Carbide lime is a sustainable alternative to the traditional lime for soil stabilization. However, its application in sulfate-rich soils can result in the formation of expansive minerals such as ettringite and thaumasite, which is counterproductive to the stabilization. Additionally, external sulfate contamination following the stabilization process poses further challenges. This study evaluates the impact of external sulfate contamination on carbide lime-treated expansive soil. The treated expansive soil samples were exposed to sulfate solutions of varying concentrations (5000 ppm, 10,000 ppm, and 20,000 ppm) to assess the changes in index properties, swell potential, swell pressure, and unconfined compressive strength. The experimental results indicated that the sulfate contamination negatively affects the performance of carbide lime-treated expansive soil. The microstructural analysis confirmed the formation of ettringite mineral, highlighting the detrimental effects of sulfate-lime-treated soil interactions post-stabilization.