## 3 Results and Discussion  
### Compressive Strength of Geopolymers  
The results in Figure 3 show the compressive strength of materials prepared with different amounts of NaOH activator and calcium hydroxide. It can be seen that the compressive strength of materials increased with increasing calcium hydroxide content up to 45 g (about 13% by weight of the solid raw materials) in the mixture. Higher content of Ca(OH)\({}_{2}\), which is over 45 g, resulted in a decrease in compressive strength. This behavior was observed for all NaOH concentrations investigated in this study. It is in agreement with the behavior of having an optimal amount of Ca(OH)\({}_{2}\) for the mechanical enhancement of geopolymer materials which has been found by [PERSON] et al. [33] and [PERSON] et al. [25]. In the study of [PERSON] et al. [33], the optimal Ca(OH)\({}_{2}\) content in the geopolymer material containing municipal solid waste incineration bottom ash and granulated blast furnace slag was 5%. The research of [PERSON] et al. [25] showed that the optimal Ca(OH)\({}_{2}\) amounts were 5.3% and 8.2% in the fly ash-based geopolymer prepared by using 8 M NaOH and 2 M Na\({}_{2}\)SiO\({}_{3}\) activators, respectively. However, [PERSON] et al. [2] found that the compressive strength of micropore-foamed geopolymer containing fly ash and ZSM-5 zeolite waste continuously increased. This might be because the amount of calcium hydroxide investigated, which was increased up to 6 g (about 2% of solid raw materials), was still small enough for the compressive strength to lay on the increasing part of the curve. When the amount of calcium hydroxide was added in excess, the reactions forming geopolymeric gel and C-S-H gel could compete with each other for soluble silicates and available space. As a result, the geopolymer materials would be more disordered, with more residual voids and holes, leading to the reduction of compressive strength [30]. Another possibility is that the crystal strength of calcium hydroxide is low, and excessive calcium hydroxide surely has a serious influence on the strength of geopolymer materials [33]. The different raw materials and their ratios in the mixtures could result in the different optimal content of calcium hydroxide added to give the highest mechanical enhancement of geopolymer products. In this study, 45 g of Ca(OH)\({}_{2}\), which is about 13% by weight of solid raw materials, was the most appropriate value for materials to obtain the best compressive strength. Compared to the results from the literature mentioned above, the optimal content of calcium hydroxide in this research was higher. It can be explained that the geopolymer materials prepared contain red mud, which has a relatively high amount of non-reactive components such as iron oxide, so the higher optimal content of calcium hydroxide was obviously measured. Comparing the compressive strength of materials prepared without the presence of calcium hydroxide to those prepared with the presence of 45 g of calcium hydroxide at different concentrations of sodium hydroxide activator, it can be seen that the compressive strength of geopolymer with Ca(OH)\({}_{2}\) presence was 580%, 173%, 170%, and 150% of that of materials prepared without Ca(OH)\({}_{2}\) when the NaOH activator concentration was 4 M, 6 M, 8 M, and 10 M, respectively. The significant high increment of compressive strength for the 4 M NaOH activator could be understood mainly because the low activator concentration in this particular case can hardly activate the silica and alumina sources of raw materials to form geopolymeric products, leading to relatively low compressive strength. The presence of calcium hydroxide can make the formation of some products, such as C-S-H, possible at low activator concentrations that can give rigidity to the material making higher compressive strength [34].