In industries such as metallurgy, cement, glass, and chemicals, castables have extensive applications. The development of high-performance castables is particularly important. Aluminum-magnesium castables with excellent high-temperature and erosion-resistant properties are commonly used in steel ladles, RH refining furnaces, cement kilns, and glass furnaces. They can dissolve a significant amount of harmful elements like Fe and Mn from steel slag without damaging their own structure. When using active alumina and magnesia as raw materials, their hydration process can mutually promote each other, and the micro-porous structure formed can eliminate expansion caused by spessartine formation at high temperatures, thereby optimizing its volume stability. The particle size distribution, morphology, and properties of the raw materials greatly affect the performance of the castable slurry, green body, and post-firing. Castables are actually a composite material consisting of a matrix of fine powder with smaller particle sizes, encasing larger aggregate particles. To facilitate molding, the castables must have excellent working properties such as fluidity and thixotropy during the casting process. After curing and firing, they must also meet the requirements of the usage environment, often requiring low porosity and high strength.

The main factors affecting the performance of casting materials are as follows:

(1) Terms of Use and Water Ratio

For the majority of casting materials, after mixing the dry powder, water must be added to achieve liquidity, ensuring they can be cast and molded on-site. The strength of casting materials before heating primarily relies on a complex hydration and dehydration process, making water a necessary factor for binder strength and an important carrier in casting materials. Casting materials often consist of a variety of complex components, which determines their highly complex microstructure. Their ultimate physical and chemical properties are closely related to the application environment and temperature.

The amount of water added can alter the construction, rheological, and mechanical properties of the casting material. Both excessive and insufficient water additions fail to yield optimal performance in the casting material. Insufficient water leads to incomplete hydration of the binder, resulting in a lower flow value of the slurry. Conversely, too much water reduces the casting material's viscosity, causing segregation of aggregates, leading to severe stratification and an ultimately uneven internal structure. Excessive water, upon baking and sintering, results in the formation of a large number of pores within the casting material due to the evaporation of free water and the separation of bound water, which weakens its strength and resistance to slag erosion. Therefore, while ensuring the casting material meets operational requirements, the amount of water added should be kept to a minimum to avoid explosions during the heating process and to prevent the formation of an excessive number of pores.

Raw Materials and Particle Size Distribution

Water added to the casting material typically resides in the gaps between particles. To reduce water usage in the casting material while maintaining its fluidity for operational requirements, fine powder within the matrix can be used to encase large aggregate particles, increasing the distance between coarse particles to act as a lubricant and alter the particle packing density. This not only improves the fluidity of the casting material but also prevents the occurrence of segregation. After adding water, uniform mixing is essential, which requires overcoming the van der Waals forces and capillary forces between fine powders; this also relates to the casting material's construction performance when in use, meaning the casting material's operational performance largely depends on the particle gradation of its dry powder. Larger aggregate particles, although they do not provide the same lubricating effect as fine powder to optimize fluidity, are cost-effective and can enhance the mechanical properties of the post-burn casting material and reduce linear expansion at higher temperatures. The presence of aggregate also provides channels for internal moisture evaporation, preventing cracking and damage to the product during heating and temperature rise.

To explore the prediction of slurry fluidity during construction and the mechanical strength after sintering through particle gradation, researchers have established numerous mathematical models. They found that, compared to water addition, the specific surface area of the matrix powder plays a dominant role in the performance of the casting material. In coarse-grained casting materials, microcracks are more prone to form during sintering due to the mismatched shrinkage between coarse and fine particles. This is the predominant way microcracks form in casting materials. Replacing medium-grained particles with coarse and fine particles will result in more microcracks.

The amount and particle size of cement significantly affect the performance of the casting material. In recent years, low-cement casting materials (LCC) and no-cement casting materials have been hot topics of research. For cement, a lower particle size is also more conducive to the final usage performance of the casting material.