In the process of concrete preparation, mineral admixtures are necessary mixed materials to improve the performance of concrete. Commonly used mineral admixtures include fly ash, slag powder, etc., but in high-strength concrete, silica fume is also a must. A mineral admixture, fly ash and silica fume are admixtures containing a large amount of silica, is it possible to replace fly ash with silica fume? Next, I will tell you whether silica fume can replace fly ash through the comparison between silica fume and fly ash.
Performance comparison between silica fume and fly ash
Liquidity comparison
(1) Natural mobility
Put a steel pipe with a height of 18cm and a diameter of 7cm on a flat plastic paper, and slowly lift the steel pipe after it is freely filled with silica fume, and measure the free fall fluidity of the silica fume. The diameter is 20cm, and the center height is 7cm; use the same device And method, measured fly ash free fall fluidity diameter is 28cm, center height is 3.5cm. It can be seen that the diameter of the free flow of silica fume is 8cm smaller than that of fly ash, and the height of the center is 3.5cm. The natural flow of silica fume is smaller than that of fly ash.
(2) Slurry fluidity
Experiments were carried out on the fluidity of silica fume and fly ash cement (according to the determination method of fluidity of cement slurry), and the fluidity of cement slurry mixed with 3% and 5% silica fume was measured to be 220mm and 210mm respectively; The fluidity of cement paste with 3% and 5% fly ash is 220mm and 220mm respectively. It can be seen that the fluidity of the cement slurry mixed with 3% silica fume is the same as that of the cement slurry mixed with 3% fly ash, and the fluidity of the cement slurry mixed with 5% silica fume is lower than that of the cement slurry mixed with 5% fly ash. Spend.
Comparison of cement performance under the same proportion
- Adding silica fume in the same proportion requires less water than the standard consistency of fly ash, but with the increase of silica fume content, the water consumption of standard consistency increases.
- The initial setting time and final setting time of fly ash mixed with the same proportion are prolonged, and the setting time is further prolonged with the increase of silica fume content.
- With the addition of silica fume in the same proportion, the 3d and 28d compressive strengths of fly ash both increase, but with the increase of silica fume content, the increase rate of cement strength shows a downward trend.
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Feasibility of silica fume replacing fly ash in the same proportion
After testing, it is found that when the same proportion of silica fume is used instead of fly ash, the fluidity loss of concrete increases, and the workability of construction becomes poor. However, since silica fume fills the voids of concrete, the amount of mixing water decreases, and the strength of concrete at 7d and 28d Both increased, indicating that silica fume can improve the strength of concrete by replacing fly ash.
Since the initial setting and final setting time will be prolonged after the addition of silica fume, it is not suitable for preparation and use in seasons with low temperatures, and the silica fume is finer and has a smaller bulk density. Storage, feeding stability and admixture should be considered when using it. According to the test data, the appropriate dosage of silica fume as an admixture is 3%, which can reduce water demand and improve early and late strength.
To sum up, under certain conditions, silica fume can be added to cement or concrete instead of fly ash, but in actual use, the actual amount of silica fume added should be determined through a reasonable ratio test, and most In some cases, in order to improve the performance of concrete in many engineering projects, silica fume, fly ash and other admixtures are used in combination.
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