CSF - Condensed silica fume

CONDENSED SILICA FUME


The raw materials for the production of silica fume are by-products from the production of silicon metal, and these by-products are further processed to produce cementitious materials for use in concrete. 

Production of silica fume :

Silica fume is a by-product of the manufacture of silicon metal and ferro-silicon alloys. The process involves the reduction of high purity quartz (SiO2) in electric arc furnaces at temperatures in excess of 2,000°C. Silica fume is a very fine powder consisting mainly of spherical particles or microspheres of mean diameter about 0.15 microns, with a very high specific surface area (15,000–25,000 m2 /kg). Each microsphere is on average 100 times smaller than an average cement grain. At a typical dosage of 10% by mass of cement, there will be 50,000–100,000 silica fume particles per cement grain. 

PHYSICAL PROPERTIES :

Fineness (m2 /kg) =15,000 to 35,000 
Bulk density(kg/m3 ) =1350-1510 
Specific gravity =2.2 

CHEMICAL PROPERTIES :

Silicon (% as SiO2)= > 85 
Aluminium (% as Al2O3) =< 2
 Iron (% as Fe2O3) =<1 
Calcium (% as CaO) =<1 
Magnesium (% as MgO) =<1 
Sodium (% as Na2O) =<1 
Potassium (% as K2O) =<1 
Titanium (% as TiO2)  =< 2 
Chloride (% as Cl) =< 0.3 
Loss on ignition (%) =< 4 
Sulfate (% as SO4) =< 0.3 
Free calcium oxide (%)  =< 1

REPLACEMENT :

Condensed silica fume is used as a replacement for portland cement. Silica fume can replace portland cement in the range of 9-15% by mass of cement.

ADVANTAGES :

Adding of silica fumes to concrete gives following advantages

1. It increases the strength of concrete.
2. It makes concrete impervious due to pore refinement.
3. It increases Alkali-aggregate resistance.
4. It increases sulfate resistance.
5. It improves abrasion resistance.
6. It reduce chloride permeability.

EFFECTS OF CONDENSED SILICA FUME ON CONCRETE PROPERTIES:

1. Setting time of concrete :
Silica fumes does not effect the setting time of concrete.

2. Need of water :
Silica fume can be expected to produce an increased water demand, which is normally countered by the use of admixtures. The water demand of concrete containing silica fume increases with increasing amounts of silica fume. This increase is caused primarily by the high surface area of the silica fume. To achieve a maximum improvement in strength and durability, silica-fume concrete should contain a high range water reducing admixture.

 3. Consistency (Slump) :
Fresh concrete containing silica fume is more cohesive and less prone to segregation than concrete without silica fume.  Silica fume addition has been used to assist in pumping long distances, especially vertically. Concrete was pumped in a single operation to a height of 601 metres at the Burj Khalifa project in Dubai; so far the world’s tallest building. 

4. Temperature according to age :
A reduction in the early age temperature rise can reduce the risk of early-age thermal cracking.However a slower release of heat can reduce the initial rate of strength gain. This may necessitate longer periods before striking formwork and/or removal of props especially when casting thin, exposed sections in winter conditions in cooler climates. 

5. Strength gain with CSF :
Concrete made with silica fume follows the conventional relationship between compressive strength and w/c but strength is increased at a given w/c ratio when silica fume is used. High early compressive strength (in excess of 25N/mm2 at 24 hours) can be achieved. With proper concrete design, very high 28-day strengths can be produced.

6.Permeability :
Silica fume can produce very large reductions in water permeability of up to one order magnitude or more, depending on the mix design and dosage of silica fume.

7. Protection to embedded steel (Carbonation) :
Steel embedded in concrete is protected against corrosion by the alkalinity of the cement paste. Despite any reduction in calcium hydroxide, resulting from the incorporation of silica fume, the pH of the cement paste remains at an adequately high level to protect steel. Carbonation can reduce the alkalinity and protection to the steel. Silica fume concrete tends to show greater carbonation than CEM mixes of equivalent 28 day strength. 

8. Protection to embedded steel (Chloride) :
If chloride permeates the concrete to the depth of the reinforcement it can initiate corrosion of the steel. Concrete made with silica fume is generally substantially more resistant to chloride diffusion than CEM I concrete and for reinforced concrete structures exposed to chlorides, its use will give enhanced durability. 

9. Sulfate resistance :
Sulfate attack causes expansion and disruption in harden concrete.Silica fume is very effective in reducing or preventing the attack from sodium sulfate.The primary mechanism by which silica fume improves sodium sulfate resistance is by reduction of permeability.

10. Abrasion resistance :
Silica fume will increase the abrasion-erosion and abrasion resistance of concrete through increased strength of the matrix and the improved bond between matrix and aggregates. 

11. ALKALI-SILICA REACTION :
 Alkali-silica reaction (ASR) is a reaction between the hydroxyl ions in the pore water within concrete and certain forms of silica, which occur as part of some aggregates.The use of silica fume in sufficient quantity and properly dispersed in concrete, either on its own or in combination with other pozzolans or GGBS, can be an effective means of combating ASR.


  

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