Fine slag powder (or ultra-fine granulated blast furnace slag powder) refers to the granulated blast furnace slag that meets the national standard "Used for Granulating Blast Furnace Slag in Cement" GB/T203-1994. Powder machine (or add a small amount of gypsum to grind together) can achieve a powder with a considerable fineness and corresponding activity index.

1. The role of ultrafine slag powder in concrete

1. Improve the microstructure of concrete. Mainly by improving the gradation of concrete fine particles, that is, improving the particle size distribution of powder materials in concrete, producing a dense packing filling effect, and optimizing the pore structure of concrete, that is, the number of large pores is reduced, the number of small pores is increased, and the average pore size is reduced. , the distribution is more reasonable; the porosity is reduced, especially the voids between cement hydration products, and the microstructure is more compact.

2. Reduce the mixing water consumption of concrete (especially the content of free harmful water). Only a small part of water is needed for complete hydration of cement, and most of the water in concrete is introduced for its workability. We call it free state harmful water. During concrete construction and hardening, only a small part of free harmful water may seep through air evaporation and formwork, but most of the free harmful water will form larger voids after concrete hardening, thus causing permanent damage to the concrete structure. When the pore structure of concrete is optimized and the porosity is reduced, the content of free harmful water can be reduced accordingly.

3. Improve the workability of concrete mixture. Since the ultra-fine slag powder is a fine spheroid, its surface is smooth and its performance is stable, it can act as a kind of microbead lubrication similar to a bearing in concrete, which reduces frictional resistance and effectively improves the mixability of the concrete mixture. Properties (i.e. fluidity, cohesiveness, water retention, etc.). The workability of the concrete mixture is good, the slump loss over time is small, and the workability (pumpability, etc.) is good.

4. Improve the strength of concrete. One is to improve the microstructure of concrete and increase the density to increase the strength of concrete. Second, the ultra-fine slag powder is rich in active SiO2, etc., which can undergo a secondary hydration reaction with the hydration product Ca(OH)2 of cement to form a silicic acid gel with higher density.

5. Reduce the amount of cement and heat of hydration of concrete. The national standard stipulates that the temperature difference between the concrete center temperature and the surface temperature shall not exceed 25°C. When the strength of concrete is constant, adding ultra-fine slag powder can greatly reduce the amount of cement. At the same time, the appearance time of the heat peak can be delayed.

6. Improve the impermeability of concrete.

7. Improve the frost resistance of concrete. A large number of studies have shown that the amount of cementitious material, water-binder ratio, and air-entraining performance of concrete are the main factors affecting the frost resistance of concrete. When the water-to-binder ratio of concrete is large, there is a lot of harmful water in its free state. After hardening, the internal pore structure is poor and the porosity is large, so the probability of being damaged by freeze-thaw cycles is high. When a large number of fine and uniform air bubbles are introduced into concrete, these fine air bubbles, that is, fine spaces, can act as a "shock valve" for volume expansion, reducing and avoiding damage caused by other physical and chemical reactions. Tests have shown that the frost resistance of concrete mixed with ultrafine slag powder is significantly higher than that of ordinary concrete.

8. Improve the crack resistance of concrete. During the hardening process of concrete, due to chemical shrinkage, cold shrinkage and dry shrinkage, the volume shrinkage will be caused, and the shrinkage value is about 0.04% of the autogenous volume. These shrinkages can cause great harm to the volume stability of concrete. Tests have shown that the concrete mixed with ultra-fine slag powder will produce moderate micro-expansion. Under the constraints of steel bars and aggregates, a certain precompressive stress can be generated to offset the tensile stress generated by the concrete during hardening, compensate for the temperature stress caused by part of the heat of hydration, and reduce and avoid the generation of concrete cracks.

9. Improve the corrosion resistance of concrete. The ultra-fine slag powder is rich in active SiO2, etc., which can undergo secondary hydration reaction with the hydration product Ca(OH)2 of cement, thus reducing the chance of cement colloid reacting with SO42-- to form ettringite after the concrete hardens , to a certain extent inhibited SO42- plasma erosion damage. This type of ettringite is the most destructive because the stress generated by the volume expansion is restrained by the hardened concrete. In addition, the addition of ultra-fine slag powder reduces the amount of cement, that is, reduces the alkali content introduced by cement, thereby reducing the possibility of alkali-aggregate reaction damage to concrete.

10. Extend the service life of concrete structures. The Mehta holism model points out: “A concrete that is impermeable but has microcracks and is porous → undergoes erosion heat and cold cycle, dry and wet cycle → microcracks in the concrete structure increase and connect → water infiltration, harmful substances corrode → concrete Expansion, corrosion of steel bars, alkali-aggregate reaction, water freezing, and sulfate attack reduce the strength and stiffness of concrete → cracking damage and loss of integrity."

The impermeability of concrete is the first line of defense in any process of physical and chemical damage. Therefore, the application of ultra-fine slag powder in concrete is of great significance to the sustainable development of concrete, that is, to save the use of concrete raw materials and improve the durability of concrete structures.

2. The best application range of ultrafine slag powder

1. High-strength grade concrete. It is quite difficult to prepare C50 concrete with the current cement. Then, the preparation of C60, C70 or even C80 concrete must be completed by adding ultra-fine slag powder and high-performance admixtures.

2. Mass concrete. It can reduce the amount of cement and heat of hydration, delay the appearance of heat peak, and effectively prevent problems such as cracks.

3. In addition to the above-mentioned optimal application range, as long as cement and concrete are used, different amounts of ultra-fine slag powder can be added.

In short, the application of ultra-fine slag powder in concrete is not only related to the quality of concrete, but also related to the big problem of "building a conservation-oriented society and taking the road of sustainable development".