178 ANNALS NEW YORK ACADEMY OF SCIENCES 



purely empirical, and not applicable to the practical waterproofing of 

 large masses of constantly wet concrete. In the interests of efficiency, it 

 is probably more economical to expend money destined for waterproof- 

 ing in the purchase of additional cement to be used in making a richer 

 concrete. 



QUALITY OF WATER AT FIRST ADDED 



Having material in solution. On adding water to cement, heat is 

 evolved, the temperature of the mix rising in some cases to above the 

 boiling point of water. It is the custom to look with suspicion upon 

 i-ements in which an excessive rise of temperature is obtained, as being 

 liable to develop unsoundness. The abnormal rise is attributed in some 

 instances to the presence of free lime, in others to an insufficient propor- 

 tion of lime. The volume changes caused by a rise in temperature have 

 been given as the reason of the difficulty encountered in joining fresh 

 cement surfaces to old, causing weakness at the plane of juncture, the 

 contraction of the mass on cooling breaking the joint before it has devel- 

 oped sufficient strength to resist the strain. 



To prevent this, it has been suggested to coat the surface to which 

 fresh cement is to be applied with a retempered mortar : that is, with a 

 cement which has been treated with water after partial setting. This 

 provides an intermediate course of material in which the temperature 

 changes are not so rapid, and upon this course the fresh cement mixture 

 is applied (35). 



Upon the same principle may be explained the use. for a fresh course 

 of cement which is to be. joined to some which has previously set, of 

 mixing water in which a quantity of cement has been stirred, thus retard- 

 ing the chemical reaction and consequent temperature changes. In both 

 cases, the active water is already charged with the soluble portion of 

 cement, its solvent power for the same material is thereby diminished 

 and the chemical action moderated, so that heat is more gradually 

 evolved and violent expansions and contractions avoided. 



The influence of dissolved electrolytes in mixing water has received 

 much careful study. Through the addition of a small percentage of 

 some soluble salt to the mixing water, many have tried to influence the 

 properties of the completed structure and to produce a mass that would 

 develop greater strength or a higher degree of imperviousness. Unfor- 

 tunately, the panacea has not as yet been discovered that is suitable for 

 practical application. 



The addition, similarly, of a soluble powder incorporated in the mass 

 of the cement comes under the same category. In this connection, our 



