Retarders for Concrete

Definition

• A retarder is an admixture that slows down the chemical process of hydration in concrete.
• Allows concrete to remain plastic and workable for a longer time compared to concrete without retarders.

Key Characteristics

• Retarders increase the setting time and delay the setting of cement.
• They can enhance compressive strength during freezing and thawing conditions.

Common Examples of Retarders

• Calcium sulfate (most frequently used)
• Sugar (effective retarding agent)
• Starch
• Cellulose
• Ammonium chlorides
• Ferrous and ferric chlorides
• Sodium hexametaphosphate
• Lignosulfonic acid and its salts
• Carbohydrates
• Hydrocarboxylic acids and their salts

Effects of Adding Retarders

• Adding 0.2% retarders by mass of cement can delay final setting time up to 72 hours.
• Adding 0.1% retarders by mass of cement can raise strength:
  • At 3 days
  • Increases 28-day strength by 30%.

Applications of Retarders

• Used in grouting for voids in:
  • Concrete arches
  • Tunnel linings
• Ensure better bond between successive lifts in concrete construction.
• Used in composite construction where steel sections carry concrete loads before it acts as a compression member.
• Allow mixing of concrete with hot water or steam injection without affecting the 28-day strength, unlike normal accelerated curing.

Conclusion

• Upcoming discussions will cover other types of admixtures.
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