Dry–wet cycling sulfate attack on RHA–GP alkali-activated concrete: Macroscopic deterioration, cyclic compression behavior and multi-scale microstructural mechanisms

This study uses rice husk ash (RHA) and glass powder (GP) as composite precursors to prepare alkali-activated concrete, and establishes an accelerated dry–wet sulfate exposure environment to conduct multiscale analyses of physical properties, mechanical performance, cyclic compressive behavior, and microstructural evolution. The results demonstrate that the material exhibits a typical three-stage deterioration pattern under dry–wet cycling: in the early stage (0–75 cycles), the filling of pores by sulfate reaction products and the secondary formation of gels lead to a temporary increase in mass, elastic modulus, and strength; in the intermediate stage (75–150 cycles), the structure transitions from densification to crack initiation, and the decline in physical properties becomes less steep; in the late stage (>150 cycles), repeated crystallization–dissolution of Na 2 SO 4 , together with the extensive formation of expansive AFt and gypsum, enhances pore connectivity, causing the compressive, splitting tensile, and flexural strengths to decrease by 55.8%, 31.7%, and 40%, respectively, while permeability and resistivity exhibit significant opposite trends. Cyclic compression tests reveal a progressive evolution from elasticity-dominated behavior to damage-induced softening, with the hysteresis loop area and peak stress gradually decreasing; parameters such as loading strain, elastic strain, residual strain, and non-closure indicate that dry–wet cycling markedly amplifies plastic accumulation. Energy analysis shows that total strain energy and elastic strain energy shift from early-stage storage dominance to late-stage dissipation dominance, reflecting the transition from structural stability to damage-induced instability. The damage index constructed from residual strain exhibits a “slow–accelerated–stable” evolution pattern, revealing the amplifying effect of the coupled action of sulfate attack and cyclic loading on damage accumulation. SEM observations confirm these mechanisms: in the early stage, needle-like AFt directionally fills the pore walls; in the intermediate stage, clustered AFt and gypsum deposit in the interfacial zone and induce microcrack propagation; in the late stage, extensive sheet-like gypsum covers the matrix surface, and disordered expansive AFt penetrates the pores, forming a multiscale crack network that fundamentally disrupts gel continuity. This study not only reveals the macro–micro coupled deterioration mechanism of RHA–GP alkali-activated concrete under sulfate dry–wet cycling, but also establishes a systematic analytical framework linking physical deterioration, static mechanical response, cyclic energy evolution, and microstructural damage, thereby extending current understanding of the durability of alkali-activated concrete prepared from agricultural-residue/municipal-solid-waste composite precursors.