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.

1. CON 123
Cementitious Materials
Session 8
Fly Ash

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

5. Hydrated Cement and Fly Ash
= Cement = Fly Ash
5

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%

11. Fly Ash Benefits
Workability Finishability Pumpability
Durability Strength
11

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.