Overview of corundum refractory castables

Usually, refractory castables with Al2O3 content (mass fraction) greater than 90% are called corundum refractory castables. High quality castables have high refractoriness and good chemical stability. The raw material combination of corundum refractories comes in various forms, such as:

(1) Use white corundum as aggregate and powder

(2) Use tabular alumina and dense (sintered) alumina as aggregate, and use dense (sintered) alumina as powder;

(3) Use subwhite corundum or brown corundum as aggregate, and use dense (sintered) alumina as powder;

(4) Use white corundum and super-grade bauxite clinker as aggregate, and use white corundum as powder, etc.

In order to improve the properties of corundum refractory castable matrix, white corundum powder or tabular alumina powder is usually used as its powder. According to the requirements of the target performance of corundum refractory castables, the particle distribution coefficient is reasonably selected as the formula fitting particle ratio. Combined with the active filler in the system, SiO2 powder (often called silica fume) or active α-Al2O3 powder is commonly used.

Depending on the application requirements, corundum refractory castables can generally be formulated with cement (LCC and UL-CC) or cement-free (NCC) compositions. The former uses cement or cement and active ultrafine powder as a binder, while the latter uses active ultrafine powder as a binder. At the same time, high-efficiency surfactants (high-efficiency dispersants and water reducers) are added to disperse binders and active fillers and reduce water consumption.

Generally speaking, the content (mass fraction) of powder + active filler (ultrafine powder) in corundum refractory castable is 30%~34%. When the material is different, the amount of active filler will be different, but the optimal amount of active filler is between 4% and 10%.

The mechanism of action of active fillers is very complicated, but its fundamental mechanism of action is filling. Because of the particle size distribution of refractory castables, the bulk density is larger and denser, but there are still many pores. If active fillers (ultrafine powder) are used to fill these pores, the pores can be greatly reduced, thereby significantly reducing the water consumption of the refractory castable, and greatly reducing the pores left after the casting member or the refractory lining is dried (baking). reduce. That is to say, adding active fillers into refractory castables can reduce the amount of water, increase its bulk density, and reduce porosity. Commonly used are SiO2 micropowder and active α-Al2O3 micropowder.

α-Al2O3 micropowder is made by calcining industrial alumina. It is characterized by good dispersion, small particles, easy sintering at high temperature and small volume effect.

The addition of α-Al2O3 to refractory castables has a significant effect on its construction properties. Adding an appropriate amount of α-Al2O3 micropowder to the castable, on the one hand, it will undergo ceramization and mullite reaction at high temperature, which can improve the refractoriness of the refractory castable; The porosity can reduce the structural defects in the castable, improve its strength and resistance to slag erosion, and improve the performance of refractory materials. At the same time, the alumina micropowder can be sintered with the cement particles, which plays an auxiliary role in connecting the surrounding particles to form a network structure. The more α-Al2O3 micropowder is added, the smaller the vibratory fluidity of the castable will be. However, when the amount of micropowder added exceeds a certain value, the strength of the castable also tends to decrease. This is because after adding excessive Al2O3, in addition to filling pores and reducing construction water consumption, the remaining part is optimized to react with the cement in the castable to generate CA2 and CA6, etc., which not only consumes a large amount of Al2O3 in the matrix, but also Accompanied by volume expansion, the castable will have structural defects after high temperature, resulting in a corresponding decrease in strength and other properties.

In refractory castables, SiO2 micropowder can improve the strength of the castable at room temperature, medium temperature and high temperature, and improve the fluidity and thermal shock resistance of the castable.

The effect of SiO2 micropowder on the fluidity of the castable is shown in Figure 1. When the amorphous SiO2 particles cover the surface of the large particles, it can not only hinder the agglomeration of the larger particles, but also play a lubricating role between the large particles, thereby effectively reducing the frictional resistance between the large particles. Therefore, adding SiO2 micropowder to the castable can significantly improve the fluidity of the castable.

SiO2 micropowder can avoid the formation of C2AH8 in the process of calcium aluminate cement hydration, and directly generate calcite C2ASH8, thereby improving the normal temperature strength of the castable.

At present, there are two main types of SiO2 micropowders used in refractory castables: one is made of high-purity silica, and the other is a by-product of metallic silicon or ferrosilicon. Both products are amorphous amorphous materials. The former is active in granular form; the latter is active in hollow spherical shape, does not agglomerate, and has good filling properties. After mixing into the castable and condensing, silanol groups are formed on the surface of SiO2, which is dried and dehydrated to form a siloxane network structure.

Jia Quanli et al. studied the effect of SiO2 micropowder content on the sintering performance, high temperature strength and thermal shock resistance of corundum ultra-low cement castables. The research shows that with the increase of SiO2 micropowder content, the apparent porosity of the samples sintered at 1100℃ decreases and the strength increases; the hot flexural strength of the castable samples increases after sintering at 1400℃. This shows that SiO2 micropowder can not only promote the sintering of castables, but also react with Al2O3 to form mullite. Because mullite has a needle-like crystal structure, it can interpenetrate with corundum aggregates to enhance and toughen. The strength and thermal shock resistance of the castable can be improved.

At the same time, studies have also shown that SiO2 micropowder can fill the gaps of the particles, significantly reducing the space occupied by the dispersion medium. For example, within the appropriate range of SiO2 micropowder addition, adding 2wt% of SiO2 micropowder in the castable will reduce the amount of water added by 1wt%. Therefore, by virtue of its own good filling performance, SiO2 micropowder reduces the amount of water added to the castable and increases the density of the castable, thereby improving the mechanical properties of the refractory castable.

In corundum refractory castables, the amount of binder, active filler and additives is very small, but they are all three very important components that complement each other and are indispensable. The selection of each component becomes a key factor in controlling the rheological properties of corundum refractory castables. The selection standard is to ensure that the corresponding corundum refractory castable meets the requirements of construction performance. This can be achieved by optimizing the method of additives, such as adding a variety of additives (composite additives), each of which has a different function, and also adding a small amount of oxide refractories such as Cr2O3, ZrO2, etc., to change Properties of corundum refractory castables.