LIME AND CEMENT INDUSTRIES 739 



28 days, one day in air, 27 days in water, 85.3 pounds per 

 square inch. 



The Roman cements correspond to those classified in this re- 

 port under the head of natural cements. 



Standards have been adopted also in Sweden and Russia. 



MIXING, ETC. 



The proportions of cement, sand and water should be care- 

 fully determined, by weight, the sand and cement mixed dry, and 

 the water all added at once. The mixing must be rapid and 

 thorough, and the mortar, which should be stiff and plastic, 

 should be firmly pressed with a trowel, without ramming, and 

 struck off level; the molds in each instance, while being charged 

 and manipulated, to be laid directly on glass, slate, or some other 

 non-absorbent material. The molding must be completed before 

 incipient setting begins. As soon as the briquets are hard enough 

 to bear it, they should be taken from the molds and kept covered 

 with a damp cloth until they are immersed. For the sake of 

 uniformity, the briquets, both of neat cement and those con- 

 taining sand, should be immersed in water at the end of 24 hours, 

 except in the case of the one day tests. 



Ordinary, fresh, clean water, having a temperature between 

 60 and 70° F, should be used for water of mixture and immer- 

 sion of samples. 



The proportion of water required varies with the fineness, age, 

 or other conditions of the cement, and the temperature of the 

 air, but is approximately as follows: for briquets of neat cement, 

 Portland, about 25$; natural, about 30$. For briquets of one 

 part cement, one part sand, about 15$ of total weight of sand 

 and cement. For briquets of one part cement, three parts sand, 

 about 12$ of total weight of sand and cement. The object is to 

 produce the plasticity of rather stiff plasterer's mortar. 



An average of five briquets may be made for each test, only 

 those breaking at the smallest section to be taken. The briquets 

 should always be put in the testing machine and broken imme- 

 diately after being taken out of the water, and the temperature 

 of the briquets and of the testing-room should be constant be- 

 tween 60 and 70° F. 



The stress should be applied to each briquet at a uniform rate 

 of about 400 pounds per minute, starting each time at 0. With 

 a weak mixture one half the speed is recommended. 



