The demand for use of castables, especially low cement castables and use of alumina magnesia spinel (MgAl 2 O 4 ) to resist slag corrotion, has been increased in recent years. Spinel can be prepared an added or made in situ by reaction between raw materials, which is cheaper and more suitable. Due to the expansion of spinel formation, development of nano size crystals is preferred. In order to obtain nano size in situ developed crystals of spinel in low cement castable in this study, kind and amount of raw materials, kind of cement and other suitable binders, different aggregates, particle size and firing temperature were considered. Different compositions were made and heated to 110°C, 900°C and 1400°C for two hours and bulk density, apparent porosity, cold crushing strength were measured and XRD and SEM studies were performed. Results indicated that by using calcined magnesia with calcined or reactive alumina and firing the castable at 1400°C for 2hr, nano spinels of 35-40nm are produced which are not destructive and yield good mechanical properties. For the first time, hexa meta phosphate was used together with hydraulic cements. XRD and SEM studies showed that MgP 2 O 7 , Mg 3 (PO 4 ) 2 and Ca 3 (PO 4 ) 2 are developed and at 800-1000°C that cement bonds are dehydrated, these bonds provide the required strength for the low cement castables. Secar 71 cement gave better properties than other cements. Calcined bauxite proved to be better than other aggregates.
