Calcium Silicate Crystal Structure Impacts Reactivity with CO₂ and Precipitate Chemistry

The reaction of CO_2(aq) with calcium silicates creates precipitates that can impact fluid flow in subsurface applications such as geologic CO₂ storage and geothermal energy. These reactions nominally produce calcium carbonate (CaCO₃) and amorphous silica (SiO_x). Here we report evidence that the crystal structure of the parent silicate determines the way in which it reacts with CO₂ and the resulting structures of the reaction products. Batch experiments were performed using two polymorphs of a model calcium silicate (CaSiO₃), wollastonite (chain-structured) and pseudowollastonite (ring-structured), at elevated temperatures (150 °C) and partial pressures of CO₂ (0-11 MPa). Reaction of CO_2(aq) with wollastonite produced CaCO₃ and SiO_x, whereas reaction of CO_2(aq) with pseudowollastonite produced platelike crystalline calcium silicate phases, along with CaCO₃ and SiO_x. A reaction mechanism that explains the observations in relation to dissolution of the parent silicate, the pH of the solution, and the presence of nucleation sites is proposed. The mechanism is supported with inductively coupled plasma optical emission spectrometry measurements and scanning electron microscopy/transmission electron microscopy-selected-area electron diffraction characterization of solid products. These findings are important for a number of reasons, among them, the fact that the crystalline silicate precipitates are more stable than CaCO₃ under low-pH conditions, which could be valuable for creating permanent seals in subsurface applications.