EECENT EESEAECHES IN THE TESTING OF CEMENT. 263 



The first effect due to the slow hydration of unslaked lime may 

 be facilitated by hot water, which rapidly disintegrates any cement 

 containing unslaked lime, either free or combined with silica. 

 The percentage of unslaked lime required to injure neat cement 

 is very small. 



In diagram 4, is represented the tensile strength obtained after 

 seven days in hot water at 180° Fahr. with various percentages 

 of slow slaking lime added. A represents the strength after 

 twenty-eight days in cold water which should with a normal 

 cement be equal to seven days in hot water. This diagram must 

 not be taken as showing absolutely correctly the proportionate 

 strength due to the addition of certain percentages of free lime, 

 because a certain proportion of silica, not ascertained, was com- 

 bined with the lime, and the samples of cement used were not 

 absolutely free from unslaked lime — it is therefore only approxi- 

 mate. 



When test briquettes are made in the usual proportion of three 

 of sand to one of cement, a small percentage of free lime has a 

 certain space between the grains of sand not filled with cement, 

 into which it can expand — with a richer proportion of cement 

 this is not so. Diagram 5 represents the progressive increase in 

 strength of briquettes made in the proportion of three to one and 

 two to one — containing each a small amount of free lime — with a 

 proportion of three to one the strength, when kept in cold water, 

 increases steadily up to twelve months, but with the proportion 

 of two to one, kept under the same conditions, there is an internal 

 strain exerted, in this instance between six and twelve months, 

 which reduces the strength so much that the proportion of three 

 to one becomes almost as strong as two to one, after twelve 

 months in water. 



When a larger proportion of free lime is present, mortar 

 briquettes made in the usual way three to one are often effected 

 in cold water. Diagrams 6 and 7 represent samples which each 

 show a partial break-down at different periods — this breaking 

 down appears to be independent of the hardening effect which 



