PORTLAND CEMENT: PRELIMINARY STATEMENTS. 305 



or sand must be absent, or present only in small quantities say 1 per 

 cent or less. If the limestone is a clayey limestone, or "cement rock", 

 the proportion between its silica and its alumina and iron should pref- 

 erably fall within the limits 



Al 2 O 3 + Fe 2 O 3 " 



A clay or shale should satisfy the above equation, and should be free 

 from sand, gravel, etc. Alkalies, sulphides, and sulphates should, if 

 present, not exceed 3 per cent or so. 



2. Physical character. Economy in excavation and crushing re- 

 quires that the raw materials should be as soft and as dry as possible. 



3. Amount available. A Portland-cement plant running on dry raw 

 materials, such as a mixture of limestone and shale, will use approxi- 

 mately 20,000 tons of raw material per year per kiln. Of this about 

 15,000 tons are limestone and 5000 tons shale. Assuming that the 

 limestone weighs 160 Ibs. per cubic foot, which is a fair average weight, 

 each kiln in the plant will require about 190,000 cubic feet of limestone 

 per year. As the shale or clay may be assumed to contain considerable 

 water, a cubic foot will probably contain not over 125 Ibs. of dry mate- 

 rial, so that each kiln will also require about 80,000 cubic feet of shale 

 or clay. 



A cement-plant is an expensive undertaking, and it would be folly 

 to locate a plant with less than a twenty years' supply of raw material 

 in sight. This would require that, to justify the erection of a cement- 

 plant on any property, 



For each, kiln of the proposed plant, there must be in sight at least 

 8,800,000 cubic feet of limestone and 1,600,000 cubic feet of clay or shale. 



4. Location with respect to transportation routes. Portland cement 

 is, for its value, a bulky product, and is therefore much influenced by 

 the subject of transportation routes. To locate a plant on only one 

 railroad, unless the railroad officials are financially connected with the 

 cement-plant, is simply to invite disaster. At least two transportation 

 routes should be available, and it is best of all if one of these be a good 

 water route. 



5. Location with respect to fuel supplies. Every barrel (380 Ibs.) 

 of Portland cement marketed implies that at least 200 to 300 Ibs. of 

 coal have been used in the power-plant and the kilns. In other words, 

 each kiln in the plant will, with its corresponding crushing machinery, 

 use up from 6000 to 9000 tons of coal per year. The item of fuel cost 

 is therefore highly important, for in the average plant about 30 to 40 



