PACIXI, METAMORPHI8M OF PORTLAND CEMENT 177 



have shown that concrete subjected to such percolation has been shorn of 

 the major portion of its ultimate strength. In this case, the solvent 

 effect of the water is only part of the influence at work, purely me- 

 chanical factors entering largely into the destructive process, as will be 

 shown late$. 



Stalactitic growths of lime salts form as the result of water percolating 

 through concrete. Micro-organisms of the algal type frequently lodge in 

 the pores of concrete and by their growth may act as a protective influ- 

 ence against the permeation of water. The effect of their products of 

 metabolism and decay upon the concrete structure has not been studied. 



Numerous waterproofing materials and processes have been devised 

 (40, 73). They may be grouped conveniently under three heads. 



Surface treatments. The application to the surface of concrete of a 

 coating similar to a paint has the disadvantage that concrete is not a 

 thoroughly dry material. Where the vehicle is a liquid immiscible with 

 water, the paint will not therefore come into contact with the concrete 

 proper. If the vehicle is miscible with water, unless insoluble products 

 are at once formed by reaction with the constituents of cement, the 

 active agent is quickly leached out. 



Membranes. These are layers of waterproof tissue interposed between 

 two layers of the concrete. There is strong probability that these never 

 actually form a bond with the concrete, and thus they necessarily intro- 

 duce an element of weakness and heterogeneity. 



Mass treatments. The active material is incorporated with the con- 

 crete at the time of mixing, either by dissolving or suspending in the 

 gaging water, or by intimately mixing with the cement or sand. These 

 treatments are many and differ widely in the agents employed. Sub- 

 stances of a waxy or fatty nature, triturated to a great fineness, are the 

 most generally offered, but the incorporation of these in a mass of con- 

 crete is generally followed by weakness of the structure. The general 

 problem of cement waterproofing has been conceded to be simply a ques- 

 tion of void-filling, yet this must be accomplished without the addition 

 of inert material that will weaken the resulting structure. 



The addition of more colloidal material has been suggested. This is 

 ingeniously effected in a recent process by the use of hydrolysed cement, 

 obtained by treating cement with an excess of water (99). The paste so 

 obtained is added to the cement during mixing. 



The still unclarified state of our knowledge of the chemistry of the 

 setting andjiardening of cement is the great handicap which has thus far 

 prevented the devising of a satisfactory waterproofing agent. A large 

 number of the waterproofing preparations on the market are therefore 



