**7. Functional group identification**

The metakaolin retains some long-range order as result of stacking of the hexagonal layers [62]. Therefore, metakaolin shows semi-crystalline to amorphous pattern with a halo at 2θ between 15 and 30° [56]. This diffuse halo represents the amorphous silica in metakaolin [63]. Marked shift in the scattering peak is observed after the geopolymer formation. The diffuse halo in metakaolin shifted to higher angle. Generally, geopolymers show completely amorphous X-ray diffraction (XRD) pattern with a diffuse halo peak at 2θ between 27 and 30° [25, 48, 57, 64, 65]. The primary binder phase in geopolymer matrix contributes to the amorphous characteristic and determines the strength of geopolymers. Also, in a study carried out by Wang et al. [59], the halo diffuse peak of metakaolin geopolymer fell at 2θ between 18 and

**Figure 10.** SEM micrographs of fly ash geopolymers cured at room temperature for 24 hours and at 80°C for another

Crystalline phases, particularly zeolites, are usually grown in geopolymers in conjunction with the amorphous binder phases (**Figure 12**) [46, 66]. As zeolites have similar chemical composition with geopolymers, geopolymers are usually deemed as the zeolitic precursor. The

25°. Increasing Si/Al ratio reduces the angle of diffuse halo [65].

**Figure 11.** SEM micrographs of metakaolin geopolymers [59].

24 hours [15].

250 Cement Based Materials

Kaolinite shows FTIR bands around 1113 cm−1 (Si-O bonds in SiO4 molecules); 994 cm−1 (Si-O bonds in SiO4 molecules); 907 cm−1 (AlIV-OH vibrations); 799 cm−1 (SiO-symmetric stretching) and 537 cm−1 (Si-O-AlVI) [24, 38, 71, 72]. On the other hand, as the kaolinite is thermally treated, the band at 1113 cm−1 shifted to lower wavenumber (~ 1031 cm−1) [23]. This is related to the amorphous SiO2 [25]. The FTIR bands associated with VI-fold coordinated Al vanished after calcination as a result of distortion of tetrahedral and octahedral sheets of kaolinite [24]. The band at around 781 cm−1 appears in metakaolin as the Al-O stretching vibration in AlO4 tetrahedral [73].

As the geopolymerization reaction progresses, shift of bands are observed. Clay-based geopolymer exhibits main FTIR absorption band at 990 cm−1 associated with the asymmetrical

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

The bulk density of metakaolin geopolymers is reported in the range between 1.20 and 1.80 g/

. 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

 [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

mers. Na-based geopolymers are generally lighter than K-based geopolymers. This is due to

composition shows longer setting and hardening times. Strength development of metakaolin

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

K-based geopolymers are denser and contain fewer pores as aforementioned [65].

the setting time was longer. Setting time is short providing that there is high Al<sup>2</sup>

**Figure 14.** Final setting times and compressive strength of metakaolin geopolymers with varying SiO<sup>2</sup>

From the result obtained by De Silva et al. [77] in **Figure 14**, high SiO2

[35] while coal fly ash geopolymers have density in the range between 1.40 to 1.80 g/

) and Na-based (1.25–1.72 g/cm3

of 3.81 became high and stabilized at a later age, even though

) metakaolin geopoly-

ratio in the initial

O3

/Al2 O3

molar ratios

content;

253

/Al2 O3

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

content. Besides, the calcium content in

geopolymers at 28 days even they set at a longer time, as reported by Rovnanik [74].

cm3

cm3

1.80 g/cm3

recorded for K-based (1.39–1.82 g/cm3

/Al2 O3

however, it will deteriorate strength due to low SiO2

leads to setting and hardening at a faster rate [55].

geopolymers with SiO2

at constant H2

O/Na2

O molar ratio of 13.6 [81].

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

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