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Silica fume for concrete

silica fume is a by-product produced by electric arc furnace used in elemental silicon or alloy containing silicon industry. the condensing of gases escaping from electric arc furnace produce silica fume. the color of silica fume ranges from light to dark grey. the main component of silica fume is SiO2. SIO2 is colorless therefor other components determining the color of silica fume such as carbon and iron oxide. the approximate specific gravity of silica fume is 2.2. using of silica fume will reduce the content of cement in the concrete mix. also, it will increase durability of structure and this, in turn, will reduce maintenance. this will have help in preserving the resources and preserving the environment. silica fume along with GGBS can be used to provide concrete with high strength and durability.




Figure 1

A.effects on fresh concrete 
adding silica fume will improve the properties of fresh concrete. silica fume increases fresh concrete cohesiveness and reduces the bleeding.

  • cohesive: using silica fume increasing concrete cohesiveness. therefore the segregation of concrete is reduced. to compensate for the increase in concrete cohesion. the slump for the same concrete mix with silica fume increase by 40 to 50 mm. using silica fume for shotcrete or sprayed concrete allows for a thicker layer for overhead shooting. also using silica fume reducing rebound significantly.

Figure 2
  • bleeding: the water content for concrete with silica fume is low. therefore the bleeding of concrete will be reduced in case of concrete bleeding. the low water content attributed to the high surface area of silica fume.







B.effects on hardened concrete
Adding silica fume to concrete will enhance the concrete mechanical properties. the permeability of concrete reduced significantly and this will be followed by the increase of concrete durability. also using silica fume will increase the compressive strength of concrete.
  • Permeability: reduction of concrete permeability will increase concrete durability greatly. the time required by aggressive chemical to penetrate the concrete will increase with decreasing permeability and this will increase durability. for example, chloride is a corrosive agent and it will adversely affect the reinforced concrete by causing steel rust. Figure 3 showing the process of corrosion formation in reinforcing steel  
Figure 3


  • compressive strength: the use of silica fume will increase concrete compressive strength significantly. most of the strength will be gained at the age of 28 days. however, the concrete will continue to gain strength after 28 days but at a slower rate. this will enhance concrete abrasion resistance. also, reducing the dimension of structure and increasing span length for concrete bridges. 



  

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