What is a retarder in concrete?

The article aims to answer the question “What is a retarder in concrete?”. It will also discuss 

What is a retarder in concrete?

A retarder in concrete is a “slow down product”. Continue reading to understand. 

An agent that slows down a chemical reaction is known as a retarder. As an example, retarders may be used to delay the chemical reaction hardening of plastic materials like wallboard and concrete.

Concrete curing is slowed down by adding sugar water to the mix. In order to remove the top layer, it may be used to delay the chemical hardening of the surface, allowing the aggregate to be washed away.

Workability during installation and a delay in concrete’s first set are the primary benefits of retarders. A little amount of air may be entrained in concrete as a result of most retarders simultaneously serving as water reducers.

How does a retarder work in concrete?

A retarder, either dissolved in the mixing water or sprayed on the surface of the concrete, momentarily halts the hydration processes, resulting in a prolonged period of dormancy. Retarders and cement have different mechanisms that arise depending on their mix.

It’s also vital to keep in mind that the processes of retardation are only in place for a short time. The effects of the mechanisms wear off after a certain amount of time, and the hydration continues.

The retardation process may take place in four distinct ways between the retarder and the cement, explained below:

Additive adsorption: the process by which a slowing agent is absorbed into the mixture. Adding a layer of retarding additive to the cement provides a protective coating around the cement particles. 

Because of this diffusion barrier, water molecules cannot reach the surface of the unhydrated cement particles, which causes hydration to be retarded. As a consequence, there is less hydration product in the cement paste, allowing it to stay flexible for a longer length of time.

Nucleation: When water is introduced to the cement, the surface of the cement particles releases calcium and hydroxyl ions into the air. They begin to crystallize when the concentration of those ions approaches a critical point.

Adding a retarding additive to the cement slows the formation of calcium hydroxide nuclei until they reach a certain degree of supersaturation.

Complexation: In the first few minutes, a kind of structure with calcium ions, produced by the cement particles, is formed. The cement becomes more soluble as a result of the creation of these compounds.

In the aqueous phase of the cement paste, an increased concentration of Ca2+, OH-, Si, Al, and Fe occurs during hydration in the presence of a retarding admixture. As a result of the calcium and hydroxyl ion buildup in the solution, calcium hydroxide cannot precipitate. Hydration is slowed as a result.

Precipitation: Unlike adsorption, precipitation produces some type of insoluble retarder derivatives by reacting with the extremely alkaline solution.. The solution’s pH climbs over 12 after just a few minutes of water-to-cement contact as a result.

It is the precipitation of these insoluble derivatives that inhibits the hydration of the cement particles. Diffusion is impeded by the protective covering, preventing water molecules from making adequate contact with the cement grains.

What is the mechanism of retarders in concrete?

Read on to understand the mechanism of retarders in concrete. Retarders are added to a concrete mix to help it set more slowly, making it easier to work with. Chemical interaction between water and cement in the concrete results in the setting or stiffening process. 

Hydration is the name given to the process that occurs as a result of this interaction. For a short period of time, chemical reactions are slowed down by retarders in the concrete (hydration). As a result, the concrete is kept in a working state for a longer period of time.

Concrete placement might take longer if the conditions are tough or if there is a delay in the shipment. At ready-mix concrete, for example, the concrete is made in a central batching facility and then shipped to the job site, which might take a considerable time depending on the method of transportation. 

Increasing the setting time of concrete or simply delaying its hydration will be necessary in this situation in order to keep the concrete pliable for optimal final placement and compaction. In hot weather concreting, concrete retarders are also employed to combat the accelerated impact of high temperatures on the setting qualities of concrete.

Retarding admixtures may be explained in an easy-to-follow manner: they generate a film surrounding cement compositions (e.g., by absorption). The water interaction is prevented or slowed by this coating. The thickness of this coating determines how much the hydration process will be slowed. 

Normal hydration begins when this coating decomposes. In rare situations, the hydration of cement compounds will never go beyond certain critical points, and the cement paste will never harden. To prevent overdosing on retarding admixture, it is essential to use it with caution.

The water-cement ratio, cement content, C3A and alkali concentration in the cement, the kind and dosage of the concrete retarder, and the stage at which the retarder is applied to the concrete mix are all elements that influence the degree of retardation. Retarding the addition of the retarder to the new concrete increases its efficacy.

What are the advantages of concrete retarders?

The advantages of concrete retarders are:

  • Maintaining the concrete’s functionality & uniformity for long periods of time is made possible by retarders.
  • A concrete retarder is used in major building projects to prevent cold joints from forming during slow lifts and to make the concrete more workable.
  • A retarder may be used to lessen delivery hazards in hot weather by reducing the increased effect of high temperatures on initial setup time.
  • Adding a retarder to the mix enhances the permeability of the concrete by extending the setting time and increasing the concrete’s workability.
  • Reducing isolation and bleeding in areas where the low sand grade is inevitable is achieved by using concrete retarder.

What are the disadvantages of retarders in concrete?

The disadvantages of retarders in concrete are:

  • In order to prevent cracks and bleeding, concrete retarders may be mixed with water.
  • The dosage of retarder additive is determined by the water-cement ratio, cement content, and C3a concentration in the cement.
  • Concrete retarders may have varied effects on different kinds of cement depending on how much they are used.

What are the uses of concrete retarders?

Continue reading to know the uses of concrete retarders.

  • During the warmer months, it is most typically employed in concrete.
  • It prevents the matrix from hardening at this interface between the formwork and the concrete, while at the same time hardening the other areas of concrete.
  • For large-scale building projects and oil well gritting, retarders are utilized.
  • In situations when concrete has to be put under challenging circumstances, such as during transportation or installation, it is often employed.
  • It is also utilized in the grouting of oil wells to pour consolidation castings without forming cold joints.
  • Even at this high temperature, cement grout must remain mobile for at least three to four hours before hardening.

What are the types of retarders in concrete?

There are two types of retarders in concrete, listed below

Organic retardersInorganic retarders
Phosphonates Borates 
Sugars Salt of Zn, Cu, As

Conclusion

Concrete retarders may be used to postpone the concrete’s initial setting period by up to an hour. When it’s hot outside, these materials are often employed to delay the quick hardening caused by the sun’s rays and the air temperature. Retarders are also water-reducing agents.

Frequently asked questions (FAQS): What is a retarder in concrete?

What is a retarder in concrete?

A retarder in concrete is a “slow down product”. An agent that slows down a chemical reaction is known as a retarder. As an example, retarders may be used to delay the chemical reaction hardening of plastic materials like wallboard and concrete.

Concrete curing is slowed down by adding sugar water to the mix. In order to remove the top layer, it may be used to delay the chemical hardening of the surface, allowing the aggregate to be washed away.

How does a retarder work in concrete?

A retarder, either dissolved in the mixing water or sprayed on the surface of the concrete, momentarily halts the hydration processes, resulting in a prolonged period of dormancy. Retarders and cement have different mechanisms that arise depending on their mix.

What is the mechanism of retarders in concrete?

Retarders are added to a concrete mix to help it set more slowly, making it easier to work with. Chemical interaction between water and cement in the concrete results in the setting or stiffening process. 

Hydration is the name given to the process that occurs as a result of this interaction. For a short period of time, chemical reactions are slowed down by retarders in the concrete (hydration). As a result, the concrete is kept in a working state for a longer period of time.

Bibliography

Concrete retarders. ConstructionOR. Retrieved from: https://constructionor.com/concrete-retarders/

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