FRESH-WATER MARLS. 341 



low as 20 per cent of water. Such a marl would then weigh about 

 2600 Ibs. per cubic yard, and a cubic yard would contain about a ton 

 of pure marl. 



In dealing with the wet marls a cement-plant may produce from 

 one and a half to three barrels of cement from each cubic yard of marl, 

 while a marsh marl might yield four barrels cement per cubic yard. 

 In estimating the life of a lake marl deposit it will be safest to assume 

 that each cubic yard of marl in place will produce only two barrels of 

 cement. 



Chemical composition of marl. Marl itself, being a chemical deposit, 

 is almost a pure carbonate of lime. During and after its deposition, how- 

 ever, foreign matter of various kinds is apt to get mixed in with the 

 marl, the principal impurities thus introduced being fine sand, clayey 

 matter, and organic material. Of these the most important, from the 

 cement manufacturer's point of view, is the organic matter. 



Sand is rarely present in sufficient amount to render the marl unser- 

 viceable, and of the 2 or 3 per cent of sand shown by most marls some 

 is fine enough to pass a 150-mesh sieve and will therefore enter into 

 combination in the kiln. The clay present in marls is principally objec- 

 tionable because of its tendency to increase the percentage of magnesia 

 and sulphur trioxide. 



Organic matter burns out in the kiln and might therefore be regarded 

 as a harmless impurity. But a high percentage of it in a marl is in reality 

 very objectionable, both negatively, because it lowers the percentage 

 of lime carbonate in the marl, and positively, because it retains moisture 

 with great avidity. It is almost impossible to dry a marl containing 

 much organic matter, and in any semi-dry or dry process this would 

 be a very serious disadvantage. Organic matter in its coarser forms 

 i.e., roots, branches, twigs, etc., interferes greatly with the grinding 

 of the marl, though the larger fragments are usually taken out by a 

 separator early in the reducing process. 



In the following table (157) are given the analyses of marls used at 

 different American cement-plants, some quoted from published sources 

 and others supplied by the chemists of the plants. A few of the quoted 

 analyses are taken from prospectuses, but in general the analyses are 

 of more satisfactory character. In all cases they are calculated dry, 

 all water below 212 being neglected. 



The analyses given in this table are mostly not picked analyses, 

 such as are usually quoted in prospectuses, in which the marl rarely 

 carries less than 98 per cent of lime carbonate. On the other hand, 

 some of them are still considerably better than can be expected 



