CEMENT. 69 



If the minimum strength is not specified, the mean of the above 

 values shall be taken as the minimum strength required. 



305. Constancy of Volume. Pats of neat cement about 

 3 in. in diameter, J in. thick at center, and tapering to a thin 

 edge, shall be kept in moist air for a period of 24 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 practicable, and observed at intervals for at least 

 28 days. 



(c) A third pat is exposed in any com&enient way in an 



atmosphere of steam, above boiling water, in a loosely 

 closed vessel for 5 hours. 



(d) These pats, to satisfactorily pass the requirements, shall 



remain firm and hard and show no signs of distortion, 

 checking, cracking, or disintegrating. 



306. Sulphuric Acid and Magnesia. The cement shall not 

 contain more than 1.75 per cent of anhydrous sulphuric 

 acid (80s) , nor more than 4 per cent of magnesia (MgO) . 



