198 ANNALS NEW YORE ACADEMY OF SCIENCES 



The normal ratio of silica to lime in unset cement may be considered 

 1 to 2.82. In this material we find the ratio 1 to 1.76. TKis indicates a 

 great loss of lime; and it was thought possible, that, by adsorption of 

 lime, this laitance might regain at least a part of its hydraulic proper- 

 ties. Accordingly it was digested for several days with lime water at 

 laboratory temperature, filtered off, carefully washed with distilled water 

 and dried, as was the previous sample, at 100° C. An analysis showed 

 the results tabulated in the second column. The ratio of Si0 2 to CaO 

 had changed to 1 : 2.30. 



Besides direct metathetical reactions between the components of ce- 

 ment and the water solution which always surrounds a mass of hardening 

 cement, adsorption of various materials from this solution is unques- 

 tionably always going on. Were the fine particles of cement inert chem- 

 ically, this would still take place, by virtue of the enormous total surface 

 which they must present. Clay, it has been demonstrated, has the prop- 

 erty of adsorbing ions of C0 3 from solutions of carbonates, and of CI 

 from solutions of chlorides (10). 



The laitance then may, by adsorption of calcium hydroxide given off 

 from the cement adjacent to it, recover some of the lime lost by it. 

 Whether the lime adsorbed restores the original status of constitution is 

 of course mere speculation. The trend of the strength tests shows that 

 this is probably not so, but that the adsorption is not entirely a reversion 

 of the hydrolytic reaction; in other words, that "drowned" cement will 

 probably never recover and attain to the strength it would have had with 

 proper hydration. 



Effect of the presence of clay and dissolved substances. — It is apparent 

 that if the decreased strength be directly referable to the action of the 

 excess water upon the cement, any means of preventing the access of 

 excess water should prevent, if only to a degree, the destructive action. 

 The colloidal nature of clay (6) has been utilized in the water-proofing 

 of concrete, the principle of its action being the formation of continuous 

 gelatinous films throughout the structure, which prevent the passage of 

 water. Although the same problem is not presented in a grout that exists 

 in finished concrete, it is probable that some blanketing action might 

 occur upon the addition of clay to the mixed mass. 



The point was investigated. To correct for the effect of absorption of 

 part of the mixing water by the admixed clay, a consistency test was 

 made upon a sample of cement to which 10 per cent of clay had been 

 added, and it was found to require 4 per cent more water than the same 

 cement used neat. 



The clay mixes were accordingly gaged with 4 per cent more water 



