Application of Metakaolin in Refractory Castables

Unshaped refractories refer to refractory materials that are composed of refractory aggregates, powders, binders, and additives of specified particle sizes. These materials do not require firing and can be used directly, offering advantages such as simplified production processes, shorter production cycles, energy savings, strong adaptability, and suitability for mechanized construction. As a result, they have seen rapid growth in the refractory industry. The development of unshaped refractories has expanded from the foundation of refractory castables. Also known simply as castables, refractory castables are currently the most widely produced and used type of unshaped refractory material. They are used to construct linings for various heating furnaces and can also be used in smelting furnaces and pre-cast components. Compared with other unshaped refractories, they have higher binder and water content, better fluidity, and can be customized in terms of material and binder based on application requirements to enhance product performance. The addition of active micro-powders has continuously improved the quality of refractory products. In recent years, pozzolanic metakaolin has attracted increasing attention from researchers. This paper provides a brief overview of metakaolin, its activation mechanisms, and research advancements in its use as a binder replacement in refractory castables. It concludes that using metakaolin in castables yields significant social and economic benefits.

Application of Metakaolin as a Binder in Castables

A binder in refractory castables refers to substances added to improve workability and provide green or dry strength. Refractory castables require binders with good dispersibility, lubricity, and high curing strength, as the density and performance of the castable largely depend on the proper choice of binder. Although the cement content in modern refractory castables has been decreasing, high-alumina cement remains a primary choice, especially pure calcium aluminate cement with around 70% Al₂O₃ content. In castables, pure calcium aluminate cement interacts with various fine and ultra-fine powders in the matrix, with active alumina and silica micro-powder commonly used as fillers. Active Al₂O₃ accelerates the hydration of pure calcium aluminate cement, while silica micro-powder slows down dissolution, extending the setting time. In castables used for ladle linings, adding alumina micro-powder and silica micro-powder significantly improves flowability, reduces water demand, enhances sintering properties, and greatly improves mechanical strength. However, these additives increase costs, and silica micro-powder can segregate in the mix, while alumina micro-powder increases the castable’s thermal expansion coefficient. Researchers have made progress in replacing micro-powders with metakaolin in castables, achieving notable results. The main findings are summarized below:

1. Replacing Silica Micro-Powder with Metakaolin

Silica micro-powder fills the gaps between cement particles and forms gels with hydration products, reacting with basic materials such as magnesium oxide to produce gel structures. In refractory castables, adding appropriate amounts of silica fume can enhance compressive strength, flexural strength, durability, density, and cost-effectiveness. When combined with Al₂O₃, it promotes the formation of the mullite phase, improving high-temperature strength and thermal shock resistance. Comparing high-alumina castables prepared with metakaolin filler to those with silica micro-powder, researchers evaluated processing characteristics, microstructure formation, and mechanical properties. Results showed that above 1300°C, castables with metakaolin exhibited lower flexural strength than those with silica fume. However, reducing the matrix content in the castables minimized shrinkage and prevented macroscopic cracks. Although fully replacing silica fume decreased high-temperature performance, partial replacement significantly reduced costs while maintaining acceptable strength. For refractory castables with approximately 70% Al₂O₃ content, where high operating temperatures are unnecessary, metakaolin replacement is deemed ideal.

2. Replacing Al₂O₃ Micro-Powder with Metakaolin

In calcium aluminate cement-bonded castables, adding silica powder can adversely affect mechanical properties, whereas substituting silica micro-powder with active alumina has been successful. Active alumina can also form ceramic bonds with other matrix components. In many cases, a combination of silica micro-powder and active alumina significantly improves castable performance. In mullite castables for ladle linings, replacing alumina micro-powder with 4%–8% ultra-fine metakaolin produced castables with higher matrix mullite content, strength, and porosity than castables with 1%–3% alumina micro-powder. This substitution significantly reduced micro-powder costs. Further experiments exploring ultra-fine metakaolin derived from coal-series kaolin showed that adding finely milled, calcined coal-series metakaolin achieved high-mullite content, low-expansion, high-strength castables aligned with performance requirements. When comparing alumina micro-powder and metakaolin, samples with metakaolin required more water but demonstrated better cohesion and water retention. Adding 8% ultra-fine metakaolin costs about one-fourth of adding 3% alumina micro-powder, making metakaolin a preferable choice in terms of performance and cost.

3. Replacing High-Alumina Cement with Metakaolin

High-alumina cement, also known as bauxite cement, is produced by calcining and finely grinding bauxite and limestone in specific ratios, resulting in an alumina-rich cementitious material, also called calcium aluminate cement. Phosphate binders are also common, bonding primarily through chemical reactions and adhesion. At room temperature, phosphate binders adhere to material particles, forming a low-strength gel film that strengthens as water evaporates and chemical bonding increases with temperature. Using metakaolin-based phosphate cement materials, experiments demonstrated that with 15% metakaolin, aluminosilicate refractory materials exhibited excellent high-temperature performance and structural stability, with compressive strength reaching 79.4 MPa after 3 hours at 1300°C, and linear shrinkage reduced to 0.1%, compared to 1.95% for the high-alumina cement sample. After 3 hours at 1370°C, the high-alumina cement sample showed melting and bubbling, while the metakaolin-based sample surface remained smooth, supporting the conclusion that 15% metakaolin addition yields superior high-temperature properties and stability.

Conclusion

China has abundant kaolin resources, with reserves exceeding 10 billion tons in coal mining areas. Recognizing the potential of metakaolin in high-tech and high-value applications offers effective resource utilization. Although research on metakaolin began later in China, initial studies on its strength performance show promising results for refractory castables. As binder and additive types continue to evolve, castables are expected to become more diverse, perform better, and increasingly use metakaolin micro-powders, replacing cement in refractory castables. Cost-effective and high-quality metakaolin holds substantial social, economic, and application potential, driving faster, more sustainable industrial growth.

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