Order and disorder in the lamellar hydrated tetracalcium monocarboaluminate compound

A single crystal X-ray diffraction study was performed on the cementitious phase 3CaO . Al2O3. CaCO3. 11H(2)O (C(4)A (C) over bar H-11 using cement ch emistry shorthand) prepared by hydrothermal synthesis. This layered compoun d crystallizes with pseudohexagonal symmetry (centrosymmetric space group P (1) over bar) with parameters a = 5.7422(4) Angstrom, b = 5.7444(4), c = 1 5.091(3) Angstrom, alpha = 92.29(1)degrees, beta = 87.45(1)degrees, gamma = 119.55(1), Dx = 2.182 g/cm(3), and F(000) = 294. Refinement of 1444 indepe ndent reflections led to a residual factor R of 0.0468. The structure is di sordered; the carbonate group and three of the five water molecules in the interlayer are distributed statistically among general positions. Where the carbonate group is absent, its O-atoms are replaced by three water molecul es. The structure is built by stacking of slabs parallel to (001) in the se quence 1/2(CO32-. 6H(2)O)-[Ca4Al2(OH)(12)](2+)-1/2(CO32-. 6H(2)O)-2H(2)O-[C a4Al2(OH)(12)](2+)-2H(2)O. Hydrogen atoms of the three disordered water mol ecules are not localized. This structure is the second modification found f or this AFm phase. The first, completely ordered in the noncentrosymmetric space group P1, was described previously. The two modifications, the ordere d called "O-C(4)A (C) over bar H-11" and the disordered called "D-C(4)A (C) over bar H-11", are compared. (C) 1999 Elsevier Science Ltd. All rights re served.