Magnesia brick mainly includes ordinary magnesia brick, medium-grade magnesia brick, high-purity magnesia brick and fused magnesia brick (combined magnesia brick). Ordinary fired magnesia brick, commonly referred to as burnt magnesia brick (magnesia brick), is the alkaline brick with the largest production and the most widely used. China enjoys a high reputation in domestic and foreign markets for its excellent magnesite mineral quality, rich reserves, high quality and low price of magnesite bricks.
The following is the analysis of sample characteristics and service performance of magnesia bricks made of magnesia aggregate matrix of different grades collected by refractory manufacturers under microstructure.
The microstructure of magnesia brick is actually the combination of magnesia microstructure. The microstructure of magnesia brick made of magnesia is the simplest, but the matrix is relatively loose and there are 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, round MgO crystal and low direct bonding rate; The content of impurities in raw materials is low. For magnesia bricks fired at ultra-high temperature, the silicate is reduced and the direct bonding rate is high. In magnesia bricks with MgO mass fraction of more than 98%, MgO crystals are self-shaped and semi self-shaped. The real direct combination of crystals can be maximized only in materials without silicate and intergranular pores. However, reflected in the high-temperature mechanical properties, it shows the opposite results. For example, the load softening temperature of seawater magnesia brick is higher than 1700 ℃, and the general sintered magnesia brick is 1500 ~ 1600 ℃.
- Ordinary magnesia brick:
Magnesia used in the production of magnesia brick mainly includes natural magnesia and seawater magnesia. Most magnesia bricks in China are made by the former. The mass fraction of MgO in magnesia is between 89% ~ 98%. The limit particle size is generally 3 ~ 5mm.
Two grades of magnesia were selected in the experiment. The common magnesia brick was made of 91 sintered magnesia and 97 high-purity magnesia particles as aggregate and high-purity magnesia as fine powder. The microstructure of ordinary magnesia fired brick is characterized by high overall density and low pore content, but mainly through pores with large pore diameter. The main crystalline phase is periclase, and the brick is mainly cemented and combined with silicate phase.
- Medium grade magnesia brick:
One grade magnesia was selected for the experiment, that is, 95 medium grade magnesia was selected for the aggregate and matrix. The microstructure of medium grade magnesia brick is relatively simple, which is close to that of medium grade magnesia. However, the brick has low density and more pores, which is due to the relatively small content of silicate phase in medium-grade magnesia, resulting in the low degree of cementation.
The bulk density and compressive strength of medium-grade magnesia brick are lower than those of ordinary magnesia brick. Obviously, it is unscientific to judge the high-temperature performance of products simply by the bulk density and compressive strength at room temperature.
- High purity magnesia brick:
The same grade of magnesia is selected for manufacturing comparison, that is, high-purity magnesia is selected for aggregate and matrix. The microstructure of high-purity magnesia brick is relatively simple, which is close to that of high-purity magnesia. However, the matrix is loose and the porosity is low.
The microstructure and morphology of high-purity magnesia brick, the overall structure of the sample is loose and the amount of pores is high. Different from medium-grade magnesia brick, the content of closed pores in the brick is high. The amount of pores in the matrix is high and the degree of cementation is low. It can be seen that the higher the purity of magnesia, the lower the content of silicate phase, the harder it is to sinter, and the higher the sintering temperature is required.
- Fused magnesia brick (recombined magnesia brick):
Taking fused magnesia as raw material is the process basis of fused magnesia brick (reconstituted magnesia brick). The microstructure characteristics of fused magnesia brick are basically the same as that of fused magnesia. The direct bonding degree between periclase and periclase is high. Therefore, reconstituted magnesia brick has high compactness, excellent high temperature performance and better hydration resistance than ordinary magnesia brick. The disadvantage is poor thermal shock stability.
In the experiment, the same grade of magnesia was selected for comparison, that is, 98 fused magnesia was selected for both aggregate and matrix. The microstructure of fused magnesia brick is relatively simple, which is close to 98 fused magnesia. The matrix part is loose and the porosity is large. The microstructure of 98 fused magnesia shows that there are obvious differences between aggregate and matrix structure, and the aggregate surface is smooth and dense; The matrix part is loose and has a large amount of pores. The morphology of the matrix of fused magnesia brick is partial, and the porosity in the matrix is high. The reason is that fused magnesia has high purity and low impurity content, which makes it difficult to sinter.