The setting and hardening of low cement refractory castables is mainly due to coagulation and bonding. Therefore, the role of ultrafine powder is crucial, being a key factor determining and influencing its performance. At the same time, the impact of admixtures on its performance cannot be ignored.
NO.01 Ultrafine Powder
The ultrafine powders in low cement castables mainly include active silica fume, α-Al₂O₃ powder, and Cr₂O₃ powder, with content (%) of 93.2%, greater than 90%, and greater than 99%, respectively. The particle size distribution of these three types of ultrafine powders is shown in the table below. The table shows that particles smaller than 1.0 mm account for more than 71%.
The refractory castables have the same mix proportion, with a CaO content of approximately 0.6%. Three groups of samples were prepared by adding active SiO₂, α-Al₂O₃ (α-alumina powder), and a composite of both ultrafine powders, respectively, in the same amount. As the heating temperature increases, the strength of castables with different ultrafine powders all increase.
It is also observed that different ultrafine powders contribute differently to the strength of the castables. The castable with an equal amount of active SiO₂ and α-Al₂O₃ composite ultrafine powder exhibits the highest strength, followed by the castable with active SiO₂ ultrafine powder, while the castable with alumina ultrafine powder has the lowest strength. At a heating temperature of 1500℃, the strength of castables with the three types of ultrafine powders is basically similar. This means that when preparing low cement refractory castables, composite ultrafine powders are best, and when used alone, active SiO₂ ultrafine powder should be preferred.
However, increasing the amount of SiO₂ ultrafine powder will reduce the Al₂O₃ content in the castable and increase free quartz, inevitably leading to a decrease in the slag resistance of the castable. For example, the mix proportion of the castable refractory for iron troughs is: 70% high-alumina aggregate, 14.2% SiC, 5.8% C, 0.2% dispersant, 6.5% water, and 10% high-alumina powder and SiO₂ ultrafine powder combined. Slag resistance tests were conducted using the crucible method under a reducing atmosphere. Test conditions: standard slag basicity 1.105, heating temperature and holding time 1500℃, 4h. With increasing SiO₂ ultrafine powder content, there is an optimal value for slag resistance; that is, the best slag resistance is achieved when the ultrafine powder content is around 5%.
With the castable mix proportion and the combined content of refractory fine powder and ultrafine powder remaining constant, the compressive strength after firing at 1600℃ also increases with increasing ultrafine powder content, but there is an optimal value. With SiO₂ ultrafine powder accounting for approximately 5%, and Al₂O₃ and Cr₂O₃ ultrafine powders accounting for approximately 7%, the strength is good, and other properties are also excellent. In terms of ultrafine powder type, SiO₂ ultrafine powder has the best reinforcing effect, followed by Al₂O₃ ultrafine powder, while Cr₂O₃ ultrafine powder has a poor reinforcing effect. It is also observed that the reinforcing effect of SiO₂ ultrafine powder is 2.5 to 4.4 times higher than the latter two types.
NO.02 Admixtures
There are many types of admixtures. Here, we take dispersants and water-reducing agents as examples to illustrate their impact on the performance of low cement refractory castables.
When the mix proportion of the castable is constant, adding different amounts of dispersant can reduce the amount of construction water required. When the amount of construction water is constant, there is an optimal value for the dried compressive strength as the amount of dispersant increases. That is, the strength is best when the dispersant content is 0.15% to 0.2%. When no water-reducing agent is added or the dosage exceeds 0.5%, the strength deteriorates or the sample cracks. This is due to the poor fluidity of the castable and the lack of density in the molded body.
There are many types of water-reducing agents, and appropriate selection should be made through testing. After determining the mix proportion of ultra-low cement corundum castable, sodium polyphosphate, polycyanamide condensates, and naphthalene sulfonate condensates were used as water-reducing agents, and suitable dosages were screened to prepare refractory castables. Castables without water-reducing agents suffer from spontaneous agglomeration of ultrafine powders, which cannot effectively fill the pores and have extremely uneven distribution. A large amount of water is trapped in the flocs or fills the pores, resulting in increased water consumption, low bulk density, high porosity, and low strength after heat treatment, and is also unfavorable for sintering. Polyphosphates have a certain dispersing and water-reducing effect, which can, to some extent, prevent the spontaneous agglomeration of ultrafine powders, allowing them to be more fully distributed in the pores, improving water utilization and reducing water consumption by approximately 17%.
Therefore, the increased bulk density and decreased porosity of the castable, compared to the untreated castable, resulted in a 0.6-1.9 times increase in compressive strength after firing and a 1.25 times increase in high-temperature flexural strength. Agents B and C are organic high-efficiency water-reducing agents, with particularly significant dispersive water-reducing effects, achieving water reduction rates of 25% and 28% respectively. Compared to the untreated castable, their bulk density increased by approximately 3.5%, porosity decreased by 15%, compressive strength after firing increased by 1-4 times, and high-temperature flexural strength increased by more than 3.5 times. It is also evident that agent C is more effective than agent B. In conclusion, water-reducing agents must be added when preparing low cement refractory castables, and organic high-efficiency water-reducing agents should be preferred.
NO.03 Aluminum Powder
In iron trough refractory castables, metallic aluminum powder is generally added to accelerate drying and strengthen the casting. Its particle size and dosage have a significant impact on the performance of the castable and should be selected appropriately.
In Al₂O₃-SiC-C ultra-low cement castables, the smaller the particle size of the aluminum powder and the higher the ambient temperature during construction, the more vigorous the chemical reaction, the more gas is produced, and the higher the material temperature. This is beneficial for the dehydration of the castable, allowing for rapid baking; however, an excessively rapid reaction can easily lead to false setting, which is detrimental to strength. The mix proportions of the castable remain the same. Large aluminum powder particle sizes are detrimental to strength, while excessively small particle sizes offer some benefit to compressive strength during drying, but other strengths decrease. A particle size of 88-44mm results in better strength. The amount of aluminum powder used should be determined based on the performance of the refractory castable and the construction conditions; it should be used as little as possible while ensuring good venting and rapid drying.
NO.04 Additives
In Al₂O₃-SiC-C ultra-low cement castables, SiC and carbon materials should be added to improve their slag resistance and thermal stability. Experiments and usage have proven that the grade and dosage of SiC and carbon materials have a significant impact on the performance of castables and should be selected rationally. Furthermore, the grade and dosage vary depending on the size of the blast furnace and the location of application. Generally, high-quality SiC and carbon materials are used in the main iron trough or slag trough of large and medium-sized blast furnaces, while lower-grade SiC and carbon materials are used in medium and small blast furnaces; the dosage of SiC is generally 5% to 35%. Carbon materials mainly include pitch, flake graphite, electrode powder, and earthy graphite, with a dosage of 2% to 6%.
In iron trough refractory castables, SiC and carbon materials are generally added in fine powder form, with ultrafine SiC being preferred. Because this material contains SiC and carbon materials, its oxidation resistance is reduced. Carbon oxidation leaves more micropores, allowing molten iron or slag to continuously penetrate the interior, forming a decarburized layer and leading to lining damage. Adding metallic aluminum powder can improve the oxidation resistance of the castable. Experiments have shown that the combined use of metallic silicon powder, i.e., Al powder and Si powder, results in better oxidation resistance and improved strength of the castable. This is because the reaction of metallic silicon and aluminum with carbon at high temperatures to form SiC and Al₄C₃ leads to a denser microstructure and surface.
In Al₂O₃-SiO₂ low cement refractory castables, the addition of 2%–8% kyanite fine powder at high temperatures of 1200–1400℃ promotes the formation of mullite, thereby increasing its strength. This means that kyanite acts not only as an expanding agent but also as a mineralizing agent.
