FEEBLY HYDRAULIC LIMES: SELENITIC LIMES. 



191 



the mortar or by the addition of powdered gypsum or plaster of Paris 

 to the ground lime. It mattered little in what form the sulphuric acid 

 was conveyed to the lime, and many soluble sulphates were found to 

 answer quite as well as the sulphate of lime. Ultimately Scott specified 

 the manufacture of a cement, which he named 'selenitic cement', by 

 the addition of 5 per cent of ground plaster of Paris to calcined hydraulic 

 lime, which was then ground to an impalpable powder and placed in 

 sacks or casks for use". 



The hydraulic lime used in the manufacture of selenitic lime is appar- 

 ently always one of the feebly hydraulic varieties such as are discussed 

 earlier in the present chapter. 



Tensile strength of selenitic limes. The following table shows the 

 results of tests * by Grant about 1880 on various selenitic limes. For 

 purposes of comparison tests are also given on two of the limes before 

 the addition of sulphate. The tests were made on briquettes having a 

 sectional area of 2J square inches; but in the table below the results given 

 are reduced to pounds per square inch. 



TABLE 86. 

 TENSILE STRENGTH OF SELENITIC LIMES. (GRANT.) 



Each of the above results represents the average of the tests of five 

 specimens. The tests were made one year after the briquettes were 

 molded. The words "wet" and "dry" refer to the fact that some 

 of the briquettes were kept in air and others in water during the entire 

 year. These results as to tensile strength are shown diagrammatically 

 in Fig. 31. 



* Proc. Institution Civil Engineers, vol. 62, p. 165. 1880. 



