282 



CEMENTS, LIMES, AND PLASTERS. 



(a) A pat is then kept in air at normal temperature. 

 (6) Another is kept in water maintained as near 70 F. as prac- 

 ticable. 



17. These pats are observed at intervals for at least twenty-eight 

 days, and, to satisfactorily pass the tests, should remain firm and hard 

 and show no signs of distortion, checking, cracking, or disintegrating. 



Comparison and Summary. 



It is of interest to compare the requirements, on various points, 

 of the specifications above printed. They differ considerably, as can 

 be readily seen, in regard to fineness, strength, setting time, etc., etc. 



Definition. Of the four specifications above printed, two give no 

 definition whatever and the others are very defective. The definition 

 given by the American Society for Testing Materials would, for example, 

 exclude almost every good natural cement in the country; for it is 

 only the very low-limed cements that are burned "at a temperature 

 only sufficient to drive off the carbonic-acid gas ". 



Specific gravity. Only one specification contains any requirement 

 in regard to specific gravity, and this places the minimum at 2.8. So 

 far as is known to the writer, not even the worst possible natural cement, 

 if burned at all, could fall much below this point. 

 , Fineness. The differences in the fineness requirements of the speci- 

 fication printed above, as well as of several other important specifica- 

 tions, are summarized in Table 137, below. The requirements of the 

 American Society for Testing Materials (90 per cent through 100-mesh, 

 70 per cent through 200-mesh) are high, and probably cannot be eco- 

 nomically attained unless modern grinding machinery is in use at the 

 mill. With tube mills, however, this fineness can be readily reached, 

 and the tensile strength of the cement is greatly improved. 



TABLE 137. 

 FINENESS REQUIRED BY VARIOUS SPECIFICATIONS. 



