826 HYDRAULIC CEMENT 



If carbonate of lime bo mixed with gelatinous silica, it forms a good hydraulic 

 cement. See SILICA. ; and STONE, ARTIFICIAL. 



If a hydraulic lime be calcined at too high a temperature, the silicate undergoes 

 partial fusion, and -will not set afterwards under water. The heat, therefore, employed 

 for burning the hydraulic limestones should only be just high enough to ezpel'the 

 water from the clay and the greater part of the carbonic acid from the carbonate of 

 lime. 



Neither clay (silicate of alumina) nor lime alone will set under water, but if we 

 carefully mix chalk and clay together, and then calcine them at a moderate heat, a 

 good hydraulic cement is obtained. 



Hydraulic limestones may be separated into two kinds, as indicated by their con- 

 stituents. One set consists of the carbonates of the earths, these yield a fat lime ; 

 the other set is composed cf silicates, or a mixture of silicates of alumina, lime, mag- 

 nesia, or potash, although of the latter less frequently, with usually much free silica ; 

 consequently these are a kind of clay. 



In burning hydraulic limestones, the silica and alumina, when the lime has once 

 parted with its carbonic acid, react upon each other. The lime enters into feeble 

 combination with the silica, producing a less stable compound than the original, and 

 which is readily acted on by acids, which was not the case previous to burning. 



Burnt hydraulic lime is, in nearly all cases, soluble in acids. The presence of a 

 silicate that can be decomposed by acids is proved by the formation of a thick jelly 

 of silica. The property of yielding gelatinous silica by solution in acids, may be 

 regarded as one of the proofs that the limestone possesses the property of ' setting,' 

 becoming hard under water. 



The degree of hardness acquired by hydraulic lime varies considerably. The time 

 required for hardening differs still more widely ; some hydraulic cements will set in 

 a few minutes, others will require weeks and months. The more of the aluminous 

 constituents which the limestone contains the more quickly will the cement prepared 

 from it solidify. 



As already remarked, too much heat in the kiln, causing partial fusion of the silica, 

 destroys the hydraulic property of the lime. In technical phrase it renders the lime 

 much weaker, and at last completely destroys its power as a water-cement. 



Hydraulic lime should acquire a certain consistence before it is immersed in water. 

 If this is not attended to, the hydraulic lime never hardens, the particles form a 

 porous mass, there is no kind of agglutination ; consequently, considerable care is 

 required in every stage of the process of making and using hydraulic cements. 



It is impossible in this work to do full justice to this very important subject ; wo 

 must therefore refer our readers to General Pasley's works on Limes and Cements, to 

 the papers of Mr. Timperly in the Transactions of the Institution of Civil Engineers, 

 and to the works especially of MM. Vicat and Belidor. See PUZZOLANA ; ROMAN CEMENT. 



Scotfs Cement. The following remarks on hydraulic cements, and on the prepara- 

 tion of a new and valuable kind of these cements, are abstracted from some memoirs 

 by Major-General Scott in the Papers on Subjects connected with the Duties of the 

 Corps of Royal Engineers. General Scott's hydraulic cement has been used in the 

 construction of the Royal Albert Hall and many other buildings, and we are given to 

 understand, generally with satisfactory results. 



The superiority of hydraulic mortars over pure lime-mortars consists very much 

 in the former containing within themselves the property of solidifying without the 

 assistance of the atmosphere ; while the pure lime-mortars do not possess such a 

 property, and in the absence of carbonic acid in the atmosphere, no solidification will 

 ensue. 



Many impure limestones contain a proportion of clay (silicate of alumina and iron) 

 intimately mingled with the carbonate of lime. By burning, the carbonic acid gas 

 is expelled from them as in the case of the pure limestones, the clay assisting in its 

 expulsion through the affinity of its silicic acid for the lime, and also parting with its 

 own combined water. A further action then takes place : the silicic acid of the clay 

 and the lime, at the high temperature of the kiln, react on one another, and either 

 form a silicate of lime, or approximate to its formation so far that on the addition 

 of water it is formed with more or less rapidity. As, however, this action is more 

 perfect in the water-cements, and there is much in common between them and the 

 hydraulic limes, it will be better here to explain in what their difference consists, and 

 then to speak of them together. 



The difference between them cannot be precisely defined with reference to chemical 

 composition, for there are some limestones which will yield cement or lime according 

 to the temperature at which they are burned. It must rather be referred to the 

 manner in which they behave when moistened with water. If a piece of cement be 

 dipped in water for a few seconds until it ceases to hiss from the escape of air from 



