• Increasing TiO2 content depolymerizes silicate and aluminate networks in blast furnace slag, reducing viscosity and improving fluidity.
• Increasing Al2O3 content polymerizes the slag structure, increasing viscosity and potentially deteriorating fluidity.
• Molecular dynamics reveals Ti exists as [TiO5]6− and [TiO6]8−, acting as a network modifier, while Al forms [AlO4]4− as a network former.
• The degree of polymerization of [SiO4]4− and [AlO4]4− is a key factor controlling slag fluidity, offering a basis for optimizing blast furnace operations.