Sustainable Stabilization of High-Plasticity Expansive Clay for Road Subgrades: Synergistic Effects of Steel Slag and Rice Husk Ash

High-plasticity expansive clay (HPC) poses significant challenges in roadbed construction due to its low strength, high swelling potential, and poor compaction characteristics. Conventional stabilization methods using cementitious materials raise environmental concerns due to high carbon emissions. This study investigates the synergistic solidification mechanisms of steel slag (SS) and rice husk ash (RHA) composites for sustainable stabilization of HPC. Laboratory tests, including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), consolidation, and California Bearing Ratio (CBR) tests, were conducted to evaluate the mechanical and microstructural properties of SS-RHA-stabilized HPC. Field monitoring and FLAC3D numerical simulations were performed to validate long-term performance in a highway expansion project. Results demonstrated that the addition of 15% SS and 9% RHA significantly enhanced the compression modulus (from 7.10 to 44.33 MPa at 28 days) and CBR values (from 7.4% to 58.2%), attributed to the formation of calcium silicate hydrate (CSH) gels and pore structure refinement. Microstructural analyses revealed reduced porosity (from 11.085% to 6.286%) and increased amorphous phases, confirming enhanced soil-cementation interactions. Simulated data and field monitoring data indicate that the settlement of SS-RHA modified HPC (maximum settlement of 11.97 mm) is lower than that of untreated soil, attributed to the synergistic activation of SS (rich in CaO) and RHA (high SiO 2 content), which effectively improves soil strength and stability. This study confirms that SS-RHA composites can serve as an effective, low-carbon stabilization solution for high-plasticity clay subgrades in highway construction, offering improved bearing capacity, reduced long-term settlement, and enhanced environmental sustainability compared to conventional cement-based treatments.