Why green bricks crack during the drying process?
The main reasons for cracking in the body during the drying process due to excessively rapid drying speed are twofold: on one hand, there are sudden changes in the drying regime during the drying process, with abrupt and significant fluctuations in temperature and humidity curves; on the other hand, the drying speed of the body is too fast before passing through the critical point.
We know that the temperature of the drying medium is one of the indicators representing its ability to remove moisture. The higher the medium temperature, the stronger its moisture-removing capacity, and the faster the dehydrating speed of the green body. However, if the temperature is too high, it will cause the surface moisture of the green body to evaporate too quickly, while the internal moisture migration speed is less than the surface evaporation speed. The green body surface contracts more, while the internal contraction is smaller, leading to internal stress against the surface. When the surface strength is less than this stress value, cracks will appear on the green body surface. During drying, if the medium temperature rises suddenly, the surface dehydration speed increases sharply at this time, the surface contracts too fast, and the internal contraction cannot keep up with the surface contraction speed. The internal stress generated against the surface exceeds the surface strength, causing cracks in the green body.
The humidity of the medium has a significant impact on the drying process of the green body. If the humidity is too high, the dehydration rate of the green body will be slow, and if handled improperly, condensation may occur, causing the green body to rewet. If the humidity of the medium is too low, its dehydration capacity is very strong, which can easily lead to rapid surface drying of the green body. The large stress difference between the surface and the interior will then cause the green body to crack. If the humidity of the medium during drying becomes either too low or too high, rewetting or cracking phenomena will occur.
The critical point is a very important process point where the drying shrinkage of the green body stops. Before the critical point, as the drying process proceeds, the green body will continue to exhibit volume shrinkage. If drying is too fast at this stage, it is easy for the green body to develop cracks. After the critical point, since the green body has stopped shrinking, even if the drying speed is very fast and more moisture is removed, it only increases the porosity of the green body and does not generate internal stress, thus having no effect on whether the green body cracks or not. Therefore, when people say that the drying speed is too fast, they usually refer to the drying speed before the critical point being too fast, which leads to cracking of the green body.
Therefore, during the drying process of the green body, as long as the temperature and humidity of the medium and the drying speed before the critical point are well controlled, it can prevent cracking of the green body caused by excessively fast drying speed.
During the drying process, the evaporation of moisture from the brick's surface creates a moisture gradient in the direction perpendicular to the drying surface, which inevitably causes internal moisture to diffuse outward. When the surface diffusion and evaporation of brick moisture are equal to the internal diffusion speed, i.e., during the constant-rate drying stage, drying proceeds most intensely. However, when surface moisture evaporates vigorously, the outer layer contracts, reducing the diameter of capillaries, thereby significantly decreasing the internal diffusion speed. Additionally, when the water level in the capillary channels drops below the surface openings, the evaporation front descends into the interior of the brick, resulting in a very dry surface while the interior remains moist, and the drying rate decreases significantly.
The result of the internal diffusion rate of brick坯 being lower than the external diffusion rate is first a reduction in the drying speed of the brick坯, and secondly the formation of a significant moisture gradient within the brick坯. When the moisture gradient phenomenon is severe, during the constant-rate drying stage, the part of the brick坯 near the drying surface contracts more in the same period, while the part away from the drying surface contracts less. As a result, cracks or body deformation occur on the brick坯 surface.
In addition to unreasonable drying conditions, the cause of cracking is also greatly related to the plasticity and drying sensitivity of the brick body. When the raw material has high drying sensitivity, drying cracks may occur even at normal room temperature.
