Alternatives to Portland cement
Practical Action
Traditionally lime renders and plasters were often mixed with animal hair to improve
cohesion. Today the addition of gypsum or Portland cement and/or pozzolanas to increase
durability and give faster setting times is usual.
Pozzolanas
Pozzolanas are materials which, although not cementitious in themselves, will combine
chemically with lime in the presence of water to form a strong cementing material. They
include:
Volcanic ash
Power station fly ash (usually known as pfa)
Burnt clays
Ash from some burnt plant materials
Silicious earths (such as diatomite)
Materials not already in a fine powdered form
must be ground, and some require calcining at
around 600–750°C to optimize their pozzolanic
properties.
Pozzolanas can be mixed with lime and/or
Portland cement and can improve quality and
reduce costs of concretes made from both
materials.
Figure 3: Hydrating of quicklime
using a watering can Chenkumbi
Limeworks, Malawi. Photo: Practical
Action / Dave Mather.
In some countries (e.g. India and Kenya),
pozzolanas are mixed with Portland cement and
sold as blended cement, which in many respects
is similar to Portland cement. In other countries
(e.g. Cuba) lime/pozzolana/Portland cement
blends are sold as an alternative to Portland
cement. Lime-pozzolana cement by itself can
make an excellent cementing material for low-rise
construction or mass concrete and in some
countries (e.g. Indonesia) is still produced
extensively.
Pozzolanas can also be mixed with lime and/or
Portland cement at the construction site but care
must be taken to ensure the pozzolana is of a
consistent quality and that the materials are
thoroughly mixed.
Gypsum plaster
Gypsum is a not an uncommon mineral, and needs only a low temperature, of around 150°C,
to convert it into a very useful binding material, known as hemi-hydrate or plaster of Paris.
On its own, plaster of Paris sets very rapidly when mixed with water. To give time for it to be
applied, around 5% of lime and 0.8% of a retarding material (such as the keratin glue-like
extracts from boiling fish bones or animal hoof and horn) are added to the plaster.
Retarded plaster of Paris can be used on its own or mixed with up to three parts of clean,
sharp sand. Hydrated lime can be added to increase its strength and water resistance.
Gypsum plasters can be reinforced with various fibrous materials from reeds to chopped glass
fibres.
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