1837.] 



FARMERS' REGISTER. 



589 



Hydraulic Cement. 



History. — The ancients were acquainted wifh a 

 pubslance whicli, when mixed with a |)iire quick- 

 lime and water, formed a mixture that possessed 

 the properly oC hardening under water. This is 

 a volcanic product havino; the iorm of powder, 

 originally lound at Puteoli in the neighhorhood oi' 

 Vesuvius, and called from the locality Tarra Pa- 

 ieolana. The material is slill used on the shores 

 of the Mediterranean and is called Puzzolana. 



The wants of the people of the low countries 

 Jed them to seek in their own vicinity for a sub- 

 stitute tor Puzzolana. This was found in a rock 

 furnished by extinct volcanoes, whose traces are to 

 be seen on the banks of the Rhine. This differs 

 from Puzzolana in requirinir a calcination to pre- 

 pare it for use, and when calcined it absorbs mois- 

 ture so readily that it must be carefully preserved 

 from contact with the air, unless it be used within 

 a short tmie of its manufacture. During the six- 

 ty years war, the people of Holland were cut off' 

 from a supply of this important article. In order 

 to replace it, earth was drawn up from bottoms of 

 their canals, tbrmed into brick, burnt, and pulver- 

 ized. This article was Ibund to be nearly as effi- 

 cacious as the native product, and both go by the 

 common name of Terras or Trass. 



Not only is there a substance which when mix- 

 ed with pure lime makes an hydraulic cement, but 

 these are varieties of hmestone, which are char- 

 acterized by refusing to slake when water is pour- 

 ed upon them, that when mixed with sand have 

 the same valuable property. The discovery of 

 this property seems to have been accidental, and its 

 date unknown. Such limestones were first work- 

 ed at Aberthaw in England, and Salenches in 

 France. When the construction of the New 

 York Canals was commenced, such limestones 

 were lound in its vicinity, and since that period 

 quarries of the same description have been o|)ened 

 in various parts of the United States. It may in- 

 deed be inferred that there is hardly any calcareous 

 formation some of the layers of wliich do not pos- 

 sess this properly. 



Even where native Puzzolana, Terras, or an 

 hydraulic lime are all wanting, the example of 

 Holland shows that artificial substitutes may be 

 found wherever clay can be obtained. 



Of all the materials tor the preparation of an 

 hydrulic cement, those which contain within them- 

 selves all the nacessary substances, and thus re- 

 quire no foreign matter to be mixed with them, 

 seem to be best. Such are the septarise found in 

 the London Clay and used in the preparation of 

 " Roman Cement." 



Rationale. — If the scale of a smiths forge, or the 

 black oxide of iron in any other form, is mixed 

 with common mortar, it acquires the property of 

 setting more rapidly, and speedily becomes so hard 

 as to resist the action of water. Clays rich in fer- 

 ruginous matter, if burnt and reduced to powder, 

 have the same effect upon lime. It was also 

 found that the native Terras contained from 10 to 

 15 per cent, of oxide of iron. It was inferred from 

 these facts, that the property possessed by a cement 

 of setting in water, was due to the presence of 

 oxide of iron. Some of the most valuable hydrau- 

 lic limes however, contain little or no iron, and 

 Puzzolana is also poor in that substance. An iso- 

 lated observation subsequently led to the ascription 



of the hydraulic property to the presence of oxide 

 of manganese. Another view of the subject led 

 to the hy|)0thesis that hydraulic cement was ob- 

 tained from a sub-carbonate of lin)e, the lime- 

 stones fi'om which it was prepared being in such a 

 state of combination that heat could not expel 

 more than half their carbonic acid. The theory 

 which is now received is that of Vicat, who infijrs 

 that hydraulic cement is obtained, whenever a sili- 

 cate of lime is either formed in the calcination of 

 the limestone, or generated rapidly by the action 

 of the substances with which a pure lime is mixed. 

 This silicate has the property of combining readily 

 with water, and then the aqueous ))art of the ce- 

 ment speedily becomes solid. To this we may 

 add that the ternarj' compounds of silica are form- 

 ed with more readiness than the binary, and thus 

 metallic oxides, alumina, and probably other earths 

 concur in liastening the ibrmation of the solid hy- 

 drated silicates. 



Puzzolana. 



Hydraulic cement may be made of Puzzolana, 

 by mixing nine measures of a pure quicklime, or 

 an equivalent quantity of that which is less pure, 

 with water, and twelve measures of Puzzolana, 

 and six measures of good sharp sand. If the 

 place where it is to be employed in the erection of 

 masonry is so situated that the stones may be laid 

 above the level of water, no other preparation is 

 necessary than in the use of common mortar. If 

 it is to be used below the usual level of a mass of 

 water, a coffer dam may be constructed. This is 

 a case formed of piles and pileplank driven into 

 the bed of the mass of water, in such manner as 

 to resist its passage. After the coffer dam is fin- 

 ished, the water inclosed within it is raised by a 

 pump, or other appropriate hydraulic engine, and 

 the work is perlbrmed, after the water is thus dis- 

 charged, precisely as if it were common masonry. 

 When the bed is level, or can be rendered so, a 

 caisson may be employed. This is a water-tight 

 vessel whose bottom is ffat, and whose sides are 

 vertical. It is made large enough to contain the 

 [iroposed structure and at the same time have 

 room for the workmen and necessary scaflfolding. 

 The caisson is buoyant, and floats at the surface 

 of the water. The building materials are intro- 

 duced into it, and covered up in regular courses 

 from its bottom. As the work advances the cais- 

 son sinks, until it rests upon the bed. Atlerthe 

 structure has been raised above the level of the 

 surface of the vv^ater, the sides of the caisson may 

 be removed, but the timber bottom remains as a 

 foundation for the masonry. Stone may also be 

 laid in hydraulic cement beneath the surface of a 

 mass of water by means of the diving bell. 



A more rapid mode of construction was em- 

 ployed by the ancients, and is still used in the 

 ports of the Mediterranean. Instead of a tight 

 coffer dam, a mere enclosure of woodwork is 

 formed in the water, inclosing a space of the fig- 

 ure of the contemplated structure. To 27 mea- 

 sures of hydraulic cement, made as has just been 

 described, 16 measures of pebbles or chip stone 

 are added, and the whole incorporated by stirring 

 it for an hout, either with a hoe in the hands of a 

 workman, or by the aid of machinery. The mix- 

 ture is then formed into a conoidal heap, and al- 

 lowed to remain at rest until a hard crust is formed 



