| Summary: | The present study aimed to investigate the effect of an expandable additive on the compressive 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 addition of slag cement as a strength enhancer, were prepared by using an elastomeric expandable additive (<i>R</i>-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 <i>R</i>-additive on the compressive strength (<i>F</i>) and linear expansion (∆<i>l/l</i><sub>0</sub>) of the samples were analyzed. When cured at 60 °C and atmospheric pressure, the highest <i>F</i> of 15.01 MPa was obtained when the concentration of <i>R</i>-additive was 10%, while the highest ∆<i>l/l</i><sub>0</sub> of 0.9985% was obtained when the concentration of <i>R</i>-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 <i>F</i> from 15.01 to 14.62 MPa and from 10.33 to 9.61 MPa, and the increase in ∆<i>l/l</i><sub>0</sub> from 0.52% to 0.63%, and from 0.99% to 1.32%, when the concentrations of <i>R</i>-additive were 10% and 25%, respectively. The findings suggest that the formulations adopted, which contain <i>R</i>-additive at concentrations ranging from 10% to 25%, fulfilled the requirements of the oil and gas industry.
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