SI-29 NMR-SPECTROSCOPY OF SILICATE ANIONS IN HYDROTHERMALLY FORMED C-S-H

Hydrothermal treatment of lime-silica mixtures under saturated steam pressures below 200-degrees-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 Ca/Si ratios of 0.3 to 2.0 and 120-degrees to 180-degrees-C for 2 h. The C-S-H was examined mainly using Si-29 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 Ca/Si 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-degrees-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.