Fly ash

File:Fly ash.jpg
Type	Byproduct
Properties	|-| | Density || |-| | Melting point || |-| | Boiling point || |-| | Thermal conductivity || |-| | Electrical resistivity ||
Discovered by	[[]]

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Fly ash is a fine powder that is a byproduct of burning pulverized coal in electric generation power plants. Composed mostly of silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), and calcium oxide (CaO), fly ash is a pozzolanic material, meaning it reacts with calcium hydroxide in the presence of water to form compounds possessing cementitious properties.

Production

Fly ash is produced during the combustion of coal in thermal power plants. The coal is pulverized and blown into the combustion chamber of a boiler where it ignites, producing heat and forming ash. The lighter particles, known as fly ash, are carried away by the flue gas and captured by electrostatic precipitators or bag filters before the flue gas reaches the chimneys of coal-fired power plants.

Composition

The chemical composition of fly ash depends on the type of coal burned. Generally, fly ash consists of:

• Silicon dioxide (SiO₂)
• Aluminum oxide (Al₂O₃)
• Calcium oxide (CaO)
• Iron oxide (Fe₂O₃)
• Magnesium oxide (MgO)
• Sulfur trioxide (SO₃)

Types

Fly ash is classified into two types based on its chemical composition:

• Class F fly ash: Produced from burning anthracite or bituminous coal, it has pozzolanic properties and contains less than 20% lime (CaO).
• Class C fly ash: Produced from burning lignite or sub-bituminous coal, it has both pozzolanic and cementitious properties and contains more than 20% lime.

Applications

Fly ash is used in a variety of applications, including:

• Concrete production: Fly ash is used as a partial replacement for Portland cement in concrete. It improves the workability, strength, and durability of concrete.
• Road construction: Used as a filler material in asphalt and as a stabilizing agent in soil stabilization projects.
• Waste management: Utilized in the stabilization and solidification of hazardous waste.
• Agriculture: Used as a soil amendment to improve soil structure and fertility.

Environmental Impact

The use of fly ash in construction reduces the need for landfill space, decreases greenhouse gas emissions, and conserves natural resources by reducing the demand for Portland cement. However, improper disposal of fly ash can lead to environmental contamination due to the presence of heavy metals and other toxic substances.

Health Concerns

Exposure to fly ash can pose health risks due to the presence of respirable crystalline silica and heavy metals. Proper handling and disposal are essential to minimize these risks.

Regulations

Fly ash is regulated by environmental agencies to ensure safe handling and disposal. In the United States, the Environmental Protection Agency (EPA) classifies fly ash as a "special waste" under the Resource Conservation and Recovery Act (RCRA).

Also see

• Coal combustion products
• Pozzolan
• Portland cement
• Concrete
• Environmental impact of coal