PACIXI, METAMORPHI8M OF PORTLAND CEMENT 



193 



TABLE 5 

 Setting Time in Laboratory Air and in Damp Closet 



From these results, it will be seen that setting in a relatively dry- 

 atmosphere takes place in a shorter time than in a damp one; also that 

 the setting time is more uniform under conditions of high atmospheric- 

 humidity. 



At the same temperature, evaporation takes place more rapidly in the 

 former case ; and allowing a cement mix to stand in such a position that 

 evaporation of the mixing water may readily take place is practically 

 equivalent to the use of an insufficient amount of mixing water. 



Effect of excess of mixing water on strength of concrete. Concrete is 

 often mixed so wet that, as it is filled into forms to a depth of several 

 feet, the water rises above the concrete and throws out considerable lai- 

 tance from the cement. The ease of mixing and placing very wet con- 

 crete is the constant incentive for its use. This practice, however, is 

 followed by a great deal of deterioration of the concrete in strength. 



The strength rapidly decreases with the increase in the quantity of 

 water used in mixing. The visible effect of this weakening is the forma- 

 tion of laitance, which has little or no setting power or strength, and 

 which represents the loss of an active part of the cement, since, as is 

 recognized, the finer parts are more hydraulically active. 



Tests were made by mixing concrete at normal consistency and shovel- 

 ing one-half the batch into a tank containing three to four inches of 

 water, the depth of concrete being about four inches. The water rose to 

 about an equal depth above the concrete. In test Xo. 1, the concrete was 

 allowed to settle in water four inches in depth for 30 minutes, when the 

 excess of water was siphoned off and the remaining material poured into 

 molds. In test Xo. 2, the depth of water in the tank was three inches, 

 and the water was siphoned off immediately while in agitation. In test 

 Xo. 3, the same process was repeated, except that the depth of the water 



