In view of the mechanism for activation of OPC that entails the absorption of water to enable cement hydration to occur, volumetric shrinkage in GPC is anticipated to be less than that of OPC [9]. Furthermore, the calcium content of GPC is considerably less than that of OPC, owing to the low calcium content of the aluminosilicate source. Conversely, the high calcium content in OPC leads to, in the course of time, elevations in porosity and permeability and, consequently, loss in mechanical strength and structural integrity of the cement sheath. Expenditure of calcium ions that are present in calcium-silicate-hydrate (C-S-H) occurs due to chemical reactions between the calcium ions and carbonic acid (H\({}_{2}\)CO\({}_{3}\)) to form calcium carbonate (CaCO\({}_{3}\)). Subsequent to the depletion of the calcium ions in C-S-H, further chemical reactions between H\({}_{2}\)CO\({}_{3}\) and CaCO\({}_{3}\) will occur [2]. Accordingly, the lower calcium content of GPC in comparison to OPC signifies that, under high exposure to CO\({}_{2}\) in CO\({}_{2}\)-rich well environments, GPC will be more resistant toward carbonation than OPC [7; 34; 35]. Apart from low shrinkage and resistance toward carbonation, a vast array of other advantages pertaining to the material properties of GPC in comparison to OPC has been reported in previous research; the advantages include, but are not limited to, high mechanical strength; high pumpability; low permeability toward the flow of water and gas, with the potential for further reduction when exposed to higher pressures, as in oil-well cementing; low Young's modulus; resistance to acid attacks; resistance to alkali-aggregate reaction; resistance to freeze-thaw cycles; stability at high temperatures; and tolerance to contamination with oil-based mud [1; 33; 36; 37; 38; 39; 40; 41]. Moreover, the adoption of GPC as an alternative to OPC presents a cost- and energy-efficient solution, as the manufacturing process of GPC consumes less energy with a carbon emission of only about 0.184 ton of CO\({}_{2}\) for every ton of GPC produced, as compared to 1 ton of CO\({}_{2}\) for every ton of OPC produced, with, coincidentally, less cost [1; 42; 43; 41; 44; 45]. GPC has been accentuated in previous research as an alternative to OPC for oil-well cementing [1; 17; 20]. Previous research on the application of geopolymer for other civil engineering applications is wide-ranging. Contrarily, research on the application of GPC for oil-well cementing is scarce, and, accordingly, studies that attempt to address the concern of volumetric shrinkage of cement in oil-well cementing are inadequate. Therefore, the present study aims to investigate the effect of an expandable additive on the compressive strength and linear expansion of GPC for the oil-well cementing application. Fly-ash-based strength-enhanced GPC samples, with the addition 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 \({}^{\circ}\)C and atmospheric pressure, and a curing chamber at 90 \({}^{\circ}\)C and 3000 psi, or approximately 20.68 MPa. Even though the adoption of styrene-butadiene as an additive in the geopolymer matrix has been explored in [PERSON] et al. [46] and [PERSON] et al. [47], the studies did not focus on the conditions for oil-well cementing. Mixability, amount of free water and slurry density were studied. Subsequently, the effects of the concentration of \(R\)-additive on the compressive strength and linear expansion of the samples were analyzed. Findings of the present study will contribute toward addressing, in general, the inadequacy of data on the application of GPC for oil-well cementing and, in particular, formulation of shrinkage-resistant oil-well cementing based on GPC.