628 CEMENTS, LIMES, AND PLASTERS. 



The increase from seven to twenty-eight days shall not be less than 

 20 per cent. 



The standard sand referred to above is to be obtained from Leighton 

 Buzzard. It must be thoroughly washed, dried, and pass through a 

 sieve of 20X20 meshes per square inch, and must be retained on a sieve 

 of 30X30 meshes per square inch, the wires of the sieve being .0164-inch 

 and .0108-inch respectively. 



Setting-time. (12) There shall be three distinct gradations of set- 

 ting-time, which shall be designated as "quick", "medium", and "slow".* 



Quick. The setting-time shall not be less than ten minutes or more 

 than thirty minutes. 



Medium. The setting-time shall not be less than half an hour or 

 more than two hours. 



Slow. The setting-time shall not be less than two hours or more 

 than five hours.* 



The temperature of the air in the test-room at the time of gauging 

 and of the water used is to be between 58 and 64 F. 



The cement shall be considered as "set" when a needle having a 

 flat end -^g- inch square, weighing in all 2 Ibs., fails to make an im- 

 pression when its point is applied gently to the surface. 



Soundness. (13) The cement shall be tested by the Le Chatelier 

 method, and is in no case to show a greater expansion than 12 milli- 

 meters after twenty-four hours' aeration and 6 millimeters after 7 days' 

 aeration. 



Note. The apparatus for conducting the Le Chatelier test consists 

 of a small split cylinder of spring brass or other suitable metal of 0.5 

 millimeter (.0197 in.) in thickness, 30 millimeters (1.1875 inches) inter- 

 nal diameter, and 30 millimeters high, forming the mold, to which on 

 either side of the split are attached two indicators 165 millimeters (6.5 

 inches) long from the center of the cylinder, with pointed ends. 



In conducting the test the mold is to be placed upon a small piece 

 of glass and filled with cement gauged in the usual way, care being taken 

 to keep the edges of the molds gently together while this operation is 

 being performed. The mold is then covered with another glass plate, 

 a small weight is placed on this, and the mold is immediately placed 

 in water at 58 to 64 F. and left there for twenty-four hours. 



The distance separating the indicator points is then measured and 

 the mold placed in cold water, which is brought to the boiling-point in 



* When a specially slow-setting cement is required the minimum time of 

 setting shall be specified. 



