234 REIBLING AND REYES. 



It was demonstrated in Parts I and II of this paper that water, or 

 water and carbon dioxide^ have no appreciable influence on perfectly 

 sintered or thoroughl)' fused Portland cement in the clinker form, and 

 that it is only the free lime in hard-burned clinker which is effected by 

 these elements. Consequently, we believe that the hydraulic compounds 

 in this hard-burned clinker, brand K, which gave to the powdered material 

 its power of setting, imderwent ver}' little decomposition during the 

 processes of seasoning employed. However, after these samples had been 

 dried and ground to the same degree of fineness, the cement produced 

 from the seasoned clinkers was decidedly different from the non-seasoned 

 product. The changes induced by this method of seasoning, in general 

 character, were essentially the same as those obtained by aerating the 

 clinker after it had been pulverized. Therefore, we attribute the changes 

 to the altered condition of the lime. The data obtained from this 

 experiment tend especially to confirm this conclusion, since as has already 

 been emphasized in this paper, the Portland cement clinker seasoned 

 more efficiently when frequently wetted than when merely exposed to 

 the atmosphere under cover. 



The changes in plasticity can also be accounted for by alterations 

 in the condition of the free lime. Aside from the fact that the presence 

 of calcium oxide tends to promote regauging to a greater or lesser 

 extent, the somewhat increased plasticity which even normal pastes 

 show after the free calcium oxide has been substituted by the hydrate, 

 can be accounted for by the now well-known fact that ignited lime may 

 hydrate to a crystalline as well as to an amorphous form.®^ 



Bechtenkirker ^® and others have shown that slow hydration at a low 

 temperature tends to produce the amorphous form, and vice versa; and 

 that the crystalline one is the least plastic. Therefore, when the free 

 lime in Portland cement remains unslaked until the cement is gauged, 

 the heat generated throughout the mass because of the immediate slaking 

 of the free lime may be sufficient to caiise some of the oxide to hydrate 

 at so high a temperature that the crystalline. product is formed. On the 

 other hand, when the hydration of the free lime is slow, as is the case 

 «-hen the cement is subjected to the ordinary processes of seasoning, 

 the hydrate foi-med may be entirely amorphous. Then too, in the latter 

 case, the water required to slake the free lime has already been added, 

 so that none of the gauging water is required for this purpose. 



Calcium hydroxide readily absorbs carbon dioxide so that if the season- 

 ing is accomplished by exposing the cement to the air more or less calcium 

 carbonate is formed. Calcium carbonate, is a crystalline substance. In 

 Portland cement it is inert and lowers the specific gravity. Therefore, 

 the substitution of calcium carbonate for the slaked lime in Portland 



==See figure 4, Part I, p. 377. ^' Rock. Prod. (1907), 7, 23. 



