The crystal structure of tobermorite 14 A (Plombierite), a C-S-H phase

The crystal structure of tobermorite 14 angstrom (plombierite) was solved by means of the application of the order-disorder (OD) theory and was refined through synchrotron radiation diffraction data. Two polytypes were detected within one very small crystal from Crestmore, together with possibly disordered sequences of layers, giving diffuse streaks along c*. Only one of the two polytypes could be refined: it has B11b space group symmetry and bell parameters a = 6.735(2) angstrom, b = 7.425(2) angstrom, c = 27.987(5) angstrom, gamma = 123.25(1)degrees. The refinement converged to R = 0.152 for 1291 reflections with F-o > 4 sigma(F-o). The characteristic reflections of the other polytype, f2dd space group, a approximate to 11.2, b approximate to 7.3 angstrom, c approximate to 56 angstrom, were recognized but they were too weak and diffuse to be used in a structure refinement. The structure of tobermorite 14, is built up of complex layers, formed by sheets of sevenfold coordinated calcium cations, flanked on both sides by wollastonite-like chains. The space between two complex layers contains additional calcium cations and H2O molecules; their distribution, as well as the system of hydrogen bonds, are presented and discussed. The crystal chemical formula indicated by the structural results is Ca5Si6O16(OH)(2) (.) 7H(2)O.