Fly ash is a byproduct of coal combustion in power plants that can be used in concrete. It is a finely divided material that is captured by electrostatic precipitators from the exhaust gases. There are two classes of fly ash - Class C contains both pozzolanic and cementitious properties, while Class F only has pozzolanic properties. Fly ash concretes can provide benefits like improved workability, strength development, permeability, and sulfate resistance compared to plain cement concrete. The use of fly ash in concrete can also provide environmental benefits through reduced CO2 emissions, less energy usage, and LEED credits for buildings.
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. 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. 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
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. Standards for Fly Ash
ASTM C 618
Class C (SiO2+Al2O3+Fe2O3 > 50%)
Class F (SiO2+Al2O3+Fe2O3 > 70%)
AASHTO M295
8. Chemical Requirements
ASTM C 618 (AASHTO M 295)
Sum of Oxides C > 50% F > 70%
SO3 < 5%
Moisture < 3%
LOI < 6%
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. Typical Amounts of Fly Ash in Concrete by
Mass of Cementing Materials
Fly ash
Class C 15% to 40%
Class F 15% to 20%
12. Fly Ash - Applications
Compressive Strength
Permeability
Sulfate Resistance
Alkali Silica Reactivity
Mass concrete - more than 50%-70% Class F
13. Compressive Strength Development
Compressive strength
development at 1, 3, 7,
28, and 90 days of
concrete mixtures
containing 517 lb/yd3 of
cementitious materials
with a fly ash dosage of
25%
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. Long Term Strength Development
Compressive strength
gain as a percentage of
28-day strength of
concretes with and
without fly ash.
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. Effect of Fly Ash on Mixing Water
Requirements
Change in mixing water
Class 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. 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. 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 5.3
Concretes had a Cementitious materials content of 517 lb/yd3 with 25% fly ash and a
slump of 3.0 +/- 1.0 in.
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 mixtures
contained 25% ash by mass of Cementitious material.
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. LEED Credits
Leadership in Energy and Environmental Design
(LEED) is a system developed by the
United States Green Building Council to rate a
building's 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.