The function of silica fume in concrete

The function of silica fume in concrete

In the process of high temperature smelting of industrial silicon and ferrosilicon, the dust escaping from the waste gas is silica fume or silicon powder, and its main component is active SiO2 (the content is about 90%, of which the free SiO2 accounts for about 10%). 

The particle is very small, which is 1% of the particle size of cement, small capacity, large specific surface area, inherent micro-silicon powder, it can directly enter the human alveoli through breathing, which does great harm to human health. 

Silica fume is widely used in construction, chemical industry, metallurgy and other industries. 

It is mainly used as active admixture in concrete to improve the performance of cement or concrete, and to prepare special concrete with ultra-high strength, wear resistance, erosion and corrosion resistance, impermeability and frost resistance, and early strength. It can be used in large hydropower, seaport, railway bridges, highways, airport runways, tunnels and urban high-rise buildings. 

Due to the physical and chemical characteristics of silicon powder, such as fine particles, large specific area, high purity of active SiO2 and strong pozzolanic activity, adding silicon powder as admixture can improve the performance of concrete in many aspects. 

1. Improve the early strength and final strength of concrete 

On the domestic research surface, when the substitution rate of silicon powder to cement is less than 30%, the steam curing temperature is 80 ℃, the one-day compressive strength of mortar is twice as high as that without silicon powder, and if autoclaved curing is adopted, it is almost 3 times; the compressive strength of mortar is also obviously improved by standard curing. 

2. Increase compactness

The addition of silicon powder into concrete increases the content of reactive silicon. Under electron microscope, there is crystal growth in the void of cement stone mixed with silicon powder. 

In addition, the silicon powder particles are very small, uniformly filled with concrete micropores, and the filling effect of microaggregate is obvious. 

3. Improve the segregation and bleeding performance of concrete

After pouring concrete, it often produces the phenomenon that water is separated from concrete, that is, a water film is formed on the surface, which is also called floating slurry, which makes the distribution of upper concrete uneven and affects the quality of the building. 

Foreign studies have proved that the more the amount of silicon powder, that is, the substitution rate Si/ (Si+C), the more difficult it is for concrete materials to segregate and bleed. 

When the substitution rate reaches 15%, the concrete slump hardly produces segregation and bleeding even if the slump reaches 15~20cm. 

When the substitution rate reaches 20% to 30%, it is not easy to separate the concrete directly into the water. 

Due to the improvement of segregation and bleeding performance of concrete with silicon powder, concrete mixed with silicon powder can be used in ports, tunnels and other underwater projects. 

4. Improve the impermeability, chemical corrosion resistance and specific resistance of concrete

When the micro-silica powder is used in concrete, on the one hand, it fills around the cement particles to make the paste denser; on the other hand, it combines with the calcium hydroxide produced by cement hydration to form hydrated calcium silicate gel, which is blocked in the capillary. make the capillaries smaller and discontinuous, greatly improve the compactness of concrete, effectively improve the chloride ion penetration resistance, and greatly enhance its corrosion resistance. 

Foreign studies have proved that when the replacement rate of silicon powder in concrete is 10%-20%, the impermeability and chemical corrosion resistance of concrete are significantly improved, and the corrosion resistance of steel bar is also improved. 

This is because the increase of compactness and the increase of SiO2 content effectively prevent the invasion and corrosion of acid ions. 

In addition, because the specific resistance of silicon powder is very high, the specific resistance of concrete can be increased by more than twice. 

5. Micro-silicon powder concrete can improve the anti-abrasion ability of the project, and the traditional anti-abrasion materials are often used in epoxy mortar and other polymer materials. 

Although this kind of material has good anti-erosion ability, it is easy to crack and fall off under the natural climate because its linear expansion coefficient is several times based on the bottom ordinary concrete and does not adapt well to the temperature of the base concrete. 

And the construction is complex, toxic and expensive, so it can not be popularized and applied in large area. 

When micro-silicon powder is mixed into concrete as an anti-abrasion material, its erosion resistance can be doubled. 

6. Micro-silica powder concrete can improve the crack resistance of the project, and the improvement of crack resistance of silica fume concrete benefits from the increase of the compactness of its concrete. however, the crack resistance of silica fume concrete must pay attention to the unfavorable factors of concrete shrinkage caused by the addition of silica fume, and grasp the choice of content in the process of practical use.

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