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Our Surroundings 



Even large ships have been made of concrete mixtures strength- 

 ened by metal. 



Cement is commonly made from certain natural rocks contain- 

 ing a proper amount of clay and limestone, or from an artificial 

 mixture of these two materials. This mixture is subjected to ex- 

 treme heat until the clay and the limestone combine to form a 

 compound. This product is ground to powder and is then ready 

 for building uses. 



In the powdered state cement is very dry. The addition of 

 water causes it to set and shortly to become very hard. In fact, 

 it becomes so hard that it is called artificial rock. 



International Newsreel. 



MODERN CEMENT CONSTRUCTION 



Cement is valuable for heavy structures because it is strong, durable, easily handled 



and easily shaped. 



Cement may be used simply with water. Usually, however, 

 it is mixed with sand, or with sand and broken stone or gravel to 

 form concrete. The proportions of these mixtures are generally 

 expressed by numbers. For example, a 1-2-3 mixture consists 

 of one part cement, two parts sand, and three parts broken 

 stone, to which the necessary amount of water is added. Ordi- 

 narily, the sand and cement are mixed dry; then water is 

 added and the stone finally included. Work must be done rapidly 

 because cement begins to set almost at once. In order to give the 



