Microstructure and Phase Evolution of Cement-Bonded High-Alumina Refractory Castables for Porous Purging Plugs

This work addresses the microstructure and phase evolution of four designed high-alumina refractory castables (containing rho-Al2O3 or calcium aluminate cement as a binder source and 0 or 1 wt% microsilica). Various experimental tests (air permeability, linear change rate, apparent porosity, cold modulus of rupture, cold crushing strength, hot modulus of rupture, pore size distribution, X-ray diffraction and scanning electron microscopy) were conducted to characterize the castables. The results show that using cement as the binder has a significant effect on the properties of the refractory castables. The cement-bonded castables exhibited a lower linear shrinkage rate, apparent porosity and air permeability, higher cold modulus of rupture and cold crushing strength, as well as a smaller mean pore size than that of the rho-Al2O3-bonded castables. The results were attributed to the formation of calcium hexaluminate. Some CA(6) was detected inside the pores, which occupied space in the pores and prevented the formation of interconnected pores, while some CA(6) existed outside the pores, which enhanced the densification of the structure and reduced the size of pores. Moreover, the addition of microsilica can facilitate interconnected pore formation in cement-bonded castables, but lower the hot modulus of rupture owing to the formation of low-melting-temperature phase (C(2)AS).