Dong Youn Chung

Amorphous magnesium carbonate (AMC) is recognized as a precursor material during the formation of crystalline magnesium carbonates, which are important materials, e.g., for next generation cements and CO2 sorbents, but its amorphous-to-crystalline transition is poorly understood. In this study, we combined in situ X-ray and neutron diffraction and total scattering to determine the activation energy of the amorphous-to-crystalline transition for AMC in water and identify the reaction mechanism. The calculated activation energy Ea of 41 ± 13 kJ/mol for the AMC to nesquehonite (MgCO3·3H2O) transformation matches previous studies indicating a dissolution–reprecipitation mechanism. Ea for AMC to hydromagnesite (Mg5(CO3)4(OH)2·4H2O) of 116 ± 19 kJ/mol is considerably higher and combined with our peak area analysis of total scattering data, we conclude that a second-order solid state transition is likely. Remarkably, we observed 2 orders of magnitude longer lifetime of deuterated AMC compared to protiated AMC in liquid.

The effects of individual component salts on MgO hydration and subsequent carbonation have not been systematically investigated despite the increase in utilization of seawater and other brines in the production of MgO-containing next-generation cements, fire retardants, and sorbents for CO2. Here, we present a study of MgO hydration in the presence of individual saltwater cations at a range of concentrations, as well as binary mixtures at seawater concentrations, and subsequent carbonation. We observed that the presence of the cations increased MgO dissolution rates and that the subsequent carbonation extent of the hydrated material remained constant or even increased. Since the presence of these salts during hydration does not show negative effects on subsequent carbonation in our experimental study, the application of seawater and other brines containing high concentrations of these salts to hydrate MgO in industrial processes is likely feasible.