Effect of curing on the coupled attack of sulfate and chloride ions on low-carbon cementitious materials including slag, fly ash, and metakaolin

Shortly after casting, offshore reinforced concrete structures are exposed to sulfate and chloride ions without undergoing any proper curing procedures, which are crucial for enhancing their strength and durability. Furthermore, there persists a lack of comprehensive understanding concerning the coupled impact of the attack of sulfate and chloride ions on cementitious materials with current existing research still yielding conflicting outcomes. This paper investigates the influence of curing on the chemical interactions between low-carbon cementitious materials (substituting 45 % of cement with industrial by-products like slag, fly ash, and metakaolin to reduce CO2 emissions) and coupled sulfate and chloride ions. To achieve this, powder samples were obtained from prismatic-shaped specimens of ordinary Portland cement (CEM I) paste, binary, ternary, and quaternary blends. Some solid specimens were ground into powder after 24 hours post-casting while others were powdered after a 90-day curing period. Subsequently, the acquired powders underwent exposure to chloride, sulfate, and sulfate-chloride solutions over a duration of 25 days. Results indicate that curing time does not alter the interplay between sulfate and chloride ions in the cementitious matrix. Sulfate presence accelerates chloride ion ingress, while chloride presence mitigates sulfate attack in both uncured and pre-cured samples. However, the curing process accelerates sulfate attack, proven through excessive ettringite and gypsum formation, high consumption of portlandite and AFm, diminution of sulfate ions concentration in the attacking solution after 5 days of exposure, increase of pH of more than 0.5 units, aspect changes observed through visual inspections, and porosity distribution changes showed by an increase of 10 % of the total porosity in the reference mix. Additionally, curing promotes the formation of Friedel's salt, thereby attenuating chloride ions ingress.