Microstructural evolution and degradation mechanisms of tricalcium silicate and tricalcium aluminate composite pastes under sulfate exposure

Sulfate attack precipitates instability and degradation within calcium silicate (aluminate) hydrate (C-(A)-S-H) gel, the primary binding phase in cementitious materials . This degradation intricately correlates with the clinker phase composition, particularly the ratios of tricalcium silicate (C 3 S) and tricalcium aluminate (C 3 A). In this study, C 3 S and C 3 A were synthesized through solid-state sintering to create C 3 S-C 3 A pastes. The research aimed to examine the impact of varying C 3 A substitution levels and Na 2 SO 4 exposure durations on the microstructure and performance of these composite pastes. The findings reveal that C 3 A markedly accelerates both the early hydration kinetics and the overall hydration degree of C 3 S within the C 3 S-C 3 A system, with this effect becoming more pronounced with increasing C 3 A content. Furthermore, the inclusion of C 3 A elevates the Al[4]/Si ratio, mean chain length (MCL), and Ca/Si of the C-S-H gel. This leads to an increase in the overall porosity of the composite matrix while simultaneously diminishing its chloride-binding capacity. Exposure to Na 2 SO 4 disrupts the incorporation of Al 3+ , released during C 3 A hydration, into the bridging sites of the C-S-H structure, causing a decrease in the Al[4]/Si of the C-(A)-S-H gel. The leached aluminum subsequently reacts with SO 4 2- to form ettringite and other deleterious compounds. Additionally, Na 2 SO 4 exposure induces a significant reduction in the Ca/Si within the composite pastes, with the magnitude of decalcification inversely proportional to the initial Ca/Si—implying that higher C 3 A content exacerbates the decalcification process. During the process of sulfate attack, the formation of gypsum and AFt complicates the pore structure of the matrix, providing more adsorption sites for the binding of Cl - .