524 CEMENT--------WITH AND WITHOUT “ EESTBAINEB. ”
THE BEHAVIOR OF CEMENTS DURING MIXING WITH AND WITHOUT THE PRESENCE OF A “RESTRAINER.”
Tests made under the preceding heading׳ on retarded sets include those on the strength of a Portland cement of the usual commercial mixture containing plaster of paris; also some of the same brand, kindly supplied by the manufacturers by request, without the addition of the “restrainer,” so-called. The setting properties, as shown by the method with indentation needles, were modified in the manner desired, although the differences in the rate of setting were less conspicuous when a mixture of thin consistency was used.
Concerning the interval of time elapsing from the gauging of grout mixtures to the time when a state of plasticity was acquired and the absorption of the full amount of water present occurred, the cement containing plaster was the earlier to attain these phases when 60 per cent or more water was used. A conspicuous difference in behavior appeared between the two kinds of cement when, in their earlier days after grinding, the material was mixed with water having its temperature near the freezing point. The cement without plaster was mixed with 40 per cent water bj׳■ weight at the temperature of about 32° F. The cement was poured into a bucket containing the water, stirring the mixture during the operation. There was a momentary interval after the full amount of cement had been added, during which the mixture had the consistency of a thin mortar or very thick grout. A few seconds later the mixture suddenly thickened, and when moved about separated into a granular, mealy mass. Using 50 per cent of water the result was the same as before; a dry, mealy mass was formed. With 60 per cent of water the mixture thickened immediately after the cement was all in, but did not attain so dry and thick a consistency as with the lower percentages of water. With higher percentages of water the mixtures became ordinary grouts.
The behavior described was characteristic of the cement without plaster. Corresponding experiments made with plaster additions present in the cement did not show the granular, mealy state in the mixtures, the results being ordinary grouts. These two kinds of cement were each 2 months old from the time of grinding, when the above observations were made. One month later the peculiar characteristics of the cement without plaster had largely disappeared. The 40 per cent mixture displayed enfeebled traits as before, and the 50 per cent mixture required 16 minutes after mixing to acquire a pasty state. Later examinations of the material showed it still retained this trait, which now displayed itself when a less amount of water was used than above mentioned. No observations were made upon cement less than 2 months old. The general results tend to associate this behavior with the absorption properties developed by cements in a greater or less degree after water has been added.
The compressive tests showed greater strength possessed by the material containing the plaster addition, so far at least as pertains to the time of setting covered by these experiments, and during the earlier periods of the series on retarded sets.