PORTLAND CEMENT.
17. Definition. This term is applied to the finely pulverised product resulting from the calcination to incipient fusion of an intimate mixture of properly pro portioned argillaceous and calcareous materials, and to which no addition greater than 3 per cent has been made subsequent to calcination.
18. Specific Gravity. The specific gravity of the cement, ignited at a low red heat, shall be not leas than 3.10; and the cement shall not show a loss on ignition of over 4 per cent.
19. Fineness. It shall leave by weight a residue of not more than 8 per cent on the No. 100, and not more than 25 per cent on the No. 200 sieve.
20. Time of Setting. It shall not develop initial set in leas than thirty minutes, and must develop hard set in not less than one hour nor more than ten hours.
21. 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: 22. Constancy of Volume. Pats of neat cement about three inches in dia
meter, 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.
23. These pats, to satisfactorily pass the requirements, shall remain firm and hard and show no signs of distortion, checking, cracking, or disintegrating.
24. Sulphuric Acid and Magnesia. The cement shall not contain more than 1.75 per cent of anhydrous sulphuric acid (SO.), nor more than 4 per cent of magnesia (MgO).