SAND AND CEMENT 193 



LIMES 



The commercial varieties of lime may be classified as 

 common, hydrated, and hydraulic. The common limes, 

 also called quicklimes, may be subdivided into rich, or fat, 

 lime, and meager, or poor, lime. 



Common Limes. The grade of common lime known as 

 fat, or rich, lime is almost pure oxide of calcium, CaO, and 

 contains only about 5% .of impurities. It has a specific 

 gravity of about 2.3 and a great affinity for water, of which 

 it absorbs about one-quarter of its weight. This absorp- 

 tion is accompanied by a great rise in temperature, by the 

 lime bursting, and by the giving off of vapor. The lime 

 finally crumbles into a powder. This powder occupies 

 from two and one-half to three and one-half times as much 

 volume as the original lime, the exact amount depending on 

 its initial purity. When the lime is in this plastic state, it is 

 said to be slaked. It is then unctuous and soft to the touch, 

 and from this peculiarity it derives the name of fat or rich. 



Meager, or poor, lime consists of from 60 to 90% of pure 

 lime, the remainder being impurities, such as sand or other 

 foreign matter. These impurities have no chemical action 

 on the lime, but simply act as adulterants. Compared with 

 fat lime, poor lime slakes more slowly and evolves less heat. 

 The resulting paste is also thinner and not so smooth, 

 greatly resembling fat slaked lime mixed with sand. Poor 

 lime is not so good for building purposes as fat lime, nor 

 has it such extensive use. 



Hydrated Lime. The class of lime called hydrated lime 

 (calcium hydrate) is merely thoroughly slaked fat lime dried 

 in the form of a fine powder, Ca(OH) 2 . It is used exten- 

 sively in conjunction with cement for making mortar, and 

 also in the sand-lime brick industry. 



Hydraulic Limes. Limes that contain enough quick- 

 lime to slake when water is added, and enough clay or 

 sand to form a chemical combination when wet, thus giving 

 them the property of setting under water, are called hydraulic 

 limes. 



Limes of this class are made by burning limestones con- 

 taining from 5 to 30% of clay or sand. They are often 



