208 



ANNALS NEW YORK ACADEMY OF SCIENCES 



may afterwards affect the metamorphism of cement. It has been pointed 

 out by Goldbeck (43) and by White (108) that the expansion' or con- 

 traction of concrete depends upon whether the concrete remains wet or 

 dry, and that the strains caused by alternate wetting and drying of con- 

 crete may be a more fruitful cause of cracks than temperature changes. 

 The presence of an optimum quantity of water is necessary, however, 

 so that the proper reactions take place in the mass of setting cement, in 

 order that the strength may increase normally. 



QUALITY OF WATER AT FIRST ADDED 



Cornpressive strengths of neat cements gaged ivitli various solutions. 

 A normal Portland cement was mixed with the proper quantity of 

 water (21 per cent by weight) in which was dissolved, in the different 

 tests, varying concentrations of the salts indicated in the subjoined table. 

 The cement was worked for one minute, and the plastic mass was tamped 

 into glass cylinders approximately one inch in diameter, with the utmost 

 precaution to avoid all air bubbles and at the same time to subject all 

 specimens to the same pressure. 



TABLE 18 



Compressi'veJStrenfftTis of Neat Cement Mixed iritli Solutions of Various Salts 

 (Age of specimens, 28 days. Average of two determinations) 



8 This water contained: CaO. 1177 parts per million. 

 MgO, 226 

 S0 3 , 408 

 Cl, 4360 



