The present study aimed to investigate the effect of an expandable additive on the com-pressive strength and linear expansion of geopolymer cement, which is an alternative to ordinary Portland cement, for oil-well cementing. Fly-ash-based geopolymer cement samples, with the ad-dition of slag cement as a strength enhancer, were prepared by using an elastomeric expandable additive (R-additive), which consists of styrene�butadiene rubber with a specific gravity of 0.945, at concentrations of 10, 15, 20 and 25 by weight of the solid blend, and cured in a water bath at 60�C and atmospheric pressure, and a curing chamber at 90�C and 3000 psi, or approximately 20.68 MPa. Mixability, amount of free water and slurry density were studied, and the effects of the concentration of R-additive on the compressive strength (F) and linear expansion (�l/l0 ) of the samples were analyzed. When cured at 60�C and atmospheric pressure, the highest F of 15.01 MPa was obtained when the concentration of R-additive was 10, while the highest �l/l0 of 0.9985 was obtained when the concentration of R-additive was 25. An increase in the curing temperature and pressure to 90�C and 3000 psi (�20.68 MPa) resulted in the reduction of F from 15.01 to 14.62 MPa and from 10.33 to 9.61 MPa, and the increase in �l/l0 from 0.52 to 0.63, and from 0.99 to 1.32, when the concentrations of R-additive were 10 and 25, respectively. The findings suggest that the formulations adopted, which contain R-additive at concentrations ranging from 10 to 25, fulfilled the requirements of the oil and gas industry. © 2021 by the authorsLicensee MDPI, Basel, Switzerland.