CHAPTER XXIII 

 CONCRETE 



187. Use. Concrete is a mixture of cement, sand, 

 crushed rock or gravel and water. It is a most excellent 

 material for sidewalks, fence posts, foundations, floors and 

 walls of buildings, beds or piers for machines and for struc- 

 tures under water. It is manufactured in enormous quan- 

 tities and its use for these and many other purposes is rapidly 

 increasing. A hundred millions of barrels are produced in 

 the United States annually. The rapid decrease of timber 

 supply has made an urgent demand for a new building 

 material which is amply met by the use of concrete. On 

 account of its economy, durability and safety from fire loss 

 and ease of manipulation it is superior to lumber, brick or 

 building stone for construction purposes. The farmer with 

 little assistance and at convenient times can use successfully 

 this most excellent material. 



188. Cement Manufacture. Cement is a powdered, 

 calcined intimate mixture which, before heating, contained 

 in definite proportion limestone or marl or chalk (CaCOs), 

 clay (HAlSiQi), and sand (SiO2). In the manufacture of 

 Portland cement these materials are mixed in the proper 

 proportions as shown by chemical analysis, pulverized so 

 that it will pass through a sieve with 100 meshes to the inch, 

 burned in inclined rotary steel cylinders from 60 to 150 feet 

 long lined with firebrick. In these furnaces the final 

 temperature rises to 1400 C.-1600 C. To the resulting 

 clinker is added a small amount of gypsum, which seems to 

 affect the time required for the setting of the cement, and 

 the material finally is ground to a very fine powder. Natural 

 cement made from rock which has the correct proportions 



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