CELL DIMENSIONS AND COMPOSITION OF NANOCRYSTALLINE CALCIUM SILICATE HYDRATE SOLID SOLUTIONS. PT.2. X-RAY AND THERMOGRAVIMETRY STUDY

XRD and TGA were used to analyse the limits of incorporation of Ca in a series of mechanochemically synthesised, nanocrystalline calcium silicate hydrate (C-S-H) phases. Results based on bulk weight loss and Rietveld refinements showed higher C/S ratios than those corrected additionally for X-ray-silent CaCO3 and Ca(OH)2. A pure C-S-H phase existed 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 be described by the so-called defect-tobermorite model. At C/S = 2/3, the C-S-H consisted of 14 Angstrom 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, XRD showed that even samples with a low Ca content (Ca/Si < 2/3) contained only the calcium silicate wollastonite. This supports the idea of a slightly defective, unbranched single chain silicate anion. Increasing the C/S ratio led 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 exhibited 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 supported the proposed model, relating the observed shrinkage with the loss of H2O molecules from the interlayer, where they coordinate calcium.