Causes and analysis of cracks in castable construction

Causes and analysis of cracks in castable construction

Raw Material Factors

Unstable Quality: The main raw materials for castables include refractory aggregates, powders, binders, and admixtures. Unstable raw material quality, such as excessive impurities or uneven particle size, can cause cracking during castable preparation and use.

Improper Ratio of Coarse to Fine Aggregates: If the proportion of fine aggregate is too high, the castable shrinkage rate will increase during the heating process after boiler operation, leading to cracks.

Mix Ratio Design Factors

Irrational: A reasonable mix ratio is crucial for ensuring castable performance. Improper mix design, such as excessive or insufficient binder, can lead to uneven shrinkage during hardening, resulting in cracking.

Improper cement dosage: Excessive or insufficient cement can reduce the castable’s strength, flexural strength, and impermeability, increasing the risk of cracking.

Construction Process Factors

Uneven Mixing: Failure to fully mix the various raw materials during the casting process can lead to uneven castable properties. Furthermore, too short a mixing time can result in uneven and loose castables, reducing their strength. Too long a mixing time can easily lead to stratification, with fines floating on the surface and aggregate sinking to the bottom. This can also reduce castable strength and make it prone to flaking and cracking.

Inadequate Vibration: Failure to adequately vibrate the castable during construction can lead to internal voids. These voids can cause stress concentrations at high temperatures, leading to cracks.

Excessive Construction Speed: Excessive construction speeds can lead to stress concentrations within the castable, causing cracks.

Improper Curing: Castables require proper curing after construction. Improper curing, such as premature demolding or insufficient watering, can lead to insufficient strength and cracking.

Construction Environmental Factors

Temperature: Excessively high or low temperatures during construction can cause thermal stresses in the castable during the curing process, leading to cracking. Furthermore, boiler temperatures fluctuate frequently during operation. If the castable is put into use directly after construction without proper baking, the rapid temperature fluctuations can generate significant internal stresses due to thermal expansion and contraction, leading to cracks.

Humidity: Ambient humidity has a significant impact on the drying process of the castable. If the ambient humidity after construction is high, the moisture in the castable will evaporate slowly, prolonging the drying time. The internal structure may become loose due to prolonged exposure to moisture, increasing the risk of cracking.

Usage Conditions

High Temperature: Castables are subject to high temperatures during use. If their high-temperature resistance is insufficient, thermal stress will occur at high temperatures, leading to cracks. High temperatures can also cause moisture within the castable to evaporate, generating shrinkage stress, which can also cause cracks.

Chemical Attack: Under certain operating conditions, castables may be subject to chemical attack, such as acid and alkali attack, and oxidation. This chemical attack can degrade the castable’s performance, increasing the risk of cracks.

Mechanical Stress: Castables are subject to various mechanical stresses during use, such as equipment vibration and thermal expansion and contraction. If the castable’s flexural strength and toughness are insufficient, it will be unable to withstand these mechanical stresses, making cracks more likely.

Design and Structural Factors

Design flaws: Flaws in the equipment design, such as excessive castable thickness or complex structure, can lead to stress concentration in the castable during use, potentially causing cracks.

Expansion coefficient mismatch: In some cases, the expansion coefficient of the castable may not match that of other materials. At high temperatures, this mismatch can cause stress, which in turn can lead to cracks.

In summary, the causes of castable construction cracks are multifaceted, and prevention and control are necessary from multiple aspects such as raw material selection, mix ratio design, construction technology, construction environment control, use conditions management, and design and structure optimization.