Unshaped refractories have advantages over shaped refractories in production, construction, energy saving and environmental protection. Their output and application proportion are important indicators for evaluating the technical development level of the refractory industry. Countries with advanced refractory technology, such as Japan, took unshaped refractories as key research objects as early as the early 1990s, and the industrial output of unshaped refractories accounted for 60% of the total refractory production at that time. Although China ranks first globally in the output of unshaped refractories, its application proportion is still much lower than that of developed countries, showing great development potential.
Refractory castables are the most widely used unshaped refractories, which are extensively applied in industrial fields including petroleum, chemical industry, steel, building materials and various thermal equipment. According to material types, they can be classified into high-alumina castables, magnesia castables, alumina-magnesia castables, etc. Generally, they consist of four components: aggregates, fine powders, binders and additives. Featuring high production efficiency, simple construction and convenient later repair, castables have been increasingly adopted in industry. They can be delivered in dry form and cast on-site after mixing with water or other liquids, suitable for kilns with complex structures.
Industrially, unshaped refractories with Al₂O₃ ≥45% are defined as high-alumina unshaped refractories. High-alumina castables are divided into three grades by Al₂O₃ content: Grade 1 with Al₂O₃＞75%, Grade 2 with Al₂O₃ ranging from 60% to 75%, and Grade 3 with Al₂O₃ between 48% and 60%. Their main mineral phases are corundum, mullite and glass phase. Traditional high-alumina castables have application limitations. With large cement addition, although high mechanical strength can be achieved at medium and low temperatures, increased CaO content reduces the refractoriness under load and deteriorates thermal shock resistance. Besides, water addition has a remarkable influence on the early strength of castables. For this reason, high-alumina castables are developing towards low-cement and cement-free castables. Low-cement castables have lower calcium content, which reduces the formation of low-melting eutectic phases. They improve refractoriness, thermal shock stability, slag erosion resistance and high-temperature strength, densifying the structure and reducing porosity.
The performance of high-alumina castables is affected by many factors, mainly raw material composition and construction conditions. In terms of raw materials, material properties can be optimized by adjusting the morphology, particle size and components of aggregates as well as binder types, together with appropriate micro-powders and admixtures. For construction, parameters such as mixing water content and baking schedule also affect service performance. This paper reviews recent domestic and international research on performance optimization of high-alumina castables, providing references for further studies of researchers in this field.
1.Research on Optimization of Aggregates in Refractory Castables
China is rich in bauxite resources, which are commonly used as the main aggregate for high-alumina castables. Driven by national policies for energy conservation and environmental protection, high-alumina castables are developing toward high efficiency and energy saving. The core approach is aggregate optimization: replacing heavy aggregates with lightweight aggregates, modifying medium-low grade bauxite, or comprehensively utilizing industrial waste to satisfy the production requirements of high-alumina castables. Such optimization can ease the shortage of high-grade bauxite reserves, improve resource utilization efficiency, and enhance various properties of high-alumina castables.
1.1 Effect of Fused Ceramsite and Bauxite Composite Aggregates on Castable Properties
With fused ceramsite (industrial waste) and bauxite as composite aggregates, super-grade bauxite powder, silica fume and reactive alumina micro-powder as fine powders, and calcium aluminate cement as binder, the effect of fused ceramsite addition (0, 10%, 19% and 28%) on the flowability and room-temperature physical properties of high-alumina castables was investigated. The results show that with the increase of fused ceramsite content, the water demand of castables decreases, flowability rises and bulk density reduces. After heat treatment at 1100 ℃ and 1450 ℃ respectively, the cold crushing strength and cold flexural strength of castables generally tend to increase, while thermal shock resistance rises first and then declines.
1.2 Effect of Homogenized Bauxite Aggregate on Castable Properties
Homogenized bauxite was prepared from medium-low grade bauxite via homogenization and purification. High-alumina castables were fabricated with homogenized bauxite and conventional bauxite clinker as aggregates respectively, and their performances were compared. The research indicates that the physicochemical properties of homogenized bauxite are less affected by particle size, showing better uniformity and stability than bauxite clinker. The high-alumina castable using homogenized bauxite aggregate contains well-crystallized mullite and corundum as main mineral phases with improved bonding inside the matrix. It delivers superior high-temperature mechanical properties and wear resistance compared with the castable based on bauxite clinker aggregate.
1.3 Effect of High-Strength Microporous Bauxite Aggregate on Castable Properties
The practical performances of high-alumina castables prepared by two types of high-strength microporous lightweight bauxite aggregates with different bulk densities were studied and compared with castables using ordinary lightweight bauxite aggregate. It is found that the lower the density of microporous lightweight bauxite aggregate, the lower the bulk density and the higher the apparent porosity of the resultant castable. Compared with ordinary lightweight bauxite aggregate, microporous bauxite aggregate significantly improves the flexural strength and crushing strength of castables after high-temperature treatment, while imposing minor influence on flexural strength under medium and low temperature. The thermal conductivity of high-strength microporous bauxite aggregate at 500 ℃ is slightly higher than that of ordinary lightweight bauxite aggregate, and the difference becomes small as temperature increases, which relates to the porosity variation of castables. High-strength microporous bauxite aggregate helps maintain volume stability and reduce linear shrinkage. Overall, increasing the Al₂O₃ content and micropore quantity of lightweight bauxite aggregate can greatly optimize castable performance.
1.4 Effect 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 explored. The results demonstrate that when multi-phase hollow spheres (0, 10%, 20%, 30% and 40%) gradually replace lightweight mullite, as the replacement ratio increases, the water demand of castables decreases, the cold crushing strength of high-alumina castables is remarkably improved, and the bulk density and thermal conductivity are reduced.
1.5 Effect of Alumina-Andalusite Aggregate on Castable Properties
The effect of andalusite addition on the properties of high-alumina castables was investigated by adding andalusite into high-alumina castables. The research shows that with rising andalusite content, andalusite undergoes in-situ reaction to form mullite. Volume expansion induces stress and microcracks inside the matrix, reducing tensile strength. The newly formed mullite also triggers thermal expansion mismatch between surrounding aggregates and the matrix, lowering the flexural strength of castables. Nevertheless, the crack propagation ductility is improved, which benefits the thermal shock resistance of high-alumina castables.
2.Research on Optimization of Fine Powders in Refractory Castables
Various fine powders are normally incorporated into castables to form the matrix under the combined effect of binders and additives. The particle size and dosage of fine powders affect the matrix structure. Since the matrix is most vulnerable to medium erosion, it tends to fail preferentially under mechanical loading. Therefore, it is necessary to optimize the state of fine powders to facilitate better reactions with other components in the matrix. After high-temperature treatment, fine powders can bond aggregates and fill voids, achieving dense packing and promoting sintering, so as to improve the room-temperature physical properties and service performance of high-alumina castables. The fine powders added into high-alumina castables mainly include silica fume, silicon carbide micro-powder, aluminum powder and alumina micro-powder.
2.1 Effect of Silica Fume on Properties of Castables
Silica fume is a common modified powder for high-alumina castables. Appropriate addition improves creep resistance and microstructure, while excessive dosage reduces flowability and increases firing deformation. Heat treatment atmospheres greatly affect performance. Oxidizing and reducing atmospheres promote stable crystalline phases and high-temperature strength, and nitriding atmosphere optimizes room-temperature strength and volume stability.
2.2 Effect of SiC Micro-powder on Properties of Castables
SiC micro-powder improves the compactness and thermal shock resistance of castables. Moderate addition enhances room-temperature strength and pore structure, while excessive SiC increases water demand and weakens medium-low temperature performance. SiC forms protective films at high temperature to improve erosion resistance, though ultra-high temperature will destroy the protective layer. The optimal dosage is around 3%.
2.3 Effect of Fly Ash Micro-powder on Properties of Castables
As a common industrial solid waste, fly ash can optimize high-alumina castables. It refines pore structure and reduces thermal conductivity, greatly improving thermal insulation performance. Although higher fly ash content slightly decreases bulk density and room-temperature strength, it effectively promotes energy-saving performance for thermal insulation refractory applications.
2.4 Effect of SiO₂ Micro-powder on Properties of Castables
SiO₂ micro-powder possesses water-reducing and densifying effects. Proper dosage improves flowability and mechanical strength, while excessive addition causes powder agglomeration and poor workability. Finer particle size benefits rheological properties. The dosage of 6% achieves the best balance of strength, compactness and wear resistance.
2.5 Effect of Andalusite on Properties of Castables
Andalusite decomposes and forms mullite with micro-expansion at high temperature, which restrains high-temperature shrinkage and improves thermal shock resistance. Moderate microcracks relieve thermal stress and enhance structural stability under alternating high-temperature conditions.
2.6 Effect of Kyanite on Properties of Castables
Kyanite produces volume expansion during high-temperature mullitization, offsetting sintering shrinkage and optimizing volume stability. It has little influence on room-temperature performance but significantly improves high-temperature strength and thermal shock resistance within a reasonable dosage range.
3.Research on Optimization of Binders in Refractory Castables
At present, the common binder systems for high-alumina castables include calcium aluminate cement, silica fume, ρ-Al₂O₃ micro-powder and sol binders, each with unique characteristics. Traditional cement bonding introduces calcium oxide, which deteriorates the high-temperature performance of castables. Ultra-low cement systems effectively reduce such adverse effects. Replacing cement with silica fume and hydrated alumina can significantly improve the high-temperature strength attenuation of traditional castables.
The cement-free ρ-Al₂O₃ bonding system contains no impurity elements and exhibits excellent high-temperature stability. However, its low hydration activity requires compound modification with other powders and additives to balance construction performance and mechanical strength. As an advanced cement-free system, sol bonding produces few hydration products and introduces no impurities. It relies on colloidal gelation to bond particles, reducing high-temperature expansion and improving volume stability.
Comparative studies show that cement-bonded castables feature fast setting and excellent medium and low-temperature strength, but low-melting phases formed at high temperature degrade high-temperature performance. Amide-based binders deliver outstanding high-temperature flexural strength and slag resistance compared with cement binders. Composite bonding of cement and ρ-Al₂O₃ achieves balanced performance in low-temperature strength, high-temperature mechanical properties and refractoriness under load.
Nano-silica bonding refines pore structure, reduces porosity and improves bulk density and compressive strength. The optimized pore structure resists melt penetration and enhances corrosion resistance and service life. Silica gel and silica-alumina gel binders show obvious dosage dependence, with excessive addition leading to loose structure and degraded performance.
Compared with silica sol, nano sols of alumina, mullite and spinel provide better mechanical properties, penetration resistance and thermal shock resistance, avoiding the thermal instability caused by free silica.
Although ultra-low cement and cement-free systems have developed rapidly in recent years, calcium aluminate cement is still the most widely used binder due to its fast hardening and high early strength. Sol bonding possesses great application potential, but its gelation process is sensitive to service conditions and usually requires coagulants to improve green strength, requiring further optimization for industrial promotion.
4.Optimization Research on Additives for Castables
Various trace additives can be applied in high-alumina castables. Water reducers improve workability, setting accelerators adjust hardening speed, and expansion modifiers optimize internal structure. A small dosage of additives can greatly enhance the overall performance of castables.
Comparative tests of dispersants prove that polycarboxylate delivers superior water reduction via electrostatic repulsion and steric hindrance. Castables with polycarboxylate achieve the highest mechanical strength after high-temperature heat treatment, yet coagulants are required to secure green strength. Sulfonated melamine-based superplasticers increase flow value. Within the optimal dosage range, bulk density and room-temperature strength rise with the increase of the superplasticer.
Setting accelerators reduce the Zeta potential of slurry and weaken inter-particle repulsion to promote agglomeration. Calcium chloride shows the strongest regulation effect and is more suitable for this high-alumina castable system.
Water reducers, setting accelerators and retarders are key admixtures. Water reducers cut mixing water, reduce porosity after firing and boost strength. Selection must balance water reduction and cement hydration to guarantee early strength. Setting regulators affect binder efficiency through different mechanisms, and Zeta potential measurement directly reflects their effect. Appropriate admixtures should be selected in practice to optimize construction and product performance.
5.Prospect
High-alumina castables are widely used monolithic refractories. Aggregates, fine powders and binders interact jointly and dominate material properties. Future research directions are listed below:
(1)Optimize the synergistic formula of aggregates, fine powders and binders;
(2)Develop low-cement and cement-free systems. New binders including micro-powders, sols and non-silica sols will gradually replace cement binders, with priority given to early strength improvement;
(3)Adopt lightweight hollow sphere aggregates combined with nano binders and additives to reduce thermal conductivity while retaining high strength;
(4)Further improve construction and hardening performance of sol-bonded cement-free high-alumina castables to realize large-scale industrial application.