Silicon powder is a kind of dust collected and treated by a special trap device in the process of smelting industrial silicon and ferrosilicon in industrial electric furnaces at high temperature. Among the fumes, the content of SiO 2 accounts for about 90% of the total amount of fumes, and the particle size is very small. The average particle size is almost nanometer, so it is called silicon powder.
Although pure cement can be used to produce HPC with compressive strength up to 100 MPa, it will be much easier to use silica fume. For concrete with strength over 100 MPa, the use of silica fume is almost indispensable. Silicon fume is used as both filling material and pozzolanic material in concrete. After using silicon powder, the pore size in the hydrated slurry is greatly reduced, and the pore size distribution is improved, thus the strength is increased and the permeability is reduced.
However, although the development of early relative strength of silica fume concrete is slower than that of pure cement concrete at the same water-binder ratio, the absolute strength of silica fume concrete is higher than that of pure cement concrete due to the addition of silica fume.
For some silica fume concrete specimens dried or cured in air with very low water-binder ratio, compressive strength reversal has been reported (De Larrard and Atitiin 1993). This strength reduction usually occurs after 90 days of age and is generally considered to be caused by internal self-drying and drying cracks. However, many other researchers'laboratory and field studies have shown that the late strength of HPC has not decreased.