Cracks in refractory castable construction are mainly related to material quality, proportion, construction process, maintenance and use environment.
1. Material quality problems
a. Insufficient raw material purity: If the purity of the raw materials of castables does not meet the standards, they will contain more impurities. These impurities will react chemically at high temperatures, causing volume changes, leading to cracks. For example, iron impurities contained in the raw materials will be oxidized at high temperatures, expand in volume, and destroy the structure of the castable.
b. Unreasonable aggregate particle size: The size and grading of aggregate particles will directly affect the performance of the castable. If the aggregate particle size is too large, it is difficult to evenly distribute large aggregate particles during the vibrating process of the castable, which can easily cause excessive local voids. When these parts are subjected to thermal stress or mechanical stress, cracks are prone to occur. On the contrary, if the aggregate particle size is too small, the specific surface area of the castable will increase, the water demand will increase, and the strength and crack resistance of the castable will be reduced.
2. Improper proportion
a. Improper amount of binder: The binder plays a key role in refractory castables. It can bond aggregates and powder together to give the castables a certain strength and plasticity. If the amount of binder is too small, the bonding strength of the castable is insufficient, and cracks are prone to occur when it is subjected to external forces during construction or use. On the contrary, if the amount of binder is too much, the water demand of the castable will increase. During the curing process, the evaporation of water will leave more pores, reducing the density and strength of the castable. At the same time, too much binder will change volume at high temperature, which is also prone to cracks.
b. Unreasonable use of admixtures: Admixtures can improve the performance of refractories castables. For example, water reducers can reduce the water consumption of castables and improve fluidity and strength; expansion agents can compensate for the volume shrinkage of castables during hardening and prevent cracks. However, if the type of admixture is not properly selected or the amount used is unreasonable, it will not only fail to improve the performance of the castable, but may also cause cracks. For example, if the amount of water reducer used is too much, the setting time of the castable will be too long. During this period, the castable is easily affected by external factors and cracks will occur.
3. Construction process problems
a. Uneven mixing: Refractory material castables need to be stirred before construction to make their components evenly mixed. If the mixing is uneven, it will lead to uneven distribution of the components in the castable, and the content of binders, admixtures, etc. in local areas will be too high or too low, thus affecting the performance of the castable. For example, if the local binder content is too high, the water in the part will evaporate too quickly during the curing process, resulting in a large shrinkage stress, which will cause cracks.
b. Uncompacted vibration: Vibration is an important part of the construction process of refractory castables. Vibration can remove the air in the castable, making it more compact, and improving its strength and crack resistance. If the vibration is not dense, there will be more voids and bubbles inside the castable, and these voids and bubbles will become stress concentration sites. When the castable is subjected to thermal stress, mechanical stress, etc., these stress concentration sites are prone to cracks, reducing the overall performance and service life of the castable.
4. Improper maintenance
a. The curing temperature is too high or too low: The curing temperature has an important influence on the performance of refractories castables. If the curing temperature is too high, the moisture inside the castable will evaporate quickly, causing the surface to dry and shrink, while the internal moisture evaporates slowly and the volume changes less, resulting in a large shrinkage stress between the surface and the inside, causing cracks. In addition, too high a curing temperature may also lead to premature hardening and crystallization of the binder, reduce the bonding strength between the binder and the aggregate, and further increase the possibility of cracks. On the contrary, if the curing temperature is too low, the hydration reaction rate of the castable will slow down or even stop, resulting in the castable being unable to harden normally, slow strength growth, and reduced crack resistance. In this case, the castable is more likely to crack when subjected to external forces or temperature changes.
b. Insufficient curing humidity: Curing humidity is one of the important conditions to ensure the normal hydration reaction and strength growth of refractory castables. If the curing humidity is insufficient, the moisture on the surface of the castable will evaporate quickly, resulting in dry surface, while the internal moisture will still maintain a certain humidity due to the slow evaporation rate, thus forming a humidity gradient between the surface and the inside. This humidity gradient will cause the surface to shrink more than the internal shrinkage, thereby generating tensile stress on the surface. When the tensile stress exceeds the tensile strength of the castable, cracks will occur. In addition, insufficient curing humidity will also affect the degree of hydration reaction of the binder, reduce the bonding strength between the binder and the aggregate, and further weaken the crack resistance of the castable.
5. Use environment issues
a. Drastic temperature changes: During the use of refractories castables, if the ambient temperature changes dramatically, it will cause large thermal stress inside the castable. When the thermal stress exceeds the strength limit of the castable, cracks will occur. For example, in some industrial kilns, it is necessary to frequently start and stop the furnace. Each time the furnace is started, the temperature inside the furnace will rise rapidly. Due to the hysteresis of heat conduction, the temperature distribution inside the castable is uneven, the surface temperature rises faster, and the internal temperature rises slower, which will generate a large thermal stress between the surface and the inside. On the contrary, when the furnace is stopped, the temperature inside the furnace will drop rapidly, the surface temperature of the castable will drop faster, and the internal temperature will drop slower, which will also generate a large thermal stress between the surface and the inside. This frequent temperature change will cause the thermal stress inside the castable to accumulate continuously, and eventually lead to the generation and expansion of cracks, seriously affecting the service life of the castable and the normal operation of the kiln.
b. Chemical erosion: In some specific use environments, refractory castables will be eroded by chemical substances, resulting in cracks. For example, in some kilns in metallurgy, chemical industry and other industries, there will be high-temperature flue gas containing acidic gases (such as sulfur dioxide, sulfur trioxide, etc.) or alkaline substances (such as sodium hydroxide, potassium hydroxide, etc.). These chemicals will react chemically with certain components in the castable to generate new compounds. These newly formed compounds may have different volumes and physical properties, which may cause stress inside the castable. When this stress exceeds the strength limit of the castable, cracks will occur. In addition, chemical erosion may also destroy the structure of the castable, reduce its density and strength, further aggravate the generation and expansion of cracks, shorten the service life of the castable, and affect the normal operation and production efficiency of the kiln.
