Alumina ultrafine powder and aluminate cement are the main factors affecting the refractory castability of corundum. Its own quality and dosage determine the properties of corundum castables.
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1. Alumina ultrafine powder.
Alumina ultrafine powder has high purity, small particle size (about D50=3μm), and high activity. It is an important part of the preparation of high-grade refractory castables, and has many similarities in its properties for reference.
That is to say, the amount of alumina ultrafine powder is generally 3% to 6%. The superfine powder added at this time can fill the pores of the material, so that the material has no defects and residues, resulting in the release of free water from the coating, wetting the surface of the particles, and having good thixotropy. Under the action of shear force, the pouring material has good fluidity, and its flow value is 114~134mm.
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2. Aluminate cement.
High-purity corundum refractory casting materials generally use fused corundum and alumina as refractory bone powder, with Ca-70 cement or Ca-80 cement as combination agent, and add admixtures. With the increase of the amount of Ca-70 cement, the flow value of the mixture increases, and the condensation time is shortened, even less than 30min, which cannot meet the technical requirements of construction. The flexural strength after drying and 1000°C burn also increased. After burn at 1500°C, the flexural strength decreases, the bulk density decreases, and the porosity increases. Due to the reaction of A-Al2O3 with cement at high temperature, a large number of mineral crystals such as CA6 and CA2 were produced, and with the expansion of the volume, the matrix produced more defects. At the same time, the amount of cement is large, the defects are many, and the bulk density and porosity are significantly reduced.
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In short, the variety and dosage of refractory castables should be reasonably selected on the premise of ensuring the workability of refractory castables, room temperature strength and high temperature three properties (that is, reactivity, sintering and swelling). The dosage is 3% to 6%. Generally speaking, the less the better, to ensure the high purity of the corundum castable.
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