Usually the refractory castable with Al2O3 content (mass fraction) greater than 90% is called corundum
refractory castable, and its main phase is corundum, because corundum has the characteristics of high melting point (2050℃) and strong resistance to slag erosion, which makes corundum High-quality castables have high refractoriness and good chemical stability. The raw material combination of corundum refractories has many forms, such as:
(1) Use white corundum as aggregate and powder
(2) Use tabular alumina and dense (sintered) alumina as aggregates, and use dense (sintered) alumina as powder;
(3) Sub-white corundum or 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 the performance of the corundum-based refractory castable matrix, white corundum powder or plate-shaped alumina powder is usually used as its powder. According to the requirements of the target performance of the corundum refractory castable, the particle distribution coefficient is reasonably selected as the matching particle ratio of the formula. In combination with the active fillers in the system, SiO2 powder (often called silica fume) or active α-Al2O3 powder is commonly used.
According to application requirements, corundum refractory castables can generally be formulated with cement (LCC and UL-CC) or non-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, high-efficiency surfactants (high-efficiency dispersants and water-reducing agents) are added to disperse the binding agent 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% 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%.
The mechanism of active filler is very complicated, but its fundamental mechanism is to play a filling role. Because of the particle size gradation of refractory castables, the bulk density is larger and denser, but there are still many pores. If these pores are filled with active fillers (ultra-fine powder), the pores can be greatly reduced, thereby significantly reducing the water consumption of the refractory castable, and greatly reducing the pores left by the cast member or refractory lining after drying (baking) cut back. In other words, 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 powder is made by calcining industrial alumina. It is characterized by good dispersibility, small particles, easy sintering at high temperature and small volume effect.
The addition of α-Al2O3 into refractory castables has a significant impact on its construction performance. Adding an appropriate amount of α-Al2O3 micropowder to the castable, on the one hand, it will undergo ceramization and mullite reaction at high temperatures, which can increase the refractoriness of the refractory castable; on the other hand, it can fill the powder and reduce the castable. The porosity of the castable reduces structural defects in the castable, improves its strength and resistance to slag erosion, and improves the performance of refractory materials. At the same time, the alumina powder can be sintered with the cement particles to play an auxiliary role in connecting the surrounding particles to form a network structure. The more α-Al2O3 powder is added, the lower the vibration fluidity of the castable will be. But when the amount of micropowder added exceeds a certain value, the strength of the castable also has a downward trend. This is because after adding an excessive amount of Al2O3, in addition to part of the role of filling pores and reducing construction water consumption, the remaining part is optimized to react with the cement in the castable to form CA2 and CA6, etc., which not only consumes a large amount of Al2O3 in the matrix, but also Accompanied by volume expansion, there are structural defects after the castable comes out of high temperature, resulting in a corresponding decrease in strength and other properties.
In refractory castables, SiO2 micropowder can increase 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 influence of SiO2 powder on the fluidity of the castable is shown in Figure 1. When amorphous SiO2 particles cover the surface of large particles, it can not only hinder the agglomeration of larger particles, but also play a lubricating effect between large particles, thereby effectively reducing the frictional resistance between large particles. Therefore, adding SiO2 powder to the castable can significantly improve the fluidity of the castable.
SiO2 powder can avoid the formation of C2AH8 during the hydration process of calcium aluminate cement, and directly generate calcein C2ASH8, thereby improving the normal temperature strength of the castable.
At present, there are two main types of SiO2 powder used in refractory castables: one is made of high-purity silica, and the other is a by-product of metallic silicon or ferrosilicon. Both of these products are amorphous amorphous materials. The former is active in granular form; the latter is active in hollow spherical form, without agglomeration and good filling properties. After being mixed with the castable and condensed, silanol groups are formed on the surface of SiO2, which are dried and dehydrated to form a siloxane network structure, which is not easy to break when the temperature rises, so the medium-temperature strength of the castable can be improved.
Jia Quanli and others studied the influence of SiO2 powder content on the sintering performance, high temperature strength and thermal shock resistance of corundum ultra-low cement castables. Research shows that with the increase of SiO2 powder content, the apparent porosity of samples sintered at 1100℃ decreases and the strength increases; the thermal flexural strength of castable samples increases after sintering at 1400℃. This shows that SiO2 powder can not only promote the sintering of the castable, but also react with Al2O3 to form mullite. Because the crystalline structure of mullite is needle-like, it can intersect with the corundum aggregate to strengthen and toughen. The strength and thermal shock resistance of the castable can be improved.
At the same time, studies have shown that SiO2 micropowder can fill the gaps between particles, significantly reducing the space occupied by the dispersion medium. For example, within a suitable range of adding SiO2 powder, adding 2wt% of SiO2 powder in the castable will reduce the amount of water added by 1wt%. Therefore, the SiO2 micropowder, with its own good filling performance, reduces the amount of water added to the castable, increases the density of the castable, and improves the mechanical properties of the refractory castable.
In the corundum refractory castable, the amount of binder, active filler and additive is very small, but they are all three very important complementary components and indispensable. The choice of each component has become a key factor in controlling the rheological properties of corundum refractory castables. The selection criterion is to ensure that the corresponding corundum refractory castable meets the requirements of construction performance. This can be achieved by optimizing additives. For example, a variety of additives (composite additives) can be added, each of which has a different function, and a small amount of oxide refractory materials such as Cr2O3, ZrO2, etc. can also be added to change Performance of corundum refractory castables.
