Growth and Transformation of Hydrated Magnesium Carbonates under Near-Ambient Conditions

Concrete composed of magnesium carbonates not only exhibits the potential for greater strength but also offers reduced carbon dioxide emissions compared with conventional concrete made with ordinary Portland cement. In a series of experiments conducted at various saturation ratios and near-ambient temperatures, hydrated magnesium carbonate phases were precipitated and subsequently analyzed by using a range of spectroscopic techniques. Hydrated magnesium carbonates, including nesquehonite (MgCO3<middle dot>3H2O) and hydromagnesite (Mg5(CO3)4(OH)2<middle dot>4H2O), formed readily from the growth solutions. Time-resolved analysis using atomic force microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, and Raman spectroscopy revealed a correlation between the degree of solution supersaturation with respect to hydromagnesite and the delay in the transition from early-stage nesquehonite to hydromagnesite, suggesting that the increased concentration of magnesium cations impeded phase evolution. Furthermore, the introduction of the additives RbCl and CsCl accelerated this transformation. These observations can be explained by considering the influences of the ions in solution on the magnesium ion's dehydration energy. These findings are significant because they demonstrate a pathway for phase selection during magnesium carbonate precipitation at near-ambient temperatures. The results of this study have implications for carbon dioxide mineralization and the design of concrete that gains strength through the precipitation of magnesium carbonates.