²⁹Si 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 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.

Solubility and structure of calcium silicate hydrate

“…In both cases, the mean chain length increases with decreasing Ca/Si ratio, in agreement with previous 29 Si NMR studies [3,4,[16][17][18][19][20]. However, for Ca/Si ratios above 1, there is a small but significant difference between the two preparations, as the mean chain lengths of the samples from the C 3 S group are consistently higher than those from the double decomposition group.…”

“…Indirect evidence for Ca-OH in C-S-H.-As also noted in C-S-H of other studies [3,12,19], the mean silicate chain lengths of samples from the C 3 S and double decomposition groups show relatively little change above Ca/Si ratios of 1.2-1.3. The constancy of the mean chain length has been interpreted by Yu et al [12] as evidence that the predominant cause of variation in Ca/Si ratios above ~ 1.3 is the chargebalancing substitution of OH for entire silicate chain segments; this substitution creates Ca-OH groups (i.e., Ca balanced by OH but not by Si anions), which the authors equated with the formation of jennite or jennite-like environments in C-S-H. Below Ca/Si ratios of ~ 1.3, they considered the dominant structural mechanism for increasing Ca/Si ratio was depolymerization of silicate chains due to the substitution of interlayer calcium (i.e., Ca balanced by Si anions) for bridging tetrahedra and protons on Si-OH.…”

“…In some of the spectra, the Q 2 peak has a shoulder near -83 ppm. This shoulder has been observed by a number of investigators [3,4,19,31,[64][65][66][67], but its assignment is still uncertain. It is generally assumed that the shoulder is produced by a subset of the Q 2 Si, and it is likely to represent at least some of those present as bridging tetrahedra [4,67].…”

“…Single dreierketten have been confirmed by 29 Si NMR and by trimethylsilylation (TMS), the latter showing that the removal of bridging tetrahedra fragments the infinite silicate chains into segments containing 2, 5, 8,…(3n-1) tetrahedra [13][14][15]. The mean chain length, measured by 29 Si NMR, increases with decreasing Ca/Si ratio, particularly below a Ca/Si ratio of 1.1-1.3 [3,4,[16][17][18][19][20].…”

Silicate Anion Structural Change in Calcium Silicate Hydrate Gel on Dissolution of Hydrated Cement

“…In the figure, Q 2 /Q 1 ratio for the synthetic C-S-H reported by Okada et al is also shown. 14) For the C-S-H gel, Q 2 /Q 1 ratio becomes larger as the Ca/Si molar ratio becomes smaller, indicating a longer silicate anion chain. From these results, a correlation between Ca/Si molar ratio and the length of the silicate anion …”