POZZOLANAS
AN INTRODUCTION
Pozzolanas are materials
containing reactive silica
and/ or alumina which on
their own have little or no
binding property but, when
mixed with lime in the
presence of water, will set
and harden like a cement.
They are an important
ingredient in the production
of alternative cementing
material to Portland cement
(OPC). (See the leaflet in
this series Alternatives to
Portland Cement - An
Introduction).
Cement or some form of
binding agent is a vital
Figure 1: Volcanic ash in Indonesia being gathered for use as
a pozzolana. Photo: Practical Action (not a Practical Action project)
element in almost all types
of construction and in
recent years the cement market has been dominated by one product, OPC. In many
countries, particularly those of the South, OPC is an expensive and sometimes scarce
commodity and this has severely limited the construction of affordable housing in much of
the Third World. Alternative cements provide an excellent technical option compared to OPC
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.
A wide variety of siliceous or aluminous materials may be pozzolanic. In this leaflet only
those of widespread availability will be discussed. Pozzolanas can be divided into two
groups: natural pozzolanas such as volcanic ash and diatomite, and artificial pozzolanas such
as calcined clays, pulverized fuel ash and ash from burnt agricultural wastes. Pozzolanas can
be used in combination with lime and/ or OPC. When mixed with lime, pozzolanas will
greatly improve the properties of lime-based mortars, concretes and renders and, in this
form, can be used in a wide range of building applications. Alternatively, they can be
blended with OPC to reduce costs considerably and to improve certain characteristics of
OPC-based concretes, such as long-term strength, resistance to sulphate attack and
workability.
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