The effects of carbonation of mechanochemically prepared C-S-H samples under ambient conditions for upto 6 months have been investigated by Raman spectroscopy and X-ray diffraction. The type and extent of carbonation are strongly dependent on the initial CaO/SiO 2 (C/S) ratio of the samples. Amorphous calcium carbonate hydrate is formed within minutes upon exposure to air. It crystallizes, over time, to give primarily vaterite at C/S ! 0.67 and aragonite at C/Sr0.50. Calcite was not observed as a primary carbonation product within the time frame investigated. Decalcification upon storage also initiates silicate polymerization. The dimeric silicate units seen in the calcium-rich phases polymerize rapidly to yield Q 2 silicate moieties. After 6 months, broad bands are seen in most spectra, ascribed to poorly ordered silica. C-S-H phases with C/S ratios of 0.75 and 0.67 are the most resistant to carbonation, and even after 6 months of storage, Q 2 silicate units still dominate their structures. The ability of Raman spectroscopy to probe the short-range order of poorly crystalline materials is ideal for investigations of C-S-H structure. Additionally, the technique's sensitivity toward the various calcium carbonate polymorphs illuminates the sequence of carbonation and decalcification processes during aging of C-S-H. Of particular importance is the identification of amorphous calcium carbonate as the first carbonation product. Additionally, the formation of aragonite as a carbonation product is related to the presence of SiO 2 gel in the aged samples.G. Scherer-contributing editor

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 À1 . 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 B1401. 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 À1 . When the samples are measured in air, the spectra exhibit carbonate bands arising from surface carbonation. The n 1 [CO 3 ] bands obscure the characteristic Raman scattering of silicate units near 1080 cm À1 , which is clearly evident in the fresh samples analyzed in closed capillaries. At C/S41.00, dimers (Q 1 ) are the main building unit of the silicate anionic structure, with a characteristic band at 889 cm À1 . At C/S ratios 1.33 and 1.50, portlandite (Ca(OH) 2 ) is also observed.G. Scherer-contributing editor

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