Combined influence of sulfate attack and wetting–drying cycles on RHA–GGBS stabilized clay

Abstract Subgrade soils in many regions are routinely exposed to harsh environmental conditions such as sulfate contamination and cyclic wetting–drying, causing structural deterioration and reduced service performance. This study examines the stabilization behavior of clayey soil (CL) treated with a geopolymer binder composed of rice husk ash (RHA) and ground granulated blast-furnace slag (GGBS), with focus on mechanical development and durability under combined exposure conditions. Strength and stiffness were measured at 7 and 28 days, with GGBS fixed at 10% and RHA (10%–30%) combined with sodium silicate–sodium hydroxide ratios of 60:40 to 90:10. The optimum mix (10% RHA, 10% GGBS, 80:20 activator ratio) exhibited the greatest durability, limiting compressive-strength loss to ~34% at 7 days and ~33% at 28 days after the first sulfate–WD cycle, and retaining the highest residual strength after six cycles, when most mixes lost more than 50%. Combined sulfate–WD exposure produced up to ~70% strength reduction relative to ambient curing, compared with ~56% under WD only, while sulfate soaking alone caused smaller losses that declined from ~8%–14% at 7 days to ~3%–5% at 28 days. A strong linear correlation (R 2 > 0.95) between UCS and G across all exposure regimes enabled reliable strength prediction. Results demonstrate that RHA–GGBS based geopolymer binders offer a sustainable and durable alternative for stabilizing subgrades exposed to chemically and physically aggressive environments.