Chaux de Theil. Blue Lias.
Per cent. Per cent.
Insoluble silicious matter 0.3 2.39
Silica (SiO2) 21.7 14.17
Alumina (Al2O3) 1.8 6.79
Ferric oxide (Fe2O3) 0.6 2.34
Lime (CaO) 74.0 63.43
Magnesia (MgO) 0.7 1.54
Sulphuric anhydride (SO3) 0.3 1.63
Carbonic anhydride (CO2) \ 0.6 / 3.64
Water (H20) / \ 2.69
Alkalis and loss · · 1.38
----- ------
100.0 100.00
Hydraulic lime contains a good deal of uncombined lime, and has to be
slaked before it is used as a cement. In France this slaking is
conducted systematically by the makers, the freshly burned lime being
sprinkled with water and stored in large bins where slaking proceeds
slowly and regularly until the whole of the surplus uncombined lime
is slaked and rendered harmless, while the cementitious compounds,
notably tricalcium silicate, remain untouched. In English practice
hydraulic lime is slaked by the user. Seeing that regular and perfect
slaking is more easily attained when working systematically on a large
scale and by storing the material for a long period, the French method
is the better and more rational. The product may then be regarded as a
cement of the Portland class mixed with slaked lime. When gauged with
water and made into a mortar it sets slowly, but ultimately becomes
almost as strong as Portland cement. Its slow setting is an advantage
for some purposes, e.g. for foundations and abutments where
settlements may occur. The structure is free to take its permanent
position before the lime sets, and cracks are thus avoided. A case in
point is the employment of hydraulic lime in place of Portland cement
as grouting outside the cast-iron tubes used for lining tunnels made
by the shield system.
_Roman Cement_ is another cement of the Portland class which came into
use shortly before the manufacture of artificial Portland cement was
attempted. 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.