Refractory castables are composed of refractory aggregates and matrix powders, combined with specific amounts of binders and additives. During installation, water is added to the mixture to ensure good flowability, followed by forming and curing processes that allow the material to set and harden. Due to advantages such as high installation efficiency and simple production processes, these materials are widely used across various sectors—ranging from ironmaking facilities (blast furnaces, hot blast stoves, and coke ovens) to steelmaking operations (converters, continuous casting units, and reheating furnaces). They are also utilized in kilns for the cement and glass industries, as well as in incinerators and other applications requiring high-temperature resistance.
While the primary raw materials are crucial, the binder is an indispensable component. The function of the binder is to bond the aggregate and fine powder particles together; after a period of curing, this enables the material to achieve sufficient strength for demolding. Binders used in refractories must not only meet specific performance requirements at ambient temperatures but also ensure the castable performs well under high-temperature conditions. Furthermore, compatibility must be considered to prevent the binder from reacting with other constituents in the castable to form low-melting-point phases. A variety of binders are used for monolithic refractories, with calcium aluminate cement, sols, and hydrated alumina being among the most common.
Calcium aluminate cement-bonded castable
Calcium aluminate cement is the most widely used binder. However, its use inevitably introduces a certain amount of CaO, which reacts with other oxides in the refractory mixture to form low-melting-point phases. For instance, in systems containing silica, the CaO and Al2O3 from the cement react with the SiO2 micropowder added to the castable to form low-melting-point phases such as CAS2 and C2AS, thereby compromising the castable’s slag resistance and high-temperature performance. Consequently, low-cement (1.0% ≤ W(CaO) ≤ 2.5%) and ultra-low-cement (0.2% ≤ W(CaO) ≤ 1.0%) castables were developed to prevent liquid phase formation at high temperatures, enhance corrosion resistance and high-temperature strength, and ultimately extend service life. Although these formulations reduce CaO content and improve high-temperature performance, their reliance on cement means that their utility at high temperatures remains limited.
Sol-bonded castable
Coagulation bonding generally refers to the process where particles of similar size aggregate and bond under the influence of van der Waals forces. Sol bonding is a form of coagulation bonding. Sols serve as binders for refractory materials based on sol-gel technology; during the mixing process, a network-like flocculated structure forms, encapsulating and connecting the refractory aggregates to create a green body, thereby providing the demolding strength required for sol-bonded castables.
Silica sol is a colloidal solution consisting of silica particles uniformly dispersed in water, and bonding with silica sol is a type of sol bonding. The condensation reaction involved is -Si-OH + HO-Si- → -Si-O-Si- + H2O, which forms a network structure that provides strength. Silica sol-bonded castables offer significant advantages in terms of mixing characteristics, strength at low to medium temperatures, sintering performance, dimensional stability, and high-temperature strength. However, because these castables exhibit relatively low demolding strength after ambient-temperature curing, their widespread application is limited.
Hydrated alumina-bonded castable
Hydratable alumina (ρ-Al₂O₃) is a crystalline form of alumina characterized by poor crystallinity and the ability to hydrate upon contact with water. At room temperature, it reacts with water to form bayerite [Al(OH)₃] and boehmite sol (AlOOH). Both bayerite and boehmite sol possess binding and hardening properties, thereby imparting strength to hydratable alumina-bonded castables. At high temperatures, Al(OH)₃ and AlOOH ultimately transform into α-Al₂O₃, resulting in high-temperature strength for the castables.
When used in silica-containing systems, hydratable alumina avoids the introduction of CaO, raises the temperature at which the liquid phase forms, and consequently enhances the castable’s high-temperature strength. In silica-free systems (such as precast blocks made of alumina-magnesia castables for ladle linings), the transformation of hydratable alumina into α-Al₂O₃ at high temperatures allows it to react with MgO in the system to form magnesium-aluminum spinel—a phase characterized by high melting point and corrosion resistance—thereby improving high-temperature performance and corrosion resistance. Furthermore, hydratable alumina-bonded castables can absorb significant amounts of CaO from slag, reducing slag alkalinity; they also form a dense CA6 layer that blocks further CaO penetration into the material, thereby enhancing resistance to slag penetration.
However, hydratable alumina also has certain drawbacks as a binder; for instance, the green strength—specifically the demolding strength—of the castables is low. Additionally, during the drying and firing process, the hydration products dehydrate rapidly within a narrow temperature range. The material exhibits low apparent porosity that remains constant as the temperature rises, resulting in poor gas permeability and susceptibility to explosive spalling.
More details about refractory castable
What is a refractory castable?
Castable refractory is a heat-resistant, cement-like material supplied as a dry mix. When mixed with water, it can be poured, pumped, or sprayed into place to form durable, seamless linings for furnaces, kilns, boilers, and pizza ovens. It hardens at room temperature without requiring heat.
What are the raw materials for refractory castables?
Refractory castables are formulated from a wide range of raw materials, including chamotte, andalusite, bauxite, mullite, corundum, tabular alumina, and silicon carbide.
What is the difference between castable and refractory plastic?
The biggest difference between refractory plastics and castables is the combination method, the binder used by refractory plastics is mostly neutral materials, and these materials have a strong ability to resist acid and alkali erosion. 5. In construction, refractory plastic has more advantages.
Is refractory cement the same as castable refractory cement?
Refractory cement is also called refractory concrete. This product can be used directly or as a raw material for other unshaped products. Refractory castable are generally made of refractory aggregates, powders and binders and are constructed by pouring.
