What are Concrete Admixtures?

An admixture is any material, other than water, aggregates, hydraulic cement, and fibres, that is used as an ingredient of concrete or mortar and added to the batch immediately before or during mixing.

Why Use Admixtures?

• Improve workability without adding extra water
• Increase compressive strength
• Reduce water-cement ratio (and hence increase durability)
• Retard or accelerate setting time
• Reduce heat of hydration
• Improve concrete durability in aggressive environments
• Provide waterproofing
• Resist freeze-thaw damage (air entrainment)
• Reduce cement consumption (economy + environmental benefit)
• Enable high-performance concrete (HPC) and self-compacting concrete (SCC)
• Improve pumpability over long distances and heights

Types of Concrete Admixtures

Admixtures fall into two broad categories:

1. Chemical Admixtures

Soluble compounds added in small quantities. Governed by IS 9103.

• Plasticisers (Water-Reducing Admixtures)
• Super-Plasticisers (High-Range Water Reducers)
• Retarders
• Accelerators
• Air-Entraining Agents
• Waterproofing Admixtures (IS 2645)
• Corrosion Inhibitors
• Shrinkage Reducing Admixtures
• Viscosity Modifying Admixtures (VMA)

2. Mineral Admixtures (Supplementary Cementitious Materials)

Finely ground materials added in larger quantities (often 5–40%).

• Fly Ash (Class F & Class C) — IS 3812
• Ground Granulated Blast-Furnace Slag (GGBS) — IS 12089
• Silica Fume (Micro Silica) — IS 15388
• Metakaolin
• Rice Husk Ash
• Calcined Clay

Plasticisers (Water-Reducing Admixtures)

Plasticisers improve concrete workability without adding extra water, or reduce water content while maintaining workability.

How they Work

Plasticiser molecules adsorb onto cement particles, neutralising their attraction and dispersing them. The freed water becomes available to lubricate the mix.

Common Base Chemicals

• Lignosulphonates (LS)
• Hydroxylated Carboxylic Acids (HCA)
• Modified Lignosulphonates

Typical Properties

Uses

• Routine RCC construction
• Improving workability without water addition
• Reducing water-cement ratio for durability

Super-Plasticisers (High-Range Water Reducers)

Super-plasticisers are highly effective water reducers that can reduce water content by 20–30% without affecting workability, or dramatically increase slump without adding water.

Common Types

PCE Super-Plasticisers

PCE-based super-plasticisers are the most advanced and widely used in high-performance concrete and RMC. They can reduce water by up to 40% and produce self-compacting concrete.

Uses

• High-strength concrete (M30 and above)
• Self-Compacting Concrete (SCC)
• Pre-stressed concrete
• Pumped concrete (high-rise)
• Ready-Mix Concrete (RMC)
• High-performance and durable concrete

Retarders

Retarders slow down the setting time of concrete, allowing more time for placing, compaction, and finishing — useful in hot weather or for long transport distances.

Common Base Chemicals

• Lignosulphonates
• Hydroxylated Carboxylic Acids
• Sugars and Sugar Derivatives
• Phosphates

Typical Properties

Uses

• Hot weather concreting
• Long-distance concrete transport (RMC)
• Continuous concreting in tall pours
• Construction joints control
• Architectural finishes requiring fine surface work

Accelerators

Accelerators reduce the setting time and increase early strength of concrete — ideal for cold weather, fast de-shuttering, and emergency repairs.

Common Base Chemicals

• Calcium Chloride (CaCl₂) — not recommended for RCC (corrodes steel)
• Calcium Nitrate
• Calcium Formate
• Sodium Silicate (for shotcrete)
• Triethanolamine

Typical Properties

Uses

• Cold weather concreting
• Emergency repair works
• Shotcrete and tunnel linings
• Pre-cast manufacturing
• Fast-track de-shuttering

Air-Entraining Agents

Air-entraining agents introduce microscopic air bubbles (10–300 µm) uniformly distributed in concrete. These bubbles provide freeze-thaw resistance and improve workability.

Common Base Chemicals

• Sulphonated Hydrocarbons
• Resin Soaps
• Vinsol Resin
• Synthetic Detergents

Typical Properties

Uses

• Cold climate concreting (freeze-thaw resistance)
• Concrete pavements
• Hill area / Himalayan construction
• Marine structures
• Improving workability of harsh mixes

Waterproofing Admixtures (IS 2645)

Waterproofing admixtures (also called integral waterproofing compounds) reduce concrete permeability and water absorption, making structures more durable in moist environments.

Common Types

• Hydrophobic / Water-repelling agents (stearates, oleates)
• Crystalline admixtures (penetrate concrete pores and crystallise)
• Pore-blocking admixtures (calcium chloride-based)
• Polymer-based admixtures

Typical Dosage

Uses

• Basements and underground structures
• Water tanks (overhead, sump, underground)
• Swimming pools
• Roof slabs and terraces
• Bathrooms and toilets
• Retaining walls
• Marine structures

Popular Brands

• Dr. Fixit Pidiproof LW+
• Fosroc Conplast WP90
• BASF MasterPel
• Sika Plastiment
• Asian Paints Damp Block

Mineral Admixtures (Supplementary Cementitious Materials)

Mineral admixtures replace a portion of cement, reducing cost, improving durability, and lowering the CO₂ footprint of concrete.

1. Fly Ash (IS 3812)

• By-product from coal-fired power plants
• Spherical particles improve workability
• Reduces heat of hydration
• Improves long-term strength and durability
• Typical replacement: 15–35% of cement

2. Ground Granulated Blast-Furnace Slag (GGBS) (IS 12089)

• By-product of iron and steel manufacturing
• Improves chemical resistance
• Reduces heat of hydration
• Better for marine and sewage structures
• Typical replacement: 30–70% of cement

3. Silica Fume / Micro Silica (IS 15388)

• By-product from silicon metal production
• Particles 100 times finer than cement
• Produces very high-strength concrete (M60+)
• Significant cost
• Typical replacement: 5–10% of cement

4. Metakaolin

• Calcined kaolin clay
• White colour — good for architectural concrete
• Improves strength and durability
• Typical replacement: 5–15%

Typical Admixture Dosages

How to Select the Right Admixture

How to Use Admixtures Correctly

1. Read the manufacturer's data sheet completely
2. Conduct trial mixes to confirm dosage and compatibility
3. Use a calibrated dispenser to measure dosage
4. Add admixture to the mixing water, then add to concrete (or as per product instructions)
5. Mix thoroughly for the recommended time
6. Test the slump and workability before placing
7. Maintain consistent dosage across all batches
8. Store admixtures in a cool, dry place away from sunlight

Admixture Compatibility Issues

• Cement-admixture compatibility: not all admixtures work well with every cement — always test
• Avoid mixing different brands of admixture in the same concrete batch
• Retarders + accelerators should not be mixed (defeats purpose)
• Calcium chloride accelerators must not be used in RCC (corrosion risk)
• Excess super-plasticiser causes bleeding and segregation
• PCE compatibility with PPC and PSC needs verification

Tests for Concrete Admixtures (IS 9103)

• Specific Gravity Test
• pH Value Test (typical range 6–9)
• Solids Content Test
• Setting Time Test on Concrete
• Water Reduction Test
• Compressive Strength Test (3, 7, 28 days)
• Flexural Strength Test
• Length Change (drying shrinkage)
• Chloride Content (for RCC use, < 0.2% by admixture weight)
• Air Content (for air-entraining agents)

Applicable Indian Standards

Popular Admixture Brands in India

Advantages of Using Admixtures

• Improves concrete workability and finishability
• Allows lower water-cement ratio (durability boost)
• Increases compressive strength
• Reduces cement consumption (economy + green)
• Enables high-strength concrete (M40+)
• Improves pumpability for high-rise concreting
• Controls setting time as per site requirements
• Improves durability in aggressive environments
• Provides integral waterproofing
• Reduces shrinkage cracks
• Improves freeze-thaw resistance
• Enables self-compacting concrete (SCC)

Disadvantages / Risks

• Adds to material cost (varies by type)
• Requires accurate dosage control
• Some admixtures (calcium chloride) corrode reinforcement
• Over-dosage can cause setting/strength problems
• May lead to segregation or bleeding if not controlled
• Compatibility issues with certain cements
• Storage requirements (cool, dry, sealed containers)
• Trial mixes required for new products

Storage of Admixtures

• Store in cool, dry place away from direct sunlight
• Temperature: 5°C – 35°C (avoid freezing)
• Keep containers tightly sealed
• Use FIFO — oldest first
• Shelf life: typically 6–12 months from manufacture
• Inspect for sedimentation, colour change, or odour
• Stir liquid admixtures gently before use

Safety Precautions

• Wear rubber gloves and safety goggles
• Avoid skin and eye contact
• Wash thoroughly after handling
• Use respirator when handling powdered admixtures (silica fume, fly ash)
• Store away from food and beverages
• Keep MSDS (Material Safety Data Sheet) for each product
• Provide first-aid eye-wash facility at site

Conclusion

Concrete admixtures are essential tools of modern concrete technology. From the routine plasticiser used in residential RCC to advanced PCE super-plasticisers in high-rise pumped concrete, the right admixture can dramatically improve workability, strength, durability, and economy.

Always select admixtures based on the specific job requirements, conduct trial mixes, follow manufacturer's dosage recommendations, and verify compatibility with your cement. Used wisely, admixtures unlock the full potential of modern concrete — producing structures that are stronger, more durable, and more sustainable.