PROPERTIES OF PORTLAND CEMENT. PART III. 231 



were required to produce a normal paste of standard consistency and set. 

 Similar treatment affected the other three cements in the same general 

 manner, the difference being that the set of brand L could be retarded 

 the most and that of K the least. 



Similar results were also secured by any process of seasoning either 

 the ground cement or clinker, provided the process slaked the free lime 

 and at the same time did not permit the cement to set or cake. (See 

 Table XXX and figure 19.) The efficiency of different methods of 

 seasoning depended entirely upon the relative amoimts of slaked lime 

 produced and maintained, the conversion of calcium hydrate into cal- 

 cium, carbonate acting so as to decrease the plasticity and the retarding 

 influence of the sulphate. 



We have already discussed the most and least efficient methods of 

 converting the uncombined calcium oxide in Portland cement into its 

 hydrate, in Parts I and II of this paper ^^ and therefore no further 

 comment on this phase of the problem is necessary. 



THE EFFECTS PRODUCED BY THE SEASONING OF PLASTERED CEMENTS. 



Xo radical difference was manifest if plaster was added before the 

 cement had seasoned. It has already been showTi that the presence of 

 calcium sulphate exerts no appreciable influence upon the slaking of 

 the lime, and the results so far obtained all indicate that the substitu- 

 tion of calcium hydroxide for calcium oxide in Portland cement tends 

 to decrease the rate of the set, and to add somewhat to the plasticity. 



The heat generated by slaking ignited lime in the presence of an 

 excess of water is very considerable."* Generally speaking, Portland 

 cements set more quickly at a high than at a low temperature other 

 conditions remaining the same. Consequently, it seems reasonable to 

 assume that if free calcium oxide is present when water is added, the 

 heat of hydration throughout the mass of the cement paste will tend 

 to increase the normal rate of the set. The extent to which the same 

 quantities of free lime may effect this activity in different cements is 

 not constant, because all hydraulic setting compounds are not affected 

 to the same extent by heat. Eesidts obtained with the three brands of 

 material under investigation showed this very clearly, brand L being the 

 least affected and brand K the most. 



However, in these experiments with the samples after aeration, it is 

 reasonable to believe that the compounds which react with water and 

 cause the cements to set might also have been affected by the moisture 

 and carbon dioxide which altered the condition of the free lime so as to 

 eliminate its heating effect. The results indicate that such was not the 

 case, or rather that if this action took place it did so only to a very 



"This Journal, Sec. A (1910), 5, 367 and 415. "269 calories. 



