²⁹ Si NMR Spectroscopy of Silicate Anions in Hydrothermally Formed C‐S‐H

Abstract

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 hydro–thermally prepared using CaO and silicic acid at Ca/Si ratios of 0.3 to 2.0 and 120° to 180°C for 2 h. The C‐S‐H was examined mainly using 29 Si 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°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.