MORTAR, CEMENT, AND CONCRETE 



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Marble is a metamorphic rock formed from limestone by 

 pressure of overlying rock and by the internal heat of the earth. 

 Marble may be white or colored, according to the limestone from 

 which it is formed (Fig. 286). It is one of the most beautiful 

 and highly prized of the building stones, but because of its scar- 

 city and cost, it is used mainly for ornamental purposes. Marble 

 does not weather so easily as 

 sandstone, but it weathers 

 more quickly than granite. 

 The chief source of marble in 

 the United States is Vermont. 



Mortar, cement, and con- 

 crete. Rocks could not be 

 used satisfactorily for build- 

 ing purposes unless there 

 were some substance to fasten 

 them firmly together. The 

 stones of a house would be 

 loose and unstable unless 



united by mortar or cement. 

 Mortar is made from lime- 

 stone which is broken up into 

 large lumps, and heated for 

 hours in strong kilns (Fig. 

 287). Under the action of 

 intense heat, the limestone 

 changes to quicklime or un- 



FIG. 287. A lime kiln. 



slaked lime. The quicklime is packed in barrels and kept in 

 a dry place. When ready to be used for mortar, it is mixed 

 with enough water to make it a thick paste, and into this thick 

 paste sand is slowly stirred. 



Limestone is also used in the making of cement. When 

 heated with clay or other substances, it forms the cement which 



