A CEMENTITIOUS COMPOUND WITH COMPOSITION 3CAO.AL2O3.CACO3.11H(2)O

The tetracalcium dialuminium hydroxide carbonate pentahydrate CA(4)Al( 2)(OH)(12)CO3.5H(2)O is a layered compound constituted by positively c harged [Ca4Al2(OH)(12)](2+) main layers and negatively charged [CO3. 5 H(2)O](2-) interlayers. The Al3+ and Ca2+ ions are six- and sevenfold coordinated by O atoms, respectively. A water molecule occupies the se venth coordination site of each of three Ca2+ ions among the four cont ained in the main layer. Thus, among the five water molecules containe d in the interlayer, two of them can be considered as only slightly bo nded. One O atom of the carbonate group occupies the seventh coordinat ion site of the remaining Ca2+ ion. Moreover, every O atom of the carb onate groups contributes to the formation of relatively strong hydroge n bonds with water molecules, providing cohesion of the interlayer. Th e planar CO32- groups are tilted by 21.8 (9)degrees from the planes fo rmed by the layers.