Structural features of C-S-H(I) and its carbonation in air - A Raman spectroscopic study. Part I: Fresh phases

The Raman spectra of a series of mechanochemically prepared calcium silicate hydrate samples of type C-S-H(I) with C/S ratios ranging from 0.2 to 1.5 reveal changes in structure with changes in the C/S ratio. These support the model of Stade and Wieker based entirely on the tobermorite structure. The main characteristic feature of the spectra is the Si-O-Si bending vibration at about 670 cm(-1). Comparisons with bending frequencies of some known crystalline phases composed of single silicate chains led to an estimation of the mean Si-O-Si angles in the C-S-H(I) phases to be similar to 140 degrees. Finite silicate chains (Q(2)) dominate the structures of the samples at C/S ratios 0.2-1.0, the spectra showing characteristic bands from 1010 to 1020 cm(-1). When the samples are measured in air, the spectra exhibit carbonate bands arising from surface carbonation. The nu(1)[CO3] bands obscure the characteristic Raman scattering of silicate units near 1080 cm(-1), which is clearly evident in the fresh samples analyzed in closed capillaries. At C/S > 1.00, dimers (Q(1)) are the main building unit of the silicate anionic structure, with a characteristic band at 889 cm(-1). At C/S ratios 1.33 and 1.50, portlandite (Ca(OH)(2)) is also observed.