the process of the future, displacing the rotatory method.

Cement clinker.

Portland cement clinker, however produced, is a hard, rock-like
substance of semi-vitrified appearance and very dark colour. The
product from a well-run rotatory kiln is all evenly burnt and properly
vitrified; that from an ordinary fixed kiln of whatever type is apt to
contain a certain amount (5 to 15%) of underburnt material, which is
yellowish and friable and is not properly clinkered. This material
must be picked out, as such underburnt stuff contains free lime or
unsaturated lime compounds. These may slake slowly in the finished
cement and cause such expansion as may destroy the work of which it
forms part. Well-burnt, well-picked clinker when ground yields good
Portland cement. Nothing is added during or after grinding save a
small amount (1 to 2%) of calcium sulphate in the form either of
gypsum or of plaster of Paris, which is sometimes needed to make the
cement slower-setting. For the same purpose a small quantity of water
(up to 2%) may be added either by moistening the clinker or by blowing
steam into the mills in which the clinker is ground. This small
addition for this specified purpose is recognized as legitimate, but
the employment of various cheap materials such as ragstone and
blast-furnace slag, sometimes added as diluents or make-weights, is
adulteration and therefore fraudulent.

Composition.

The composition of Portland cement varies within comparatively narrow
limits, and for given raw materials the variations are tending to
become smaller as regularity and skill in manufacture increase. The
following analysis may be taken as typical of cements made from chalk
and clay on the Thames and Medway:--

Per cent.
Silica (SiO2) 22.0
Insoluble residue 1.0
Alumina (Al2O3) 7.5
Ferric oxide (Fe2O3) 3.5
Lime (CaO) 62.0
Magnesia (MgO) 1.0
Sulphuric anhydride (SO3) 1.5
Carbonic anhydride (CO2) 0.5
Water (H2O) 0.5
Alkalis 0.5
-----
100.0

There may be variations from this composition according to the nature
of the raw materials employed. Thus the silica may range from 19 to
27%, the alumina and ferric oxide jointly from