176 ANNALS NEW YORK ACADEMY OF SCIENCES 



leaching out of calcium hydroxide from the mass of the cement ; but this 

 diminishes as the mass grows more and more impermeable, by the coagu- 

 lation of the colloidal cell walls and by the carbonation or other precipi- 

 tation of lime salts in the pores. 



This deposition of lime salts in the pores is evidently the cause of 

 higher strength in specimens which are allowed to dry out a few hours 

 before testing. It is analogous to the higher strength developed by sea- 

 soned stone than by freshly quarried stone, occasioned by the evaporation 

 of the "quarry sap." In addition, the carbon dioxide conveyed to the 

 material in a gaseous form is absorbed by the lime and may be considered 

 a positive factor towards strength, while that conveyed in solution (where 

 the cement is under water) is a negative factor, in that it accelerates the 

 solvent effect of the water coming into contact with the cement. On the 

 other hand, cement specimens which are entirely air-hardened are un- 

 questionably weaker, by reason of the absence through evaporation of the 

 requisite amount of water for proper hydration. 



When the action of water upon set cement is intermittent, the solvent 

 effect manifests itself by unsightly incrustations and discolorations (3), 

 caused by dissolved material brought to the surface through capillary 

 action and there deposited by evaporation. When the mass is perma- 

 nently under water, these salts are merely washed away. The danger 

 from these incrustations, although slight, is the disintegrating effect pro- 

 duced by their increase in volume, through crystallization or efflorescence, 

 and the consequent disruption of the denser surface skin, rendering- 

 easier the action of frost upon the entire mass. 



This surface skin is improved by troweling the cement while in a, 

 plastic state, and consists of a closely packed layer of fine particles, which 

 offers high resistance to permeation by water and comparative immunity 

 from the solvent action favored by a rough, porous or fractured surface. 



If the mass be placed in water before setting, it is more liable to hy- 

 drolysis, as evidenced by the copious formation of laitance ; and if greatly 

 exposed, as by agitation under water, it may fail to develop the greater 

 portion of its normal ultimate strength. To prevent this, care is taken, 

 in laying concrete under water, so to convey it that it offers the least 

 possible surface to water action during its descent; and to this end it is 

 either lowered in cloth bags, or filled in through a chute, so as to escape- 

 all avoidable exposure to hydrolysis. 



If the water which comes in contact with a cement structure be under 

 considerable pressure, so that its tendency is to percolate through the 

 mass, the solvent effects will of course be magnified, proportionally to the- 

 porosity of the mix; and experiments made by the Board of Water Supply 



