What is lime?
Until the invention of the Ordinary Portland Cement which is used as a binding material these days but when we look back before the invention of Ordinary Portland Cement “Lime” was used as a binding material because it also produces cementing properties.
It was used as a mortar or as a plaster. Nowadays its use as a binding material is reduced drastically. So if you get a question like what is lime so the answer is mentioned above.
What is calcium carbonate?
Calcium carbonate is a compound that results from calcium ions reacting with carbonate ions in the water. Its chemical symbol is CaCo3. That means it is made up of calcium carbon and oxygen.
This reaction creates a solid particle that sinks to the ocean floor. From there organisms can use it to build their shells, skeletons, and reef structures. Corals use calcium carbonate for their skeletons. Snails, clams, and oysters secrete it to create their shells.
Turns and sea stars use it for their exoskeletons and spines. Crustaceans create a hard shell as armor or so they can attach to rocks, bottoms of boats, and other stable surfaces in the ocean. Even microscopic organisms like plankton and phytoplankton create tests or shells from calcium carbonate.
Limestone is a sedimentary rock made of calcium carbonate. Sedimentary rock is a type of rock that forms from the accumulation of small pieces of sediments compacting together.
Limestone is mostly made up of shells, corals, and sand forming together. Limestone can be broken apart in washing away by three types of room like physical, biological, and chemical.
Fresh lime water is slightly acidic and as it falls on the limestone it dissolves it away. As the rocks dissolve it leaves behind pockets where the dissolved sediments fall into it in pools of water.
As the ocean comes splashing over the top it picks up that dissolved sediments and mixes it back into the ocean. That dissolved sediment can then be broken into parts and the calcium carbonate part can be used to form new limestone or it can be used by organisms in the ocean.
Calcium carbonate is really important to our oceans; it is one of the major building blocks that create the ecosystems we know and love today.
Types of lime
- Pure lime.
- Impure/Kankar lime.
- Shell lime.
Pure limestone is also known as Calcite (CaCo3) and in that, if magnesite (MgCo3) is present then it is known as Dolomite. For obtaining quick lime, pure limestone containing calcium carbonate is burned.
If in pure limestone only CaCo3 is present then it is known as Calcite. But in the limestone the minerals containing calcium carbonate and magnesium carbonate in the equimolecular quantities then it is known as Dolomite which is denoted as [CaMg(Co3)2]. In pure limestone, if the present magnesite percentage is more than 45% then it is known as Dolomite.
Impure / Kankar lime
It is an impure lime. These occur in the form of nodules and compact blocks. It is used to obtain hydraulic lime.
It is also a pure lime. It is obtained from the calcination of shells of sea animals and corals. It is used for lime punning, whitewash, soil stabilization, and glass production.
So, I guess that you have got a clear idea of what is lime.
How is lime obtained?
When we burn pure limestone i.e (CaCo3) in the presence of oxygen at 800°C which is known as the calcination process. This process is carried out until the pure limestone turns red hot.
Then we get the byproduct that is quick lime i.e (CaO) and carbon dioxide i.e (Co2) is released in the air. When it comes out from the kiln this lime is in the form of lumps. So it is known as lump lime. It is also known as caustic lime because it is very pure.
When we carry out the same process as mentioned above for pure limestone. The same process is carried out for impure limestone then we get the byproduct that is hydraulic lime.
What is quick lime?
As mentioned above that quick lime is known as caustic lime. Quick lime when obtained from the kiln is in the form of lumps. So it is also known as lump lime. It is very unstable because initially when the pure limestone is burned the carbon dioxide is released in the air to form quick lime but as time passes it again starts absorbing carbon dioxide. Due to which it again turns into calcium carbonate. That’s why it is termed as highly unstable. The Specific gravity of quick lime is G = 3.4.
What is slaked lime?
Hydrated lime / Slaked lime
If we take quick lime and then it is crushed finely. On this finely crushed quick lime if we sprinkle a little amount of water over it, then it reacts with the water and forms a fine homogeneous powder. This powder is known as hydrated lime or slaked lime.
CaO + H2O → Ca(OH)2
Which lime is prepared from shells of sea animals?
As quick lime, fat lime also has a high amount of calcium oxide (CaO). Also as same as quick lime if it is kept open to the atmosphere it starts absorbing carbon dioxide. Due to this it sets and hardens after the absorption of carbon dioxide (Co2). It is manufactured by burning marble, seashells, and coral, etc.
Which lime is prepared from an impure limestone?
Hydraulic lime contains small quantities of silica, alumina, iron oxide in chemical composition with calcium oxide components. Due to all the impurities present in this lime the property of hydraulicity is developed in it. Due to this it can set and harden in underwater conditions. Hydraulic lime is manufactured by burning impure limestone / Kankar lime.
Impurities in lime
Different types of impurities in lime are as follows:
- Soluble silicates.
- Magnesium carbonates.
It is that property of lime due to which lime tends to set in a damp place with no air circulation or underwater setting is known as hydraulicity.
- Imparts hydraulicity in lime.
- The presence of clay in lime in the form of impurities ranges from 8% – 30%.
- As the percentage of clay increases in the lime hydraulicity and setting properties increases.
- As the percentage of clay increases in the lime slaking property decreases.
2. Soluble silicates
- Silicates of calcium, aluminum, and magnesium impart hydraulicity.
- As the percentage of soluble silicates increases in the lime hydraulicity and setting properties increases.
- As the percentage of soluble silicates increases in the lime slaking property decreases.
3. Magnesium carbonates
- Imparts strength in lime.
- As the percentage of magnesium carbonates increases in the lime slaking and setting property decreases.
- As the percentage of sulphates increases in the lime slaking property decreases.
- As the percentage of sulphates increases in the lime setting property increases.
Classification of lime (Depending on purity and impurity)
- Pure lime or Rich lime.
- Hydraulic lime.
- Poor lime.
1. Pure lime or Rich lime
- The presence of calcium oxide in pure lime is more than 95%.
- The presence of impurities in pure lime is less than 5%.
- It is obtained after the calcination of pure limestone.
- It can be used for plastering work, whitewashing, and where strength is not a major concern.
- The plasticity of pure lime is very high.
- Its color is perfectly white.
2. Hydraulic lime
- Presence of calcium oxide in hydraulic lime is more than 70%.
- Presence of impurities in hydraulic lime is less than 30%.
- It is obtained after the calcination of impure limestone or kankar lime.
- It can be used where strength is required brick masonry, stone masonry, and under water constructions.
- Hydraulicity of hydraulic lime is very high.
- Its colour is not perfect white.
Hydraulic lime is further classified into three types they are as follows:
- Feebly hydraulic lime.
- Moderately hydraulic lime.
- Eminently hydraulic lime.
1. Feebly hydraulic lime
- Impurities present in feebly hydraulic lime is upto 5 to 10%.
- Setting time is generally 21 days.
2. Moderately hydraulic lime
- Impurities present in moderately hydraulic lime are up to 10 to 15%.
- Setting time is generally 7 days.
3. Eminently hydraulic lime
- Impurities present in eminently hydraulic lime are up to 20 to 30%.
- Its initial setting time is 2 hours and the final setting time is 48 hours.
4. Poor lime
- Presence of calcium oxide in poor lime is less than 70%.
- Presence of impurities in poor lime is more than 30%.
- It is obtained after the calcination of impure limestone.
- It is used in sub structures.
- Setting and slaking of poor lime is very low.
- Its colour is not perfect white.
Classification of lime (According to IS 712)
According to the IS 712 lime is classified into 6 different classes. They are as follows:
- Class A – Eminently Hydraulic Lime.
- Class B – Semi Hydraulic Lime.
- Class C – Fat Lime.
- Class D – Magnesium / Dolomite Lime.
- Class E – Kankar Lime.
- Class F – Siliceous Dolomite Lime.
1. Class A – Eminently Hydraulic Lime
- The presence of calcium oxide in eminently hydraulic lime is up to 70%.
- The presence of impurities in eminently hydraulic lime is up to 20 to 30%.
- Its color is grey.
- Compressive strength after 14 days is 1.75 N/mm2 and after 28 days is 2.8 N/mm2.
- Its initial setting time is 2 hours and the final setting time is 8 hours.
- It is used as a mortar and to prepare lime concrete.
- Its modulus of rupture is 1.05 N/mm2.
2. Class B – Semi Hydraulic Lime
- Presence of calcium oxide in semi hydraulic lime is upto 70%.
- Presence of impurities in semi hydraulic lime is upto 30%.
- Its colour is also grey.
- Compressive strength after 14 days is 1.25 N/mm2 and after 28 days is 1.75 N/mm2.
- It is used as a mortar and to prepare lime concrete.
- Its modulus of rupture is 0.7 N/mm2.
3. Class C – Fat Lime
- Presence of calcium oxide in fat lime is upto 93 to 95%.
- Presence of impurities in fat lime is upto 5 to 7%.
- Its colour is perfect white.
- Its volume increases after slaking by 2 to 2.5 times.
- It is used for white washing and plastering.
4. Class D – Magnesium / Dolomite Lime
- The presence of calcium oxide in dolomite lime is up to 85%.
- The presence of impurities in dolomite lime is up to 15%.
- Its color is white.
- It is used for whitewashing and plastering.
- Its setting process is slow.
5. Class E – Kankar Lime
- The kankar lime is an impure lime.
- Its colour is grey.
- It is used to make hydraulic lime by the calcination process.
6. Class F – Siliceous Dolomite Lime
- Siliceous dolomite lime is used for applying the final coat of plaster.
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