What is Fly ash?
Fly ash is a heterogeneous by-product material produced in the combustion process of coal used in power stations. It is a fine grey colored powder having spherical glassy particles that rise with the flue gases. As fly ash contains pozzolanic materials components which reach with lime to form cementitious materials. Thus Fly ash is used in concrete, mines, landfills, and dams
 |
Fly ash Generation |
Chemical composition and classification
Classification of Fly Ash
The classification of fly ash is done differently as per the codes used. They are1.
Type of Fly Ash as per IS Codes (IS 3812-1981)
A. Grade I
This grade of Fly ash is derived from bituminous coal having fractions
SiO2+Al2O3+Fe2O3 is greater than 70 %.
B. Grade II
This grade of Fly ash is derived from lignite coal having fractions
SiO2+Al2O3+Fe2O3 is greater than 50 %.
 |
Chemical composition and classification table |
Fly Ash Properties
Fineness.
The fineness of fly ash affects the rate of pozzolanic activity and the workability of the concrete. Specifications require a minimum of 66 percent passing the 0.044 mm (No. 325) sieve.
Specific gravity.
Although specific gravity does not directly affect concrete quality, it has value in identifying changes in other fly ash characteristics.
Chemical composition.
The reactive aluminosilicate and calcium aluminosilicate components of fly ash are routinely represented in their oxide nomenclatures such as silicon dioxide, aluminum oxide and calcium oxide. The variability of the chemical composition is checked regularly as a quality control measure. The aluminosilicate components react with calcium hydroxide to produce additional cementitious materials.
Carbon content.
LOI is a measurement of unburned carbon remaining in the ash. It can range up to five percent per AASHTO and six percent per ASTM. The unburned carbon can absorb air entraining admixtures (AEAs) and increase water requirements. Also, some of the carbon in fly ash may be encapsulated in glass not affect the mix. Conversely, some fly ash with low LOI values may have a type of carbon with a very high surface area, which will increase the AEA dosages.
Further, if the fly ash has a very high carbon content, the carbon particles may
float to the top during the concrete finishing process and may produce dark-colored surface streaks.
Fly ashes tend to contribute to concrete strength at a faster rate when these components are present in finer fractions of the fly ash.
Sulfur trioxide content is limited to five percent, as greater amounts have been shown to increase mortar bar expansion.
Available alkalis in most ashes are less than the specification limit of 1.5 percent. Contents greater than this may contribute to alkali-aggregate expansion problems.
The use of fly ash in portland cement concrete (PCC) has many benefits and improves concrete performance in both the fresh and hardened state. Fly ash use in concrete improves the workability of plastic concrete and the strength and durability of hardened concrete. Fly ash use is also cost-effective. When fly ash is added to concrete, the amount of portland cement may be reduced.
Use of fly ash in portland cement concrete
Benefits to Fresh Concrete.
Generally, fly ash benefits fresh concrete by reducing the mixing water requirement and improving the paste flow behavior.
The resulting benefits are as follows:
Improved workability.
The spherical-shaped particles of fly ash act as miniature ball bearings within the concrete mix, thus providing a lubricant effect. This same effect also improves concrete pumpability by reducing frictional losses during the pumping process and flatwork finish ability.
Benefits to Fresh Concrete
- Workability
Decreased water demand.
The replacement of cement by fly ash reduces the water demand for a given slump. When fly ash is used at about 20 percent of the total cementitious, water demand is reduced by approximately 10 percent. Higher fly ash contents will yield higher water reductions. The decreased water demand has little or no effect on drying shrinkage/cracking. Some fly ash is known to reduce drying shrinkage in certain situations.
Reduced heat of hydration.
Replacing cement with the same amount of fly ash can reduce the heat of hydration of concrete. This reduction in the heat of hydration does not sacrifice long-term strength gain or durability. The reduced heat of hydration lessens heat rise problems in mass concrete placements.
- Water demand & heat of hydration
Cement Reaction: C3S + H → C-S-H + CaOH (hydration)
Pozzolanic Reaction: CaOH + S → C-S-H (silica from ash constituents)
One of the primary benefits of fly ash is its reaction with available lime and alkali in concrete, producing additional cementitious compounds. The following equations illustrate the pozzolanic reaction of fly ash with lime to produce additional calcium silicate hydrate (C-S-H) binder:
- Hardened Concrete.
Increased ultimate strength.
The additional binder produced by the fly ash reaction with available lime allows fly ash concrete to continue to gain strength over time. Mixtures designed to produce equivalent strength at early ages (less than 90 days) will ultimately exceed the strength of straight cement concrete mixes
- Ultimate strength
Reduced permeability.
The decrease in water content combined with the production of additional cementitious compounds reduces the pore interconnectivity of concrete, thus decreasing permeability. The reduced permeability results in improved long-term durability and resistance to various forms of deterioration
 |
| Benefits to Fresh Concrete - ultimate strength graph |
- Permeability
Improved durability.
The decrease in free lime and the resulting increase in cementitious compounds, combined with the reduction in permeability enhance concrete durability. This affords several benefits:
Improved resistance to ASR. Fly ash reacts with available alkali in the concrete, which makes them less available to react with certain silica minerals contained in the aggregates.
Effect on the durability of concrete
Improved durability.
Improved resistance to sulfate attack. Fly ash induces three phenomena that improve sulfate resistance:
- Fly ash consumes the free lime making it unavailable to react with sulfate
- The reduced permeability prevents sulfate penetration into the concrete
- Replacement of cement reduces the amount of reactive aluminates available
Improved resistance to corrosion. The reduction in permeability increases the resistance to corrosion.
Post a Comment