Although several models for the surface tension of molten oxide slag have been proposed, suitably
accurate predictions of this property are not yet possible. The surface tension, or surface excess
free energy, of molten oxides is largely determined by the ionic coordination induced by surface
relaxation. However, the associated mechanism of surface structural relaxation for molten oxide
mixtures such as slag is complex and not yet fully understood. To allow a better evaluation of surface
relaxation characteristics, the present work analysed the ionic coordination associated with the
surface-relaxed states of several oxide mixtures. This work especially focused on calcium aluminate
slags because these materials exhibit complex relationships between composition and surface
tension. The effects of incorporating low concentrations of SiO2 as a surface active component on
the surface ionic coordination were also examined. X-ray absorption techniques were used to assess
oxygen and cationic elements in glassy samples after surface relaxation treatments, and molecular
dynamics simulations based on a polarisable ion model of molten slag with vacuum/melt interfaces
were also performed. The X-ray absorption techniques provided data related to electron yields
(which are primarily associated with surface features) and fluorescence yields (which evaluate the
bulk material). Thus, the ionic coordination characteristics of both regions were investigated. The
results of molecular dynamics calculations allowed statistical analysis of the distributions of Al- or Sicentred
polymorphs as well as the bond angles between cations and oxygen anions in the vacuumexposed
and bulk regions in the equilibrated melt. The results indicated that bridging oxygen ions
were preferentially distributed in the relaxed surface region rather than non-bridging oxygens. These
bridging oxygen ions represented vertex connections between AlO4 or SiO4 tetrahedra. In the case
of the calcium aluminate slags, the addition of one Ca2+ ion as charge compensation for two Al3+ ions
was found to be necessary to form two AlO4 tetrahedra. This specific feature of surface ionic
coordination could be responsible for the complex trends observed in surface tension data. The
incorporation of low concentrations of SiO2 reduced the proportion of tetrahedrally-coordinated Al
ions such that higher coordination states become dominant in the bulk of the material, whereas AlO4
tetrahedra were preferentially distributed in the surface region. SiO4 tetrahedra were the preferred
Si-centred polymorph and were likely connected to one another to form bridging oxygen ions at the
surface. On the basis of these results, a model to predict slag surface tension was proposed, and
the energy of the formation of the ionic clusters resulting from surface relaxation was taken into
account.