The activity of Na
2O in CaO-based slags has been measured employing a galvanic cell technique with a β-Al
2O
3 solid electrolyte. The iso-activity lines for Na
2O were obtained over the liquidus composition range in the CaO–Na
2O–SiO
2 system at 1 400°C. The activity of Na
2O in CaO–based slag is strongly controlled by the SiO
2 content in slag and the activity coefficient of Na
2O increases as the CaO content increases. The activity of Na
2O in a 50%CaO–40%Al
2O
3–10%SiO
2 (typical ladle slag composition) is extremely high. At a Na
2O concentration of about 8 wt% in this slag, the activity of Na
2O is nearly equal to that of 50wt%Na
2O–SiO
2 slag at the same temperature. The Richardson solution model fits the experimental data well and this model was used to compute the activity of CaO in these systems. The addition of Na
2O significantly increases the activity of CaO.
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