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Structure refinements have been carried out on the pure tricalcium silicate (C 3 S) and the hydrated C 3 S with the D 2 O-C 3 S mass ratio, which is 0.5, using neutron powder diffraction (NPD). The multi-phase Rietveld analysis of the hydrated C 3 S revealed the coexistence with the Ca(OD) 2 (trigonal phase) and the unhydrated C 3 S (triclinic one). The Ca(OD) 2 phase was hardly observed on the NPD patterns in the first ,6 h of hydration, while the several Bragg reflections of Ca(OD) 2 appeared drastically from ,6 to ,24 h, and then the hydration reaction rate was gradually suppressed. We could apply the Avrami-model to the initial hydration reaction process of C 3 S.