ed. It is still in use, though only for special purposes where a
quick-setting material is required. It is made from septaria nodules
which are dredged up on the Kent and Essex coasts and consist of about
60% of calcium carbonate mixed with clay, the mass being sufficiently
indurated to remain coherent under water. The nodules are not prepared
in any way, but simply burned at a moderate red heat.

The resulting cement varies somewhat in composition, but approximates
to the following figures:--

Per cent.
Insoluble silicious matter 5.86
Silica (SiO2) 19.62
Alumina (Al203) 10.30
Ferric oxide (Fe2O3) 7.44
Manganese dioxide (MnO2) 1.57
Lime (CaO) 44.54
Magnesia (MgO) 2.92
Sulphuric anhydride (SO3) 2.61
Carbonic anhydride (CO2) 3.43
Water (H2O) 0.25
Alkalis and loss 1.46
------
100.00

The most characteristic constituent is the oxide of iron, which gives
the cement a reddish colour, and the presence of manganese also
differentiates Roman from Portland cement, which rarely contains
appreciable quantities of that element. The high percentage of alumina
causes the cement to be quick-setting, and it becomes hard in about
five minutes. It resists the action of water, salt or fresh, very
well, and is therefore useful in situations where the work is likely
to be submerged immediately after it has been put in place.

The term _Natural Cements_ is applied to cements made by burning
mixtures of clay and carbonate of lime naturally occurring in
approximately suitable proportions. They may be regarded as badly-mixed
Portland cements, and need no special description. American "natural"
cements are of a somewhat different class. They are usually made from a
silicious limestone containing magnesia, and are comparatively lightly
burned.

The following analysis is typical of a cement of this kind:--

Per cent.
Silica (SiO2) 24.30
Alumina (Al203) 7.22
Ferric oxide (Fe2O3) 5.06
Lime (CaO) 33.70
Magnesia (MgO) 20.94
Water, carbonic anhydrid