European Cement Association, up to 5% of primary
raw materials in cement clinker can be replaced
by mineral ashes produced during incineration
of the waste-derived fuel, leading to reduction of
primary raw materials usage and avoidance of
1.4 mega tons of mineral ashes that otherwise
would have to be landfilled. Coal fly ash has been
successfully used in Portland cement concrete
(PCC) as a mineral admixture, and more recently
as a component of blended cement for nearly 60
years. Figure 9 illustrates the process of cement
production.
Two major standards, specifically the US
standard ASTM C150 and the EU standard
EN-197, are used for classification of cement.
The most widely used specification for fly ash
in North America is ASTM C618 standard
specification for coal fly ash and raw or calcined
natural pozzolan for use in concrete. This
specification divides fly ash into two classes
based on the sources of origin and composition
as described in Table 5. In order to produce
cement which meets these major standards,
the ashes need to possess physicochemical
properties similar to conventional raw materials,
which favour the pozzolanic reaction. The
cement clinkers are generally pozzolanic due
to the silicon, iron, aluminium, calcium and/
or sulphur contents in their compounds, which
react with calcium hydroxide in the presence
of moisture to form complexes that possess
cementitious properties.
The calcium content of the fly ash is perhaps
the best indicator of how the fly ash incorporation
influences the concrete properties. In Canada,
the specification covering fly ash is CSA A3001
cementitious materials for use in concrete. It
separates fly ash into three types based on the
fly ash’s calcium content as shown in Table 6.
These fly ashes will react and harden when mixed
with water due to the formation of cementitious
hydration products.
It is possible to produce concrete with
moderate strength using the fly ash as the sole
cementing material given that the ash contains
high calcium content.
The first step in cement manufacture is to
mix a variety of raw ingredients to produce a
mixture with desired chemical composition. These
ingredients are ground (to increase reactivity),
blended together, and then the resulting raw
mix is heated in a cement kiln at extremely high
temperatures. Under such temperature, many
chemical components in the raw mix are oxidised.
Table 7 displays raw ingredients that can be used
to provide the main cement components.
In view of the well-established development
in coal ashes recovery through cement
production, there is a large potential in
the recycling of ashes produced from SW
incineration in a similar way. Despite the
benefits of incorporating HW incineration ash
in the cement, the presence of chloride in the
ash threatens the cement’s long-term durability.
It is well-known that chlorine in the cement
often reacts with water to form HCl, which
catalyses the oxidation of steel (rusting) in the
steel-reinforced concrete, hence reducing the
cement durability. Such factors account for the
frequent structure failures at locations where
chlorine or chlorides are abundant (for example
in coastal areas). Therefore, it is important to
minimise the presence of chlorine in cement,
water, aggregate and admixtures, and thus
the replacement of cement raw materials by
incineration ash is often limited below 20%.
In order to apply zero waste emission concept
in Malaysia, an in-depth study is necessary to
determine its potential and understand the
limitation of cement production from the SW
incineration ashes.
CONCLUSION
Proper management of SW is important to
minimise the potential hazards to human health
and the environment. A paradigm shift from
Cradle-to-Grave to Cradle-to-Cradle approach
will ensure sustainable SW management, which
focuses on resource sustainability and the zeroemission
concept. The technologies introduced in
this article will effectively reduce the amount of
SW sent to landfills, provided there is co-operation
from the public and private sector in sustainable
waste management.
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