Effect of calcium-silicon ratio on microstructure and nanostructure of calcium silicate hydrate synthesized by reaction of fumed silica and calcium oxide at room temperature

A C-S-H series with calcium-silicon ratio 0.6-3.0 was synthesized by pozzolanic reaction. Phase composition, nanostructural and morphological characteristics were determined using XRD, XRF, SEM and ²⁹Si NMR. Most of the samples were phase-pure, poorly crystalline C-S-H. Significant changes in the nanostructure of the C-S-H samples were observed when the calcium-silicon ratio reached values of 0.8, 1.0 and 1.5. At calcium-silicon ratio 0.8 the basal XRD peak began to develop, crosslinking between layers was seen below this ratio but not above, and there was a substantial decrease in mean silica chain length at this ratio. At calcium-silicon ratio 1.0 there was a pronounced microstructural change from granular to reticular and another substantial decrease in mean chain length (indicated by an abrupt increase in the Q¹ peak intensity and decrease in the Q² peak intensity). At calcium-silicon ratio 1.5 the basal XRD peak began to diminish again, the mean silica chain length decreased further, and isolated tetrahedra (Q⁰) were observed.