47. Specific Gravity. The specific gravity of cement is lowered by under-burning, adulteration, and hydration, but the adulteration must be in considerable quantities to affect the results. Since the dif ferences in specific gravity are usually very small, great care must be exercised in making the tests. When properly made, the tests afford a quick check for under-burning or adulteration. The determination of specific gravity is conveniently made with Le Chatelier's apparatus. This consists of a flask D, Fig. 1, of 120-cu. cm. (7.32-cu. in.) capac ity, the neck of which is about 20 cm. (7.S7 in.) long; in the middle of this neck is a ball C, above and below which are two marks F and E; the volume between these marks is 20 cu. cm. (1.22 cu. in.). The neck has a diameter of about 9 mm. (0.35 in.), and is graduated into tenths of cu. cm. above the mark F. Benzine (62° 13aume naphtha), or kerosene free from water, should be used in making the determina tion.
The specific gravity may be determined in two ways: First. The flask is filled with either of these liquids to the lower mark E, and 64 gr. (2.25 oz.) of powder, previously dried at 100° Cent. (212° Fahr.) and cooled to the temperature of the liquid, is gradually introduced through the funnel B (the stem of which extends into the flask to the top of the bulb C) until the proper mark F is reached. The difference in weight between the cement remaining and the original quantity (64 gr.) is the weight which has displaced 20 cu. cm.
Second. The whole quantity of powder is introduced, and the level of the liquid rises to some division of the graduated neck. This reading pluS 20 cu. cm. is the volume displaced by 64 gr. of the powder. The specific gravity is then obtained from the formula: The flask during the operation: is kept in water in a jar A in order to avoid variation in the temperature of the liquid. The results should agree within 0.01.
48. Fineness. It is generally accepted that the coarser mate rials in cement are practically inert, and it is only the extremely fine powder that possesses adhesive cementing qualities. The more finely cement is pulverized, all other conditions being the same, the more sand it will carry and produce a mortar of a given strength. The de gree of pulverization which the cement receives at the place of manu facture is ascertained by measuring the residue retained on certain sieves. Those known as No. 100 and No. 200 sieves are recommended for this purpose. The sieve should be circular, about 20 cm. (7.87 inches) in diameter, 6 .cm. (2.36 inches) high, and provided with a pan 5 cm. (1.97 inches) deep, and a cover. The wire cloth should be woven from brass wire having the following diameters: No. 100, 0.0045 inches; No. 200, 0.0024 inches. This cloth should be mounted on the frame without distortion. The mesh should be regular in
spacing and be within the following limits: 50 grams (1.76 oz.) or 100 gr. (3.52 oz.) should be used for the test and dried at a temperature of 100° Cent. or 212° Fahr., prior to sieving.
The thoroughly dried and coarsely screened sample is weighed and placed on the No. 200 sieve, which, with pan and cover attached, is held in one hand in a slightly inclined position, and moved forward and backward, at the same time striking the side gently with the palm of the other hand, at the rate of about 200 strokes per minute. The operation is continued until not more than of 1 per cent passes through after one minute of continuous sieving. The residue is weighed, then placed on the No. 100 and the operation repeated. The work may be expedited by placing in the sieve a small quantity of large shot. The results should be reported to the nearest tenth of 1 per cent.
49. Normal Consistency. The use of a proper percentage of water in making the pastes, cement and water, from which pats, tests of setting, and briquettes are made, is exceedingly important, and af fects vitally the results obtained. The determination consists in meas uring the amount of water required to reduce the cement to a given state of plasticity, or to what is usually designated the normal consist ency. Various methods have been proposed for making this deter mination, none of which has been found entirely satisfactory. The Committee recommends the following: The apparatus for this test consists of a frame K, Fig. 2, 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, aid cylinder weighing 300 gr. (10.5S oz.). The rod, which can be held in any desired position by a screw F, carries an indicator, which moves over a scale (graduated to centimeters) attached to the frame K. The paste is held by a conical, hard-rubber ring I, 7 cm. (2.76 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 be subsequently used for each batch in making the briquettes (but not less than .500 grams) is kneaded into a paste, as described later in paragraph on "Mixing," and quickly formed into a ball with the hands, completing the operation by tossing it 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 placed under the rod bearing the cylinder, which is brought in contact with the surface and quickly released.