Mineralogical and microstructural studies of C-S-H exposed to sulphate attack

Calcium-silicate-hydrate (C-S-H), the most important phase of hydrated portland cement, is responsible for the cementitious system's mechanical properties performance and durability. In a severe environment, C-S-H often suffers from sulphate attack, leading to severe concrete deterioration. The present work describes a detailed mechanism of sulphate attack on C-S-H from 28 to 270 days of sulphate curing (10% Na2SO4 solution) using XRD, TGA, FTIR, Si-29-NMR and FESEM/EDX techniques. The results show that gypsum is the only sulphate-bearing product formed by decalcification of calcium hydroxide first and then of C-S-H. Decalcification enhances polymerization in the C-S-H structure as an additional Q(3) peak was observed in FTIR and Si-29-NMR analysis. The microstructure of C-S-H becomes more crystalline as the sulphate curing period increases, as observed in FESEM micrographs. In addition, sulphate adsorption on C-S-H was identified through FESEM/EDX, which could play an important role in the deterioration of cementitious materials.