Calcium carbonate (CaCO₃) has six different phases whose formation is affected strongly by precipitation conditions. Among them, pH is considered to be one of dominating factors controlling polymorphism during calcium carbonate precipitation. Furthermore, the Ca/CO₃ ratio in solution was recently shown to strongly affect CaCO₃ growth rate (e.g. Nehrke et al., 2007). However, the change of solution pH is at the same time associated with a change in hydrogen ion concentration (H⁺) and the Ca/CO₃ ratio in solution, making it difficult to separate pH and solution stoichiometry effects. The objective of this study is to investigate whether the H⁺ or Ca/CO₃ ratio has a control on calcium carbonate polymorphism.

Experiments were carried out at 0^oC and two different pH conditions (pH = 12.6 and 9.0). Under each pH condition, different concentrations of CaCl₂ and NaHCO₃ were applied to achieve ratios of 1:1 and 10:1 for Ca/CO₃. CaCl₂ and NaHCO₃ solutions were pumped into different pH solutions with or without phosphate at different pumping rates. Results showed that, at high pH (pH = 12.6), only ikaite formed, independent of PO₄, pumping rate or the ratio of Ca/CO₃. At low pH (pH = 9.0), the precipitate was vaterite in the absence of PO₄ and ikiate in the presence of PO₄ regardless of the ratio of Ca/CO₃. These results indicate that at low temperature, the H⁺ concentration determined which polymorph was precipitated (ikaite or vaterite) rather than Ca/CO₃ ratio.

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1. The effect of pH on calcium carbonate polymorphs precipitated from aqueous solutions at 0^oC