PORTLAND CEMENT 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 cal careous 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 S 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: 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-fou• hours.
(a) A pat is then kept in air at normal temperature and ob served 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 2S days.
(c) A third pat is exposed in any convenient way in an atmos phere 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 con tain more than 1.75 per cent of anhydrous sulphuric acid 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 "home made" machines which serve their purpose with sufficient accuracy.
One very common type of machine is illustrated in Fig. 6. B is a reser voir containing shot, which falls through the pipe I, which is closed with a valve at the bottom. The briquette is carefully placed be tween the clips, as shown in the figure, and the wheel P is turned until the indi cators 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 re sults will be obtained by allowing the shot to run slowly into the cup. The crank is then turned with just suf ficient 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.