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·        SILICAFUME

Silicafume is a by-product resulting from the reduction of high purity quartz with coal or coke and wood chips in an electric arc furnace during the production of silicon or Ferro silicon alloys. The silicafume, which condenses from the gases escaping from the furnaces has  very fine spherical particles having diameter of 0.1 micro meter.

Ferro silicon alloys are produced with nominal silicon contents of 61% to 98 %. As the silicon content increases in the alloy, the SiO₂ content increases in the silicafume. Generally more than 85% SiO₂ in silicafume is satisfactory for the use of silicafume in concrete.

Comparison of size and chemical

composition of different particles

·        PHYSICAL PROPERTIES AND CHEMICAL COMPOSITION OF SILICAFUME

Ø      Colour

Generally the colour of silicafume ranges from light to dark gray. Because SiO₂ is colourless, the colour is determined by non silica components. Which include carbon and iron oxide. Higher the carbon content in silicafume, the darker the colour of silicafume.

Ø      Density

The specific gravity of silicafume is about 2200 kg/ m³, as compared to 3100 Kg./m³ for normal Portland cement. Variations in density are attributed to the non silicafume components of the various silicafume.

Ø      Bulk density

Bulk densities of different types of silicafume are as under.

Normal silicafume or As produced silicafume     ----      130 to 430 Kg/m³

Slurried silicafume                                             ----      1320 to 1440 Kg/m³

Densified silicafume                                           ----      200 to 500 Kg/m³

Ø      Fineness, particle shape and over size material

Silicafume consists primarily of very fine smooth spherical particles with a surface area of approximately 20,000 m2/Kg. It is measured by nitrogen adsorption method. The nitrogen adsorption method is currently the most common to estimates the surface area of silicafume particles.

The particle size distribution of a typical silicafume shows most particles to be smaller than 1 micro meter with an average diameter of about 0.1 micro meter. This is approximately 1/100 of the size of an average cement particle. The particle size distribution of silica fume may vary depending upon the furnace gas exhaust temperature.

Scanning Electron Microscope picture      The amount of oversize material is strongly influenced by the a      gives visual indication  of  particle size                                    silicafume collection system; and this amount of oversize material may distribution               of        silicafume                                vary from one system to another.

Ø      Chemical Composition

Table [ 1 ]gives the chemical composition of typical silicafume. The silicafume generally contain more than 85 percent of Sio₂. The chemical composition of the silicafume varies with the type of alloy that is being produced. Ferro silicon alloys are produced with nominal silicon content of 50%, 75% or 90%. The Sio₂ content in silicafume increases with the silicon content in the alloy or metal. Table gives the limit value of different standards on silicafume. Other requirements are also given in table.

Table 1 : Chemical composition of silicafume and requirement of different standard.

Table 2: Requirements of silicafume for use in concrete according to different standards.   

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