Among silicon-aluminum refractory castables, refractory castables with Al₂O₃ content greater than 90% belong to corundum refractory castables.
Design principles of corundum castable
Its raw material combinations come in many forms, such as
(1) Use white corundum as aggregate and powder
(2) Use platy alumina and dense (sintered) alumina as aggregate, and use dense (sintered) alumina as powder;
(3) Sub-white corundum/brown corundum is used as aggregate, and dense (sintered) alumina is used as powder;
(4) Use white corundum and super bauxite clinker as aggregate, and white corundum as powder.
In order to improve its matrix properties, white corundum powder or plate-shaped alumina powder is usually used as its powder composition. According to the requirements of the target performance, the particle distribution coefficient (q value) is reasonably selected as the matching particle ratio of the formula. Combined with the active fillers in the system, SiO₂ powder (often called silica fume, abbreviated as uf-SiO2) or active α-Al₂O₃ powder (abbreviated as uf-Al₂O₃) is commonly used. According to application requirements, it can generally be formulated with cement (LCC and ULCC) or without cement (NCC). The former uses cement or cement and active superfine powder as the binding agent, while the latter uses active superfine powder as the binding agent. At the same time, a high-efficiency surfactant (high-efficiency dispersant and water-reducing agent) is added to disperse the binding agent and active filler and reduce water consumption.
Generally speaking, the total amount (mass fraction) of powder + active filler (ultrafine powder) is 30% to 34%. When the material is different, the amount of active filler will be different, but the optimal amount of active filler is between 4%~10%. In addition, it is necessary to add high-efficiency surfactants (high-efficiency dispersant and water-reducing agent) to disperse the binding agent and active filler, so that the slurry can obtain good fluidity and better construction performance.
In corundum refractory castables, the amount of binder, active filler and additives is very small, but they are all three very important complementary components and are indispensable. The choice of each component has become a key factor in controlling rheological properties. This can be achieved by optimizing additives (high-efficiency dispersants and water-reducing agents). For example, multiple additives (composite additives) can be added, each of which has a different function to change the rheological properties of the corundum refractory castable.
Its drying process
The temperature rise during the drying process is a very important process and requires careful operation. In the initial stage of drying, f-H₂O is mainly discharged from the pores. As the temperature continues to rise, the hydrate will undergo a series of dehydration processes and changes in the microstructure of the binding phase. Take the low cement corundum refractory castable as an example to illustrate the dehydration process and the microstructure changes of the binding phase during the entire heating and drying process: ①When the temperature is between room temperature and 100℃, the cement hydration products gradually transform into more stable AH₃ And C₃AH₆ phase, and discharge f-H2O; ②between 100~300℃/350℃, AH₃ and C₃AH6 phases gradually decompose into some amorphous anhydrous products, and discharge f-H₂O(g) at the same time; ③over 800~ When the temperature is above 900°C, the decomposition products of cement hydration products continue to react with certain mineral phases of the matrix, and finally form a ceramic bonded phase. In the whole process, the strength of the material is continuously increasing, so as to obtain an ideal refractory castable body.
Cement-containing corundum refractory castable
The main products of Al₂O₃-CA₆ refractories are cement-bonded corundum refractory castables and low-cement-bonded corundum refractory castables. These two types are common bonding systems based on calcium aluminate cement, which are solidified by hydration at ambient temperature. . CAC is actually composed of aluminates (CA, CA₂, etc.) with a high content of Al₂O₃.
In the standard CAC, that is, when the stoichiometric composition meets CA₂, the corresponding reaction is:
CA+Al₂O₃→CA₂
Corundum refractory castables (CC/LCC) containing CAC will be converted into α-Al₂O₃ and CA₆ through high-temperature treatment or during high-temperature use. At 1500°C, all CaO components contained in CAC will be converted into CA6.
Through experiments, it can be found that the Al₂O₃-CA₆ refractory castable with silica micropowder added to the matrix will start to produce a liquid phase when the temperature reaches 1345℃, and its stable phase is mullite-anorthite-cristobalite. When the material is used for a long time under the condition of higher than 1350℃, the situation of stable phase needs to be considered. It should be noted here that the Al₂O₃-CA6 refractory castable with micro-silica powder added to the matrix has the advantage of producing a liquid phase at a lower temperature, which can make the material production have a coating effect and can effectively reduce the penetration of the medium. Improve the elastoplastic properties of high-aluminum products.
The combination between CA6 and sintered Al₂O₃ or plate-like Al₂O₃ is better than that between it and fused corundum, indicating that the surface of sintered Al₂O₃ aggregate has higher activity than the surface of fused corundum. At the same time, the experimental results also show that the refractory castable with sintered Al₂O₃ or plate-shaped Al₂O₃ as the main raw material has higher room temperature mechanical strength, high temperature flexural strength and thermal shock resistance than refractory castable with fused corundum as the main raw material.
