Geopolymer cement (GPC) is an aluminosilicate-based binder that is cost-effective and eco-friendly, with high compressive strength and resistance to acid attack. It can prevent degradation when exposed to carbon dioxide by virtue of the low calcium content of the aluminosilicate source. The effect of the concentration of calcium chloride (CaCl2) as the accelerator on the compressive strength and acoustic impedance of GPC for well cement, while exposed to high pressure and high temperatures, is presented. Fly ash from the Tanjung Bin power plant, which is categorized as Class F fly ash according to ASTM C618-19, was selected as the aluminosilicate source for the GPC samples. Sodium hydroxide and sodium silicate were employed to activate the geopolymerization reaction of the aluminosilicate. Five samples with a density of 15 ppg were prepared with concentrations of CaCl2 that varied from 1 to 4 by weight of cement. Findings revealed that the addition of 1 CaCl2 is the optimum concentration for the curing conditions of 100 °C and 3000 psi for 48 hours, which resulted in the highest compressive strength of the product. Results also indicate that GPC samples that contain CaCl2 have a smaller range of acoustic impedance compared to that of ordinary Portland cement. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.