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

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 (Q^x). 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.