584 



CEMENTS, LIMES, AND PLASTERS. 



the purely external agencies, and that these agencies may be either 

 physical or chemical. 



This brief outline will serve to give some idea of the wide scope 

 which might be given to a discussion of the properties of Portland 

 cement. 



ALL PASSED NO. 100 SIEVE 



FIG. 143.* Variation of tensile strength with fineness. 



Value of fineness tests. The reason for testing the fineness of a 

 cement depends on the facts that (a) the strength of the cement, and 

 particularly its tensile strength when mixed with sand, increases with 

 the fineness, and (6) the soundness of the cement may be improved 

 by fine grinding. The second point is one that concerns the manu- 

 facturer more than the user, because an unsound cement will usually 

 fail to pass other tests and will therefore be rejected. 



The increase in strength consequent on increased fineness is well 

 shown in Figs. 143 and 144, both showing the results of tests on 1:3 

 mixtures, the tests of Fig. 143 having been made at four months while 

 those in Fig. 144 are at various ages. 



The value of fine grinding is evident, and engineers are constantly 

 raising the standard of fineness in specifications. Unfortunately, how- 

 ever, they fail to make proper use of this fine cement after they hav 

 paid extra for getting it. They insist, for example, in obtaining cement 

 which will pass 92 or 95 per cent through a 100-mesh sieve, and then 

 use it in the same sand mixtures that they would if it were an English 

 cement passing perhaps 85 per cent through 100-mesh. 



The actual fineness of a number of typical American Portlands is 

 shown very exactly in the tests given in Table 222. 



* From Johnson's " Materials of Construction", p. 409. 



