• Equimolar substitution of MgO for CaO in high-alumina slag leads to a viscosity minimum at 15 mol% MgO, offering a strategy to optimize slag fluidity.
• Spectroscopic analyses (FTIR, Raman, XPS) reveal progressive polymerization of the silicate network with increasing MgO substitution, evidenced by increased bridging oxygen and decreased non-bridging oxygen.
• The viscosity minimum arises from a complex interplay between structural polymerization and bond strength evolution of non-bridging oxygens, challenging the simple depolymerization model.
• The findings provide theoretical and data support for utilizing high-alumina ores in blast furnace operations, potentially expanding the allowable Al2O3 content range.