Influence of the Ca/Mg Ratio on Mineralogical Transformations during Carbonation

Ex-situ mineral carbonation, utilizing industrial by-products or natural calcium- and magnesium-bearing rocks, presents a promising avenue for CO₂ removal. Both calcium and magnesium can form carbonates independently or in conjunction. While magnesium is a significant mineralization source, its presence can negatively influence carbonate precipitation and crystal stability when co-precipitated with CaCO₃. This study investigates the effects of varying Mg²⁺/Ca²⁺ molar ratios on the mineral carbonation process under controlled conditions, quantitatively evaluating the resulting precipitate phases. Calcite consistently emerged as the dominant mineral phase across all tested Ca/Mg ratios. The results demonstrate that at higher Mg²⁺/Ca²⁺ ratios (≥1.0), substantial distortions occur within the calcite lattice, with magnesium ions substituting up to 6.51% of calcium ions. These distortions manifest as reduced crystal size, increased internal strain, and enhanced solubility, collectively indicating a decrease in structural stability. Furthermore, the formation of secondary phases, including aragonite and brucite, was observed at elevated Mg²⁺ concentrations (Mg²⁺/Ca²⁺ = 2.0), accompanied by the development of amorphous Mg-rich phases. These findings underscore the dual role of magnesium ions in carbonate mineralization: while they facilitate the formation of diverse mineral phases, they simultaneously introduce structural instabilities that may compromise the long-term durability of the mineralized products. This research provides crucial insights for optimizing mineral carbonation processes to achieve a balance between reactivity and stability, ultimately contributing to the development of more effective and reliable applications in both industrial and environmental settings.