Effects of carbonate and phosphate admixtures on MgO dissolution and Mg(OH)2 precipitation

The effects of sodium hexametaphosphate (NaHMP) and sodium carbonate (Na 2 CO 3 ) on MgO dissolution and Mg(OH)₂ precipitation from 30 min to 35 days were examined through solution analyses (ICP-OES, DIC, pH) and mineralogical methods (XRD, TGA, FTIR, TGA-Infrared gas analysis, 13 C ssNMR, and 31 P ssNMR). Both admixtures raised the Mg concentration above the theoretical Mg(OH) 2 solubility. Strong depletion of total dissolved phosphorus and moderate loss of dissolved inorganic carbon indicate adsorption or secondary phase formation. NaHMP delayed Mg(OH) 2 nucleation by adsorbing onto ≡Mg(OH) + sites, producing pronounced brucite oversaturation and subsequent direct precipitation from solution. MgO dissolution was also slowed down, likely due to adsorption of dissolved phosphate species or a transient quasi-equilibrium under oversaturated conditions with respect to Mg(OH) 2 . In contrast, Na 2 CO 3 promoted rapid formation of Mg(OH) 2 and hydrous carbonate-containing brucite. Morphological analyses showed idiomorphic hexagonal plates in pure water, while carbonate admixtures limited c-axis growth of Mg(OH) 2 , generating thin flakes through incorporation or surface adsorption. Phosphate admixtures yielded irregular blocky crystals, consistent with rapid precipitation and surface adsorption.