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 (C3S) and tricalcium aluminate (C3A). In this study, C3S and C3A were synthesized through solid-state sintering to create C3S-C3A pastes. The research aimed to examine the impact of varying C3A substitution levels and Na2SO4 exposure durations on the microstructure and performance of these composite pastes. The findings reveal that C3A markedly accelerates both the early hydration kinetics and the overall hydration degree of C3S within the C3S-C3A system, with this effect becoming more pronounced with increasing C3A content. Furthermore, the inclusion of C3A 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 Na2SO4 disrupts the incorporation of Al3+, released during C3A 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 SO42- to form ettringite and other deleterious compounds. Additionally, Na2SO4 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 C3A 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-.