38 



METHODS OF MANUFACTURE. 



If, as in the present volume, we exclude from con- 

 sideration the so-called "natural Portlands," Portland ce- 

 ment may be regarded as being an artificial product, obtained 

 by burning to semi-fusion an intimate mixture of pulverized 

 materials, this mixture containing lime, silica and alumina, 

 varying in proportion only with certain narrow limits ; and by 

 crushing finely the clinker resulting from this burning. 



If this restricted definition of Portland cement be accepted, 

 four points may be regarded as being of cardinal importance in 

 its manufacture. These are : 



1 i ) The cement mixture must be of the proper chemical 

 composition. 



(2) The materials of which it is composed must be carefully 

 ground and intimately mixed before burning. 



(3) The mixture must be burned at the proper temperature. 



(4) After burning, the resulting clinker must be finely 

 ground. 



The first named of these points the chemical composition 

 of the mixture can be more advantageously discussed after 

 the other th/ree points have been disposed of. The subjects 

 will, therefore, be taken up in the following order : 



Preparation of the mixture for the kiln. 

 Burning the mixture. 



Grinding the clinker, addition of gypsum, etc. 

 Composition and properties of Portland cement. 



PREPARATION OF THE MIXTURE FOR THE KILN. 



The preparation of the mixture for the kiln involves the re- 

 duction of both of the raw materials to a very fine powder, and 

 their intimate mixture. In practice the raw materials are 

 usually crushed more or less finely, and them mixed, after which 

 the final reduction to powder takes place. Two general methods 

 of treatment the dry and the wet are in use at different 

 plants. Unless the limey constituent of the mixture is a marl, 

 already full of water, the dry method is almost invariably fol- 

 lowed. This consists merely in keeping the materials in as dry 



