Replacement of Portland cement with eco-friendly cementitious materials may reduce carbon dioxide emission and unfavourable chemical reactions between cement and carbon dioxide, which, in oil well cementing, negatively affect downhole cement integrity. Geopolymer cement is an alternative cementitious material, and therefore, its formulation for oil well cementing has to be studied. In oil well cementing, setting time of the cement has to be considered and, a retarder, such as lignosulfonate, can be added to lengthen the setting time. However, addition of lignosulfonate may negatively affect compressive strength. The present study investigates the effect of lignosulfonate, with varying concentrations of between 0.2 and 0.5 by weight of fly ash, on the compressive strength of fly ash-based geopolymer cement cube samples, with exposure to curing conditions of 3,000 psi and 100 °C for 8 and 24 h. Compressive strengths of samples cured for 8 h indicate that lignosulfonate increased early strength by 39 to 100, while, even though compressive strengths of samples cured for 24 h reduced by 9 to 28, their values still exceed 2,000 psi, hence suggesting that lignosulfonate can be employed as a retarder in geopolymer cement for oil well cementing. © 2022