Stabilization and solidification of lead, chromium, and cadmium-contaminated clayey sand using calcium hydroxide and sodium carbonate-activated materials

Soil pollution by heavy metals has emerged as a critical environmental issue, posing significant threats to human health, food security, and environmental sustainability. Particularly in industrial and mining areas, the high levels of pollutants such as lead, chromium, and cadmium degrade soil quality and create potential risks for ecosystems. This study explores the stabilization and solidification of lead, chromium, and cadmium-contaminated clayey sand using Ca(OH)₂ and Na₂CO₃-activated waste materials, specifically slag and natural zeolite, aimed at improving mechanical strength, reducing leachability, and enhancing environmental sustainability. Unconfined compressive strength tests revealed that soil samples treated with 20 wt% Ca(OH)₂ and Na₂CO₃-activated slag exhibited a UCS of 4449 kPa after 28 days, a 75-fold increase over untreated soil, far exceeding the U.S. EPA's minimum requirement for landfill stabilization. X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) analyses confirmed the formation of calcium-aluminosilicate hydrate (C(A)SH) gels and secondary products like ettringite, which contributed to the substantial strength gain. Toxicity characteristic leaching procedure (TCLP) leachability tests, showed that Ca(OH)₂ and Na₂CO₃-activated slag achieved up to 95 %, 92 %, and 98 % reductions in lead, chromium, and cadmium leachability, respectively, all below EPA regulatory thresholds. Permeability tests demonstrated a significant decrease in hydraulic conductivity, with up to a 97 % reduction, indicating the treated soil's improved resistance to contaminant migration. Life cycle assessment (LCA) showed that the proposed Ca(OH)₂ and Na₂CO₃-activated slag and zeolite-based system reduced CO₂ emissions by 75 % compared to conventional Portland cement stabilization, while maintaining equivalent performance in UCS and metal immobilization.