Brief analysis of five physical properties of brick raw materials
10-09
Or particle gradation. Although the finer the particle size of the raw material, the larger the specific surface area, the better the water penetration, and the better the plasticity of the raw material; but the brick raw material is by no means the finer the better. Because, all too fine raw materials are not conducive to the drying and roasting of products. The role of raw materials with different particle sizes in products is different: powder with particle size less than 0.05mm is called plastic particles, which are used to produce the plasticity required for molding.
Of course, these fine particles must be clay or shale, coal gangue or other material with clay-like properties. Otherwise, as for river sand, no matter how fine it is ground, it is not plastic. Followed by the particle size of 0.05~1.2mm part called filler particles, its role is to control the excessive shrinkage of the product, cracks and in the plastic forming to give the body a certain strength. As for the coarse particles with a particle size of 1.2~2mm, they play a skeleton role in the green body, which is beneficial to the discharge of water in the green body during drying. Hollow brick production raw material particles should not be greater than 2mm. Reasonable particle composition should be plastic particles accounted for 35 ~ 50%; filling particles accounted for 20 ~ 65%, coarse particles less than 30%.
The plasticity index is an important parameter for evaluating brick-making raw materials. In the brick-making industry, plasticity refers to the ability of clay-water material to be extruded at its maximum consistency and to maintain the shape after the pressure is released. The magnitude of this ability is expressed in terms of the plasticity index. High plasticity index is conducive to extrusion molding, but it is easy to crack when drying and roasting: low plasticity index, although it is conducive to drying and roasting, but it will bring difficulties to molding. If the plasticity index is lower than 7, it is not only difficult to extrude, but also the strength of the product is low. In general, a suitable plasticity index is 7 to 15. The higher the hole rate of the product, the more complex the hole type, the thinner the wall, and the higher the plasticity index required for molding. The plasticity index of clay is high, some can reach more than 25, coal gangue is low, sometimes less than 7, argillaceous shale is in the middle, often 7~18. Through certain technical means, the plasticity index of raw materials can be adjusted within a certain range or the technical performance of extrusion molding can be improved. For raw materials with high plasticity index, coal ash, slag, fly ash or waste brick powder can be appropriately mixed for refinement. For raw materials with poor plasticity index, it should be improved by mining in advance, natural weatherization, appropriate improvement of fineness, water drenching loosening, stirring, rolling, aging, grinding, mixing, extrusion and other methods. The clay and shale powder with higher plasticity index can also be mixed appropriately to adjust the overall plasticity index of the mixture. In the production of fly ash bricks, bentonite can also be added as a binder, so that the surface of each fly ash is stuck with a thin layer of bentonite, through mixing, homogenization, extrusion to improve plasticity, to meet the molding.
It must be pointed out that when using a variety of raw materials for production, the primary condition is to fully mix. Otherwise, it will not only fail to achieve the expected effect, but will cause drying and shrinkage cracks due to the different shrinkage rates of various raw materials. Because the quality of the extruded brick not only depends on the plasticity of the raw material, but also has a direct relationship with the friction coefficient between the raw material particles. Particles with a smooth and spherical surface are more likely to flow closer to each other when squeezed than particles with a rough rather than spherical surface. By adding some additives to the raw materials and the black liquor of the paper mill, the waste acid liquid of the chemical plant and the electroplating plant, due to its special ion adsorption and lubrication with the surface of the mud particles, the flow performance of the mud is greatly improved when it is extruded, and it is easier to be squeezed and dense to produce a smooth and dense brick.
During the drying process, the particles naturally converge due to water evaporation. The volume shrinkage is so-called drying shrinkage. The shrinkage length and the percentage of the original length of the body is called the drying line shrinkage rate. For raw materials with large shrinkage rate of the drying line, the products should be dried slowly, otherwise, the body will have serious drying cracks and become waste blanks. In production, the linear shrinkage rate of raw materials is required to be less than 6%, otherwise, the raw materials should be refined. During roasting, due to a series of physical and chemical changes and the ablation of certain substances in the raw materials, the finished product is not only lighter than the brick blank, but also slightly shrunk in volume called firing shrinkage. The length of its shrinkage is called the percentage of the length of the green body after drying.
4 Drying sensitivity factor
During the drying process, the volume of the body is gradually reduced while the moisture in the body is gradually evaporated. Since the drying speed and shrinkage speed inside and outside the body are always fast outside and slow inside, that is, the surface layer has dried and started to shrink, while the inside is still "intact". Once the shrinkage quantity exceeds the elastic coefficient of clay (1 ~ 2%), the surface of the body will be "burst" and mesh cracks will occur. This is called the drying sensitivity of mud and is expressed by the drying sensitivity coefficient. The greater the drying sensitivity coefficient, the more serious the threat of cracks in the drying process. When the drying sensitivity coefficient is less than 1, the problem in the drying process is small. Once the drying sensitivity coefficient is greater than 2, the risk of cracks in the drying process is very serious. Generally speaking, the higher the plasticity index of the mud, the higher the drying line shrinkage and the drying sensitivity coefficient.
5 Firing temperature range
More than the firing temperature continues to heat up, the body will slowly soften deformation, or even melt, the stack collapse. Obviously, there will be raw bricks or underfire bricks below the firing temperature. Obviously, the wider the sintering temperature range, the easier it is to master the roasting. For hollow bricks, this temperature range should not be less than 50 ℃. This is because the cross-sectional temperature of the kiln chamber cannot be exactly the same. It is often that the temperature in the middle is higher and the temperature on the side is lower, as well as the top and bottom, inside and outside, near the kiln wall and kiln door, there will be a certain temperature difference. If the firing temperature range is too narrow, the middle brick will be burnt and the side brick will be short of fire on the same section. Or the side brick is burnt and the middle brick is burnt: or this part is under fire and the other part is over fire. The sintering temperature of clay brick is low, about 900 ℃, while that of coal gangue brick is high, about 1000~1100 ℃, and the shale is in the middle.
