A new phase of
dicalcium silicate (Ca(2)SiO(4)) was formed by heating alpha-
dicalcium silicate hydrate [alpha-Ca(2)(SiO(4)H)
OH = alpha-C(2)SH] at temperatures of approximately 663-763 K, and it was transformed into alpha(L)'-Ca(2)SiO(4) (= alpha(L)'-C(2)S) above approximately 1193 K. The crystal structure of the new phase (hereafter called x-C(2)S) has been determined by simulated annealing and refined by the Rietveld method using
synchrotron radiation
powder diffraction data. The structure consists of isolated SiO(4) tetrahedra and a three-dimensional CaO(n) polyhedral network, forming a new structural type of
dicalcium silicate. A structural change from alpha-C(2)SH to x-C(2)S is compelled by large displacements of SiO(4) tetrahedra, accompanied by
dehydration, in the direction perpendicular to the two-dimensional Ca(O,
OH)(n) polyhedral network in alpha-C(2)SH. With increasing temperature, sizes of CaO(n) polyhedra in x-C(2)S become too large to confine Ca atoms at the sixfold to eightfold coordination sites. Then the structure of x-C(2)S is transformed into alpha(L)'-C(2)S, having eightfold to tenfold coordination sites for the Ca atoms.