Structural transformation of tricalcium silicate during hydration

The structures of anhydrous calcium silicates Ca₃SiO₅(C₃S), Ca₂SiO₄ (β-C₂S and γ-C₂S) and CaSiO₃ (CS) have been examined and compared whereas the hydration products of the main cement component C₃S have been studied by radial electron density distribution (RED) and infrared (IR) spectroscopy. The polymerization of the initially isolated silica tetrahedra in the course of the hydration process of C₃S has been recognized by several authors as the responsible agent of the mechanical resistance developed during the hardening. In this work, the invoked polycondensation has been directly proven by the appearance in the RED curves, between 12 and 48 h, of Si–Si vectors at 3.1 Å and of half-a parameter contributions at 5.65 Å, which represent typical structural features of the poorly crystallized tobermorite-like CSH residue. Independently, this structural transformation was also followed in IR spectra by the shift of the orthosilicate (C₃S) Si—O stretching doublet 935–890 cm⁻¹ toward a single higher-frequency band at 975 cm⁻¹. Finally, assuming a linear hydration process, synthetic mixtures of the RED profiles of the three main components, i.e. C₃S, tobermorite and CH, account reasonably, within the first 6 Å region, for the sample's RED profiles observed after up to 5 d hydration (63%).