224 CEMENTS, LIMES, AND PLASTERS. 



It is probable that some natural cements are burned at tempera- 

 tures not exceeding 900 C., in which case their strength depends largely 

 on the formation of aluminates and ferrites. When, as in most cases, 

 the temperature is carried to 1100-1 300 C., the silica is attacked and 

 lime and magnesia silicates are formed. 



Relation of composition to degree of burning. It may be set down 

 as a general principle that: 



The lower the Cementation Index the higher the temperature that must 

 be reached to secure thorough combination. 



A cement with an index of 2.00, for example, can be burned at a 

 temperature little, if any, above that of an ordinary lime-kiln (900 C.), 

 while a cement whose index falls below 1.10 will require a temperature 

 almost equal to that (2300 F.) attained in a Portland-cement kiln. 



There are therefore distinct economic advantages, so far as fuel con- 

 sumption is concerned, in making a cement with a high Cementation 

 Index. It must, however, be recollected that if the burning is properly 

 done, a cement of low index will be stronger than one of high index. 



By far the most satisfactory proposition to handle, from the manu- 

 facturer's point of view, is a product whose index falls between 1.20 

 and 1.60. Such a rock, if properly burned, will give a cement strong 

 enough to compare favorably with the best American or foreign naturals, 

 while, on the other hand, there is no particular danger of making an 

 unsound product. With an index between these limits the burning 

 temperature may vary considerably, one way or the other, without any 

 danger of leaving too much free lime or magnesia in the clinker. With a 

 cement whose index falls below 1.10 this is not true, for the margin of 

 safety is so narrow that the temperature must be kept up to its highest 

 point under penalty of producing unsound cement. 



Losses in burning, etc. If all the rock fed to the kiln were perfectly 

 burned, the loss in burning would correspond directly to the percentage 

 of carbon dioxide (C02) plus water present in the raw rock. On this 

 basis one ton (2000 Ibs.) of rock would produce the number of barrels 

 (280 Ibs.) of cement given in Table 102. 



In actual practice, however, a very large additional percentage must 

 be deducted for losses by everburning or underburning. Bad weather 

 and bad management may carry this loss from clinkering or under- 

 burning to a point where one third of the product of the kiln is spoiled. 

 Improved kilns may reduce the loss from these causes to about 10 

 per cent, and anything between these limits (10 per cent and 33 J per 

 cent) may be expected at a natural-cement plant. Under average con- 

 ditions I should say that 25 per cent would be a safe allowance. 



