Due to their tendency to cause structural damage such as fractures and foundation failure, expansive soils, which have high volume changes with changing moisture levels, are a serious concern for civil engineers. In this research, the effectiveness of alkali-activated fly ash, an industrial by-product, as a replacement for traditional cementitious binders is investigated in response to the urgent need for sustainable soil stabilization. Class F fly ash activated using sodium hydroxide (10, 12.5, and 15 M) and sodium silicate, with varying liquid-to-solid ratios and concentrations of fly ash, was used on expansive clay from Bhubaneswar, India. Unconfined compressive strength tests were conducted on 81 combinations after 3, 7, and 28 days of curing. With an unconfined compressive strength of 4.8 MPa after 28 days, greater than twice that of untreated soil, the data indicate unconfined compressive strength increases with curing time and is best at 12.5 M NaOH concentration and 1.25 liquid-to-solid ratio. Due to over-activation effects, more strength was not contributed by increased alkali concentrations. Relative to cement or lime, fly ash-based stabilization reduces costs and carbon footprint, based on economic and environmental analyses, and microstructural studies confirmed the creation of a compact geopolymer matrix. This study provides an economically viable, green solution for problematic soils through systematically optimizing the parameters of geo-polymerization in expansive soil stabilization. It also presents quantitative evidence for the utilization of binders from industrial waste in geotechnical applications.