Degradation mechanisms of alkali-activated binders in sulfuric acid: The role of calcium and aluminum availability

Abstract

Although alkali-activated binders (AABs) show a stronger resistance against sulfuric acid (H2SO4) attack than ordinary Portland cement (OPC), it remains unclear how the resistance capacity is affected by the compositional variability of gel products, i.e., sodium calcium aluminosilicate hydrate (N-C-A-S-H). In this work, the compositional and molecular alteration of synthetic N-C-A-S-H exposed to 1.0%, 2.5%, and 5.0% H2SO4 solutions are studied. The results show that the formation of ettringite is less favored than gypsum when the calcium availability is limited, despite sufficient aluminum sources. The calcium and aluminum dissolved from N-C-A-S-H can contribute to the ettringite formation even though the AFm phases are present, particularly for those with relatively high Ca/Si and Al/Si ratios. The aluminum content in AABs controls the AFm formation and Al/Si ratio of N-C-A-S-H, thus affecting the condition for ettringite and gypsum precipitation. The presence of calcium in the atomic structure of N-C-A-S-H makes it more susceptible to the acid-induced degradation and polymerization.