e, and loss 8.78
------
100.00

These irregular cements of the Portland class are good building
materials for ordinary purposes, but are not so suitable as good
artificial Portland cement for heavy and important undertakings.

_Passow Cement _is a recent product which is in a class by itself. It is
made by granulating blast furnace slag of suitable composition and
finely grinding the product, either alone or with an admixture of about
10% of Portland cement clinker. It differs from ordinary slag cement
(see above) in that it is not a pozzuolanic cement depending on the
interaction of granulated slag and lime. The particular method of
granulating slag for Passow cement produces a material which sets _per
se_ and attains a strength comparable with that of Portland cement.
Passow cement has been successfully made from slag of different
compositions in Germany, England and America.

Uses of hydraulic cements.

The chief use of hydraulic cements, whether of the pozzuolanic or
Portland class, is to act as an adhesive material in work which is to be
exposed to water. No doubt in times of remote antiquity it was found
that the jointing of masonry which was to be immersed required the use
of a cement indifferent to the action of water. Ordinary mortar failed
in such positions; mortar made from lime prepared from limestones or
chalks containing a little clay was found to stand; mortar made from
lime mixed with trass or similar active silicious material was also
found to stand. On this observation rests the whole of the present
enormous employment of hydraulic cements. It was a natural transition to
utilize these cements not merely for jointing masonry but also for
making concrete, and the only reason why hydraulic cements, as distinct
from cements which are not hydraulic (e.g. ordinary mortar), are used
for the latter purpose is their great mechanical strength. Their use in
above-water work is checked by the low price of common brick. Even in
such work, where it would be thought that masses of burnt clay would be
the cheapest conceivable material, concrete is at least on level terms
with its rival. It must be remembered that one of the great advantages
of concrete is that five-sixths of its total mass may be provided from
local sand and gravel, on which no carriage has to be paid. The cement,
on which alone freight is to be reckoned