Silica Fume and Fly ash

1. 5 minute Flash Silica Fume and Fly ash 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. 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 : 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