Introduction to concrete building products
Practical Action
silicon, aluminium and iron that react chemically with the lime (calcium hydroxide) generated by
the hydration of cement to produce additional cementitious components. Some examples of
pozzolanas include rice husk ash, fired clay brick dust, volcanic ash, activated kaolin and
diatomaceous earth. Addition of pozzolanas in this way does not adversely affect the properties of
the concrete and can confer some additional advantages such as increased chemical resistance
in harsh environments. Note however, that as pozzolanas are very fine materials they may
increase the water requirement slightly as well as increase the curing time slightly.
Low strength blocks can also be made entirely with a lime-pozzolana cement. Mix proportions to
use can be 1:2:2:4 or 1:3:2:6 lime to pozzolana to sand to gravel.
See the Practical Action technical brief Pozzolanas: An Introduction for further information about
pozzolanas and their use in cement
Blocks for use in dry conditions can also be made using gypsum as the binder. Mix proportions of
1:1:2 gypsum to sand to coarse aggregate may be used. Note that gypsum mixes cannot carry
large aggregate particles as well as OPC concrete so that it may be useful to limit the maximum
aggregate size to 5 to 10mm across. Gypsum (plaster of Paris) concrete can be made more
durable to occasional exposure to damp conditions with the addition pulverised fuel ash that can
replace 10 to 20 per cent of the sand fraction of the mix.
See the Practical Action technical brief Gypsum Processing and Use for further information
about gypsum as a binder and its technical properties -
OPC properties are relatively similar throughout the world, wherever this cement is produced.
However, aggregate properties vary widely. Therefore it is best to treat the above guidelines only
as approximate and to carry out trials on the best mix proportions to use for particular products
with the locally available aggregates. This applies in particular if lime and pozzolanas are used in
the mix as these materials can vary widely in properties even over quite a small area.
Equipment for Concrete Products and Production Site Layout
Equipment Requirements
The following equipment may be seen at a medium-scale concrete products yard :-
(i) Silo for cement
(ii) Drying kiln for aggregate
(iii) Conveyors for transporting material
(iv) Automatic batching boxes (by weight)
(v) Industrial Concrete Mixer
(vi) Moulding Machine
(vii) Water tanks, steam cabinet or autoclave
(viii) Trolley or fork-lift truck for transporting the products.
A smaller yard may just have some moulds and a two or three wheelbarrows or trolleys or small
hand-held trucks, a builders’ concrete mixer, a set of weighing scales, and a water tank. Optionally
such a yard may also have a small moulding machine for blocks or a vibrating table or electrically-
powered vibrating plates that are attached to moulds. At the very lowest level of production, if only
producing blocks, it may be enough just to use hand tamping of the concrete in the moulds.
However for most small to medium scale producers the costliest items for would be the concrete
mixer and the moulding / vibrating unit; so some further details are given on these.
Concrete Mixers
Concrete may be mixed manually using shovels on a hard level surface. This is completely
acceptable for small quantities, up to about two wheelbarrow loads, provided the mixing is carried
out thoroughly, but becomes very cumbersome for larger quantities if done by only one or two
people. In practice, except when very small quantities are used, concrete is normally mixed in a
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