Binders,include hydrate and chemical ones,are used in low cement refractory castables to compensate low cement content effects.Since the main problem in using hydrate binders is breaking their bonds at 800-1000oC,chemical binders,especially phosphate ones,are suggested to substitute in these products with hydrate types.The aim of this research is to investigate the effect of using sodium hexametaphosphate on alumina-magnesia low cement castables.