Hydrothermal treatment of lime-silica mixtures under saturated steam pressures below 200°C usually gives C-S-H as an initial product, which reacts further to give crystalline calcium silicate hydrates. In this paper, C-S-H was hydrothermally prepared using CaO and silicic acid at CdSi ratios of 0.3 to 2.0 and 120" to 180°C for 2 h. The C-S-H was examined mainly using 29Si NMR by the magic angle spinning gate proton decoupling and cross polarization magic angle spinning methods. XRD for all of the C-S-H showed bands at 0.304, 0.280, 0.183, and 0.166 nm. NMR results showed that all of the C-S-H contained single chains of silicate anion, which became progressively longer as the CdSi ratio decreased, i.e., as the system became richer in silica. This was independent of the preparation temperature. The 0.8 ratio preparations at 180°C contained small amounts of double-chain structure of 1.1-nm tobermorite. The reaction processing in the lime-silicic acid mixtures is also discussed.
ol-C,SH can be synthesized by hydrothermal treatment of lime and silicic acid for 2 h at 200°C. When heated to 390-490"C, a-C,SH dissociates through a two-step process to form an intermediate phase plus some y-C,S. This appears to be a new dicalcium silicate different from known dicalcium silicate-, a'", p, and y phase-and is stable until around 900°C. At 920-96OoC, all the phases are transformed to the atL 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 a' phase at higher temperature yields p-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 p-C,S at the lowest temperature.
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