CHAPTER XLII. 

 SLAG CEMENT. REQUISITES AND TREATMENT OF THE SLAG, 



SLAG cement is at present by far the most important member of the 

 group of puzzolan cements, so that its manufacture will be described 

 in some detail. 



Summary of general methods of manufacture. Slag cement is com- 

 posed of an intimate mechanical mixture of slaked lime and granu- 

 lated blast-furnace slag of suitable chemical composition; both mate- 

 rials being finely pulverized before, during or after mixing. The process 

 of manufacture includes the granulating and drying of the slag, the 

 slaking of the lime, the mixing of these materials, and the grinding 

 of the resulting cement, together with any means which may be employed 

 for the regulation of the setting time of the cement. 



These different factors in the manufacture will be described in the 

 order named above. In the present chapter, the character and treat- 

 ment of the slag will be taken up. 



Composition of the Slag. 



Requisite chemical composition of slag. The slag used in cement- 

 manufacture must be a basic blast-furnace slag. Tetmajer, the first 

 investigator of slag cements, announced as the results of his experi- 

 ments (a) that the hydraulic properties of the slag increased with the 

 proportion of lime contained in it, and that slags in which the ratio 



nr- was so low as to approach unity were valueless for cement-manu- 



bl(J2 



facture; (6) that, so far as the alumina content of the slag was con- 

 cerned, the best results were obtained when the ratio .^ 3 gave a value 



of 0.45 to 0.50; and (c) that with any large increase of alumina above 

 the amount indicated by this value of the alumina-silica ratio, the ten- 

 dency of the cement to crack (when used in air) was increased. 



Prost, at a later date, investigated the subject, using for experi- 



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