Improving sulfate and chloride resistance in eco-friendly marine concrete: Alkali-activated slag system with mineral admixtures

Abstract

This study investigates the effects of various mineral admixtures, such as fly ash, metakaolin, and limestone powder, on the mechanical, chloride, and sulfate resistance properties of alkali-activated slag (AAS). The resistance of alkali-activated concrete (AAC) to chloride and sulfate is evaluated using the rapid chloride migration (RCM), wetting-drying cycles, SEM and XRD, TG/DTG. The results reveal that introducing fly ash improves workability but degrades the strength and durability of AAS. The 15% slag substitution by metakaolin is recommended to improve durability. Incorporating 15% limestone powder increases the compressive strength and reduces porosity due to filling and nucleation effects. The replacement ratio of 30% for each mineral admixture induces a considerable Ca2+ dilution, resulting in reducing the strength and durability of AAC. Under short-term wetting-drying cycles, the continued hydration of raw materials under MgSO4 erosion and the subsequent production of gypsum, hydrotalcite, and brucite from the matrix initially increase the AAC strength. However, the continued substitution of calcium in the C-(A)-S-H gel by magnesium and precipitation of gypsum under excessive cycles adversely deteriorates the compressive strength. The chloride migration degree in AAC is closely related to the porosity. Although the porous gel N-A-S-H facilitates chlorine migration in accelerated tests, it prevents further chlorine penetration by adsorbing chloride ions in large quantities, exhibiting the dominant effect on chloride immobilization. The formation of Friedel's salt, stemming from the chemical bonding of chloride, is present only in the high-calcium specimen G100 and the highly reactive aluminum specimen G85M15. Improvements in the gel chemistry and porosity are key factors in bolstering the durability of AAC. The findings of this study contribute to advancing the application of eco-friendly AAC in marine environments.