Cement may be delivered in bulk or in bags. Bulk cement is delivered by tanker, usually in loads of more than 25 tones and blown into storage silos by compressed air. Bagged cement is usually supplied in bags containing 25 kg or, very rarely 50 kg, whilst 1 tone bags are also available from some suppliers for special purposes. It is often convenient to use bags on a smaller site, but cement is cheaper in bulk.Cement should be kept dry during storage as moist air leads to the phenomenon of air-setting, which results in the formation of lumps of hydrated
cement. Air-set cement should not be used, as concrete made from it could have a much reduced strength.
Silos have to be weatherproof but, during prolonged periods of
storage, some air setting may occur due to condensation in the
silo. This is minimized by aeration,
which should be done frequently in periods of prevailing damp weather. In addition,
the weatherproofness of the silo should be checked if there is any
evidence of the formation of lumps in the cement.
Regular maintenance of cement silos is essential. All moving parts should be kept free from coatings of cement by cleaning
at least at
the end of every day. Weigh hoppers should also be cleaned every
day,
both inside and out, since a build-up of cement can result in top little cement being dispensed,
and weigh gear checks should be carried out at least once a month. Silo air filters
must be cleaned
after every cement delivery to prevent them from becoming choked; this is done by giving them a thorough shaking
or,
preferably, by replacing old filters with reverse-air jet units that
prevent contamination of the environment.
Bagged cement
should be stored on a raised floor in a weather-
tight
shed in order to prevent
deterioration. Failing this, it should
be,
stacked on a raised timber platform
and covered by waterproof covers with generous
overlaps (Figure 1). The bags should be used
in
the order in which they are received;
thus each delivery should
be kept separate
to avoid confusion. To avoid 'warehouse set', which results
from the compaction of cement, bags should not be stacked higher
than about 1.5 m. The paper bags used for packing cement are not vapour proof, so undue exposure should be
avoided. Even when stored under good conditions,
bagged cement may lose 20% of its strength after
two
months' storage. To avoid
risk
of accidental confusion, cements of different types should be stored separately.
Sampling and testing of cement
The testing of cement requires the resources of a well-equipped laboratory with strictly controlled
temperature and humidity, which are seldom achieved on site. Manufacturers
in
the UK produce cement
whose conformity is certificated by a third party in a
scheme based on a strict regime of inspection and independent audit testing. Cement test reports showing results of physical and chemical tests are forwarded to users of the cement and it is general practice for concrete producers to monitor cement quality by continuously assessing the data and thereby avoiding unnecessary duplication of costly tests on cement.
