## 3 Fresh and Hardened Properties of GPC
### Compressive Strength  
The compressive strength of FA-based geopolymer is directly affected by source material and its particle size distribution [130]. [PERSON] et al. [56] studied the compressive strength of GGBFS and POFA-based geopolymers and found the maximum compressive strength of approximately 66 MPa at the binder composition of 70% GGBFS and 30% POFA, respectively. [PERSON] et al. [131] determined that while increasing the dosage of POFA, the SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio increases, and this result reduced compressive strength due to delayed geopolymerization process. According to the authors, it can be explained by the reaction of alumina at the early stages and thus there exists a scarcity of Al\({}_{2}\)O\({}_{3}\) at the later reaction stage. Compressive strength also depends on the sand type and binder to sand ratio. The effect of limestone sand on compressive strength of geopolymer was studied by [PERSON] [132] and it was observed that compressive strength of geopolymer mortar decreased with the inclusion of limestone sand; however, by using limestone sand with sand/binder ratio of 7:1, the green construction material can be produced. In addition, [PERSON] et al. [133] inspected the strength and density of ambient cured geopolymer samples and found that with the rise in the binder to sand ratio from 0.25 to 0.5, the compressive strength increased while it decreased when this ratio was above 0.5. [PERSON] et al. [134] studied the effect of fly ash/slag ratio on the compressive strength of GPC and found that the strength was higher at fly ash/slag ratio of 2:1 when compared to 4:1 and 3:1. This increase was due to the denser microstructure. Figure 12: Effect of sodium silicate/FA ratio and hydroxide/FA ratio on the temperature, adopted from [126]; SciELO, 2014. Several researchers [63; 135] have found that the compressive strength of FA-based GPC increases with the increase in the molarity due to improved polycondensation process. [PERSON] et al. [135] further considered the effect of curing temperature Na\({}_{2}\)SiO\({}_{3}\)/NaOH and alkaline liquid/FA ratio on the compressive strength of GPC. While increasing curing temperature, the 7 th day's compressive strength increases; conversely, with a rise of Na\({}_{2}\)SiO\({}_{3}\)/NaOH ratio, the compressive strength decreases. However, the alkaline liquid/FA ratio has shown no significant effect on compressive strength, as shown in Figure 13.  
Figure 13: \(Cont.\)  
[PERSON] et al. [2] summarized the different factors that affect the compressive strength of GPC. The authors defined the distinct factor levels as 0, 1, and 2 while determining the compressive strength as presented in Table 6. The effect on the compressive strength of different factor levels is displayed in Figure 14. It can be seen that with an increase in NaOH molarity and Na\({}_{2}\)SiO\({}_{3}\)/NaOH ratio, the compressive strength of alkali-activated slag increases while an increase in alkaline solution to slag ratio results in a decrease in compressive strength. However, heat curing is not recommended for slag-based GPC because the increment in dry curing temperature leads to more water evaporation, which negatively affects the amount of CSH formation. In addition, the increase in alkali content from 35-45% of the total binder decreases the compressive strength of samples [53]. Likewise, the inclusion of nanoparticles enhances the compressive strength of geopolymer mortar at different curing age [136]. From Table 5, it is concluded that the strength development of FA-based GPC at ambient temperature enhanced due to the addition of external agents like superplasticizers or additives. The addition of calcium hydroxide improved the dissolution process of FA in alkaline activator media due to precipitation of CSH or CASH phases [100]. Further, the incorporation of additives in GPC up to optimum content enhanced the strength, flow, and other mechanical properties.