The construction industry is currently switching to geopolymer cement, a greener product from power plant waste. While the utilization of geopolymer cement in the construction industry is well-known, its application as well cement is still being researched. Current study analyzes fly ash composition from two power plants, Tanjung Bin and Manjung Power Plant, on compressive strength and thickening time under downhole conditions per API RP 10B-2. Findings on compressive strength reveal that silica to alumina ratios of 2.0 to 2.5 resulted in higher compressive strength than ratios of less than 2.0. In addition, compressive strength increases as iron oxide and calcium oxide concentrations increase from 7.79 to 15.9 and 6.73 to 22.2, respectively. Fly ash composition had almost no effect on geopolymer cement thickening time at 60 °C and 2000psi. The thickening time is more likely to be influenced by extrinsic factors, including temperature, pressure, and the presence of a retarder or accelerator. © 2024, Institute of Technology PETRONAS Sdn Bhd.