Pumice soil stabilisation using alkali-activated waste glass for sustainable road subgrade applications

This research explores the development of glass-based alkali-activated cement (GBAAC) for stabilising New Zealand pumice sand as a subgrade in road construction. Using GBAAC in road construction offers a sustainable solution by reducing waste glass (WG) accumulation in landfills and providing an effective alternative to conventional chemical soil stabilisers. Several mixtures were prepared by systematically optimising both the activator-to-precursor ratio (A/P), using a 10 M KOH solution as the activator, and the particle size distribution of the WG precursor to achieve optimal strength and performance characteristics. The maximum compressive strength of 37.9 MPa was obtained at an A/P of 0.4 after curing at 65 °C for 48 h. The microstructural analysis confirmed the formation of alkali/earth-alkali-silicate-hydrate gels involving Na⁺, K⁺, Ca²⁺, and Mg²⁺ ions. An interesting aging phenomenon was observed, with compressive strength decreasing at 28 days due to depolymerisation, then surpassing initial strength at 56 days. Initial curing at 65 °C for 48 h resulted in a 56-day strength 2.6 times higher than that achieved with room temperature curing. An unground-to-ground WG unit ratio was selected as the optimal precursor composition, balancing strength requirements and production efficiency. California Bearing Ratio (CBR) tests demonstrated that mechano-chemical stabilisation using GBAAC significantly enhanced the stress-strain behaviour of pumice sand. The highest average CBR5.0 value of 64.70% was achieved with heavy compaction of GBAAC-stabilised samples, a substantial improvement from 21.67% for lightly compacted untreated samples.