y mixed. The
usual proportions are three of slag to one of slaked lime by weight. The
product termed slag cement sets slowly, but ultimately attains a
strength scarcely inferior to that of Portland cement. Although it is
cheap and suitable for many purposes, its use is not large and tends to
decrease. Pozzuolanic cements are little used in England. Generally
speaking, they are only of local importance, their cheapness depending
largely on the nearness and abundance of some suitable volcanic deposit
of the trass or tufa class. They are not usually manufactured by the
careful grinding together of the pozzuolana and the lime, but are mixed
roughly, a great excess of pozzuolana being employed. This excess does
no harm, for that part which fails to unite with the lime serves as a
diluent, much as does sand in mortar. In fact, ordinary pozzuolanic
cement made on the spot where it is to be used may be regarded as a
better kind of common mortar having hydraulic qualities. Good hydraulic
mortars may be made from lime mixed with furnace ashes or burnt clay as
the pozzuolanic constituent.

Portland Cement

Cements of the Portland type differ in kind from those of the
pozzuolanic class; they are not mechanical mixtures of lime and active
silica ready to unite under suitable conditions, but consist of definite
chemical compounds of lime and silica and lime and alumina, which, when
mixed with water, combine therewith, forming crystalline substances of
great mechanical strength, and capable of adhering firmly to clean inert
material, such as stone and sand. They are made by heating to a high
temperature an intimate mixture of a calcareous substance and an
argillaceous substance. The commonest of such substances in England are
chalk and clay, but where local conditions demand it, limestone, marl,
shale, slag or any similar material may be used, provided that the
correct proportions of lime, silica and alumina are maintained. The
earliest forms of cements of the Portland class were the hydraulic
limes. These are still largely used, and are prepared by burning
limestones containing clayey matter. Some of these naturally possess a
composition differing but little from that of the mixture of raw
materials artificially prepared for the manufacture of Portland cement
itself. Although hydraulic limes have been in use from the most ancient
times, their true nature and the reason of their resistance to water
have only become known since 1