fly ash

WordReference Random House Unabridged Dictionary of American English © 2025

fly′ ash′, 

1. Energy fine particles of ash of a solid fuel carried out of the flue of a furnace with the waste gases produced during combustion.
2. Energy such ash recovered from the waste gases, used chiefly as a reinforcing agent in the manufacture of bricks, concrete, etc.
• 1930–35

Collins Concise English Dictionary © HarperCollins Publishers::

fly ash noun

1. fine solid particles of ash carried into the air during combustion, esp the combustion of pulverized fuel in power stations