MASONRY 



307 



Pozzuolana, or puzzolan, cement is a material resulting from 

 grinding together, without subsequent calcination, an intimate 

 mixture of slaked lime, and a puzzolanic substance, such as 

 blast-furnace slag or volcanic scoria. The only variety of 

 puzzolan cement employed extensively in American practice 

 is slag cement. This cement is made by grinding together a 

 mixture of blast-furnace slag and slaked lime. The slag used 

 for this purpose is granulated, or quenched, in water as soon as 

 it leaves the furnace, which operation drives off most of the 

 dangerous sulphides and renders the slag puzzolanic. 



AVERAGE WEIGHTS OF HYDRAULIC CEMENTS 



Portland cement may be distinguished by its dark color, 

 heavy weight, slow rate of setting, and greater strength. 

 Natural cement is characterized by lighter color, lighter weight, 

 quicker set, and lower strength. Slag cement is somewhat 

 similar to Portland, but may be distinguished from it by its 

 lilac-bluish color, by its lighter weight, and by the greater fine- 

 ness to which it is ground. 



Portland cement is adaptable to any class of mortar or 

 concrete construction, and is unquestionably the best mate- 

 rial for all such purposes. Natural and slag cements, how- 

 ever are cheaper, and under certain conditions, may be sub- 

 stituted for the more expensive Portland cement. All heavy 

 construction, especially if exposed, all reinforced-concrete 

 work, sidewalks, concrete blocks, foundations of buildings, 

 piers, walls, abutments, etc., should be made with Portland 

 cement. In second-class work, as in rubble masonry, brick 



