| Chaux de Theil. Per cent. | Blue Lias. 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.
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 anhydride, and loss | 8.78 |
| ——— | |
| 100.00 |
These irregular cements of the Portland class are good building materials for ordinary purposes, but are not so suitable as good artificial Portland cement for heavy and important undertakings.