Ash
for Concrete and other Building
Materials
In the 1930s when pulverised fuel boilers were developed, it was
discovered that the fly ash produced had properties similar to the
volcanic ash used by the Romans to make "concrete" some two
thousand years ago. Such materials which harden when mixed with lime and
water are called pozzolans. Unlike volcanic ash, most of the particles
in fly ash are spherical and given the shape can enhance the handling
properties of concrete without the requirement for additional
add-mixtures.
Fly ash, as a partial replacement of the sand and cement in concrete,
economically enhances performance in regard to workability, shrinkage
creep and durability. Production of High Performance Concrete generally
involves the use of fly ash today.
Several lightweight aggregates for concrete can be produced from ash. In
addition to the use of furnace bottom ash in concrete masonry, pellets
of fly ash can be bound by thermal fusion or chemically, using cement or
lime. Such materials have many desirable properties.
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Bottom
Ash as backfill |
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Structural Fill and Road Works
Ash products have a long history of successful use both locally and overseas. Most of the pioneering work was done in the United Kingdom by the Central Electricity Generating Board
who have published project reports since the 1960s. Structural fills have been successfully completed with conditioned fly ash, furnace bottom ash and pond ash (a mixture of fly ash and
furnace bottom ash reclaimed from settlement ponds). For example in Australia, at Vales Point Power Station near Newcastle NSW, the capacity of the ash settlement pond has been increased
by building above the dam wall with compacted, conditioned fly ash. |
This range of applications also includes flowable fills and controlled
low strength materials. Fly ash is widely used in the binder
component of roadbase mixtures and in 5 MPa concrete subbase in NSW.
At Port Augusta Power Station, roads and cover over coal stockpiles
have been constructed successfully using cement bound mixtures of
fly ash and furnace bottom ash. Queensland Transport requires the
use of fly ash in concrete used in roads and bridges as insurance
against alkali-silica reaction (ASR). The ADAA has also sponsored
research into the use of ash in road pavements. Other Uses - Agriculture
There are conditions where fly ash and furnace bottom ash can be
used to improve soils for agriculture. Heavy clay can be lightened
at one extreme and sandy soil can be given body and water
retention at the other.
The ADAA are joint research sponsors of a four (4) year test
program with the University of Western Australia. The research
program which is being conducted in Western Australia, has already
demonstrated substantial improvement in water retention and
pasture growth in sandy soils.
Some of the findings have highlighted FA ability to improve water
retention in soils with low moisture holding capacity (i.e. sandy
soils) to over 30% compared untreated soils. Furthermore fly ash
provided increased plant growth without the need for additional
fertilizers.
Waste treatment and Fixation
Fly ash finds a use both as a reactive absorbent for industrial
waste liquors and as a component of containment concretes. ADAA
are reviewing the potential for this use prior to proving-up
appropriate fixation advice for Australia.
Chemical Source
The technical literature abounds with schemes for removal of
alumina and other materials from fly ash. Magnetite removal has
been done on a commercial scale from fly ash rich in iron.
Conversion of fly ash to more reactive zeolites shows promise for
enhanced performance as a pozzolan and for noxious waste fixation
and effluent treatment. |
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Asphalt
Roadway sub-base |
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Environmental Benefits
The reuse or recycling of fly ash into identified applications can
have substantial environmental benefits. These include:
1. Waste stream reduction and associated reductions in
requirements for landfill.
2. When used as a cement replacement, fly ash use contributes to
the conservation of resources such as gypsum, limestone and
natural gas which would otherwise be used in cement production.
3. The use of fly ash as a cement replacement also leads to
reduction in Greenhouse Gas (GHG) emissions from cement production
of almost 1 tonne of CO2 per tonne of cement.
Highlighting this point, since 1975 emissions of some 16 million tonnes of GHG
has been avoided by the use of fly ash in concrete.
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