791. Next in antiquity to hydraulic lime
is Roman cement, prepared by heating an indurated marl occurring
naturally in nodules. Its name must not be taken to imply that it was
used by the ancients; in point of fact the manufacture of this substance
dates back only to 1796.

With the growth of engineering in the early part of the 19th century
arose a great demand for hydraulic cement. The supply of materials
containing naturally suitable proportions of calcium carbonate and clay
being limited, attempts were made to produce artificial mixtures which
would serve a similar end. Among those who experimented in this
direction was Joseph Aspdin, of Leeds, who added clay to finely ground
limestone, calcined the mixture, and ground the product, which he called
Portland cement. The only connexion between Portland cement and the
place Portland is that the cement when set somewhat resembles Portland
stone in colour. True, it is possible to manufacture Portland cement
from Portland stone (after adding a suitable quantity of clay), but this
is merely because Portland stone is substantially carbonate of lime; any
other limestone would serve equally well. Although Portland cement is
later in date than either Roman cement or hydraulic lime, yet on account
of its greater industrial importance, and of the fact that, being an
artificial product, it is of approximately uniform composition and
properties, it may conveniently be treated of first. The greater part of
the Portland cement made in England is manufactured on the Thames and
Medway. The materials are chalk and Medway mud; in a few works the
latter is replaced by gault.

The composition of typical samples of chalk and clay is shown in the
following analyses:--

+-----------------------------------+----------------------------------------------------------------------------+
| Chalk. | Clay. |
+-------------------------+---------+----------------------------+---------+------------------+-------+----------+
| |Per cent.| |Per cent.| | | |
|Silica (SiO2) | 0.92 | Insoluble silicious matter | 26.67 |Consisting of | | |
|Alumina+ferric oxide | | Silica (SiO2) | 31.24 | Quartz (SiO2) | 19.33 | |
| (Al2O2 + Fe2O3) |