Si-29 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 Si-29 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 beta-C2S and aqueous reaction of fumed silica with CaO and separately with highly reactive beta-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: Ca-4.5[Si3O8(OH)](OH)(4) . nH(2)O. 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.