Si NMR Study of the Hydration of Ca3SiO5 and ß-Ca2SiO4 in the Presence of Silica Fume

²⁹Si magic-angle spinning nuclear magnetic resonance (MASNMR) was used to study the room-temperature hydration of C₃S, ß-C₂S, and reactive ß-C₂S mixed with different amounts of silica fume (SF) that had been hydrated up to nine months and longer. The overall CaO:SiO₂ molar ratios of the mixes were 0.12, 0.20, 0.35, 0.50, and 0.80. NMR spectroscopy was used to quantify the remaining starting materials and the resulting hydration products of different species. A broad peak assigned to Q³, appearing in both the fourier transform (FT) and the cross-polarization (CP) modes, increased in intensity with increased SF content and with age. This Q³ species was attributed to two sources: (1) the surface hydroxylation of SF and (2) the cross-linking of dreierketten (chains of silicate tetrahedra arranged in a repeating three-unit conformation) in the calcium silicate hydrate (C-S-H) structure. A Q⁴ species also appeared in the CP spectra of samples with large SF additions after extended hydration and was attributed to cross-polarization by adjacent hydroxylated Q³ species at the surface of amorphous SiO₂.