The effect of CaO on the sulfide capacity of CaO‐Al2O3‐SiO2 slags was studied from the viewpoint of the ionic structure of alumina in slag. The aluminum coordination number was analyzed using 27Al 500‐MHz solid nuclear magnetic resonance spectroscopy and the results were compared with those of the sulfide capacity analysis. The sulfide capacity of slag, in the peralkaline region (XnormalCaO/XnormalAl2normalO3>1.0), exhibited a linear relationship with respect to basicity (anormalO2−) as excess free Ca2+ formed a 4‐coordinated aluminum unit structure ([IV]Al; AlO45−) and stabilized the sulfide ions (γnormalS2−). However, sulfide capacity in the peraluminous region (XnormalCaO/XnormalAl2normalO3<1.0) exhibited a nonlinear relationship with respect to basicity (anormalO2−) owing to the structure of higher‐coordinated aluminum units ([V]Al, [VI]Al; Al3+) and the relative lack of Ca2+. Therefore, the sulfide capacity of high Al2O3‐bearing slags strongly depended on the basicity (anormalO2−) and stability of sulfide ions (γnormalS2−), which depended on the competitive behavior of Ca2+ owing to the structural changes in Al2O3. The effect of the aluminum coordination number on the sulfide capacity was discussed in detail using an analysis of the slag structure and thermodynamics model.

The present study investigated the relationship between sulfide capacity and the average nonbridging oxygens to tetrahedral cations (NBO/T) of CaO‐SiO2‐Al2O3‐MgO slags. The sulfide capacity and NBO/T increase with increasing Al2O3/SiO2 ratio and XCaO due to increasing basicity. The inflected dependence of sulfide capacity and NBO/T observed at a eutectic composition. Thermodynamic investigations revealed that the inflected dependence is caused by the effects of mixing enthalpy and entropy components in eutectic composition. Quantitative analysis of sulfide capacity and NBO/T revealed a proportional linear relationship between the two variables. Based on these experimental results, the corresponding structure‐property relationships concerning sulfide capacity, viscosity, and stability are discussed in detail.

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