Sika Egypt

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Silica Fume and Fly ash

Mohamed Wagih

BU Manager Contractors

Sika Qatar

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Flash Agenda :

•Definition .

•Specification Standards.

•Chemical and physical Characteristics.

•Advantage and Disadvantage of Fly Ash in Fresh and

hardened concrete.

•Reaction of Silica Fume in Concrete

•Physical Contribution.

•Chemical Contribution.

•Effect of Silica Fume on fresh concrete and how those

effects improve constructability and the final concrete.

•Effect of Silica Fume on Hardened Concrete and how those

effects are used in concrete applications.

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Definition

Silica Fume Fly Ash

Fly ash is a group of materials that can vary significantly in composition. It is residue left from burning coal, which is collected on an electrostatic precipitator or in a bag house. It mixes with flue gases that result when powdered coal is used to produce electric power.

Very fine non-crystalline silica produced in electric arc furnaces as a by product of the production of elemental silicon or alloys containing silicon-ACI 116R-Grey colored powder similar to Portland cement or some fly ash.

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Fly Ash standards:

•Class ” F “ is made from burning anthracite and/or

bituminous coal .

•Class “ C “" is produced from lignite or sub bituminous coal.

In Canada, there is a further distinction.

•When the lime content is 8-20 percent, it is classified Cl, and

when it is higher, it is class C.

Silica Fume ( ASTM C1240 ) :

•Minimum Silicon Dioxide of 85 %.

•Maximum Moisture content to 3 % .

•Maximum loss in ignition to 6 % .

•The Oversize materials retained on a 45- m ( No.325) sieve to

a maximum of 10 %.

•The maximum variation from average be no more than 5%

points .

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Comparison of chemical and physical Characteristics Fly Ash,

Silica Fume and Portland cement.

Property Portland

Cement

Fly Ash

Class F

Fly Ash

Class C

Silica

Fume

SiO2 % 21 52 35 85 to 97

CaO % 62 5 21 < 1

Fineness as

surface area ,

m2/Kg

370 420 420 15,000 to

30,000

Specific gravity 3.15 2.38 2.65 2.22

General use in

Concrete

Primary binder Cement

replacement

Cement

replacement

Property

enhancer

Silica Fume is typically much more reactive, particularly at

early ages , because of its higher silicon dioxide content and

because of its very small particle size .

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Advantages of Fly Ash :

In Fresh Concrete In Hardened Concrete

Fly Ash particles are spherical and in the

same size range as Portland cement, a

reduction in the amount of water needed for

mixing and placing concrete can be obtained .

Good quality fly ash generally improves

workability or at least produces the same

workability with less water. The reduction in

water leads to improved strength.

In precast concrete , this can be translated

into better workability , resulting in sharp and

distinctive corners and edges with better

surface appearance , it also reduce

permeability which is leading cause of

premature failure .

Fly Ash has been suspected of contributing to

corrosion because it reacts with Calcium

Hydroxide.

Also, it reduces permeability that help to

protect the concrete from Chloride penetration.

The use of Fly Ash can result in better

workability , pump ability ,Cohesiveness ,

finish, ultimate strength and durability .

A super plastizer combined with Fly Ash can

be used to make high-performance and high

strength concrete.

The fine particles in fly ash help to reduce

bleeding and segregation .

Concrete containing fly ash generally

performs better than plain concrete in drying

shrinkage tests.

Cost of fly ash

Fly ash typically costs approximately 1/2 to 1/3 that of Portland cement as delivered, assuming a suitable means of batching is already in place.

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Disadvantages of Fly Ash :

•Poor-quality fly ash can have a negative

effect on concrete (increase permeability ).

•Some concrete will set slowly when fly ash

is used. Though this might be perceived as

a disadvantage, it can actually be a benefit

by reducing thermal stress .

•Freeze-thaw durability may not be

acceptable with the use of fly ash in

concrete.

•High-carbon fly ash materials tend to use

more water and darken the concrete as

well.

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Reaction of Silica fume in Concrete :

The benefit seen from adding Silica Fume are

the result of changes to the microstructure of

the concrete.

Physical Contribution Chemical Contribution

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Physical Contribution :

•Adding Silica Fume brings millions and millions of

very small particles to concrete mixture.

•Just like fine aggregates fills in the spaces between

coarse aggregate particles, Silica Fume fills in the

spaces between cement grains.

•This phenomenon is frequently referred to as particle

packing or micro-filling .

Even if Silica Fume didn’t react chemically, the micro-filler

effect would bring about significant improvement in the nature

of the concrete .

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Chemical Contribution :

•It is a very high amorphous silicon dioxide content

because of that silica fume is a very reactive

Pozzolanic material in concrete.

•As the Portland Cement in concrete begins to react

chemically it releases Calcium hydroxide.

•The Silica Fume reacts with this Calcium hydroxide

to form additional binder material called Calcium

Silicate hydrate , which is very similar to the Calcium

hydrate formed from the Portland cement .

•It is largely this additional binder that gives silica-

fume concrete its improved hardened properties.

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Effect of Silica Fume on fresh concrete and how those effects improve constructability and the final concrete.

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Effect of Silica Fume on Hardened Concrete and how those effects are used in concrete applications.

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We hope You got the information

you need from

this flash

By: Eng.:Mohamed Wagih

Business Unit Contractors Manager Sika Gulf B.S.C - Qatar Division

Return to the Technical Data sheet for more details