Atomistic-Level Insights into MgO and Na2O Modifications of Molten Aluminosilicate Slag: A Molecular Dynamics Research on Structural Evolution and Properties

Molecular dynamics simulations were employed to systematically investigate the synergistic effects of Na2O and MgO on the atomistic-scale structural evolution and properties of CaO–SiO2–Al2O3-based slags. By constructing slag models with varying Na2O/MgO ratios, the variations in pair distribution functions, oxygen structural units, coordination environments, diffusion coefficients, and viscosity were analyzed in detail. Compared with Na2O, MgO exhibits a stronger ability to disrupt oxygen structural units. The relative content of Na2O and MgO does not significantly affect the bond lengths within the basic network structure. As the MgO content increases, a greater proportion of bridging oxygens and tricluster oxygens are converted into non-bridging oxygens and free oxygens, markedly reducing the degree of polymerization in the slag network. Although MgO also promotes the formation of tetrahedrally coordinated Al (Al4) more effectively than Na2O, its dominant role in enhancing slag fluidity is primarily attributed to its impact on the oxygen structural units. Despite the much higher self-diffusion coefficient of Na+ compared to Mg2+, MgO more significantly reduces the overall viscosity and enhances the fluidity of the melt than Na2O. Therefore, although the number of Na atoms is greater under equal mass conditions, Mg demonstrates a considerably stronger capacity to depolymerize the slag structure.