85.6 " "
100.0
This strong solution was employed in order that the probable
effect of immersing the cement in sea-water might be
ascertained very much quicker than could be done by observing
samples actually placed in ordinary sea-water, and it is worthy
of note that the various mixtures which failed in this
accelerated test also subsequently failed in ordinary sea-water
within a period of twelve months.
Strong solutions were next made of the individual salts
contained in sea-water, and pats were immersed as before, when
it was found that the magnesium sulphate present in the water
acted upon the calcium hydrate in the cement, forming calcium
sulphate, and leaving the magnesium hydrate free. The calcium
sulphate combines with the alumina of the cement, forming
calcium sulpho-aluminate, which causes swelling and cracking of
the concrete, and in cements containing a high proportion of
alumina, leads to total destruction of all cohesion. The
magnesium hydrate has a tendency to fill the pores of the
concrete so as to make it more impervious to the destructive
action of the sea-water, and disintegration may be retarded or
checked. A high proportion of magnesia has been found in
samples of cement which have failed under the action of sea
water, but the disastrous result cannot be attributed to this
substance having been in excess in the original cement, as it
was probably due to the deposition of the magnesia salts from
the sea-water; although, if magnesia were present in the cement
in large quantities, it would cause it to expand and crack,
still with the small proportion in which it occurs in ordinary
cements it is probably inert.
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