Microstructure and mechanical properties of magnesia refractories containing metallic Al by the incorporation of glucose and citric acid as binders

Abstract A study of magnesia refractories containing glucose, citric acid, and metallic Al and their influences on structure, mechanical strength, and thermal shock resistance, is presented. Combined with thermodynamic calculation, microstructure, and properties analysis, the results show that formation of gluconates and citrates from the introduced glucose and citric acid enhanced the bonding of raw materials and the curing strength of samples. The pyrolysis of gluconates and citrates, and the oxidation and nitridation of metallic Al had significant effects on the microstructural evolution of magnesia refractories after high temperature treatments, mainly AlN and MgAl2O4 whiskers in pores and particle gaps. The formation and development of whiskers give rise to samples improved toughness and thermal shock resistance. Increasing the metallic Al content from 0 wt% to 2 wt% enhanced the fracture surface energy of samples heat treated at 1400 °C from 82.22 kJ/m2 to 119.75 kJ/m2. Meanwhile, after three thermal-shock cycles, the residual strength ratios of heat treated samples were elevated from 52.8% to 67.9%. On the basis of influences of phase transformation on the structure and properties of samples at different temperatures, considering the structural compactness and ability to withstand external stress for magnesia refractories used in metallurgical field, it is suggested that when glucose and citric acid are used as binders, the addition of metallic Al should be less than 3 wt%.