Testing of Cement

This consists of a frame K, Fig. 3, bearing a movable rod L, with the cap A at one end, and at the other the cylinder B, 1 cm. (0.39 in.) in diameter, the cap, rod, and cylinder weighing 300 grams (10.58 oz.). The rod, which can be held in any desired position by a screw F, carries an indicator, which moves over a scale (gradu ated to centimeters) attached to the frame K. The paste is held by a conical, hard-rubber ring I, 7 cm. (2.75 in.) in diameter at the base, 4 cm. (1.57 in.) high, resting on a glass plate J, about 10 cm. (3.94 in.) square.

In making the determination, the same quantity of cement as will subsequently be used for each batch in making briquettes for testing strength (but not less than 500 grams), is kneaded into a paste in the manner described below under the heading "Mixing for Tests." This paste is quickly formed into a ball with the hands, the operation being completed by tossing the ball six times from one hand to the other, maintained 6 inches apart. The ball is then pressed into the rubber ring, through the larger opening, smoothed off, and placed (on its large end) on a glass plate, and the smaller end smoothed off with a trowel. The paste, confined in the ring, resting on the plate, is then placed under the rod bearing the cylinder, which is brought in contact with the surface, and quickly released.

The paste is of normal consistency when the cylinder penetrates to a point in the mass 10 mm. (0.39 in.) below the top of the ring. Great care must be taken to fill the ring exactly to the top.

The trial pastes are made with varying per centages of water until the correct consistency is obtained.

The Committee has recommended, as normal, a paste consistency of which is rather wet, because it believes that variations in the amount of compression to which the briquette is sub jected in moulding are likely to be less with such a paste.

Having determined in this manner the proper percentage of water required to produce a paste of normal consistency, the proper percentage required for the mortars is obtained from an empirical formula.

The determination of such a formula is a matter on which no universal and final agree ment has yet been reached. In the meantime, until a standard formula has been worked out, the amount of water to be used may be taken as that indicated in Table IX, which shows the percentage of water required for a 1 : 3 mixture.

This percentage will vary, where different cements are used, according to the percentage that may be necessary for giving normal con sistency to the neat cement paste, as already determined.

For example, if the neat cement requires 18 per cent of water for normal consistency, then 8.5 per cent of water will be needed for the 1 : 3 mortar mixture. The percentages of water indi cated are percentages by weight, being based on the weight of the dry material.

Time of Setting.

The object of this test is to determine the time which elapses from the moment water is added until the paste ceases to be fluid and plastic (called the initial set), and also the time required for it to acquire a certain degree of hardness (called the final or hard set). The former of these is the more important, since, with the commencement of setting, the process of crystallization or hardening may be said to begin. As a disturbance of this process may produce a loss of strength, it is desirable to com plete the operation of mixing and moulding or incorporating the mortar into the work, before the cement begins to set.

It is usual to measure arbitrarily the begin ning and end of the setting by the penetration of weighted wires of given diameters. For this purpose the Vicat needle, already described, should be used. A paste of normal consistency is moulded and placed under the rod L, Fig. 3, this rod bearing the cap D at one end, and the needle 1 mm. (0.039 in.) in diameter, at the other, weighing 300 grams (10.58 oz.). The needle is then carefully brought in contact with the surface of the paste, and quickly released.

The setting is said to have commenced when the needle ceases to pass a point 5 mm. (0.20 in.) above the upper surface of the glass plate, and is said to have terminated the moment the needle does not sink visibly into the mass.

The test pieces should be stored in moist air during the test. This is accomplished by placing them on a rack over water contained in a pan and covered with a damp cloth, the cloth to be kept away from them by means of a wire screen; or they may be stored in a moist box or closet.