LIMESTONES. 313 



In prospectuses and in the reports of "cement experts", analyses 

 of limestones averaging 98 or 99 per cent of lime carbonate are quite 

 common, but in real life a quarry that will steadily turn out limestone 

 94 per cent pure is about as good as can be hoped for. With a lime- 

 stone of this degree of purity little attention need be paid to the char- 

 acter of the remaining 6 per cent of impurities. But when a limestone 

 carrying 90 per cent or less of lime carbonate (equivalent to about 50 

 per cent of lime) is in use or under consideration, the character of the 

 impurities becomes of the first importance. 



Of course objectionable percentages of sulphur compounds or mag- 

 nesia would be enough to debar a limestone from use, but even when the 

 impurity consists of clayey matter (silica, alumina, and iron oxide) its 

 exact composition is a matter of importance and should be carefully 

 studied. Tha matter of interest is the ratio given for the formula 



_ Percentage silica (SiQ 2 ) __ 

 Percentage alumina (A1 2 O 3 ) + percentage iron oxide (Fe 2 O 3 )' 



It is to be noted that the importance of this question increases as 

 the limestone becomes less pure. The reason for this is obvious. Sup- 

 pose we are dealing with two limestones of respective composition: 



A. B. 



Lime carbonate ....................... 95.00 80.00 



Silica ................................ 4.00 16.00 



Alumina ............................. 0.70 2.80 



Iron oxide ..................... . ..... .30 1 .20 



The ratio . n ^ 1 * will in each case give a value of 4.0; but 



the result to the cement manufacturer will be very different. If he 

 uses limestone A, its silica-alumina ratio is of little importance, for 

 as the limestone is very pure (95 per cent CaC0 3 ) it will require the addi- 

 tion of considerable clay. The silica-alumina ratio of the mix will there- 

 fore be determined by that of the clay, not by the ratio shown by the 

 limestone; and the manufacturer can select a clay which will give what- 

 ever he considers a desirable ratio for the mix. 



But if he should use limestone B, it would require but little clay, 

 since it is already very clayey; and it would be almost impossible to 



find a clay sufficiently aluminous to reduce the AJ Q + p Q ra ^ 



much below the 4.0 which is fixed by the limestone. 



For this reason it may be taken as a safe rule that when a limestone 



