352 



CEMENTS, LIMES, AND PLASTERS. 



Slags used as Portland-cement materials. The slags used in Port- 

 land-cement manufacture are iron blast-furnace slags of the more basic 

 types i.e., those in which the lime (CaO) reaches 30 per cent or over. 

 The higher the lime, up to say 50 per cent, the more valuable the slag 

 for this use. The composition of the slags will usually be controlled, 

 however, by the requirements of the furnaces, not by the needs of the 

 cement-plant. 



The following shows the range in composition of the slags used at 

 a German Portland-cement plant. 



ANALYSES OF SLAG USED IN PORTLAND-CEMENT MANUFACTURE. 



Per Cent. 



Silica (SiO 2 ) . . 30 to 35 



Alumina (A1 2 O 3 ) 10 



Iron oxide (FeO) 0.2 



Lime (CaO) 46 



Magnesia (MgO) 0.5 



Sulphur trioxide (SO 3 ) 0.2 



14 



1.2 

 49 



3.5 



0.6 



As a Portland-cement material slag possesses one great advantage 

 in addition to its cheapness. This advantage is chemical, and is due 

 to the fact that the lime contained in the slag is present in the form 

 of oxide (CaO), instead of carbonate (CaCOs), as in limestones. It does 

 not require to be decarbonated, and therefore a mixture made up of 

 slag and clay will clinker with less fuel than one consisting of limestone 

 and clay. 



Opposed to this chemical advantage is a physical disadvantage. 

 If the slag is allowed to cool as it issues from the furnaces, it solidifies 

 into very hard and tough masses much more resistant than the hard- 

 est of limestones. In order to avoid this difficulty, it is the common 

 practice to "granulate" the slag, i.e. to run it direct from the furnace 

 into cold water. This proceeding breaks up the slag into little porous 

 granules -fa to J inch in diameter, and incidentally removes part of 

 the sulphur contained in the slag. But to offset these gains, it intro- 

 duces a large amount of water into the product, so that a granulated 

 slag may carry from 20 to 40 per cent of water, and this greatly increases 

 the cost of drying. 



As the chemical and physical properties of slag introduce certain 

 interesting features into the manufacture of Portland cement from a 

 limestone-slag mixture, this mixture will be discussed separately on 

 later pages. 



