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Concrete is made up of three ingredients: aggregates, cement and water. Thanks to the adjuvants added to it, concrete can become more manoeuvrable, waterproof and lend itself better to the needs of construction. It can even be said that the adjuvants have become the fourth component of concrete.

Concrete admixtures

Concrete admixtures

Why add an adjuvant to concrete?

The additives chemicals that are added at low doses to concrete (less than 5% of the volume) modify its properties in the fresh state or in the cured state. They are added at the time of mixing.

Adjuvants are classified according to their effects:

  • Retarders or accelerators.
  • Plasticizers that increase the workability of fresh concrete.
  • Water repellents that waterproof.
  • The air trainers.
  • Curing products that protect the concrete during curing.
  • Milling agents.

Use of concrete retarders

They slow down taking delaying the hydration of concrete.
They are useful in hot weather to prevent concrete from hardening too fast or when transporting concrete over long distances. When the installation conditions are difficult or long, they make it possible to perform this or make a recovery more easily.

Use of concrete setting accelerators

They act in accelerating the hydration of concrete. They are of two kinds: setting accelerators or hardening accelerator. Note that their use may lead to less mechanical strength of the concrete.

Use of plasticizer for concrete

They facilitate the laying of concrete in the making it more manageable. The plasticizers-water reducer give it a better resistance.
Superplasticizers, which are synthetic polymers, have a greater effect than traditional plasticizers for less product. Some are specifically designed for self-leveling concretes (BAN) or self compacting (BAP).

Use of water repellents for concrete

They make it possible to waterproof the concrete by stopping the capillary absorption. They are more likely to be used in coatings rather than in all concrete.
They are of two kinds: the water repellent that is incorporated into the mix when mixing concrete or the surface water repellent that is applied to the hardened concrete. In the latter case, the water repellent is no longer really an adjuvant.

Air trainers

Their role is to create microscopic air bubbles in the concrete. They allow it to better resist frost and especially freeze / thaw cycles.

Cure products

These are not properly adjuvants since they are sprayed on the concrete once it is poured. They prevent or compensate for too rapid moisture loss and control the temperature of the concrete. Too fast drying results in less mechanical strength of the concrete and shrinkage phenomena or cracks.

The grinding agents

They are added during the manufacture of the cement at the time of grinding. They avoid the cement to agglomerate and ensure better quality of the finished product.

Did you know? The addition of admixtures in concrete dates from the Roman era. Egg yolk or blood was added to the lime concrete. Since the development of concrete in the nineteenth century, various additive tests have been carried out. Until trials of adding sugar as a retarder!

Video Instruction: Effects and Usage of Admixtures [Year-3]