47 



GRINDING AND MIXING WET METHODS. Wet methods of 

 preparing Portland cement mixtures date back to the time when 

 millstones and similar crude grinding contrivances were in use. 

 With such imperfect machinery it was almost impossible to 

 grind dry materials fine enough to give a good Portland cement 

 mixture. The advent of good grinding machinery has practi- 

 cally driven out wet methods of manufacture in this country, 

 except in dealing with materials such as marls, which naturally 

 carry a large percentage of water. One or two plants in the 

 United States do, it is true, deliberately add water to a lime- 

 stone-clay mixture; but the effect of this practice on the cost 

 sheets of these remarkable plants is not encouraging. 



In preparing cement mixtures from marl and clay, a few 

 plants dry both materials beifore mixing. It seems probable 

 that this practice will spread, for the wet method of mixture is 

 inherently expensive. At present, however, almost all marl 

 plants use wet methods of mixing, and it is therefore necessary 

 to give some space to a discussion of such methods. 



Certain points regarding the location, physical condition, and 

 chemical composition of the marls and clays used in such mix- 

 tures have important effects upon the cost of the wet process. 

 As regards location!, considered on a large scale, it must be 

 borne in mind that marl deposits of workable size occur only 

 in the Northern States and in Canada. In consequence the cli- 

 mate is unfavorable to continuous working throughout the 

 year, for the marl is usually covered with water, and in winter 

 it is difficult to secure the material. In a minor sense location 

 is still an important factor, for marl deposits necessarily and in- 

 variably are found in depressions ; and the mill must, therefore, 

 just as necessarily, be located at a higher level than its source 

 of raw material, which involves increased expense in transport- 

 ing the raw material to the mill. 



Glacial clays, which are usually employed in connection with 

 marl, commonly carry a much larger proportion of sand and 

 pebbles than do the sedimentary clays of more southern regions. 



The effect of the water carried by the marl has been noted on 

 an earlier page. The material as excavated will consist approxi- 

 mately of equal weights of lime carbonate and of water. This 

 on the face of it would seem to be bad enough as a business pro- 

 position ; but we find that in practice more water -is often added 

 to permit the marl to be pumped up to the mill. 



