SLAG BRICKS AND SLAG BLOCKS. 679 



in proportion to the slag, and the bricks made from this slag require 

 a longer time to dry and harden than is needed by those made from 

 the ordinary slag. 



Slag bricks are made at Ekaterinoslav,* Russia, from blast-furnace 

 slags showing the following range in composition: 



Per Cent. 

 Silica (Si0 2 ) 22.5 to 35.0 



Alumina (A1 2 O 3 ) . 14 



Iron oxide (Fe 2 O 3 ) i i 



Manganese oxide (MnO) 0.0 



Lime (CaO) 45 



Magnesia (MgO) tr. 



Sulphur (S) 0.3 



Loss on heating 2.3 



15.0 

 3.3 

 0.3 



51.0 

 1.4 

 0.4 

 7.5 



The slag is granulated, sieved on a revolving screen, dried, and 

 ground in a ball mill. Lime is slaked and sieved. Enough of this 

 slaked-lime powder is added to the slag to bring the lime (CaO) con- 

 tent of the mixture up to about 55 per cent. With slags of the range 

 in composition above indicated, this would require the mixture to con- 

 sist of from 5 to 12 parts of lime to 100 parts of slag. The mixing is 

 carried on in a screw mixer, and the powdered mix is then pressed into 

 brick in a dry press. On issuing from this press the bricks are set aside 

 to harden, and at the end of six days are usually hard enough for use. 

 Their tensile strength is about 312 Ibs. per square inch; and the crush- 

 ing strength varies from 1250 to 5600 Ibs. per square inch; both, of 

 course, increasing with age. The bricks are gray in color, well shaped, 

 weigh less than stone, and require little mortar in laying up. They 

 withstand temperature changes well, and are particularly well adapted 

 for use in damp situations or under water. 



Toldt has described the manufacture of slag bricks at Bilbao, 

 Spain, where the blast-furnace slag from the Vizcaya furnaces is used. 

 Slag cement is made by mixing three parts of granulated and dried 

 slag with one part powdered slaked lime, and grinding this mixture 

 in a ball mill. The bricks are then made by mixing one part by volume 

 of this cement with four parts of wet granulated slag, and pressing 

 this mixture into shape in a brick -press. A Belgian form of press 

 with twelve molds is used. This turns out twenty bricks per minute, 

 with thirteen workmen. 



It will be noted that in a slag brick made in this fashion the strength 

 of the brick must be almost entirely derived from the slag cement used 

 in the mixture, for the uncrushed slag will be almost inert. 



* Engineering and Mining Journal, 1896. 



