Mechanism of sulfate attack on alkali-activated slag: The role of activator composition

In this work, the degradation mechanisms of alkali-activated slag (AAS), prepared with various activator compositions, exposed to 5% Na2SO4 and MgSO4 solutions are studied. The results show that upon Na2SO4 attack, little ettringite formation and compositional alteration occur in AAS with little length expansion. However, upon MgSO4 attack, brucite, gypsum, and magnesium-aluminosilicate-hydrate (M-A-S-H) are observed in altered layers of hardened AAS pastes. The sulfate-activated slag shows a weaker resistance against MgSO4 attack than NaOH- and Na2CO3 -counterparts due to a lower pH value in pore solution and thus absence of brucite protective layer. The reacted products in hardened AAS pastes have a high capacity against dealumination in sulfate environments, contributing to superior sulfate resistance over traditional ordinary Portland cement (OPC).