Editing Fly Ash (section)

== 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.