silica fume (SF) is a byproduct of the smelting process in the silicon and ferro- silicon industry. The reduction of high-purity quartz to silicon at temperatures up to 2,000°C produces SiO2 vapours, which oxidizes and condense in the low- temperature zone to tiny particles consisting of non-crystalline silica. By-products of the production of silicon metal and the ferrosilicon alloys having silicon con- tents of 75% or more contain 85–95% non-crystalline silica. The by-product of the production of ferrosilicon alloy having 50% silicon has much lower silica content and is less pozzolanic. Therefore, SiO2 content of the silica fume is related to the type of alloy being produced .
Silica fume is also known as micro silica, condensed silica fume, volatilzed silica or silica dust. defines silica fume as a ‘‘very fine non- crystalline silica produced in electric arc furnaces as a by product of production of elemental silicon or alloys containing silicon’’. It is usually a grey colored powder, somewhat similar to Portland cement or some fly ashes. It can exhibit both poz- zolanic and cementitious properties.Silica fume has been recognized as a pozzolanic admixture that is effective in enhancing the mechanical properties to a great extent. By using silica fume along with superplasticizers, it is relatively easier to obtain compressive strengths of order of 100–150 MPa in laboratory. Addition of silica fume to concrete improves the durability of concrete through reduction in the permeability, refined pore structure, leading to a reduction in the diffusion of harmful ions, reduces calcium hydroxide content which results in a higher resistance to sulfate attack. Improvement in durability will also improve the ability of silica fume concrete in protecting the embedded steel from corrosion.
Vitreous silica, mainly of cristobalite form. Silica fume has a very high content of amorphous silicon dioxide and consists of very fine spherical particles. Silica fume generally contains more than 90% SiO2. Small amounts of iron, magnesium, and alkali oxides are also found.
Because of its extreme fineness and very high amorphous silicon dioxide content, silica fume is a very reactive pozzolanic material. As the Portland cement in concrete begins to react chemically, it releases calcium hydroxide. The silica fume reacts with this calcium hydroxide to form additional binder material called cal- cium silicate hydrate which is very similar to the calcium silicate hydrate formed
High density silica fume powder can reduce conductivity？
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