148 



UEIBLING AND SALINGEK. 



Taule X. — After exposure for sixlij-five and one-half ilni/s. 



Sample No. 



Loss on 

 igriition. 



Carbonic 



acid 



(C0»). 



FG-1 



Per ami. 

 •i.2o 



Per cent. 

 2.40 

 2.42 



L208 .. _. . 





The rapidity with which this action will progress depends upon 

 climatic conditions^ upon the nature of tlie cement itself and upon the 

 ratio between the volume of the cement and tlie surface exposed. The 

 action niaj' talce jDlace very rapidly on tlie exposed surfaces, and yet 

 penetrate into tlie mass very slowly. 



A well-mixed cement was stored in a uncovered can for one month. Cement 

 taken not lower than half an inch from the upper surface set in twenty minutes, 

 while cement taken 6 inches beloAV this surface did not set until two hours. It 

 is therefore absolutely essential in order to secure uniform results from the same 

 sample thoroughly to mix the cement before weighing; otherwise a wide dis- 

 crepancy between the specific gravity, time of setting, and tensile strength may 

 result which could not otherwise be accounted for. 



The above data are considered sufficient to illustrate tlie effects of 

 aeration, although in this laboratory we liave many more experiments 

 proving the same facts. It may be well to state that cements have been 

 encountered which do not change to an}^ appreciable extent after exposure 

 for several weeks. 



It has been shown (Table VI, can 8; eleven and twenty-four days) 

 that cement, otherwise susceptible to a marked change by exposure 

 to air, when preserved in closely covered galvanized-iron cans will be 

 little affected by storage; and that thin paper bags do not eliminate the 

 atmospheric influences. It has also been shown that the characteristics 

 of a cement often undergo a cliange upon exposure which may be suSicient 

 to make failures of otherwise acceptable tests. Therefore, it is evident 

 that no system of cement testing, however accurate, will insure uniform 

 or even comparative results until a proper, specified preservation of the 

 samples after they are taken from stock is made compulsory. 



As a result of the considerations given above, it would seem necessary 

 so to modify the ordinary procedure that tlie quantity of the cement 

 deemed necessary for the desired tests should be freely exj)osed to tlie 

 atmosphere of the laboratory for seven days in a layer 1 inch deep, in 

 order to determine tlie effects of aeration. A comparison of the -specific 

 gravity, setting time, and loss on ignition of the cement Ijefore and after 

 exposure, will give valuable indications as to its nature. The determina- 

 tions made before exposure will be consistent with the quality of the 

 cement at the time tlie stock was sampled, and the second treatment will 

 show the qualities liable to be developed by subsequent storage. If the 



