CON 123 Session 9 - Silica Fume

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  • in slide 12, it is mensioned that Silica Fume increased chloride permeability. I think this point needs to be corrected. Silica fume provides a dramatic improvement in chloride penetration resistance.
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  • Cement grains and silica fume particles at the same magnification. In a 15 percent silica fume replacement of cement, there are approximately 2,000,000 particles of silica fume for each grain of portland cement.The longer white bar in the silica fume side is 1 µm long.ACI 234R-96, Guide to the Use of Silica Fume in Concrete.conc696
  • SI version of previous slide.These values are typical for concrete containing about 360-390kg/m3 of cement with a w/cm < 0.40.The “strength trap” is the situation in which strength achieved with silica-fume concrete will greatly exceed most project specifications. This factor may cause concrete producers and contractors to relax their control over the concrete production and curing. While strength may be adequate for structural design, rapid chloride permeability values may not be within specifications.
  • SI version of previous slide.This slide summarizes many years of experience using silica fume for improved durability in bridge decks and parking structures.More and more concrete in this application is now including either fly ash or slag along with the silica fume. See the examples in Chapter 5 for typical mixture proportions.ASTM C 1202, “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration.”
  • CON 123 Session 9 - Silica Fume

    1. 1. CON 123Cementitious MaterialsSession 9Silica Fume
    2. 2. What is Silica Fume?“Very fine non-crystallinesilica produced in electricarc furnaces as abyproduct of theproduction of elementalsilicon …” -- ACI 116R
    3. 3. What is Silica Fume? Cement grains and silica fume particles at the same magnification. In a 15 percent silica fume replacement of cement, there are approximately 2,000,000 particles of silica fume for each grain of Portland cement. The longer white bar in the silica fume side is 1 µm long. ACI 234R-96, Guide to the Use of Silica Fume in Concrete.
    4. 4. Specification for Silica Fume ASTM C 1240Silica Fume—finelydivided residue resultingfrom the production ofsilicon, ferro-silicon, orother silicon-containingalloys that is carried fromthe burning surface areaof an electric-arc furnaceby exhaust gases.
    5. 5. Silica Production & Silica Fume Silica fume is a byproduct of producing silicon metal or ferrosilicon alloys in an electric-arc furnace. The raw materials going into the furnace, the electric-arc furnace (2000 C), and the smoke collection system are shown (next slide).
    6. 6. Silica Production & Silica Fume
    7. 7. Silica Fume The densified material is on the left and the as- produced material is on the right and slurried on top. Silica Fume  Powdered  Densified
    8. 8. Effect of Silica Fume in Concrete
    9. 9. Physical PropertiesParticle size (typical) < 1 micronBulk Density (as-produced) 130 – 430 kg/m3 (densified) 480 – 720 kg/m3Specific Gravity 2.2Surface Area (BET) m2/kg 13,000 – 30,000
    10. 10. Silica Fume in Concrete Silica Fume Provides  Technical Advantages  Life-cycle Cost Advantages  Resource Conservation  Reduced Solid Waste Disposal  Reduced Greenhouse Gas Production from PC
    11. 11. Technical Benefits of Silica Fume Benefits  Pore Refinement  High Early Age Strength  Increased Stickiness  Black-Gray Color  ASR Resistance  Sulfate Resistance  Consumes CH
    12. 12. Effects on Concrete Properties High Strength Development Water Demand Increased No Bleeding of Concrete Reduced Permeability High Range Water Reducers required Increased Chloride Permeability Increased Plastic Shrinkage
    13. 13. Silica-Fume Concrete: Typical StrengthsCompressive Strength, MPa 15% 70 10% 60 5% 50 0% 40 30 Control mixture cement: 390 kg/m3 20 w/c: 0.41 10 air: 5% 0 0 3 7 28 60 Age, days SI
    14. 14. Silica-Fume Concrete: Typical Strengths (SI Version) This slide shows the effects of adding increasing amounts of silica fume to a low water- cementitious ratio, air-entrained concrete. Note that:1. Increased strength will be seen at all ages.2. There is a diminishing return that governs the addition of silica fume. Strength will not continue to increase at the same rate with increased additions.
    15. 15. Silica-Fume Concrete: Typical Strengths (SI Version) This concrete mixture is typical of what has been used in many parking structures and bridge decks, where the amount of silica fume added would be approximately 7.5 percent. This mixture would not be optimal for high- strength applications. See the following slides for high-strength concrete performance.
    16. 16. Advantages of Silica Fume & Pozzolans Socket where a sand grain has been pulled away from cement paste in 1-day old mortar. The sand grain was originally at the top of the picture. Note the massive CH layer engulfing the sand grain and by some channel type gaps Below: Socket where a sand grain has been pulled away from cement paste in 28-day old mortar containing silica fume. The sand grain was originally at the top of the picture.
    17. 17. Advantages of Silica Fume & Pozzolans Enhance Interfacial Zones Reaction W/Calcium Hydroxide
    18. 18. Corrosion-Resisting Concrete Within the general area of improving durability, by far the greatest use of silica fume has been in concrete susceptible to corrosion caused by chloride ion attack of the reinforcing steel. This category includes concrete exposed to either deicing salt or to marine salts. The protection mechanism in this case is primarily the reduction of the permeability of the concrete, which significantly increases the time that it takes for the chlorides to reach the level of the reinforcing steel.
    19. 19. Corrosion-Resisting Concrete Parking structures Bridge decks Marine structures
    20. 20. Silica-Fume Concrete: Typical ValuesSilica Fume RCP Compressive Strength(by mass of cement)0% > 3,000coulombs = 35 MPa7-10% < 1,000 coulombs > 50 MPa> 10% < 500 coulombs > 65 MPa Don’t fall into strength trap! SI
    21. 21. Silica-Fume Concrete: Typical Values (SI Version) These values are typical for concrete containing about 360-390kg/m3 of cement with a w/cm < 0.40. The “strength trap” is the situation in which strength achieved with silica-fume concrete will greatly exceed most project specifications. This factor may cause concrete producers and contractors to relax their control over the concrete production and curing. While strength may be adequate for structural design, rapid chloride permeability values may not be within specifications.
    22. 22. Silica-Fume Concrete: Corrosion Protection 3-7% silica fume added by mass of cement Mixture may include fly ash or slag w/cm < 0.40: use HRWRA Total cementitious materials < 415 kg/m3 Permeability estimated using ASTM C 1202 SI
    23. 23. Silica-Fume Concrete: Corrosion Protection (SI Version) This slide summarizes many years of experience using silica fume for improved durability in bridge decks and parking structures. More and more concrete in this application is now including either fly ash or slag along with the silica fume. See the examples in Chapter 5 for typical mixture proportions. ASTM C 1202, “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration.”
    24. 24. Concrete with Silica Fume 3-7% of Mass of  Cautions Cement  > Plastic Shrinkage Used with HRWRA  High Water Demand Adjust Admixtures  Stickiness  Black-Gray Color Must be Moist Cured
    25. 25. Silica Fume and the Environment Recovered/ recycled material Replaces Portland cement  Reduction of CO2  Reduction of Sox and NOx Reduced Material extraction
    26. 26. LEED Credits Leadership in Energy and Environmental Design (LEED) is a system developed by the United States Green Building Council to rate a buildings environmental performance. This system has become the principal method by which buildings can achieve green building certification. The system is based on credits earned in five major categories. Silica Fume can positively impact in two of the credit categories.

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