SPECIFICATIONS FOR PORTLAND CEMENT. 631 



which no addition greater than 3 per cent has been made subsequent 

 to calcination. 



19. Specific gravity. The specific gravity of the cement, thoroughly 

 dried at 100 C., shall be not less than 3.10. 



20. 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. 



21. Time of setting. It shall develop initial set in not less than 

 thirty minutes, but must develop hard set in not less than one hour, 

 nor more than ten hours. 



22. 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 hi strength within the periods 

 specified: 



NEAT CEMENT. 



Age. Strength. 



24 hours in moist air 150 200 Ibs. 



7 days (1 day in air, 6 days in water) . 450 550 ' ' 



28 days (1 day in 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 ' ' 



23. 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. 



(6) Another pat is kept in water maintained as near 70 F. as prac- 

 ticable, 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. 



24. These pats, to satisfactorily pass the requirements, shall remain 

 firm and hard and show no signs of distortion, checking, cracking or 

 disintegration. 



25. Sulphuric acid and magnesia. The cement shall not contain 

 more than 1.75 per cent of anhydrous sulphuric acid (SOs), nor more 

 than 4 per cent of magnesia (MgO). 



