α-Dicalcium silicate hydrate: Preparation, decomposed phase, and its hydration

α-C₂SH can be synthesized by hydrothermal treatment lime and silicic acid for 2 h at 200 °C. When heated to 390-490 °C, α-C₂SH dissociates through a two-step process to form an intermediate phase plus some γ-C₂S. This appears to be a new dicalcium silicate different from known dicalcium silicates - α, α′_L, α′_H, β, and γ phase - and is stable until around 900 °C. At 920-960 °C, all the phases are transformed to the α′_L phase. The intermediate phase has high crystallinity and is stable at room temperature. ²⁹Si MAS NMR measurements indicate the possibility that it contains both protonated and unprotonated monosilicate anions. The intermediate phase that has passed through the α′ phase at higher temperature yields β-C₂S on cooling. The intermediate phase is highly active, and completed its hydration in 1 day (w/s = 1.0, 25 °C). Among the crystalline calcium silicate hydrates with Ca/Si = 2.0, it is hillebrandite that yields β-C₂S at the lowest temperature.