A research study was designed and conducted to evaluate the applicability of using crystalline silica-rich material as a co-additive with calcium-based stabilisers to treat sulfate-rich expansive soils. Extensive laboratory tests were conducted on lime-treated sulfate-rich soils, with and without crystalline silica admixture, to evaluate the improvements in engineering properties, including swell strain, unconfined compressive strength and resilient moduli properties. The influence of co-additive dosage, curing period and duration of moisture exposure was studied to gain insights into the behaviour of treated soils. Supplementary mineralogical and microstructural studies including X-ray diffraction, scanning electron microscope imaging and differential scanning calorimetry studies were performed on the treated soils to detect the formation of ettringite mineral and cementitious gel compounds. Test results indicated that the use of crystalline silica facilitated enhancement of both the strength before and after capillary soaking, and the resilient moduli properties, while suppressing ettringite mineral formation and associated free swell strains in the treated soils. Mineralogical and engineering studies showed the dominance of pozzolanic activity between lime and crystalline silica over ettringite formation and hence contributed to the effective stabilisation of sulfate-rich soils. Overall, the paper provides a comprehensive understanding of the role of crystalline silica admixture for mitigating deleterious sulfate heaving in lime-treated soils.