To address the stability issues associated with highly gypseous soils under water exposure, the effects of nano-calcium carbonate (NCC), Class C fly ash (FA), and their combination on the shear strength and compaction characteristics of highly gypseous soils containing 56% gypsum were systematically investigated. In laboratory tests, three FA concentrations (5%, 10%, and 15%) and three NCC concentrations (0.5%, 1%, and 1.5%) were used as well as a blend of 0.5% NCC and 10% FA. Results indicate remarkable improvements in cohesion and internal friction angle of all treated samples, with the percentage of improvement being most pronounced under soaked conditions. The best combination (0.5% NCC + 10% FA) showed the highest after-soaking cohesion (39.2 kPa) and friction angle (41.8°) with the improvement ratios of 600% and 55.97%, respectively, compared to the untreated soil. Maximum dry density (MDD) increased with NCC content, reaching 1.782 g/cm³ at 1.5% NCC, while increasing FA content led to a decrease in MDD from 1.67 to 1.605 g/cm³. The blend had an MDD of 1.70 g/cm³, effectively optimizing the opposing effects of the two additives on compaction. The improvements are primarily attributed to the void-filling effect of FA, nanoscale densification and enhanced particle bonding by NCC, and the synergistic physicochemical interaction between the two additives. Early pozzolanic reactions may also have contributed. The improvements were further validated by microstructural analyses (XRD and SEM), which confirmed the formation of cementitious products. The mixture proved to be more resistant to the loss of strength due to soaking and more durable in different moisture conditions. The results of this study fill a critical gap in the literature on the combined stabilization of NCC and Class C FA and provide practical suggestions for improving the mechanical and compaction properties of gypseous soils in arid and semi-arid regions.
