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The setting and hardening of aluminate cement are related to curing temperature. As the curing temperature increases, the setting and hardening speed of the CA2 phase accelerates, while the CA phase behaves differently. It is faster at 20°C, slower at 30°C, and then becomes faster again above 30°C.
During the hydration of aluminous cement, there is a heat release phenomenon. When using aluminous cement containing quick-setting phases as a binder, appropriate cooling measures should be taken, especially in summer, to prevent excessive heat concentration from affecting the internal and surface quality of aluminate cement bonded castables. In contrast, cement with CA2 as the main phase has a slower setting speed. During winter construction, steam curing is used to promote hydration and accelerate setting and hardening speeds. The measures generally involve adding appropriate amounts of accelerators and retarders to adjust the setting and hardening speeds of aluminous cement.
In high-temperature industries, the use of aluminate cement must meet the following three points:
Appropriate setting time to ensure sufficient operation time, with initial setting greater than 0.5 hours and final setting less than 8 hours.
Sufficient early strength, with a one-day cured strength reaching 60%-70% of the grade, and over 90% in three days.
Having certain refractory temperature and high-temperature usability.
Aluminate cement is primarily used as a binder for refractory castables and spray materials. Ordinary calcium aluminate cement is used for medium and low-grade refractory castables, such as clay-based castables and high-alumina castables; high-grade refractory castables use pure calcium aluminous cement as the binder. Examples include corundum enhanced castables, mullite-based castables, and corundum-spinel castables. The addition rate of calcium aluminate cement in ordinary refractory castables is 10%-20%, while low cement castables have an addition rate of 5%-7%, and ultra-low cement castables have an addition rate of less than 3%.
The usage temperature of clay-based castables is 1300-1450°C, generally used as linings for steel reheating furnaces, various heat treatment furnaces, boilers, shaft kilns, and rotary kiln preheat zones.
The usage temperature of high-alumina castables is 1400-1550°C, suitable for linings and burners of various heat treatment furnaces. Applications include electric furnace steel spouts, high-temperature zones of lime shaft kilns, rotary kiln ends, and linings of power plant boilers.
The usage temperature of corundum castables is 1500-1650°C, mainly used for linings of various high-temperature furnaces and high-temperature components such as immersion pipes of steel ladle vacuum degassing devices, spray metallurgy and argon blowing integral lances, electric furnace top triangle zone linings, LF furnace covers, and high-temperature wear-resistant linings of catalytic cracking reactors in the petrochemical industry.
Low cement and ultra-low cement castables are widely used in industrial kilns in metallurgy, petrochemical, machinery manufacturing, electric power, and building materials industries to replace masoned refractory bricks for linings.
Precautions for the Construction of Castables Bonded with Aluminate Cement
After the castable is formed (before and after demolding), it should be covered with a thin plastic film to prevent rapid evaporation of moisture, which can leave unhydrated cement resulting in a powdery layer or peeling surface.
During the migration of moisture in the castable, Na+ ions (mainly from dispersants) and Ca+ ions (mainly from calcium aluminate cement) can move to the surface of the castable. They react with CO2 in the air to form carbonates, which can cause "whitening" or peeling on the castable surface, damaging the overall surface structure.
