Commonly used admixtures for monolithic refractory materials and their functions
2025-09-02 16:27:29
Admixtures
Admixtures are substances that can be added in appropriate amounts to refractory materials to improve the physical properties, molding or construction properties and performance of refractory materials. They are also called additives. Admixtures can be used in both shaped refractory products and monolithic refractory materials, and are most commonly used in monolithic refractory materials. They are generally added when or before the monolithic refractory materials are mixed. The amount of admixture added varies with the properties and functions of the admixture, and the amount added is generally very small.
Classification of admixtures
Admixtures for refractory materials are generally classified according to their chemical composition, properties and functions. According to their chemical composition and properties, they are divided into two categories: inorganic and organic.
(1) Inorganic. There are various inorganic electrolytes, some metal elements, inorganic minerals, oxides and hydroxides, etc.
(2) Organic. Most of them are surfactants, which have hydrophilic and hydrophobic groups. The hydrophilic groups that can ionize in water are called ionic surfactants, while those that do not ionize are called nonionic surfactants. The ionic surfactants can be further divided into anionic, cationic and amphoteric surfactants. In addition, there are some high molecular surfactants, organic acids, etc.
According to the function, there are 5 types:
(1) Change rheological properties (workability) type. Including water reducers (dispersants), plasticizers (plasticizers), gelling agents (flocculants), disintegrators (deflocculants).
(2) Adjust the coagulation and hardening speed type. Including coagulants, retarders, delayed coagulants, etc.
(3) Adjust the internal structure type. Including foaming agents (air entraining agents), defoamers, shrinkage inhibitors, expansion agents, etc.
(4) Maintain the construction performance of materials type. Including inhibitors (anti-expansion agents), preservatives, antifreeze agents, etc.
(5) Improve the performance type. Including sintering aids, mineralizers, quick-drying agents, etc.
Admixtures used in refractory materials, especially monolithic refractory materials, have diverse properties and come in a wide variety of types. Often, a single admixture's function varies depending on the dosage, binder type, and ingredients used. In other words, the same substance can function as one admixture or another, depending on the conditions.
01
Water Reducers
Water reducers (also known as water reducers or dispersants) can significantly reduce the mixing water requirement while maintaining the flow value of refractory castables. Water reducers themselves do not chemically react with the material components to form new compounds; they only act on the surface physically and chemically. Therefore, they are surfactants or electrolytes. When dissolved in water, they adsorb onto the surface of particles, increasing the surface potential of the particles in the solution, enhancing the repulsive forces between the particles, and releasing free water trapped in the aggregated structure of the particles, thereby providing wetting and dispersing properties. Therefore, they are also called dispersants. Because it releases free water from the aggregate structure, it can reduce the specific water content while maintaining the castable's rheological properties (workability). Alternatively, without changing the specific water content, it can increase the material's rheological properties, improve workability, and make the material easier to construct and form.
Common water reducers (dispersants) used in refractory castables using calcium aluminate cement, binding clay, and oxide fine powder as binders include inorganic sodium pyrophosphate, sodium tripolyphosphate, sodium tetrapolyphosphate, sodium hexametaphosphate, sodium superpolyphosphate, and sodium silicate; organic water reducers include sodium lignosulfonate, calcium lignosulfonate, naphthalene-based water reducers, and water-soluble resin-based water reducers.
02
Plasticizers
Plasticizers (also known as plasticizers) increase the plasticity of refractory mixtures and improve the strain capacity of the mixture (clay) under stress. Plasticizers are commonly used as admixtures in refractory plastics and ramming materials, and are also used in other monolithic refractory materials. Commonly used materials include plastic clay, bentonite, ultrafine oxide powder, soy flour, methylcellulose, and wood sulfonate.
03
Gelling Agents
Gelling agents (also known as flocculants) can cause colloids (or suspended particles) in colloidal solutions (or suspensions containing solid particles) to agglomerate. Gelling agents are primarily inorganic electrolytes and inorganic acids.
Flocculants are primarily organic polymers, available in both natural and synthetic forms. Based on their functional groups, they can be categorized as cationic, anionic, and nonionic.
Gelling agents are primarily used in refractory castables with clay-bonding, sol-bonding, and ultrafine powder-bonding properties.
04
Deflocculating Agents
Deflocculating agents (also known as deflocculating agents or dispersing agents) can convert agglomerated or agglomerated colloids (or particles) into sols or evenly dispersed suspended particles. They can transform thick colloids into free-flowing sols and are also known as dispersants or peptizers. The mechanism of action of deflocculating agents is similar to that of water reducers. Therefore, a water-reducing agent also acts as a debonder. Adding a debonder to clay-bound, sol-bound, and ultrafine powder-containing refractory castables can improve their fluidity.
05
Setting Accelerators
Setting accelerators can shorten the setting and hardening time of monolithic refractory materials. The mechanism of action of setting accelerators is relatively complex and varies depending on the properties of the binder and setting accelerator used. Different binders require different setting accelerators. The setting accelerators used in calcium aluminate cement binders are mostly alkaline compounds: NaOH, KOH, Ca(OH)2, Na2CO3, Na2SiO3, etc.; setting accelerators used with phosphoric acid and aluminum dihydrogen phosphate binders include activated aluminum hydroxide, talc, magnesium oxide, and calcium aluminate cement; and setting accelerators used with water glass (sodium silicate) binders include sodium fluorosilicate, aluminum phosphate, sodium phosphate, lime, dicalcium silicate, and glyoxal.
06
Retarders
Retarders can slow the setting and hardening of monolithic refractory materials. The mechanism of action varies depending on the properties of the binder and retarder used. Retarders are primarily used in monolithic refractory materials bonded with calcium aluminate cement containing a rapid-hardening mineral. Commonly used retarders include low-concentration NaCl, citric acid, tartaric acid, ethylene glycol, glycerol, starch, phosphates, and lignosulfonates.
07
Delayed-Accelerators
Delayed-Accelerators delay the setting and hardening of the binder in monolithic refractory materials. For castables using ultrafine silica powder as the primary binder, calcium aluminate cement can be used as a delayed-acting accelerator. This is because calcium aluminate cement slowly releases Al?? and Ca?? ions during hydrolysis. These ions, when adsorbed on the surface of silica particles, deprive the particles of their electrical properties for coagulation, thus achieving a delayed-acting accelerating effect.
08
Foaming Agents, Air-Entraining Agents, and Air-Entraining Agents
Foaming agents reduce the surface tension of liquids, producing large quantities of uniform, stable foam. Examples of foaming agents include rosin soap, resin soap esters, aluminum petroleum sulfonates, and hydrolyzed blood; air-entraining agents include rosin thermopolymers, nonionic surfactants, and alkylbenzene sulfonates. Another type of air-entraining agent, aerated by chemical and physical reactions with water during castable mixing and pouring, produces bubbles that are evenly distributed throughout the slurry. A commonly used air-entraining agent is aluminum powder. Aluminum powder is highly reactive and reacts with water to release hydrogen. The released hydrogen forms bubbles within the castable. However, the amount of aluminum powder added must be appropriate; otherwise, the strength of the monolithic refractory can be significantly reduced or uneven interconnected pores can be created. This type of admixture can be used to formulate lightweight, porous monolithic refractory materials.
09
Defoaming agent
Defoaming agent can make the bubbles generated during the mixing or vibration molding of amorphous refractory materials escape quickly. Defoaming agent is also a surfactant. Its ability to reduce the surface tension of liquid is much stronger than that of foaming agent. It greatly reduces the strength of the liquid film formed, makes the bubbles lose stability, and spreads faster on the liquid surface. The faster the spreading speed, the stronger its defoaming effect. Types of defoaming agents include alcohols, fatty acids and fatty acid salts, organic silicon compounds, etc.
10
Shrinkage inhibitors
Shrinkage inhibitors can reduce or prevent the shrinkage of refractory materials during heating and use after molding. They are also called volume stabilizers or expansion agents. The shrinkage inhibitor principle is:
(1) Thermal decomposition.
After being heated at high temperature, the shrinkage inhibitor will undergo pyrolysis. The molar volume of its product is greater than the molar volume of the original substance before pyrolysis, thereby compensating for the sintering shrinkage of the material. For example, kyanite shrinkage inhibitor.
(2) Chemical reaction.
After the added shrinkage preventer undergoes a high-temperature chemical reaction, the molar volume of the new phase is greater than the molar volume of the original reaction phase, thereby compensating for sintering shrinkage. For example, when α-Al2O3 is added to a magnesia castable as a shrinkage preventer, the high-temperature reaction generates alumina-magnesia spinel, which can produce a volume expansion effect. Another example is when clay or silica powder is added to a high-alumina castable, the volume expansion effect of mullite generated at high temperature can be used to compensate for sintering shrinkage. (3) Crystal transformation. The added shrinkage preventer can undergo a crystal transformation that expands during high-temperature heating, which can also compensate for sintering shrinkage. For example, when an appropriate amount of silica powder is added to an aluminum silicate castable or plastic, the volume expansion when quartz is converted into tridymite or cristobalite can be used to compensate for the sintering shrinkage of the matrix. 11 Preservatives Preservatives can keep the construction performance of amorphous refractory materials unchanged or slightly changed after a certain period of storage. For example, Al2O3-SiO2 refractory plastics and ramming materials, which are typically bonded with phosphoric acid or aluminum dihydrogen phosphate, react with the Al2O3 in the material to form insoluble aluminum orthophosphate, causing the mix to dry out prematurely and lose workability. Therefore, a sequestering agent that forms a complex with Al3+ ions is added to inhibit the formation of insoluble aluminum orthophosphate and extend the shelf life. Preservatives used for plastics and ramming materials bonded with phosphoric acid or aluminum dihydrogen phosphate include oxalic acid, citric acid, and tartaric acid. Sulfosalicylic acid and dextrin also serve as preservatives.
12
Inhibitors
Inhibitors suppress the expansion caused by the reaction of iron and iron compounds in the monolithic refractory mix with the acidic chemical binder to produce hydrogen gas. Therefore, they are also called anti-expansion agents. When using acidic chemical binders (such as phosphoric acid, aluminum dihydrogen phosphate, aluminum sulfate, aluminum chloride, and polyaluminum chloride) as a binder, the mixture generally needs to be trapped to allow the acidic binder to fully react with the metallic iron or iron-containing compounds in the mixture to release hydrogen before molding. Otherwise, expansion will occur after molding, making the material loose and porous, and reducing its strength. If it is used directly without being trapped, an inhibitor must be added. An inhibitor is a chelating agent that reacts with metallic iron to form a complex, thereby inhibiting the expansion effect. Such inhibitors include CrO3, diacetone alcohol, and iron phosphate.