Purpose: This research was intended to evaluate Nano silica as an additive to improve the sulphate resistance of cement used in geothermal wells. Design/ Methodology/ Approach: Sulphate resistance was determined by measuring the longitudinal change in cement cube specimens that were cured in sodium sulphate solution for 21 days. Cube specimens with varied concentrations of Nano silica (0%, 0.3%, 0.6%, 0.9% and 1.2%) were used in the study. Five separate solutions were maintained at 23℃, 40℃, 65℃, 70℃ and 80℃ for 21 days. Final length measurements were taken and compared as a percentage of initial length measurements. Findings: Beyond 65℃, the sulphate resistance of cement improved for each percentage concentration of Nano silica replacement. Control specimens with 0% Nano silica had the most inferior performance at all temperatures. Higher concentrations of 1.2% and 0.6% Nano silica replacement gave the most resistance between 23℃ and 65℃. A lower concentration of 0.3% proved more suitable between 65℃ and 80℃. Research limitation: The results of the experiment indicate performance in low-temperature geothermal wells. Practical implications: The application of this additive can improve the durability and strength of the cement, reducing the potential for degradation due to exposure to sulphates. This can lead to a longer lifespan of the geothermal well, reducing maintenance costs and increasing its overall efficiency. Social implications: Improved cement designs that create longer-lasting cement sheaths can be developed from this research, thereby fostering geothermal energy development. The option to replace certain volumes of cement with Nano silica contributes to a reduction of the carbon footprint by minimising the demand for, and therefore the production of cement.   Originality/ Value/ Novelty: Previous research that tested cement at high temperatures analysed mechanical resistance. This research examined the sulphate resistance of cement at high temperatures.