Stabilization of Expansive Soil in a Road Subgrade with a Ladle Refining Furnace Slag- Water Treatment Sludge Hybrid Binder

This study presents a sustainable hybrid stabilization approach for subgrade soil using ladle refining furnace (LRF) slag and dried water treatment sludge (WTS). Soil was initially treated with LRF slag at replacement levels of 5–20 % by dry weight, followed by the incorporation of dried WTS to evaluate the synergistic effects of hybrid stabilization. A comprehensive laboratory program-including strength, durability, swelling, consistency, and leachability tests-was conducted on both individual and hybrid stabilized mixtures. The best-performing mixture among those tested, containing 15 % LRF slag and 12 % WTS, achieved a peak unconfined compressive strength of 1.57 MPa, representing a 726 % improvement over untreated soil. This mixture also demonstrated excellent durability, with only 9.29 % cumulative mass loss after 12 wetting–drying cycles and reduced the free swell index by up to 97 % after 28 days of curing. Microstructural analyses confirmed the formation of cementitious hydration products, including calcium silicate hydrate and calcium aluminosilicate hydrate gels, which contributed to enhanced mechanical and volumetric performance. Toxicity leaching test results showed that heavy metal concentrations in the best-performing stabilized samples remained within regulatory limits, indicating safe environmental behavior. Life cycle assessment demonstrated that the hybrid LRF-WTS system offers a low carbon, efficient alternative to conventional cement and lime-based stabilization. Overall, the findings highlight the potential of LRF slag and WTS binder as an effective and environmentally sustainable solution for stabilizing expansive subgrade soil.