HYDRAULIC CEMENT 825 



of Pouilly is prepared from a Jurassic (secondary) limestone, which contains 39 

 per cent, of silica, with alumina, magnesia, and iron oxide. Vicat forms a factitious 

 Roman cement by making bricks with a pasty mixture of 4 parts of chalk and 1 

 part of dry clay, drying, burning, and grinding them. River-sand must be added 

 to this powder ; and even with this addition, its efficacy is somewhat doubtful ; though 

 it has, for want of a better substitute, been much employed at Paris. 



Prof. Kuhlmann, of Lisle, has made certain improvements in the manufacture of 

 lime-cement, and he has prepared artificial stone possessed of a hardness and solidity 

 fit for the sculptor. See STONE, ARTIFICIAL. 



In operating by tho dry method, instead of calcining the limestone with sand and 

 clay alone, as has been hitherto commonly practised, this inventor introduces a small 

 quantity of soda, or preferably, potash, in the state of sulphate, carbonate, or muriate 

 salts susceptible of forming silicates when the earthy mixture is calcined. Tho 

 alkaline salt, equal in weight to about one-fifth that of the lime, is introduced in solu- 

 tion among the earths. 



All sorts of lime are made hydraulic in the humid way, by mixing slaked lime with 

 solutions of common alum or sulphate of alumina ; but the best method consists in 

 employing a solution of the silicate of potash, called liquor of flints, or soluble glass, 

 to mix in with the lime, or lime and clay. An hydraulic cement may also be made 

 which will serve for the manufacture of architectural ornaments, by making a paste of 

 pulverised chalk, with a solution of the silicate of potash ; the said liquor of flints 

 likewise gives chalk and plaster a stony hardness, by merely soaking them in it 

 after they are cut or moulded to a proper shape. On exposure to the air they get 

 progressively indurated. Superficial hardness may be readily procured by washing 

 over the surface of chalk, &c., with liquor of flints, by means of a brush. This 

 method affords an easy and elegant method of giving a stony crust to plastered walls 

 and ceilings of apartments ; as also to statues and busts, cast in gypsum, mixed with 

 chalk. 



The essential constituents of every good hydraulic mortar are caustic lime and 

 silica ; and the hardening of this compound under water consists mainly in a chemical 

 combination of these two constituents through the agency of the water, producing a 

 hydrated silicate of lime. But such mortars may contain other bases besides lime, as, 

 for example, alumina and magnesia, whence double silicates of great solidity are formed ; 

 on which account dolomite is a good ingredient of these mortars. But the silica must 

 be in a peculiar state for these purposes ; namely, capable of affording a gelatinous 

 paste with acids ; and if not so already, it must be brought into this condition, by cal- 

 cining it, along with an alkali or an alkaline earth, at a bright red heat, when it will 

 dissolve, and gelatinise in acids. Qnartzose sand, however fine its powder may be, 

 will form no water-mortar or lime ; but if the powder be ignited with the lime, it then 

 becomes fit for hydraulic work. Ground felspar or clay forms with slaked lime no 

 water-cement ; but when they are previously calcined along with the lime, the mix- 

 ture becomes capable of hardening under water. 



Hamelin's Cement is composed of ground Portland stone (roe-stone), sand and litharge, 

 in the proportion of 62 of the first, 35 of the second, and 3 of the third, in 100 parts ; 

 but other proportions will also answer the purpose. Considerable dexterity is required 

 to make good work with it. 



Limestone, which contains as much as 10 per cent, of clay, comports itself after cal- 

 cination, if all the carbonic acid be expelled, just as pure limestone would do. When 

 it is less strongly burned, it affords, however, a mass which hardens pretty speedily in 

 water. If the argillaceous proportion of a marl amounts to 18 or 20 per cent., it still 

 will slake with water, but it will absorb less of it, and forms a tolerably good hydraulic 

 mortar, especially if a little good Roman Cement be added to it. When the proportion 

 of clay is 25 to 30 per cent, after burning, it heats but little with water, nor does it 

 slake well, and must therefore be ground by stampers or an edge millstone, when it is 

 to be used as a mortar. This kind of marl yields commonly the best water-cement 

 without other addition. Should the quantity of clay be increased farther, as up to 40 

 per cent., tho compound will not bear a high or long-continued heat without being 

 spoiled for making hydraulic mortar after grinding to powder. When more strongly 

 calcined, it forms a vitriform substance, and should, after being pulverised, be mixed 

 up with good lime, to make a water-mortar. If the marls in any locality differ much 

 in their relative proportions of lime and alumina, then the several kinds should be 

 mixed in such due proportions as to produce the most speedily-setting, and most 

 highly-indurating hydraulic cement. 



Hydraulic limes or cements require the presence of carbonate of lime with silica, 

 or silicate of alumina, or magnesia. It is stated that dolomite calcined at a moder.itf 

 heat exhibits the property of a hydraulic lime. See DOLOMITE. 



