Interaction between dissolved silica and calcium carbonate: 1. Spontaneous precipitation of calcium carbonate in the presence of dissolved silica

The kinetics of spontaneous precipitation of CaCO3 from aqueous solution in the presence of dissolved silica was investigated by recording pH as a function of time. The presence of dissolved silica, at concentrations below saturation with respect to the amorphous phase, decreases induction time for CaCO3 nucleation, but does not affect CaCO3 polymorphism. For a “pure” system without silica, the surface free energy, σ, determined from classical nucleation theory is 42 mJ m−2. This agrees well with values reported in the literature for vaterite and indicates some degree of heterogeneous nucleation, which can occur because of the relatively low degree of supersaturation used for the experiments. In the presence of 1 and 2 mM silica, σ is 37 and 34 mJ m−2, indicating an increasing degree of heterogeneous nucleation as the amount of polymeric silica increases. The ratio of Ca2+ to CO32− activity was a governing parameter for determining which CaCO3 polymorph precipitated. At high Ca2+ to CO32− activity ratios, almost all initial solid was vaterite, whereas at low ratios, a mixture of vaterite and calcite was observed. In solutions with low Ca2+ to CO32− activity ratios, the presence of silica at concentrations above saturation with respect to amorphous silica led to formation of only calcite and strongly influenced the crystalline structure and morphology of the precipitates. At high Ca2+ to CO32− ratios, system behaviour did not differ from that without silica.