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

C-S-II 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.