An eco-efficient soil stabilization method: Synergistic use of calcium carbide residue and EAF slag for bentonite soil

• Expansive clay is stabilized using calcium carbide residue and electric arc slag. • Optimal CCR–EAF mix improves strength, reduces swelling, and enhances soil structure. • UCS of bentonite soil increased from 380 kPa to 9060 kPa; swelling potential decreased to 1%. • Microstructural analysis confirms the formation of C–S–H and C–A–S–H gels. • CCR and EAF slag offer a sustainable, affordable treatment method for expansive soil. Bentonite-rich expansive clay soils, which undergo substantial volume changes with moisture variation, present serious challenges for civil infrastructure, including buildings, pavements, and bridges. Conventional stabilizers such as lime and cement are effective but raise concerns over cost, carbon emissions, and environmental performance. Consequently, attention has shifted toward resource-efficient and low-impact alternatives. This study evaluates the stabilization of bentonite soil using calcium carbide residue (CCR) and electric arc furnace (EAF) slag, partially substituted with CCR. Laboratory tests including compaction, unconfined compressive strength (UCS), ultrasonic pulse velocity (UPV), electrical conductivity (EC), and pH were performed after 7, 28, and 56 days of curing. Microstructural analyses were conducted using SEM, XRD, FTIR, and EDX. The optimal mixture, consisting of 25% CCR combined with 10% EAF slag, enhanced the bentonite soil’s UCS from 380 kPa to 9060 kPa and increased the UPV from 1300 m/s to 2285.2 m/s. Furthermore, the combination of 4% CCR and 10% EAF effectively minimized bentonite swelling to 1% after 56 days. Accordingly, ‘optimal’ mix varies depending on the desired performance: a lower CCR content (around 4%) is adequate for controlling swelling, while higher CCR dosage is required to achieve maximum UCS. Microstructural characterization confirmed the formation of calcium silicate hydrate (C–S–H) and calcium aluminate silicate hydrate (C–A–S–H) gels, supported by higher Ca, Si, and Al concentrations. These hydration and polymerization processes enhanced the soil structure in an alkaline environment. Overall, CCR and EAF slag are effective and alternative agents for improving the performance of bentonite soils.