29Si chemical shift anisotropies in calcium silicates from high-field 29Si MAS NMR spectroscopy

Si-29 chemical shift anisotropy (CSA) data have been determined from Si-29 MAS NMR spectra recorded at 14.1 T for a number of synthetic calcium silicates and calcium silicate hydrates. These are beta- and gamma-Ca2SiO4, Ca3SiO4Cl2, alpha-dicalcium silicate hydrate (alpha-Ca-2(SiO3OH)OH), rankinite (Ca3Si2O7), cuspidine (Ca4Si2O7F2), wollastonite (beta-Ca-3- Si3O9), pseudowollastonite (alpha-Ca3Si3O9), scawtite (Ca-7(Si6O18)CO3.2H-2H2O), hillebrandite (Ca2SiO3(OH)(2)), and xonotlite (Ca6Si6O17(OH)(2)). The Si-29 MAS NMR spectra of rankinite and wollastonite clearly resolve manifolds of spinning sidebands from two and three Si sites, respectively, allowing the CSA parameters to be obtained with high precision for each site. For the Si-29 Q(1) sites in rankinite and cuspidine, the CSA asymmetry parameters (eta(sigma) approximate to 0.6) contrast the general expectation that sorosilicates should possess small eta(sigma) values as a result of the nearly axially symmetric environments of the SiO4 tetrahedra. The Si-29 CSA parameters provide an improved insight into the electronic and geometric environments for the SiO4 tetrahedra as compared to the values solely for the isotropic chemical shift. It is shown that the shift anisotropy (delta(sigma)) and the CSA asymmetry parameter (eta(sigma)) allow a clear distinction of the different types of condensation of SiO4 tetrahedra in calcium silicates. This relationship may in general be valid for neso-, soro-, and inosilicates. The CSA data determined in this work may form a valuable basis for Si-29 MAS NMR studies of the structures for tobermorites and calcium silicate hydrate phases resulting from hydration of Portland cements.