CHM 110 - CHEMISTRY AND ISSUES IN THE ENVIRONMENT

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Limestone - Rock of the Ages

 

 
Limestone which is a sedimentary rock, also called calcite, calcium carbonate, CaCO3, is a very commonly mined or quarried by blasting. The two most common forms of limestone as pure calcium carbonate are calcite and aragonite - and are formed as fossilized remains of marine animal shells. Natural limestone may also have significant quantities of dolomite present, magensium carbonate, MgCO3. Limestone has a variey of uses including construction, steel making, lime and cement, and in the synthesis or use of other chemicals.
 
Construction and Aggregates
Limestone has been used directly in buildings as load bearing walls and also in facades. Crushed limestone, also called aggregate, is used as a filler in concrete, as a base in road construction, and as a filler in asphalt.
 
Steelmaking
Limestone is used tin making steel. The limestone is mixed with iron ore and coke, a form of coal, and all are melted in bl is converted to lime, CaO, combines with the impurities, mostly silicon dioxide, in the iron ore or hot molten metal to form a material called slag which has a general formula of caclium silicate, CaSiO3. The slag which now is in the form of a calcium silicate floats on top of the molten metal because it is lighter. Then the molten iron which sinks to the bottom of the furance. About 100 pounds of limestone are needed to make a ton of iron.
 
Lime and Cement
 
Limestone is converted to lime, CaO, calcium oxide, or also called "quicklime" by heating the limestone rock to about 800 degrees Celsius. Lime may be used on an industrial scale as an inexpensive base to adjust pH in chemical processes, water treatment, and adjusting soil pH.
 
Portland cement is made by heating a mixture of silica, clay, and limestone to about 1500 C. The cooled mass is then crushed and some gypsum is added (CaSO4.2H2O). When mixed with water, sand, and gravel, complex reactions results in a very strong hardened material called concrete.
 
Chemicals
 
Calcite and Dolomite, when heated and in some cases slurried or combined with salt, are used in making many common everyday products such as paper, glass, paint and varnish, soap and detergents, textiles, refractories, baking powder, and pharmaceuticals, including milk of magnesia and bicarbonate of soda. Finely ground, they are used to control coal mine dust, to collect sulfur dioxide from power plant exhaust, to sweeten soils, and as ingredients in fertilizer and stock feeds, to name a few.