Sintered products remain crucial local building materials used in current industrial and civil construction projects. The quality of sintered products is closely related to the quality of construction projects and the safety of human lives and property. Based on years of experience in wall reform work, this article briefly discusses common quality issues and solutions in the production process of sintered bricks (blocks).
Quality issues that are prone to occur during the production process of sintered porous bricks and hollow bricks (blocks)
Sintered bricks, previously known as "baked bricks," primarily depend on the sintering process (earth kiln, shaft kiln, rotary kiln). With technological advancements and changes in the building system, modern sintered bricks are mainly produced using tunnel kiln technology, with products mainly consisting of perforated bricks, hollow bricks, and blocks. However, regardless of the technology and production process used, to produce a good brick, "raw materials are the key, technology is the condition, and management is the guarantee.".
In ancient times, people attached great importance to the selection and disposal of raw materials in brick making. In this ancient industry, there have always been two "terminologies", namely, "three parts craftsmanship and seven parts raw materials, three parts firing and seven parts baking". According to Ming Zhang's "Illustrations on Brick Making", the disposal of raw materials for brick making is as follows: "It takes seven turns to obtain soil, six turns to make mud, and eight months to form the brick billet. It takes a hundred and thirty days to immerse the water and take the brick out of the kiln. The brick must be free of dry cracks on the surface and back, no broken corners, and when knocked, the sound should be clear and resonant. Only then can it be considered qualified.".
Based on the analysis of the quality of sintered products, it is evident that most of the quality issues are caused by improper handling of raw materials, despite the multiple quality inspections conducted by the country and our province on sintered products.

1. Dimensional deviation
Reason: The main issue lies in the insufficient particle size distribution and inadequate aging time of the raw materials.
Solution:
(1) Perform sieve analysis on the raw materials to address the particle size distribution of the raw materials;
(2) Increase the storage time of raw materials and enhance their plasticity index. Generally, based on the production capacity of the enterprise, the minimum area of the storage warehouse should be able to accommodate seven days' worth of production;
(3) Improve the extrusion port size of the brick machine. Conduct a comprehensive analysis of the existing raw materials, especially focusing on their plasticity index, to ensure that the size of the extruded clay strip from the brick machine is close to the range of deviation values corresponding to the plasticity of the raw materials.

2. Edge cracks
Reason: The proportion of large particles in the raw material is relatively high, and the molding moisture content is relatively low, which increases the resistance of the brick machine to extruding the mud strip.
Solution:
(1) Increase the crushing fineness of raw materials or reduce the aperture of the sieve, appropriately adjust the gradation of raw materials, and reduce the resistance of the mud strip;
(2) Check the gap between the high-speed counter-rollers and adjust it to 3mm;
(3) Properly adjust the moisture content of the raw materials.
3. Horizontal length cracks on ribs and walls
It is commonly seen in sintered porous bricks, sintered hollow bricks, and blocks. The main reason is that the proportion of large particles in the raw materials is relatively high, which leads to uneven shrinkage of coarse and fine particles in the raw materials, resulting in shrinkage cracks. Another reason could be that the temperature of the drying kiln is too high or the drying speed is too fast.
Solution:
(1) Reasonably adjusted raw material gradation;
(2) Properly adjust the temperature of the drying kiln. Generally, the temperature control range for the drying kiln is as follows: the temperature of the clay billet entering the kiln should be controlled at around 40℃, and the temperature of the bricks exiting the kiln should be controlled at around 110℃;
(3) Based on the temperature and humidity inside the drying kiln, adjust the drying time of the clay billets and the speed of the kiln's fan accordingly.

4. Under-fired brick
During the production of sintered bricks in tunnel kilns, it is often observed that there are severely under-fired bricks on both sides of the kiln car's bottom, which are whitish in color and have low strength, resulting in a high crushing rate of finished bricks.
Reason:
(1) The kiln car is not sealed tightly, allowing cold air to enter the kiln from the bottom, resulting in a temperature drop on both sides of the bottom of the brick stack. This is the main reason for under-fired bricks;
(2) The pressure, temperature, and atmosphere within the tunnel kiln are imbalanced, with the pressure inside the kiln being lower than that beneath the kiln car;
(3) The internal heat content in the raw material is insufficient;
(4) The production is insufficient due to inadequate sintering time and holding time.
Solution:
(1) Check the sand seals on both sides of the kiln walls that seal between the kiln cars, promptly repair any air leaks in the sealing grooves of the kiln cars, and ensure that there is sufficient sand in the sand seal grooves to prevent cold air from entering the kiln through the bottom of the cold air car;
(2) Adjust the balance between positive pressure, zero pressure, and negative pressure within the kiln, maintain the temperature of each zone inside the kiln, reduce the temperature difference between the upper and lower parts of the billets on the kiln car, and ensure normal firing and heat preservation of the brick billets;
(3) Analyze the calorific value of the raw materials and appropriately increase the roasting temperature;
(4) Strictly control the roasting and holding time, ensuring that roasting and holding are carried out with "fixed zone, fixed temperature, and fixed time", and do not compromise quality for the sake of increasing production.

5. Brick blasting
During the production of sintered bricks in a tunnel kiln, the bricks in the middle and upper parts of the stack may undergo bending deformation or develop bubbles, commonly known as "burst bricks" or "over-fired bricks".
Reason:
(1) The calorie content of the raw materials is set too high;
(2) The stacking of brick blocks is unreasonable;
(3) The baking zone moves forward, and the baking and holding time are too long.
Solution:
(1) Analyze the mixing ratio of raw material heat and allocate the calorific value reasonably;
(2) Improve the palletizing method by appropriately increasing the spacing between brick pallets;
(3) Strictly control the roasting and holding time.
6. Weather resistance
In the inspection of sintered bricks, many enterprises have encountered issues with poor weather resistance. Although our province is not classified as a severely wind-prone area, the problem still persists.
Reason: Due to changes in the raw materials used for brick making, many enterprises do not store the raw materials for a sufficient period of time. Some enterprises fail to take measures for homogenization and storage after crushing the raw materials, and directly feed them into the brick making machine for brick making. When the brick billets are sintered at the same temperature in the kiln, the solid and liquid phases of the raw material particles in the billets fail to meet the sintering requirements, which we refer to as "uncooked rice" or "uneven sintering".
Solution:
(1) The raw materials must undergo homogenization and aging treatment;
(2) Increase the firing temperature inside the kiln;
(3) For inferior raw materials, the particle fineness of crushing should be improved