172 REIBMNG AND SALINGER. 



assertion of the paragraph headed "General observations" of the "com- 

 mittee on standard specifications for cement." This committee states : 



"iSpecilie gravity is useful in detecting adulterations and underburning. Tlie 

 results of tests of specific gravity are not necessarily conclusive as an indicator 

 of the quality of a cement, but when in combination with the results of other 

 tests may afford valuable indications." -° ( Italics are supplied. ) 



The specific gravity is useful in detecting adulterations because certain 

 adulterations will alter the specific gravity beyond the limits of specifica- 

 tions. However, the adulteration of Portland cement is so readily de- 

 tected by competent chemists and testers that it is now seldom indulged 

 in by manufacturers. The real problem of cement testing concerns itself 

 with the pure product; and for the valuation of this we find the specific- 

 gravity determination to be a great aid. Of course, its importance is 

 limited. Like the chemical analysis, it gives definite aid only to a 

 limited degree. Cliemieal analysis will not show the degree of burning 

 nor the compounds that exist in a cement; and the specific gravity will 

 not always disclose adulteration or underburning. However, both these 

 tests give valuable aid in tracing causes of defects which by other tests 

 have been found to exist. For instance, it was the relation between the 

 specific gravity, the tensile strength and the setting time of the cement 

 recorded in diagrams 1 and 2 which gave us the first clue to the cause 

 producing the variations which prevailed throughout these tests and 

 which led us more fully to investigate the effects of aeration on high 

 alumina cements. Now that we understand the nature of this cement, 

 the specific-gravity determination alone enables us to predict very ac- 

 curately what the results of the other tests will be and to suggest how 

 the cement may be improved. 



Failure to pass the soundness tests may be due to two causes — excess 

 of lime or underburning. Unsoundness in conjunction with low specific 

 gravity proves that underburning alone is the cause of the warping and 

 disintegrating. 



Cements may attain a low specific gravity as a result of prolonged 

 seasoning. If this benefits the cement, well and good; but if it injures 

 it, then the material should not be allowed to season, or, if seasoning has 

 already developed dangerous properties, it should be rejected. The 

 specific gravity, before and after ignition, will 'indicate to what extent 

 seasoning has effected a well-burned cement, and a record of tests com- 

 pared with the corresponding specific gravities will show the quality of 

 the cement developed by the absorption of various amounts of water and 

 carbonic acid. 



If a cement shows little change in its specific gravity before and after 

 ignition, and also gives unsatisfactory tests in tensile strength and setting 



^ Meade, loc. cit. 



