Simulated annealing structure solution of a new phase of dicalcium silicate Ca2SiO4 and the mechanism of structural changes from α-dicalcium silicate hydrate to αL′-dicalcium silicate via the new phase

Abstract

A new phase of dicalcium silicate (Ca2SiO4) was formed by heating α-dicalcium silicate hydrate [α-Ca2(SiO4H)OH = α-C2SH] at temperatures of ∼663–763 K, and it was transformed into \alpha _{\rm L} ^\prime-Ca2SiO4 (= \alpha _{\rm L} ^\prime-C2S) above ∼1193 K. The crystal structure of the new phase (hereafter called x-C2S) has been determined by simulated annealing and refined by the Rietveld method using synchrotron radiation powder diffraction data. The structure consists of isolated SiO4 tetrahedra and a three-dimensional CaO n polyhedral network, forming a new structural type of dicalcium silicate. A structural change from α-C2SH to x-C2S is compelled by large displacements of SiO4 tetrahedra, accompanied by dehydration, in the direction perpendicular to the two-dimensional Ca(O,OH) n polyhedral network in α-C2SH. With increasing temperature, sizes of CaO n polyhedra in x-C2S become too large to confine Ca atoms at the sixfold to eightfold coordination sites. Then the structure of x-C2S is transformed into \alpha _{\rm L} ^\prime-C2S, having eightfold to tenfold coordination sites for the Ca atoms.