CON 123 Session 8 - Fly AshPresentation Transcript
CON 123Cementitious MaterialsSession 8Fly Ash
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.”
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
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
Hydrated Cement and Fly Ash = Cement = Fly Ash 5
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
Standards for Fly Ash ASTM C 618 Class C (SiO2+Al2O3+Fe2O3 > 50%) Class F (SiO2+Al2O3+Fe2O3 > 70%) AASHTO M295
Chemical Requirements ASTM C 618 (AASHTO M 295)Sum of Oxides C > 50% F > 70%SO3 < 5%Moisture < 3%LOI < 6%
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
Typical Amounts of Fly Ash in Concrete by Mass of Cementing Materials Fly ash Class C 15% to 40% Class F 15% to 20%
Fly Ash - Applications Compressive Strength Permeability Sulfate Resistance Alkali Silica Reactivity Mass concrete - more than 50%-70% Class F
Compressive Strength DevelopmentCompressive strengthdevelopment at 1, 3, 7,28, and 90 days ofconcrete mixturescontaining 517 lb/yd3 ofcementitious materialswith a fly ash dosage of25%
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.
Long Term Strength Development Compressive strength gain as a percentage of 28-day strength of concretes with and without fly ash.
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%.
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
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
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.
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.
Fly Ash and the Environment Recovered/ recycled material Replaces Portland cement Reduction of CO2 Reduction of energy impact Reduced Material extraction Energy Saving
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.