Sulfate resistance of recycled powder-slag-based geopolymer mortars: Effects of recycled powder content and alkali equivalent

Recycled powder-slag-based geopolymers (RSG) offer route for the high-value reuse of construction waste; however, their durability under sulfate exposure requires further investigation. In this study, the sulfate resistance of RSG mortars with varying recycled powder (RP) contents and alkali equivalents was investigated by immersing the specimens in a 5 wt% Na₂SO₄ solution for up to 180 days. Workability, compressive strength, visual appearance, and mass change were evaluated, while the degradation mechanisms were further elucidated by XRD, TG-DTG, SEM, FT-IR, and MIP analyses. It was found increasing the RP content reduced the flowability and initial compressive strength of the system, whereas a higher alkali equivalent could effectively compensate for these drawbacks. During sulfate immersion, both the strength and mass of RSG exhibited a two-stage evolution, characterized by an initial increase within the first 30–60 days, depending on the mixture composition, followed by gradual deterioration. This behavior primarily resulted from ettringite and gypsum filling the pores during the initial phase of exposure, along with subsequent damage caused by crystallization expansion. Microstructural analyses confirmed that a high alkali equivalent (8%) promoted the extensive formation of C-(A)-S-H gels, hindering sulfate ion ingress. In contrast, a high RP content (≥50%) diluted the reactive precursors, increased the porosity of the system, and rendered the matrix highly susceptible to severe deterioration during prolonged exposure. Overall, when the RP content was controlled within 10%-30% and combined with an appropriately high alkali equivalent, RSG exhibited excellent resistance to sulfate attack under the investigated 5 wt% Na₂SO₄ full-immersion condition.