Alumina-magnesia Castables Research Articles

Agentes ligantes e seus efeitos em concretos refratários alumina-magnésia

Concretos 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 (MgAl2O4). Quando ligados por cimentos, reações adicionais (formação de CA2 e CA6) 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.

Calcium aluminate cement source evaluation for Al 2O 3–MgO refractory castables

Alumina–magnesia refractory castables are commonly bonded with calcium aluminate cements. At temperatures above 1200 °C, these materials present an expansive behavior due to the in situ spinel (MgAl 2O 4) formation and the CA 2 and CA 6 ones. One alternative to control the volumetric stability is by replacing the typical cements used (30 wt.% CaO) for another containing less lime (20 wt.% CaO). Nevertheless, when changing the cement sources, the castable's properties would be affected. Among them, the most relevant are the green mechanical strength, the drying behavior, the properties at intermediate and high temperatures, the expansion behavior, the hot modulus of rupture and creep. Considering these aspects, the objective of this work was to evaluate the impact of different cement sources in the processing steps and properties of alumina–magnesia castables. The results pointed out specific behaviors for each cement source during the whole processing stages, highlighting the importance of its proper selection in order to attain the required properties in working conditions.

Microsilica or MgO grain size: Which one mostly affects the in situ spinel refractory castable expansion?

Microsilica is commonly added to alumina–magnesia castables to counterbalance the in situ spinel expansion. This effect is attained by the generation of a low-melting temperature phase, which also affects the expansive reaction kinetics. Additionally, the MgAl 2O 4 formation depends on the grain size of the reactants. The use of coarse magnesia grains results in lower Mg 2+ dissolution and could lead, at 1500 °C, to forsterite development (Mg 2SiO 4). For finer MgO, silica was detected at the edge of the spinel grains. Considering these aspects, this work evaluated the effect of microsilica content for different magnesia grain sizes (<45 or <100 μm). Due to a faster spinel formation for the fine MgO source, microsilica counterbalanced the MgAl 2O 4 expansion. Conversely, for the coarser MgO, silica increased the Mg 2+ dissolution, speeding up the spinel formation and expansion. Therefore, microsilica presented opposite roles, pointing out that it does not always counterbalance the spinel expansion. This work also indicated the need for a systemic approach for the expanding design of alumina–magnesia refractory castables.

Selection of binders for in situ spinel refractory castables

The expansive behavior of alumina–magnesia refractory castables is usually associated with in situ spinel formation. Nevertheless, when bonded with calcium aluminate cement (CAC), this class of materials can present additional expansion reactions due to CA 2 and CA 6 formation. Considering that these reactions impart a further contribution to the material's overall volumetric change, the objective of this work has been to analyze the effect of partial or complete replacement of CAC by hydratable alumina (HA). Taking into account that this substitution would affect various castable processing steps, properties such as the mechanical strength (during curing, intermediate or high temperatures), linear change behavior during heating, creep and thermal shock resistance were evaluated. In general, CAC-containing castables led to better mechanical strength and thermal shock resistance, whereas HA-containing castables presented higher creep resistance, lower apparent porosity and better volumetric stability. Due to the substantial reduction of the overall expansion of alumina–magnesia castables, the addition of hydratable alumina was pointed out as an interesting alternative to attain designed expansion levels.

Magnesia grain size effect on in situ spinel refractory castables

Alumina–magnesia castables present an expansive behavior due to in situ spinel formation and the reaction is affected by the magnesia source and its grain size. In this study, castables were prepared with different magnesia grain sizes (<45 or <100 μm) and the samples were evaluated by an assisted sintering technique, mechanical strength, creep resistance and microstructural evaluation. The expansion, the resulting phases and properties scaled with the magnesia grain size. The use of coarse MgO grain (<100 μm) led to phases which were predicted by local equilibrium phase diagrams, such as forsterite and monticellite. Due to a completely different microstructure developed for the castable containing large grains, a large number of cracks were generated, worsening the mechanical strength and creep resistance. Magnesia grain size selection is thus a key issue for alumina–magnesia castable design, as it affects the material's performance during service. As the change of a single parameter affected the final castable microstructure and properties, this study is a typical example concerning the complexity of refractory ceramic systems.

A novel approach for magnesia hydration assessment in refractory castables

Magnesium oxide (MgO) or magnesia is one of the most important raw materials in the refractory industry. Because of its high refractoriness (melting point of 2800 °C) and corrosion resistance, the presence of this oxide in refractory compositions promotes an increase in the performance of pre-shaped linings for steel production [C.M. Peret, J.A. Gregolin, L.I.L Faria, V.C. Pandolfelli, Patent generation and the technological development of refractories and steelmaking, Refract. Appl. News, submitted for publication; M. Rigaud, Z. Ningsheng, Major trends in refractories industry at the beginning of the 21st century, China's Refract. 11 (2) (2002) 3–8; W.E. Lee, R.E. Moore, Evolution of in-situ refractories in the 20th century, J. Am. Ceram. Soc. 81 (6) (1998) 1385–1410; A. Nishikawa, Technology of monolithic refractories, Tokyo: Technical report No. 33-7, PLIBRICO Japan Co. Ltd, 1984, pp. 98–101]. Nevertheless, in refractory castables formulations, magnesia additions are restricted to coarse particles (usually above 50 μm) and contents of up to 10 wt.% due to their high tendency to react with water and the following great volumetric expansion promoted by this reaction [G.K. Layden, G.W. Brindley, Kinetics of vapor-phase hydration of magnesium oxide, J. Am. Ceram. Soc. 46 (11) (1963) 518–522; A. Kitamura, K. Onizuka, K. Tanaka, Hydration characteristics of magnesia, Taikabutsu Overseas 16 (3) (1995) 3–11; A. Yoschida, T. Nemoto, A. Kaneyasu, Evaluation method for hydration resistance of magnesia fine powder and effect of B 2O 3 content in magnesia raw materials, in: Proceedings of UNITECR’ 2003, 2003, pp. 21–30; R.A. Wogelius, K. Refson, D.G. Fraser, G.W. Grime, J.P. Goff, Periclase surface hydroxylation during dissolution, Geochim. Cosmochim. Acta 59 (9) (1995) 1875–1881; A. Kaneyasu, S. Yamamoto, A, Yoshida, Magnesia raw materials with improved hydration resistance, Taikabutsu Overseas 17 (2) (1996) 21–26; P. Brandão, G.E. Gonçalves, A.K. Duarte, Mechanisms of hydration/carbonation of basic refractories—Part I, Refract. Appl. News 3(2) (1998) 6–9; P. Brandão, G.E. Gonçalves, A.K. Duarte, Mechanisms of hydration/carbonation of basic refractories—Part II: investigation of the kinetics of formation of brucite in fired basic bricks, Refract. Appl. News 3 (2) (1998) 9–11; P. Brandão, G.E. Gonçalves, A.K. Duarte, Mechanisms of hydration/carbonation of basic refractories—Part III, Refract. Appl. News 8 (6) (1998) 23–26; S. Chatterji, Mechanism of expansion of concrete due to the presence of dead-burnt CaO and MgO, Cem. Concr. Res. 25 (1) (1995) 51–56] . Despite the great number of studies describing the behavior of powdered magnesia or high-magnesia bricks, not much research has systematically been done related to its hydration behavior in castables [A. Yoschida, T. Nemoto, A. Kaneyasu, Evaluation method for hydration resistance of magnesia fine powder and effect of B 2O 3 content in magnesia raw materials, in: Proceedings of UNITECR’ 2003, 2003, pp. 21–30; S. Chatterji, Mechanism of expansion of concrete due to the presence of dead-burnt CaO and MgO, Cem. Concr. Res. 25 (1) (1995) 51–56; T.A. Bier, C. Parr, C. Revais, H. Fryda, Chemical interactions in calcium aluminate cement based castables containing magnesia, in: Proceedings of UNITECR’ 1997, 1997, pp. 15–21; D. Jeong, H. Kim, Y. Kim, S. Lee, Development and application of basic dam block for Tundish, J. Tech. Assoc. Refract. Japan 23 (1) (2003) 4–10; K.G. Ahari, J.H. Sharp, W.E. Lee, Hydration of refractory oxides in castable bond systems—I: alumina, magnesia and alumina–magnesia mixtures, J. Eur. Ceram. Soc. 22 (2002) 495–503; K.G. Ahari, J.H. Sharp, W.E. Lee, Hydration of refractory oxides in castable bond systems—II: alumina–silica and magnesia–silica mixtures, J. Eur. Ceram. Soc. 23 (2003) 3071–3077; Y. Koga, M. Sato, K. Sekeguchi, S. Iwamoto, Effects of alumina cement grade and additives on alumina–magnesia castable containing aluminum lactate, Taikabutsu Overseas 18 (1) (1997) 43–47] . In this paper, aspects of magnesia hydration were briefly reviewed. Novel insights concerning magnesia hydration products generated under different curing conditions were attained adapting techniques already used in castables study.

Effects of cement and magnesia fines on in situ spinel formation in alumina–magnesia castable

The present study was carried out to determine the effects of cement content along with varying additions of medium and fine particles in added magnesia on in situ spinel formation in an alumina–magnesia castable. The effects of cement and magnesia on spinel and hibonite formation in the castable, alumina content of spinel, and associated physical changes after exposure to high temperature were studied. The presence of a larger quantity of fines in added magnesia was found to help increase spinel formation, with greater permanent linear change (PLC), provided cement content was not too high. Considering spinel formation and PLC, it appears that optimum levels are 3% cement for 50% of fines in magnesia or 4% cement for 34% of fines in magnesia, for a total addition of 6% magnesia. No clear combined effect of cement content and fines content in added magnesia on alumina content of spinel could be found. It is observed in general only that alumina content of spinel increases with increasing firing temperature. From these findings it is evident that increase in liquid phase to a certain level helps in increasing spinel formation and expansion characteristics. However for increasing alumina content of the spinel formed, it is found that temperature is more effective than increase in the liquid phase content.