Fly Ash and Its Classification





A Short Course In Fly Ash Technology


Fly Ash and Its Classification


Fly ash is comprised of the non-combustible mineral portion of coal consumed in a coal fueled power plant. Fly ash particles are glassy, spherical shaped “ball bearings” — typically finer than cement particles — that are collected from the combustion air-stream exiting the power plant.


There are two basic types of fly ash: Class F and Class C.


Both types react in concrete in similar ways. Both Class F and Class C fly ashes undergo a “pozzolanic reaction” with the lime (calcium hydroxide) created by the hydration (chemical reaction) of cement and water, to create the same binder (calcium silicate hydrate) as cement. In addition, some Class C fly ashes may possess enough lime to be self‑ cementing, in addition to the pozzolanic reaction with lime from cement hydration.


The main benefit of fly ash in concrete is that it not only reduces the amount of non-durable calcium hydroxide (lime), but in the process converts it into calcium silicate hydrate (CSH), which is the strongest and most durable portion of the paste in concrete. Fly ash also makes substantial contributions to workability, chemical resistance and the environment.