Although the thermodynamics of the MgO–CO2–H2O system is well-known, the kinetics of the dissolution–precipitation reactions that characterize the system is less clear. Magnesium-based cements including carbonates and hydrated silicates are observed in weathering products in nature. Under ambient to slightly elevated temperatures, various magnesium carbonate phases have been shown to occur with distinct stoichiometries and morphologies. Although magnesite is the most thermodynamically stable phase, it does not readily form at modest temperatures because of the slow water exchange around the strongly hydrated Mg2+ ions. Instead, one of the metastable hydrated phases, including nesquehonite (MgCO3●3H2O), hydromagnesite (Mg5(CO3)4(OH)2●4H2O), and dypingite (Mg5(CO3)4(OH)2●5H2O) have typically been observed to form. In addition, competition for Mg2+ ions by brucite (Mg(OH)2) may also occur. In this study, we used a fast-scanning environmental atomic force microscope equipped with a sealed fluid cell and a cooler-heater stage for real-time control of temperature to determine the various phases that grow on single crystal MgO surfaces upon exposure to carbonate-containing solutions at temperatures between 25 and 75 °C. Chemical characterization of the precipitates was performed using infrared spectroscopy. Dissolution–precipitation experiments revealed the influences of the solution composition, temperature, and reaction time on the phase evolution of overgrowths on MgO, which were identified as either brucite, nesquehonite, or hydromagnesite. These findings provide insights into the kinetics of carbonation of Mg-bearing oxides and silicates for the permanent sequestration of carbon dioxide.

Phase selection during hydration and carbonation of MgO surfaces

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