The activities of components in the CaO-SiO2-AlO1.5-MgO slags at 1 823 and 1 873 K were directly measured by equilibrating the slags with Si based alloys. For the CaO-SiO2, CaO-SiO2-AlO1.5 and CaO-SiO2-MgO systems, thermodynamic properties of Si based alloys and the activity data of SiO2 obtained from the distribution of each element between slags and the alloys in the previous paper were used to derive the activities of AlO1.5, CaO and MgO. For the CaO-SiO2-AlO1.5-MgO quaternary system, the activities of components other than SiO2 on the 10 mass% MgO plane were determined at 1 873 K by employing the SiO2 activities available in the literature. Accordingly, it was confirmed that the activities of components other than SiO2 in multi-component silicate stag systems can be determined by this technique as long as the activity datum of at least one component in the systems is available.
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