## 4 Conclusions  
The present study aimed to investigate the effect of an expandable additive on the compressive strength and linear expansion of GPC, which is an alternative to OPC, for oil-well cementing. Fly-ash-based strength-enhanced GPC samples, with the addition of slag cement as the strength enhancer, were prepared by using different concentrations of \(R\)-additive and cured at 60 \({}^{\circ}\)C and atmospheric pressure. Mixability, amount of free water and slurry density were studied, and the effects of the concentration of \(R\)-additive on the compressive strength and linear expansion of the samples were analyzed. All formulations exhibited stability during mixing, mixability and homogeneity were adequate and the amount of free water collected was zero. An increase in the concentration of \(R\)-additive from 10% to 25% led to the reduction in the slurry density from 1.76 to 1.64 g/cm\({}^{3}\) and an increase in the rheological properties of the slurry, where \(PV\) increased from 48 to 104 cP and \(YP\) also increased from 3.8 to 12.3 N/m\({}^{2}\). An addition of \(R\)-additive at a concentration of 20% is recommended for optimization of the rheological properties. As the curing time was extended, \(F\) and \(\Delta\)/\(l_{0}\) increased with varying gradients. When cured at 60 \({}^{\circ}\)C and at atmospheric pressure, the highest \(F\) of 15.01 MPa was obtained when the concentration of \(R\)-additive was 10%, while the highest \(\Delta\)/\(l_{0}\) of 0.9985% was obtained when the concentration of \(R\)-additive was 25%. An increase in the curing temperature and pressure to 90 \({}^{\circ}\)C and 3000 psi (\(\approx\)20.68 MPa) resulted in the reduction in \(F\) from 15.01 to 14.62 MPa and from 10.33 to 9.61 MPa, and the increase in \(\Delta\)/\(l_{0}\) 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 formulations adopted in the present research, which contain \(R\)-additive at concentrations ranging from 10% to 25%, fulfilled the requirements of the oil and gas industry. Future research on the effect of adding elastomeric expandable additives to GPC to address volumetric shrinkage concerns in oil-well cementing can be augmented by performing an extensive study on the fluid flow for a wider and higher temperature range with cement qualification tests. In addition, the effect of downhole pressure can also be included in the investigations to assure that GPC can be adopted for a wide range of oil-well types. Furthermore, in view of the inconsistent properties that fly ash produced from different plants or batches in the same plant, the development of pretreatment methods to standardize the quality of fly ash is recommended to facilitate the generation of findings that are consistent and reliable for future research that may employ fly ash from various sources. Conceptualization, S.H.A.R. ; methodology, Y.A.S. ; software, S.H.A.R. ; validation, L.H.S. ; formal analysis, N.N.Z. ; investigation, N.H.; resources, M.F.H. ; data curation, A.I.A.H. ; writing--original draft preparation, S.A.F. ; writing--review and editing, S.H.A.R. ; visualization, L.H.S. ; supervision, N.S. ; project administration, Y.A.S. ; funding acquisition, N.S. All authors have read and agreed to the published version of the manuscript. This research was funded by PETRONAS Research Sdn. Bhd., Malaysia, via Project Cost Centre 015 MD0-064, as administered by the Research Management Centre of Universiti Teknologi PETRONAS, Malaysia, and the Project Cost Centre E.025.GST.02019.003, as per the record of PETRONAS Research Sdn. Bhd. Not applicable. The data that support the findings of this study are available from PETRONAS Research Sdn. Bhd. Restrictions apply to the availability of these data, which were used under license for this study. Data are available from the corresponding author, S.H.A.R., with the permission of PETRONAS Research Sdn. Bhd. The authors are thankful to the technologists of Universiti Teknologi PETRONAS and PETRONAS Research Sdn. Bhd.