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

Synchrotron‐based X‐ray diffraction has been used to analyze a series of mechanochemically prepared calcium silicate hydrate (C–S–H) phases with aimed Ca/Si ratios from 1/5 to 3/2. Fumed silica and CaO were used as starting materials. All samples contain 3‐dimensionally ordered C–S–H phases. Pure C–S–H phases are present in samples with Ca/Si ratios from 2/3 to 6/5. The samples with C/S ratios 1/5 and 1/2 contain unreacted silica, while those with C/S ratios 4/3 and 3/2 contain portlandite as minor component. A new approach has been used to follow structural changes with C/S ratio, involving assignment of an orthorhombic space group (I2mm) to the C–S–H phase followed by refinement of the unit cell parameters by the whole powder pattern decomposition (WPPD) method. The results reveal a discontinuity in the c parameter at C/S=5/6–1/1, indicating that at least two different structural types separated by a miscibility gap are needed to describe C–S–H, there being two ordered end members with C/S ratios of 2/3 and 5/4, respectively. Nevertheless the structure of C–S–H phases within this interval may be well described by the defect‐tobermorite model. At C/S=2/3 it consists of tobermorite slabs linked via H‐bonds without interlayer Ca. At this C/S ratio, the layer thickness is 13.2 Å. Increasing the C/S ratio leads to little change in the layer thickness, but increased disorder due to competitive omission of bridging tetrahedra and incorporation of Ca in the interlayer in samples with 2/3

X‐ray diffraction and thermogravimetry have been used to analyze the limits of incorporation of Ca in a series of mechanochemically synthesized, nanocrystalline calcium silicate hydrate (C–S–H) phases. Results based on bulk weight loss and Rietveld refinements show higher C/S ratios than those corrected additionally for X‐ray‐silent CaCO3 and Ca(OH)2. A pure C–S–H phase exists over the C/S range 2/3–5/4, with two ordered end members. The structure of C–S–H phases within the interval 2/3–5/4 may well be described by the so‐called defect‐tobermorite model. At C/S=2/3, the C–S–H consists of 14 Å tobermorite slabs linked via H‐bonds without interlayer Ca, resulting in the formula Ca4[H2Si3O9]2·xH2O, where x=4. After heating up to 1000°C, X‐ray diffraction has shown that even samples with a low Ca content (Ca/Si <2/3) contain solely the calcium silicate wollastonite. This supports the idea of a slightly defective, unbranched single chain silicate anion. Increasing the C/S ratio leads to increased disorder due to the competitive omission of bridging tetrahedra and the incorporation of Ca into the interlayer in samples with 2/3 < C/S <5/4. The Ca‐rich end member with C/S=5/4 exhibits structural features of a tobermorite‐based dimer: {Ca4[HSi2O7]2} . Ca . xH2O, where x=4. The observed change in the d‐value of the basal reflection upon X‐ray irradiation further supports the proposed model, relating the observed shrinkage with the loss of H2O molecules from the interlayer, where they coordinate calcium.

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