334 REVIEWS — GEOLOGICAL SURVEY OF CANADA. 



placed under -water, however, and excluded from the influence of carbonic acid, 

 mortars thus composed do not harden, but become dissolved or disintegrated ; they 

 cannot therefore be employed for constructions which are submerged. 



Certain limestones have long been known to yield mortfirs or cements, «vhieh 

 have the property of hardening under water; and pozzuolanas of Italy and some 

 other countries, when mingled with ordinary lime, yield mortars which are pos- 

 sessed of similar properties. Pozzuolanas, and these peculiar limestones are com- 

 paratively rare ; but Vicat has shown that it is possible to imitate them in a very 

 simple manner, and with materials which are everywhere present, to prepare 

 hydraulic cements. The limestones which yield hydraulic cements are those 

 which are mingled with a certain proportion of clay, and by calcining an artificial 

 mixture of carbonate of lime and clay, we may prepare hydraulic cements, vary- 

 ing in character according to the proportions of the mixture. When the limestone 

 contains 10, 15, or 25 per cent, of clay, it becomes more and more hydraulic, and 

 when the mixture amounts to one-third of the lime, we obtain a mortar which har- 

 dens almost immediately in air or under water. The proportion of clay may even 

 rise to 60 per cent. 



The name of Roman cement is applied to a mixture of this sort, but incorrectly, 

 as the preparation of such a cement was unknown to the Romans. The poszuolana 

 or trass, which was employed by them to give hardness to their mortars, is a fel- 

 spathic or argillaceous matter, which has been calcined by volcanic heat, and has 

 thus acquired the property of rendering ordinary lime hydraulic. It suffices, in 

 fact, to calcine any ordinary clay, especially with the addition of a little alkali, to 

 obtain an artificial pozzuolana. 



The well-known Portland cement (so called because its colour resembles that of 

 the Portland stone,) is prepared by calcining a mixture in proper proportions, of 

 chalk with the clayey mud of the Thames ; but similar and equally good cements 

 are now manufactured elsewhere in England and France by mixing chalk or marl 

 with other clays. The materials are reduced to fine powder, and intimately mixed 

 with the addition of water. The resulting paste is moulded into bricks, which are 

 dried and burned. It is of importance that the heat in calcining be sufficiently eleva- 

 ted, otherwise the carbonic acid and water may be expelled without that reaction be- 

 tween the lime and clay which is required for the production of a cement. It is 

 necessary to employ a white heat, which shall agglutinate and frit the mixture. 

 After this operation the material is assorted, and the portions which are scorified 

 by too much heat, as well as those insufficiently calcined, being set aside, the ce- 

 ment is pulverized for use. It is often advantageous to grind to powder the native 

 mixture of limestone and clay before burning them, in order to ensure greater 

 homogeneousness. It will also be seen that a calcination at a very elevated tem- 

 perature is frequently required to develope the hydraulic character of limestones; 

 the greater the temperature employed, the more slow is the solidification of the 

 cement, but the harder does it become. 



The portions of cement which have been over-heated and converted into a slag, 

 as well the semi-vitrified masses obtained in the calcination of ordinary lime, over- 

 burned bricks and tiles, and the scoriaj of iron furnaces, may all be used with 

 advantage to give hydraulic properties to ordinary lime, either by mingling witb 

 it before burning, or by employing them as pozzuolanas to mix with the slaoketj 



