• Finer magnetite particles generally reduce faster and more completely, but gangue composition can override this size effect.
• MgO gangue forms magnesio-wustite solid solutions that create dense microstructures, hindering hydrogen diffusion and limiting reduction.
• CaO gangue promotes calcium ferrite formation, leading to porous microstructures and enhanced reducibility.
• Gangue composition is a dominant factor in reduction kinetics, phase formation, and product morphology, crucial for optimizing hydrogen-based ironmaking.