43 



GRINDING AND MIXING SLAG-LIMESTONE MIXTURES. 

 While the manufacture of Portland cement from a mixture of 

 slag and limestone is similar in general theory and practice to 

 its manufacture from a limestone-clay mixture, certain inter- 

 esting difference occur in the preparation of the mixture. In 

 the following paragraphs the general methods of preparing 

 mixtures of slag and limestone for use in Portland cement manu- 

 facture will first be noted, after which certain processes pecu- 

 liar to the use of this particular mixture will be described sep- 

 arately. 



General methods. After it had been determined that the 

 pozzuolanic cement made* by mixing slag with lime without 

 subsequent burning of the 'mixture, was not an entirely satisfac- 

 tory structural material, attention was scon, directed toward the 

 problem of making a true Portland cement from such slag. The 

 blast-furnace slags commonly available, while carrying enough 

 silica and alumina for a cement mixture, are too low in lime. 

 to be suitable for Portland cement. Additional lime must be 

 added, usually in the form of limestone ; the slag and limestone 

 must be well mixed and the mixture properly burned. The 

 general methods for accomplishing the proper mixture of the 

 materials vary in details. It seems probable that the first meth- 

 od used in attempting to make a true Portland cement from 

 slag, was to dump the proper proportion of limestone, broken 

 into small lumps, into molten slag. The idea was that both 

 mixing and calcination could thus be accomplished in one stage ; 

 but in practice it was found that the resulting cement was vari- 

 able in composition and always low in grade. This method has 

 accordingly fallen into disuse, and at present three different 

 general processes of preparing the mixture are practiced at dif- 

 ferent European and American plants. 



1. The slag is granulated, dried, and ground, while the 

 limestone is dried and ground separately. The two materials 

 are then mixed in proper proportions, the mixture is finely pul- 

 verized in tube mills, and the product is fed in a powdered state 

 to rotary kilns. 



2. The slag is granulated, dried, and mixed with slightly 

 less than the calculated proper amount of limestone, which has 

 been previously dried and powdeired. To this mixture is added 



*See Municipal Engineering, vol. 24, p. 335, May, 1903. 



