The particle size of the silicon powder is less than 1 μm with the average particle size 0.1 to 0.3 μm and the specific surface area is 20 to 28 m2 / g. Its fineness and specific surface area are of about 80 to 100 times than that of cement.
Although HPC with compressive strength up to 100 MPa can be made with pure cement, it will be much easier when using silica fume. For the preparation of concrete strength of more than 100 MPa, the use of silicon powder is almost indispensable. Silica fume in the concrete at the same time from the filling material and volcanic ash material used. The use of silica fume greatly reduces the pore size in the hydrated slurry and improves the pore size distribution, thus increasing the strength and decreasing the permeability. For example, the results of the study (CEB2FIP1988) show that for cement to be 70 MPa concrete, the cement ratio is 0.35 for pure cement, and 0.50 for 8% silica. Because the silica fines are very fine, they can react with volcanic ash in the early hours. According to Carette and Malhotra (1992), the contribution of silica fume to concrete strength is mainly before 28 days. Therefore, the long-term strength growth, the general view that the silica fume concrete than pure cement concrete or fly ash concrete. Almad (1994) cited the results of the development of NSC strength by silica fume, which indicates that the increase in silica fume content leads to a decrease in early relative strength development. Sandvik 1992 also found this phenomenon in 65 MPa concrete.
However, despite the relatively early development of silica fume concrete at the same water / cement ratio than pure cement concrete, the strength of silica fume concrete is much higher than that of pure cement concrete, due to the addition of silica fume. On the other hand, experience has shown that the early strength development of HPC is faster than that of NSC, although the setting time of HPC may be slightly delayed, and the hydration after coagulation can be greatly accelerated by superplasticizers and silica fume. The result is usually a very rapid development of strength after condensation.
There are reports of compressive strength collapse for certain silica fume concrete specimens that have been dried or cured in the air at very low water / cement ratios (De Larrard and Aiticin 1993). This decrease in strength usually occurs after 90 days of age and is generally believed to be caused by internal self-drying and dry fractures. However, laboratory and field studies by many other researchers have shown that the late strength of HPC does not decrease. For example, all cores tested from six different HPC samples from 3 months to 3 years showed increased strength. Of course, the long-term strength growth potential of HPC is smaller than that of NSC.
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