TESTS ON CEMENT 227 



Results of Specific-Gravity Tests. The specific gravity 

 of well-burned Portland cement averages about 3.15 and 

 should not fall below 3.1. If it falls below 3.1, tests should 

 also be made on dried and on ignited samples to ascertain 

 whether or not this condition has been produced by reason 

 of excessive seasoning. As a rule, low specific gravity 

 merely indicates well-seasoned cement, and if sound and 

 sufficiently strong, such cement is the best sort of material 

 for use, as its durability is scarcely open to question. 



TESTS OF NATURAL AND SLAG CEMENTS 



The methods of conducting tests of natural and slag 

 cements are, in all important particulars, identical with those 

 employed for Portland cement, although the results obtained 

 and the interpretation to be put on them are often radically 

 different. In the testing, the only essential difference is in 

 the amount of water required by these cements to produce 

 normal consistency, natural cement requiring from 23 to 

 35% and slag cement taking about 18%, or an average 

 of 2 or 3% less than Portland. Tests of natural cement for 

 tensile strength are also frequently made on 1-1 and 1-2 

 mortars, but recent practice is to test mortars of all kinds 

 of cement in 1-3 mixtures. For these cements, moreover, 

 the specific-gravity test has practically no significance, 

 except in determining the uniformity with which the dif- 

 ferent brands are made. 



CEMENT SPECIFICATIONS 



The common requirements for high-grade Portland, 

 natural, and slag cements are given in the table on page 

 228, and following the table is given a good example of a 

 complete modern specification for Portland cement. 



SPECIFICATIONS FOR PORTLAND CEMENT 



Kind. All cement shall be Portland of the best quality, 

 dry and free from lumps. By Portland cement is meant the 

 finely pulverized product resulting from the calcination 

 to incipient fusion of an intimate mixture of properly pro- 



