REENFORCED CONCRETE 157 



combined with it is said to be slaked. Slaked lime when dry falls 

 into a fine powder. 



Lime mortar is formed by mixing slaked lime with a large pro- 

 portion 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 lime- 

 stone. 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, 

 that is, combine with carbon dioxide, under water. 



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 F., 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 aluminium 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 finding 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 propor- 

 tions as to form perfect crystals, and consequently it is not as strong 

 as Portland cement. 



The artificial mixing of the limestone and clay in the manufac- 

 ture of Portland cement is accomplished in different ways. Through- 

 out the north central portion of the United States large beds of marl 

 are found, and also in the same localities beds of suitable clay. 

 This marl is nearly pure limestone and is mixed with the clay 

 when wet. (These materials are also mixed dry.) Both the marl 

 and clay are pumped to the mixer, where they are mixed in the 

 proper proportions. The product is then dried, roasted, and ground. 

 Most American Portland cements, however, are made by grinding 

 a clay-bearing limestone with sufficient pure limestone to give the 



