Acta Crystallographica Section B

Structural Science

Volume 68, Part 5 (October 2012)

research papers

Acta Cryst. (2012). B68, 493-500 [ doi:10.1107/S010876811203042X ]

Crystal structures of calcium hemicarboaluminate and carbonated calcium hemicarboaluminate from synchrotron powder diffraction data

T. Runcevski, R. E. Dinnebier, O. V. Magdysyuk and H. Pöllmann

Abstract: One of the main phases formed at the beginning of the carbonation reaction of cementitious building materials is the calcium hemicarboaluminate (abbreviated as Hc). This AFm (shorthand for hydrated calcium aluminate phases structurally related to hydrocalumite) phase was synthesized, crystallized and then studied by synchrotron X-ray powder diffraction and micro-Raman spectroscopy. At room temperature and standard experimental conditions two major cementitious phases were detected, the Hc phase (as a major phase) and carbonated calcium hemicarboaluminate (abbreviated as cHc). By increasing the temperature the Hc form transforms into cHc. The crystal structures of these important AFm phases were successfully solved and refined in the $R\bar 3c$ space group of the trigonal crystal system. Hc has the unit-cell parameters a = 5.7757 (1) and c = 48.812 (2) Å, and cHc the unit-cell parameters a = 5.7534 (1) and c = 46.389 (1) Å. The two crystal structures are composed of positively charged main layers, [Ca₄Al₂(OH)₁₂]²⁺, and negatively charged interlayers, [OH_2n(CO₃)_{1 - n}·4H₂O]^2-. The structure of the main layers is typical of the AFm family. Conversely, the interlayer region has a characteristic structure built up from water molecules and statistically distributed anions. In the interlayer, the Hc carbonate and hydroxyl anions are distributed in a 0.25:0.5 ratio, whereas the ratio of the anions in the cHc interlayers is 0.4:0.2.

Keywords: synchrotron powder diffraction; cementitious building material; micro-Raman spectroscopy.


	Structure factor file (CIF format) (19.4 kbytes) [ doi:10.1107/S010876811203042X/eb5017Hcsup2.hkl ] Contains datablock Hc

	Structure factor file (CIF format) (18.6 kbytes) [ doi:10.1107/S010876811203042X/eb5017cHcsup3.hkl ] Contains datablock cHc

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