Influence of Al2O3 and SiO2 on the structure and viscosity of iron-compound bearing calcium-aluminosilicate slags

The effect of the addition of Al2O3 on the viscosity of the iron-compound bearing CaO-SiO2-Al2O3-MgO slags with different SiO2 content (= 30, 40, and 50 wt%) was investigated at high temperatures. The addition of Al2O3 increased the viscosity of iron-compound bearing calcium-aluminosilicate melts. The addition of Al2O3 to the slag at low-SiO2 content (= 30 wt%) increased the fraction of Fe2+ at a fixed iron content because Fe3+ cations prefer to exist as tetrahedral unit, i.e., Fe3+ [IV], in the melt. On the other hand, the addition of Al2O3 to the slag at high-SiO2 content (= 50 wt%) increased the fraction of Fe3+ at a fixed iron content due to the preference of octahedrally coordinated Fe3+ [VI] in the slag. Although the redox equilibrium reaction of iron in the CaO-SiO2-Al2O3-FetO-MgO system was strongly affected by the basicity of the melt, the addition of Al2O3 polymerized silicate networks regardless of the SiO2 content in the slags. The activation energy for the viscous flow of the slags had a linear correlation with the non-bridged oxygen per tetrahedron (NBO/T) in the aluminosilicate network. Therefore, the viscosity of the iron-compound bearing calcium-aluminosilicate melt was determined by the degree of polymerization within the aluminosilicate networks.