e said that
before using Portland cement very careful tests should be made to
ascertain its quality and condition. Moreover, it should be kept in a
damp-proof store for a few weeks; and when taken out for use it should
be mixed and placed in position as quickly as possible, because rain, or
even moist air, spoils it by causing it to set prematurely. The oldest
of all the matrices is lime, and many splendid examples of its use by
the Romans still exist. It has been to a great extent superseded by
Portland cement, on account of the much greater strength of the latter,
though lime concrete is still used in many places for dry foundations
and small structures. To be of service the lime should be what is known
as "hydraulic," that is, not pure or "fat," but containing some
argillaceous matter, and should be carefully slaked with water before
being mixed with the aggregate. To ensure this being properly done, the
lumps of lime should be broken up small, and enough water to slake them
should be added, the lime then being allowed to rest for about
forty-eight hours, when the water changes the particles of quicklime to
hydrate of lime, and breaks up the hard lumps into a powder. The
hydrated lime, after being passed through a fine screen to sort out any
lumps unaffected by the water, is ready for concrete making, and if not
required at once should be stored in a dry place. Other matrices are
slag cement, a comparatively recent invention, and some other natural
and artificial cements which find occasional advocates. Materials like
tar and pitch are sometimes employed as a matrix; they are used hot and
without water, the solidifying action being due to cooling and to
evaporation of the mineral oils contained in them. Whatever matrix is
used, it is almost invariably "diluted" with sand, the grains of which
become coated with the finer particles of the matrix. The sand should be
coarse-grained and hard. It should be free from dirt--that is to say,
free from clay or soft mud, for instance, which prevents the cement
adhering to its particles, or again from sewage matter or any substance
which will chemically destroy the matrix. The grains should show no
signs of decay, and by preference should be of an angular shape. The
sand obtained by crushing granite and hard stones is excellent. When
lime is used as a matrix, certain natural earths such as pozzuolana or
trass, or, failing these, powdered bricks or tiles, may be used instead
of san