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Abstract: The formation behaviour of calcium zirconate (CaZrO3) at the interface between the CaO–SiO2–MgO–CaF2(–ZrO2) slags ((=(mass% CaO) /(mass% SiO2)) = 2.0) used in the AOD converter and the MgO refractories has been computed by employing a commercial thermodynamic software. The solubility of zirconia (ZrO2) in the liquid CaO–SiO2-7 mass% MgO slag phase is relatively small, viz. about 2–3 mass% and the CaZrO3 phase is formed at about . The region of fully liquid phase extends to the composition saturated by dicalcium silicate (Ca2SiO4) and cubic ZrO2 (). The effect of 7 mass% CaF2 addition on the solubility of ZrO2 in the liquid slag phase was computed to be negligible, while the liquid phase exists through the entire compositions. In addition, the region of fully liquid phase extends to the more basic composition range (), where the saturating phases are the Ca2SiO4 and CaZrO3. The thermodynamic calculations indicate CaZrO3 not to form at (mass% ZrO2) /(mass% MgO) ((=Z/M))<0.6. In the “ Liquid+Ca2SiO4+MgO” region, the activity of SiO2 in the liquid phase is nearly fixed because the activity of CaO in the liquid phase is unaffected by the activity of ZrO2. However, with higher ZrO2 activity from the increase in the Z/M ratio, the activity of CaO in the liquid phase is expected to decrease due to the formation of CaZrO3. The formation behaviour of CaZrO3 in the slags, computed based on the Gibbs energy minimization principles, could experimentally be confirmed by employing the XRD and SEM–EDS analysis. [Copyright &y& Elsevier]