The binder for mullite and refractory castables is a composite binder, mainly composed of α-Al2O3 and SiO2 fine powder, clay, and hydrated alumina. It is a combination of fine powders and hydrated alumina that bond at low temperatures. At medium to high temperatures, Al2O3 reacts with SiO2 to form mullite, creating a binding material, hence referred to as the binder for mullite and refractory castables. It is well known that using secondary mullite in amorphous refractories is a common technique to improve their performance. Refractory plastics, clay composites, and micro-expansion refractory castables are all manufactured using the concepts of self-reaction, self-sintering, and self-expansion of Al2O3 and SiO2 and can be used in industrial furnaces below 1400°C to achieve good results.
Corundum and silicon carbide are high-purity refractory raw materials used for preparing high-performance refractory castables. When aluminate cement is used as a binder, it introduces harmful impurities and reduces its performance. Currently, corundum and silicon carbide refractory castables are generally prepared using a composite binder, combining mullite with corundum and silicon carbide refractory castables, as shown in Table 1 for their performance.
Grade 1 and Grade 2 use corundum with 98% Al2O3 as refractory aggregates, with a particle size of 8 to 1mm, in a dosage of 550. Silicon carbide with >98% SiC purity is used as fine aggregates, with a particle size of 2 to 1mm, in a dosage of 100. The powder consists of synthesized mullite powder, α-Al2O3, and SiO2 fine powders, with dosage percentages of 15%, 14%, and 6% for component 1, and 10%, 17.5%, and 7.5% for component 2. Component 3 uses silicon carbide with >97% SiO2 purity as refractory aggregates, with a dosage of 65%. Silicon carbide with >99% SiC purity is used as refractory powder, with a dosage of 10%. A binder and powder consisting of SiO2 ultrafine powder with 91% content of β-Al2O3, α-Al2O3, and SiO2 is used, with a dosage of 25%. The mass ratio of refractory aggregates to refractory powder is 65:35.
It should be noted that in the design of the mixing ratio, the materials in the matrix form a high-temperature bonding phase of mullite, resulting in good workability, structural integrity, and resistance to thermal shock.