656 CEMENTS, LIMES, AND PLASTERS. 



ing his experiments for the Maryland Steel Company, states that the 

 best results were secured by the use of 25 parts of lime to 100 parts of 

 slag, by weight. At another American plant the proportions used are 

 20 Ibs. lime to 100 Ibs. slag. In the manufacture of slag brick, which 

 is in reality merely a branch of the slag-cement industry, the amount 

 of lime added may fall as low as 10 Ibs. to 100 Ibs. of slag. 



These rules are, of course, purely empirical; and it is time that some 

 better method of calculating the mixture should be presented, Thi:. 

 of course, can be accomplished by the use of the same device which 

 has been previously discussed in connection with hydraulic limes, 

 natural cements, and Portland cements. 



Calculating the mixture. If we determine the Cementation Index * 

 of a series of representative American slag cements, such as is given 

 on page 667, we will find that the value obtained ranges from about 

 1.6 to 1.9 



Accepting these values as fairly typical the information thus gained 

 can be employed in devising a method for determining accurately the 

 proportions in which any given slag should be mixed with any given 

 lime in order to secure a good slag cement. 



Operation 1. Slag. Multiply the percentage of silica in the slag by 

 2.8, the alumina by 1.1, and the iron oxide by 0.7; add all the products 

 together. From the sum subtract the percentage of lime in the slag 

 plus 1.4 times the magnesia. Call the result "ra". 



Operation 2. Limestone. -Multiply the percentage of silica in the 

 unslaked quicklime by 2.8, the alumina by 1.1, and the iron oxide by 

 0.7, and add the products together. Subtract this sum from the total 

 percentage of lime (CaO) plus 1.4 times the magnesia. Call the result 

 "". 



Operation 3. Divide 100 Xm by 1.7Xn. The quotient, m , will 



equal the number of parts of quicklime to be used for each 100 parts 

 of slag. The factor by which n is to be multiplied is here taken as 

 1.7, a very satisfactory value. Values as low as 1.6 and as high as 

 1.9 would, however, give the proportions used in practice at various 

 plants. 



* As previously explained in detail (pp. 170-171), the Cementation Index is 

 the value obtained from the formula 



(2.8 X percentage silica) + (1. 1 X percentage alumina) 



Cementation Index = __ + (0.7 X percentage iron oxide) ^_^ 



(Percentage lime) + (1.4 X percentage magnesia) 



