Concrete admixtures and additives are among the most popular materials used to modify several properties of concrete in plastic and hardened states. They can help increase the strength, workability, unit weight and hardening of concrete.

Some of the most common admixtures include concrete retarders, concrete accelerators, superplasticisers, plasticisers, and air entraining admixtures.

How Concrete Retarders Work

The main aim of using retarders in concrete is to extend or delay the cement paste’s setting time. The stiffening or setting is due to the chemical reaction between the cement and water in the concrete. The reaction is referred to as hydration.

Concrete retarders slow down the hydration process. This keeps the concrete in its plastic stage and more workable for longer. Concrete retarders are used when more time to place the concrete is needed during transportation or a complex placing condition. The retarders are helpful when putting concrete at increased temperatures or using concrete transported over a long distance.

When water is added to cement, there is a fast initial hydration reaction, after which there is a slight formation of hydrates for about 2-3 hours. The exact reaction time depends on the temperature and cement type, the time before the cement starts to harden is called the dormant period. The dormant period is characterised by an increase in hydration rate and increase in the rate at which calcium hydroxide is formed.

The retarding admixture slows down the end of the dormant period and the commencement of the hardening and setting phase. When used alone, retarders allow concrete vibration to avoid the formation of cold joints when the concrete is placed between layers.

It is essential to note that the retardation mechanism is short-term. After a certain period, the effects of the retardation mechanisms stop, and hydration continues.

Setting of Concrete

The concrete setting is the transition process of converting concrete from plastic to a hardened state. The cement properties significantly affect the setting time.
Other factors affecting the setting include:

  • Suitable temperature
  • Cement content
  • Water cement ratio
  • Admixtures
  • Type of cement
  • Type and amount of aggregate
  • Relative humidity

Common types of concrete retarders

The most commonly used concrete retarder is calcium sulphate. Sugar is also an effective and common retarding admixture used for delaying the setting time of concrete without a detrimental effect on the strength of the concrete.

Other organic and inorganic materials used as retarders include:

  • Acids (tartaric and citric)
  • Phosphonates
  • Cellulose products (sucrose, glucose)
  • Sodium gluconate
  • Rochelle salt and cream of tartar
  • Lignosulphonates
  • Starches
  • Hydroxycarboxylic acids
  • Borates

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