Effects of Nano Calcium Carbonate on Strength and Microstructure of Corundum-based Castables

The castables specimens were prepared using white fused alumina particle and powder,α-Al2O3 micropowder,hydrated alumina,nano calcium carbonate or calcium aluminate cement as starting materials.Effects of nano calcium carbonate addition on phase compositions,strength and microstructure of corundum based castables were studied.The calcium aluminate cement-containing corundum based castables with the same CaO amount was also tested for comparison.The results show that,when temperature is higher than 900 ℃,the phase compositions of nano CaCO3-containing mixture and the calcium aluminate cement containing mixture are the same,but the forming mechanism,modality and distribution of new phases in the castables are different.With temperature rising,the hydration cement dehydrates and reacts inside cement forming calcium aluminate until the alumina in cement is not enough for the reaction(temperature is≥1 400 ℃),then reacts with the surrounding alumina forming cluster CA6 in the castables.The change process of nano CaCO3 in corundum based castables is that nano calcium carbonate decomposes to CaO after firing at 800 ℃ which reacts with Al2O3 forming amorphous calcium aluminate that causes an in-situ bonding.With temperature rising,the formed calcium aluminate reacts with Al2O3 in matrix and wholly forms tabular CA6 at 1 600 ℃,which distributes uniformly in the castables.The cold and hot strength of the castables with nano calcium carbonate are obviously higher than those of the castables without nano calcium carbonate,especially at 800-1 000 ℃ due to smaller size and higher dispersion of the nano calcium carbonate and its different reaction mechanism with Al2O3.