DEFINITION AND RELATIONS OF NATURAL CEMENTS. 199 



setting and high in tensile strength, including the " natural 

 Portlands " and allied products. If not burned high enough, 

 however, cements of such low index will necessarily contain 

 large amounts of free lime and magnesia. 



B. Cements with an index between 1.15 and 1.60. These include 



most American natural cements. As the index is higher than 

 in Class A, it is not necessary to burn these products at so 

 high a temperature. Practically all of the European " Roman" 

 cements will also fall in this subgroup. 



C. Cements with an index exceeding 1.60. These include the rela- 



tively low-limed natural cements, which carry so much clayey 

 material that only a light burning is required in order to com- 

 bine all their lime and magnesia. As the index rises above 

 2.00, the products become feebler in hydraulic properties, 

 until at about 3.00 they can be considered only as artificial 

 pozzuolanas. 



