²⁹Si chemical shift anisotropies in hydrated calcium silicates: A computational study

The ²⁹Si chemical shift anisotropies are investigated for calcium silicate hydrates. The focus is on the naturally occurring minerals, jennite and 14 Å tobermorite and models derived from them to simulate calcium-silicate-hydrate gel, the main component of Portland cement. Our theoretical results show that the analysis of anisotropy and asymmetry of the ²⁹Si chemical shift discriminates between different Si types, even if their isotropic chemical shifts are similar. Terminal and pairing silica tetrahedra are clearly distinguished and the chemical shift anisotropies set apart the Si tetrahedra that are hydroxylated. The chemical shift anisotropy measurements, although more challenging than the usual isotropic chemical shift experiments, could greatly improve our knowledge of not only cement materials, but silicate hydrates, in general.