246 



REIBLING AND REYES. 



Meade entirely neglected to consider the fact that the coarser particles 

 even in well-seasoned cements usually contain microscopic particles of 

 free lime which remain inactive and anhydrous until their protective 

 coatings of slag or calcium carbonate are destroyed or split open by 

 finer grinding. (See Table VI, Part II.) Obviously, if a cement is 

 reground and tested before the newly liberated calcium oxide has become 

 converted into its hydrate, the heat produced by the slaking of this 

 lime when water is added will tend to quicken the set. Obviously also, 

 if a cement contains material which is inert only because of the coarse- 

 ness of its particles, and later is reground until this material is fine 

 enough to become active, the amount of retarder in relation to the active 

 cement is decreased by the process of grinding. Both of these influences 

 tend to quicken the set. However, the former can be removed simply 

 by seasoning the cement until the newly liberated calcium oxide, or its 

 equivalent, has hydrated, and it is our experience that after this has 

 been done, reground commercial Portland cements of standard quality 

 usually regain their normal,- slow set even though the regrinding at first 

 made them abnormally quick setting. 



A characteristic instance in which the influence of the newly liberated 

 free lime was capable in itself of causing a normal cement to become 

 quick setting is given in Table XXXV. 



Table XXXV. — The influence of regrinding in the rate of set of cement N. 

 (Temperature 27°. 5 to 30°. 0.) 



Time 

 aer- 

 ated 

 in 

 days. 



Cement N, not reground. 



Fineness. 



Specif- 

 ic 

 grav- 

 ity. 



Condition of the free 

 lime (microscopic test). 



Water 

 required 

 for a nor- 

 mal con- 

 sistency 



paste. 



1 

 Time of setting 

 in minutes. 



200- 

 mesh. 



100- 

 mesh. 



Initial 



set. 



Final 

 set. 







3 



17 



30 







3 



14 





 3 



76.8 



95.2 



3.10 

 3.077 

 3.012 

 2.942 



CaOandCa(OH)2 



Ca(OH)o-- - - - . . 



Per cent. 

 20 

 21 

 21.5 

 23 



70 



150 



70 



30 



140 

 280 

 220 

 250 







CaCOa 







do - - 









Cement N, reground. » 



96.7 



-100 



3.10 

 3.077 

 3.026 



CaO and Ca(0H)2 



Ca(OH)o 



22 



19.5 



23 



8 

 110 

 25 



15 

 170 

 150 







CaCOs 









Cement N aerated 30 days and then reground. 



100 



100 



2.93 

 2.922 



CaO and Ca(0H)2 



Ca(0H)2 - - • 



22.5 

 21 



10 

 30 



160 

 180 











» The dry cement was reground in an air-tight ball mill. 



