Stabilization of highly expansive clay using synergistic EAF slag and water treatment sludge: a mechanical performance study

This study addresses the challenges of highly expansive clay soils by stabilizing them with two industrial by-products: electric arc furnace (EAF) slag and water treatment sludge (WTS), thereby addressing the limited research on their combined (synergistic) use in soil stabilization. The potential of these additives to enhance soil strength and stiffness while mitigating swelling was evaluated using clay samples treated with 0, 10, 20, and 30% EAF slag and 0, 5, 10, 20, and 30% WTS replacement, after curing for 7, 28, and 56 days. Unconfined compressive strength (UCS) and ultrasonic pulse velocity (UPV) tests were employed as primary indicators of mechanical performance, while free swelling tests were carried out to assess volume stability. In addition, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) analyses were performed to provide micromechanical insights into the mechanisms underlying the observed improvements. Results indicate that the combined use of 30% EAF slag and 20% WTS replacement produced the most favorable outcomes in terms of strength, stiffness, and swelling reduction. UCS increased by more than fourfold compared with untreated soil, reaching approximately 2800 kPa, while UPV rose to nearly 1700 m/s. The swelling potential of the optimized mix decreased dramatically from 270% in the natural clay to only 8% after 56 days of curing. Microstructural analyses confirmed the development of cementitious gels, primarily calcium-silicate-hydrate (C–S–H) and calcium-aluminate-hydrate (C–A–H), which filled voids, bonded particles, and created a denser, more stable matrix resistant to moisture-induced expansion. These findings demonstrate that the combined use of EAF slag and WTS provides an effective approach to improving the mechanical and volumetric performance of expansive soils.