Enhancement of highly expansive subgrade with metabentonite and nano-waste additive-based geopolymers

In this study, the geopolymerisation method was employed to enhance the properties of highly expansive subgrade soil. Two precursors, namely coconut shell ash (CSA) and metabentonite, were mixed with NaOH alkali solution at various mix ratios generated using the Taguchi orthogonal array. The study aimed to improve geotechnical properties such as compaction, California bearing ratio (CBR) and unconfined compressive strength (UCS). The results from the experimental data revealed significant enhancements in UCS and CBR compared to natural soil, with increases of 425.79% and 691.97% for UCS after 7 and 28 days of curing, and a 132.46% increase in unsoaked CBR. These improvements were attributed to the complete dissolution of silica (Si) and alumina (Al) from the precursors, which encourages significant polymerisation and subsequently results in the formation of geopolymeric gels of N-A-S-H and C-A-S-H. Additionally, the pozzolanic reaction and presence of Ca(OH)2 contributed to the formation of secondary calcium silicate hydrate (C-S-H) gel. The optimal soil-geopolymer mix for CBR and UCS was determined as 10% CSA + 20% metabentonite + 2M NaOH. Using paveXpress software, pavement design analysis under heavy traffic loading conditions revealed a 28.57% reduction in pavement thickness compared to the natural subgrade soil.