Cell Dimensions and Composition of Nanocrystalline Calcium Silicate Hydrate Solid Solutions. Part 2: X‐Ray and Thermogravimetry Study

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 CaCO 3 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 Ca 4 [H 2 Si 3 O 9 ] 2 · x H 2 O, 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: {Ca 4 [HSi 2 O 7 ] 2 } . Ca . x H 2 O, 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 H 2 O molecules from the interlayer, where they coordinate calcium.