Low Temperature Synthesis of High Alumina Cements by Gel-Trapped Co-Precipitation Process and Their Implementation as Castables

摘要

Use of high alumina cement (HAC) as low cement castables (LCC) and ultra-cement castables (ULCC) has increased for refractories applications over the last few years. HAC is being prepared commercially, by fusing or sintering a mixture of argillaceous and calcareous materials above 1500℃ and subsequent grinding to obtain a fine powder. Powders produced by this method have low specific surface area. Therefore, the present work was aimed to prepare nano structured HAC powders at lower temperatures than the conventional one. The powder process adopted here was modified gel-trapped precipitation method as described earlier. Two compositions of calcium alu-minate powders containing 70 and 80 wt% A1_2O_3, respectively, were selected. The remaining material was CaO in both mixtures. The gel powders obtained by the abovementioned process were calcined at different temperatures with varying soaking periods. The prepared cements were characterized for structural, mechanical, and cementing properties. Finally, the calcination temperature and time were optimized to obtain the desired phases in HAC. The prime cementing phases observed were CA, CA_2, CA_6, C_3A_5, and C_4A_3SO_4. These cements were then used with bauxite of different grain sizes to prepare LCC and then their physical properties were studied. The XRD patterns of the bauxite-based castables indicated the formation of the corundum phase along with a mullite phase in all samples. Castable containing HAC and 10 wt% zirconia substituted for bauxite improved physico-mechanical and refractory properties. Mullite formed at high temperature acts as a bonding phase and is accounted for high CCS values. These excellent properties of such castables may enable their use in various applications such as refractory lining for fabrication of steel, aluminum, copper, glass, cement, chemicals, and ceramics.