Ca-OH bonding in the C-S-H gel phase of tricalcium silicate and white portland cement pastes measured by inelastic neutron scattering

Inelastic neutron spectroscopy (INS) was used to determine the content of Ca-OH bonds in a series of mature tricalcium silicate (3CaO.SiO2) and white portland cement (WPC) pastes that were decalcified to calcium/silicon molar ratios (Ca/Si) as low as 1. These measurements indicate that the band in the INS data at 41 meV, which originates from lattice modes involving the translational oscillations of OH groups attached to Ca, has intensity contributions from both Ca(OH)(2) and C-S-H gel, the main hydration product of cement paste. Quantitative analysis of the INS spectra show that similar to23% of the Ca in C-S-H gel with Ca/Si = 1.7 is charge-balanced by OH groups, with the remainder charge-balanced by silicate groups. This substantial concentration of Ca-OH bonds supports the hypothesis that C-S-H gel has a local structure resembling a disordered form of the mineral jennite. On decalcification, the concentration of Ca-OH bonds in C-S-H gel decreases and is eliminated at Ca/Si ratios slightly above 1. The observed trend is shown to be consistent with charge balance calculations and interpretations of recent phase equilibrium findings.