Fly Ash

Fly ash, also known as pulverised fuel ash (PFA), is a fine powdery residue generated primarily from the burning of pulverised coal in coal-fired thermal power plants. It is captured from flue gases by electrostatic precipitators or fabric filter bags before the gases are released into the atmosphere. Smaller quantities are also produced from certain industrial kilns that burn coal or other solid fuels (excluding plastics).

Composition

Fly ash is a pozzolan—a siliceous or siliceous-aluminous material that reacts with calcium hydroxide in the presence of water to form cementitious compounds. Its chemical composition depends on the type of coal burned, but major oxides typically include:

Silicon dioxide (SiO₂): 40–65%
Aluminium oxide (Al₂O₃): 15–30%
Iron oxide (Fe₂O₃): 4–20%
Calcium oxide (CaO): 1–35%
Magnesium oxide (MgO), sulfur trioxide (SO₃), and trace heavy metals (arsenic, cadmium, lead, mercury, etc.)

Fly ash is classified into two main types according to ASTM C618:<ref name="ASTM C618">Template:Cite web</ref>

Class F – normally produced from anthracite or bituminous coal; CaO < 10–18%; pozzolanic properties only.
Class C – produced from lignite or sub-bituminous coal; CaO > 20%; both pozzolanic and some self-cementing properties.

Physical properties

Fly ash particles are mostly spherical and range in size from 0.5 μm to 300 μm. The material has a low bulk density (typically 1.9–2.8 g/cm³) and high surface area, which improves workability and reduces water demand when used in concrete.

Uses

Fly ash is one of the most widely reused coal combustion products. Common applications include:

Partial replacement of Portland cement in concrete (10–40% by mass)
Production of Portland Pozzolana Cement (PPC) and blended cements
Soil stabilisation and road base construction
Manufacture of fly ash bricks, blocks, and lightweight aggregates
Raw material for geopolymer concrete and zeolites
Fill material in embankments and mine reclamation

Global utilisation rates range from approximately 10–15% in India to over 90% in several European countries.<ref>Template:Cite journal</ref>

Environmental and health aspects

Improper disposal of fly ash can lead to contamination of soil, surface water, and groundwater through leaching of heavy metals. Modern regulations and beneficial reuse practices have significantly reduced these risks.<ref>Template:Cite web</ref> Inhalation of fine particles may cause respiratory irritation, but risks are low with proper handling.

Standards

ASTM C618 – Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete<ref name="ASTM C618" />
EN 450-1:2012 – Fly ash for concrete (European standard)<ref>Template:Cite web</ref>
IS 3812 (Part 1):2013 – Indian Standard for Pulverised Fuel Ash<ref>Template:Cite web</ref>

References

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