## 2 Experimental
### Geopolymer Preparation and Characterization
The determination of the optimum value for control variables (factors) is one of the main objectives of RSM, which can have maximum (or a minimum) response over a specific region of interest. Having a \"good\"-fitting model is necessary to adequately represent the mean response because such a model can be utilized to determine the optimum value [15]. In the second step, a total of 30 mixtures were prepared at different percentages of RH, RHA, MK, POFA, and GGBS using NaOH (alkali activation) based on Design of Experimental software (DOE) version 7, response surface mode, and central composite design, by introducing each component's limitations and factors (Table 4). The samples were manually mixed, and the pastes were poured into a 20 \(\times\) 20 \(\times\) 20 mm\({}^{3}\) models to set, forming the specimens that will be characterized. For each geopolymer paste mixture, eighteen specimens were cast in a 20 mm\({}^{3}\) cube to determine its compressive strength, density, porosity, and water absorption capabilities. The samples in the molds were tightly wrapped in plastic films to prevent any loss of moisture. All of the samples were heated in an electric oven at 80 \({}^{\circ}\)C for 2 h after casting. Later, the prepared samples were kept at an ambient temperature of 30 \(\pm\) 2 \({}^{\circ}\)C prior to being characterized. \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/cm\({}^{3}\)** & **28 Days** \\ \hline  
& 1.387 & 2.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._ \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Source** & **Sum of** & **df** & **Mean** & **\(F\) Value** & **\(p\)-Value** & **Yield** \\  & **Squares** & **dif** & **Square** & **\(F\) Value** & **Prob \(>F\)** & **Yield** \\ \hline Model & 8.90 & 6 & 1.48 & 5.38 & 0.0013 & significant \\ NaOH Concentration (Molar) & 0.029 & 1 & 0.029 & 0.11 & 0.7469 & - \\ Sodium Silicate/NaOH & 1.25 & 1 & 1.25 & 4.54 & 0.0439 & - \\ Liquid/Solid & 3.69 & 1 & 3.69 & 13.39 & 0.0013 & - \\ Lack of Fit & 6.21 & 22 & 0.28 & 2.26 & 0.4873 & not significant \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Software sequential model sum of squares for compressive strength and the lack of fit tests for flavonoid yield. The water absorption and porosity of the specimens were calculated based on ASTM D 570-98 and ASTM C 642-06, respectively. The density of the samples was measured using an Electronic Densimeter (model MD-300S, Alfa Mirage, Miyakojima-ku, Osaka, Japan). The capillary rise of the samples was measured by immersing one side of the prepared cube in water at room temperature at various times, as per ASTM 1585. The compressive strength of mortar is one of the principal parameters that represent the quality of the binder, where its used to gauge the potential success of geopolymerization. The test is regarded as being cost effective and simple to administer, and the development of strength is used as a primary measure of its utility for application in the construction industry [16]. It was determined using a 20 mm\({}^{3}\) cube sample, as per ASTM C 109 (2009) testing standards, with a PC base automatic SSM-AJ-10 KN compression testing machine. An average was taken from three values for samples after 3, 7, and 28 days post-curing. To investigate the effect of the concentration of NaOH on compressive strength in the prepared geopolymers, two samples were randomly selected and prepared at different concentrations of NaOH (8, 10, and 14 M). The compressive strength of the samples has been tested after curing at room temperature for 28 days. The samples' components and their water absorption, porosity, density, and compressive strengths are tabulated in Table 3.  
As per Table 4, the residual compressive strength of the specimens is inversely proportional to both water absorption and porosity.