386 



REIBLING AND REYES. 



providing data for Table IV was arranged so that only an upper layer 

 was fully exposed at any given time. 



The absence of individual needles and the presence of aggregates form- 

 ing plumes at times during the course of aeration supports White's state- 

 ment that "experiment on the aeration of cement in thin layers in the 

 laboratory indicate that the conversion into carbonate goes practically 

 as fast as hydration." It will be noted that as the length of the time 

 of aeration increased, that necessary for the formation of calcium hy- 

 droxide-phenol crystals also increased. Since sintered calcium oxide 

 may hydrate very slowly and since the slaked lime changes to carbonate 

 very rapidly, it seems reasonable to suppose that particles of sintered 

 lime gradually became coated with a film of calcium carbonate which 

 prevented the immediate action of the testing solution. It also is very 

 probable that this outer coating of carbonate might become so dense as 

 finally to prevent hydration and combination with phenol in the inner 

 portions of the particles. The work recorded in Table VI was therefore 

 undertaken to ascertain if this conclusion is correct. 



Conditions of the experiment recorded in Table VI. — The cements recorded 

 in Table V, which had been aerated until no crystals could be formed on the 

 microscope slide, were each reground in an agate mortar and again subjected to 

 the test for free lime. 



Table VI. — Microscopic tests of reground, aerated cements. 



Description of Port- 

 land cement. 



Microscopic test (duplicate samples). 



Soundness 

 (5-hour steam 



test). 



Well-burned clinker 

 aerated 8 days 



No crystals formed in 1 hour 



Sound. 





(Brand 0, Table 







III). 







The same reground.. 



An almost immediate formation of crystals. In 5 min- 

 utes the slide presented a network of clusters, long and 

 short, single and radiating needles and of plumes. 



Do. 



Underburned clink- 



No crystals formed in 1 hour.. ... 



Do. 



er aerated 8 days 







(Brand 0, Table 







IV). 







The same reground. . 



Indications of plumes in 2 minutes. In 5 minutes the 



Slightly dis- 





slide presented a network of long and short needles 



integrated. 





and well-formed plumes. This underburned cement 







shows a greater quantity of plumes and needles than 







the reground, well-burned material. 





These results make it evident that free lime may continue to be 

 present in considerable quantity even in thoroughly aerated, finely ground 

 cement, although such .cement may have ceased to form calcium hy- 

 droxide-phenol crystals. The significance of this fact will become more 

 apparent elsewhere. 



