650 CREATIVE CHEMISTRY The vaporizing-liquid type of extinguisher contains carbon tetrachloride, which is forced onto the fire by a pump in the cylinder. Carbon tetrachloride is a compound in which the carbon has been completely oxidized by chlorine rather than by oxygen. Carbon tetrachloride forms a heavy vapor that smothers certain types of fires such as oil and gasoline fires quite efficiently if the vapors are confined over the surface of the fuel. Fig. 284. Note how the foam floats on the liquid shutting off the oxygen and extinguishing the fire, and how the water goes to the bottom of the receptacle allowing the burning liquid to overflow and the fire to spread. (Courtesy of the American-La France and Foamite Industries, Inc.) Combustion Furnishes the Heat to Transform Many Inorganic Materials into Useful Products. 1. Limestone Is Heated to Produce Lime. Any naturally occurring carbonate, such as calcium carbonate (limestone, chalk, or marble) or magnesium carbonate (magnesite), when heated to a high enough temperature in large furnaces, or kilns, will decompose to form carbon dioxide and the corresponding metallic oxides — lime, CaO, in the case of limestone. Lime, CaO, called quicklime, will react with water to form hydrated lime, Ca(0H)2. This process, called "slaking," evolves considerable heat. When lime is left exposed to the air, it combines with the carbon dioxide to form calcium carbonate, CaCOa. This product is called "air-slaked" lime. Because of its low cost, hydrated lime (calcium hydroxide, Ca(0H)2)
COMBUSTION 651 is much used in industries whenever a mild alkah is desired. Hydrated Hme is an important constituent of mortar, plaster, and stucco. Ordinary mortar consists of a mixture of hydrated lime, sand, and water; when exposed to the air, the water evaporates, and the mortar slowly sets as the calcium hydroxide reacts with the carbon dioxide of the air to form artificial limestone, CaCOs, containing sand. Interior plasters contain plaster of Paris as their principal constituent. Plaster of Paris is obtained from naturally occurring gypsum, CaS04 • 2 H2O, by heating it to drive off a part of its water to form CaS04 • |- H2O. When plaster sets, the reverse reaction slowly takes place. As plaster of Paris sets, it swells slightly and is therefore very useful for preparing casts of various kinds. Stucco, i.e., exterior plaster, usually contains both lime and cement. 2. Portland Cement May Be Made by Heating the Proper Mixtures of Certain Very Abundant Rocks. The invention of Portland cement revolutionized the building and construction industries by enabling man to produce artificial stone in any shape desired, thus saving immense amounts of labor that would otherwise have been expended in cutting stones. Modern Portland cement is produced by intermingling raw materials rich in lime, CaO, with other materials rich in alumina, AI2O3, and silica, Si02. The lime is usually obtained from low-grade limestone, chalk, or shells. The alumina and silica are furnished by clay, shale, slate rock, volcanic ash, or other materials such as blast-furnace slag. These raw materials are analyzed and mixed in the proper proportions. The manufacturing process consists in grinding the material very finely, mixing it intimately, partially fusing it in a rotary kiln at a very high temperature, and then grinding it to an extremely fine powder in rotating tubes partially filled with steel rolls or balls. When Portland cement is mixed with water or with a mixture of water, sand, and gravel to form concrete, it takes an initial set within a few hours and continues to harden slowly for months or years. This hardening process consists, in part at least, in the hydration, i.e., chemical combination with water, of the anhydrous compounds produced during the burning of the cement, to produce synthetic silicate rocks. These synthetic rocks may be made stronger than natural rocks by the use of reinforcing steel. Concrete should be kept wet when setting in order to provide plenty of water for the hydrating process. 3. Ceramic Products Have Also Replaced Natural Stone and Provided Entirely New Building Materials. The manufacture of brick and other clay products such as pottery was among the first advances made by man in the realm of material transformations.