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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