CRYSTAL-STRUCTURE OF HILLEBRANDITE - A NATURAL ANALOG OF CALCIUM SILICATE HYDRATE (CSH) PHASES IN PORTLAND-CEMENT

The crystal structure of hillebrandite, Ca2SiO3(OH)(2), was solved and refined in space group Cmc2(1), a = 3.6389, b = 16.311, c = 11.829 An gstrom, to R = 0.041 using single-crystal X-ray data. The structure co nsists of a three-dimensional network of Ca-O polyhedra that accommoda tes wollastonite-type Si-O tetrahedral chains. Each of the wollastonit e-type chains is an average of two symmetrically equivalent chains rel ated by the mirror plane perpendicular to a. In a given structural cha nnel of the Ca-O polyhedral network, only one chain orientation can be occupied to give reasonable Si-O distances. The O3 and O4 sites corre sponding to each vacant Si2 site are occupied by OH groups to achieve charge balance. The wollastonite-type Si-O tetrahedral chains in the h illebrandite structure resemble those reported for many calcium silica te hydrate (CSH) phases.