LIME AND CEMENT INDUSTRIES 673 



tliej could obtain a stiff paste which possessed valuable prop- 

 erties. 



Pure lime has 71.4^ calcium and 28.6^ oxygen. It is a porous, 

 earthy, white solid, which when pure resists a high degree of heat. 

 It absorbs both moisture and carbonic acid from the air with the 

 greatest avidity. 



Richardson gives the following requirements for caustic lime 

 when used for mortar.-^ 



Except when made from coarsely crystalline marble or from 

 marl or shells it should be in hard lumps. 



It should be white, or nearly so, in color. Lime of a yellow 

 or brownish color with veins of silicious matter is inferior. 



It should be free from fused or semi-fused stone which shows 

 over-burning, and from unburnt ash of fuel or clinker. 



It should contain less than 10^ of impurities but often has 

 more. 



It should slake rapidly, showing that it is rich and fresh. 



Good lime in lumps should weigh, as packed, with about 40;^ of 

 voids, 60 lb a cubic foot, 75 lb a bushel, and from 220 to 

 230 lb per 3 bushels. If ground or in powder, it will weigh less 

 when packed loosely, but when well shaken dowm it will weigh 

 as much as 270 lb a bbl. A lump of hard lime one foot cube 

 would weigh about 95 lb, having a density of 1.52. 



Slaking 



Lime combines with water with evolution of heat and every 

 100 parts of lime takes 32 parts of water. If 33^ of its weight 

 in water is sprinkled on lime it heats, cracks open and falls to 

 powder. 



The increase in volume in slaking is caused by the expansive 

 force of the steam, but lime may be slaked without increasing its 

 volume by passing dry steam over it in a tube. The energy of 



1 Brickbuilder. 1897. p. 78. 



