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< prev - next > Construction Cement and binders KnO 100516_Introduction to concrete building products (Printable PDF)
Use of Concrete as a Building Material
Concrete can be considered to be any material which has been fabricated from a binder and a
filling material. As such it is a composite material formed of two or more primary components.
Each of the components contributes something to the characteristics and properties of the
The best known binder for making concrete is Ordinary Portland Cement (OPC). Less commonly,
lime-pozzolana cement and gypsum have been used as binders. Binders such as sulphur, synthetic
organic resins and high alumina cement have also been used for certain specialist applications.
The filler normally makes up the majority of the concrete. It forms the skeletal structure of the
concrete and is integrally linked with the binder to produce a material which can be given shape
and acquire hardness and strength. The filler normally used in concrete products is a mixture of
gravel or crushed rock and sand, collectively known as aggregate. Good quality aggregate can,
however, be expensive and its extraction from the ground can cause loss of agricultural land or even
changes in the flow of watercourses if taken from river beds, so attention in recent years has been
focused on alternative aggregates, including recovery from demolition wastes and using residues
from process industries and agriculture.
In truth, however, most alternative aggregates have proved to provide inferior properties for the
concrete compared with conventional aggregates of rock, stone and sand extracted from the
ground. Most agricultural and plant wastes, such as rice husks, wood chips and broken nut husks
generally produce quite a poor quality concrete, unless they have been treated in special ways.
Others, such as metallurgical slags and mining and mineral processing wastes may offer more
promise. A few wastes, such as pulverised fuel ash (pfa) from coal burning power stations and
silica fume from silicon production, can even improve conventional concrete properties if used in
specific proportions; mainly due to their pozzolanic properties.
Pozzolanas are materials that react chemically with the lime present in the concrete to give
additional hardened materials. Pozzolanas consist of silicon, aluminium and iron oxides and are
finely pulverised and treated by heat to make them active. As well as pfa and silica fume other
types of pozzolana include rice husk ash, some types of volcanic ash, brick dust from fired clay
brickworks and pulverised and heat-treated diatomaceous earth.
In this brief waste materials as aggregate in concrete are given little further consideration, except in
the case of potential pozzolanas and demolition wastes. It is assumed that most producers of
concrete items would be using conventional stone or rock and sand as aggregate.
Water is the final very important ingredient for concretes made with hydraulic binders such as OPC,
lime-pozzolana or gypsum, which set by chemical reaction with water.
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