The 2 Materials That Define Corundum Refractory Castable Quality

Corundum Refractory Castables

In complex refractory systems, especially in unshaped refractory materials, the factors that determine the material's performance during later use include not only the quality of raw materials but also the type and quality of binders and additives.

The main role of the binder is to bind the particles together to form a cohesive product. Additives in refractory materials generally serve to regulate the composition of the physical phases, refine the microstructure, and improve overall performance.

The following presents the commonly used binders and additives in the production of corundum refractory castables, along with their respective functions, for reference:

01 Binding Agent

The differences between various types of bonding agents lie mainly in the type of bond they form, such as "carbon bonding," "silicate bonding," "phosphate bonding," "mullite bonding," "cement bonding," and so on.

In-situ bonding systems for amorphous refractories have always been at the forefront of bonding system development. For amorphous refractories and many other products, consideration is given not only to the bonding strength of the agent but also to its phase changes during heating, its reactions with bonded particles at high temperatures, and its influence on phase composition and microstructure. Therefore, it is often necessary to select a suitable bonding agent based on the specific system and service environment.

Bonding Agents Commonly Used in Corundum Castables：

Hydration bonding, chemical bonding, cohesion bonding, adhesion bonding, and high-temperature liquid phase bonding are all common types of bonding in corundum-based castables, with each corresponding to a specific bonding mechanism.

Hydration bonding is one of the most common types. In the presence of water, the hydratable phase of the binder reacts exothermically at room temperature to form various hydration products, leading to coagulation and hardening, which results in strength development. Common binders of this type include ρ-Al₂O₃ and aluminate cements.

Chemical bonding occurs when binders such as phosphates, water glass, and others chemically react with the base material or additives (e.g., coagulants) to form a binding phase. For example, aluminium phosphate–bonded corundum castables exhibit high strength, excellent abrasion resistance, a high load softening temperature, and strong slag erosion resistance, making them widely used in hot blast furnaces.

In solid aluminium phosphate–bonded corundum castables, the addition of MgO as a hardener can shorten the setting time and improve workability during construction.

Cohesive bonding refers to the combination formed by adding a coagulant or adjusting the pH value, which causes fine solid particles in a solid–water suspension system to coalesce. The most common example of cohesive bonding is the use of silica sol as a bonding agent. Other types of colloidal or suspension-based bonding agents also fall into this category.

After heating to 1100 °C, corundum castables bonded with silica sol show superior strength compared to those bonded with calcium aluminate cement. They also exhibit excellent resistance to rapid temperature changes, molten steel and slag erosion, and maintain high strength at medium temperatures.

The SiO₂ nanoparticles in the silica sol fill the gaps between the particles, reducing void size. At high temperatures, a liquid phase forms, which helps relieve stress at crack tips and significantly improves the material's ability to withstand mechanical stress and rapid thermal changes.

Adhesive bonding involves joining solids through physical or chemical interactions at low temperatures. This includes physical adsorption (e.g., Van der Waals forces), chemical adsorption, bonding through membrane interconnection via penetration and diffusion of the binder on the particle surface, and bonding through electrostatic attraction at the interface between the binder and particles.

High-temperature liquid phase bonding, often referred to as ceramic bonding, is another important mechanism. Common binding agents used for this purpose in corundum refractories include boron anhydride (B₂O₃) and boric acid.

At low temperatures, B₂O₃ forms a liquid phase that binds the corundum particles. At high temperatures, it reacts to form 9Al₂O₃·2B₂O₃, a compound with a high melting point that strengthens the bond. Another common ceramic bond is the Sialon bond. Corundum bricks produced with this method exhibit excellent resistance to alkali metal attack, high strength, thermal shock resistance, and oxidation resistance, making them suitable for blast furnace linings.

Adding a small amount of flux during preparation can induce controlled linear expansion, allowing Sialon-bonded castables to meet the performance requirements of permeable bricks.

02 Additives

Additives can be classified in several ways. Based on their chemical composition, they can be divided into organic and inorganic additives.

(1)Inorganic additives include inorganic salts, minerals, inorganic electrolytes, oxides, and hydroxides.

(2)Organic additives are generally surfactants. Surfactants, also known as water-reducing agents, include ionic (anionic and cationic) types, polymer-based water reducers, and organic acid-based water reducers.

According to their function and efficacy, additives can also be classified as follows:

(1)Additives used to improve the workability of the casting slurry, including flocculants, anti-flocculants, plasticizers, and water-reducing agents (dispersants).

(2)Additives that modify the setting and hardening speed, typically including accelerators and retarders. Accelerators can be further divided into early- and late-stage types.

(3)Additives that alter the internal structure, such as air-entraining agents and defoamers.

(4)Additives that enhance construction performance, including water-retaining agents, anti-settling agents, and preservation agents.

Additives Commonly Used in Corundum Refractory Castables

(1) Water Reducers

Due to various factors-such as the different charges of cement hydration products, van der Waals forces between fine powders, and other interactions-fine particles in castables can form a flocculated structure. In this structure, the fine particles trap free water, increasing the overall water demand of the castables. Generally, the smaller the particle size, the more water can be trapped, and the easier it is for flocculation to occur, thereby increasing the water demand of the slurry.

The primary function of water-reducing agents is to break up this flocculated structure through electrostatic repulsion (electric double-layer protection) and steric hindrance (hydration film), releasing the trapped water and significantly reducing the water requirement. During the mixing process, dispersants are added to achieve water reduction. However, while reducing the water content, it is important that the additive does not negatively impact the early strength through its effect on coagulation and hardening. Therefore, the type and dosage of the water-reducing agent must be carefully selected according to actual conditions.

(2) Coagulants and Retarders

Coagulants (also known as accelerators) are used to speed up the setting and hardening of castables, thereby shortening the required setting time. If the setting effect occurs with a delay rather than immediately upon addition, it is referred to as a delayed coagulant or delayed accelerator.

Retarders, on the other hand, are used to prolong the setting and hardening process. Both accelerators and retarders fall under the category of setting control additives but function in opposite ways.

Conditioning agents in castables generally act on the binder, so different types of binders and bonding mechanisms require corresponding additives to achieve optimal performance. Cement bonding is the most commonly used bonding method in corundum castables; thus, many coagulants are designed specifically for hydration bonding.

Common accelerators include lithium salts, Ca(OH)₂, Na₂CO₃, K₂CO₃, and Na₂SiO₃. Common retarders include citric acid and its salts, phosphoric acid and phosphates, acetic acid, carboxylic acids, Mg(OH)₂, Ba(OH)₂, Na₂SO₄, NaCl, starch, sugar, seawater, etc.

The effects of these additives on setting and hardening are quite complex and not yet fully understood. Their effectiveness is influenced by additive structure, chemical properties, cement composition, and hydration conditions. Therefore, practical application requires a case-by-case selection to ensure optimal casting conditions.

(3) Mineralising Agents

In castables, small amounts of mineralising agents are often added to promote sintering and enhance certain properties of the final product. These agents are typically used to control phase formation or chemical reactions during firing.

TiO₂ is a common mineralising agent in the synthesis of magnesium-aluminium spinel. Studies have shown that in alumina–magnesia castables, TiO₂ promotes sintering and facilitates the formation of CA₆ (CaAl₁₂O₁₉). Its use in corundum castables has also been reported.

For example, in calcium aluminate cement-bonded corundum castables, the addition of MgCl₂ promotes CA₆ formation. The voids created by MgCl₂ decomposition provide space for CA₆ crystal growth, which helps reduce expansion during processing and leads to larger grain sizes and improved volume stability.