The setting of cement under the
action of water always frees a portion of the lime which was
combined, but over twice as much is freed when the cement sets
in sea-water as in fresh water. The setting qualities of cement
are due to the iron and alumina combined with calcium, so that
for sea-coast work it is desirable for the alumina to be
replaced by iron as far as possible. The final hardening and
strength of cement is due in a great degree to the tri-calcium
silicate (3CaO, SiO2) which is soluble by the sodium chloride
found in sea-water, so that the resultant effect of the action
of these two compounds is to enable the sea-water to gradually
penetrate the mortar and rot the concrete. The concrete is
softened, when there is an abnormal amount of sulphuric acid
present, as a result of the reaction of the sulphuric acid of
the salt dissolved by the water upon a part of the lime in the
cement. The ferric oxide of the cement is unaffected by sea-
water.
The neat cement briquette tests showed that those immersed in
sea-water attained a high degree of strength at a much quicker
rate than those immersed in fresh water, but the 1 to 3 cement
and sand briquette tests gave an opposite result. At the end of
twelve months, however, practically all the cements set in
fresh water showed greater strength than those set in sea-
water.
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