Strength and microstructure evolutions of calcium aluminate cement bonded alumina-spinel castables containing Cr2O3 in the presence of ZnO

Unfired calcium aluminate cement (CAC) bonded alumina-spinel based plug precasts have many advantages including energy saving, shorter placement period, lower cost and good thermal shock resistance in comparison with fired purging plugs. Unfortunately, their strength was drastically decreased in the range of 700°C–1000°C, which was attributed to the destruction of the hydrates network of the castables, and structural spalling was also occurred because of their lower strength. To respond these, a feasible method to fabricate higher strength of unfired precast was proposed by adding ZnO powders, and the effect of the ZnO content on the microstructure and properties of the castables was studied in this paper. Results demonstrate that cold strength of the specimens after heat-treating at different temperatures increases significantly by rising of the ZnO level, permanent linear change of the castables after firing at 1400°C and 1600°C decreases, and hot strength value noticeably increases from 36.3 to 58.4 MPa, exhibited that cold and hot strength of CAC bonded castables significantly improve via adding ZnO powders. Strengthening mechanism on the strength of ZnO-bearing castables is ascribed to formation of ZnAl₂O₄ and ZnCr₂O₄, and their strengthening effect is much higher than that of the weakening effect by CAC hydration products fired at 600°C–1000°C, and this strength reinforcing effect is intensified with rising of heat-treating temperature. In addition, ZnCr₂O₄ and ZnAl₂O₄ can dispense in MA spinel grains to form spinel solid solution at 1400°C–1600°C, (Al_1−xCr_x)₂O₃ solid solution is formed via Cr₂O₃ reacts with Al₂O₃. Those favor the densification process of the matrix structure, creating strength reinforcing effect of the ZnO-containing castables, thereby significantly increase their strength.