrinkage to 1% [85]. In addition, based on Rickard et al. [86], foamed geopolymers reinforced with polypropylene fibers achieved fire rating of at least 1 h (**Figure 15**).

Foamed geopolymers have good potential for ambient application as thermal insulator while exhibiting low density and compressive strength. For fire resistance application, materials must have very low thermal conductivity and resistance to thermal damage as to achieve the similar fire rating. Contradict result was reported by Elimbi et al. [87], whereby metakaolin geopolymers decreased in strength when heated between 300 and 900°C. It was explained due to the progressive transformation of geopolymer matrix into crystalline phases. The metakaolin geopolymers were warped and glazed with cracks at 1000°C.

Geopolymers possess high perseverance in acidic and alkaline media [68, 88]. Comparatively, they are more stable under alkaline medium. No deterioration in mechanical properties when

the other hand, geopolymers were severely attacked when immersed in HCl solution for long period. Compression strength decreased while mass loss of samples increased. This was probably due to the de-aluminum of geopolymer structure in highly acidic medium. De-aluminum leads to mass loss of geopolymer structure as the consequences of SiAOAAl bonds break that form more silicic acid ions in acid medium. The microstructure of the produced geopolymers

Drying shrinkage is shrinkage of the geopolymer matrix as a result of the loss of unbounded water during the curing process. As aforementioned, the addition of filler minimizes shrinkage of geopolymer samples. In general, shrinkage occurs in greater tendency in materials with higher content of finer materials than those with high content of coarser materials [89]. For instance, for geopolymers with sand filler, the drying shrinkage recorded was 0.01% at 180 days. However, for geopolymers without sand filler, the drying shrinkage fluctuated

Geopolymers have great potential for variety of applications. Some applications have been successfully commercialized and marketed such as PYRAMENT blended cement and GEOPOLYMITE binders. GEOPOLYMITE binders have been used in several fields such as molding, tooling, foundry work, building's thermal insulation and furnace insulation while

SO4

Clay-Based Materials in Geopolymer Technology http://dx.doi.org/10.5772/intechopen.74438 255

) up to 360 days. On

immersed in sea water (pH = 8) and sodium sulfate solution (5% Na2

**Figure 16.** SEM micrograph of kaolinite geopolymers subjected to acid attack test after 90 days [30].

became more porous (**Figure 16**) [30].

between 0.03 and 0.04% [30].

**9. Applications of geopolymers**

**Figure 15.** Cold side temperatures during the fire testing of four mixes of metakaolin geopolymers [86].

**Figure 16.** SEM micrograph of kaolinite geopolymers subjected to acid attack test after 90 days [30].

Geopolymers possess high perseverance in acidic and alkaline media [68, 88]. Comparatively, they are more stable under alkaline medium. No deterioration in mechanical properties when immersed in sea water (pH = 8) and sodium sulfate solution (5% Na2 SO4 ) up to 360 days. On the other hand, geopolymers were severely attacked when immersed in HCl solution for long period. Compression strength decreased while mass loss of samples increased. This was probably due to the de-aluminum of geopolymer structure in highly acidic medium. De-aluminum leads to mass loss of geopolymer structure as the consequences of SiAOAAl bonds break that form more silicic acid ions in acid medium. The microstructure of the produced geopolymers became more porous (**Figure 16**) [30].

Drying shrinkage is shrinkage of the geopolymer matrix as a result of the loss of unbounded water during the curing process. As aforementioned, the addition of filler minimizes shrinkage of geopolymer samples. In general, shrinkage occurs in greater tendency in materials with higher content of finer materials than those with high content of coarser materials [89]. For instance, for geopolymers with sand filler, the drying shrinkage recorded was 0.01% at 180 days. However, for geopolymers without sand filler, the drying shrinkage fluctuated between 0.03 and 0.04% [30].
