824 



HYDRAULIC CEMENT 



hardens too soon, as in 12 hours, it is apt to crack; it is bettor when it takes 8 days 

 to' concrete. Through the agency of the water, silicates of lime, alumina (magnesia), 

 and oxide of iron are formed, which assume a stony hardness. 



Besides the above two volcanic products, other native earthy compounds are used 

 in making water-cements. To this head belong all limestones which contain from 

 20 to 30 per cent, of clay and silica. By gentle calcination, a portion of the carbonic 

 acid is expelled, and a little lime is combined with the clay, while a silicate of clay 

 and lime results, associated with lime in a subcarbonated state. A lime-marl con- 

 taining less clay will bear a stronger calcining heat without prejudice to its qualities 

 as a hydraulic cement : but much also depends upon the proportion of silica present, 

 and the physical structure of all the constituents. 



The mineral substance most used in England for making such mortar is vulgarly 

 called cement-stone. It is a reniform limestone, which occurs distributed in single 

 nodules, or rather lenticular cakes, in beds of clay. They are mostly found in those 

 argillaceous strata which alternate with the limestone beds of the Oolite formation, as 

 also in the clay strata above the chalk, and sometimes in the London clay. On the 

 coasts of Kent, in the isles of Sheppey and Thanet, on the coasts of Yorkshire, 

 Somersetshire, and the Isle of Wight, &c., these nodular concretions are found in con- 

 siderable quantities, having been laid bare by the action of the sea and weather. 

 They were called by the older mineralogists Septaria and Ludus Helmontii (Van 

 Helmont's coits). When sawn across, they show veins of calc-spar traversing the 

 siliceous clay, and are then sometimes placed in the cabinets of virtuosi. They are 

 found also in several places on the Continent, as at Neustadt-Eberswalde, near 

 Antwerp, near Altdorf in Bavaria ; as also at Boulogne-sur-mer, where they are 

 called Boulogne pebbles (galets}. These nodules vary in size from that of a fist to a 

 man's head ; they are of a yellow-grey or brown colour, interspersed with veins of 

 calc-spar, and sometimes contain cavities bestudded with crystals. Their specific 

 gravity is 2'59. 



The Blue Lias cement-stones are considered the strongest water- limes of this country, 

 and are found on opposite sides of the Bristol Channel, near Watchet in Somerset- 

 shire, and Aberthaw in Glamorganshire, and also in North Wales and at Lyme Regis 

 in Dorsetshire. The Dorking or Merstham lime 'and the Hailing lime, so termed 

 from a village on the left bank of the Medway above Rochester, but which is also 

 found near Burnham on the opposite side of the riverj though not possessing such 

 strong hydraulic properties as the lias, are also much esteemed. 



Analyses of several cement-stones, and of the cement made with them : 



No. 1, English cement-stone analysed by Berthier; No. 2, Boulogne stone, by 

 Drapiez ; No. 3, English ditto, by Davy ; No. 4, reniform limestone-nodules from 

 Arkona, by Hiihnefeld ; No. 5, cement-stone of Avallon, by Dumas. 



In England the stones are calcined in shaft-kilns, or sometimes in mound-kilns, 

 then ground, sifted, and packed in casks. The colour of the powder is usually 'lark 

 brown-red. When made into a thick paste with water it absorbs little of it, evolves 

 hardly any heat, and soon indurates. It is mixed with a sharp sand in various pro- 

 portions, immediately before using it; and is employed in all marine and river em- 

 bankments, for securing the seams of stone or brick floors or arches from the percolation 

 of moisture, and also for facing walls to protect them from damp. 



HYDRAULIC CEMENT. This is a kind of mortar used for building piers or 

 under, or exposed to, water, such as those of harbours, docks, &c. The cement 



