Chemical Analysis

To determine the rate of setting, points have been arbitrarily fixed at which the set is said to begin and to end. It is very difficult to determine these points with exactness, particularly the latter; but an exact determination is not necessary to judge of the fitness of a cement for a particular use. For this purpose it is ordinarily sufficient to say that a mortar has begun to set when it has lost its plasticity, i.e., when its form can not be altered without producing a fracture; and that it has set hard when it will resist a slight pressure of the thumb-nail. Cements will increase in hardness long after they can not be indented with the thumb-nail.

To obtain uniform results the mortar should have a definite plasticity, and to obtain results comparable with those found by others, mortar of a standard plasticity should be employed. For the method of making a mortar of standard plasticity, see * 161-65.

There are two methods or forms of instruments used in making this test, viz.t Gillmore's and Vicat's. The former is more frequently used, apparently because of the cheaper and simpler apparatus required; but the latter is used in the better equipped laboratories. Both forms of apparatus are made by all manufac turers of cement-testing appliances.

Gillmore's Test.

Mix the cement with water to a plastic mortar (see § 161-65), and make a cake or pat 2 or 3 inches in diameter and about inch thick. The mortar is said to have begun to set when it will just support a wire -inch in diameter weighing } pound and to have "set hard" when it will bear a -inch wire weighing 1 pound. The interval between the time of adding the water and the time when the light wire is just supported is the time of beginning to set, and the interval between the time the light wire is supported and the time when the heavy one is just supported is the time of setting.

Vicat's Test.

The apparatus consists of two parts: 1. A stand supporting an arm through which a rod weighing 300 grams (10.58 oz:) slides freely and carries in its lower end a penetrating needle having a cross section of a square millimeter (0.0006 sq. in.). The rod carries an index which moves over a graduated scale and by which the depth of penetration is read. 2. A vulcanite ring having a height of 4 centimeters (1.57 inches), and a diameter at one end of 7 centimeters and at the other of 6 centimeters.

In making the test, not less than 500 grams of cement are mixed to a paste of the normal consistency (see § 161-65) and formed into a ball; and then the mixing should be completed by tossing the ball six times from one hand to the other, a distance of 6 inches. The

ball is next pressed into the vulcanite ring, through the larger open ing, and smoothed off; then the ring is placed, small end up, on a glass plate and the top is smoothed off with a trowel. The paste, confined in the ring and resting on the glass plate, is now placed on the base of the instrument under the arm carrying the sliding rod, and the penetrating needle is brought into contact with the surface of the paste and quickly released. At first the needle will penetrate to the glass, in which case the index should read zero, provided glass of the proper thickness has been used; but as a precaution the index should be read and recorded when the needle rests upon the glass. The set is said to have commenced when the needle comes to rest 5 millimeters above the glass; and the cement is said to have "set hard" when the needle no longer sinks visibly into the mass. Care should be taken to keep the needle clean, as the collection of cement on its sides decreases the penetration, while cement on the point reduces the area and tends to increase the penetration.

As a rule the values by the Vicat needle are only about two thirds as great as those by the Cillmore needles. Usually specifications do not say which method is to be employed. However, the exact time of set is of no great importance, and a determination by either method is subject to a considerable error; besides, in practice mortars are mixed wetter than in laboratory practice and are also mixed with sand, and for these two reasons the mortar used in ordinary con struction will require six to eight times as long to set as that em ployed in laboratory tests.

Elements Affecting Activity.

The determination of the time of set is only approximate, since it is affected by the temperature of the mixing water, the temperature and humidity of the air during the test, the percentage of water used, and the amount of moulding the paste receives. It is usually specified that the water and air shall be from 60° to 65° F. The higher the temperature, the more rapid the set. The test pieces should be stored in moist air during the test, either by being placed on a rack over water contained in a pan and covered with a damp cloth, the cloth being kept away from the test pieces by a wire screen, or better by being stored in a moist box or closet.