SILICA FUME

Silica fume is a fine-grained powder obtained by the reduction of high-purity quartzite with coal and wood particles in electric arc furnaces used during the production of silicon metal or ferrosilicon (FeSi) alloys. In the upper parts of the furnaces at low temperatures, SiO gas is rapidly oxidized by contact with air and condenses into amorphous SiO2 to form almost all of the silica fume composition. The amount of SiO2 in the silica fume increases depending on the silicon content in the alloy.

Silica fumes used in research and practice are generally obtained from alloys containing ferrosilicon in 75% and higher ratios. There is little information on the use of lower-rate silica fumes.

CHEMICAL PROPERTIES In the composition of silica fumes obtained from silicon metal and 75% ferrosilicon alloys or mixtures thereof, the amount of SiO2 exceeds 85%, usually around 90%.

The amount of chlorine dissolved in acid in silica fumes varies between 0.016-0.025% by weight X-ray diffractograms showed that the silica fume was completely amorphous.

PHYSICAL PROPERTIES Particle Size The silica fume particle size is only one micron, which is about 100 times smaller than ordinary cement particles. Because of its extreme fineness and high silica content, Silica Fume is a highly effective pozzolanic material.

Specific Surface Specific surface is the total surface area of a given mass of a material. Because the particles of silica fume are very small, the surface area is very large. We know that water demand increases for sand as the particles become smaller; the same happens for silica fume. This fact is why it is necessary to use silica fume in combination with a water-reducing admixture or a superplasticizer. Specific surface determinations based on sieve analysis or air-permeability tesing are meaningless for silica fume.

Color Silica fume is often gray in different shades Color Silica fume is often gray in different shades. Since SiO2 is colorless, the darkness of the color increases depending on the amount of carbon. The excess iron oxide gives the color a light brown tone.

Specific gravity The specific gravity of silica fume is on average around 2.20. This value may vary slightly depending on alloy type and welding. Adding silica fume to concrete mix will not density the concrete.This expression means that silica fume isn’t a substance that is crystalline. A material will not dissolve before the substance could react in concrete, which must occur. Don’t forget that there is a crystalline material in concrete that’s chemically similar to silica fume. That substance is sand, while sand is basically silicon dioxide (SiO2), it doesn’t respond because of its crystalline nature

Unit Weight The loose unit weight of silica fume ranges from 130 to 430 kg / m3. This value can be increased to 1300 - 1400 kg / m3 in the form of a slurry, usually containing 50% silica fume. In the case of silica fumes delivered as compressed, the average unit weight is 550 kg / m3.

Grain Properties Silica fume generally consists of glassy, ​​smooth surface spherical particle. Since it is very fine-grained and light, its fineness cannot be determined by Blaine method on specific surface basis. However, the following values ​​still give an idea of ​​the fineness of silica fume.

REFERENCES Superior, Chemical and Physical Properties of Silica Fume Yeğinobalı, A., “Silis dumanının beton katkı maddesi olarak değerlendirilmesi”, Endüstriyel Atıkların İnşaat Sektöründe Kullanılması Sempozyumu, Bildiriler Kitabı, 149-167, İMO, Ankara, 1993. Eti Elektrometalurji A.Ş. İşletmesi yetkililerinden alınan bilgiler, Antalya, 1993, 2001. Silis Dumanı ve Çimento ile Betonda Kullanımı, TÇMB/ AR-GE / Y 01.01, Prof. Asım YEĞİNOBALI Türkiye Çimento Müstahsilleri Birliği, Temmuz 2009, ANKARA Newreach, What Is the Silica Fume?