Quantitative study of hydration of C₃S and C₂S by thermal analysis: Evolution and composition of C-S-H gels formed

This research is part of a European project (namely, CODICE project), main objective of which is modelling, at a multi-scale, the evolution of the mechanical performance of non-degraded and degraded cementitious matrices. For that, a series of experiments were planned with pure synthetic tri-calcium silicate (C₃S) and bi-calcium silicate (C₂S) (main components of the Portland cement clinker) to obtain different calcium-silicate-hydrate (C-S-H) gel structures during their hydration. The characterization of those C-S-H gels and matrices will provide experimental parameters for the validation of the multi-scale modelling scheme proposed. In this article, a quantitative method, based on thermal analyses, has been used for the determination of the chemical composition of the C-S-H gel together with the degree of hydration and quantitative evolution of all the components of the pastes. Besides, the microstructure and type of silicate tetrahedron and mean chain length (MCL) were studied by scanning electron microscopy (SEM) and ²⁹Si magic-angle-spinning (MAS) NMR, respectively. The main results showed that the chemical compositions for the C-S-H gels have a CaO/SiO ₂ M ratio almost constant of 1.7 for both C₃S and C ₂S compounds. Small differences were found in the gel water content: the H₂O/SiO₂ M ratio ranged from 2.9 ± 0.2 to 2.6 ± 0.2 for the C₃S (decrease) and from 2.4 ± 0.2 to 3.2 ± 0.2 for the C₂S (increase). The MCL values of the C-S-H gels, determined from ²⁹Si MAS NMR, were 3.5 and 4 silicate tetrahedron, for the hydrated C₃S and C₂S, respectively, remaining almost constant at all hydration periods.