Strength and environmental assessment of fly ash-slag-stabilised kaolin for road sub-base

The stabilisation of weak soils, such as kaolin, is essential for improving their engineering properties for geotechnical applications. This study investigates kaolin stabilisation using fly ash (FA) and ground granulated blast furnace slag with 12 mixes at 10%, 20%, and 30%. Laboratory tests evaluated compaction, California bearing ratio, and unconfined compressive strength (UCS) under ambient curing at 1, 3, 7, 14, 28, and 60 days. The optimum strength was achieved with KFS746, reaching 2.4 MPa at 28 days with 30% precursor content. Microstructural analyses (X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy) were conducted and correlated with strength development. Cradle-to-gate life cycle assessment estimated a carbon footprint of 25.535 kg CO2-equivalent per m³ of treated soil. Heavy metal concentrations from toxicity characteristic leaching procedure were below permissible limits. A response surface methodology-based quadratic model predicted UCS with high accuracy (R2 = 98.73%, p ≤ 0.05). Based on slag CaO content, the theoretical CO2 sequestration potential was 348.6 g/kg. FA and slag proved effective for sustainable kaolin stabilisation.