Unshaped refractories offer advantages over shaped refractories in production, construction, energy conservation, and environmental protection. Their production volume and application proportion within the overall refractory materials industry reflect the technological development of the refractory materials sector.
Refractory castables are the most widely used type of unshaped refractories, primarily applied in industries such as petroleum, chemical, steel, building materials, and thermal equipment. They are classified by material into high-alumina castables, magnesia castables, and alumina-magnesia castables, typically composed of four parts: aggregate, fine powder, binder, and additives. Due to their advantages of high-efficiency production, simple construction, and ease of subsequent repair, castables are increasingly used in industry. They can be delivered dry and then poured on-site with water or other liquids, facilitating the casting of kilns with complex structural shapes.
High-alumina Unshaped Refractories
Industrially, unshaped refractories with Al₂O₃ ≥ 45% are generally referred to as high-alumina unshaped refractories. High-alumina castables are also classified into three grades based on their Al₂O₃ content: Grade 1 has an Al₂O₃ content greater than 75%; Grade 2 has an Al₂O₃ content of 60%–75%; and Grade 3 has an Al₂O₃ content of 48%–60%. Their main mineral composition is corundum, mullite, and a glassy phase. Traditional high-alumina castables have certain limitations in application. Due to their high cement content, although they can achieve high mechanical strength under medium and low temperature conditions, the increased CaO content lowers the load softening temperature and worsens the thermal shock resistance. Furthermore, controlling the amount of water added is crucial for high-alumina castables; otherwise, the initial strength of the castable will be significantly affected.
Currently, high-alumina castables are trending towards low-cement and cement-free castables. Low-cement castables have a lower calcium content than traditional high-alumina castables, which reduces the formation of eutectic phases in the material. Low-cement castables improve the refractoriness, thermal shock stability, slag erosion resistance, and high-temperature strength of traditional high-alumina castables, resulting in a denser structure and reduced porosity.
Many factors influence the performance of high-alumina castables, primarily the composition of the raw materials and construction conditions. Regarding raw material composition, the performance of the castable can be optimized by controlling the structure, particle size, component content, and type of binder of the aggregates, and by adding appropriate amounts of fine powder and admixtures. In terms of construction, factors such as the amount of water added and the baking conditions also affect the performance of the castable.
The Influence of Aggregates on the Performance of Castables
China has relatively abundant high-alumina bauxite resources, which are typically used as the main aggregate in high-alumina castables. With the vigorous development of energy conservation and environmental protection, high-alumina castables are also evolving towards high efficiency and energy saving. The most important aspect of this is the optimization of aggregates. This is achieved by replacing heavy aggregates with lightweight ones, optimizing low- and medium-grade high-alumina bauxite, or comprehensively utilizing industrial waste to better meet the production needs of high-alumina castables. This not only addresses the current shortage of high-grade bauxite reserves and improves the utilization rate of related resources, but also optimizes and improves the various properties of high-alumina castables.
The Effects of Electrofused Ceramsite Sand and High-Alumina Bauxite Composite Aggregates on Castable Properties
Using industrial waste electrofused ceramsite sand and high-alumina bauxite as composite aggregates, and premium bauxite powder, silica fume, and activated alumina powder as fine powders, and calcium aluminate cement as binder, this study investigated the effects of the addition amount of the substitute aggregate, electrofused ceramsite sand (0, 10%, 19%, and 28%), on the flowability and room-temperature physical properties of high-alumina castables. The results showed that with increasing electrofused ceramsite sand content, the water requirement of the castable decreased, flowability increased, and bulk density decreased. After heat treatment at 1100℃ and 1450℃, respectively, the room-temperature compressive strength and flexural strength of the castables tended to increase, while thermal shock resistance initially increased and then decreased.
The Influence of Alumina Homogenized Aggregate on Castable Properties
Alumina homogenized aggregate was prepared from low- to medium-grade bauxite raw materials through a homogenization and purification process. High-alumina castables were then prepared using both the homogenized aggregate and traditional bauxite clinker as aggregates, and their properties were compared. The study shows that the physicochemical properties of the homogenized aggregate are not affected by particle size, and it is more uniform and stable than bauxite clinker. Furthermore, the high-alumina castable prepared using homogenized aggregate as aggregate contains well-crystallized mullite and corundum as main mineral phases, resulting in better matrix bonding. Compared to the high-alumina castable prepared using bauxite clinker as aggregate, it exhibits better high-temperature mechanical properties and wear resistance.
The Influence of High-Strength Microporous Bauxite Aggregate on Castable Properties
The properties of high-alumina castables prepared from two types of high-strength microporous lightweight bauxite aggregates with different bulk densities were studied in practical applications and compared with those of high-alumina castables prepared from ordinary lightweight bauxite aggregates. The study found that the lower the density of the microporous lightweight bauxite aggregate, the lower the bulk density and the higher the apparent porosity of the prepared high-alumina castable. Compared with ordinary lightweight bauxite aggregate, microporous bauxite aggregate significantly improved the flexural strength and compressive strength of the castable after high-temperature treatment. However, it had little effect on the flexural strength under medium- and low-temperature treatment. The thermal conductivity of high-strength microporous bauxite aggregate at 500℃ was slightly higher than that of ordinary lightweight bauxite aggregate. However, as the temperature increased, the difference in thermal conductivity between the two became negligible, which is related to the change in porosity of the castable. High-strength microporous bauxite aggregate is more conducive to maintaining the volumetric stability of the castable and reducing the linear shrinkage rate. Taking all factors into consideration, increasing the Al₂O₃ content and micropore number of lightweight bauxite aggregate can significantly improve the performance of castables.
The Influence of Bauxite-Mullite-Hollow Sphere Aggregate on Castable Properties
The influence of bauxite-mullite-hollow sphere aggregate on the properties of high-alumina castables was studied. The results show that gradually replacing lightweight mullite with multiphase hollow spheres (0%, 10%, 20%, 30%, and 40%) reduces the water requirement of the castable as the replacement ratio increases. Simultaneously, it significantly improves the room-temperature compressive strength of high-alumina castables while reducing their bulk density and thermal conductivity.
The Influence of Alumina-Andalusite Aggregate on Castable Properties
The effect of adding andalusite to high-alumina castables on their properties was investigated. The results show that with increasing andalusite content, andalusite reacts in situ to form mullite, and the resulting volume expansion generates stress, causing microcracks within the matrix and reducing tensile strength. The formation of mullite also leads to a mismatch in thermal expansion between the surrounding aggregate and the matrix, reducing the flexural strength of the castable. However, it improves crack propagation resistance, which is beneficial for enhancing the thermal shock resistance of high-alumina castables.
In summary, in applications where temperature and castable strength requirements are not high, high-alumina castables are typically prepared using lightweight bauxite aggregate. Microporousization of high-strength bauxite aggregate can improve the refractoriness and service strength of the castable, indicating that reducing the pore size of the aggregate and ensuring its uniform distribution is beneficial for improving the strength and thermal insulation performance of the castable. Hollow spherical aggregate can also be used to optimize traditional high-alumina castables, creating uniformly distributed closed pores within the material to improve various performance characteristics.
