Stability and phase relations of dicalcium silicate hydrates under hydrothermal conditions

In the system of CaO-SiO₂-H₂O, there are three crystalline compounds with calcium/silicon ratio of 2.0, α-dicalcium silicate hydrate (α-C₂SH), hillebrandite, and dellaite. In this paper, these three compounds were synthesized and their stability and phase transformation were investigated under hydrothermal conditions up to 400°C. Hydrothermal treatments of α-C₂SH indicated that Ca-rich jaffeite (C/S = 3.0) and reinhardbraunsite (C/S =2.5) were formed as intermediate phases before complete transformation to dellaite at 350°C for 1 d. The phase transformation from α-C₂SH to hillebrandite was not observed. On the other hand, a reversible phase transformation from dellaite to hillebrandite was observed at low temperatures below 300°C with formation of a small amount of the C-S-H compounds with C/S ratio of 1.5. The fact that α-C₂SH was not formed form dellaite suggests that α-C₂SH is a metastable phase.