PROPERTIES OF PORTLAND CEMENT. PART III. 



247 



An instance in which the effect of the newly liberated lime was suffi- 

 cient to quicken the set, but not to make the reground cement abnormally 

 quick setting, is given in Table XXXYI : 



Table XXXVI. — The influence of regrinding on the rate of set of cement G. 

 (Temperature 28° to 30°.) 



Condition. 



Time 

 aer- 

 ated 

 in 



days. 



Fineness. 



Specif- 

 ic 

 grav- 

 ity. 



Condition of the 



free lime 



(microscopic test). 



Water 

 required 

 for a nor- 

 mal con- 

 sistency 



paste. 



Time of setting 

 in minutes. 



200- 

 mesh. 



100- 

 mesh. 





Initial 

 set. 



Final 

 set. 



• Not reground 



Reground 



Regro u n d— 1. 

 per cent of 

 plaster. 



Reground and 

 aerated. 





 





 3 



75.6 

 96.0 



X 

 X 



91.8 

 100. 



X 

 X 



3.10 

 3.095 



X 

 3.04 



CaOandCa(OH)o-_ 

 More CaO and 



Ca(0H)2. 

 do 



Per cent. 

 19.5 

 22.0 



21.0 

 18.5 



130 



45 



100 

 130 



300 

 150 



210 

 300 



Ca(0H)2 „- 



The other influence, namely that due to the increase in the amount of 

 active cement can not be removed without seriously injuring the strength 

 of the cement. It can be realized that this influence might be sufficiently 

 great to cause a reground cement to remain quick setting, not only after 

 the newly liberated calcium oxide had become slaked, but also, after 

 additional retarder had been added. However, fortunately most manu- 

 facturers have already succeeded in producing a cement which makes 

 such an instance so exceptional that our experience does not include an 

 example. 



Therefore, it is evident that the influence of fineness on the rate of 

 set introduces no new or insurmountable factors into the problem of 

 the control of the set, but Ave are convinced that most manufacturers 

 can control the set of their Portland cement even though it has been 

 ground to an impalpable powder. ^Ye also believe that the possibility 

 of so doing will become more universal as the practice of grinding the 

 raw material to a high degree of fineness and of seasoning cement by 

 the most efficient methods, increases. 



CONCLUSIONS. 



We have attributed the fundamental cause of changes in the setting 

 properties of Portland cement to alterations in the condition of the free 

 lime. Unfortunately, we have found it impossible except in quantities 

 much too small to meet the requirements of experimental purposes to 

 obtain commercial cements which contain no free lime. Such material 

 could only be secured from a few selected clinkers which in no wise 



