## 2 Geopolymer Concrete (GPC)
### Constituents of GPC
#### 2.1.1 Aluminosilicates  
The aluminosilicates used in GPC are a natural and industrial by-product which contains amorphous silica and alumina. These are various mineral admixtures including SF, GGBFS, BA, WSA, FA [25, 26], and chemicals in particular like calcium silicate and sodium aluminate [27] which have been used as aluminosilicates [28]. In developing countries, the industrial and agro-waste materials containing amorphous silica and alumina are commonly used for energy generation [29]. The resulting ash disposal problem of these wastes is resolved by using them in the cement-based materials and in the production of geopolymer technology. Mostly six groups of aluminosilicates sources are used in GPC. These are FA-based, MK-based, SG-based, RHA-based, HCWA-based, and a combination of either two of the earlier mentioned aluminosilicates [9]. However, different GPCs show different performances depending on the content of alumina and silica in the aluminosilicates used in geopolymer. The various types of aluminosilicates, along with their sources and chemical composition are tabulated in Table 1. It is shown that different aluminosilicates have different ions concentrations. However, the performance of an aluminosilicate depends mostly on optimum percentage of silica and alumina available in the chain reaction of the geopolymerization process. The most commonly used aluminosilicate binder is fly ash (obtained from coal combustion stations), which has been found to improve (or substantially improve) the mechanical and durability properties of GPC [9]. It is actually the physical, chemical, and microstructural properties of FA, \begin{table}
\begin{tabular}{c c c c c c} \hline
**Chemical Composition** & **FA** & **POFA** & **MK** & **Dolomite** & **Kaolin** \\ \hline Sources from raw Materials & From coal mining & From oil palm industry & Natural Resources & Natural Resources & Natural Resources \\ \hline SiO\({}_{2}\) & 52.11 & 51.18 & 55.90 & 15.37 & 52.00 \\ Al\({}_{2}\)O\({}_{3}\) & 23.59 & 4.61 & 37.20 & 1.69 & 35.00 \\ Fe\({}_{2}\)O\({}_{3}\) & 7.39 & 3.42 & 1.70 & 0.51 & 1.00 \\ TiO\({}_{2}\) & 0.88 & - & 2.40 & 0.015 & 0.90 \\ CaO & 2.61 & 6.93 & 0.11 & 23.00 & \textless{}0.05 \\ MgO & 0.78 & 4.02 & 0.24 & 17.20 & 0.70 \\ K\({}_{2}\)O & 0.80 & 5.52 & 0.18 & 0.195 & 2.00 \\ Na\({}_{2}\)O & 0.42 & 0.06 & 0.27 & 0.013 & 0.05 \\ SO\({}_{3}\) & 0.49 & - & 0.02 & - & - \\ P\({}_{2}\)O\({}_{3}\) & 1.31 & - & 0.17 & 0.019 & - \\ Loss in ignition & - & 21.6 & 0.80 & - & - \\ \hline \end{tabular}
\end{table} 
Table 1: X-ray fluorescence (XRF) analysis of different aluminosilicates modified from [30]. Figure 2: Constituents of geopolymer concrete (GPC): (**a**) Ordinary Portland Cement (OPC), (**b**) fly ash (FA), (**c**) silica fume (SF), (**d**) NaOH pellets, (**e**) Na\({}_{2}\)SiO\({}_{3}\), (**f**) GPC aggregate. which are responsible and improve the mechanical properties in GPC. The particle shape and fineness of FA in GPC plays an important role as the particle size distribution and fineness of FA, significantly affect the reactivity of precursor and, thus, to the geopolymerization process [31]. As a result of the higher surface area, the silica and alumina leached down quickly on the FA surface before condensation that will result in a higher geopolymerization rate [32]. Increasing the fineness of FA results in the lessening of porosity and thus provides superior resistance against aggressive environments [33]. It is known that both silica and alumina content are the main constituents in the geopolymerization reaction. The fast dissolution of reactive silica and alumina helps in the synthesis of aluminosilicate gel, which in turn, enhances the mechanical strength of GPC [32]. Generally, the presence of silica and alumina oxides in GPC is usually expressed by silica to alumina ratio. According to [PERSON] et al. [34], with the increase in silica to alumina ratio, the compressive strength of FA-based GPC increases. The second most widely used aluminosilicate is slag.