29Si NMR in Cement: A Theoretical Study on Calcium Silicate Hydrates

The NMR spectra of Si-29 in cement-based materials are studied through calculations of the isotropic shielding of silicon atoms within the density functional theory. We focus on the main component of cement, the calcium-silicate-hydrate gel, using widely accepted models based on the observed structures of jennite and tobermorite minerals. The results show that the Si-29 chemical shifts are dependent not only on the degree of condensation of the (SiO4) units, as commonly assumed, but also on the local arrangement of the charge compensating H and Ca cations. We find that the NMR spectra for models of the calcium-silicate-hydrate gel based on tobermorite are in better agreement with experiment than those for jennite-based models.