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Since calcium aluminate hydrate undergoes dehydration and phase transformation during the heating process, its hydration bonds are destroyed, and at the same time, it is converted from low-density hydrate to high-density hydrate, the molar volume is reduced, and the voids are enlarged. The strength of the treated calcium aluminate cement-bonded castables decreased significantly. Only when heated to a high temperature, the material sinters and produces a ceramic bond, and the strength increases again.
1) Appropriate setting time to ensure sufficient working time. Generally, the initial setting is greater than 1 h and the final setting is less than 8 hours. The shorter the time interval between the initial setting and the final setting, the better.
2) Sufficient early strength , that is, 60% to 70% of the strength specified by the cement label can be achieved in one day of curing, and more than 90% can be achieved in 3 days of curing. In addition to the above two points, pure calcium aluminate cement is also required to have a certain degree of refractoriness and good workability to meet construction requirements and high temperature use requirements.
Calcium aluminate cement is mainly used as a binder for refractory castables and shotcrete. Medium and low-grade refractory castables, such as clay-based, high-aluminum castables, use ordinary calcium aluminate cement as a binder. High-grade refractory castables, such as corundum, mullite, chromium-containing corundum, corundum-spinel and other castables use pure calcium aluminate cement as a binder. The addition amount of calcium aluminate cement for ordinary refractory castables is 10% to 20%, the addition amount of low cement refractory castables is 5% to 7%, and the addition amount of ultra-low cement castables is less than 3%.
The key to the application of calcium aluminate cement in marine engineering is the durability of calcium aluminate cement concrete. However, with the continuous development of marine engineering, improving the anti-corrosion performance of calcium aluminate cement in seawater can help the development of marine engineering.
The way to improve the performance of calcium aluminate cement is to avoid the formation of metastable phase hydration products (CAHo and C2AH&). At present, with the improvement of cement production technology, calcium aluminate cement is modified with various modification additives, and inhibiting the phase transformation of calcium aluminate cement hydration products is an effective way to modify calcium aluminate cement.
After the refractory castable is formed, the surface of the castable should be covered with a plastic film to prevent the water from evaporating too quickly and leaving unhydrated cement, resulting in a powdery layer or peeling on the surface; it can also prevent the inside of the castable from being dissolved. When Na+ ions (from dispersant) and Ca+ ions (from cement) in water migrate to the surface of the castable with water, and react with the CO2 gas contained in the atmosphere to form carbonate, causing the surface of the castable to appear hoarfrost or rise. skin and destroy the overall structure of the surface layer.
