What is an alkali resistant castable?
Castable refractories that can resist the erosion of alkali metal oxides (such as K2O and Na2O) at medium and high temperatures are called alkali resistant refractory castables. The composition of this type of castable is similar to that of ordinary calcium aluminate cement combined castable, which is a mixture of alkali resistant refractory aggregate and powder, binder and admixture.
What are the types of alkali resistant castables? What are the characteristics of each?
According to different use environments and conditions, alkali resistant refractory castables are divided into light-weight and heavy-weight castables. The heavy weight can be divided into medium temperature alkali resistant castable and high temperature alkali resistant castable.
The aggregates used in light alkali resistant refractory castables include alkali-resistant ceramsite, clay porous clinker, waste porcelain ware, high-strength expanded perlite, etc., and calcium aluminate cement or water glass is used as the binding agent. High siliceous refractory raw materials can be used for alkali-resistant materials. Such materials will react with alkali metal oxides at high temperatures to form a high-viscosity liquid phase, forming an axialized dense protective layer to prevent further penetration of alkali metal melts. and erosion. Its chemical composition is generally: Al2030%~55%, SiO25%~45%. The traditional alkali-resistant castable cement dosage is 25%~30%, and the water amount is 20%-25%. Due to the damage of the cement glue structure at medium temperature (800~1000℃0), the strength is greatly reduced, which is only about 50% of the drying strength. Therefore, ultrafine powder and suitable dispersant can be introduced, the cement dosage can be reduced to 10%~20%, and the water addition amount can be reduced to 15%~20%. The physical properties of typical low-cement light alkali resistant refractory castables are as follows: the temperature is 110°C, drying for 16h, the bulk density is 1.5~1.6g/cm3, the flexural strength is 3~6MPa, and the compressive strength is 30~40MPa; the temperature is 1100°C, After 3h burning, the bulk density is 1.4~1.5g/cm3, the flexural strength is 6.0~6.5MPa, the compressive strength is 40~45MPa, the linear change rate after burning is 0.3%~-0.5%; the temperature is 350℃, the thermal conductivity is 0.4~0.5 W/(m・K).
The aggregates used in heavy alkali resistant refractory castables include bauxite clinker, clay clinker, etc. The binder and alkali-resistant powder are the same as the light alkali-resistant castables. Its main chemical composition is: A2O35%~60%, SiO235%~60%. The traditional heavy alkali resistant refractory castables use 20% cement, and add 10% to 15% of water: low-cement type heavy alkali-resistant refractory castables use 5% to 15% of cement, and add 6.5% to 7.5% of water. Calcium aluminate cement and silica micropowder (smoky silica) are used as binders for low-cement heavy-duty refractory castables. Adding silica micropowder not only helps to improve the medium-temperature bonding strength of the castable, but also helps to lining the castable during use. The anti-penetration shaft layer is formed on the surface of the body, which is characterized in that the strength after burning at medium temperature (1000~1200℃) is equivalent to the strength after drying (110℃), and the resistance to alkali corrosion is good. The physical indicators of typical low-cement heavy alkali resistant refractory castables are as follows:
The temperature is 110°C, drying for 16h, the bulk density is 2.20~2.59g/cm3, the flexural strength is 4~8MPa, and the compressive strength is 40~60MPa; the temperature is 1100°C, after 3h burning, the bulk density is 2.20~2.40g/cm3, the resistance The flexural strength is 6~10MPa, the compressive strength is 35~55MPa, the linear change rate after burning is -0.3%~0.4%, the thermal shock stability (1100℃-water cooling is more than 20 times; the temperature is 350℃, the thermal conductivity is 1.2~1.3W /(m・K)