49 



sary. The average charge was about 25 per cent, clay and 

 about 75 per cent. marl. 



The mixing was carried on in a mixing pan 12 feet in diame- 

 ter, in which two large rolls, each about 5 feet in diameter, and 

 1 6-inch face, ground and mixed the materials thoroughly. The 

 mixture was then sampled and analyzed, after which it was 

 carried by a belt conveyor to two pug mills, where the mixing 

 was completed and the slurry formed into slabs about 3 feet 

 long and 4 to 5 inches in width and height. These on issuing 

 from the pug mill were cut into a number of sections, so as to 

 give bricks about 6 inches by 4 inches by 4 inches in size. The 

 bricks were then placed on slats, which were loaded on rack 

 cars and run into the drying tunnels. The tunnels were heated 

 by waste gases from the kilns and required from twenty- four 

 to thirty-six hours to dry the bricks. 



After drying the bricks were fed into dome kilns, twenty of 

 which were in use, being charged with alternate layers of coke 

 and slurry bricks. The coke charge for a kiln was about four 

 or five tons, and this produced 20 to 26 tons of clinker at each 

 burning, thus giving a fuel consumption of about 20 per cent, 

 as compared with the 40 per cent, or so required in the rotary 

 kilns using wet materials. From thirty-six to forty hours were 

 required for burning the charge. After coaling, the clinker was 

 shoveled out, picked over by hand, and reduced in a Blake 

 crusher, Smidth ball mills, and Davidsen tube mills. 



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. Sev- 

 eral analyses of actual cement mixtures are given in the follow- 

 ing table. Analysis No. i, with its relatively high percent- 

 age of magnesia, is fairly typical of Lehigh Valley practice. 

 Analyses Nos. 2 and 3 show mixtures low in lime, while analy- 

 sis No. 4 is probably the best proportioned of the four, especial- 

 lv in regard to the ratio between silica and alumina plus iron. 

 This ratio, for ordinary purposes, should be about 3.-, as the 

 cement becomes quicker setting and lower in ultimate strength 

 as the percentage of alumina increases. If the alumina percent- 

 age be carried too high, moreover, the mixture will give a fusi- 

 ble, sticky clinker when burned, causing trouble in the kilns. 



