Silica Fume

The use of pozzolanic materials is an old as that of  the art of concrete construction. it was recognised long time age, that the suitable pozzolns used in appropriate amount, modify certain properties of fresh and hardened mortars and concretes.
It has been amply demonstrated that the best pozzolans in optimum proportions moxed with portland cement improves many qualities of concrete, such as :  
·                     Lower the heat of hydration and thermal shrinkage ;
·                     Increase the watertightness;
·                     Reduce the alkali- aggregate reaction;
·                     Improve resistance to attack by sulphate soils and sea water;
·                     Improve extensibility;
·                     Lower gusceptibility to dissolution and leaching;
·                     Improve workability;
·                     Lower costs.
Pozzolanic materials are siliceous or siliceous and aluminous materials, which in themselves possess little or no cementitious value, but will , in finely divided form and in the presense of moisture, chemically react with calcium hydroxide liberated on hydration, at ordinary temperature, to form compounds, possessing cementitious properties.
What is Silica fume?
Silica fume is a by-product from electric are furnaces used in the manufacture of silicon metal of silicon alloys. The material, which contains more than 80% silica in no crystalline state and in the form of  extremely fine particles (0.1 um average diameter), is highly pozzolanic. This product is excellent for use as a Portland cement supplement In addition to economic and energy saving potential, the use of pozzolanic admixtures in concrete leads to several technical advantages, such as reduction in thermal cracking caused by heat of cement hydration, improved durability to attack by sulphate and acidic waters, and high ultimate strength.
Unlike other by- product pozzolans such as fly ashes, a unique feature of CSF is that it has a better faster pozzolonic action.

CEMENTITIOUS COMPOUNDS
In cementitious compounds, silica fume works on two levels, the first one described here is a chemical reaction called the "pozzolanic" reaction. The hydration (mixing with water) of Portland cement produces many compounds, including calcium silicate hydrates (CSH) and calcium hydroxide (CH). The CSH gel is known to be the source of strength in concrete. When silica fume is added to fresh concrete it chemically reacts with the CH to produces additional CSH. The benefit of this reaction is twofold; increased compressive strength and chemical resistance. The bond between the concrete paste and the coarse aggregate, in the crucial interfacial zone, is greatly increased, resulting in compressive strengths that can exceed 15,000 psi. The additional CSH produced by silica fume is more resistant to attack from aggressive chemicals then the weaker CH.

Chemical composition of silica fume

Chemical composition of silica fume will mostly depend upon compositon of the principle product being made by furnace.additionally,the composition is also influnced by the furnace design; generally a furnace with a heat recovery system producessilica fume with lower ignition loss.the sio2 content of silica fume varies with the silica content of alloy being producer.unlike other by product pozzoloans such as fly ash,silica fume from a single source is little or no varition inchemical composition from one day to another,

Item

Requirement

       Sio2
   >90%
       mgo
  <1.5%
       SO3
  <1.1%
       H2O
  <0.4%
       K2O
  <2.25%
       Na2O
  <1.4%
       CaO
  <0.35%
        Si
  <0.5%
        Cl
  <0.06%
       Fe2O3
  <2.0%

Physical characteristics

The specific gravity of  CSF from silicon metal of ferrosilicon alloy industries is close to that of amorphous silica, that is approximately 2.2
The amorphous silica structure and the fine particle size re the principle reasons for the excellent pozzolanic activity of CSF, the surface area by Blaine air permeability method ranged from about 3.3 t 7.7 m2 / g The spherical shape of CSF can be confirmed by scanning electron microscopy of well- dispersed particles Figure 5.5 ) generally, the particles exist in the form of agglomerates.
Properties of fresh concrete using silica fume

Workability:-

Water demand increases in proportion to the amt of silica fume added. The increase in water demand of concrete containing silica fume is about 1% for every 1% cement substituted Workability is the ease with which a concrete mix can be handled. Water measures can be taken to avoid this increase by adjusting aggregate grading and use of super plasticizer. The addition of silica fume will lead to a cohesive mix due to more solid-to-solid contact and will have a lower slump. This reduces bleeding and segregation.

Bleeding and segregation:-

The effect of silica fume on the rheology of fresh concrete is considered a stablising affect in the sense that addition of fine particle to a concrete mix tends to reduce segregation and bleeding. When very fine particles of silica fume are added to concrete the size of flow channel is greatly reduced because these particle are able to find their way into empty spaces between 2 cement grains causing drastic segmentation of bleed water flow and reduce bleeding. Due to increase in the number of solid-to-solid contact, the cohesiveness of the concrete mix is greatly improved when silica fume is added. This makes the concrete highly attractive for pumping, shotcreting as it reduces segregation.
Time of set:-
 Silica fume in small amounts (10% by weight of cement) to ordinary concrete mixtures (250 to 300 kg/ m3 cement) either has no significant effect or alters the time of set of reference concrete only slightly. For example, the data reported by pistilli et al,27 showed that the presence of 24 kg/m3 CSF (FeSi - 75 type) in a concrete mixture containing 237 kg m3 Type 1 Portland cement increased the initial and the final times of set (ASTM Method (403) by 26 min and 29 min respectively.
Plastic Shrinkage:-
Freshly placed concrete mixtures that have not vet set (i.e., that are still in a plastic state ) are prone to surface cracking due to the phenomenon known as plastic shrinkage. Since concrete containing CSF shows little or no bleeding, there are several reports confirming the sensitivity of this concrete to plastic- shrinkage, cracking when exposed to drying conditions at early age. In order to overcome this problem, the surface of concrete must be protected against evaporation as soon as possible after placement. This precaution is a part of the standard concrete curing practice in hot weather.

Properties of hardened silica fume concrete

Drying shrinkage:-
The date from drying shrinkage tests by different researchers shows that the long- term shrinkage of concrete is not affected significantly by the addition of CSF, especially when the water content of the concrete mixture has not been changed.

Creep:-

Creep of concrete is inversely proportional to strength. Since CSF is a highly pozzolanic material, it is expected that the creep of concrete containing CSF will be lower than the corresponding Portland cement concrete a reference concrete made of a calcareous aggregate and normal Portland cement, and a concrete made of the same amount of the aggregate but with 25% (by weight) cement replaced by CSF. The ratio between water to cementations material was 0.435
Examination of the 1- year creep data showed that basic creep strains were similar in both the cases: however, the drying creep strain was about 370 X 10-6 for the reference concrete compared to 300 X 10-6 for the CSF concrete

Strength charecteristic:-.
If CSF is used as an addition, there is no deleterious effect on early strengths (i.e. I- day and 3 day strengths), and a noticeable strength increase is recorded during the 3 to 28 days moist- curing period 30 when most of the pozzolanic reaction takes place. Consequently, the relative strength increase during the 28 to 90 - day period is relatively low. From the results of the investigation reported the addition of 24 kg/m3 CSF to the concrete mixture containing 297 kg/m3 ASTM Type I Portland cement caused a strength increase of about 10% and 20% at 7 and 28 days, respectively; there were no differences in the strength of reference concrete and the CSF - concrete at the test ages I and 2 days.
Three series of both non air- entrained and air- entrained concrete mixtures were designed. The first contained 284kg / m3 of ASTM Type I Portland cement 0.6 ratio between water to cementations materials (i.e., Portland cement + CSF), and 0.5, 10, and 15 % cement replacement replacement by weight with CSF (94 % SiO2, 20 m2/g surface area) The second contained 340kg/m3 Portland cement 0.5 ratio between water to cementations materials, and similar levels of cement replacement with CSF as the first series. The third contained 431 kg /m3 Portland cement, 0.4 ratio between water to cementations materials.
The incorporation of CSF did not result in a significant change in compressive strength at 3 days with concretes of 0.6 and 0.5 ratio of water/ (cement+ CSF); however, the concrete with 0.4 ratio of water / (cement + CSF); showed increase in strength with increasing amount of the fume used in the test (i.e. 5 to 15 % cement replacement)
Regardless of water / (cement + CSF) ratio at 7 days and 28 days the compressive strength of concrete were increased.
All air- entrained concretes, both with and without: CSF, showed strength loss when compared with the corresponding non- air entrained concrete
DURABILITY ASPECTS
Permeability:-
Durability o a concrete to aggressive water is generally a direct function of its permeability. Manmohan and Mehta observed that highly reactive pozzolans, such as rice husk ash, are able to reduce the size of voids in hydrated cement pastes, thus making them almost impermeable even at an early age (7- 28 days) with as low as 10% additio of the pozzolan by weight of cement. a concrete moxture containing 100 kg / m3 portland cement 20% CSF, and a superplasticizer showed approximately the same permeability as a concrete containing 250 kg/m3 Portland cement but no CSF or plasticizer.
Abrasion resistance
Generally, in high- strength concrete there is a direct relationship between strength and abrasion resistance. It may be noted that exceptionally high strength in concrete cannot be achieved unless concrete is dense (low water cement ratio) and a strong aggregate has been used. The factors that have a beneficial effect on strength have also a beneficial effect on abrasion resistance. HENCE SILICA FUME CONCRETE HAS HIGH RESISTACE TO CONCRETE. A major reason for the improved resistance of concrete to acidic and sulphate waters is the reduction in the calcium hydroxide content of the cement paste which decreases linearly with the amount of CSF added. With 20% CSF by weight of cement, very little of the calcium hydroxide produced by the Portland cement hydration is left in a well hydrated cement paste.


Alkali- aggregate reaction

The amount of pozzolan needed for reducing the alkali - aggregate expansion depends on the reactivity of the pozzzolan. Whereas many researchers have reported that 30 to 40% replacement of a high- alkali Portland cement by a class F fly ash may be needed to control the alkali- aggregate expansion according to the ASTM Method C 227, even less than 10% CSF is found adequate for this purpose.

Resistance Corrosion of embedded steel

The service life of reinforced and prestressed concrete is often adversely affected by corrosion of the embedded steel. A strongly adherent iron - oxide film is usually present on the surface of steel.  and this film must get disrupted before the anodic and the cathodic processes associated with the corrosion phenomenon can make sufficent progress. In some situations, such as in the presence of chloride ions, it somehow gets disrupted: this explains why cholorides are known to accelerate the corrosion of steel in concrete.
The ability of concrete to protect embedded steel from corrosion also depends on electrical resistively.

Freeze – thaw resistance

Air void stability of concrete incorporating silica fume was studied. The test results indicated that the use of silica fume has no significant influence on the production and the stability of the air void system. However freeze-thaw testing on silica fume concrete showed acceptable results. The durability was considerable greater as compared to normal concrete
Industrial Use:
Case Study 1:
The construction of New Tjorn Bridge (Figure 5.15) in Sweden is believed to be the first industrial use of CSF for obtaining both a high strength and a lower heat of hydration than otherwise would have been possible According to Rockne and Svensson45 the bridge was designed to contain some very large structural members (Pylons) of a high - strength concrete  (50MPa). The use of a high content of Portland cement in the concrete mixture would have caused excessive heat of hydration with risk of thermal cracking. The principal reason for using CSF was that this permitted a deduction of the cement content without loss of strength. The average strength of concrete was 62 MPa : reduction in the Portland cement content that was made possible by the use of CSF puls the post- cooling of concrete by circulation of cold air through imbedded pipes helped to lower the peak concrete temperature by 10 to 12 C. Case Study 2CSF was used to produce a very high- strength concrete foe a high - rise building in Montreal,canada. The average compressive strength of concrete cylinders was reported to be apa at 3 days. 75 MPa at 7 days, and 90 MPa at 28 days.
Case Study 2: -
In 1980.SKW Canada. Inc., xonstructed an experimental sidewalk in Becancour (Quebec) with a concrete containing CSF. Until the time of his report in1983, thesidewalkhassurvived three Canadian winters and numerous de-icing salt applications proving durability of silica fume concrete.

Cementitious application of silica fume

High Strength Concrete
High Performance Concrete (HPC) enhanced with silica fume is so strong it becomes an economical alternative to steel. Besides providing architects and engineers with greater design flexibility, there is more usable space, since smaller columns and beams can be used in high rise buildings and long span structural designs. Other advantages include:
  • Tremendous compressive strengths up to 20,000 psi (140 MPa).
  • High modulus of elasticity exceeding 6 million psi (40,000 MPa). High flexural strengths (2,000 psi @ 28 days) for airport pavements.
  • Greatly reduced permeability to moisture, chlorides and chemical attack.
  • Increased resistance to abrasion, erosion, corrosion.
  • High early strengths for fast-track construction projects and precast applications.
Shotcrete 
Greater economy, greater timesaving and more efficient use of sprayed concrete. Norchem silica fume produces superior shotcrete for use in rock stabilization; mine tunnel linings, and rehabilitation of deteriorating bridge and marine columns and piles. Greater bonding strength assures outstanding performance of both wet and dry process shotcreting, with greater impermeability and less rebound loss. Norchem silica fume provides improved cohesion that resists washout in tidal rehabilitation of piles and sea walls. In addition, thicker applications--up to 12 inches-- can be achieved with each pass of the nozzle. Superior performance characteristics are:
  • High compressive and flexural strengths.
  • Reduction of rebound loss up to 50%.
  • Improved production time, with one-pass application thicknesses up to 12 inches           higher bonding strength.
  • High electrical resistivity and low permeability
Oil Well Grouting
Whether used for primary (placement of grout as a hydraulic seal in the wellbore) or secondary applications (remedial operations including leak repairs, splits, closing of depleted zones), the addition of silica fume enables a well to achieve full production potential. Besides producing a blocking effect in the oil well grout which prevents gas migration, it provides these advantages:
  • Improved flow, for easier, more effective application
  • Dramatically increased impermeability, for better control of gas leakage
  • Easier handling
  • Better results

Conclusion:-

The use of CSF in concrete offers many advantages; however for several reasons the indiscriminate use of the material is not recommended. Incorporation of any additional ingredients into concrete requires extra handling and storage facilities Besides cost, special precautions will be needed for batching unusual materials such as CSF, and for curing concrete containing this admixture.
Cost. Although CSF is an industrial waste, due to its fine particle size and very low bulk density the cost of shipping and handling the material is rather high. The material that is being marketed in the form of a water suspension probably costs two to three times as much.
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