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.

坯体在干燥过程中因干燥速度过快产生开裂的原因主要有两方面:一方面是干燥过程中干燥制度有突然的变化,温度,湿度曲线有突然的较大变动；另一方面是坯体的干燥在没有经过临界点前的干燥速度过快.

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.

我们知道,干燥介质的温度是表示干燥介质带走水分能力标志之一,介质温度越高,带走水分的能力越强,坯体脱水速度越快.但如果温度过高,会造成坯体表面水分蒸发太快,而内部水分移动速度小于表面水分蒸发速度,坯体表面收缩大,而内部收缩小,造成内部对表面产生应力,当表面强度小于此值时,坯体表面就产生裂纹.在干燥时,如果介质温度突然升高,这时表面脱水速度急剧增加,表面收缩太快,内部收缩赶不上表面的收缩速度,内部对表面产生的应力大于表面强度,使坯体产生裂纹.

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.

介质湿度对坯体干燥过程的影响也是巨大的,如果湿度太高,则坯体脱水速度缓慢,若处理不当,就可能出现凝露现象,使坯体回潮.如果介质湿度太低,则脱水能力很强,易使坯体表面干燥过快,表面与内部有较大的应力差,从而导致坯体产生开裂.如果干燥时介质的湿度变得很低或很高,就会出现回潮或开裂现象.

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.

临界点是坯体干燥收缩停止的一个很重要的工艺点,在临界点以前,随着干燥过程的进行,坯体会一直有体积收缩现象,这时如果干燥过快,就很容易使坯体产生裂纹.而在临界点以后由于坯体停止了收缩,就是再快的干燥速度,脱去再多的水分,也只是增加了坯体中的孔隙率,不会产生内应力,对坯体开裂与否没有任何影响.所以,通常说的干燥速度太快就是指临界点前的干燥速度太快,导致坯体开裂.

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.

故在坯体干燥过程中,只要控制好介质的温度,湿度及临界点前的干燥速度,就能防止因干燥速度过快而导致的坯体开裂.

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.

在干燥过程中,砖坯表面水分的蒸发,使砖坯在垂直干燥面的方向上形成了水分梯度,这就必然引起内部水分的外扩散.当砖坯水分的表面扩散蒸发和内扩散具有相等的速度时,即在等速干燥阶段,干燥进行最为强烈.但表面水分强烈蒸发时,外层发生收缩,毛细管的直径就缩小,因此,使内扩散的速度大大降低.另外,当毛细孔道中的水位降落到低于表面的开口时,蒸发面就下降到砖坯的内部,结果使得砖坯表面很干,而内部仍然潮湿,同时干燥速度显著降低.

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.

砖坯的内扩散速度低于外扩散速度的结果,首先是降低了砖坯的干燥速度,其次是在砖坯内部形成了明显的水分梯度.当水分梯度现象十分严重时,在等速干燥阶段,砖坯在同一时间内接近砖坯干燥的表面部分的收缩大,而远离砖坯干燥表面部分的收缩小,因此,砖坯表面就形成了裂纹或坯体变形.

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.

产生裂纹的原因除干燥制度不合理外,和砖坯的塑性,干燥敏感性有很大的关系.当原料干燥敏感性较高时,即使在正常室温下也会出现干燥裂纹.