CALCINED CLAYS AND SHALES, AND VOLCANIC ASH
Pozzolanas are materials containing
reactive silica and/or alumina
which on their own have little or no
binding property. However, when
mixed with lime, in the presence of
water, they will set and harden like
cement. Pozzolanas are an
important ingredient in the
production of alternative binding
materials to Portland cement
(OPC). (See the leaflets in this
series Alternatives to Portland
Cement - An Introduction and
Pozzolanas - An Introduction).
Alternative cements provide an
excellent technical option to OPC
Figure 1: house plastered with lime-pozzolana mix,
at a much lower cost and have the
potential to make a significant contribution towards the provision of low-cost building
materials and consequently affordable shelter.
Pozzolanas can be used in combination with lime and/or OPC. When mixed with lime,
pozzolanas will improve the properties of lime-based mortars, concretes and renders for use
in a wide range of building applications. Alternatively, they can be blended with OPC to
improve the durability of concrete and its workability, and reduce its cost considerably.
A wide variety of siliceous or aluminous materials may be pozzolanic, and historically the two
most widely used of these are calcined clays and volcanic ash. Calcined clay in the form of
crushed fired clay bricks, tiles or pottery has traditionally been used for improving the
properties of lime mortars and renders. (It is known as surkhi in India, homra in Egypt and
semen merah in Indonesia.) Shales are harder than clays and have similar mineral contents
resulting in similar pozzolanic properties.
Volcanic ash was first used as a pozzolana by the Romans from deposits close to the village
of Pozzuoli, near Naples, hence the name pozzolana.
Clays and shales
Clays or shales suitable for use as a pozzolana are very widespread and are readily available
in almost all regions of the world. They have been used as cement replacement materials on
large-scale construction programmes in a number of countries, particularly the US, Brazil,
Egypt and India. For example, in Egypt, a lime-calcined clay mortar was used in the core of
the first Aswan dam built in 1902 and an OPC-calcined clay mixture was used in the
construction of the Sennar dam in Sudan.
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