A study of the Na2O–CaO–P2O5–SiO2 system with respect to the behaviour of phosphate bonded basic refractories at high temperature

Investigating various phosphate bonded basic refractories by means of powder X-ray diffraction, energy dispersive spectroscopy and X-ray fluorescence analyses, reveals a relation between good mechanical properties at high temperature and the presence of a silicophosphate bond (T>1200°C). The silicophosphate formation mechanism was studied using X-ray thermodiffractometry. From room temperature up to 500°C, a mixed calcium sodium phosphate gel is formed. It transforms around 500°C into a crystalline calcium sodium phosphate which is not stable at room temperature. Above 1200°C, the silicophosphate belongs to a Na2−xCa5+x(PO4)4−x(SiO4)x type solid solution with x>0 identified by studying the Na2O–CaO–P2O5–SiO2 system. The Na2−xCa5+x(PO4)4−x(SiO4)x solid solution structure was shown to be a superstructure of the glaserite-type structure. Good mechanical properties of the refractory material are maintained as long as the silicophosphate glaserite-type unit cell superstructure is kept. That is achieved using starting raw materials with a high CaOSiO2 ratio and a high calcium content.