Types of Admixtures Used in Concrete and Why They Are Used

Concrete is one of the most widely used construction materials in the world. While basic concrete is made of cement, fine aggregate (sand), coarse aggregate (stone), and water, modern construction often demands special properties such as higher strength, better workability, faster or slower setting, durability, and resistance to water or chemicals.

To achieve these special properties, admixtures are used.

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What Is an Admixture?

An admixture is a chemical or mineral substance added to concrete before or during mixing to modify its properties in the fresh or hardened state.

Admixtures are not mandatory, but they are used when concrete needs to meet specific performance requirements. Today, admixtures are widely used in:

Ready Mix Concrete (RMC) plants

In-plant batching systems

Large construction sites

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Why Are Admixtures Used in Concrete?

Admixtures are used to:

Improve workability without adding extra water

Increase or decrease setting time

Increase strength and durability

Reduce water-cement ratio

Improve resistance to weather, chemicals, and corrosion

Reduce shrinkage and cracking

Improve pumping and placing of concrete

👉 Important Note for Students:

Admixtures are selected based on site conditions and project requirements, not randomly.

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Classification of Admixtures

Admixtures are broadly classified into two categories:

1. Chemical Admixtures

2. Mineral Admixtures

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1. Chemical Admixtures

Chemical admixtures directly modify the behavior of concrete by chemical action.

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1. Plasticizers (Water Reducers)

Purpose:

Reduce water content without reducing workability

Benefits:

Lowers water-cement ratio

Increases strength

Improves workability

Water reduction: ~15–20%

Plasticizers make concrete easier to place and finish while producing stronger concrete.

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2. Superplasticizers (High-Range Water Reducers)

Purpose:

Achieve very high workability or very high strength

Benefits:

Convert low-slump concrete into flowing concrete

Allows easy placement in congested reinforcement

Reduces shrinkage and thermal cracking

Water reduction: ~30% or more

Widely used in high-strength concrete and RMC projects.

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3. Retarders and Retarding Plasticizers

Purpose:

Slow down the setting time of concrete

Used when:

Hot weather concreting

Long-distance transportation of concrete

Large pours (rafts, dams)

Did you know?

👉 Sugar is a natural retarder and can delay concrete setting in emergencies.

⚠️ Not recommended in cold weather.

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4. Accelerators and Accelerating Plasticizers

Purpose:

Speed up the setting and early strength gain

Common accelerator: Calcium Chloride

Uses:

Cold weather concreting

Early formwork removal

Emergency repairs

Calcium chloride is usually limited to 2% by weight of cement.

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5. Air-Entraining Admixtures

Purpose:

Introduce microscopic air bubbles into concrete

Benefits:

Improves freeze-thaw resistance

Increases workability

Reduces bleeding

These air bubbles prevent internal cracking during freezing conditions.

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6. Damp-Proofing and Waterproofing Admixtures

Purpose:

Prevent water penetration into concrete

Applications:

Basements and substructures

Water tanks and dams

Damp-proof courses above plinth level

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7. Water-Retaining Admixtures

Purpose:

Reduce bleeding and segregation

Benefits:

Improves surface finish

Increases durability

Useful with poor-quality aggregates

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8. Dual or Multifunction Admixtures

A single admixture or combination of admixtures performing multiple functions, such as:

Water reduction + retardation

Workability improvement + strength gain

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9. Corrosion Inhibitors

Purpose:

Protect steel reinforcement from corrosion

Used in:

Marine structures

Bridges

RCC structures exposed to moisture and salts

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10. Shrinkage-Reducing Admixtures

Purpose:

Reduce drying shrinkage

Benefits:

Minimizes cracking

Improves dimensional stability

These are used in larger quantities compared to other admixtures.

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11. Pozzolanic Admixtures (Chemical Type)

Purpose:

Partial replacement of cement

Benefits:

Improves durability

Reduces heat of hydration

Enhances long-term strength

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12. Gas-Forming Admixtures

Purpose:

Produce lightweight concrete

Materials used:

Aluminium powder

Hydrogen peroxide

Activated carbon

These create gas bubbles, reducing density and preventing settlement.

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2. Mineral Admixtures

Mineral admixtures are cement-replacing materials that improve concrete properties and sustainability.

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1. Fly Ash

By-product of coal combustion

Improves workability and durability

Reduces cement consumption

Enhances long-term strength

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2. Silica Fume (Micro Silica)

Very fine amorphous silica

Increases strength significantly

Reduces permeability

Used in high-performance concrete

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3. Blended Hydraulic Cement

Combination of OPC and pozzolanic materials

Reduces clinker content

Improves durability

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4. Metakaolin

Calcined clay material

Highly reactive pozzolan

Improves strength and resistance to chemicals

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5. Rice Husk Ash & Volcanic Ash

Agricultural and natural by-products

Used as supplementary cementitious materials

Improve durability and sustainability

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6. GGBFS (Ground Granulated Blast Furnace Slag)

By-product of iron manufacturing

Enhances durability and sulfate resistance

Reduces heat of hydration

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Final Notes for Students

Every admixture has a specific purpose

Dosage depends on project requirements

Improper use can harm concrete performance

Always follow IS codes and manufacturer guidelines

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Key Takeaway

👉 Admixtures help concrete meet modern construction demands by improving strength, durability, workability, and performance, making them an essential part of today’s civil engineering practice.