Study on Mechanical Strength and Mechanism of Coal Gangue–Magnesium Oxide-Stabilized Expansive Soil

This study investigates the effectiveness of magnesium oxide (MgO) and coal gangue in improving expansive soil roadbeds through wet–dry cycle tests, unconfined compressive strength tests, and direct shear tests. The results demonstrate that coal gangue effectively suppresses non-load-induced swelling rates in expansive soil. At 15% coal gangue content, the unconfined compressive strength peaked at 0.6 MPa. Further incorporation of MgO at 2%, 4%, 6%, 8%, and 10% concentrations showed that 10% MgO significantly enhanced strength to 3.1 MPa, surpassing both the untreated soil (0.07 MPa) and coal gangue-only treatment (0.6 MPa). While wet–dry cycle tests revealed a gradual decrease in strength with repeated cycles, increasing MgO content mitigated this decline, resulting in coal gangue–MgO-stabilized soil with excellent water stability. Scanning electron microscopy revealed MgO-hydrated cementitious materials encapsulating expansive soil particles and connecting coal gangue particles, though fine cracks were observed near coal gangue surfaces.