646 CEMENTS, LIMES, AND PLASTERS. 



As already noted, the bottom of the earthen trench is about 6 inches 

 above the bottom of the iron trough. This is done to allow the inser- 

 tion at this end of the trough of a 3-inch water-pipe. Slag from the 

 furnace flows through the trench and into the trough, which is set at 

 an inclination of about 1 inch in 10. Water is injected through the 

 3-inch pipe, under 10 or 15 feet head, into the trough. If enough 

 water is used, the slag will be granulated as soon as it enters the 

 trough, and will be readily carried down it into the car below, rarely 

 flowing with a greater depth than 6 inches in the trough. If insuffi- 

 cient water is used the slag puffs up and fills the trough, so that the 

 slag-mass has to be broken into with an iron rod and pushed along. 



The car into which the slag flows is provided with four 3-inch holes 

 in its sides, to allow the surplus water to escape. 



At another slag-cement plant recently visited by the writer, the 

 granulated slag is caught in cylindrical masonry tanks, 15 feet in 

 diameter and 10 feet in depth. The stream of molten slag flows from 

 the furnace to and over the edge of the tank and through a semi-circular 

 trough about 10 inches in diameter, which enters the tank at its top 

 rim and projects 6 inches over the edge. About 6 inches below the 

 bottom of this trough a pipe, carrying cold water under slight pressure, 

 enters the tank, projecting into it for 4 inches. This pipe is 3 

 inches in diameter for most of its length, but the portion projecting 

 into the tank is flattened so as to give an orifice 4 or 5 inches wide 

 and about half an inch high. The stream of slag, flowing slowly along 

 the trough and over the edge of the tank, is struck by the jet of cold 

 water from the pipe, and is granulated. The granulated slag is taken 

 from the tank by bucket elevators running continuously. 



Effects of granulating the slag The physical effect of causing hot 

 slag to come in contact with cold water is to break the slag up into 

 small porous particles. As this materially aids in pulverizing the slag, 

 it is probable that granulation would be practiced on this account 

 alone. But as a matter of fact, granulation has in addition to its purely 

 physical result two important chemical effects. One is to make the 

 slag, if it be of suitable chemical composition, energetically hydraulic; 

 the other is to remove a portion of the sulphides contained in the slag 

 in the form of hydrogen disulphide. 



Le Chatelier states that the hydraulic properties of granulated slag 

 are due to the presence of a silico-alumino ferrite of calcium correspond- 

 ing in composition to the formula 3CaO, A1 2 3 , 2Si02. This com- 

 pound appears also in Portland cements, but in them it is entirely inert, 

 owing to the slow cooling it has undergone. When, however, as in 



