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STONE: NATURAL AND ARTIFICIAL. 99
mass in which they happen to be incorporated in a porous and laminated
condition.
The matrix is almost universally Portland cement, though hydraulic lime
has been, and is still used, and also Roman cement. Hydraulic lime of a
special character—the Teil lime—is a favourite with French engineers for sea
work. It has been very largely employed in their works on the Mediter-
ranean coast and along the English Channel, and, with one or two exceptions,
seems to have answered satisfactorily. The use of Roman cement is limited
to situations where rapidity of execution is essential, and where the hardening
of the mortar is required to take place within a very short period. Neither
hydraulic lime nor Roman cement has anything like the strength and
durability of Portland cement.
Portland cement is an artificial product obtained by calcining, at a high
temperature, an intimate compound of clay or shale with chalk or other
limestone. In this condition, it contains a number of ingredients, of which
the principal are lime, silica, alumina, and oxide of iron. These form about
nineteen-twentieths of the whole, within the following limits, viz. : —
Lime, . . . . 60 to 64 per cent.
Silica, .... 20 to 24 „
Alumina, . . . 6 to 10 „
Oxide of Iron, . . . 3 to 5 „
The remaining ingredients are magnesia, sulphuric acid, certain alkalies,
and moisture. Of these, the magnesia should not be permitted to exceed
5 per cent., nor the sulphuric acid 1 per cent.
So great a variation has been manifested in the character of the different
brands of cements emanating from the numerous manufacturers both in this
country and abroad, and so much divergence of opinion has been exhibited in
regard to standards and tests to be adopted for reference and comparison,
that it was recently felt desirable, and even necessary, to draw up a spéci-
fication for general use among engineers. This has been done by the
Engineering Standards Committee, and the result of their deliberations is
embodied in the spécification at the end of this chapter. It is not necessary,
therefore, at this stage, to enter into the more general requirements of the
model spécification.
Effect of Sea-water on Conerete. —The most vital considération in
regard to the matrix is the effect of sea-water upon conerete. On this point
there is scope for much discussion and some ground for difference of view.
On the one hand, there is abundant practical exemplification to demonstrate
that Portland cement conerete is, in general, a thoroughly sound and durable
material, in every way adaptable to maritime situations as elsewhere; on the
other hand, there are indubitable instances of deterioration and failure.
These instances obviously demand a searching inquiry, for in the absence of
definite and authoritative refutation they must inevitably produce a feeling of
doubt and uncertainty as to the propriety of using Portland cement in situa-