Effect of Combining Hydraulic and Phosphate Bonds on Alumina-Spinel Low Cement Castables

A basic alumina-spinel low cement castable (LCC) composed of sintered bauxite, reactive alumina, calcined alumina, calcined magnesia, microsilica, and high alumina refractory cement was prepared (castable A) to which 5% sodium hexa-meta phosphate was later added (castable B) to prepare samples for heat treatment at 110ºC, 900ºC, and 1400ºC. Bulk Density (BD), Apparent Porosity (AP), and Cold Crushing Strength (CCS) were measured and XRD and SEM studies were performed. It was shown that after heat treating at 110ºC, CCS of castable B increased by more than 3 times while its AP value was less than half that of castable A. The main difference between the two samples was found to lie in the presence of Mg2P2O7 and AlPO4.2H2O which are responsible for increases in CCS. At the temperature range of 800-1000ºC where the hydraulic bond reverses to the dehydrated condition and castable A becomes weak with high porosity, castable B showed a CCS value of more than 4 times. Needles of magnesium phosphate were found to be responsible for reinforcing the microstructure of castable B at 900ºC. After firing at 1400ºC, castable B showed extraordinary CCS values of up 100 MPa (about 1000 Kg/Cm2), 5 times stronger, and AP% of below 1/5 times that of castable A. The phases present after firing were alumina, Al2O3, spinel, MgAl2O4, calcium phosphate, Ca3 (PO4)2 and Enstatite, and MgSiO3. Magnesium phosphate melted at 1300-1400ºC and aided the formation of ceramic bonds. The Ca coming from calcium aluminate cement reacted with phosphate and developed refractory calcium phosphate.