394 



CEMENTS, LIMES, AXD PLASTERS. 



its arrival at the mill. Four determinations are made on each sample 

 of marl and three on the clay. These are: 

 Marl. 1. Percentage of water; 



2. Weight per cubic foot; 



3. Percentage of insoluble matter; 



4. Percentage of carbonates. 

 Clay. 1. Percentage of water; 



2. Percentage of insoluble matter; 



3. Percentage of carbonates. 



From these determinations the mix is proportioned in such a way 

 that the ratio 



Carbonates 

 Insoluble matter 



shall fall within certain numerical limits. At the plant in question, 

 which runs a high-testing cement which is also very high in silica, the 

 above formula is made to give a value of 4.2. In the majority of plants 

 it would fall about 3.0 to 3.4. 



Composition of mixture. The cement mixture ready for burning 

 will commonly contain from 74 to 77.5 per cent of lime carbonate, or 

 an equivalent proportion of lime oxide. Several analyses of actual 

 cement mixtures are given in the following table. The ratio of silica 

 to alumina plus iron for ordinary purposes should be about 3:1, for the 

 cement becomes quicker setting and lower in ultimate strength as the 

 percentage of alumina increases. If the alumina percentage be carried 

 too high, moreover, the mixture will give a fusible, sticky clinker when 

 burned, causing trouble in the kilns. 



TABLE 171. 

 COMPOSITION OF ACTUAL MIXES. 



