Role of porous aggregate types on the sulfate attack resistance of cement mortar

Sulfate attack remains a critical durability challenge for cementitious materials, especially in aggressive environments such as coastal regions or sulfate-rich groundwater areas. While existing research predominantly focuses on optimizing the pore structure of the cementitious matrix, the role of aggregate porosity in mitigating sulfate-induced degradation remains underexplored. This study systematically investigates the influence of two porous aggregates, coral sand (3D interconnected pore network) and volcanic sand (interconnected sac-like pores), on the sulfate resistance of cement mortar. They were used to partially replace standard sand at varying substitution levels (20–80 % for coral sand; 20–50 % for volcanic sand). Mortar specimens were exposed to a 5 wt% sodium sulfate solution for 12 months. Results demonstrate that the 3D network pores in coral sand promote AFt growth, mitigating structural damage. Specimens with 50–80 % coral sand substitution exhibited minimal expansion (around 0.02 % at 12 months) and sustained strength improvement, attributed to the dual mechanisms of pore-filling and interfacial transition zone (ITZ) strengthening. In contrast, volcanic sand accelerated sulfate ingress due to its permeable pore structure, leading to rapid ettringite and gypsum formation, severe expansion (up to 0.66 % at 12 months), and substantial strength loss (67–70 % reduction in flexural strength after 12 months). The findings of this study highlight the critical role of pore structure in determining the sulfate resistance of cementitious materials. These insights provide valuable guidance for the selection and optimization of aggregates in cementitious materials and design for durable infrastructure in chemically aggressive settings.