The Crystal Structure of Tobermorite 14 Å (Plombierite), a C–S–H Phase

The crystal structure of tobermorite 14 Å (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 B 11 b space group symmetry and cell parameters a =6.735(2) Å, b =7.425(2) Å, c =27.987(5) Å, γ=123.25(1)°. The refinement converged to R =0.152 for 1291 reflections with F o >4σ( F o ). The characteristic reflections of the other polytype, F 2 dd space group, a ≈11.2 Å, b ≈7.3 Å, c ≈56 Å, were recognized but they were too weak and diffuse to be used in a structure refinement. The structure of tobermorite 14 Å 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 H 2 O 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 Ca 5 Si 6 O 16 (OH) 2 ·7H 2 O.