190 



CEMENTS, LIMES, AND PLASTERS. 



The results have been reduced to give the values for compressive strength 

 in pounds per square inch. 



TABLE 85. 

 COMPRESSIVE STRENGTH OF HYDRAULIC-LIME MORTARS. (GRANT.) 



The values given are the average of ten specimens tested. 



Selenitic Lime: Scott's Cement. 



The cementing material known as Scott's cement, selenitic cement, 

 or selenitic lime consists essentially of lime (CaO) plus a small per- 

 centage of sulphur trioxide (S0 3 ). The lime used as a basis for this 

 cement is always a more or less hydraulic variety, while the sulphur 

 trioxide may be added to it in the form of either plaster of Paris or 

 sulphuric acid. The resulting selenitic lime or Scott's cement shows a 

 markedly higher strength, both in compression and tension, than the 

 lime from which it was made. 



Manufacture of selenitic limes. In his earlier patents Scott pro- 

 vided for the manufacture of this product by exposing lime to the 

 fumes of burning sulphur. This was accomplished * "by reheating 

 calcined lump lime in an oven having a perforated floor, beneath which 

 were placed pots of burning sulphur. The sulphurous-acid fumes from 

 the sulphur rose among the red-hot lumps of lime, leading to the 

 formation of calcium sulphite (CaSOs), and this in turn became oxidized 

 into calcium sulphate (CaSO*). The amount of sulphurous acid thus 

 absorbed by the whole bulk of the lime was small, rarely exceeding from 

 2 to 3 per cent, and of course only the exterior surfaces of the lumps 

 became coated with the sulphur compound; but when the cement was 

 ground, to prepare it for use, the sulphate of lime became evenly dis- 

 tributed throughout the mass. 



In course of time General Scott found that he could obtain the same 

 results, either by adding sulphuric acid to the water used in preparing 



* Redgrave, G. R. Calcareous cements, p. 176. 1895. 



