1. Silica Fume and Fly ash 5 minute Flash Mohamed Wagih
BU Manager Contractors
Sika Qatar

2. 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.

3. 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.

4. 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 .

5. Comparison of chemical and physical Characteristics Fly Ash, Silica Fume and Portland cement.
Property
Portland Cement
Fly Ash
Class F
Class C
Silica Fume
SiO2 % 21 52 35
85 to 97
CaO % 62 5
< 1
Fineness as surface area , m2/Kg
370
420
15,000 to 30,000
Specific gravity
3.15
2.38
2.65
2.22
General use in Concrete
Primary binder
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 .

7. 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.

8. 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.

9. Reaction of Silica fume in Concrete :
The benefit seen from adding Silica Fume are the result of changes to the microstructure of the concrete.
Chemical Contribution
Physical Contribution

10. 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 .

11. 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.

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

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

14. 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