Timing is crucial in construction, especially regarding pouring and placing concrete. The setting of concrete is a delicate chemical reaction, and every now and then, we need to take our time to develop the perfect mix. This is where retarding admixtures can come into play.
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Retarding admixtures are a great addition to concrete. They help to slow down the setting time of concrete without impacting its viability. Whether it is hot weather, long transportation, or a large pour, admixtures provide a little more flexibility and control on the project.
Let’s take a closer look at retarding admixtures, how they work, and where they fit into modern construction.
What is a Retarding Admixture?
A retarding admixture is a chemical compound added to cement compositions that postpones the chemical process of setting. Under normal circumstances, once cement is mixed with water, hydration begins, and the mix will harden.
But sometimes, you need that process to slow down. If you’re working in hot climates or building a huge structure, it is quite common that there will be some delay in the placement of concrete.
These admixtures do not usually make concrete more workable, like plasticizers or superplasticizers might, which brings us to retarding plasticizers – and advanced admixtures which can slow down set time and increase workability by reducing it.
Why Use Retarding Admixtures?
Think of pouring a large slab of concrete on a sunny afternoon. The heat could be enough to start the concrete setting before you finish spreading it. This is where a retarder can make a significant difference.
The intended use of a retarding admixture is to extend the overall setting time, allowing for ease of finishing, providing better bonding, and eliminating cold joints between layers.
They are particularly useful in:
- Hot climates where hydration is accelerating quickly.
- Long-distance transportation of ready-mix concrete.
- Mass concrete placements such as foundations, bridges, and dams.
- Complex structural elements require extended work periods.
In simple terms, they can buy you longer periods of setting time when you need it.
How Do Retarding Admixtures Function?
Simply put, retarding admixtures function to slow down the hydration of cement, which is the chemical reaction between water and cement that causes concrete to harden. They achieve this in two basic ways:
Form a thin film around the cement particles that shields them from the water until hydration can occur.
Change the chemical environment in the mix to slow down the growth of the hydration products.
Some work slowly, whilst others work more aggressively, depending on what the retarding admixture is and how much is added.
The effectiveness of a retarder is going to depend on:
- Water-to-cement ratio.
- Cement composition (especially C3A and alkali content)
- Temperature and environmental conditions.
Types of Retarding Admixtures
Retarding admixtures can have different chemical compositions, and they can be classified in two main groups:
1. Regular Set Retarders
Regular set retarders are typically used to extend the setting time of concrete by a few hours. You can use a regular set retarder for a relatively short period between mix and pour if there are delays due to transport or other controlled factors, or for poor working conditions like hot weather.
2. Extended-Set (Stabilizing) Admixtures
Extended Set (or Stabilizer) admixtures are used to extend the setting time of concrete systems for many hours or even days. These admixtures are used whenever concrete needs to remain workable, even overnight, and can be reactivated later with an activator.
Extended set admixtures are also used in precast applications, especially when developing an exposed aggregate finish. In these applications, the workability is delayed by adding the admixture to the surface of the mold, so that only the surface of the concrete is delayed at the mold surface. This will allow an easy wash off the top layer of concrete, to expose the decorative finish.
Examples of Common Chemicals Used as Retarders
Organic Compounds
- Sodium Gluconate: Strong retarding agent used extensively in advanced admixture packages.
- Lignosulfonates: Wood pulp-based, low-cost retarder; most widely used in commercial applications.
- Sugars and derivatives: Small doses of sugar have been used theoretically to retarding concrete; a relevantly effective natural retarder.
- Hydroxycarboxylic acids & salts: Useful to re-adjust both retarding and workability characteristics.
Inorganic Compounds
- Borates
- Phosphonates
All of these chemicals will have different impacts on the setting process, and the decision to use one depends on some site conditions and the type of work involved.
Effects of Retarding Admixtures on Concrete Properties
Retarding admixtures can impact the following properties of concrete:
1. Strength
Strength may initially develop more slowly with a retarder. In time, ultimate strength is similar to standard concrete, provided proper curing.
2. Bleeding
Since the setting of the concrete is delayed, bleeding may be more significant than normal. A good mix design and proper finishing will help to mitigate bleeding.
3. Durability
Retarded concrete will be equally durable if properly cured.
4. Slump Retention
Retarders are very useful for retaining slump (i.e., workability) for longer periods. This is beneficial when pumping concrete or when adding to existing work.
5. Available Workability
Workability will usually be improved with a retarder. Retarders commonly provide a typical slump increase of 60-100 mm (depending on the mix).
6. Heat of Hydration
Retarders do not reduce the total amount of heat produced; they delay the heat’s peak temperature. This can be advantageous in mass pours to mitigate cracking.
7. Air Entrainment
Most retarders do not add air to the mix, but can, depending on the type, reduce the quantity of air. Long-term shrinkage and creep remain unaffected, but plastic shrinkage (early-age deformation) may slightly increase.
Benefits of Using Retarding Admixtures
Here are some reasons why contracting and engineering teams may choose to use retarders in a demanding situation:
- Allows for a longer setting time and therefore provides more opportunity for placing, compacting, and finishing.
- Allows for large or layered pours without cold joints.
- Increases pumpability at long distances for the concrete.
- Reduces the chance of premature stiffening in hot conditions.
- Contributes to workability in transit mixes or if delivery is outside the site.
- Enhances the surface finish and appearance.
- Minimizes segregation and bleeding of poorly graded sand mixes.
In short, it provides for greater control, increased quality and will allow improved execution of concrete work.
Uses of Retarding Admixtures
With some limitations, these admixtures can be used in a variety of types of concrete construction, including:
- Large pours (e.g,. slabs, foundations)
- Hot weather concrete work.
- Ready-mixed concrete travelling long distances.
- Mass concrete structures such as bridges, dams, or retaining walls.
- Precast elements with decorative surfaces.
- Overnight storage of returned concrete.
- Layered concrete pours where time intervals are expected.
In hot weather, they help avoid flash setting, in complicated structural works, they ensure continuity of bonding, and in mass pours, they limit thermal cracking associated with heat rise.
Classification by ASTM Standards
As per ASTM C494, the retarding admixtures are classified as follows:
Type B – Retarding Admixture
Type D – Retarding and Water-Reducing Admixture
The use of Type D adds to the water reduction, which creates a stronger density in concrete.
How Sodium Gluconate and Lignosulfonates Help?
Both of these materials are typically used in the construction of retarding admixtures.
Sodium Gluconate offers high efficiency in dosage and performs well in hot weather concrete placement.
Lignosulfonate is inexpensive and also a mild plasticizer, making a compound that offers a better set time and flow.
Both may be included depending on the application of the concrete to achieve an improved performance.
Precautions When Using Retarders
Do not overdose – too much of the retarder can leave the concrete without a chance of setting.
Always mix well to achieve a consistent combination.
There should always be trial mixes done to better gauge the amount of dosage required, depending on the weather conditions, mix design, and scope of work.
When using retarders, select the type based on ambient temperature, type of cement, and timing of the application.
Final Thoughts
Modern concrete construction would not be where it is today without retarding admixtures. Retarding admixtures allow construction to be flexible, efficient, and reliable, especially in challenging environments and in situations involving complex works.
Whether it is delaying early setting in hot and high temperatures, delaying setting in high relative humidity, managing long-distance haulage, or improving finishing in large areas, retarders are available to resolve the real-world concreting problems.
With the right mix design and application by professionals, they also improve the quality and durability of concrete applications. For large-scale or time-sensitive projects, they are not just useful; they are a necessity!
Sandip Agrawal, Polymer Engineer and MD of Sakshi Chem Sciences Pvt. Ltd., leads innovation in construction chemicals, shuttering oils, and industrial lubricants. With expertise in polymer science and eco-friendly solutions, he drives R&D and sustainable advancements, ensuring high-performance products for India and global markets.