Editing 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">{{cite web |title=ASTM C618-23 Standard Specification for Coal Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Concrete |url=https://www.astm.org/c0618-23.html |publisher=ASTM International |year=2023}}</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>{{cite journal |last1=Blissett |first1=R.S. |last2=Rowson |first2=N.A. |title=A review of the multi-component utilisation of coal fly ash |journal=Fuel |volume=97 |pages=1–23 |year=2012 |doi=10.1016/j.fuel.2012.03.024}}</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>{{cite web |title=Coal Ash Basics |url=https://www.epa.gov/coalash/coal-ash-basics |publisher=U.S. Environmental Protection Agency |access-date=2025-12-01}}</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>{{cite web |title=EN 450-1:2012 Fly ash for concrete |url=https://standards.iteh.ai/catalog/standards/cen/0e7a3d8b-9f2d-4e6d-9c70-5e5e5f5e5f5e/en-450-1-2012 |publisher=European Committee for Standardization}}</ref>
* IS 3812 (Part 1):2013 – Indian Standard for Pulverised Fuel Ash<ref>{{cite web |title=IS 3812 (Part 1):2013 Pulverised Fuel Ash – Specification |publisher=Bureau of Indian Standards}}</ref>

== References ==
{{Reflist}}

== Further reading ==
* American Coal Ash Association (ACAA). ''Coal Combustion Products Utilization Statistics''. {{cite web |url=https://www.acaa-usa.org |access-date=2025-12-01}}
* Yao, Z.T., et al. (2015). "A comprehensive review on the applications of coal fly ash". ''Earth-Science Reviews'', 141, 105–121. {{doi|10.1016/j.earscirev.2014.11.016}}

[[Category:Coal]]
[[Category:Concrete]]
[[Category:Waste]]
[[Category:Building materials]]
[[Category:Air pollution]]