RECENT RESEARCHES IN THE TESTING OF CEMENT. 265 



little also — but when hydrated and kept in air, the decomposition 

 and formation of carbonate of lime is very great. 



How to distinguish between good and indifferent cement has 

 received a good deal of criticism, and there are a few facts that 

 have been gained by experience which to a certain extent upset 

 some of the ideas that have been formed without sufficient data 

 to work upon. 



There is a very general impression that the stronger a cement 

 is, during the first month, the better suited it is for general work ; 

 this is not so, a true Portland cement cannot be above a certain 

 strength, and it is only by adding an excess of lime that the tensile 

 and crushing strength can be raised, and then from time to time 

 the cement is strained by the action of the lime. 



In every case where there is a partial break-down as shown on 

 diagrams 6 and 7, there is always a corresponding indication of 

 free or feebly combined lime when tested. The development of 

 heat while setting indicates free lime which slakes quickly. 



Slower slaking lime is indicated by the cement cracking when 

 allowed to set in cold water, and very slow slaking lime will show 

 no indications at first except in hot water. The disintegrating 

 effect of the hot bath is often due to unslaked silicate of lime, but 

 when the silicate of lime is air slaked an abnormally high tensile 

 strain is obtained in hot water ; this silicate of lime when slaked 

 can do no harm, but on the contrary will make an excellent 

 hydraulic lime, hence the necessity of thoroughly air slaking 

 cement likely to contain simple silicate of lime before use. When 

 the cement contains free lime already slaked, the only indication 

 at first is a large amount of carbon di-oxide that can be absorbed by 

 ■a given quantity of cement; the limit for good cement is put down 

 by the German Association of Cement Manufacturers at two 

 milligrams in three grams. 



The action of hot water on the briquettes might appear to be 

 sometimes very contradictory, unless all the other indications are 

 taken into consideration ; the rule is that a normal cement will 

 show approximately the same strength after seven days in hot 



