Even though the effectiveness of sulfate-resistant cement (SRC) in stabilizing the high sulfate-bearing expansive soils is proven, its effectiveness in controlling the volume change of expansive soils when exposed to external sulfate contaminants is not known. The physico-chemical and index properties provide basic insight into the volume change behavior of clays. Therefore, this study brings out the effect of external sulfate contamination on the physico-chemical and index properties of SRC-treated expansive soil. Three SRC contents of 5, 10, and 15% were added to the expansive soil separately and reconstituted with distilled water and cured for 1–28 days. After the desired curing period, the SRC-treated expansive soil was reconstituted with sulfate solutions of 5000, 10,000, and 20,000 ppm separately and moisture equilibrated for 1 day for the determination of the properties. The experimental results showed that the SRC treatment increased the pH from 8.75 to 11.95–12.21 and the subsequent sulfate contamination decreased the pH to 9.33–11, where the decalcification of calcium silicate hydrate occurred. Further, the effect of sulfate contamination on liquid limit of SRC-treated soil was negligible, while the plastic and shrinkage limits increased upon sulfate contamination. The increase in the shrinkage limit is attributed to the formation of ettringite/thaumasite in the voids of SRC-treated samples contaminated with 10,000–20,000 ppm sulfate solutions, whereas the monosulfate formation and destruction of cementation gels occurred in samples contaminated with 5000 ppm. These formations are evidenced with the scanning electron microscopy, energy dispersive X-ray analysis, and X-ray diffraction. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.