prepared from materials free from iron. Such cement, if free also from
manganese, is white, and its manufacture has been proposed for
exterior decorative use. Magnesia, if present in Portland cement in
quantity not exceeding 5%, appears to be inert, but there is evidence
that in larger proportion, e.g. 10-15%, it may hydrate and set after
the general setting of the cement, and may give rise to disruptive
strains causing the cement to "blow" and fail. In so-called natural
cement which is comparatively lightly burnt, the magnesia appears to
be inert, and as much as 20 to 30% may be present. Another constituent
of Portland cement which influences its setting time is calcium
sulphate, naturally formed from the sulphur in the raw materials or
fuel, or intentionally added to the finished cement as gypsum or
plaster of Paris. It has a remarkable retarding effect on the
hydration of the calcium aluminate, and consequently on the setting of
the cement; thus it is that a little gypsum is often added to convert
a naturally quick-setting cement into one which sets slowly. It will
be observed that in the hydration of tricalcium silicate, the main
constituent of Portland cement, a large portion of the lime appears as
calcium hydroxide, i.e. slaked lime. It is evident that this will form
a pozzuolanic cement if a suitable silicious material such as trass is
added to the cement. The ultimate product when set may be regarded as
a mixed Portland and pozzuolanic cement. The use of trass in this
manner as an adjunct to Portland cement has been advocated by W.
Michaelis, and undoubtedly increases the strength of the material, but
it has not become general.

Testing.

The quality of Portland cement is ascertained by its analysis and by
determining its specific gravity, fineness, mechanical strength and
soundness. A good sample will usually have a composition within the
limits cited above and approximating to the typical figures given
above. It will be ground so finely that not more than 3% will be left
on a sieve of 76 X 76 meshes per sq. in., the wires of the sieve being
0.005 in. in diameter. It will have, when freshly burned, a specific
gravity not lower than 3.15, and briquettes made from it and kept in
water will possess a tensile strength of 400-500 lb. per sq. in. seven
days after they are made, while briquettes made from a mixture of 3
parts by weight of san