LIME, CEMENT, AND CONCRETE 249 



much free lime, the ball will disintegrate and show signs of crumbling. 

 The ball of cement is kept under a damp cloth for twenty-four 

 hours before boiling. This test is not regarded with favor by many 

 engineers. 



201. Test of fineness. If the grinding has not been properly done, 

 large particles of clinker remain, which act as a sand or other foreign 

 substance and thus weaken the cement. The test for fineness is made 

 by sifting the cement through different sieves ; usually all of it is 

 required to pass through a sieve of 50 meshes to the inch, and a 

 smaller amount through sieves of 80 and 100 meshes. About 75 per 

 cent should pass through a 200-mesh sieve (see Article 206). 



202. Test of time of setting. It is important that a cement should 

 not set too quickly or too slowly. A test for time of setting, known 

 as Gillmore's test, has been standardized in the United States, and 

 consists in applying to a small cement pat given weights supported 

 by points of specified area (Fig. 154). The cement pat is made by 

 mixing a portion of neat cement with the proper amount of water, 

 mounting this on a piece of glass, and smoothing it until the middle 

 is half an inch thick and the edges are smooth and tapering. The 

 pat is then kept under a damp cloth to prevent injury by sudden 

 chants in temperature, or too high temperature, of the surrounding 

 air. When this pat will hold without appreciable indentation a 

 quarter-pound weight supported by a wire ^ in. in diameter, it is 

 sai<l to have acquired its initial set. It is said to have acquired its 

 final set when a one-pound weight supported by a wire ^ in. in 

 diainrtcr will not appreciably indent the surface. 



When a pat prepared as indicated above checks or warps, it 

 indiraU's that the cement in setting changes volume too rapidly. 

 l-'.-r many pieces of work a slow-setting cement cannot be used; but 

 a cement which sets too quickly is likely to contain too much free 

 lime, and should be very carefully tested before being used. In 

 general, the time of final setting for natural cement should not be 

 less than thirty minutes nor more than three hours. 



The table given on page 250 shows the time of setting of different 

 1 .rands of cement* The student is also referred to the standard speci- 

 fications for cement given in Article 206. 



*Watertown Arsenal Report, 1901. 



