First-principles study was carried out to clarify theoretically an effect of the substitution of Sr atoms for Ca atoms in CaO polyhedra on the change in crystal structure and their
hydraulic activity of beta-form Belite, ?-C2S. The large supercell sizing up to 504 atoms was employed in this work. Two Sr atoms-doped ?-C2S forming edge to edge bond between Sr(1)O polyhedron with 7 Sr-O bonds and Sr(2)O polyhedron with 8 Sr-O bonds lies in most unstable state in total energy of the crystal. The replacement of Ca(1) atom having 7 Ca-O bonds with Sr atom contributes to shorten Ca-Ca bond length of the Belite crystal compared with the replacement of Ca(2) atom having 8 Ca-O bonds with Sr atom. This study reveals that the Ca-Ca bond length based on first-principles calculation becomes a yardstick to predict the hydraulic activity of ?-C2S.