COMPOSITION AND PROPERTIES OF NATURAL CEMENTS. 263 



Weight and specific gravity. The specific gravity of American 

 natural cements appears to be greatly underestimated by most engi- 

 neering authorities. In a recent report,* for example, it is stated that 

 "natural cement has a specific gravity of 2.5 to 2.8". In reality very 

 few of our American cements ever fall as low in specific gravity as 2.8, 

 and it would be nearer the truth to say that, as a class, the natural 

 cements range between 2.8 and 3.2. 



In the following table (126) a number of careful determinations 

 are given, selected from various sources so as to cover as many cement 

 districts and brands as possible. 



TABLE 126. 

 SPECIFIC GRAVITY OF AMERICAN NATURAL CEMENTS. 



Rapidity of set. Natural cements are normally much quicker- 

 setting than Portlands, but this rapidity of set may be changed by 

 aeration, the use of plaster, etc., to a very remarkable degree. 



In 1894-95 Sabin tested the effect of aeration on the setting time 

 of natural cement, with the following results : t 



* Professional Paper No. 28, Corps of Engineers, U. S. A., p. 11. 

 f Report Chief of Engineers, U. S. A., 1895, p. 2937. 



