Types of Lime Based on Calcium Oxide Content
Lime is generally classified based on the calcination process of limestone, specifically focusing on the calcium oxide (CaO) content. The three main types of lime based on CaO content are:
- Fat Lime
- Hydraulic Lime
- Poor Lime
1. Fat Lime
Fat lime, also known as high-calcium lime, pure lime, rich lime, or white lime, is characterized by its high calcium oxide content (typically around 95%) and low levels of impurities (less than 5%). Fat lime is called “fat” because it slakes vigorously when mixed with water, leading to a two- to three-fold increase in volume. This type of lime is white in color and is primarily composed of calcium oxide, which hardens only in the presence of carbon dioxide from the air.
Properties of Fat Lime:
- Very slow hardening: Takes longer to set, especially in thicker applications.
- High plasticity: Provides ease of workability in construction applications.
- Solubility in water: Dissolves when exposed to moisture.
Uses of Fat Lime:
- Lime Mortar: Commonly used in lime mortar (a mixture of lime and sand) for plastering and pointing work on walls.
- Whitewashing: Used for whitewashing plastered surfaces.
- Lime Surkhi Mortar: Ideal for thick masonry walls and foundations. Surkhi, a powder obtained by grinding burnt bricks, is often mixed with fat lime to improve the durability of thick masonry work.
- Concrete Matrix and Distemper Base: Serves as a base for distemper and matrix in concrete.
- Other Applications: Used in cement manufacturing, metallurgy, and for producing Ca(OH)₂ powder for other industrial applications.
Specific Characteristics of Fat Lime:
- Slaking: Hydration is vigorous, resulting in a high heat release and forming a cloud of steam as it hydrates.
- Setting Mechanism: Hardens slowly by absorbing CO₂ from the atmosphere, making it unsuitable for humid or wet conditions.
- Specific Gravity: The specific gravity of pure fat lime is about 3.4.
Fat lime is ideal for applications requiring a high degree of workability, plasticity, and a fine finish, such as plastering and whitewashing. However, it is less suitable for environments where rapid setting or water resistance is necessary.
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2. Hydraulic Lime
Hydraulic lime, also known as water lime, contains a mixture of silica, alumina, and iron oxide in combination with calcium oxide. This composition allows it to harden and set even without carbon dioxide (CO₂) exposure, making it suitable for underwater or damp environments.
Types of Hydraulic Lime
Hydraulic lime is further classified based on its clay content, which impacts its hydraulic (water-setting) properties:
i. Feebly Hydraulic Lime
- Clay Content: Less than 5-10%.
- Slaking: Slakes slowly, typically after 5-15 minutes.
- Setting Time: Takes about 21 days to fully set.
- Applications: Suitable for damp locations and less critical construction works.
ii. Moderately Hydraulic Lime
- Clay Content: 10-20%.
- Slaking: Slakes more slowly, usually after 1-2 hours.
- Setting Time: Reaches full setting within 7 days.
- Applications: Commonly used in moderately damp areas and for more substantial structures.
iii. Eminently Hydraulic Lime
- Clay Content: 20-30%.
- Slaking: Requires careful slaking with minimal water; slakes with difficulty.
- Setting Time: Initial setting in 2 hours, final setting within 48 hours.
- Applications: Ideal for damp areas and for significant structural work.
Properties of Hydraulic Lime
- Setting Ability: Capable of setting and hardening in underwater or low-oxygen conditions.
- Workability: Forms a thin paste with water; slaking requires precise water management to prevent hardening before use.
- Hydraulic Strength: The higher the clay content, the stronger the hydraulic properties.
Uses of Hydraulic Lime
- Underwater Construction: Can set underwater, making it suitable for bridges, dams, and other submerged structures.
- Thick Masonry Walls: Effective in thick masonry, where limited air circulation prevents the use of other lime types.
- Civil Engineering Works: Due to its cement-like properties (with around 30% clay content), it is used in various civil engineering applications where durable, water-resistant lime is needed.
Hydraulic lime provides versatility in conditions where traditional lime is inadequate, making it valuable for foundational work, underwater applications, and structures exposed to high moisture levels.
3. Poor Lime (Lean Lime)
Poor lime, also known as impure or lean lime, contains a high percentage of clay impurities (over 30%), which causes it to slake slowly and limits its binding properties. This type of lime has reduced calcium oxide (CaO) and magnesium oxide (MgO) content, along with a significant presence of silica, alumina, and iron oxide.
Properties of Poor Lime
- Slow Slaking: Requires a longer slaking time due to high clay content.
- Thin Paste Formation: Forms only a thin paste with water, reducing its effectiveness as a binder.
- Slow Setting and Hardening: Sets and hardens very gradually by absorbing CO₂ from the atmosphere, making it unsuitable for structural applications.
- Color Variation: Usually appears in shades of yellow to gray.
Composition
- Calcium and Magnesium Oxide (CaO + MgO): 80-85%, with MgO content less than 5%.
- Clay Impurities: Contains over 7% clay (silica, alumina, and iron oxide), which impedes fast setting and reduces binding quality.
Uses of Poor Lime
- Inferior Mortar: Primarily used to produce low-quality mortar, suitable for non-critical or temporary structures.
- Less Important Constructions: Recommended for less significant or temporary construction projects due to its weak binding and slow setting characteristics.
Poor lime is best suited for applications that don’t demand strong bonding or durability, making it ideal for minor or temporary work rather than essential structural projects.
Types of Lime Based on IS:712–1984
1. Class A: Eminently Hydraulic Lime
Class A lime is known for its high hydraulic strength and is available in a hydrated form. It is used primarily in structural applications, where durability and compressive strength are required.
Characteristics of Class A Lime
- Color: Typically grey.
- Composition: Contains 60-70% calcium oxide and approximately 25% clay content.
- Slaking: Requires careful slaking due to its high hydraulic properties.
- Setting and Hardening: Sets and hardens effectively underwater, with an initial setting time of 2 hours and a final setting time of 48 hours.
Compressive Strength
Lime-sand mortar (1:3 by weight) made from Class A lime achieves a compressive strength of 1.75 N/mm² at 14 days and 2.8 N/mm² at 28 days.
Uses of Class A Lime
- Suitable for structural works such as arches, domes, and foundations.
- Commonly used in making mortar and concrete for construction and foundation work where stability and durability are essential.
Class A lime’s high strength and water-setting properties make it ideal for significant construction projects, especially where water exposure is likely.
2. Class B: Semi-hydraulic Lime
Class B lime, also known as semi-hydraulic lime, is available in two forms: quicklime (produced by calcining pure limestone) and hydrated lime (a dry powder obtained by treating quicklime with water). This type of lime has moderate hydraulic properties and is primarily used in applications that require slower setting times but still demand some resistance to moisture.
Properties of Class B Lime
- Color: Typically grey.
- Composition: Contains about 70% calcium oxide and 15% clay.
- Slaking and Setting: Slakes and sets at a slow rate, taking approximately a week to set underwater.
Compressive Strength
Lime-sand mortar (1:3 by weight) made from Class B lime achieves a compressive strength of 1.25 N/mm² at 14 days and 1.75 N/mm² at 28 days.
Uses of Class B Lime:
- Masonry Mortar: Commonly used in masonry work, providing a strong bond for brick and stone structures.
- Flooring and Concrete: Suitable for concrete in ordinary constructions and as a base for flooring.
- Plaster Undercoat: Used as an undercoat for plastering, offering a durable foundation for finishes.
Class B lime is ideal for ordinary construction where moderate hydraulic properties are sufficient, making it suitable for applications such as masonry and plastering.
3. Class C: Fat Lime
Class C lime, also known as fat lime, is available in both quicklime and hydrated lime forms. It is primarily used in finishing applications where a smooth, white finish is desired, as well as in masonry mortars when mixed with pozzolanic materials (such as volcanic dust containing about 80% clay along with lime, magnesia, and iron oxide).
Characteristics of Class C Lime:
- Color: White.
- Slaking: Slakes vigorously upon contact with water, expanding up to three times its original volume.
- Composition: Contains approximately 93% calcium oxide and 5-7% clay.
Uses of Class C Lime:
- Finishing Coat for Plastering: Provides a smooth and fine finish for plastered surfaces.
- Whitewashing: Commonly used in whitewashing due to its bright white color and high calcium content.
- Masonry Mortar with Pozzolana: When mixed with pozzolanic material, it creates a durable masonry mortar for specific construction applications.
Class C lime’s high calcium content and rapid slaking make it ideal for finishing applications, particularly where aesthetic appeal is desired, such as in plastering and whitewashing.
4. Class D: Magnesium or Dolomite Lime
Class D lime, also known as magnesium or dolomite lime, is available in both hydrated and quicklime forms. This type of lime is mainly used in finishing applications, providing a smooth, white finish for plastering and whitewashing.
Characteristics of Class D Lime:
- Color: White.
- Composition: Contains around 85% calcium and magnesium oxides.
- Slaking: Slakes promptly upon contact with water.
- Setting: Sets slowly, making it suitable for finishing applications.
Uses of Class D Lime:
- Plaster Finishing Coat: Used as a finishing coat in plastering, providing a refined and durable surface.
- Whitewashing: Commonly applied in whitewashing for a bright, smooth finish.
Class D lime’s unique magnesium content and smooth finish make it ideal for aesthetic applications like plaster finishing and whitewashing.
5. Class E: Kankar Lime
Class E lime, also known as kankar lime, is produced by burning lime nodules found in soils, such as black cotton soil, which contains silica. This type of lime is typically available in hydrated form and is primarily used for masonry mortar, plastering, and whitewashing.
Characteristics of Class E Lime:
- Color: Grey.
- Composition: Contains about 20% calcium oxide, 5% magnesium oxide, with the remaining composition being various impurities.
- Slaking and Setting: Slakes and sets slowly, requiring more time to hydrate and harden.
Uses of Class E Lime:
- Masonry Mortar: Suitable for creating masonry mortar, providing a strong bond for building materials.
- Plastering: Used as a base for plastering, offering a durable foundation.
- Whitewashing: Can be used for whitewashing applications, although it may not offer the same aesthetic quality as other limes.
Class E lime is commonly used in construction where a slower setting time is acceptable and where the lime’s lower purity does not impact the required strength for basic mortar and plastering tasks.
6. Class F: Siliceous Dolomite Lime
Class F lime, also known as siliceous dolomite lime, is available in both quicklime and hydrated forms. This type of lime is primarily used as an undercoat and finishing coat in plastering applications.
Characteristics of Class F Lime:
- Form: Available as both quicklime and hydrated lime.
- Uses: Mainly used as an undercoat and finishing coat for plastering, providing a smooth and durable surface for decorative finishes.
Class F lime’s composition makes it suitable for creating strong, refined plaster surfaces, particularly in applications requiring both a base and finishing layer.
Frequently Asked Questions
Lime can be categorized into three main types based on calcium oxide content:
- Fat Lime (High-calcium lime)
- Hydraulic Lime (Water-setting lime)
- Poor Lime (Lean lime)
Additionally, according to IS:712–1984, lime is classified into six classes: Class A (Eminently Hydraulic Lime), Class B (Semi-hydraulic Lime), Class C (Fat Lime), Class D (Magnesium/Dolomite Lime), Class E (Kankar Lime), and Class F (Siliceous Dolomite Lime).
The types of lime include:
- Fat Lime: High calcium content, used for plastering, whitewashing, and finishing.
- Hydraulic Lime: Contains clay impurities, suitable for damp conditions and underwater constructions.
- Poor Lime: High in clay content, primarily used for non-critical or temporary construction.
The IS:712–1984 classification provides further subcategories for specialized applications.
Limestone is a sedimentary rock primarily composed of calcium carbonate (CaCO₃). It forms from the accumulation of marine organisms’ shells, corals, and other calcium-rich debris.
