14 



Iron oxk';~, within reasonable limits, seems to. act PS a substi- 

 tute for alumina, and the two may be calculated together. Mag- 

 nesium carbonate is rarely entirely absent from limestones or 

 clays, and magnesia is therefore almost invariably present in the 

 finished cement. Though magnesia, when magnesium carbon- 

 ate is burned at low temperature, is an active hydraulic material, 

 it does not combine with silica or alumina at the clinkering heat 

 employed in Portland cement manufacture. At the best it is an 

 inert and valueless constituent in the cement ; many regard it as 

 positively detrimental in even small amounts, and because of 

 this feeling manufacturers prefer to carry it as low as possible. 

 Newberry has stated that in amounts of less than 3^% it i c 

 harmless, and American Portlands from the Lehigh district 

 usually reach well up toward that limit. In European practice 

 it is carried somewhat lower. 



