Surface ionic coordination of Al 2 O 3 –CaO–based molten slag induced by structural relaxation

The surface tensions of molten oxides depend strongly on the structural relaxation of the surface region. The mechanism of surface structural relaxation for molten oxide slags is complex. The surface tension of calcium aluminate slag is minimal at an intermediate composition, although the critical reason has not been identified. Here, two novel approaches were used to evaluate the features of surface ionic structures in the molten state for the range of 25–50 mol% Al 2 O 3 : (1) X‐ray absorption analysis of oxygen and cationic elements in glass samples after surface relaxation treatment, and (2) molecular dynamics simulations, based on a polarizable‐ion model, of the ionic distribution in a molten slag with vacuum/melt interfaces. The results indicate that bridging oxygen (BO) ions are preferred to non‐BOs in the surface region. In calcium aluminate slag, BOs are formed by connecting two AlO 4 tetrahedrons with charge compensation of two Al 3+ ions with one Ca 2+ ion. Additionally, the above approaches were used to qualify the effect on the surface ionic structure of adding 1–20 mol% SiO 2 to the calcium aluminate slag. The results indicated that the SiO 4 tetrahedrons incorporate the vertex connection with AlO 4 tetrahedrons to form BOs in the surface region.