When clay refractory materials come into contact with glass liquid at high temperature, the surface of the brick is first infiltrated by molten glass. After that, due to the action of capillaries, the molten glass is sucked into the pores of the brick, and the alkali metal-rich ions in the melt gradually diffuse into the gaps between the clinker phases around the pores, and the replacement reaction occurs.
The glass liquid first dissolves the free SiO2 in the refractory, and the mullite is dissolved at a lower rate, so it gathers at the interface between the glass liquid and the refractory. After that, the small crystal mullite is dissolved, and the recrystallized (secondary) mullite is produced near the pores or on the interface in contact with the glass liquid. The clinker may also be transformed and decomposed into β-Al2O. Since the replacement is not deep, the size of these grains is mostly very fine. In addition, the replacement reaction also forms a part of glass phase with different composition from the original glass liquid. This is because the clinker is partially dissolved, and SiO2 and Al2O2 components are added to the melt. These melts will diffuse into the rest of the glass liquid. As the replacement reaction develops, the clinker particles will gradually disintegrate and become residual aggregates. Mullite and β-Al2O3 may grow around them, and β-Al2O crystals will aggregate in the interface layer. Nepheline and glass phases may also appear. At this stage, Na2O and K2O have penetrated into the middle of the clinker aggregates. It reacts with mullite in the clinker as follows:
3Al2O3・2SiO2+Na2O=NaO・Al2O3・2SiO2+2Al2O3
When mullite coexists with R2O, it will decompose at a lower temperature. The more R2O there is, the lower the temperature at which mullite decomposes into corundum and nepheline liquid phases. The alkaline components gradually diffuse inward from the replacement interface into the interior of the brick body. Therefore, the alkaline components are high in the interface layer, and BAl2O3 crystals are more aggregated.
As the replacement reaction develops further, the clinker particles may be transformed into scattered broken layers, or even all transformed into secondary or new mineral phases, which mainly include secondary mullite and BAl2O transformed by decomposition, as well as nepheline, triclinic nepheline, leucite, orthoclase, albite, etc. generated by the replacement reaction. After further erosion, they will melt into the glass liquid and become high-aluminum stripes or lumps.
This erosion is most intense at the glass liquid surface, where not only the temperature is high, but also it is at the junction of gas, liquid and solid phases, and it will be affected by alkali solution and nitrate water in the batch. Nitrate water reacts with clay bricks to generate SiS. When this substance decomposes, it produces gas to foam the metamorphic layer of the brick, thereby accelerating erosion. The volatiles of alkali metal oxides can react with clay bricks at 1000~1100℃, that is, R2O reacts with mullite in clay bricks to form corundum and nepheline glass phases. The latter continues to be affected by R2O to form feldspar glass phases. If the main component of R2O is K2O, a strong protective layer of potassium ore and BAl2O2 in the high viscosity area can be formed on the surface of the clay brick. However, if the brick contains less SiO2, the reaction with R2O will cause this glassy glaze layer to fall off.
After the clay bricks in the heat storage chamber are eroded by the dust of the batch material, a layer of glaze is formed. In addition to the glass phase, this layer of glaze also contains nepheline, leucite, feldspar and mullite in complex twins. This glaze layer will be lost at higher temperatures. The residue solidifies at a lower temperature of the checker brick, which is easy to block the checker hole, which will greatly reduce the heat storage effect of the checker brick in the heat storage chamber. It is usually difficult to use clay checker bricks for more than 30 months.
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