Aluminum-magnesium castable is a refractory castable with bauxite and magnesium oxide as its main components. It is widely used in the industrial practice of out-of-furnace refining and ladle refractory materials. The MgO can be added in the form of magnesia or pre-synthesized spinel. Aluminum-magnesium castables can be divided into different types. According to different raw materials, it is divided into high-purity corundum-spinel castables and low-purity bauxite-spinel castables. According to the different ways of introducing spinel, it can be divided into aluminum-spinel castable (Al2O3-MA castable) and aluminum-magnesium castable (Al2O3-MgO castable). According to the bonding method, it can be divided into cement-bonded, hydrated bonded, silica powder bonded, magnesium silicon water bonded, and other castables.
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| Aluminum-Magnesium Castables |
How to Combine Aluminum-Magnesium Castables
Binders are also an important component of aluminum-magnesium castables. The bonding methods used in aluminum-magnesium castables are generally calcium aluminate cement bonding, silica powder bonding, hydrated alumina bonding, sol bonding, magnesium silicon water bonding, etc.
(a) Cement bonding
In the early stages of the development of aluminum-magnesium castables, pure calcium aluminate cement was used for bonding. This is because cement calcium aluminate reacts with Al2O3 in the matrix to form CA6, which is a highly refractory phase. However, as the amount of cement added increases, the fluidity of the castable decreases. At the same time, due to excessive CaO content at high temperatures, it will react with SiO2, Al2O3, and MgO in the matrix to form a low melting phase. As a result, the high-temperature performance of the castable is reduced. Zhang Wenjie et al. studied the effect of the amount of water added to cement on the thermal expansion behavior of aluminum-magnesium castables after high-temperature firing. The results show that as the amount of cement added increases, the CA2 and CA6 phases in the sample increase, and the linear change rate of the sample after high-temperature heat treatment increases. Gu Huazhi et al. studied the effects of silica powder, magnesia, and cement on the properties of high-purity aluminum-magnesium castables. Studies have pointed out that pure calcium aluminate cement and Al2O3 fine powder in the matrix begin to react at about 1000°C, and the reaction generates CA2 and CA6. The reaction is accompanied by volume expansion, which can reduce the strength ratio after high-to-medium temperature firing and also contribute to the improvement of the high-temperature performance of the sample.
(b)MgO-SiO2-H2O combination
There are many studies on MgO-SiO2-H2O bonding and condensation bonding. The combination of MgO-SiO2-H2O is used in aluminum-magnesium castables. On the one hand, due to the polymerization of SiO2 gel formed by the interaction between SiO2 powder and water, the amount of water added is reduced and the fluidity of the castables is greatly improved. On the other hand, the combination of MgO-SiO2-H2O can inhibit magnesia hydration. However, if too much silica powder is added, it will form a low melting point phase with impurities in the matrix. At the same time, the forsterite phase formed by the reaction of magnesia and silica powder in the matrix has poor thermal shock stability at high temperatures.
