cement particles include particle gradation, particle shape, fineness and specific surface area, which have an important impact on the strength of cement, in which the shape and size of cement particles have a significant impact on the strength of cement. How to improve the strength of cement by adjusting the shape and size of cement particles has become an important issue to be solved urgently.
1. The characteristics
of cement particles are the core factors affecting the strength of cement. The specific surface area, fineness and particle morphology of cement particles are the key indicators to characterize its characteristics. In the field of domestic building materials, specific surface area and fineness have been widely used to describe the characteristic differences of cement particles. The results show that the larger the specific surface area of cement particles and the smaller the fineness of cement particles, the higher the strength of cement when the external conditions such as ambient temperature, humidity and curing process remain unchanged.
Taking the widely used ordinary 32.5 grade cement as an example, its fineness is accurately defined as 80 μm, the square hole sieve residue is strictly controlled within the range of not more than 6.0%, while the specific surface area is stable in the specific range of 350 to 380 m ²/kg, and the precise setting of each parameter is the same as that of the ordinary 32.5 grade cement. Based on a large number of experimental optimization and the crystallization of quality control practices, the fineness of 42. 5 grade cement is also accurately anchored at 80 μm, and the square hole sieve residue follows a more stringent standard, which is not more than 4.
In addition, the morphology and gradation of cement particles also have a significant impact on the strength of cement. As far as high-grade Portland cement is concerned, its optimal particle composition structure has been finely analyzed: in the key range of 9-25 μm, the particle content, as the core supporting force, steadily occupies 38% of the proportion, laying a solid foundation for the construction of cement strength; The content of 3-9 μm is like an agile "tactical coordination unit", which can be flexibly deployed in the range of 22% -36% according to the actual working conditions to achieve efficient coordination with other particle sizes; The content of 0-3 μm is used as an auxiliary supplementary force, accounting for 10% -20%. Each part performs its own duties and complements each other to build a solid structure of cement strength. Some scholars have proposed that the particle roundness should be accurately controlled at 0.
2. Cement particle morphology characterization parameters
The morphology of cement particles has an important impact on the strength of cement, and sphericity and roundness are the core parameters to describe its morphology. In actual research and production, the methods of obtaining sphericity and circularity data include manual method and image instrument method. Through careful observation and accurate measurement by scientific researchers, the sphericity and circularity of cement particles are counted one by one. With the help of high-precision microscope imaging technology, image analysis software and data processing algorithm, the image instrument method can quickly scan a large number of cement particle samples, and automatically count the detailed parameters of sphericity and circularity. These methods provide data support for the study of the internal relationship between cement strength and particle morphology.
3. Effect
on cement strength (1) Effect
of cement fineness and specific surface area on cement strength The fineness and specific surface area of cement particles have a significant effect on cement strength. Generally speaking, the finer the cement particles are, the larger the specific surface area is, and the higher the cement strength is. In the early stage of hydration reaction, the water reaction surface area is the key factor to determine the amount of hydrate in the early stage of hardening. With the progress of hydration, the hydration product structure around the cement particles is gradually formed, and the reaction rate is gradually slowed down. Therefore, increasing the grinding fineness can significantly improve the early cement strength. The results show that the particles less than 3 μm play a decisive role in the early strength development, but the later role is limited; the particles of 3-9 μm and 9-25 μm are the "backbone" to support the final strength of cement, and continue to contribute to the strength growth; increasing the content of 3-32 μm particles can significantly improve the strength of cement. CEMI32.
(2) The effect
of particle size distribution and particle morphology on the performance of cement The particle size distribution and particle morphology of cement have a significant effect on the strength of cement. In the case of similar specific surface area of cement, the subtle change of particle size distribution is the key factor to determine the strength of cement. The results show that the cement strength increases with the increase of the content of the particles with the particle size not more than 32 μm. Among them, the particle of 3-32 μm is the key to enhance the strength of cement, its content should not be less than 65%, and the particle size distribution should be continuous; the particle size of less than 3 μm should be controlled within 10%; the particle size of more than 65 μm should be the less the better; the particle of 16-24 μm has a great impact on the strength, the more the better. In addition, in the case of similar specific surface area, the cement with higher sphericity has unique advantages, the water consumption is significantly reduced, and the 28 d compressive strength is greatly improved.
(3) The effect
of particle morphology and distribution on the pore structure and strength of cement paste The morphology and distribution of cement particles have a significant effect on the pore structure and strength of cement paste. Studies have shown that spherical cement particles reduce the water requirement for standard consistency due to their excellent fluidity, while exhibiting excellent strength performance. Even if the particle composition and specific surface area are the same, the difference in circularity can lead to significant differences in cement strength. This is because the improvement of roundness optimizes the pore structure of hardened cement paste, reduces the spatial porosity, increases the number of small pores, reduces the number of large pores, and makes the pore structure of hardened cement paste more dense. Therefore, in the production practice, optimizing the grinding process to improve the roundness is the key to enhance the strength of cement.
(IV) Effect
of particle size distribution on cement setting time Cement setting time is significantly affected by particle size distribution. The setting time is determined by the amount of hydration products and the space between cement particles. When the hydration product is sufficient, the setting process is smooth; if the interparticle space is large, more hydration product is needed to fill it. When the particle size distribution is wide, the accumulation gap between particles is small, the water requirement for standard consistency is reduced, a large number of hydration products are produced in the early stage, the distance between particles is reduced, and the setting time is shortened. On the contrary, when the particle gap distribution is narrow, the water consumption is increased, the early output of hydration products is reduced, the distance between particles is increased, and the setting time is prolonged.
(V) Effect
of particle size distribution on cement hydration rate Cement particle size distribution has a significant effect on hydration rate. Under the same time and hydration depth, the smaller the particle size is, the higher the hydration rate is. When the hydration depth is shallow, the hydration rate of the cement with wide particle distribution is high, and the early strength advantage is obvious; while when the hydration depth is deep, the hydration rate of the cement with narrow particle distribution increases in the later period, and the strength advantage is obvious. The game between particle size distribution and hydration rate has a profound impact on the hydration process and strength development of cement.
(6) The research on the influence
of particle size distribution on cement strength shows that adjusting the particle size distribution of cement is the key to improve the compactness and strength of the pore structure of cement stone. The optimal particle gradation is that the content of particles smaller than 3 μm is controlled within 10%, the content of particles between 3 μm and 32 μm is more than 65%, and the content of particles larger than 65 μm and smaller than 1 μm is zero. Increasing the content of 3-32 μm particles can significantly improve the strength of cement. Reasonable planning of particle size distribution can make the cement paste achieve the best density accumulation state, significantly reduce the porosity and pore size of cement stone, and make the structure uniform and dense. Through fine adjustment of particle distribution, the bulk density and reaction area can be optimized, the optimal particle size distribution can be excavated, and the optimization and upgrading of cement strength can be realized.
4. Methods
to optimize the shape and size of cement particles (1) Selecting appropriate equipment
to improve the efficiency of powder selection is the key to optimize the shape and size of cement particles. The higher the separation efficiency is, the more concentrated the particle distribution is, which can effectively reduce the content of particles with particle size less than 3 μm and more than 45 μm. Cyclone, centrifugal and O-sepa separators are commonly used in the loop mill system, but the separation efficiency is significantly different. Under the same cycle load, the efficiency of centrifugal classifier is 50%, that of cyclone classifier is 64%, and that of O-sepa classifier is 78%. Therefore, in the selection of equipment, priority should be given to the equipment with high separation efficiency, such as O-sepa separator, in order to improve the quality of cement particle distribution and lay the foundation for the production of high-quality cement.
(II) Adjustment
of the internal structure of the mill The internal bin length ratio of the mill is the key parameter, which is usually 1:2. Deviation from this ratio will lead to abnormal operation of the mill and imbalance of particle size distribution. Through scientific transformation, the proportion of bin length is reduced to 1:2, which can improve the output of mill, optimize the particle size distribution, and provide guarantee for the production of high-strength cement.
(3) grinding the mixed materials together and grinding the mixed materials respectively
; grinding the fly ash, Limestone and other mixed materials with good grindability are ground together with clinker, which can increase the particles with particle size less than 3 μm, optimize the particle gradation and improve the strength of cement. However, for the mixed materials which are more difficult to grind than clinker, the strategy of separate grinding should be adopted to grind them separately to the appropriate particle size before blending, so as to ensure the stability of cement quality.
(4) Addition
of mineral admixtures Mineral admixtures are widely used in cement production because of their small particle size. The particle size of mineral admixtures is usually required to be less than 0 of the cement particle size. For example, the specific surface area of fly ash is controlled at about 500m ²/kg, which can effectively fill the particle gap, optimize the cement particle gradation, and significantly improve the durability and strength of cement.
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