Civil Engineering Reference
In-Depth Information
Depending on their generation, wastes can be separated into two types: those
that directly result from industry as industrial by-products and those that can be
named recycled wastes. The first type includes coal ash, various slags from metal
industries, industrial sludge, waste from industries like pulp and paper mills, mine
tailings, food and agriculture, and leather. The second type includes different
plastic and rubber wastes.
A broad classification of industrial waste aggregate can be made depending on
the chemical nature of wastes. Some waste aggregates come from production and
use of organic materials. Plastics, rubber, leather and some food industries wastes
are organic wastes. On the other hand, industrial slags, mining wastes, coal
industry wastes and others are inorganic wastes. Glass reinforced plastics and
some industrial sludge may contain both organic and inorganic materials.
Another classification of industrial waste aggregate can be done depending on
the weight of waste aggregates. Some aggregates are lightweight by nature.
Plastics, rubber, most food and agricultural industries wastes and coal bottom ash
are of this kind. On the other hand, most of the industrial slags are heavier than
conventional aggregates.
2.3 Coal Ash as an Aggregate in Concrete
Burning of coal generates two types of waste materials: fly ash and bottom ash.
There are two types of bottom ashes, wet bottom boiler slag and dry bottom ash
depending on both the boiler type and its design.
Coal fly ash, also known as pulverised fuel ash, is the finest fraction of these
ashes, which are released from combustion chamber and transported by flue gases.
Fly ash contains the non-combustible matter in coal along with a small amount of
carbon that remains from incomplete coal combustion. Fly ash consists mostly of
silt-sized and clay-sized glassy spheres. When pulverised coal is burned in a dry
bottom boiler about 80 per cent of the unburned material or ash is entrained in the
flue gas and is captured and recovered as fly ash. The remaining 20 % ash that is
collected from the bottom of furnaces is called coal bottom ash (CBA), which is a
coarse, incombustible by-product with a grain size similar to that of fine and coarse
sized natural aggregates. Bottom ash is produced as a granular material and
removed from the bottom of dry boilers.
Boiler slag, a coarse grained product, is produced from two types of wet bottom
boilers, slag-tap and cyclone boilers. The slag-tap boiler burns pulverised coal
while the cyclone boiler burns crushed coal. Both boiler types have a solid base
with an orifice that can be opened to allow molten ash to flow into a hopper, which
contains quenching water. When the molten slag comes in contact with the
quenching water, the ash fractures instantly, crystallises, and forms pellets. High-
pressure water jets wash the boiler slag from the hopper into a sluiceway, which
then transmits the ash to collection basins for dewatering and further processing.
Boiler slag is a coarse, angular, glassy, black material. When pulverised coal is
