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 angstrom. 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. 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.mH(2)O with a layer thickness of 11.7 angstrom. Its powder pattern fits the B11b space group. Indexing according I2mm sub cell and B11b true cell are also presented.