Blocks stabilised with cement should be kept moist for 4 or 5 days. Lime stabilised blocks are
kept moist for up to 1 week. One good way to do this is to sprinkle water on the top layer of
blocks once each day and cover the stack with plastic sheeting. If blocks are not kept damp,
the clay in the soil will shrink too fast and the blocks will crack and be weak.
When blocks have been kept moist for the right number of days they can be stacked tightly
together. It is still a good idea to keep the blocks in shade, perhaps under a thick layer of
thatch grass if covered building space is short.
The blocks are still not fully cured. They are still setting and growing stronger. Blocks
stabilised with OPC should not be sold or used for at least 28 days after they are made. At
this time the block should have about 75 % (3/4) of its final strength. The blocks should keep
growing stronger for more than a year!
Lime stabilised blocks grow stronger more slowly. They should not be sold for at least 37 days.
Lime blocks will probably have 75% of their final strength after this time and will also keep
growing stronger.
Good curing is very important. Blocks that are not cured properly will tend to crack and be
weak. It is a good idea to leave a few blocks to dry in the sun and not wet them. These sun
dried blocks will probably crack badly and perhaps not harden at all. If these blocks are tested
with others from the same day that have been cured properly there should be a big difference
in their strength.
BLOCK TESTING
Cured blocks can be sent to the Standards Association of Zimbabwe (SAZ) or some
commercial companies for testing. The tests that can be carried are: Dry compressive
strength; Water absorption and wet compressive strength. The Model Building By-Laws
(available from SAZ) give standards for SSBs. In a sample of 10 concrete blocks the minimum
average compressive strength must be 2.8
MPa (compare this to a common fired clay brick which will have a compressive strength
around 7 MPa). In a sample of 10 blocks, no single block should be weaker than 2.2 MPa.
Increase in mass after 24 hours should not exceed 12% of dray mass. Linear expansion after
immersion in water after 24 hours should not exceed .05% for structural walls and .1 % for
internal walls.
It is expensive to send blocks for testing to government laboratories or commercial companies.
If you are making blocks in an area where there is not a restriction on what can be used, there
are some tests that you can do yourself. These tests should be done to make sure that
building made with the blocks are safe and will not fall down, especially when the blocks get
wet.
(a) All your blocks should be the same sizes. If your block should be 295mm
long x 140mm wide x 88mm high, then the measured length should always be
between 292 and 295mm, the width between 138 and 141 mm and the height
between 87 and 90mm.
(b) The density of a wet block should be at least 1,870 kg/cubic metre and it
would be better at 2,200 kg/ cubic metre.
If your block measures the same as the example in (a) above then its volume is 29.5cm x
14.0cm x 8.8cm = 3,634 cubic cm. So its weight should be at least 1,870 x
(3634/1,000,000) = 6.80 kg.
(c) For a dry block (after at least 28 days for OPC) the density should be at
least 1,700 kg/cubic metre and it would be better at 2,000 kg/cubic metre.
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