472 SMITH'S INTERMEDIATE CHEMISTRY 



powdered material is first burnt or raised to a temperature of 

 1400-1600, at which temperature it fuses partially and forms 

 lumps or clinkers. When these have cooled, they are mixed with 

 2-3 per cent of gypsum and pulverized again. The resulting 

 product is Portland cement, the manufacture of which in the 

 United States alone now exceeds 90,000,000 barrels (of 380 pounds) 

 yearly. 



Portland cement is essentially a mixture of calcium silicate 

 and calcium aluminate, with excess lime. The calcium silicate is 

 simply a filler. The calcium aluminate is hydrolyzed on addition 

 of water, according to the equation: 



Ca 3 (A10 3 ) 2 + 6H 2 - 3Ca(OH) 2 + 2H 3 A1O 3 . 



The calcium hydroxide thus formed slowly crystallizes, connecting 

 the particles of the calcium silicate. The aluminium hydroxide 

 fills the interstices and renders the whole compact and impervious. 



The small amount of gypsum added regulates the setting tune 

 of the cement. The iron oxide is necessary to assist in the burning 

 and to lower the temperature at which the mixture begins to fuse. 

 If too small a quantity of excess lime is present, the cement will be 

 unsound and crack on drying. Too little excess lime gives a 

 cement which sets too quickly and is lacking in strength. 



Concrete is a mixture of cement with sand and crushed stone or 

 gravel, all made into a paste with water. It sets to a solid mass, 

 suitable for walks, and for the foundations, walls, and floors of 

 buildings. Since no carbon dioxide from the air is required in the 

 hardening process (contrast with p. 386), it sets equally well 

 under water (hence hydraulic cement), and is employed in con- 

 structing dams, levees, and the foundations of bridges. Rein- 

 forced concrete contains twisted rods of iron, embedded in the 

 mass, and is much used in building construction. 



Blast-furnace slag, when pulverized and heated with limestone, 

 has been found to yield an excellent quality of cement, and a 

 valuable use has thus been found for what was formerly an annoy- 

 ing encumbrance. 



