Geopolymer Stabilization of Industrial Wastes for Sustainable Pavement Base and Subbase Layers: Strength, Durability, and Environmental Assessment

This study investigates the geopolymer stabilization of industrial wastes—coalmine overburden (OB), basic oxygen furnace slag (BOFS), and granulated blast furnace slag (GBFS)—as a sustainable substitute for pavement base/subbase layers. The effect of the molar concentration ratio (MCR) of NaOH to Na2SiO3 (1, 2, and 4) on strength and durability was analyzed. A blend of 40% OB, 40% BOFS, and 20% GBFS at MCR=1 achieved the highest 28-day unconfined compressive strength (UCS) of 6.5 MPa, meeting Indian Road Congress standard. Durability assessments confirmed maximum mass loss of 6.38% and UCS retention of 4.71 MPa after 12 wet–dry cycles for MCR=4, along with low water absorption (6.97%) and capillary rise (<25% in 24 h). X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and energy-dispersive X-ray (EDX) analyses identified calcium-aluminosilicate hydrate (C-A-S-H) gel as the primary strength contributor. Toxicity tests confirmed heavy metal leaching below regulatory limits. A comparative cost and carbon footprint estimate demonstrated a 9.7% reduction in construction costs and a 28.4% decrease in CO2 emissions for constructing a 1-km highway. These findings highlight geopolymer-treated OB-BOFS-GBFS as a feasible, environment-friendly alternative to cement-based roadway materials.