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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.

Understanding the correlation between material deterioration and quality is required to control the slag composition with increasing gangue components (Al 2 O 3 /SiO 2) 1-3 and impurity elements (S and P). According to recent studies, 1,3,4 the flexibility of slag usage can be secured using the structure-property relationships of slag containing moderate CaO contents and amphoteric oxides such as Fe t O and Al 2 O 3. Detailed observations of the ionic structure of slags aid the fundamental understanding of the physical and chemical properties of slags. Understanding the structure-property relationship enables the prediction of properties via the slag-property model with strong reliability. 1,3,4 Mysen and Richet 5 reviewed the relationship between the ionic structure and the properties of silicate for geological magma or glass, and Keene and Mills 6 summarized the physical properties of BOS slags. Recently, the relationships between various physical properties of slag, such as the viscosity, heat capacity, conductivity, surface tension, and foaming index, with the ionic structure of slags have been reviewed. 1

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