Concrete Retarders: It's Uses, Mechanism & Pros-Cons - GharPedia

The modern-day material chemistry has revolutionized all industries, and concrete is also not an exception. Adding few ml of chemicals to concrete modifies the property of concrete to a larger extent, and hence its application is increasing day by day.
Concrete is the most used material in the construction, made of cement, sand, aggregate, water, and concrete admixtures. The concrete admixtures or concrete additives are the important materials which modify one or more than one properties of concrete in the plastic state and the hardened state, such as an increase or decrease in setting time, increased hardening, workability, strength, unit weight etc.

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There are many kinds of admixtures available which can be used in concrete to modify the properties of concrete such as concrete accelerator, concrete retarder, plasticizers, Superplasticizer, air entraining admixtures etc. These admixtures are added before mixing or during mixing of concrete itself.
Here, we are going to discuss the concrete retarders in detail, so that you can get an idea about its working mechanisms, uses, advantages and disadvantages

Concrete Retarders: How it Works?

According to ‘Indian Standard, IS : ’ (Concrete Admixtures- Specification (First Revision)), “An admixture which delays the setting of cement paste, and hence of mixtures, such as mortar or concrete containing cement is called retarders”. Concrete retarders are also Known as retarding admixtures or retarders.
Concrete retarders are added in the concrete to delay the setting time of the concrete. As you know, the procedure of setting or stiffening is a result of a chemical reaction between water and cement in the concrete. This reaction is called hydration. When retarders are used in the concrete, it temporarily slows down the chemical reaction (hydration). Hence the concrete remains in plastic stage and remains workable for a longer time.
Sometimes we need more time to place concrete because of difficult placing condition or delay may occur in transportation. Take an example, in ready mix concrete, concrete is manufactured in the central batching plant and transported over a long distance and often to the work sites which may take significant time depending on the mode of transportation. In that case, if you want the concrete in a plastic state for perfect final placing and compaction, then the setting time of concrete will have to be increased or simply delayed by retarding its hydration process.

Concrete retarders are also used to overcome the accelerating effect of high temperatures on setting properties of concrete in hot weather concreting.

Mechanisms of Concrete Retarders

‘The European Federation of Concrete Admixture Associations (EFCA)’ stated that, “Retarding admixtures are used to slow down the speed of the reaction between cement and water by affecting the growth of the hydration products and/or reducing the rate of water penetration to the cement particles”.
The mechanism of the concrete retarders can be understood in a simple way: the retarding admixtures form a film around the cement compounds (e.g., by absorption). This film prevents or slows the reaction with water. How much the rate of hydration will be retarded is governed by thickness of this film. When this film breaks down, then normal hydration starts. If in some cases when the dosage of concrete retarders exceeds a certain critical point, hydration of cement compounds will never proceed beyond a certain stage, and the cement paste will never set. Hence, it is important to avoid overdosing and take proper care while using a retarding admixture.
The factors influencing the degree of retardation are water-cement ratio, cement content, C3A and alkali contents present in the cement, type and dosage of the concrete retarder, and the stage at which the retarder is added to the concrete mix itself. The efficiency of retarder is increased if its addition to the fresh concrete is delayed for a few minutes.

Advantages of Concrete Retarders  

Considered a boon for the construction process in the long run, concrete retarders assist with multiple advantages and some major advantages are being discussed below for your reference,

• Retarding admixtures extend the setting time of concrete and maintain the workability and cohesion for extended period of time
• In the large construction projects, concrete retarder prevents cold joints formation in successive lifts and make concrete workable throughout the concrete placing
• With the use of concrete retarders, you can compensate the accelerating effect of high temperature towards the initial setting time and minimize risks of long-distance delivery in hot weather.
• Use of retarding admixtures improves pumpability of concrete by delayed setting period and improved workability of concrete.
• Concrete retarders help to reduce segregation and bleeding in concrete in which poor sand grading is unavoidable.

Disadvantages of Concrete Retarders

Everything has a slice of precautions or you may say disadvantages. Same stands true in case of concrete retarders. Laid below are some common disadvantages that one should be aware of before using concrete retarders for the construction process.

• The concrete retarders can be used with water-reducers but care should be taken while curing to avoid cracks and bleeding.
• The dosage of retarding admixtures should be within limits depending upon its water-cement ratio, cement content and amount of C3A in the cement.
• Use of gypsum as a retarding admixture for the purpose of retarding setting time is only recommended when adequate inspection and control is available, otherwise, the addition of excess amount may cause undesirable expansion and an indefinite delay in the setting of concrete.
• Concrete retarders may have variable action on different types of cement when used in different quantities.

Uses of Concrete Retarders  

Concrete retarders are abundantly used or applied in various construction settings; however, some most popular uses or applications are given over here,

• Retarding admixtures are most commonly used in hot weather concreting.
• Retarder sprayed to the surface of the formwork to prevent the hardening of the matrix at the interface of concrete and formwork, whereas the rest of the concrete gets hardened.
• Retarders are used for large or massive construction work.
• Retarding admixtures are used in grouting oil wells.

According to ‘M.S. Shetty’ (Author of the Concrete Technology Theory and Practice 16th edition), oil wells are sometimes taken up to a depth of about -meter-deep where the temperature may be about 200°C. The annular spacing between the steel tube and the wall of the well will have to be sealed with cement grout. Sometimes porous rock strata may also require grouting to prevent the entry of gas or oil into some other strata. For all these works, cement grout is required to be in workable condition for around 3 to 4 hours and not get set at that high temperature. For meeting these challenging requirements retarding admixtures are commonly applied and mostly yield preferable results.

Some Common Types of Concrete Retarders  

The most commonly known concrete retarder is calcium sulphate – gypsum. Sugar is also one of the most common and effective retarding admixture which is used for delaying the setting time of concrete without detrimental effect on the ultimate strength of concrete itself.
There are number of other organic and inorganic materials used as concrete retarders, these are:

• Starches,
• Cellulose Products,
• Acids or Salts of Acids like mucic acids, calcium acetate etc.
• Lignosulphonates
• Hydroxycarboxylic Acids and their salts
• Phosphonates
• Borates
• Salts of Pb, Zn, Cu, As (Lead, Zinc, Copper, Arsenic)

Lignosulphonic acids and their salts and Hydroxylated carboxylic acids and their salts which in addition to the retarding effect also reduce the quantity of water required for the given workability.
Nowadays, admixtures are manufactured to combine a set of retarding and water reducing properties. They are usually made by the mixing of conventional water reducing admixtures and sugars or hydroxylated carboxylic acids or their salts.

In the summary, the knowledge about the concrete retarders is essential for their proper application. It’s beneficial for maintaining workability in hot weather concreting, in difficult placing conditions, for long transportation etc. But it is highly recommended that before retarder is used, trial mixes with the actual cement to be used in construction, should be tested. Any mistake may Cause delay in construction, wastage of time and money too.

Also Read:

Pozzolanic or Mineral Admixtures! Eco-Friendly and Inexpensive Cementing Material

All you Need to Know Before Using Air Entraining Admixtures for Concrete

Concrete admixtures and retarders are popular materials used to enhance the mechanical properties of concrete and mortar. Nowadays, concrete retarders are highly preferred in hot weather to allow time to properly mix, transport, and place concrete wherever necessary.

Table of Contents

Many construction chemical manufacturers provide concrete retarders but to get the best results it is always recommended by the experts to choose the best ones. Nevertheless, in this blog, we will explore all about concrete retarders including their mechanisms, types, benefits, and more.

What is a Concrete Retarder?

To give idea about the concrete retarders, it is additives that are added to cement-based mixtures like mortar or concrete to delay the setting time of the cement paste. These are commonly known as concrete retarders, which are water-soluble additives that get easily dissolved in water.

When you add the concrete retarder in the concrete mix can result in extending the setting time of the concrete by approximately up to one hour. It basically slows down the hardening in warm or hot weather to allow construction workers more time to mix, deliver and pour the concrete. In fact, concrete retarders reduce the pace of the process while conserving water.

Mechanism of Retardation

A concrete retarder is a type of admixture that temporarily stops the hydration process upon addition to the mixing water or on application to the surface of fresh concrete, thereby allowing additional time for the concrete to rest before it begins to harden.

The mechanism involved in such retarders depends on both the type of retarder and cement involved. It is worth noting that the retarder effects are temporary or for a short period of time, and after a certain period, its mechanism fades, and the watering goes back to normal.

Are you interested in learning more about retarder in concrete? Contact us today to secure an expert consultation!

Below are the four key ways retarders slow down the setting process:

1. Adsorption:

A retarding additive forms a protective coating on the surfaces of the cement particles. The barrier reduces access by water molecules to the unhydrated cement, hence it retards the hydration reactions. As a result, the cement paste thus remains workable for a longer time due to the insufficient hydration produced to initiate hardening. The retarder complex with the C3A of cement and comes out of solution, incorporating into the hydrating material.

2. Nucleation:

When water is added to it, the surfaces of cement particles release calcium and hydroxyl ions into the mix. When their concentration increases up to a certain critical amount, C2S and CS start crystallizing. In this process, the crystal nuclei of calcium hydroxide absorb the retarding admixture that delays further crystal growth until the point where the cement becomes supersaturated.

3. Complexation:

At the start of the process, calcium ions given out by the cement grains make some complex with other ions. It is easier to dissolve cement when these complex forms. In the liquid phase of the cement paste, during hydration with a retarding admixture, the concentrations of Ca²⁺, OH⁻, Si, Al, and Fe are increased. This buildup of calcium and hydroxyl ions interferes with the association between these ions and thus slows down the hydration process.

4. Precipitation:

During precipitation, the retarder interacts with the highly alkaline solution to form insoluble derivatives. The phenomenon resembles with adsorption. In a few minutes of mixing water with cement, the pH of the solution goes beyond 12. These insoluble compounds create protective coatings around the cement particles, which slows down the hydration process. The coating forms a diffusion barrier that restricts the ability of water to reach the cement particles effectively.

This is all about the mechanism of concrete retarders and now let’s check out the types of concrete retarders. 

Types of Concrete Retarders

There are broadly two categories of concrete retarders which are as follows:

1. Organic Retarder

• Phosphonates
• Sugars
• Lignosulphonates
• Hydroxycarboxylic acids and their salts

2. Inorganic Retarder

• Phosphonates
• Borates
• Salts of Pb, Zn, Cu, As, Sb

Now, let’s understand how retarding additives acts on the concrete’s properties.

How Retarding Additives Affect Concrete’s Properties?

1. Effect on Strength: When used with concrete, retarding additives decrease the initial compressive strength compared to non-treated concrete.
2. Effect on Durability: As retarding additive primarily slows down the setting time of the concrete, without affecting other properties of the concrete. Therefore, after curing retarded concrete shows the exact durability as the ordinary concrete.
3. Effect on Slump Loss: It is evident that the concrete treated with retarding additive shows great workability of the concrete by lowering slump loss.
4. Volume Deformation: Retarding admixtures has little effect on drying shrinkage but it may lead to an increase in plastic shrinkage.
5. Effect on Workability: The concrete workability is slightly affected by the retarding additives which led to a 60-100 mm more initial slump.
6. Effect on Air Entrainment: Most of the retarding additives do not accompany air, although some are. Particularly those based on Hydroxycarboxylic acid can reduce the air content.
7. Bleeding Risk: Whenever the concrete retarders are used in the concrete mixture, the risk of bleeding in the concrete grows.
8. Freeze-Thaw Cycles: The use of air-entraining admixtures often elevates the concrete’s resistance to freezing and thawing.
9. Heat of Hydration: Although retarding additives doesn’t eliminate the heat production of concrete. However, it delays the increase in peak temperature for the amount of time the concrete was retarded.

The above-mentioned effects can be observed when the concrete retarders are added to the concrete mixture. However, the best concrete retarder doesn’t affect the concrete properties after the setting time effect. Keep reading to know the various uses of concrete retarders.

Uses of Concrete Retarder

There are various applications of concrete retarders to get the desired outcome in particular situations. The following are the uses of concrete retarder: 

• Concrete retarders are mostly used with concrete admixtures when temperatures are hot or warm.
• Builders, developers, or construction companies often use concrete retarders when they aim to build big buildings or infrastructure projects.
• In some situations, the retarder is used on the outer surface of the formwork to stop the matrix from hardening.
• In certain instances, the retarder is applied to the outer surface of the formwork to prevent the matrix from hardening. However, the remaining concrete part gets hard due to the non-application of the concrete retarder.
• Admixtures that delay the reaction rate are utilized during the grouting process in oil wells.

Now that you are aware of retarder additive uses, let’s have insights into the advantages of concrete retarders.

Benefits of Concrete Retarders

There are several advantages of concrete retarder which are as follows:

• By using the concrete retarding admixtures, the workability and cohesion ability of concrete can be preserved. Resulting in increasing the time requires to set.
• Concrete retarders can help lower the risks associated with long-distance transportation in hot weather by targeting the acceleration of the initial setting time caused due to high temperatures.
• Due to the concrete retarder’s properties, it slows down the hardening process between concrete pours and helps avoid the formation of cold joints in large construction projects.
• In certain situations, concrete retarders can be used to decrease segregation and bleeding when poor sand grading is inevitable.
• When the concrete setting time is delayed and the workability of concrete is increased then concrete retardants improve permeability.

Limitations of Concrete Retarder

Each material has its own set of advantages and disadvantages when applied. Let’s explore some of the limitations of using concrete retarders.

• It is possible to use water reducers along with concrete retarders, but careful monitoring of the curing process is essential to avert bleeding and cracks.
• In the case of gypsum, it can positively delay the setting time of concrete when the process is properly monitored and managed. If gypsum addition is not monitored then it could result in unwanted expansion and an indefinite delay in the concrete setting.
• It is possible that concrete retarders can have different effects on various types of cement at different concentrations.
• The appropriate dosage of retarding admixtures must be determined based on the cement’s water-cement ratio, cement content, and C3A concentration.

The best concrete retards are always used in many great construction projects, however, you should also choose a branded one to get the best results in the construction process. Let’s have a look at one of the top-quality dry mix raw materials that can be used with the concrete admixtures.

Introducing Sodium Gluconate by Sakshi Chem Sciences Pvt Ltd

Sodium Gluconate which is also known as Sodium Salt of Gluconic Acid which is a dry mix raw material, can be used as a setting time retarder. Our Sodium Gluconate is formulated under strictly controlled conditions through the fermentation of glucose. To give an idea about it, it is a white crystalline powder and easily soluble in water.

Our Sodium Gluconate is non-toxic, non-flammable, and non-corrosive. In short, Sodium Gluconate is an environment-friendly and biodegradable construction chemical. When it is added to cement, it inhibits hydration and slows down the setting time of the concrete mix.

Conclusion

To wrap up, a concrete retarder’s role is impressive in the construction industry as it delays the hydration process. It keeps the concrete workable and easier to handle for an extended period. Concrete retarders are specifically useful in hot and warm temperatures to allow time for workers to deliver, transport, and pour the concrete. However, it is very essential to buy this construction chemical from authorized and genuine manufacturers like Sakshi Chem Sciences Pvt Ltd. 

We are one of the leading construction chemical manufacturers in India offering 200+ construction chemicals to construction companies, developers, builders, and various clients. Our trained team offers the best consultation, customized quotations, and assistance on quality products. So, if you are also searching for the best quality construction chemicals then get in touch with us.

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