PHYSICAL PROPERTIES OF PORTLAND CEMENT. 613 



72. Normal test. A pat is immersed in water maintained as near 

 210 C. (70 F.) as possible for twenty-eight days, and observed at 

 intervals; the pat should remain firm and hard and show no signs of 

 cracking, distortion, or disintegration. A similar pat is maintained 

 in air at ordinary temperature and observed at intervals. 



73. Accelerated test. A pat is exposed in any convenient way in an 

 atmosphere of steam, above boiling water, in a loosely closed vessel, 

 for three hours. 



74. To pass these tests satisfactorily, the pats should remain firm 

 and hard, and show no signs of cracking, distortion, or disintegration. 



75. Should the pat leave the plate, distortion may be detected best 

 with a straight-edge applied to the surface which was in contact with 

 the plate. 



76. In the present state of our knowledge it cannot be said that 

 cement should necessarily be condemned simply for failure to pass the 

 accelerated tests, nor can a cement be considered entirely satisfactory 

 simply because it has passed these tests. 



