66. Definition

This term is applied to the finely pulverized product resulting from the calcination to incipient fusion of an intimate mixture of properly proportioned argillaceous and calcareous materials, to which no addition greater than 3 per cent has been made subsequent to calcination.

67. Specific Gravity

The specific gravity of the cement, thoroughly dried at 100° C, shall be not less than 3.10.

68. Fineness

It shall leave by weight a residue of not more than 8 per cent on the No. 100 sieve, and not more than 25 per cent on the No. 200 sieve.

69. Time Of Setting

It shall develop initial set in not less than thirty minutes, and must develop hard set in not less than one hour nor more than ten hours. .

70. Tensile Strength

The minimum requirements for tensile strength for briquettes one inch square in section, shall be within the following limits, and shall show no retrogression in strength within the periods specified:

Neat Cement

Age

Strength

24 hours in moist air..

150-200

lbs.

7 days (1 day in moist air, 6 days in water).

450-550

,,

28 days (1 day in moist air, 27 days in water).

550-650

,,

One Part Cement, Three Parts Sand

7 days (1 day in moist air, 6 days in water).

150-200

,,

28 days (1 day in moist air, 27 days in water).

200-300

,,

Constancy of Volume. Pats of neat cement about three inches in diameter, one-half inch thick at the center, and tapering to a thin edge, shall be kept in moist air for a period of twenty-four hours.

(a) A pat is then kept in air at normal temperature and observed at intervals for at least 28 days.

(b) Another pat is kept in water maintained as near 70° F. as practicable, and observed at intervals for at least 28 days.

(c) A third pat is exposed in any convenient way in an atmosphere of steam, above boiling water, in a loosely closed vessel, for five hours.

These pats, to pass the requirements satisfactorily, shall remain firm and hard, and show no signs of distortion, checking, cracking, or disintegrating.

71. Sulphuric Acid And Magnesia

The cement shall not contain more than 1.75 per cent of anhydrous sulphuric acid (S03), and not more than 4 per cent of magnesia (MgO).

72. Testing Machines

There are many varieties of testing machines on the market. Many engineers have constructed "homemade" machines which serve their purpose with sufficient accuracy. One very common type of machine is illustrated in Fig. 6. B is a reservoir containing shot, which falls through the pipe I, which is closed with a valve at the bottom. The briquette is carefully placed between the clips, as shown in the figure, and the wheel P is turned until the indicators are in line. The hook lever Y is moved so that a screw worm is engaged with its gear Then open the automatic valve J so as to allow the shot to run into the cup F. By means of a small valve, the flow of shot into the cup may be regulated. Better results will be obtained by allowing the shot to run slowly into the cup. The crank is then turned with just sufficient speed so that the scale beam is held in position until the briquette is broken. Upon the breaking of the briquette, the scale beam falls, and automatically closes the valve J. The weight of the shot in the cup F then indicates, according to some definite ratio, the stress required to break the briquette.