What is Concrete Retarder – Types, Uses, Advantages and Disadvantages

Concrete retarders, also known as retarding admixtures, play a crucial role in construction by slowing down the setting process of cement-based mixtures. Let’s delve into the mechanisms, types, and effects of these additives.

Mechanism of Retardation

Concrete retarders function by temporarily pausing the hydration reactions in cement paste. Four mechanisms contribute to this process:

i) Adsorption

• Retarder forms a protective skin on cement particles.
• Slows hydration by hindering water access.

ii) Nucleation

• Calcium ions and hydroxyl ions expelled from cement particles.
• Inhibits further crystallization until super saturation.

iii) Complexation

• Complex formation with ions increases solubility.
• Slows down clumping, delaying hydration.

iv) Precipitation

• Reacts with alkaline solution, forming insoluble compounds.
• Acts as a diffusion barrier, hindering hydration.

Types of Concrete Retarders

Concrete retarders come in two main types:
1. Organic Retarder
Phosphonates, lignosulphonates, sugars, hydroxycarboxylic acids.

2. Inorganic Retarder
Salts of Pb, Zn, Cu, As, Sb; borates; phosphonates.

How Retarding Additives Affect Concrete

Retarding admixtures impact various properties of concrete:

Strength : Reduces initial compressive strength.

Slump Loss : Improves initial workability by decreasing slump loss.

Durability : Comparable durability to untreated concrete when properly cured.

Workability and Rheological Values : Marginally affects workability, leading to a higher initial slump.

Volume Deformation : Does not significantly alter creep or drying shrinkage.

Air Entrainment : Some retarding admixtures may reduce air content.

Bleeding : Increases the risk of bleeding in concretes.

Freeze-Thaw Cycles : Air-entraining admixtures enhance resistance to freezing and thawing.

Heat of Hydration : Delays the peak temperature rise in concrete.

Uses of Concrete Retarder

Concrete retarders find applications in various scenarios:

• Added in hot weather conditions.
• Sprayed on formwork surface to prevent matrix hardening.
• Common in large-scale constructions.
• Utilized in oil well grouting processes.

Advantages

• Preserves workability and cohesion.
• Mitigates risks during long-distance concrete delivery.
• Prevents cold joint formation in major projects.
• Reduces segregation and bleeding.
• Enhances permeability by delaying setting time.

Disadvantages

• Requires close monitoring when used with water reducers.
• Improper use of gypsum may lead to undesirable expansion.
• Dosage depends on various factors.
• Effects on different cement types may vary.

Conclusion

Concrete retarders offer a valuable solution to manage the setting time of concrete in challenging conditions. While they bring several advantages, careful evaluation and dosage consideration are essential to avoid construction setbacks. Trial mixes with the specific cement used are recommended for optimal results.