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Generally, the raw material used in the manufacture of
artificial aggregate is pulverised fuel ash (fly ash). This is the waste
material produced from electricity production in coal-fired power stations.
Synthetic artificial aggregate produced from environmental waste, like fly ash,
is a viable new source of structural aggregate material. Fly ash is a residual
material of energy production using coal, which has been found out have a few
advantages for use in the concrete to reduced permeability, increased ultimate
strength, reduced bleeding, better surface finish and reduced heat of hydration
(Davidovits, 1988). According to fly ash’s property which has strong silica
alumina glassy chain, it has been used as supplementary cementing material to
substitute Ordinary Portland Cement (OPC). Portland cement is the most used
material in the worldwide construction industry but it also has a high level of
CO2. Its use tends to become less competitive compared to alternative
ecological new binders like geopolymers (Davidovits, 1991 and Mustafa, 2011).

Fly ash (FA) and bottom ash (BA) are produced as a
coal fuelled power stations. FA is a highly dispersible powder. It contains
mainly aluminosilicate and ferriferous glassy spherical particles (about 60 –
80%) and irregularly shaped grains of amorphous clay, mullite quartz and
unburned metamorphic fuel (Malhotra and Ramezaniarpour, 1994). The chemical
compositions of FA and BA ashes from the same power plant are similar (Yun et al.,
2004).

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Fly ash obtained from coal combustion is frequently
used in concrete as a cost-effective substitute for portland cement. The
pozzolanic properties of fly ash improve the strength of concrete and its small
spherical particles make the concrete mixture more workable (Peiwei et al.,
2007). Extensive research and development works have been done on the use of
fly ash as a component of concrete (Chindaprasirt et al., 2007) and on the
changes that its incorporation induces in both mechanical (Topcu and Canbaz,
2007) and thermal (Demirboga, 2007) properties. Moreover, Lingling et al.
(2005) found that fly ash improves the compressive strength of bricks and makes
them more resistant to frost attack. Cicek and Tanriverdi (2007) also observed
the positive effect of fly ash on the compressive strength of bricks.

There are some advantages of using fly ash as a raw
material for bricks, such as the firing energy can be saved because of the
carbon content in fly ash. Several studies have been carried out in Germany,
England, and China to produce bricks from fly ash (Lingling et al., 2005). The
above studies revealed that the use of fly ash in fired bricks as a replacement
of clay effectively saves land and energy, and decreases environmental
pollution. Kumar (2003) conducted studies to produce bricks (solid and hollow)
and blocks (solid and hollow) using fly ash with lime and phosphogypsum. He
found that bricks and blocks of sufficient strength can be produced from the
fly ash-limephosphogypsum mixture. These bricks and blocks have the potential
for use in place of conventional burnt clay bricks and blocks.

Bottom ash is produced in a dry-bottom coal boiler
often found in coal-fired electric power plants. When coal is pulverized to the
consistency of powder, blown into the boiler and burned, two by-products are
generated. The fine, particulate matter rises with the flue gases and is
collected by electrostatic precipitators (ESP). This is called fly ash. The
larger fused particles fall to the bottom and are called bottom ash. Bottom ash
is a coarse, angular material of porous surface texture and size ranging from
fine gravel to fine sand, predominantly sand-sized. This material is composed
of silica, alumina, and iron with small amounts of calcium, magnesium, and sulphate;
as a whole, the quality of the material is governed by the coal source, not by
the type of furnace. In the present study, lightweight expanded aggregates were
produced by using clay and bottom ash at various temperatures. Test results
showed that lightweight aggregates can be produced from clay and bottom ash. (Karasu, Arioz
et al. 2007)

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