ALPHA-DICALCIUM SILICATE HYDRATE - PREPARATION, DECOMPOSED PHASE, ANDITS HYDRATION

Alpha-C2SH can be synthesized by hydrothermal treatment of lime and si licic acid for 2 h at 200-degrees-C. When heated to 390-490-degrees-C, alpha-C2SH dissociates through a two-step process to form an intermed iate phase plus some gamma-C2S. This appears to be a new dicalcium sil icate different from known dicalcium silicates-alpha, alpha(L)', alpha (H)', beta, and gamma phase-and is stable until around 900-degrees-C. At 920-960-degrees-C, all the phases are transformed to the alpha(L)', phase. The intermediate phase has high crystallinity and is stable at room temperature. Si-29 MAS NMR measurements indicate the possibility that it contains both protonated and unprotonated monosilicate anions . The intermediate phase that has passed through the alpha' phase at h igher temperature yields beta-C2S on cooling. The intermediate phase i s highly active, and completed its hydration in 1 day (w/s = 1.0, 25-d egrees-C). Among the crystalline calcium silicate hydrates with Ca/Si = 2.0, it is hillebrandite that yields beta-C2S at the lowest temperat ure.