CHAPTER VIII. 

 COMPOSITION AND PROPERTIES OF LIME. 



THE lime (or " quicklime ") resulting from the burning of a pure 

 limestone is a white solid, with a specific gravity of 3.09 to 3.15. As 

 packed, it will weigh about 60 Ibs. per cu. ft., or 70-75 Ibs. per bushel. 

 When made from a limestone rock, the lime should be in lumps, the 

 occurrence of powder or dust proving that the lime has been exposed 

 to the air so much since burning that air-slaking has begun. When 

 the lime has been made from shells, marl, highly crystalline marbles, 

 soft chalk, or shelly limestones, however, it will often come from the 

 kilns in small fragments, which in this case is no sign of deterioration. 



If the raw material is impure, containing much clayey matter or 

 iron oxide, the resulting lime will not be white, but will vary from 

 yeilowish to gray or brown in color, according to the amount and kind 

 of impurities present. It will also, in general, slake much slower than 

 would a purer product. 



High-calcium vs. magnesian limes. The relative merits of these 

 two classes have been frequently discussed in text-books and technical 

 journals, and are still subjects of controversy. The facts of the case, 

 however, seem to be simple enough and may be summarized as follows: 



High-calcium limes slake rapidly on the addition of water, and evolve 

 much heat during slaking They also expand greatly, giving a large 

 bulk of slaked lime. Magnesian limes slake very slowly, and evolve 

 very little heat during the process. Their expansion is also less; so 

 that, taking equal weights, they give less bulk of slaked lime. 



Owing to the slowness and coolness with which the magnesian limes 

 slake, there is some danger that the average mortar-mixer * will not 

 give them sufficient time to slake thoroughly. Owing to the fact that 

 they make less bulk of slaked product than do the high-calcium limes, 

 the average contractor or builder thinks that they are too expensive. 

 But, on the other hand, they are very much stronger in long-time tests 



* A human, not mechanical, mortar-mixer is here spoken of. 



115 



