The present paper provided a fundamental investigation on the effect of Al 2 O 3 on the viscosity and structure of CaO-SiO 2 -MgO-Al 2 O 3 -Fe t O slags for the purpose of efficiently recycling the valuable elements from the steelmaking slags. The results show that the viscosity of CaO-SiO 2 -Al 2 O 3 -MgO-Fe t O slags slightly increases with increasing Al 2 O 3 content. The degree of the polymerization (DOP) of quenched slags, determined from Raman spectra and magic angle spinning–nuclear magnetic resonance, is also found to increase with increasing Al 2 O 3 content. It can be deduced that the increasing DOP can promote the formation of gehlenite phase (Ca 2 Al 2 SiO 7 ), thus facilitating the formation of higher phosphorous (or vanadium) contained solid solution ( n ′Ca 2 SiO 4 ·Ca 3 ((P or V)O 4 ) 2 ). As Al 2 O 3 content increases up to a specific value, the charge compensating ions which present near [AlO 4 ]-tetrahedra and [FeO 4 ]-tetrahedra are not fully supplied due to the scarcity of Ca 2+ . In this case, the existing Fe 3+ in the melt cannot completely form [FeO 4 ]-tetrahedra and part of Fe 3+ would form [FeO 6 ]-octahedra to substitute Ca 2+ to modify the slags.