Lime is not freely available in nature but is obtained by burning naturally available materials such as:
- Limestone found in limestone hills
- Limestone boulders found in the bed of old rivers
- Kankar found below ground and
- Shells of sea animals.
Lime is more or less pure calcium carbonate. The main sources of obtaining lime are the stones which possess clay, soluble silica, magnesium carbonate, alkalis and metallic oxides, sulphates, iron compounds and carbonaceous matter.
Clay
It is the material which gives the property of producing hydraulicity in lime. In small quantity it retards slaking and is also capable of arresting slaking when it is in excess. It gives the property of insolubility for lime in water. In order to get the best quality lime it is recommended to have 8–30% of clay.
Soluble Silica
In order to develop hydraulicity in lime it is necessary to have silica and alumina during chemical combination with limestone. Hydraulicity in lime is caused due to silicates of calcium, magnesium and aluminium. These silicates are inert at low temperatures and they become active to combine with lime at a high temperature.
Magnesium Carbonate
Hydraulicity is rendered to lime when the magnesium carbonate content is about 30% and this can happen even with the absence of clay. But limes containing large proportions of magnesium carbonate are liable to crack. It has the important property of increasing the setting process but reducing slaking.
Alkalis and Metallic Oxides
These materials convert as soluble salts at a low temperature and thereby cause hydraulicity when present in small quantities (5%).
Sulphates
The presence of sulphates in small quantities tend to accelerate the setting and reduces the slaking action.
Iron
The presence of iron compounds lowers the temperature of calcination of limestone. But excess of iron is objectionable.
Pyrites
It is undesirable to have pyrite in limestone as it is harmful and liable to produce poor-quality lime.
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