Calcium

Calcium is used as a reducing agent in preparing other metals, viz., thorium, uranium and zirconium.   It is also used as a deoxidizer, desulfurizer and decarburizer for various ferrous (iron) and nonferrous alloys.   Calcium is alloyed with aluminum, beryllium, copper, lead and magnesium to remove impurities.   Likewise, it is a good "getter" of residual gases in vacuum tubes.   Calcium compounds are highly important in many industries:

Limestone is sedimentary rock consisting of at least 50% of calcite, CaCO₃, and/or dolomite, CaMg(CO₃)₂.   These minerals also contain sand, clay, and chert impurities.   The most important use of limestone is to produce lime, CaO, (also called "burnt lime", caustic lime", or "quicklime" because of its high reactivity) for the steel, paper-making, glassmaking, and tanning industries.   Limestone is quarried in open pits or in underground mines.   Drilling and blasting produce a rubble that is hauled to a plant where it is crushed and screened for size to produce lime.   To make lime, crushed limestone is heated in a kiln to 2,000°F in a process called calcination ("lime-burning"), which drives off carbon dioxide, CO₂.

This process is reversible, since once the quicklime product has cooled, it immediately begins to absorb carbon dioxide from the air until it is completely converted back to calcium carbonate.

Calcination is an ancient process.   Later, lime was used by the Romans in the first century to make mortar.   They called it calx.   Mortar is a mixture of sand, a binder such as cement or lime, and water.   It is applied as a paste which then sets hard and used in masonry to fill the gaps between blocks in construction.

Because carbon dioxide is the main constituent of greenhouse gases, lime production contributes to global warming.

Lime is changed into slaked lime, Ca(OH)₂, by adding water.   It is widely used in chemical and other industries as a cheap base (alkali).   It is mixed with other chemical ingredients to supply nutritional calcium.   It also is used in water purification, smokestack sulfur removal, and sewage treatment.

Cement, in a general sense, is any binder; that is, a substance that sets and hardens and used to bind other materials together.   As used here, it is a fine grey powder of lime, CaO, often made from dolomite because of its desirable magnesium content.   Cement is mixed with sand, gravel, and small stone (aggregate) to produce concrete.   When water is added to the cement, it changes chemically to bind all the ingredients into a very hard and durable product, a process called hydration.   It can be formed into various shapes by pouring it into molds when wet and then left to set and harden.   It is often used for bridge and building foundations, since it can withstand great loads under compression.   There are two categories of cement, hydraulic and non-hydraulic, each based on hydraulic or non-hydraulic lime.   Hydraulic in this context means able to harden under water.

Hydraulic lime (cement) has the ability of lime to set under water.   Hydraulic lime is produced by heating (calcining) limestone that contains clay silicates (Al₂0₃•2Si0₂•2H₂0, etc.) and other impurities.   These impurities may be natural or added.   Calcium reacts in the kiln with the clay minerals to produce silicates that enable the lime to set without exposure to air.   Any unreacted calcium is slaked to calcium hydroxide.   Hydraulic lime is used for providing a faster initial set than ordinary lime in more extreme conditions, including under water. ^Wiki

Hydraulic lime can be considered, in terms both of properties and manufacture, as between non-hydraulic lime and Portland cement.   The limestone used contains sufficient quantities of clay and/or silica and is heated to a higher temperature than in the production of non-hydraulic lime.   The resultant product will contain dicalcium silicate, but unlike Portland cement, not tricalcium silicate.   It is slaked enough to convert the calcium oxide to calcium hydroxide but not with sufficient water to react with the dicalcium silicate.   It is this dicalcium silicate which in combination with water provides the setting properties of hydraulic lime. ^Wiki

Portland cement is a artificial (synthetic, "manmade") hydraulic lime with specific combinations of calcium oxide (61%-67%), and silicon, aluminum and ferric oxides made from silicates, aluminates, and gypsum.   It is the most common ingredient in construction concrete.   Portland cement raw materials consist of at least 2/3 by mass of calcium silicates (3CaO•SiO₂ and 2CaO•SiO₂), the remainder consisting of aluminium- and iron-containing other compounds.   The ratio of CaO to SiO₂ is not be less than 2.   The magnesium content (MgO) does not exceed 5% by mass.   Portland cement is made by heating in a kiln a homogeneous mixture of raw materials to a sintering temperature of about 1450 °C for modern cements.   The aluminium oxide and iron oxide are present as a flux and contribute little to the strength.     The major raw material usually limestone (CaCO₃).   Normally, an impure limestone which contains SiO₂ is used, the CaCO₃ content can be as low as 80%.   Non-limestone raw materials added depend on the purity of the limestone.   Some of the secondary raw materials used are clay, shale, sand, iron ore, bauxite, fly ash and slag.   When a cement kiln is fired by coal, the ash of the coal acts as a raw material. ^Wiki

Portland cement was developed from cements (hydraulic limes) made in Britain in the early part of the 19th century.   Its name is derived from its similarity to Portland stone, a type of building stone that was quarried on the Isle of Portland in Dorset, England.   Joseph Aspdin, a British bricklayer, in 1824 was granted a patent for a process of making a cement that he called Portland cement.   His cement was an artificial hydraulic lime similar in properties to the material known as "Roman Cement" that was patented in 1796 by James Parker.   His process was similar to that patented in 1822 and used since 1811 by James Frost who called his cement "British Cement".   The name "Portland cement" is also recorded in a directory published in 1823 being associated with a William Lockwood and possibly others.   Aspdin's son, William, in 1843 made an improved version of this cement and he initially called it "Patent Portland cement" although he had no patent.   In 1848, William Aspdin improved his cement and in 1853 moved to Germany where he was involved in cement making.   Many people have claimed to have made the first modern Portland cement in the modern sense, but it is generally accepted that it was first manufactured by William Aspdin at Northfleet, England in about 1842.   The German Government issued a standard on Portland cement in 1878. ^Wiki

Non-hydraulic lime (cement) is produced by slaking fresh quicklime (CaO) in an excess of water which is then left to mature.   It requires exposure to air to carbonate and does not set under water. ^Wiki   Non-hydraulic lime is primarily composed of calcium hydroxide, generally greater than 95%.   Non-hydraulic lime is produced by the heating of sufficiently pure limestone (calcium carbonate) to between 954° and 1066°C, driving off carbon dioxide, to produce quicklime (CaO). This is done in a lime kiln. The quicklime is then slaked; that is, thoroughly mixed with water to produce lime putty (calcium hydroxide).   If the quicklime is slaked with an excess of water then putty or slurry is produced.   If less water is used, then the result is a dry material, the excess water escaping as steam during heating.   This is ground to make hydrated lime.   Hydrated non-hydraulic lime can be mixed with water to form lime putty.   Before use it must be left in the absence of carbon dioxide (usually under water) to mature.   Putty can be matured for anything from 24 hours to many years, an increased maturation time improving the quality of the putty. ^Wiki

Pozzolans can be added to the mix of lime mortar.   These are substances which when combined with lime produce a hydraulic set.   They include powdered brick, heat treated clay and volcanic materials.   The chemical set imparted ranges from very weak to almost as strong as Portland cement. ^Wiki

Non-hydraulic lime mortar sets by reaction with atmospheric carbon dioxide.   This is in contrast to the setting of Portland cement and hydraulic lime which sets by reaction with water in the mix.   The reaction with carbon dioxide produces calcium carbonate, the raw material used at the start of the process to create lime.   This process is slower than that in Portland cement.   Depending on the thickness of the mortar and climate conditions, an initial skin hardening may take from a couple of hours to several days to gain an initial hardness.   The full drying and hardening of the thickness process can continue at a slower rate for many years with the mortar continuing to gain strength.   One of the advantages of non-hydraulic lime mortar is that in the event of cracking, mortar deeper in the join is exposed and reacts with the air to reduce the effect of the crack. ^Wiki

Gypsum (calcium sulfate dihydrate), CaSO₄•2H₂O, is used to make plaster, stucco, gypsum board, wallboard (drywall, sheetrock), surgical and dental casts, toothpaste, and blackboard chalk.

Plaster (plaster of Paris), is made like cement, but with fine sand and/or dolomite.   It is used in home and building wall construction.   Plaster is made by heating gypsum to about 150 °C.

2CaSO₄•4H₂O ——> 2CaSO₄•H₂O + 3H₂O (released as steam)

Limestone dimension stone is cut into shape and polished for use in building structures.   The residual crushed limestone is used for aggregate and filler in construction work.

Fluorite, CaF₂, is used as a flux in steel and aluminum processing, a source of fluorine for hydrofluoric acid, fluorinated water, and optics.

Calcium carbide, CaC₂, used to make acetylene gas.

Calcium chloride, CaCl₂, used to dry gases, melt ice on roads and sidewalks, preserve foods, control dust, provide a refrigerating brine.

Calcium cyanamide, CaCN₂, used as a fertilizer, weed killer, and a source of other nitrogen compounds

Calcium hypochlorite, Ca(OCl)₂, used to provide chlorine to bleach textiles, wood pulp, and paper, to clean and disinfect water supplies and swimming pools, to sanitize homes, public buildings, and food processing plants.

Calcium nitrate, Ca(NO₃)₂, used as a fertilizer and rubber coagulant.

Calcium is essential to life.   Leaves, bones, teeth and shells are mostly calcium.   Calcium builds and strenghtens bones and teeth; therefore, higher daily doses are required for children and pregnant and lactating women.   It assists muscle contraction, which includes the heart, maintains cell membranes, blood clotting, absorption of vitamin B₁₂, activates enzymes, regulates heartbeat, and nerve propagation. ^{Brody 184}   Calcium is often deficient in women and the elderly, leading to a condition known as osteoporosis (brittle bones).   Vitamin D aids the absorption of calcium.   Since calcium is a natural tranquilizer, it alleviates insomnia.   The best natural sources of calcium are milk and milk products, cheese, sardines, soybeans, salmon, peanuts, sunflower seeds, dried beans, and green leafy vegetables.   Too much calcium intake can result in bone and tissue calcification and interfers with nerve and muscular action. ^{Netzer 800}

Limestone provides plants with calcium, an essential element.   Calcium activates enzymes, is a structural component of cell walls, influences water movement in cells and is necessary for cell growth and division.   Some plants must have calcium to take up nitrogen and other minerals.   Calcium is very important to plant life, more so than phosphorus, but it normally is available in sufficient quantities in the soil through rock weathering.   Calcium, once deposited in plant tissue, is immobile (non-translocatable) so there must be a constant supply for growth.   Occasional deficiencies causes stunting of new growth in stems, flowers and roots.   Symptoms range from distorted new growth to black spots on leaves and fruit.   Yellow leaf margins may also appear. ^{Morgan n.p.}     Limestone, which contains calcium, is an important plant nutrient.   Soils lying above limestone were known since antiquity to produce good grains.

Calcium was discovered and isolated in impure form in 1808 by Sir Humphrey Davy, a British chemist, by the electrolysis of lime and mercuric oxide, HgO.   Calcium occurs in nature as limestone, gypsum, and fluorite.  It is prepared by the electrolysis of fused chloride to which calcium fluoride is added to lower the melting point. ^{Lide 4-6}