CHEMICAL NAME = calcium carbonate CAS NUMBER = 471–34–1
MOLECULAR FORMULA = CaCO3 MOLAR MASS = 100.1 g/mol
COMPOSITION = Ca(40.0%) C(12.0%) O(48.0%) MELTING POINT = 825°C
BOILING POINT = decomposes DENSITY = 2.7 g/cm3
Calcium carbonate is a naturally occurring compound found in organisms and throughout the earth’s crust. After quartz, calcium carbonate, primarily in the form of calcite, is the most common mineral found in the crust. Geologically, calcium carbonate exists in several mineral forms: calcite, aragonite, and vaterite. Calcite is the most common calcium carbonate mineral, whereas vaterite is a very rare form. Th e diff erent mineral forms of calcium carbonate are based on their crystalline structure. Th e form of calcium carbonate depends on the conditions at its formation such as temperature and pressure. Th e structure of calcium carbonate in minerals can be viewed as a triangle, with calcium in the center and a carbonate group at each of the vertices. In calcite, the carbonates lie in the same plane, whereas in aragonite the carbonates lie in two diff erent planes. Aragonite is more closely packed than calcite and therefore has a greater density. Th e density of calcite is about 2.7 g/cm3; that of aragonite is about 2.9 g/cm3. Calcium carbonate is found in many rocks, but it is most commonly associated with chalk, limestone, and marble, which is metamorphic limestone.
Numerous organisms extract and synthesize calcium carbonate from seawater, which is used in skeletal structures. Marine organisms including mollusks, sponges, foraminiferans, coccolithophores, coralline algae, and corals use dissolved calcium and carbon dioxide to make calcium carbonate. Corals secrete calcium carbonate in the form of aragonite skeletons, which are formed from the reactions of calcium and bicarbonate: Ca2+(aq) + 2HCO3-(aq) ↔ CaCO3(s) + CO2(g) + H2O(l). Pearls are formed by oysters secreting calcium carbonate to encase foreign
60 | Th e 100 Most Important Chemical Compounds
objects lodged inside their shells. Th e remains of marine organisms accumulated over millions of years form sedimentary deposits of chalk and limestone. Humans primarily use calcium carbonate as a primary source of calcium to combat osteoporosis.
Th e abundance of limestone deposits throughout the world has resulted in the use of calcium carbonate as a primary building material since antiquity. Th e ancient pyramids and the Sphinx in Egypt were made almost exclusively with limestone 5,000 years ago. Most limestone is used today as construction material. One estimate is that the United States used a billion tons of crushed limestone for roads, dams, fi ll, buildings, and various other construc-tion uses in 2005.
In addition to its use as a construction material, calcium carbonate is also used in numer-ous industrial processes. Two forms commonly used are ground calcium carbonate (gcc) and precipitated calcium carbonate (pcc). Ground calcium carbonate is pulverized limestone that has been reduced in particle size, with diameters of a fraction of a micron to several microns.
Precipitated calcium carbonate is made by subjecting calcium carbonate to processes in order to produce a product with specifi c characteristics with respects to form (calcite or aragonite), size, and properties. Precipitated calcium carbonate is made by heating (calcining) lime-stone to calcium oxide (lime, CaO) at temperatures between 600°C and 900°C: CaCO3(s)→ CaO(s) + CO2(g). Calcium oxide is then slacked with water to produce calcium hydroxide (Ca(OH)2): CaO(s) + H2O(l)→ Ca(OH)2(aq). Th e calcium hydroxide is then combined with carbon dioxide, which is produced when the limestone is calcined, to give a suspension of calcium carbonate and water: Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l). Th e calcium carbon-ate produced in the last step is separcarbon-ated by fi ltration and various other separation methods to give precipitated calcium carbonate. By modifying the process of making pcc using diff erent temperatures and limestone sources, numerous forms of calcium carbonate that vary in struc-ture, size, and properties can be made for various applications.
Calcium carbonate is used widely in papermaking as fi ller and coating pigment to whiten paper. Papermaking plants often contain a satellite plant devoted to the production of pcc.
Calcium carbonate is used in place of more expensive optical brightening agents in paper and as a fi ll to replace more expensive wood pulp fi ber; it also helps control the pH in an alkaline range. Th e second most common industrial use of calcium carbonate (after papermaking) representing the largest use of gcc is in the production of plastics. It is used in the production of polyvinyl chloride (PVC), thermoset polyesters, and polyolefi ns. Calcium carbonate can be used to replace resins that are more expensive. Similar to its use in the paper industry, it is used as an optical brightener and whitening agent. It also is used to increase strength and absorb heat during exothermic processes. Calcium carbonate is also used in the production of polyethylene and polypropylene. It is an additive to paints and coatings for several purposes including particle size distribution, opacity control, weather resistance, pH control, and anti-corrosion.
Calcium carbonate is used to buff er acidic soils. In soils that contain sulfuric acid calcium carbonate, it will react with the acid to produce calcium sulfate (CaSO4), carbon dioxide, and water: H2SO4(aq) + CaCO3(s) → CaSO4(s) + CO2(g) + H2O(l). Th e ability of vari-ous limes to neutralize acid in a soil is given in terms of calcium carbonate equivalents. In this system, limestone has a calcium carbonate equivalent of 100. If a slaked lime (calcium hydroxide) has a calcium carbonate equivalent of 150, then only two-thirds as much of slaked lime would be needed to achieve the same neutralizing eff ect. Calcium carbonate
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has also been used to mitigate the eff ects of acid precipitation on water bodies. Another environmental application of calcium carbonate is for gas desulfurization in scrubbers used to reduce sulfur emissions from air pollution sources. In the most popular of these methods, called wet scrubbing, process calcium carbonate reacts with sulfur dioxide to produce calcium sulfi te (CaSO3), which can be further oxidized to gypsum (see Calcium Sulfate).