182 ANNALS NEW YORK ACADEMY OF SCIENCES 



moval of the calcium hydroxide, and in some cases to its replacement by 

 material occupying greater volume, as before shown, and consequent 

 disintegration of the structure. 



That some such reactions occur is indubitable, and that the mechanical 

 factors are a large influence in the disintegration is equally certain. An 

 additional cause which may be of great importance has hitherto been 

 neglected. The electrolytes in these natural waters may act as acceler- 

 ators of hydrolysis, and, in effect, cement which is in contact with sea 

 water is subject to the same action as that of an excess of water from any 

 cause. By the presence of these electrolytes the hydrolysis of a larger 

 proportion of the cement is effected; and the results are increase in tlia 

 volume of the hydrolysed portion, and production of a larger proportion 

 of inert colloids. It has been found that a larger amount of cement can 

 be converted into colloidal matter by the presence of an electrolyte in 

 the water with which it is treated (99), and also that the speed of hydra- 

 tion of cement is affected by the presence and proportion of electrolytes 

 present (84). The fact that a larger amount of laitance appears to be 

 formed in sea-water construction also seems to bear out this theory. 



Besides the reactions mentioned, set cement is subject to the replace- 

 ment of silicic acid by carbonic acid, as are the natural rocks. Especially 

 is this true in cases where the cement comes into contact with marsh and 

 peaty waters and waters containing ferrous carbonate, which by transfor- 

 mation to the hydroxide liberates carbon dioxide (24), which has been 

 found to act, not only upon the calcium hydroxide but also upon the 

 silicates and aluminates (28). 



The presence of free acids in water which acts upon the cement is 

 quite destructive, in proportion to the concentration of the acid and to 

 its strength or weakness as an acid. It is quite probable, however, that 

 the liberation of colloidal silica by the action of acids would serve to a 

 great extent as a protective influence against their further action. 



Sewage gases are generally effective by reason of the hydrogen sulphide 

 which they contain. This gas is readily oxidized to sulphuric acid, and 

 then its action is the production of soluble calcium and aluminum sul- 

 phates, which are subsequently leached out from the mass. This action 

 has been found greatest at the surface of the liquid (106). Hydrogen 

 sulphide may also act by converting the iron of the cement into sulphide, 

 and this becomes oxidized into ferrous sulphate and is leached out, or by 

 its expansion causes disruption (28). 



The action of many other inorganic and organic solutions has been 

 observed, but they do not come within the scope of this paper, since they 

 are not met with in natural processes. 



