This study addresses the engineering geological disaster resulting from the degradation of mechanical properties of expansive soil due to changes in environmental humidity along the Middle Route of the South-to-North Water Transfer Project. Calcium carbide slag and slag are utilized as curing materials to improve the expansive soil. Comparative tests were conducted on the unconfined compressive strength, split tensile strength, and water stability of untreated and treated expansive soil to analyze the performance differences pre- and post-treatment. The strength enhancement mechanism of the calcium carbide slag-slag cured soil was investigated through the X-ray diffraction (XRD), electron microscope scanning (SEM), thermogravimetric analysis (TGA) test and nuclear magnetic resonance (NMR) test, revealing its microscopic mechanism of action. The results showed a significant increase in the overall strength and water resistance of the calcium carbide slag-slag composite modified cured soil with different slag dosage based on 6% dosage of calcium carbide slag, and a maximum value was reached when the slag dosage was 9%. Over time, the unconfined compressive strength and split tensile strength improved, while the water stability coefficient decreased notably. Hydration of calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H) generated by the hydration of calcium carbide slag-slag composite cured soil led to the formation of tightly bonded soil particles, enhancing the soil's pore structure distribution and strength. The evident effectiveness of the composite curing method for calcium carbide slag-slag treated soil suggests promising engineering applications.
