Magnesia bricks mainly include ordinary magnesia bricks, mid-range magnesia bricks, high-purity magnesia bricks, and fused magnesia bricks (recombined magnesia bricks). Ordinary fired magnesia bricks, usually referred to as fired magnesia bricks (magnesia bricks). my country's magnesite is of good quality and abundant reserves, and magnesia bricks are of high quality and low price, and enjoy a high reputation in domestic and foreign markets. The following is an analysis of the characteristics and performance of the samples under the microstructure of magnesia bricks made of different grades of magnesia aggregate matrix collected by refractory manufacturers.
The microstructure of magnesia brick is actually a combination of magnesia microstructure. A magnesia brick made of magnesia has the simplest microstructure, but the matrix part is relatively loose and has many pores. The microstructure of magnesia bricks made of different grades of magnesia is obviously different. The magnesia brick made of magnesia with high impurity content has many silicate phases, the MgO crystal is round, and the direct bonding rate is low; the raw material has less impurity content, and the magnesia brick fired at ultra-high temperature has less silicate and is directly bonded In the magnesia brick with a high MgO mass fraction of more than 98%, the MgO crystal is euhedral or semi-hedral. The true direct bonding between crystals can only be maximized in materials that do not contain silicates and intergranular pores. However, it shows the opposite results in terms of high-temperature mechanical properties. For example, the softening temperature of seawater magnesia bricks under load is higher than 1700 °C, and generally sintered magnesia bricks are 1500 to 1600 °C.
1. Ordinary magnesia brick:
The magnesia used in the production of magnesia bricks mainly includes natural magnesia and seawater magnesia. The vast majority of magnesia bricks in my country are made by the former. The mass fraction of MgO in magnesia is between 89% and 98%. The limit particle size is generally 3-5mm.
In the experiment, two grades of magnesia were selected, and 91 sintered magnesia and 97 high-purity magnesia particles were used as aggregates, and high-purity magnesia was used as an ordinary burnt magnesia brick made of fine powder. The microstructure characteristics of ordinary burnt magnesia bricks, the overall density of the brick is high, the content of pores is low, but mainly through pores, the pore size is large, the main crystal phase is periclase, and the cementation of silicate phase in the brick Mainly combined.
2. Mid-range magnesia brick:
In the experiment, one grade of magnesia was used, that is, 95 mid-grade magnesia was used for both the aggregate and the matrix. The microstructural characteristics of mid-grade magnesia bricks are relatively simple, similar to those of mid-grade magnesia. However, the density of the brick body is low and the porosity is high, which is due to the relatively small content of silicate phase in the mid-grade magnesia, resulting in a low degree of cementation.
The bulk density and compressive strength of mid-grade magnesia bricks are lower than those of ordinary burnt magnesia bricks. Obviously, it is unscientific to judge the high temperature performance of products simply by their bulk density and compressive strength at room temperature.
3. High-purity magnesia brick:
The same grade of magnesia is used for manufacturing comparison, that is, high-purity magnesia is selected for the aggregate and matrix parts. The microstructural characteristics of high-purity magnesia bricks are relatively simple, similar to those of high-purity magnesia. However, the matrix part is relatively loose, and the porosity is relatively high.
The microstructure and morphology of the high-purity magnesia brick, the overall structure of the sample is loose, and the porosity is high. Unlike the mid-grade magnesia brick, the content of closed pores in the brick is relatively high. The matrix part has higher porosity and lower degree of cementation. It can be seen that the higher the purity of magnesia, the lower the content of silicate phase, the more difficult to sinter, and the higher the required sintering temperature.
4. Fused magnesia brick (recombined magnesia brick):
Using fused magnesia as raw material, it is the technological basis of fused magnesia bricks (recombined magnesia bricks). The microstructure characteristics of fused magnesia bricks are basically the same as those of fused magnesia. The degree of direct bonding is high, so the recombined magnesia brick has high density and excellent high temperature performance, and its hydration resistance is also better than that of ordinary magnesia bricks. The disadvantage is poor thermal shock stability.
In the experiment, the same grade of magnesia was used for comparison, that is, 98 fused magnesia was used for both the aggregate and the matrix. The microstructure characteristics of fused magnesia bricks are relatively simple, which are close to 98 fused magnesia. The matrix part is loose and the porosity is larger. The microstructure characteristics of 98 fused magnesia, the structure of aggregate and matrix are obviously different, the surface of the aggregate is smooth and the degree of density is high; the matrix part is loose and the amount of pores is large. The morphology of the matrix of fused magnesia bricks and the high porosity in the matrix are due to the high purity of fused magnesia and low impurity content, which makes it difficult to sinter
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