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The tetracalcium dialuminum hydroxide carbonate pentahydrate Ca4Al2(OH)12CO3.5H2O is a layered compound constituted by positively charged Ca4Al2(OH)122+ main layers and negatively charged CO3, 5H2O2− interlayers. The Al3+ and Ca2+ ions are six- and sevenfold coordinated by O atoms, respectively. Water molecules occupy the seventh coordination site of three Ca ions among the four contained in the main layer. Thus, among the five water molecules contained in the interlayer, two of them can be considered as slightly bonded. One O atom of the carbonate group occupies the seventh coordination site of the remaining Ca ion. Moreover, every O atom of the carbonate groups contributes to the formation of relatively strong hydrogen bonds with water molecules, providing cohesion of the interlayer. The planar CO32− groups are tilted by 21.8 (9)° from the planes formed by the layers.
François et al. (Tue,) studied this question.