, converts these from loose
incoherent material into a solid stone. Thus it comes about that the
largest use of cement is for manufacturing concrete for dock and harbour
work, and for the making of foundations. It is also employed for the
building of light bridges, floors, and pipes constructed of cement
mortar disposed round a skeleton of iron rods. Such composite structures
take advantage at once of the high tensile strength of iron and of the
high compressive strength of cement mortar. (See also CONCRETE.)

Good hydraulic cements are highly permanent materials provided certain
conditions be observed. It might be supposed that hydraulic cements from
their nature would be indifferent to the action of water, but this is
only true if the structures of which they form part are sufficiently
compact. In this case the action of the water is checked by the film of
carbonate of lime which eventually forms oh the surface of calcareous
cement. This, together with the compactness of the mortar, hinders the
ingress and egress of water, and prevents the dissolution and ultimate
destruction of the cement. But where the concrete or mortar is not well
made and is porous, the continual passage of water through it will
gradually break up and dissolve away the calcareous constituents of the
cement until its strength is utterly destroyed. This destructive action
is increased if the water contains sulphates or magnesium salts, both of
which act chemically on the calcareous constituents of the cement. As
sea-water contains both sulphates and magnesium salts, it is especially
necessary in concrete for harbour work to take every care to produce an
impervious structure. There are various minor external causes for the
failure and ultimate destruction of cement mortar and concrete, but
their discussion is a matter for the specialist. Failure from inherent
vice in the cement has been already touched on; it can always be traced
to want of skill and care in manufacture.

_Calcium Sulphate Cements._--Under this term are comprehended all
cements whose setting properties primarily depend on the hydration of
calcium sulphate. They include plaster of Paris, Keene's cement and many
variants of these two types. The raw material is gypsum (q.v.). This may
be almost chemically pure, when it is generally used for Keene's cement;
or it may contain smaller or greater quantities of impurities, in which
case it is suitable for the preparation of cements of the p