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BY

RAHIM

SEMINAR ON “SILICA FUME AS AN

ADMIXTURE IN CONCRETE”

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What is Silica Fume?

• Silica Fume is an artificial pozzolona having high pozzolonic activity

• It is a By-product from an Electric Arc Furnace used in manufacture of Silicon metal or Silicon alloy.

• Has a high Silica content of more than 80%

• It is excellent for use as a Portland cement supplement.

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Silica Fume

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How Silica Fume works?

• In cementations 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.

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CHEMICAL COMPOSITION• Chemical Composition depends on the product

being made by the Furnace.• Composition is also influenced by the Furnace

Design.• It is mostly made of Silica having Silica percent

more than 80.• The other Chemical Composition include Fe2o3,Al2o3,CaO,MgO,Na2O,K2O in small

percentages.• Unlike other By-products like Fly Ash, Silica

fume from a single source has little or no variation in chemical composition from one day to another

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PHYSICAL CHARACTERSTICS

• Specific Gravity :- 2.2

• Specific surface area :- 20,000Sq.meters /Kg

• Particle size :- < 1 micron with Avg. Dia of 0.1 micron

• Particle Spherical in shape.

• Amorphous in nature.

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Available FormsAvailable Forms

• Undensified forms with bulk density 200-300 Kg/m3

• Densified forms with bulk density 500-600 Kg/m3

• Pelletised forms with Bulk density 600 -800 Kg/m3.

• Slurry form.

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Properties of Fresh Concrete using Silica fume

Workability:

• Reduced Workability

• Water demand increases in proportion to Silica fume added.

• Water demand is 1% for every 1% replacement of cement.

• Lower Slump & more cohesive mix.

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Bleeding & Segregation:

• Bleeding is reduced as Silica fume particles find their way in between two cement grains.

• Segregation is reduced has the concrete mix is more cohesive due to increase in number of solid to solid contact points.

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• Time of Set: Test results have shown that the initial

setting time and final setting time is not greatly influenced. The increase may be 30 min or so.

• Plastic Shrinkage: Fresh concrete which is placed is

subjected to surface cracking Know as plastic shrinkage. Since Silica Fume concrete show no bleeding, fresh concrete is subjected to plastic shrinkage. Precaution should be taken as per normal concrete

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Properties of Harden concrete

• Drying Shrinkage:

Drying shrinkage tests by different researcher show that long term shrinkage of concrete is not affected significantly by the addition of Silica Fume

• Creep:

Creep of concrete is inversely proportion to

to strength of concrete. Since Silica fume is highly pozzolonic material , the creep of concrete containing silica fume will be lower than corresponding Portland cement concrete.

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STRENGTH:1. Concrete containing Silica Fume showed

outstanding characteristics in development of strength.

2. Strength of 62 -80 Mpa can be easily achieved

3. If Silica Fume is used as an addition ,there is no deleterious effect on early strength and noticeable strength increase is recorded during the 3 to 28 days moist curing period when most of the pozzolonic reaction takes place.

4. Silica fume forms backbone of high performance concrete due to early high strength properties

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Durability aspects:

• Permeability

Durability of concrete is a direct function of its permeability. silica fume reduce the size of voids in hydrated cement paste, thus making them almost impermeable even at early ages with 10% addition of silica fume by weight of cement .

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• Freeze-thaw effect

The effect of silica fume concrete on freeze thaw affect is not very significant. but as compared to normal concrete ,it is better as silica fume concrete in hardened state is dense.

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• Abrasion resistance

Generally in high strength concert there is a direct relationship b/w strength and abrasion resistance. silica fume concrete is used to produce high strength concrete.

The factors that have a beneficial effect on

strength( like dense concrete, strong aggregates) also have a beneficial effect on abrasion resistance. silica fume concrete caters to all this needs.

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• Chemical resistance

A major reason for the improved resistance of concrete to acidic and sulphate waters is the reduction in the ca(oh)2 content of the cement paste, which decreases linearly with the amount of silica fume added. with 20% silica fume by weight of cement ,very little of ca(oh)2 produced by the Portland cement hydration is left in well hydrated cement paste.

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• Alkali aggregate reaction

The amount of pozzolona needed for reducing the alkali aggregate expansion depends on the reactivity of pozzolona. studies have shown that less than 10% of silica fume is found adequate for this purpose as compared to fly ash which requires 30%-40% replacement

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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 advantagesinclude:

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• 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.

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Case study 1• Construction of new tjorn bridge in

Sweden.• First industrial use of silica fume for

obtaining high strength and low heat of hydration.

• Bridge designed to contain very large structural members (pylons) of high strength concrete.

• Mix proportion used for pylons were Portland cement 370 kg/cu.m ,silica fume 37 kg/cu.m ,water 205 kg/cu.m,f ine aggregate 785 kg/cu.m, coarse aggregates 970 kg/cu.m.

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Case study 1

• Average strength was 62mpa

• Cement replacement by silica fume and post cooling of concrete by circulation of cold air through imbedded pipes,helped to lower the concrete temperature by 10-12 degrees centigrade.

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Case study 2

• Side walk constructed in bacancour(quebec) during the year 1980.

• Exterme cold climatic condition• Silica fume was used for the construction

purpose as replacement.• The side walk was inspected in the year

1983• The side walk had survived three winters

and numerous de-icing salt applications.

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conclusions• Silica fume can be used to manufacture

concrete of very high strength at early age.

• Forms the back bone of modern day high performance.

• Strengths upto 200 mpa can be achieved using silica fume.

• New type of concrete called reactive powder concrete developed using silica fume can give strengths mpa with required ductility.

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conclusions• Due to fine particle, low bulk density the

cost of shipping and handling is high.

• Slurry form costs 2 to 3 times more.

• Inspire of high cost of silica fume ,the demand for the material in the concrete construction industry is expected to grow due to the technical benefits offered by the material

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High-rise buildings using silica fume

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Girders reduced using silica fume concrete

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New TJORN Bridge in Sweden

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