What Are The Main Binders for Alumina Magnesia Castables
In recent years, with the rapid development of secondary refining and continuous casting technologies, the ladle has served both as a container for molten steel and as a refining unit. This has greatly prolonged the residence time of molten steel in the ladle and has been accompanied by higher tapping temperatures and harsher smelting conditions. Consequently, the service life of the ladle lining has decreased significantly. The use of monolithic linings for ladles with alumina-magnesia castables (including spinel-containing castables) has long been a major development trend. There are five main categories of binders commonly used in alumina-magnesia castables:
Water Glass Bonding System
Water glass consists of alkali metal silicates and provides excellent bonding strength. Depending on the type of alkali metal oxide, it can be classified as sodium water glass (Na2O⋅nSiO2), potassium water glass (K2O⋅nSiO2), or potassium-sodium water glass (K2O⋅Na2O⋅nSiO2). It primarily forms a gel through air-drying and heat-induced dehydration, which imparts its bonding strength.
Phosphoric Acid and Aluminum Phosphate Bonding System
Industrial phosphoric acid, with the molecular formula H3PO4⋅0.5H2O, exists as prismatic crystals and is completely soluble in water. There are three types of phosphoric acid, among which orthophosphoric acid (commonly referred to as phosphoric acid, H3PO4) is the most stable. The bonding mechanism of phosphoric acid involves reacting with oxides in the raw materials to form compounds, thereby providing the castable with bonding strength.
Aluminum phosphate can also be used as a refractory binder. It is generally prepared by reacting phosphoric acid with aluminum hydroxide, producing aluminum monohydrogen phosphate and aluminum dihydrogen phosphate.
When high-alumina bauxite clinker and fused spinel fine powder are used as the main raw materials with phosphoric acid as the binder, the resulting spinel high-alumina castable exhibits excellent thermal shock resistance and slag corrosion resistance.
Phosphoric acid reacts with free MgO in spinel to form magnesium dihydrogen phosphate, which then undergoes spontaneous polymerization into magnesium phosphate to achieve mechanical strength. This bonding method also extends the construction time and improves the castable's resistance to hydration.
MgO+2H3PO4=Mg(H3PO4)2+H2O (1)
nMg(H2PO4)→nMg·2nPO3+2nH2O (2)
MgO·SiO₂-H₂O Bonding System
This bonding system is a fine-powder coagulation type and is commonly used in bauxite-based castables.
The addition of CaO should be avoided, as it can form low-melting-point phases at high temperatures, which significantly reduces the material's high-temperature performance.
Fine silica fume can react with water as follows:
SiO2+H2O=Si-OH++OH- (3)
The advantages of this system include high material strength after heat treatment at moderate temperatures. M-S-H contains less crystalline water, which is beneficial for rapid heating and drying. At high temperatures, SiO₂ reacts with MgO to form forsterite, enhancing the high-temperature performance of the castable. In addition, SiO₂ can improve the flowability of castables.
The main disadvantage of this system is its relatively low resistance to slag erosion.
Hydratable Alumina Bonding
Among the various crystal forms of Al2O3, only ρ−Al2O3 can undergo spontaneous hydration at room temperature. As a binder for castables, its bonding mechanism relies on hydration to form bayerite and boehmite sols. The reaction is shown as follows:
ρ-Al2O3+2H2O=AI(OH)3+AlOOH (4)
ρ−Al2O3 is an amorphous substance. The disordered arrangement of internal Al–O bonds and its valence deficiency endows it with high chemical activity and a rapid hydration reaction rate. At room temperature, the hydration rate of ρ−Al2O3 follows an autocatalytic reaction, increasing with rising temperature. However, the hydration reaction is relatively intense and difficult to control, leading to poor stability of the resulting material.
A comparative study on alumina-magnesia castables bonded with hydratable alumina and calcium aluminate cement shows that castables containing 3 wt% hydratable alumina deliver better slag resistance, penetration resistance, and thermal shock resistance than cement-bonded castables.
The characteristics of alumina-magnesia castables with different bonding systems were further investigated. The results indicate that slight shrinkage occurs around in-situ formed ρ−Al2O3 at high temperatures due to the dehydration of hydratable alumina. The annular zone formed by this shrinkage can hinder crack propagation and relieve internal stress, thus effectively improving the thermal shock resistance of the material.
Aluminate Cement Bonding System
Currently, calcium aluminate cement is the most widely used binder for industrial alumina‑magnesia ladle purging plugs and other castables.
Castables bonded with aluminate cement form 2CaO·Al2O3・8H2O and Al(OH)3 colloids at room temperature, providing high demolding strength. After heat treatment at moderate temperatures, the strength decreases; when heated above 1400 °C, CA6 is formed, and the material strength increases significantly. However, excessively high service temperatures can cause structural spalling, limiting its range of application.
Low Cement Castables (LCC) and Ultra‑Low Cement Castables (ULCC) have been developed to reduce cement dosage. In conventional refractory castables, cement addition accounts for 10–15%, while in ULCC, the cement content is only 2–3 wt%.
The strength evolution of aluminate cement–bonded refractory castables can be observed by examining the relationship between relative compressive strength (using the compressive strength after drying at 110 °C as 100%) and heating temperature. As shown in Figure 1, after initial setting and molding, aluminate cement castables can achieve high room-temperature strength under standard curing. The strength declines after drying, due to the dehydration of hydration products 2CaO\cdotpAl2O3⋅8H2O and Al(OH)3.
The high-temperature compressive strength characteristics of aluminate cement–bonded refractory castables are as follows. When heat-treated below 1000 ℃, their high-temperature compressive strength is close to the cold compressive strength. As the temperature rises, a liquid phase appears, reducing the material's high-temperature strength. When the temperature further increases to 1350 ℃, the high-temperature compressive strength drops to only 2 MPa.
Around 300 ℃, rapid crystal transformation occurs, and a significant amount of free water is released, resulting in a considerable reduction in relative strength, generally ranging from 18% to 25%. In the temperature range of 300–900 ℃, free water and most of the bound water are released, causing a significant increase in apparent porosity.
Between 900 and 1200 ℃, chemical reactions form CA and CA₂, generating a new mineral structure accompanied by volume shrinkage. However, due to the relatively low temperature and insufficient sintering, the internal structure loosens, and the strength decreases markedly to about half of the strength after drying.
Microscopic observation of specimens treated at 1200 ℃ shows that the microstructure consists of isolated, block-like particles of similar size, resulting in the lowest strength. After heating to 1300–1400 ℃, the strength rebounds substantially due to the formation of the stable CA₆ phase and the development of ceramic bonding (Figure 2).
Research results indicate that at 1300 ℃, corundum and calcium dialuminate are the main crystalline phases of the synthesized material, and the reaction CA2+Al2O3→CA6 begins. At 1400 ℃, the content of CA2 decreases significantly, while a large amount of CA6 forms. At 1500 ℃, the reaction is complete, and the main crystalline phases of the material are corundum and CA6.
Since CA6 grain growth is anisotropic, its crystal morphology is mostly plate-like or needle-like. It has a thermal expansion coefficient close to that of alumina and is highly compatible with it. When added to alumina-based ceramics, coatings, and other materials, it can significantly improve the mechanical properties of the matrix.
In calcium aluminate cement–bonded castables, CA6 forms through high-temperature chemical reactions within the matrix. Its plate-like crystals interlock with spinel grains to create a network structure, effectively enhancing the strength of the refractory material.