Pozzolanas: An introduction
Practical Action
The history of pozzolanas
The Greeks, pre-400BC, followed by the Romans, were the first civilisations to use
pozzolanas in lime mortars. The Romans used not only crushed pottery, bricks and tiles that
formed the first artificial pozzolanas, but also found that some volcanic soils were excellent
for producing a hydraulic mortar when mixed with lime. (A hydraulic mortar or concrete is
one which will set and harden under water. They are generally much more durable than
ordinary lime mortars.) One such soil was found close to the town of Pozzuoli, near Naples -
hence the name pozzolana or pozzolan.
The development of hydraulic cements based on lime-pozzolana mixtures led to radical
changes in building during the Roman era. The increased strength of lime-pozzolana
mixtures, their hydraulic properties and good resistance to seawater, permitted the
construction of not only arches and vaults but also marine structures. Lime-pozzolana
mortars were also used as waterproofing renders in the lining of baths, tanks and aqueducts.
The durability of the material is attested to by the many remains of Roman structures still in
evidence today.
More recently, over 100,000 tonnes of pozzolana was used in the construction of the Los
Angeles aqueduct from 1910 to 1912. Since then, pozzolanas have been used in the
construction of many mass concrete and marine structures such as dams and harbours,
particularly in Europe, North America and Japan.
With a few notable exceptions such as Indonesia, Oman, India and China, the commercial
exploitation of pozzolanas in the Third World has generally been slower. Considering the need
for low-cost construction materials, this is perhaps surprising but in recent years there have
been projects to develop pozzolanas in several countries, including Kenya, Tanzania, Cuba,
Trinidad and Dominica.
Why use pozzolanas
The addition of a pozzolana in either a lime or OPC-based product has two major ad-
vantages. Firstly, the properties of the cement will be improved, and secondly, as the costs of
a pozzolana are usually low and certainly well below that of lime or OPC, overall cost will be
significantly reduced assuming the pozzolana does not have to be transported too far.
Lime-pozzolana
The addition of a pozzolana will decrease setting times and increase the strength of lime-
based concretes, mortars and renders. It can produce a hydraulic cement which has the
ability to set under water. While lime-pozzolana cements are unlikely to achieve the strengths
of OPC they are more than adequate for all forms of low-cost shelter construction. Lime-
pozzolana cements can often be produced for less than half the cost of OPC.
The use of low-cost lime-pozzolana cements in small-scale building works is common in
many parts of Asia.
OPC-pozzolana
In OPC-based concretes pozzolanas are used to replace up to 30 per cent of OPC for use in
structural applications and up to 50 per cent for non-structural purposes. As OPC is an
expensive and sometimes scarce commodity, this can represent a significant cost saving. In
addition a Portland-pozzolana blended cement has a number of significant technical
advantages over plain OPC. These are:
Improved workability
Improved water retention/reduced bleeding
Improved sulphate resistance
Improved resistance to alkali aggregate reaction
Lower heat of hydration
Enhanced long-term strength
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