Materials · waste

Fly ash

Meaning statusEstablishedSource recordDirect source requestedWhy these are different

Definition

Fly ash is the finely divided mineral residue carried out of the boiler with flue gases when pulverised coal is burned, captured by electrostatic precipitators or baghouses. Chemically rich in silicates and aluminates, it is pozzolanic — it reacts with calcium hydroxide in the presence of water to form cementitious compounds — making it a standard supplementary cementitious material that can replace a substantial share of Portland cement in concrete. Together with flue-gas desulphurisation gypsum in wallboard, fly ash in concrete accounts for about half of the beneficial use of coal combustion residuals in the United States, where the Bevill amendment shaped their regulatory treatment.

References

U.S. Federal Highway AdministrationFly ash (materials chapter)

definition as finely divided residue of pulverised-coal combustion, pozzolanic properties, capture by precipitators/baghouses

U.S. EPACoal combustion residuals — beneficial use FAQ

~50% of beneficial use as fly ash in concrete plus FGD gypsum in wallboard, Bevill amendment context

Overview

What it means

Fly ash is both a waste-liability problem (storage in ash ponds has caused major spills and groundwater contamination) and a circular-economy resource: each tonne used in concrete avoids roughly a tonne of clinker-related emissions. Declining coal generation is shrinking supply, prompting interest in harvested ash from legacy ponds.

How it is used

Used in concrete and cement, structural fills and waste-stabilisation; regulated as coal combustion residuals with disposal and beneficial-use rules.

Why it matters

It links three sustainability agendas — coal-phaseout legacies, construction decarbonisation and industrial by-product valorisation — in one material stream.