## 2 Experimental
### Geopolymer Preparation and Characterization  
Taking into account the ratio of alkali liquid to solid, the ratio of sodium silicate to sodium hydroxide (Na\({}_{2}\)SiO\({}_{3}\)/NaOH), and the concentrations of sodium hydroxide on geopolymers performance, in the first step, the effects of these factors on a selected sample (10% RH, 15% RHA, 35% MK, 10% POFA and 30% Slag) based on literature review and trial-and-error was investigated. In this step, the Design of Experimental software (DOE) version 7 (Stat-Ease cooperation, Minneapolis, MN, USA, 2015), response surface mode, and central composite design software were employed for studying these factors and determining water absorption, density, and compressive strength for the purpose of obtaining the optimal molar ratio and preparing the geopolymer paste. As per the literature, the ratios of alkali liquid to solid were set at 0.3, 0.4, 0.5; the ratios of sodium silicate to sodium hydroxide (Na\({}_{2}\)SiO\({}_{3}\)/NaOH) were set at 1, 1.5, and 2.5; the concentrations of sodium hydroxide (NaOH) were set at 8, 10, 14 M; all of which were within range of the DOE. The results are presented in Table 2, while the analysis of variance (ANOVA) table is tabulated in Table 3 (Figures S1-S3). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Chemical Composition (wt \%)** & **RH** & **RHA** & **MK** & **POFA** & **Slag** & **WG** & **Sample 16** & **Sample 16** \\ \hline SiO\({}_{2}\) & 23.77 & 91.04 & 50.29 & 48 & 29.24 & 26.5 & 54.46 & 51.97 \\ Al\({}_{2}\)O\({}_{3}\) & 0.06 & 0.33 & 45.75 & 2.30 & 11.71 & - & 30.95 & 19.88 \\ Fe\({}_{2}\)O\({}_{3}\) & 0.13 & 1.89 & 0.38 & 2.70 & 0.34 & - & 0.86 & 1.32 \\ CaO & 0.25 & 0.54 & - & 4.95 & 42.43 & - & 0.86 & 7.58 \\ MgO & 0.08 & 0.58 & - & 4.64 & 5.06 & - & 0.64 & 1.13 \\ K\({}_{2}\)O & 1.08 & 2.4 & 0.17 & 10.99 & 0.27 & - & 2.00 & 1.65 \\ SO\({}_{3}\) & 0.27 & 0.53 & 0.08 & 2.47 & 4.67 & - & 0.42 & 0.89 \\ TiO\({}_{2}\) & - & 0.02 & 1.56 & 0.15 & 0.46 & - & 0.82 & 0.67 \\ Na\({}_{2}\)O & - & 0.05 & 0.27 & 0.12 & 0.2 & 10.6 & 0.17 & 8.46 \\ Others & 0.81 & 2.62 & 0.3 & 5.47 & 0.14 & 62.9 (water) & 1.16 & 0.7 \\ Loss on ignition (LOI) & 78 & 2.86 & 1.27 & 8.73 & 0.6 & - & 5.36 & 5.81 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of rice husk (RH), rice husk ash (RHA), metakaolin (MK), palm oil fuel ash (POFA), Slag, water glass (WG), and selected geopolymer sample before and after casting (dry mix before and after activation) using XRF (mass %). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sample** & **NaOH Concentration** & **Sodium** & **Liquid/Solid** & **\% Water Absorption** & **Density** & **Strength (MPa)** \\  & **(Molar)** & **Silicate/NaOH** & & **after 24 h** & **g/cm3** & **28 Days** \\ \hline
1 & 8 & 2.5 & 0.5 & 40.9 & 1.166 & 3.3 \\
2 & 10 & 1.75 & 0.4 & 46.2 & 1.181 & 1.4 \\
3 & 14 & 1 & 0.5 & - & 1.277 & 1.9 \\
4 & 14 & 2.5 & 0.3 & 45.4 & 1.202 & 2.3 \\
5 & 8 & 1 & 0.3 & 49.9 & 1.162 & 2.2 \\
6 & 8 & 1 & 0.5 & 45.9 & 1.216 & 2.2 \\
7 & 14 & 1 & 0.3 & 49.0 & 1.213 & 2.2 \\
8 & 14 & 2.5 & 0.5 & 40.8 & 1.173 & 2.3 \\
9 & 8 & 2.5 & 0.3 & 48.2 & 1.111 & 1.4 \\
10 & 14 & 1.75 & 0.4 & 45.0 & 1.216 & 2.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The result of DOE software considering three factors (ratios of alkali liquid to solid; the ratio of sodium silicate to sodium hydroxide and the concentrations of sodium hydroxide) on selected sample for water absorption, density, and the compressive strength. ANOVA values confirmed that all main factors significantly influence the three responses. The model's \(F\)-value of 5.38 implies that it is significant. There is only a 0.13% chance that a \"Model \(F\)-Value\" this large could occur due to noise. The \"Lack of Fit \(F\)-value\" of 2.261 implies that the Lack of Fit is not significant relative to pure error (Table 3).