The effect of phosphate additives on alumina, alumina-calcia, alumina-silica refractory castable systems

The effects of the addition of phosphate chemicals to Al2O3, Al2O3-CaO, and Al2O3-SiO2 systems have been studied concentrating on selected dry and fired properties. The various refractories evaluated were formulated from all alumina aggregate systems based on 1) a mix of tabular and calcined fine aluminas, 2) a combination of calcium aluminate cement bonded-tabular and calcined fine aluminas, and 3) an alumina aggregate-silica fume mixture. Specimens were vibration-cast, dried at 110°C and fired at 1450°C. It was found that each system has different property responses to different phosphate additions depending on the chemical nature of the phosphate. The phosphates used in this study were (NaPO3)x, Na3PO4, (NH4)2HPO4, NH4H2PO4, and K4P2O7∙(NaPO3)x was the most effective in reducing viscosity. Only 0.05-0.20 weight percent of phosphate additions are required to obtain the maximum fired strength of the above refractory systems. Microstructural studies were performed to aid in explaining the specific additive behavior with the respective phosphate chemicals. Microstructural studies showed that the phases formed between the interfaces of the aggregate systems have an important effect on the fired strength