PROPERTIES OF PORTLAND CEMENT, PART III. 245 



effects of storage on the setting properties of Portland cement which has 

 been plastered with a known amount of retarder. 



THE INFLUENCE OF FINENESS ON THE RATE OF SET. 



Another subject of considerable importance to both manufacturer and 

 consumer is the influence of fineness upon the rate of set. Generally 

 speaking, it has long been known that the finer a cement is ground the 

 greater is its possible efficiency. Nevertheless, the attempts made, to 

 enforce finer grinding met \\dth such decided opposition from the begin- 

 ning that many cements are able to meet the requirements of our modem 

 standard specification even though they contain 20 per cent and more 

 of inert material which, if ground fine enough^ would correspondingly 

 increase their value. 



For a long time advocates against fine grinding maintained that if 

 Portland cement was completely pulverized to form an impalpable 

 powder, it would become so active that its set could not be controlled. 

 However, eventually, it became known that, although the general effect 

 of finer grinding is to quicken the set, in some instances this is almost 

 unnoticeable. 



However, scientific knowledge iipon this subject is extremely limited. 

 This is readily accounted for if we consider that the majority of investi- 

 gators failed to take into consideration the influence which changes in 

 the condition of the free lime had upon the set. Many endeavored to 

 ascertain the effects of fine grinding by using material obtained by 

 separating the fine from the coaiser particles in ordinary Portland 

 cement. Others failed to consider the fact that finer grinding or 

 regrinding often increased the amount of active free lime. 



Our work on this problem soon convinced us that the general in- 

 fluence of fine grinding, namely the quickening of the set, is due to the 

 presence of "some undesirable element in the coarser and at present 

 inert particles of the cement which is liberated or rendered active by the 

 grinding." ^^ However, we do not believe that the undesirable element 

 in the coarser and harder particles of Portland cement is "owing to the 

 fact that they are lower in lime and are burned to a high degi'ee of 

 vitrifaction." 



In the first place Meade's conclusion in this respect is based upon 

 doubtful evidence because he did not prove that the coarser particles in 

 cement contained less combined lime than the separated, impalpable 

 powder. The usually higlier total amount of calcium oxide in the latter 

 is readily accounted for by the fact that the free lime liberated by the 

 grinding, the product into which the free lime has become converted and 

 most of the retarder, are included in the fine powder. Furthermore, 



«» Meade, R. K., Am. Soc. Test. Mat. (1908), 8, 410. 



