525
CEMENT--ABSORPTION OP WATER.
Absorption op Water bt Grout Mixtures and Tests• op Grouts.
Cement to which water at ordinary atmospheric temperatures has been added in sufficient quantities to at first produce a grout, may subsequently thicken, the water disappearing apparently by absorption in the mixture. The interval of time necessary to accomplish this result varies with the amount of water used in a given kind of cement.
Experiments with Star Portland cement, with and without plaster, gave the following results:
Cement with plaster.		Cement without plaster.	
Quantity of water used.	Interval required for absorption.	Quantity of water used.	Interval required for absorption.
Per cent.	Hours.	Per cent.	Hours.
40		40	if
50		50	74
60	11	60	15
		70	30£
80	34	80	44
100	45		
In the above cases the full amount of water disappeared below the surface of the mixtures at the times stated. The mixtures were each stirred occasionally after the water was added. A moderate amount of stirring promoted the absorption of the water, while vigorous stirring sensibly retarded this action, and temporarily reduced the consistency of the grout. The interval of time required for the absorption of the water is necessarily approximately given, since the period is influenced by the manipulation, and some uncertainty exists as to the time when the period shall be judged to have reached its end.
Bearing upon this kind of phenomena, an examination of the details of the tests on retarded sets shows that at certain stages copious quantities of water were needed to maintain the desired consistency of the mixture. Thus with the Star Portland cement, with plaster, the interval between 4J and 9£ hours after initial mixing was characterized by the addition of considerably more water than at any subsequent period. A large quantity of water was initially used with the same brand of cement without' plaster in its composition, and this does not show so marked a period subsequently. With the Whitehall, 64 hours series, the interval of time between 4 hours and 20 hours was one in which much water was added. In the ordinary mixing of cement similar behavior is met with, and while in some instances definiteness is lacking, there appears reason for believing that these intervals mark different stages in the chemical activity of the material.
The tests which next follow were upon the material which was mixed with different percentages of water and allowed to remain in the mixing bed until the water used was absorbed. The mixtures were then put into molds, set in air, and tested when 30 days old.