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CON 123 Session 8 - Fly Ash

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CON 123 Session 8 - Fly Ash

  1. 1. CON 123Cementitious MaterialsSession 8Fly Ash
  2. 2. What is Fly Ash? “Finely divided material that is removed from the exhaust gas of a high temperature combustion process of coal burning power plants.”
  3. 3. Source of Coal Fly Ash Coal Source and Ash Composition Stack Gases Contain Liquefied Minerals Cooling of Gases form Spherical Particles Electrostatic or Bag- House Precipitators
  4. 4. Cement and Fly Ash Particles = Cement = Fly Ash Reaction of Cement releases Ca(OH)2 Chemical control changes to diffusion control Some unreacted cement Surface area is important
  5. 5. Hydrated Cement and Fly Ash = Cement = Fly Ash 5
  6. 6. Specifications and Classes of Fly Ash ASTM C 618 (AASHTO M 295) Fly ash, a powder resembling cement, has been used in concrete since the 1930s. Class F—Fly ash with pozzolanic properties Class C—Fly ash with pozzolanic and cementitious properties
  7. 7. Standards for Fly Ash ASTM C 618  Class C (SiO2+Al2O3+Fe2O3 > 50%)  Class F (SiO2+Al2O3+Fe2O3 > 70%) AASHTO M295
  8. 8. Chemical Requirements ASTM C 618 (AASHTO M 295)Sum of Oxides C > 50% F > 70%SO3 < 5%Moisture < 3%LOI < 6%
  9. 9. Physical Requirements ASTM C 618 Fineness 325 Mesh retained: 34 % Max Pozzolanic Activity 7 day & 28 day: 75% Water Demand 105% max Soundness, Autoclave 0.80% max Density Variation: 5% from average 325 Mesh Fineness Variation: 5% from average
  10. 10. Typical Amounts of Fly Ash in Concrete by Mass of Cementing Materials Fly ash  Class C 15% to 40%  Class F 15% to 20%
  11. 11. Fly Ash BenefitsWorkability Finishability Pumpability Durability Strength 11
  12. 12. Fly Ash - Applications Compressive Strength Permeability Sulfate Resistance Alkali Silica Reactivity Mass concrete - more than 50%-70% Class F
  13. 13. Compressive Strength DevelopmentCompressive strengthdevelopment at 1, 3, 7,28, and 90 days ofconcrete mixturescontaining 517 lb/yd3 ofcementitious materialswith a fly ash dosage of25%
  14. 14. Cold Weather Strength Development Compressive strengths for concretes cured at 23 C (73 F) for the first 24 hours and 4 C (40 F) for the remaining time. Control had a cement content of 560 lb/yd3 and w/c of 0.45. The fly ash curves show substitution for cement (S), partial (equal) substitution for cement.
  15. 15. Long Term Strength Development Compressive strength gain as a percentage of 28-day strength of concretes with and without fly ash.
  16. 16. Effect of Fly Ash on Mixing Water Requirements Effect of Fly Ash on Mixing Water Requirements for Air-Entrained Concrete All mixtures had Cementitious materials contents of 564 lb/yd3, a slump of 5 1 in., and an air content of 6 1%.
  17. 17. Effect of Fly Ash on Mixing Water Requirements Change in mixing waterClass of fly Fly ash content, % by mass requirement compared to ash of cementing material control, % C 25 -7 F 25 -5 C 50 -15 F 50 -10
  18. 18. Effect of Fly Ash on Bleeding of Concrete ASTM C 232 (AASHTO T 158) Bleeding Fly Ash Mixtures Percent mL/cm2 Average of: Class C 0.34 0.011 Class F 1.31 0.044 Control mixture 1.75 0.059
  19. 19. Effect of Fly Ash on Air-Entraining Admixture Dosage and Air Retention Air content, % Percent of air-entraining Fly ash mixtures admixture relative to Minutes after initial mixing control 0 30 60 90 C 152 6.5 6.0 5.8 5.8 F 299 6.3 5.3 4.7 4.5 Control mixture 100 6.6 6.0 5.6 5.3Concretes had a Cementitious materials content of 517 lb/yd3 with 25% fly ash and aslump of 3.0 +/- 1.0 in.
  20. 20. Effect of Fly Ash on Setting Time of Concrete Retardation Setting time, relative to Fly ash test mixtures hr:min control, hr:min Initial Final Initial Final Average of: Class C 4:40 6:15 0:30 0:45 Class F 4:50 6:45 0:35 1:15 Control mixture 4:15 5:30 — —Concretes had a Cementitious materials content of 517 lb/yd3. Fly ash mixturescontained 25% ash by mass of Cementitious material.
  21. 21. Fly Ash and the Environment Recovered/ recycled material Replaces Portland cement  Reduction of CO2  Reduction of energy impact Reduced Material extraction Energy Saving
  22. 22. 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. Fly ash can positively impact in three of the credit categories.

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