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.

中文翻译：

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硅酸钙晶体结构影响与CO ₂和沉淀化学反应

CO _2（aq）与硅酸钙的反应产生沉淀，这些沉淀会影响地下应用（例如地质CO ₂储存和地热能）中的流体流动。这些反应通常产生碳酸钙（CaCO ₃）和无定形二氧化硅（SiO _x）。在这里我们报告的证据表明，母体硅酸盐的晶体结构决定了它与CO ₂反应的方式以及反应产物的结构。使用模型硅酸钙（CaSiO ₃），硅灰石（链结构）和假硅灰石（环结构）的两种多晶型物，在升高的温度（150°C）和CO ₂分压下进行了批量实验。（0-11 MPa）。CO _2（aq）与硅灰石的反应生成CaCO ₃和SiO _x，而CO _2（aq）与假硅灰石的反应生成板状结晶硅酸钙相以及CaCO ₃和SiO _x。提出了一种反应机理，该机理解释了与母体硅酸盐的溶解，溶液的pH和成核位点有关的观察结果。该机制由电感耦合等离子体发射光谱测量和固体产品的扫描电子显微镜/透射电子显微镜选择区域电子衍射表征支持。这些发现很重要，其原因有很多，其中包括在低pH条件下结晶硅酸盐沉淀比CaCO ₃稳定的事实，这对于在地下应用中形成永久密封非常有价值。