### Limestone

Limestone is a sedimentary rock composed chiefly of calcium carbonate CaCO3. Marble, chalk, sea shells and coral are mainly made of CaCO3. Various types of limestone are used as building stone. Marble is metamorphosed limestone.

### Lime and Slaked Lime

Lime or quicklime is calcium oxide, CaO. It is white powdery substance used in making mortar and cement. It is highly infusible since it melts at 2850°C. Calcium oxide is prepared by heating calcium carbonate at about 900°C.

• $\displaystyle CaC{{O}_{3}}+heat\to CaO+C{{O}_{2}}$

Pure calcium oxide is a white amorphous powder. It is used as a flux in the smelting of metals, in the manufacture of glass, in the production of calcium carbide, and as a source of calcium hydroxide.

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Lime reacts vigourusly with water, and forms calcium hydroxide, Ca(OH)2 which is known as slaked lime. This process is called the slaking lime.

• $\displaystyle CaO+{{H}_{2}}O\to Ca{{\left( OH \right)}_{2}}$

A saturated solution of calcium hydroxide in water is called limewater. Limewater is a base. In suspension, it is whitewash or milk of lime. This watery lime mixture is used to paint walls white.

Calcium hydroxide has many important uses. It is an important component of mortar, plaster, stucco, whitewash, and other such building materials. Acidic soil may be neutralized with lime. A mixture of lime and sulfur is used as an insecticide. A solution made up of equal quantities of copper(II) sulphate and limewater (known as Bordeaux mixture) is used in horticulture and in the wine industry as a fungicide. Calcium hydroxide is also used in removing hair from hides preparatory to making leather.

### Mortar, Cement, Stucco, and Plaster

When one part of slaked lime is mixed with three parts of sand and stirred into a pasty mass with water a mortar is formed. Mortar is used extensively in building, since good mortar improves with age.

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The hardening of mortar is due to loss of water, the formation of calcium carbonate when carbon dioxide of the air reacts with lime, and the slow formation of calcium silicate.

The reactions representing hardening of the mortar are:

• $\displaystyle Ca{{\left( OH \right)}_{2}}+C{{O}_{2}}\to CaC{{O}_{3}}+{{H}_{2}}O$
• $\displaystyle Ca{{\left( OH \right)}_{2}}+Si{{O}_{2}}\to CaSi{{O}_{3}}+{{H}_{2}}O$

### Cement

Cement is a mixture of limestone and clay (an aluminum silicate) heated in a rotary kiln. The reaction between these materials forms small, pebble-sized lumps of calcium aluminum silicates, called clinkers. When cool, the clinkers are ground into a fine powder and mixed with about 2% of powdered gypsum (aluminum sulfate)

### Stucco

Stucco is an outside covering for the walls of buildings. It consists of lime, sand, and cement. When properly applied, stucco finishes are very durable and attractive.

The inside walls of many houses are covered with plaster containing lime, sand, and hair or wood fiber to make the mixture stick better. The newer types of plaster are principally plaster of Paris, CaSO4 . 2H2O.