construction terms

Calcium, the Element of Construction
Compounds of this Metal Support Mountains, Buildings and Our Bodies
Calcium is the fifth most abundant element in the earth's crust. Among metals, only aluminium and iron are more plentiful.
   
In 1808, Humphrey Davy isolated tiny quantities of calcium by passing an electric current through molten lime, though it was not until the 20th century that significant amounts could be obtained. Nowadays, some 120 million tonnes of lime are produced annually together with 2000 tonnes of calcium metal.

Calcium Metal
The metal itself has a density slightly more than half that of aluminium. It is sometimes alloyed with aluminium, copper or lead, and is used in the extraction of several other metals. It is very reactive, and rapidly becomes coated with a layer of white calcium oxide when exposed to air. If it comes into contact with water, it reacts quickly, releasing bubbles of hydrogen and forming a solution of calcium hydroxide known as lime water.

Calcium in Rocks
Calcium occurs most commonly in rocks as calcium carbonate. This is mainly found in chalk and limestone, and when mixed with magnesium carbonate, forms the mineral dolomite. Where these sedimentary rocks are subjected to the high pressures and temperatures of volcanic activity, they change into the metamorphic rock, marble. Other minerals, such as gypsum and anhydrite, consist of calcium sulphate and are formed by the evaporation of waters in which they are dissolved.

Uses of Calcium Compounds
Deposits of calcium carbonate are almost unlimited in their availability, and are extensively quarried. Some is used directly to build houses and roads. Most is heated in kilns to temperatures in excess of 900 degrees C, when it decomposes into carbon dioxide and quicklime, calcium oxide. When water is added to quicklime, a vigorous reaction occurs in which calcium hydroxide, slaked lime is produced.

Both quicklime and slaked lime are alkaline. Quicklime is therefore used in industrial chimneys to remove acidic gases like sulphur dioxide and nitrogen dioxide from smoke. Slaked lime is scattered onto fields to neutralise acids that build up in soil. Sometimes, powdered limestone is added to lakes which have become acidic as a result of acid rain.

Mixing calcium oxide with sand makes mortar, a building material that has been in use for more than 2000 years. When water is added to this, the calcium hydroxide produced slowly absorbs carbon dioxide from the air to make hard, interlocking crystals of calcium carbonate. The sand is an inert filler that adds strength to the mortar. Cement is a similar material made by heating limestone with clay.

Limestone Features of the Landscape
Calcium carbonate does not normally dissolve in water. However, rain water contains carbon dioxide, which renders it acidic, and therefore able to react with limestone. Where the rain forms pools, it dissolves the rock until, over many centuries, small depressions deepen into cracks, known as grikes, which are a common feature of limestone pavements. Further action over geological time scales, as the water trickles through underground fissures, hollows out caves in the limestone. At the same time, water dripping from the roofs of these caves slowly evaporates, depositing its dissolved salts as stalactites and stalagmites, which given enough time, would grow to refill the cave.

Hard Water
The calcium present in water that has flowed over limestone reacts chemically with soap, making washing very difficult and leaving a ‘scum‘ floating on the surface of the water. This water is referred to as ‘hard’. Despite this disadvantage, hard water is good to drink, being a source of the calcium that is essential to nearly all living creatures.