Low-Temperature Synthesis of Portlandite for Carbon Dioxide Mineralization

The incorporation of CO₂ within stable metal carbonates such as calcium and magnesium carbonates is one strategy for the mitigation of atmospheric CO₂ concentrations. The precipitation of CaCO₃ (e.g., calcite) from a Ca-rich solution requires the consumption of alkalinity and is favored by high Ca concentrations and high pH. The alkaline Ca salt, portlandite (Ca(OH)₂) is a potent CO₂ sorbent that can uptake 0.59 g CO₂ per g Ca(OH)₂ through the net reaction: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. However, current means of portlandite production entails a high energy consumption and high CO₂ emissions from the calcination of limestone (primarily CaCO₃) at elevated temperatures of about 800 °C to form lime (CaO), which is subsequently reacted with water to produce Ca(OH)₂. In this study, we develop a low-temperature process for portlandite synthesis which does not involve calcination and the associated release of CO₂. The precipitation of portlandite at sub-boiling temperatures is induced through the steps of: Ca extraction, Ca concentration enhancement, and portlandite crystallization via temperature ramping. A Ca-rich solution obtained by dissolving crystalline slag in deionized water is filtered through a membrane which selects for the divalent Ca²⁺ ions to increase the solution Ca concentration, thus reaching portlandite saturation. Upon reaching equilibrium with respect to portlandite at 25 °C, the solution is heated to temperatures of up to about 70 °C. Because the solubility product of portlandite decreases with increasing temperature, the temperature increase causes the precipitation of the phase out of the solution. Solid portlandite yields corresponding to the theoretical value were obtained. Moreover, the total energy consumption and CO₂ footprint of the process were estimated and compared with those for the traditional production method, revealing that the presented alternative route for the synthesis of portlandite features a potentially smaller environmental impact.