Preparation Of Micro-porous CA₆-MA Aggregate And Its Use In Al_O3-CaO-MgO Castable

More adoption of light weight refractory castables is helpful for energy savingand environmental friendliness, thanks to their high porosity, low thermal conductivityand thermal capacity. For some severe applications, it is critical whetehr the insulatingcastables have high refractoriness and good resistances to reducing atmosphere andalkali attack. As both calcium hexaluminate （CA₆） and spinel （MA） are featured byhigh melting point, good resistances to alkali and reducing atmosphere attack, it ismeaningful to develop micro-porous CA₆-MA aggregate and use it to develop Al₂O₃-CaO-MgO light weight castables, so as to overcome the weakness of conventionalinsulating castables in the Al₂O₃-SiO₂system to this connection. The present workconsists of two parts, i.e., on the one hand, the synthesis of micro-porous CA₆-MAaggregates, and on the other hand, the development of Al₂O₃-CaO-MgO light weightcastables by using the synthesized micro-porous aggregates.Using commercially available aluminum hydroxide, magnesite and calciumcarbonate as starting materials, and anthracite, sweet potato starch, anthracite+sweetpotato starch（1:1） as burning-out materials, micro-porous CA₆-MA aggregates wereprepared through wet co-grinding the starting maetrials, slip casting, drying andcalcining. The influences of the type and amount of the burning-out materials onproperties of the micro-porous aggregates were investigated, and properties of theaggregates were characterized by AP, BD, strength, pore size distribution, phasecomposition and microstructure. Based on the experimental formulation led to thelowest bulk density, the influences of adding such additives as MgCl₂, CaCl₂and silicafume （uf-SiO₂） on aggregate properties were investigated. Micro-porous CA₆-MAaggregates with fairily low bulk density were accordingly prepared. With30%ofsweet potato starch addition, the preppared CA₆-MA aggregates reached76.8%of APand0.78g/cm³of BD, with bi-model pore size distribution,6.17μm in median diameter and5.93μm in averaged pore diameter. Adding different levels of MgCl₂、CaCl₂and uf-SiO₂respectively, the phase composition and apparent porosity remainedalmost unchanged, while bulk density increased by6%-13%, and strength increasedslilghtly as well.Using the micro-porous CA₆-MA aggregates （size fractions:8-5mm,5-3mmand3-1mm）, fast-sintered conrundum （size fractions:8-5mm,5-3mm,3-1mm,under0.074mm and under0.044mm）, ultrafine alumina powders and CA cement（Secar71） as raw materials, a series of Al₂O₃-CaO-MgO lightweight castables withvarying densities were prepared. Adding12%,24%,36%and42%of the microporousaggregates respectively to replace accordingly the corundum aggregates in the Al₂O₃-CaO-MgO light weight castables, the influences of the microporous aggregate additionon properties in terms of BD, AP, CCS, CMOR, HMOR, thermal shock resistance,thermal conductivity, resistances to reducing atmosphere and alkali attack wereinvestigated, and the microstructure of the related samples was observed and analysed.On the basis of12%addition of the micro-porous aggregate, the influence of CAcement addition on properties of the castables at ambient temperature was investigated.At42%of the micro-porous aggregate addition, AP of the castable heated at1600℃is59.8%，BD1.51g/cm³，CMOR and CCS2.7MPa and7.3MPa respectively,HMOR at1400℃1.4MPa, and the thermal conductivity at1000℃0.22W/m·K. Littleeffect of the CA cement addition between4%and10%is found on the AP, BD andstrength of the castables, while lower cement addition can reduce permanent linearchange.