Understanding Lime Production from Kankar and Clayey Limestone

The question asks about the type of lime obtained when burning materials like kankar or clayey limestone. To answer this, we need to understand how different types of lime are produced and what properties they have.

What is Lime?

Lime generally refers to calcium oxide ($\text{CaO}$), which is produced by heating limestone ($\text{CaCO}_3$) to high temperatures (around $900-1000^\circ\text{C}$). This process is called calcination, where carbon dioxide ($\text{CO}_2$) is driven off from the calcium carbonate.

Types of Limestone and Their Impact on Lime Properties

The type of lime produced depends heavily on the composition of the original limestone:

• Pure Limestone: Consists mainly of calcium carbonate ($\text{CaCO}_3$) with minimal impurities (less than 5% clay or silica).
• Clayey Limestone / Kankar: Contains significant amounts of clay and other impurities like silica ($\text{SiO}_2$), alumina ($\text{Al}_2\text{O}_3$), and iron oxides ($\text{Fe}_2\text{O}_3$) in addition to calcium carbonate. Kankar is often a nodular form of impure limestone found in alluvial soils. These impurities range from 5% to 30% or more.

Burning Process and Resulting Lime Types

When limestone is burned, the calcium carbonate is converted to calcium oxide. The impurities present in clayey limestone or kankar also react during the burning process, typically forming calcium silicates, aluminates, and ferrites.

• Burning Pure Limestone: Results in Quick lime ($\text{CaO}$), also known as Fat lime or Rich lime. This lime is nearly pure white, slakes vigorously with water (hence "fat" lime), and expands significantly upon slaking. It sets slowly and only in the presence of air by absorbing $\text{CO}_2$ (carbonation). It does not set under water.
• Burning Clayey Limestone / Kankar: Results in Hydraulic lime. The presence of silicates, aluminates, and ferrites formed from the impurities gives this lime its hydraulic properties. Hydraulic lime slakes less vigorously than fat lime, expands less upon slaking, and importantly, it has the ability to set both in air and under water. The degree of hydraulicity depends on the percentage of clayey impurities.

Comparing Lime Types

Here is a simple comparison of Quick/Fat lime and Hydraulic lime:

Based on this comparison, the lime obtained by burning kankar or clayey limestone, which contains significant impurities, is Hydraulic lime because these impurities provide the necessary compounds for setting underwater.

Conclusion

Burning kankar or clayey limestone produces Hydraulic lime due to the presence of clayey impurities that form silicates and aluminates upon calcination, enabling the lime to set in the presence of water.

Revision Table: Key Facts About Lime Types

Additional Information: Hydraulic Lime Properties and Uses

Hydraulic lime is classified based on its degree of hydraulicity, which depends on the percentage of clayey impurities:

• Feebly Hydraulic Lime: Contains 5-10% clay impurities. Sets slowly under water (in several weeks). Used for general masonry work.
• Moderately Hydraulic Lime: Contains 10-20% clay impurities. Sets relatively faster under water (in about a week). Used for more important masonry work, like thick walls.
• Eminently Hydraulic Lime: Contains 20-30% clay impurities. Sets quite rapidly under water (in a few days). Used in damp conditions, foundation work, and where quick setting is required.

Hydraulic lime is particularly valuable in construction where the mortar or plaster will be exposed to moisture or submerged, as it does not rely solely on atmospheric carbon dioxide for setting. Its natural composition also makes it more flexible and breathable than Portland cement, often preferred in restoration work for historical buildings.