## 2 Materials and Methods  
### Materials  
The materials utilized for this research were kaolin clay (K), hydrated lime (L), calcium sulfate (gypsum) (G), (M), (RHA), and (GGBS). Kaolin clay was supplied by KAOLIN(Malaysia) SDN BHD under the brand name MK40 as a white, finely ground, odorless powder. Kaolin clay was used due to the following considerations: (1) it is one of the major compositions of natural clay minerals; (2) it has a uniform composition, minimal organic content, and consistent and uniform mineralogy; (3) it has a low cation exchange capacity [4; 20]; (4) it has a higher alumina content than most other costly minerals and can thus release more alumina in high pH environments, thereby participating in ettrinigite mineral formation and increasing susceptibility of sulfate attack [12]. For these reasons, kaolin clay is a suitable control soil in the soil stabilization process. Table 1 summarizes the major properties of kaolin clay. A hydrometer test of clay was conducted according to BS EN ISO 17892-4:2016 [34] with the grading curve shown in Figure 1.  
L, G, and RHA were collected from Sungai Jawi, 14200 Penang, Malaysia. However, G (CaSO\({}_{4}\times 2\)H\({}_{2}\)O) was selected in this research due to its much lower solubility compared to other sulfate types (potassium sulfate, magnesium sulfate, and sodium sulfate), as presented in Table 2. Moreover, it is one of the sulfates that contain calcium, and it is logical that it would form ettrinigite if a non-calcium-based stabilizer had been used, as presented in Table 2. \begin{table}
\begin{tabular}{c c} \hline \hline
**Properties** & **Value** \\ \hline Liquid limit (\%) & 57.78 \\ Plastic limit (\%) & 38.13 \\ Shrinking limit (\%) & 4.4 \\ Plasticity index & 19.65 \\ Sand (\%) & - \\ Silt (\%) & 88.35 \\ Clay (\%) & 11.65 \\ Electric conductivity (\(\mu\)S/cm) & 320 \\ pH & 5 \\ Specific gravity (G\({}_{\text{s}}\)) & 2.46 \\ Water content (\%) & 1.01 \\ Optimum moisture content (\%) & 29 \\ Maximum dry density (Mg/m\({}^{3}\)) & 1.326 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of the kaolin clay used in this study. Figure 1: Particle size distribution (PSD) of the kaolin clay used in this study. RHA is a byproduct of the rice milling industry that is produced from the burning of rice husk. The processing of 1000 kg of rice grain is estimated to generate about 200 kg of RHA, which, when burnt, should produce about 40 kg of RHA [35]. Currently, RHA is commercially used in the extraction of silica and as a pozzolanic material [36]. The phase of the silica content of RHA (amorphous or crystalline) is a function of the duration and temperature of the burning process. Fine particle-sized amorphous silica is more reactive, and many researchers [36; 37] have shown that burning RHA at a temperature range of 500-650 \({}^{\circ}\)C results mostly in the crystalline form of silica, as it can be effectively transformed from its active state into a crystalline form. In this research, RHA was prepared by burning RHA at a temperature range of 350 to 550 \({}^{\circ}\)C for three hours to retain the amorphous silica content. GGBS is a byproduct of the pig-iron manufacturing process and is formed through the rapid cooling of molten iron slag to retain its amorphous structure, followed by grinding to increase its specific surface area. The GGBS used in this study was collected from MDC Sungai Pentani Company, Malaysia. It was chosen to reduce the rate of ettrinigite formation through the provision of more Al and Si, which react with Ca\({}^{2+}\) to form complex cementing gels [12; 31; 32]. The denser structure and lower calcium ion content of GGBS should contribute to superior sulfate resistance [6; 38]. In this research, M was chosen, as it is a green and low-carbon stabilizer for clay soil. It was collected from Hang Zhou Jiu Peng New Material Co., Ltd., China as a white, fine, crystalline powder.