Ordinary calcium aluminate cement refractory castables can be divided into three categories. A type A cement castable with a mineral composition of CA-based cement as a binder. The mineral composition is mainly composed of CA2 (about 60%), and the rest is B type cement castable with cement of CA and a small amount of C2AS as a binder. The C-type cement castable with a mineral composition of CA2 and a cement containing almost no CA as a binder.
Type A refractory castable: All grades of clay or high alumina aggregate and powder are blended in a certain proportion, and A type cement is added. Because type A cement has a faster setting and solidification rate at normal temperature, it releases a large amount of heat in the initial stage of hydration (within 10 hours) (about 80% of total hydration heat), so it must be sprayed and cooled at any time during the summer maintenance process. During the winter maintenance process, it should be covered with wet linen or plastic film to keep the surface of the castable moist. The purpose is to avoid the temperature of the poured lining or precast block is too high, the surface moisture evaporates too fast, and the hydration cannot be achieved. Looseness occurs, resulting in a decrease in strength. After the casting of A type cement castables, the initial setting can be achieved after 1~3h. After 6~8h, the final setting will be achieved, and the strength will increase rapidly. The curing 1d can reach 75% of the ultimate strength, and 3d can reach 85%~95%. After 7d, the ultimate strength is basically reached.
The quality of the A-type cement castable depends on the quality of the aggregate used, the particle size ratio of the aggregate, the amount of cement added, the water consumption and the curing temperature. The aggregate is required to have sufficient sintering, compactness, low porosity and low impurity content. Aggregate particle size ratio design requires the most compact packing, so multi-level ratio should be adopted. The commonly used grading is: 15~7mm20%, 7~3mm25.%, 3~0.088mm20%, <0.088mm (or <0.074 Mm) 10~15%. A type of cement 15~25%, can also add a micro-water reducing agent to reduce water consumption, improve construction performance and increase bulk density.
The effect of the amount of cement added is: with the increase of the amount of cement added, the strength after curing at room temperature also increases, but after the addition amount is more than 25%, the strength growth is not large, and when the amount of cement added is too much, the CaO in the cement will be related to A12O3. And SiO2 generates a large amount of C2AS or CAS2 low-melting phase, which greatly reduces the high temperature performance of the castable. Therefore, the general cement addition amount is 12-15%.
The effect of the amount of water added is: the lower the water consumption value, the higher the strength of the wound material while maintaining the preferred flow value of the castable. In order to reduce the amount of water, 0.01 to 0.2% of a water reducing agent can be added.
The effect of curing temperature on the strength of the A-type cement burnt material is obvious. Good strength is usually obtained by curing at 20 ° C. Below this temperature, the hydration is incomplete and the strength is difficult to reach the highest value; above this temperature, the strength is reduced after reaching the highest value.
The mixture prepared by using ordinary calcium aluminate cement as a binder and refractory aggregate and powder in a certain ratio can be directly used after adding water, stirring, vibrating casting, curing and baking. Ordinary calcium aluminate cement has a low iron type and a high iron type. China produces low-iron type, which is made of natural low-silicon bauxite (SiO2 content less than 8%) and limestone.
Ordinary calcium aluminate cement combined castables have a wide application range, and different types of calcium aluminate cement can be used as binders according to the use temperature and use conditions, and different refractory aggregates and powders are added. Commonly used in various metallurgical, petrochemical, mechanical, building materials and other industrial sectors of heat treatment furnaces, heating furnaces, soaking furnaces, tapping and tapping tanks, boilers, rotary kiln heads and kiln tails, petrochemical reactors and The heat pipe and the inner lining of the valve.
