Cell Dimensions and Composition of Nanocrystalline Calcium Silicate Hydrate Solid Solutions. Part 1: Synchrotron-Based X-Ray Diffraction

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 A˚ . 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<C/S<1/1. For C/S ratios above 1/1, there is an increased structural ordering accompanied by a reduction in layer thickness, which reaches a theoretical minimum at C/S=5/4. This end member has structural features of a tobermorite-based dimer: {Ca4H2[Si2O7]2}.Ca.mH2O with a layer thickness of 11.7 A˚ . Its powder pattern fits the B11b space group. Indexing according I2mm sub cell and B11b true cell are also presented.