semble somewhat irregular Portland cement.

Analyses of the two classes of hydraulic lime are as follows:--

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
attempt