Effects of pH on precipitation of quasi-crystalline calcium silicate hydrate in aqueous solution

Abstract

C-S-H samples were precipitated from aqueous solution containing Ca(NO3)2 and Na2SiO3 with an initial Ca/Si molar ratio of 1·3, when the pH value in the solution was varied by adding HNO3 or NaOH. A pH > 10·9 was required to obtain quasi-crystalline C-S-H, and the CaO/SiO2 ratio of the precipitate was increased with increasing pH, reaching a maximum close to the initial ratio at pH > 13. C-S-H was also precipitated with varying initial Ca/Si molar ratios at pH > 13. It was shown that the CaO/SiO2 ratios can be controlled by either varying the pH value at given initial Ca/Si ratios, or by varying the initial Ca/Si ratios at pH > 13. The C-S-H samples were investigated using X-ray diffraction and 29Si nuclear magnetic resonance spectroscopy, which identifies the connectivity of silica tetrahedra (Qx). The interlayer spacing decreased linearly, and Q1/(Q1 + Q2 + Q3) ratios increased with increasing CaO/SiO2 ratios. A good correlation between Q1/(Q1 + Q2 + Q3) ratios and interlayer spacing was observed.

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.…”

“…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].…”

Composition, silicate anion structure and morphology of calcium silicate hydrates (C-S-H) synthesised by silica-lime reaction and by controlled hydration of tricalcium silicate (C3S)

“…29 Si magic angle spinning nuclear magnetic resonance (MAS NMR) and Raman spectroscopic studies of synthetic C-S-H with varying Ca/Si ratios have shown that samples with low Ca/Si ratios have, on average, long silicate chains and the silicate structure is dominated by Q 2 tetrahedra, while samples with high Ca/Si have, on average, shorter chains and their silicate structures are predominantly formed by Q 1 tetrahedra. 8,9,[16][17][18][19][20][21] The percentage of Q 1 becomes greater than the percentage of Q 2 when 1vCa/Siv1.2. 8,[16][17][18] A recent model for C-(A)-S-H(I) 7 has shown constraints for Ca/Si and the average silicate chain length in terms of the fraction of vacant tetra-hedral sites.…”

Spontaneous precipitation of calcium silicate hydrate in aqueous solutions

“…It has been reported that below saturation with respect to Ca(OH) 2 and up to 2 mM calcium concentration levels the Ca:Si ratio is between 0.85-1.1 [9]. Moreover, since calcium addition in the Ca(NO 3 ) 2 -Na 2 SiO 3 -H 2 O system, often used for the investigation of the precipitation of C-S-H [10,11] causes changes in the solution pH, it is very interesting to examine the effect of this parameter on the kinetics of C-S-H formation in aqueous solutions.…”