CHAPTER XIII 



LIME, CEMENT, AND CONCRETE 



197. Quicklime. If calcium carbonate (ordinary limestone) is 

 heated to about 800 F., carbon dioxide is driven off, leaving an 

 oxide of calcium, which is known as quicklime. This has a great 

 affinity for water and slacks upon exposure to moisture. Slacked 

 lime when dry falls into a fine powder. 



Lime mortar is formed by mixing slacked lime with a large propor- 

 tion of sand. Upon exposure to the air this mortar becomes hard by 

 reason of the lime combining with carbon dioxide and forming again 

 calcium carbonate, the product being a sandy limestone. Lime mortar 

 is used in laying brick walls and in structures where the mortar will 

 not be exposed to water, since it will not set, i.e. combine with carbon 

 dioxide, under water. 



198. Cement. When limestone contains a considerable amount of 

 clay, the lime produced is called hydraulic lime, for the reason that 

 mortar made by using it will harden under water. If the limestone 

 contains about 30 per cent of clay and is heated to 1000 K, the 

 carbon dioxide is driven off, and the resulting product, when finely 

 ground, is called natural cement When about 25 per cent of water is 

 added, this cement hardens, because of the formation of crystals of 

 calcium and aluminum compounds. 



If limestone and clay are mixed in the proper proportions, usually 

 about three parts of lime carbonate to one of clay, and the mixture 

 roasted to a clinker by raising it to a temperature approaching 3000 F., 

 the product, when ground to a fine powder, is known as Portland cement 

 The proper proportion of limestone and clay is determined by find- 

 ing the proportions of the particular clay and stone that will make 

 perfect crystallization possible. In the case of natural cement the 

 lime and clay are not present in such proportions as to form perfect 

 crystals, and consequently it is not as strong as Portland cement. 



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