200 ANNALS NEW YORK ACADEMY OF SCIENCES 



From these two series of experiments, it is qualitatively apparent that 

 the presence of clay does prevent a certain amount of hydrolysis. From 

 the first series, it is seen that this effect only begins to show itself as 

 higher percentages of water are present, which would indicate that the 

 clay may have taken up much more water than the constituency test 

 revealed, and that, in the relatively drier mixes with clay, the cement 

 suffered in strength because of insufficient water. On the other hand, 

 experiments at this laboratory in which clay was used, replacing up to 

 10 per cent of cement in normally gaged material, showed that no signifi- 

 cant decrease in strength was thereby obtained ; hence the loss in strength 

 in the 54 and 79 per cent grouts cannot be due to this cause. 



It is more probable that the colloidal nature of the added clay is 

 brought into play more effectively at the concentrations in which in- 

 creased strength is observed, and that the latter is due to the coagulation 

 of the clay by electrolytes adsorbed at this optimum concentration. 



The same result would obtain where additional saline material has 

 been added to the mixing water, as in the series where a 5 per cent solu- 

 tion of magnesium sulphate was used. The clay here prevents the accel- 

 eration of hydrolysis by the magnesium sulphate through adsorption of 

 part thereof, and possibly by coagulating, forming an impenetrable bar- 

 rier to the further action of water upon the remainder of the cement. 



QUANTITY OF WATEE THAT MAY SUBSEQUENTLY COME INTO CONTACT 

 WITH THE SYSTEM 



Permeability. — The solvent effect of water coming into contact with 

 cement structures is best studied by the permeability test. This consists 

 in forcing water through a mortar or concrete at a known pressure and 

 observing the amount of leakage through the specimen. In detail, the 

 specimen is generally made up in the form of a cylinder, and this is 

 cased with a thick coating of neat cement on all sides but the bottom. 

 The water, under pressure, is applied on the full cross-section of the 

 specimen and forced through, dripping from the bottom, whence it may 

 be collected. 



With neat cement, of course, this method is inapplicable, because of 

 the density of the material and the consequently enormous pressure nec- 

 essary to force water through it, and moreover because of the mechanical 

 difficulty in confining the water strictly to a passage through the speci- 

 men. The specimens tested, then, are lean mortars and concretes. 



Although this test is designed to ascertain the resistance which these 

 materials offer to the flow of water, it is evident that this resistance is 

 not a constant quantity in the case under consideration. 



