Activities of Aluminum and Calcium Oxides in the CaO–Al 2 O 3 and CaO–SiO 2 –Al 2 O 3 Melts

The thermodynamic activities of aluminum and calcium oxides in the CaO–Al 2 O 3 and CaO–SiO 2 –Al 2 O 3 systems are calculated using the concepts of a polymer model of oxide melts, which takes into account the variable functionality of a monomer. The calculations based on the polymer theory equations are used to calculate the polymerization constants in binary modifying oxide–complexing oxide systems. The activity of aluminum oxide is considered as effective: it is determined with allowance for the fraction of aluminum located in the tetrahedral interstices between four oxygen ions to form AlO_4^5 - anions and the fraction of aluminum located in the octahedral interstices between six oxygen ions to form Al 3+ cations, which play the role of a modifier, in a melt. The activities of the aluminum and calcium oxides in the CaO–Al 2 O 3 system are calculated on the assumption that, at a mole fraction X of calcium oxide less than 0.7, aluminum is present in the tetracoordination and exhibits acidic properties. At higher CaO contents, aluminum exhibits both acidic and basic properties. In CaO–SiO 2 –Al 2 O 3 slags, agreement between the calculated and experimental activities of the components is achieved by varying the fraction of aluminum in the tetra- and hexacoordination with respect to oxygen. The dependence of the activity of aluminum oxide on the mole fraction of silicon dioxide is found to have a maximum at X_SiO_2 = 0.3. This trend is explained by the fact that, when the fraction of silicon dioxide increases to 0.3, the amount of Al 3+ , which has high degrees of freedom and characterizes the manifestation of basic properties, increases and the presence of aluminum in the tetracoordination decreases, which affects the manifestation of acidic properties. When the mole fraction of SiO 2 increases above 0.3, aluminum is more able to form its own isolated AlO_4^5 - complexes and the activity of aluminum decreases correspondingly because of an increase in its binding to oxygen atoms; as a result, the acidic properties of Al 2 O 3 again prevail.