Waste glass can be disposed of by either storage or recycling.
The raw materials required for the production of glass are very
common and inexpensive consisting of sand, soda ash and limestone
and through practice it was further confirmed that it is
cheaper to store than to recycle glass due to the conditioners
required for the recycling process [1]. Apart from the chemicals
required for the recycling of glass, transportation to recycling
plants together with the need of high quality glass with no colour
mixing makes such a process even more elaborate. Mixed glass
cannot be used back in the glass industry and is usually land-filled.
In order to have a material of sufficiently high quality for recycling,
waste glass had to pass through thorough washing in high temperatures
in order to remove labels and other contaminants. The
difficulties encountered with glass processing lead to an ever
increasing interest to use waste glass in alternative areas mainly
in the construction industry especially in concrete. In the last decades
concrete became one of the most attractive options due to the
large quantities produced, relatively low quality of glass required
and long term use. Reusing it in concrete converts a material which
was a burden to society into a source [2]. Concrete exhibits different
properties and uses which differ from other materials that use
more specific sources. Concrete is the most widely used man-made
material in the world with a production of about one ton per person
in the world per year with consumption still rising [3]. This
creates a constant demand for a binding and filling material which
is usually satisfied by Portland cement and natural aggregate.
The introduction of glass in concrete provides an ideal solution
where less natural materials would be depleted and also creating
a constant demand for waste glass. In addition while recycled glass
would again become a waste after a relatively short period of time,
its use in concrete provides a long term solution. The use of waste
glass in concrete in various forms and amounts created both solutions
and problems. The initial problem was devising a method of
processing the waste glass and turning it into usable glass cullet.
The first glass crushing equipment evolved from equipment used
to crush rock and other minerals. Two types are available namely
the abrasion and impact crushers. Glass is more abrasive than most
natural aggregates. This leads to excessive wearing of the equipment
used and high maintenance costs. Furthermore most crushers
produce sharp elongated cullet which is both dangerous to handle
and also unsuitable for use [4]. Abrasion crushing equipment such
as jaw crushers and cone crushers use friction and compression
to reduce the material. Due to the abrasive nature of glass this type
of equipment is the most inappropriate of all for processing waste
glass. The most suitable glass crushing equipment available on
the market mainly consists of impact crushers. There exist a wide
range of impact crushers due to their wide range of use in the industry
however these still suffer from excessive wear. In the mid-1990s
the implosion technology was introduced as a new way to ‘crush’
glass making it free of sharp edges. A harmonic resonance is created
through a high-speed rotor, a gravity chamber and feed system [5].
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While imploding the glass other materials present in the batch
such as metal or cork tops, labels and straw are left untouched and
therefore can be easily be collected through sieving. In impact
crusher such a process is not possible as it would lead to losses
of valuable materials such as paper which would be disintegrated
and glass would get stuck in any flexible material such as cork and
metal leading to the loss of both. In such a process glass has to be
separated and cleaned before processing. Separation and cleaning
can be done manually or through automated systems. Apart from
the advantages of producing sharp free glass cullet, this system requires
less energy to operate when compared to traditional
crusher. In addition the implosion technology can be considered
maintenance free.
Waste glass has been used to replace coarse aggregate [6,7], fine
aggregate [8,9] and cement [8,10]. The use of glass as coarse aggregate
produced poor quality concrete [6]. Glass aggregate coarser
than 1.5 mm produced poor quality concrete with a 50% reduction
in strength reported with 1