TESTS ON CEMENT 225 



part is most liable to cause unsoundness in the cement, and 

 although specifications do not call for tests with the No. 50 

 sieve, it is usually employed for the same reason as the No. 

 100 sieve. Any appreciable residue on this sieve indicates 

 that the material is much more liable to unsoundness. Any 

 cement failing to pass the fineness test should be watched 

 more carefully in the soundness and strength tests, but if 

 these tests show good results up to 28 da., the cement can, as 

 a rule, be used safely. It must be remembered, however, that 

 only that part passing the No. 200 sieve is really cement, so 

 that a coarsely ground shipment is practically equivalent to 

 one adulterated with weak and unsound material. 



TESTS FOR SPECIFIC GRAVITY 



The object of the specific gravity test is to furnish indica- 

 tions of the degree of burning, the presence or absence of 

 adulteration, and the amount of seasoning that the cement 

 has received. When Portland cement is burned, the separate 

 ingredients are in close contact and gradually combine by a 

 process of diffusion. The greater the amount of this burn- 

 ing the more thoroughly are the elements combined. Thus, 

 by their contraction they give, in volume, a higher density 

 or specific gravity. Since, to secure good cement the burn- 

 ing must have been made within definite limits, it fouows 

 that the specific gravity must also lie within fixed limits if 

 the cement has been properly manufactured. 



The common adulterants of Portland cement, namely, 

 limestone, natural cement, sand, slag, cinder, etc., all have 

 specific gravities ranging from 2.6 to 2.75, while the specific 

 gravity of Portland cement averages about 3.15. An appre- 

 ciable amount of adulteration, therefore, is at once indi- 

 cated in the results of the test. 



Seasoning is indicated because the cement on standing 

 gradually absorbs water and carbonic acid from the air. 

 These ultimately combine with it and thus lower the 

 specific gravity. 



Apparatus for Specific-Gravity Test. Of the many forms 

 of apparatus employed for the specific-gravity test, the 

 Le Chatelier flask, shown in Fig. 7, is the one most com- 



