The carbonation of γ-Ca
2SiO
4 (γ-C
2S) and mechanism of vaterite formation have been investigated by evaluating the crystal structures of both γ-C
2S and vaterite. The samples used were autoclaved calcium silicate hydrate hardening bodies prepared from Ordinary Portland Cement (OPC), γ-C
2S and α-quartz and then subjected to accelerated carbonation. The ratio of OPC to γ-C
2S was varied. Ca
2+ in both γ-C
2S and vaterite was found to be coordinated to six O
2-. In addition, both γ-C
2S and vaterite have similar atomic arrangements of O
2- and Ca
2+ and Ca-O bond distances. Therefore, it is proposed that vaterite mainly forms from γ-C
2S via a topotactic reaction during accelerated carbonation.
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