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< prev - next > Construction Cement and binders KnO 100366_Testing Methods for Pozzolanas (Printable PDF)
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 cement. They are an important ingredient in the production of alternative
cementing materials to ordinary Portland cement (OPC).
The Greeks and the Romans were the first civilisations known to use pozzolanas in lime
mortars. The Romans used not only crushed pottery, bricks and tiles which formed the first
artificial pozzolanas but also found that some volcanic soils were excellent for producing a
hydraulic mortar. Nowadays, a wide variety of siliceous or aluminous materials are used for
producing pozzolanas, the common materials being calcined clays, pulverised fly ash,
volcanic ash and ash from agricultural residues such as rice husks. (For more general
information on pozzolanas see the leaflet in the ‘Low-cost cements’ series entitled
Cement or some form of binding agent is a vital element in all types of construction and in
recent years the cement market has been dominated by one product, OPC. In many countries
of the developing world, OPC is an expensive and sometimes scarce commodity and this
factor has severely limited the construction of affordable housing. Many of these countries
have large supplies of pozzolanic materials readily available. In volcanic areas such as
Central Africa, Central America and Indonesia there are enormous deposits of volcanic ash.
In India and other Asian countries rice husk ash (RHA), (the husks can often be difficult to
dispose of), provides a good source of pozzolana for use in masonry cements. Although still
limited in comparison with the growth of the use of pozzolanas in Europe and the USA, there
are a growing number of developing countries now using more and more pozzolanas in
combination with lime or OPC.
Pozzolanas, by their diverse and varied nature, tend to have widely varying characteristics.
The chemical composition of pozzolanas varies considerably, depending on the source and
the preparation technique. Generally, a pozzolana will contain silica, alumina, iron oxide and
a variety of oxides and alkalis, each in varying degrees. This presents problems for small-
scale manufacturers wishing to use pozzolanas in a lime or OPC - pozzolana mix. Where there
are no laboratory facilities available for testing the raw materials, then it is difficult to
maintain standards and produce a consistent product.
It is also generally agreed that although the chemical content of a raw material will determine
whether or not it is pozzolanic and will react when mixed with lime or OPC, the degree of
reaction and subsequent strength of the hydrated mixture cannot be accurately deduced from
just the chemical composition (except for a small number of known pozzolanas - see
reference 5). In most cases no direct correlation can be found between chemical content and
reactivity. Other characteristics of the pozzolana also affect its reactivity, such as fineness
and crystalline structure.
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