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 ∼140°. 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 ν 1 [CO 3 ] 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.