What are the commonly used additives and binders for refractory castables? Additives are also called admixtures, which generally play a role in adjusting the phase composition, microstructure and improving performance in refractories. Additives using different classification methods the additives for refractory castables can be divided into several types:
According to the chemical composition, it can be divided into organic additives and inorganic additives.
(1) The types contained in the refractory castables inorganic additives are inorganic salts, minerals, inorganic electrolytes, oxides, and hydroxides.
(2) Refractory castables Organic additives are generally surfactants. Surfactants and water reducing agents include ionic (anionic, cationic), polymer water reducing agents and organic acid water reducing agents.
According to the function and efficacy of the admixture, it can be classified into the following types.
(1) Additives for improving the workability of castable slurry. Generally includes flocculants, deflocculants, plasticizers and water reducing agents (dispersants).
(2) Additives that can change the rate of coagulation and hardening. Generally includes coagulants and retarders, and coagulants include both fast and delayed effects.
(3) Additives that can change the internal tissue structure. For example, air-entraining agents, defoamers, and so on.
(4) Additives that can improve workability. Common are water-retaining agents, anti-settling agents and preservatives.
- Water reducing agent( refractory castables)
Cement hydration products have different charges and van der Waals forces between fines, and many other reasons may cause the flocculation structure of small particles in the castable, that is, the fine particles wrap the free water, resulting in Demand for water has increased. Generally speaking, the smaller the particle size, the larger the amount of water that can be wrapped, and the more prone to flocculation, resulting in an increase in the water demand of the slurry. The main function of the water reducing agent is to destroy the flocculent structure in the castable through electrical protection (electrostatic repulsion) and space protection (hydration film), so that the free water wrapped in the structure can be released, thereby significantly reducing the castable Demand for water.
In the construction process of the castable, a dispersant needs to be added to reduce water in the initial stage. However, while reducing the amount of water used, it cannot affect its initial strength through condensation and hardening. Therefore, the type and amount of water reducing agent must be combined with the actual situation. Li Zi et al. Explored the effects of different types and amounts of dispersants on the properties of corundum castables, and finally discovered that different types of additives have different optimal amounts
- coagulant and retarder( refractory castables)
The role of the accelerator is to accelerate the setting and hardening of the castable and reduce the time required for setting and hardening. If the accelerating effect does not manifest immediately after the addition, but it takes a period of time to have the effect of promoting coagulation, it is called a delayed-acting coagulant. The effect of retarder is to make the process of setting and hardening of the castable longer. Accelerators and retarders are both coagulants, but they are different types. Regulating agents in casting materials generally act on the binding agent, so different types and combinations of binding agents need to correspond to different setting agents in order to achieve the best results.
Cement bonding is the most common bonding method for corundum castables, so there are many coagulants used for hydration bonding. Common accelerators include lithium salts, Ca (OH) 2, Na2CO3, K2CO3, Na2SiO3 and so on. Common retarders include citric acid and citrate, phosphoric acid and phosphate, acetic acid, carboxylate, Mg (OH) 2, Ba (OH) 2, NaSO4, NaCl, starch, sugar, seawater and so on.
The accelerating and retarding effects of additives on castables are very complicated. There are many places that have not been thoroughly studied. They are related to the structure and properties of additives, and the composition of cement, hydration conditions and other factors. In applications, it is only reasonable based on actual conditions The selection can obtain better construction conditions.
- Mineralizer( refractory castables)
In castables, it is often used to add a small amount of additives to achieve the purpose of sintering and improve certain properties of the product. Mineralizers are often used to control the formation or reaction of substances. TiO2 is a mineralizer commonly used in the synthesis of magnesia-alumina spinel. The previous research results show that for aluminum-magnesium castables, the addition of TiO2 is beneficial to sintering and the formation of CA6. There are also reports of adding it to corundum castables. For example, when added to the corundum castable combined with calcium aluminate cement, MgCl2 can promote the formation of CA6, and the voids generated by the decomposition of MgCl2 provide sufficient space for the growth of CA6 to reduce its expansion during production and make its grains more Larger and more volume stable. Wu Ran et al. Added CaCO3 to the corundum castable and found that the amount of CA6 produced in the fired sample is larger and the grains are larger, but the expansion generated during the sintering process leads to an increase in the apparent porosity of the castable and a decrease in strength . The addition of 0.5wt% MgCO3 can make the β-Al2O3 phase in the corundum castable more stable, and promote the formation of a plate-like structure, so that the castable has better high temperature strength.
Hydration bonding, chemical bonding, cohesive bonding, adhesive bonding and high temperature liquid phase bonding are all common bonding types in corundum-based castables, and each bonding type corresponds to a bonding mechanism.
Hydration bonding is a common type of bonding. In the presence of water, the hydration phase in the binder can undergo an exothermic reaction with water at normal temperature, forming different hydration products, and condensing and hardening at the same time to obtain strength.
Common such binders include p-Al2O3, aluminate cement, and the like.The binding agent such as phosphate, water glass and the like is formed through a chemical reaction with the material to be bound or an admixture (such as a coagulant) to form a binding phase. This type of binding agent is a chemically bound binding agent. Phosphoric acid-containing corundum castable has the advantages of high strength, high wear resistance, high load softening temperature, and excellent resistance to slag erosion. It is widely used in hot air furnaces. The addition of MgO as a hardener in the solid aluminum phosphate combined corundum castable can shorten its hardening time and optimize the construction performance.
Adding a coagulant or adjusting the pH value causes the particles in the suspension of the solid small particle-water system to agglomerate to produce binding, which is called coagulation binding. The most common cohesive bond is a silica sol binder. In addition, various other colloidal binders and suspension binders also belong to this binding mode. After calcination at 1100 ° C, the strength of corundum castables using silica sol as a binding agent is due to the corundum castables combined with calcium aluminate cement, and it can resist the rapid changes in temperature, the erosion of molten steel and slag, and the high intermediate temperature strength. Zeng Wei et al. Added organic acid to the silica sol and corundum castable and found that it can improve the dispersion stability and optimize the physical properties of the castable. The nano-SiO2 particles in the silica sol are filled between the particles, reducing the size of the gap, and the liquid phase appears at high temperature, so that the stress at the crack tip is released, and the ability of the sample to withstand stress and sudden temperature changes is significantly optimized.
Adhesive bond is a bond formed by connecting solids together by physical or chemical action at low temperature. Including physical adsorption (van der Waals force), chemisorption, binding produced by the infiltration and diffusion of the binder to form a film interconnect on the particle surface, and the binding caused by the electrostatic attraction of the electric double layer at the interface between the binder and the particle. .
High temperature liquid phase bonding is often referred to as ceramic bonding. Commonly used boric anhydride or boric acid as a binder for corundum refractories. At low temperatures, B2O3 forms a liquid phase, which bonds the corundum material, and at high temperatures, 9Al2O3 · 2B2O3 is formed with a high melting point, thereby producing a binding effect. Common ceramic bonding is also Sialon bonding. Corundum bricks prepared by this bonding method have great advantages in terms of resistance to alkali metal corrosion, strength, thermal shock resistance and oxidation resistance. They are commonly used in blast furnace linings. in. In the preparation process, adding a small amount of scorching aid can make Sialon combined castables have an appropriate linear change and can be applied to breathable bricks.