312 CEMENTS, LIMES, AND PLASTERS. 



related salts of lime. In this manner a natural cement will be pro- 

 duced. An artificial mixture of certain and uniform composition 

 burned at a higher temperature will give a Portland cement the details 

 of whose manufacture are discussed in the present section of this 



book. 



Pure Hard Limestones. 



Under this heading are grouped limestones of normal hardness (ex- 

 cluding the soft chalky limestones and the marls) which carry no less 

 than 80 per cent of lime carbonate and less than 6 per cent of magne- 

 sium carbonate. Limestones carrying less than 80 per cent of lime 

 carbonate are described in the next chapter under the heading of Cement 

 Rock. The boundary between the two classes is of course an arbitrary 

 limit, and 80 per cent of CaC0 3 has been selected for convenience. As 

 a matter of fact, most of the limestones used in cement-plants are much 

 purer than the lower limit above fixed, ranging usually from 90 to 95 

 per cent of lime carbonate. 



Soon after the American Portland-cement industry had become 

 fairly well established in the Lehigh district, attempts were made in 

 New York State to manufacture Portland cement from a mixture of 

 pure limestone and clay. These attempts were not commercially suc- 

 cessful, and although their lack of success was not due to any defects 

 in the limestone used, a certain prejudice arose against the use of the 

 hard limestones. In recent years, however, this has disappeared, and 

 a very large proportion of the American output is now made from mix- 

 tures of limestone with clay or shale. (See page 304 for comparative 

 figures.) This reestablishment in favor of the hard limestone is doubtless 

 due in great part to recent improvements in grinding machinery, for 

 the purer limestones are usually much harder than argillaceous lime- 

 stones like the Lehigh district " cement rock". 



Composition of hard limestones actually used. In Table 146 analyses 

 of a large number of limestones used at American cement-plants are 

 given. On examination it will be seen that most of these limestones 

 range from 49 to 54 per cent of lime (CaO) and thus represent quite 

 pure rocks, since a theoretically pure limestone composed entirely of 

 lime carbonate (CaCOs) will contain only 56 per cent of lime (CaO), the 

 remaining 44 per cent being carbon dioxide (C02)- With few excep- 

 tions the limestones analyzed carry less than 1 per cent of magnesia 

 (MgO). Their sulphur percentages are also low, which appears to be 

 more commonly the case in dealing with a hard limestone than when 

 a soft limestone or marl is in question. The same may be said in regard 

 to alkalies. 



