## 2 Geopolymer Concrete (GPC)
### Constituents of GPC
#### 2.1.2 Activators  
Alkaline activators in GPC are used for polymerizing the aluminosilicates. These are strong alkaline solutions such as sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium silicate (Na\({}_{2}\)SiO\({}_{3}\)), potassium silicate (K\({}_{2}\)SiO\({}_{3}\)), or combination of these hydroxides and silicates for dissolving Al and Si atoms [40]. The geopolymerization process depends on the reactivity and concentration of alkaline activator. The alkali-enrichment in GPC increases shrinkage and pore structure development of the cementitious system, which considerably affects the sample with less water to binder ratio. Furthermore, the increase in alkalinity results in the degradation of mechanical strength [41; 42]. Therefore, the quantity of alkaline activators needs to be considered wisely while designing the GPC mix. The list of different alkaline activators used by several researchers is given in Table 2.  
Several researchers [59; 60; 39; 61] have concluded that the composition and type of activator is the major factor responsible for the strength development of GPC. According to these studies, the effect of ratios of activator/binder and silicate to hydroxide plays an important role in geopolymerization process and hence, a critical step in preparing the GPC/GPM. Generally, the most commonly used  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Reference** & **Activator Used** & **Molarity of Hydroxide** & **Activator to Binder Ratio** & **Silicate to Hydroxide Ratio** \\ \hline
[43] & Na\({}_{2}\)SiO\({}_{3}\) + NaOH & 10 M & 0.6 & 2 \\
[44] & Na\({}_{2}\)SiO\({}_{3}\) + NaOH & 13 M & 0.4 & - \\
[45] & Na\({}_{2}\)SiO\({}_{3}\) + NaOH & 14 M & 0.45 & 0.25 \\
[46] & Na\({}_{2}\)SiO\({}_{3}\) + NaOH & 10 M & - & 2.5 \\
[47] & Na\({}_{2}\)SiO\({}_{3}\) + NaOH & 10 M & 0.43 & - \\
[48] & Na\({}_{2}\)SiO\({}_{3}\) + NaOH & 10 M & 0.7 & - \\
[49] & Na\({}_{2}\)SiO\({}_{3}\) + NaOH & 8 M + 10 M & - & 0.5–2.5 \\
[50] & Na\({}_{2}\)SiO\({}_{3}\) + NaOH & 12 M & 0.5 & 2.5 \\
[51] & Na\({}_{2}\)SiO\({}_{3}\) + NaOH & 10 M + 20 M & & 0.67–1.0 \\
[52] & Na\({}_{2}\)SiO\({}_{3}\) + NaOH & 10 M & - & 0.2 \\
[53] & Na\({}_{2}\)SiO\({}_{3}\) + NaOH & 14 M & - & 1.5–2.5 \\
[54] & NaOH & 6 M & 0.35 & - \\
[55] & Na\({}_{2}\)SiO\({}_{3}\) + NaOH & 14 M & - & 15-2.5 \\
[56] & Na\({}_{2}\)SiO\({}_{3}\) + NaOH & 12 M & 0.4 & 1.25 \\
[57] & Na\({}_{2}\)SiO\({}_{3}\) + NaOH & & 0.35 & - \\
[58] & Na\({}_{2}\)SiO\({}_{3}\) + KOH & 7 M & 0.3 & - \\ \hline \end{tabular}
\end{table}
Table 2: List of alkaline activators with various dependent parameters.  
activators in GPC are Na\({}_{2}\)SiO\({}_{3}\) and NaOH. The GPC sample prepared with Na\({}_{2}\)SiO\({}_{3}\) and NaOH showed better mechanical, durable, and microstructural properties [62]. The GPC, containing only hydroxides as an activator, results in poor mechanical properties, contains pores and shrinkage cracks because it only helps in the dissolution of Si and Al ions of precursor. However, inclusion of silicate source as an activator in combination with hydroxide source results in the promotion of the condensation process of GPC [63; 64; 65]. According to [PERSON] et al. [66], the excess use of silicates in GPC inhibits the geopolymerization while the excess of hydroxides led to early and vigorous precipitation of aluminosilicate precursors hindering the further formation of geopolymeric gel [66]. It has been found by [PERSON] et al. [55], with the decrease in silicate to hydroxide ratio, the workability and compressive strength of GPC have been found to increase.