SAND AND CEMENT 195 



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

 puzzolan cement employed at all 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. 



Mixed cements cover a wide range of products obtained 

 by mixing, or blending, the foregoing cements with each 

 other or with other inert substances. Sand cements, im- 

 proved cements, and many second-grade cements belong 

 to this class. Mixed cements, however, are of comparatively 

 little importance. 



Properties of Cements. The hydraulic cements differ 

 from the limes in that they do not slake after calcination, 

 and that they set, or harden, under water. They can be 

 formed into a paste with water without any sensible increase 

 in volume and with little, if any, disengagement of heat. 

 They do not shrink appreciably in hardening, so that the sand 

 and broken stone with which they are mixed are employed 

 merely through motives of economy and not, as with limes, 

 of necessity. 



The color of the different grades of cement is variable, 

 but in certain cases it is distinctive. Portland cement is a 

 dark-bluish or greenish gray; if it is a light yellow, it may 

 indicate underburning. Natural cement ranges in color 

 from a light straw, through the grays, to a chocolate brown. 

 Slag cement is gray with usually a tinge of lilac. In general, 

 however, the color of cement is no criterion of its quality 



Cement is packed either in wooden barrels or in cloth or 

 paper bags, the latter being the form of package most com- 

 monly employed. A barrel of Portland or of slag cement 

 contains the equivalent of 4 bags, while but 3 bags of natural 

 cement equals a barrel. The average weights of the various 

 cements are given in the table on page 196. 



In proportioning mortar or concrete by volume, the com- 

 mon assumption is that a bag of Portland cement occupies 

 .9 cu. ft. 



