Property Optimization Of Light-weight Al₂O₃-CaO-MgO Castables Using CA₆-MA Light-weight Aggregates

Lightweight refractories, high performance lightweight monolithic refractories inparticular, are useful and meaningful to heat insulation and safety of refractory liningsunder severe operating conditions, Research and development work has focused onnew type lightweight refractory raw material and their castables to meet specifichigher end applications. Calcium hexaluminate （CA₆） based refractories are receivingmore importance and attention due to their good resistances to high temperature andattack by alkali or reducing atmosphere. The synthesis of CA₆-spinel combined porousaggregates and preparation of lightweight Al₂O₃-CaO-MgO castables using suchaggregates are of a new attempt in insulating refractories, for which the optimizationof composition, microstructure and property is of great necessity.Based on previous related work, this thesis work focused on the propertyoptimization of the synthesized CA₆-MA porous aggregate with a CA₆to MA massratio of7over3and the Al₂O₃-CaO-MgO lightweight castables adopting theaggregates. In view of the limited dispersion of CaO and MgO in the previouslyprepared CA₆-MA lightweight aggregates, using solid starting materials, this worktried using soluble salts. i.e. MgCl₂·6H₂O and CaCl₂, to replace the ever usedmagnesite and calcium carbonate respectively and investigated such effect on bulkdensity （BD）, strength and microstructure of the lightweight aggregates. The addedsalts participate in the reaction to form targeted products after decomposition whensubjecting to heating. Such partial or complete replacement of the calcium carbonateand magnesite lead to appreciable micronized pores in the CA₆-MA lightweightaggregate. With increased addition of the soluble salts, BD of the lightweightaggregate tends to decrease, and meanwhile apparent porosity （AP） to increase. At9%addition of MgCl₂·6H₂O, a BD of1.05g/cm3and an averaged pore size0.7μm afterfiring at1450℃for3h were achieved; With7%addition of CaCl₂, a BD of1.1g/cm3and averaged pore size of0.9μm were obtained. The amount of the salt additions can significantly change the microstructure. The MgO and CaO derived from theMgCl₂·6H₂O and CaCl₂respectively are dispersed more homogeneously, and can helpform tiny pores down to nanosize after decomposition, which is helpful to inhibit masstransfer and sintering, so as to maintain low density and good insulation property athigh temepratures.To tackle with the weak strength of the CA₆-MA lightweight aggregates, theeffect of calcination parameters on BD and strength is investigated, i.e. calcinated at1450℃for3h, at1600℃for0h,0.25h and0.5h respectively. Higher temperature isfavorable for strength. Compared with MgCl₂·6H₂O, introduction of CaCl₂is moreeffective to improve volumetric stability at high temperature of the lightweight CA₆-MA. With its addition increasing, the BD increasing rate from1450℃for3h to1600℃for0.5h tend to decrease. By the substitution of CaCl₂for calcium carbonate,the contradiction of maintaining low BD and good strength can be well compromisedand the lightweight CA₆-MA with relatively low BD and enhanced strength can beobtained by calcination at high temperature. When the CaO source is completely fromCaCl₂, the high temperature properties of the lightweight CA₆-MA can be remarkablyimproved.To appropriately reduce harsh decomposition and help sintering, α-Al₂O₃ultrafines were added to partially replace the adopted Al（OH）3to help reach higherstrength; meanwhile appropriate burn-out material was adopted to help achieve lowerBD. At the additions of α-Al₂O₃ultrafines25%and burn-out material20%,micropored lightweight CA₆-MA aggregates with BD1.08g/cm3, strength index92%,CA₆and MA as principal crystalline phases and a little corundum can be obtained afterfiring at1450℃for3h, its strength being significantly improved compared with theoriginal counterpart with the same composition. Batched lightweight CA₆-MA wereproduced, adopting the strength improved formulation and technological parameters,and the Al₂O₃-CaO-MgO lightweight castables using the aggregates were thenprepared.In the castable containing36%of the lightweight CA₆-MA aggregate, the effectof CA cement addition of4%,6%,8%and10%on BD, strength and other propertieswas investigated, indicating that appropriate CA cement addition should be6⁸%. Forreducing BD of the lightweight castables, some decomposable matters as precursors ofrelated oxides were introduced in the matrix. In the castable with42%of the lightweight CA₆-MA aggregate,8%of CA cement,1.1%calcium carbonate of andmagnesium carbonate of3.9%, properties of the castables with Al（OH）3additions of0,1%,3%and5%respectively were compared. It is evidenced that introduction ofdecomposable precursor can help form micropored structure in the matrix and theoccurred in-situ reactions are also beneficial to compromise properties. When Al（OH）3addition is3%, the castable with BD of1.48g/cm3, cold crushing strength of14MPa,cold modulus of rupture of5.2MPa can be achieved after firing at1600℃,correspondingly comparing with1.51g/cm3,7.3MPa, and2.7MPa obtained bypreviously related work, putting it in evidence that properties have been significantlyimproved.This research work has met the predetermined objectives to optimize properties ofboth the CA₆-MA lightweight aggregates and the Al₂O₃-CaO-MgO lightweightcastables. The achieved results can provide technical support and guidance for furtherrelated research and application of insulating refractory materials.