Expansive soils pose significant challenges to civil engineering infrastructure due to their high plasticity, excessive swelling, and shrinkage behavior under fluctuating moisture conditions. These characteristics often lead to structural distress, including pavement cracking, foundation settlement, and loss of bearing capacity, particularly in regions such as the Niger Delta where seasonal moisture variation is pronounced . This study investigated the effectiveness of industrial waste materials, specifically fly ash and desulphogypsum, as sustainable stabilizing agents for improving the geotechnical properties of expansive soils. Laboratory experiments were conducted on untreated and treated soil samples with varying proportions of fly ash (5–15%) and desulphogypsum (5–15%), including their combined application. The natural soil was classified as highly plastic clay (CH) with a liquid limit of 92% and plasticity index of 71%, indicating high swelling potential. The addition of stabilizers significantly modified the soil behavior. The plasticity index decreased progressively with increasing stabilizer content, reaching a minimum value of 24% for the combined mix of 10% fly ash and 15% desulphogypsum. Compaction characteristics improved, with maximum dry density increasing from 1.485 g/cm³ to 1.556 g/cm³, while optimum moisture content reduced from 24.8% to 19.6%. Unconfined compressive strength (UCS) results demonstrated substantial improvement in strength due to stabilization. The untreated soil exhibited a UCS value of 89.2 kPa, which increased to 456.3 kPa after 28 days of curing for the combined stabilizer mix, representing a 411% increase. Regression models developed for predicting UCS and plasticity index reduction yielded strong correlations with coefficients of determination (R²) of 0.887 and 0.862, respectively, and mean absolute error below 8%. Sensitivity analysis revealed that incremental increases in fly ash and desulphogypsum content contributed positively to strength development, with fly ash having a slightly higher influence. Optimization analysis identified an optimal stabilizer combination of 16.1% fly ash and 10.9% desulphogypsum, corresponding to a predicted UCS of 468.3 kPa and a minimum plasticity index of 22.1%. Economic evaluation showed that the combined stabilization method cost approximately ₦3,600/m³ compared to ₦11,700/m³ for conventional lime stabilization, resulting in a cost reduction of about 69%. The results demonstrated that the combined use of fly ash and desulphogypsum significantly improved the engineering performance of expansive soils by reducing plasticity, enhancing strength, and improving compaction characteristics. The study established that these industrial by-products offer a cost-effective and environmentally sustainable alternative for ground improvement in the Niger Delta, with strong potential for application in road construction and foundation engineering.