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 powde r X-ray diffraction, energy dispersive spectroscopy and X-ray fluorescence analyses, reveals a relation between good mechanical properties at high tem perature and the presence of a silicophosphate bond (T>1200 degrees C). The silicophosphate formation mechanism was studied using X-ray thermodiffract ometry. From room temperature up to 500 degrees C, a mixed calcium sodium p hosphate gel is formed. It transforms around 500 degrees C into a crystalli ne calcium sodium phosphate which is not stable at room temperature. Above 1200 degrees 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-S iO2 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 mechanic al properties of the refractory material are maintained as long as the sili cophosphate glaserite-type unit cell superstructure is kept. That is achiev ed using starting raw materials with a high CaO/SiO2 ratio and a high calci um content. (C) 2000 Elsevier Science Ltd. All rights reserved.