422 ROCKS AND BUILDING STONES OF THE EARTH 



is widely used for pavements, linings of reservoirs, and flooring 

 of stables and dairies. Cement has added greatly to the clean- 

 liness and to the safety of the world. Cement floors and trim- 

 mings can be more thoroughly and more easily cleaned than 

 wooden floors and trimmings and are safe from fire. 



Cement is expensive and cannot be used alone for the con- 

 struction of buildings. Its place for that purpose is taken by 

 concrete, which is simply a mixture of cement, sand, gravel, 

 and water. Concrete is not only reasonably cheap, but it is 

 also easy to handle. This is largely because it takes the form 

 of the mold into which it is poured and quickly sets or hardens. 

 Concrete has greatly simplified the construction of underwater 

 structures. For example, if an underwater concrete column 

 for a bridge is to be erected, a hollow wooden mold is sunk 

 into the water and concrete is poured into it. The concrete 

 displaces the water and fills the mold, where it quickly hardens. 

 The mold is withdrawn and the concrete support is finished. 



Concrete is often strengthened or reenforced by metal forms 

 placed within the molds. The concrete hardens around the 

 metal supports as around a core, and forms a strong durable 

 structure. 





