192 



ANNALS NEW YORK ACADEMY OF SCIENCES 



Table 4 

 Twenty-eight-day Tests of Grouts Mixed for Varying Lengths of Time 



Duration of mixing 



Compressive strength, pounds 



per square inch, average 



of duplicate tests 



50 perct grout 



100 per ct.grout 



1 minute 



5240 

 5545 

 5710 



5875 

 6075 

 4775 



3095 

 4725 

 4955 

 4840 

 4320 

 4538 



15 minutes 



30 minutes 



1 hour 



2 hours 



5 hours 





The effect of mechanical agitation, when thus prolonged, is equivalent 

 to that of the use of excess water — the strength of the cement is pro- 

 gressively diminished as the working proceeds. It is noteworthy that 

 the effect is only reached after a certain optimum period is passed. Be- 

 fore this time, increased working increases the strength. We may con- 

 clude that there occurs within this period a process which neutralizes the 

 effect of hydrolysis; and this process is probably the formation of the 

 network which constitutes the setting. 



As will be seen later, the effect of excess water is to reduce the ulti- 

 mate strength. The effect, then, of mechanical agitation must be to 

 bring more cement into contact with water and, therefore, to increase 

 hydrolysis. This is probably accomplished by stripping off the protective 

 film of gelatinous material which envelops each cement particle when it 

 comes into contact with water, which film regulates the hydration of 

 cement and causes it to proceed in a regular manner. This film being 

 stripped off, the cement is subject to the destructive action of hydrolysis. 



Where more water is originally present, the destructive action is sooner 

 attained, as will be seen by comparing the 100 per cent grout with the 

 50 per cent. Evidently, the setting process proceeds best at high concen- 

 trations, when the amount of water is low. This may be so regulated 

 that the setting process will not take place at all, by using a large excess 

 of water and much mechanical agitation, as has been repeatedly observed 

 by the writer. 



Setting time of cement in laboratory air and in damp closet. — The 

 standard specifications for setting-time tests call for storing the specimen 

 in the damp closet, whereas the tests as generally conducted in most 

 laboratories are made in the open laboratory air. A series of experi- 

 ments was made, for the purpose of noting the deviation from standard 

 results caused by this departure from the rule. 



