LIME AND CEMENT INDUSTRIES 725 



difference of the readings on the index before and after the im- 

 pact. The falling body is a hollow metal cylinder, weighing 1 

 pound, of which the exterior diameter is about equal to the length. 

 This cylinder in its descent passes freely over the spindle and 

 strikes on the shoulder attached just above the screw. 



Another device used by Gen. Gilmore was a y^- inch wire with 

 & flat end, and loaded with J of a pound, and a ^ inch wire also 

 loaded with 1 pound. These were used on cakes of neat cement, 

 2 or 3 inches in diameter, -J inch thick at the center, and ^ at the 

 edges. One cake was left in the air and one in the water. The 

 time at which the loaded wire ceased to penetrate the pat was 

 noted. 



In England those Portland cements are called quick setting 

 which will bear the yV inch needle loaded with 4 ounces in 10 

 minutes after mixing with water, and to be slow setting if they 

 require 30 minutes or more, up to 5 hours. If they will not 

 bear the weight of the needle after this period, the cement is 

 rejected. 



Still another test is that with Vicat's needle. The needle has 

 a cross-section of 1 millimeter and bears a weight of 10J ounces. 

 The depth to which the needle penetrates the cement is read off 

 on a scale. 



A quick setting cement may begin within a few minutes after 

 wetting, while a slow setting one may not begin till 24 hours 

 after it has been wet, though, when once begun, the setting usu- 

 ally goes on rapidly. 



Setting is always accompanied by a slight rise in temperature, 

 which continues while the setting is going on. The rise in tem- 

 perature is less in slow setting cements. As the setting of cement 

 is also influenced by the temperature of the air, and water, it is 

 recommended by Gary that, in order to obtain comparable re- 

 sults, the tests should be made at a mean temperature of 15° to 

 18° C. 1 



1 Trans. Am. soc. civ. eng. 30 : 11. 



