## 2 Materials and Methods  
### Preparation of Geopolymer Cement (GPC) Samples  
GPC samples were prepared by using fly ash as the aluminosilicate source. The fly ash was obtained from the Tanjung Bin power plant in Johor, Malaysia, which is a 2100-MW coal-fired power plant [48] that implements denitrification technology. Figure 1 presents the particle size distributions of the fly ash sample obtained. According to the American Society for Testing Materials (ASTM), the fly ash can be classified as a Class F fly ash with a maximum calcium oxide (CaO) content of 6.72%, as per ASTM C618-19 [49]. The chemical composition of the fly ash, which was determined by using X-ray fluorescence of model S8 Tiger developed by Bruker (Billerica, MA, USA), is presented in Table 1. Micrographs of the fly ash were produced by using a scanning electron microscope of model Evo LS15 VPSEM developed by Zeiss (Jena, Germany), at magnifications of 1000\(\times\) and 10,000\(\times\), as disclosed in Figure 2. \begin{table}
\begin{tabular}{c c} \hline
**Element/Parameter** & **Weight (\%)** \\ \hline Silicon Dioxide (SiO\({}_{2}\)) & 46.47 \\ Aluminum Oxide (Al\({}_{2}\)O\({}_{3}\)) & 25.95 \\ Iron (III) Oxide (Fe\({}_{2}\)O\({}_{3}\)) & 8.31 \\ Calcium Oxide (CaO) & 6.88 \\ Magnesium Oxide (MgO) & 4.95 \\ Potassium Oxide (K\({}_{2}\)O) & 2.11 \\ Sodium Oxide (Na\({}_{2}\)O) & 1.72 \\ Titanium (IV) Oxide (TiO\({}_{2}\)) & 1.16 \\ Sulfur Trioxide (SO\({}_{3}\)) & 0.63 \\ Chlorine (Cl) & \textless{}0.1 \\ Moisture & 0.11 \\ Loss of Ignition & 1.61 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the fly ash employed in the present study. Figure 1: Particle size distribution of the fly ash obtained from the Tanjung Bin power plant. In order to enhance the strength of the cement to obtain the desired strength range for oil-well cementing, slag cement with a strength grade of 32.5 [50] was added to the mix as a strength enhancer, as implemented in previous studies [51; 52; 53], with a concentration of 10% of the weight of the fly ash and slag cement. Furthermore, \(R\)-additive, which is an elastomeric expandable additive that consists of styrene-butadiene rubber with a specific gravity of 0.945, was incorporated into the mix to induce expansion in the cement in an attempt to obstruct volumetric shrinkage. Four (4) mix formulations of the samples, namely R10, R15, R20 and R25, were specified, which were composed of, by weight of the mix, 65% of a solid blend that comprises fly ash, slag cement and \(R\)-additive, and 35% of an alkaline-activator solution that comprises 8M NaOH and Na\({}_{2}\)SiO\({}_{3}\) with a Na\({}_{2}\)SiO\({}_{3}\)-to-NaOH molar ratio of 0.25. Concentration of the \(R\)-additive was increased from 10% to 15%, 20% and 25% by weight of the solid blend for formulations R10, R15, R20 and R25, respectively. Preliminary tests were conducted on trial mix designs prior to selecting the concentration range of \(R\)-additive, which was specified with the aim of attaining cement expansions that are acceptable to perform cementing for various well operations. Concentrations of the fly ash and slag cement, also by weight of the solid blend, were adjusted accordingly, as clarified in Table 2, with the percentage of solid blend fixed at 65% by weight of the mix. Cement slurry with a volume of up to 600 mL was prepared for each mix. Mixing of slurry was performed by using a constant speed mixer of Model 3260, which was developed by Ametek Chandler Engineering (Tulsa, OK, USA), at an initial rotational speed of 4000 rpm for 15 s, which was subsequently increased to a final rotational speed of 12,000 rpm for 35 s after the solid constituents were completely poured into the mixer. Mixing was performed in accordance with API RP 10B-2 [54], which was also adopted to perform free-water and rheology tests on the slurry.