Agentes ligantes e seus efeitos em concretos refratários alumina-magnésia

Abstract

ResumoConcretos refratários alumina-magnésia são comumente ligados por cimentos de aluminato de cálcio. Estes materiais apresentam, em temperaturas elevadas, uma reação de caráter expansivo decorrente da espinelização in-situ (MgAl 2 O 4 ). Quando ligados por cimentos, reações adicionais (formação de CA 2 e CA 6 ) contribuem para a expansão. Uma alternativa para controlar a estabilidade volumétrica é a substituição do cimento tipicamente utilizado (~30%-p CaO) por outro de menor teor de cálcio (~20%-p CaO) ou por alumina hidratável. Tais substituições podem afetar as propriedades dos concretos, como a resistência mecânica a frio, a hidratação da magnésia e as propriedades durante e após a sinterização. Deste modo, o objetivo deste trabalho é avaliar o impacto destes ligantes no processamento de concretos alumina-magnésia. Sendo assim, pretende-se utilizar a fonte de ligante como uma ferramenta para a obtenção de concretos refratários com expansão engenheirada, visando-se aumentar a flexibilidade na seleção de materiais aplicados em panelas de siderurgia. Palavras-chave: cimento de aluminato de cálcio, alumina hidratável, concretos alumina-magnésia. Abstract The usual binder of alumina-magnesia castables is the calcium aluminate cement. Due to in-situ spinel (MgAl

Effect of Micro‐Sized Calcium Carbonate Addition on Volumetric Stability and Strength of Corundum‐Based Castables

“…that the castable with 2.5 wt% or more CaCO 3 shows decreasing in the strength after firing at 1100 and 1450°C, compared with the strength of the castables with 2 wt% CaCO 3 . This is attributed to the presence of too much micro‐sized CaCO 3 that generate porosity due to the decomposition of CaCO 3 and also volumetric expansion of the castable due to the formation of CA 2 or CA 6 , which would be unfavorable for the strength of the castable . Therefore, the additions of micro‐sized CaCO 3 should be lower than 2.5% with regard to the strength of the castables.…”

“…Refractory castables used in steel ladles are exposed to the elevated temperatures, mechanical wear, and intense temperature fluctuation . Therefore, the castables are required to possess high strength and thermal shock resistance . Moreover, as the linings of steel ladles are also subjected to high temperatures, the castables may shrink due to the sintering of the castable matrix .…”

“…Refractory castables used in steel ladles are exposed to the elevated temperatures, mechanical wear, and intense temperature fluctuation . Therefore, the castables are required to possess high strength and thermal shock resistance .…”

“…The shrinkage could induce crack generation in the castables and consequently deteriorate the strength of the castables and facilitate slag penetration at high temperatures . Therefore, the castables applied in steel ladles are required to have sufficient volumetric stability …”

“…CaO·2Al 2 O 3 (CA 2 ) produced through the reaction between CaO·Al 2 O 3 and free Al 2 O 3 could develop ceramic bond in the matrix, leading to increase of strength . Also, the interlocked platelet CaO·6Al 2 O 3 (CA 6 ) phases formed between CA 2 and free Al 2 O 3 could provide bridging toughening and consequently improve strength and thermal shock resistance of the castables . The required volumetric stability of the castables for steel ladles could be achieved through the formation of calcium aluminates in which the formation of CA 2 and CA 6 is accompanied by the volumetric expansion of 13.6% and 3%, respectively, and could accordingly counterbalance the sintering shrinkage of the castables at high temperatures …”