29Si MAS NMR study of the structure of calcium silicate hydrate

This paper presents the results of a comprehensive investigation of single-phase calcium silicate hydrate (CSH) with known compositions using the combined capabilities of 29Si magic angle spinning (MAS) NMR, powder X-ray diffraction (XRD), and chemical analysis of the solution and solid. CSH gels with C/S ratios ranging from 0.4 to 1.85 have been synthesized by hydration of highly reactive β-C2S and aqueous reaction of fumed silica with CaO and separately with highly reactive β-C2S. The main findings include the following. (1) CSH shows continuity and diversity in both composition and structure and forms a continuous structural series. (2) Phase-pure CSH has C/S ratios between 0.6–1.54. (3) SiOH and CaOH bonds both occur in CSH, with the abundance of the former decreasing and that of the latter increasing with increasing C/S ratio. Model calculation indicates that there are between 0.13–0.43 SiOH bonds per tetrahedron and the CaOH/Ca ratio varies between nearly 0 and 0.64. An ideal formula for CSH with a C/S ratio of 1.5 is: Ca4.5[Si3O8(OH)](OH)4 · nH2O. A defect-tobermorite structural model is proposed for CSH in which individual layers have the basic structure of 1.4-nm tobermorite but contain a significant concentration of defects and are more disordered. Stacking disorder between adjacent layer and disorder within individual layers may both contribute to the local and long-range disorder and thus to the diversity of CSH.