etter to use the former, because, although cement is the more
expensive matrix, a smaller proportion of it will suffice for use. Lime
should never be used in work exposed to sea-water, or to water
containing chemicals of any kind. Portland cement concrete, on the other
hand, may be used without fear in sea-water, provided that certain
reasonable precautions are taken. Considerable alarm was created about
the year 1887 by the failure of two or three large structures of
Portland cement concrete exposed to sea-water, both in England and other
countries. The matter was carefully investigated, and it was found that
the sulphate of magnesia in the sea-water has a decomposing action on
Portland cements, especially those which contain a large proportion of
lime or even of alumina. Indeed, no Portland cement is free from the
liability to be decomposed by sea-water, and on a moderate scale this
action is always going on more or less. But to ensure the permanence of
structures in sea-water the great object is to choose a cement
containing as little lime and alumina as possible, and free from
sulphates such as gypsum; and more important still to proportion the
sand and stones in the concrete in such a way that the structure is
practically non-porous. If this is done there is really nothing to fear.
On the other hand, if the concrete is rough and porous the sea-water
will gradually eat into the heart of the structure, especially in a case
like a dam, where the water, being higher on one side than the other,
constantly forces its way through the rough material, and decomposes the
Portland cement it contains.

Convenience and appearance.

As regards its convenience for building purposes it may be said roughly
that in "mass" work concrete is vastly more convenient than any other
material. But concrete is hampered by the fact that the surface always
has to be formed by means of wooden or other framing, and in the case of
thin walls or floors this framing becomes a serious item, involving
expense and delay. In appearance concrete can rarely if ever rival stone
or brickwork. It is true that it can be moulded to any desired shape,
but mouldings in concrete generally give the appearance of being
unsatisfactory imitations of stone. Moreover, its colour is not
pleasing. These defects will no doubt be overcome as concrete grows in
popularity as a building material and its aesthetic treatment is better
understood. Concrete pavings are bein