160 The Philippine Journal of Science ins 



plained — and that if this material after hardening were reground 

 another set could be obtained. 



This leads us to the conclusion that the presence of more water 

 when cement is gauged facilitates hydration and should, there- 

 fore, result in greater strength. This last is contrary to the 

 general opinion on the subject. As a rule, especially for short 

 periods, the addition of more water means lower strength ^^ for 

 briquettes. In concrete practice, very wet mixes are not recom- 

 mended.^* 



Here again are other factors to be considered. The water 

 that remains mixed with the concrete or mortar until setting 

 is complete reduces the strength, because it decreases the density 

 of the material and consequently the cohesion. The water that 

 separates, either by leaking through the forms or rising to the 

 top, carries calcium hydroxide, one of the products of hydration, 

 in solution. Previous work by one of us '^^ has indicated that 

 an agency that removes dissolved calcium hydroxide or inter- 

 feres with the cohesion will lower the strength. Therefore we 

 believe that, although the strength is increased by the use of 

 a higher percentage of water, other factors have a still greater 

 tendency to lower the strength, and consequently the latter is 

 the net result. A series of experiments just started indicates that 

 this conclusion is correct, although sufficient data have not been 

 obtained to justify any definite statement as yet. A series of 

 mortar briquettes was made with gradually increasing amounts 

 of water, starting with the amount calculated from normal con- 

 sistency tests. There was a decrease in strength with increase 

 of water. A second series was made with the same amounts 

 of water, but, before a given mix was molded, it was placed in 

 a metal vessel, and the water was evaporated until the weight 

 showed that the amount indicated by the normal consistency tests 

 was reached. It was assumed that the extra water temporarily 

 present would facilitate hydration and dissolve more calcium 

 hydroxide and that this hydroxide would remain in solution even 

 after a portion of the water was evaporated, because the solu- 

 tion was not near the saturation point, even though the solubility 

 of this compound decreases with a rise in temperature. After 

 the evaporation each mix was immediately regauged and placed 

 in molds. In general, the strength increased as the water in- 

 creased, contrary to the first series. 



^Cf. Larned, E. S., Proc. Am. Soc. Test. Mat. (1903), 3, 401. 

 " Cf. Taylor and Thompson, A Treatise on Concrete. John Wiley and 

 Sons, New York (1917), 251. 



"Witt, J. C, This Journal, Sec. A (1916), 11, 288. 



