What is Fly Ash?

By : Branden Drew


Fly ash is a by-product of coal fuel combustion. Clay is typically entrapped during the formation
of coal. When coal is burnt, the incombustible clay particles are left behind as ash. In grate
boilers, incombustible ash agglomerates as cinders through prolonged residence time. The
ground clay escapes along with flue gases, settling as ash in bag filters or electrostatic
precipitators (ESPs). This gives rise to the name ‘fly ash’. The main use of fly ash is as a partial
replacement of Portland cement in concrete mix.
There are two kinds of fly ash; Type F and Type C. Type F has high silica content and comes
from burning anthracite or bituminous coal (high carbon content). Type C has high calcium
content and comes from burning lignite or subbituminous coal (low carbon content). The main
use of fly ash is as a partial replacement of Portland cement in concrete mix. The makeup of
these two types affects the overall properties of the concrete. The silica content of Type F reacts
with the alkali components of the Portland cement to reduce expansive reactions, which in turn
makes a denser concrete. Type F fly ash also increases the setting time, but in return allows for a
longer lasting concrete. Type C fly ash can reduce setting time but can be questionable on
concrete longevity. While it has similar attributes to workability, bleeding, and resistance to
freezing and thawing, it does not have the additional advantages that Type F has. For this reason,
Type F is the more commonly used fly ash for Portland cement replacement. It is recommended
that when using Type F, you should replace 15 to 25 percent of Portland cement, and 20 to 35
percent for Type C.
To be used in cement, the fly ash must be dry. At Vulcan® Drying Systems, we dry fly ash by
using a direct fired rotary dryer system. This system can include feed/discharge conveyors, a
drum dryer with burner, fuel train, and pollution control. Vulcan® Drying Systems designs each
system to fit the specific needs of our customers. This will ensure that they are not paying for
over or undersized systems. Each Vulcan® customer receives a manual that is designed around
their system which includes initial setup, safety, startup and shutdown sequence, and all other
information to get their system up and running.
The artlicle below talks more on fly ash and its possible uses.
Related Article: https://breakingenergy.com/2014/02/18/can-coal-fly-ash-waste-be-put-to-gooduse

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