PACINI, METAMORPHISM OF PORTLAND CEMENT 205 



Comparison of these two sets of figures indicates that the cement of the 

 concrete is more attacked than the aggregate. In fact, the flow obtained 

 in this specimen was the highest but one of a series of sixteen, and the 

 total lime content of the effluent water was also the highest but one. 



The visible effect upon examination of the interior of the specimens 

 was a bleaching of the mortar, with evident solution of the cement. The 

 original percentage of lime in the mortars was 12.8. Analysis of mortar 

 from the granite specimen showed a content of 4.8 per cent, indicating 

 that nearly two-thirds of the lime had been dissolved out. Further evi- 

 dence of the loss of lime was found in the heavy white crust which 

 formed on the exposed bottoms of the concrete specimens during the test. 

 Small stalactites, quite soft to the touch, were abundant. The quantity 

 of this deposit was not visibly different in the different tests. 



The calculated loss in lime of the mortar was greater than the loss 

 computed from periodical chemical analyses of the effluent water, and 

 this is due to the fact that much of the dissolved lime was deposited upon 

 the bottoms of the specimens as the stalactitic growth above mentioned. 



There was no evidence that suspended impurities in the water had 

 been carried into the interior of the concrete, and it is therefore supposed 

 that the one-inch layer of sand by which the latter was screened from 

 the direct flow of the water was an efficient filter for the purpose. The 

 clogging action resulting from this source may therefore be dismissed as 

 negligible. 



It may be concluded from these tests that concrete of this density tends 

 to protect itself automatically from the action of percolating water, so 

 that, for the period investigated at least, the flow tends to diminish to a 

 minimum. The action of the water seems to be confined to the cement 

 of the mortar, leaving the aggregate relatively unaffected. 



It is evident that, notwithstanding the utmost precaution in mixing 

 concrete test specimens, wide differences in permeability may obtain in 

 specimens mixed under the same conditions of handling and by the same 

 workman, owing to structural differences in the resulting mass. How- 

 ever, the results obtained are fairly comparable. 



- The most sensitive test for the internal changes which the concrete has 

 undergone during percolation is the resulting strength of the concrete. 



Concretes containing different cements. — A series of tests was under- 

 taken in which the specimens were made up in the same proportions, 

 1 : 2.5 : 6, using in each specimen the same coarse aggregate, a crushed 

 granite, and the same fine aggregate, a standard quartz, but using differ- 

 ent brands of cement. The specimens were stored in damp sand for a 

 period of 28 days, then subjected to continuous water pressure of about 



