Concrete Retarders
Concrete retarders are used to postpone the concrete’s first setting period by up to one hour. They are usually employed in hot weather conditions to prevent the temperature from hardening too quickly, giving mixing, transporting, and placing more time. Retarders function as water reducers as well.
Mechanisms of Concrete Retarders
A longer dormant period can be produced by adding a retarder, which can be sprayed on the concrete’s surface or dissolved in the mixing water to temporarily stop the hydration reactions. The mechanisms that manifest are contingent upon the mix of cement type and retarder kind.
It’s also critical to understand that the retardation mechanisms are transient. The mechanisms’ effects wear off after a certain amount of time, and the hydration continues.
The Process of Retardation Happing in between Retarders and Cement
1. Adsorption
In this form, a retarding additive is deposited on the cement particle surface. The cement particles are covered in a protective coating made of this retarding additive layer. This diffusion barrier slows down the hydration process by preventing water molecules from reaching the surface of the unhydrated cement particles.
As a result, the cement paste stays plastic for a longer amount of time because there aren’t enough hydration products to provide it firmness.
2. Formation of Nucleation
Calcium and hydroxyl ions are liberated from the cement particle surfaces when water is introduced. The hydration products C2S and CS begin to crystallize when the concentration of those ions reaches a threshold amount.
After being added to the cement, a retarding admixture is absorbed by the calcium hydroxide nuclei, halting their growth until a certain point of supersaturation.
3. Intricacy
Some sort of compounds are created with calcium ions released by the cement grains within the first few minutes. The cement becomes more soluble as a result of those compounds’ production.
The aqueous phase of the cement paste will see a rise in the concentration of Ca2+, OH-, Si, Al, and Fe during the hydration process if a retarding admixture is present. The solution’s buildup of calcium and hydroxyl ions inhibits those ions’ precipitation to produce calcium hydroxide. That delays the process of hydration.
4. Precipitation
Although precipitation and adsorption are almost identical, precipitation results in the formation of some insoluble retarder derivatives through a reaction with a strongly alkaline solution. As a result, after the initial few minutes of water and cement interaction, the pH of the solution climbs to above 12.
Cement hydration is inhibited by the precipitation of protective coatings made of these insoluble derivatives around the cement particles. Water molecules are prevented from making good contact with the cement particles by the protective layer, which functions as a diffusion barrier.
Different Types of Retarders
Depending on the type of retarder, there are two categories into which they fall:
1. Organic Retarders
- Lignosulphonates
- Phosphonates;
- Hydroxycarboxylic acids and their salts
- Sugars;
2. Inorganic or Chemical Retarders:
- Borates,
- Phophonates,
- Salts of Pb, Zn, Cu, As, Sb
Retarding Admixtures’ Effects on Concrete’s Properties
1. Power Strength
When retarding admixtures are added to concrete, the initial compressive strength of the treated concrete is less than the compressive strength of the same concrete without the retarding admixtures.
2. Values of Rheology and Workability
Admixtures that retard concrete also have a negligible effect on how workable it is. They may result in a 60–100 mm rise in the initial slump.
3. Decline in Success
Admixtures that retard have demonstrated remarkable efficacy in mitigating slump loss and thereby enhancing the initial workability.
4. The Entrainment of Air
Although certain retarders entrain air, retarding admixtures typically do not. Particularly those that are hydroxycarboxylic acid based may actually have less air in them.
5. Cycles of Freeze-Thaw
Admixtures that entrain air are frequently used to increase concrete’s resistance to freeze-thaw cycles. The air cells serve as tiny expansion chambers for the freezing water when it begins to freeze in the concrete.
6. Gore
Concretes that have been treated with retarding admixtures are always more prone to bleeding because they postpone the onset of the setting process.
7. Hydration Heat
Retarding admixtures cause the peak temperature to climb more slowly than it did when the concrete was retarded, but they do not lessen the heat output of the concrete.
8. Distortion of Volume
The addition of retarding admixtures has no discernible effect on creep or drying shrinkage, but it may marginally increase plastic shrinkage.
9. Sturdiness
Retarding concrete should have the same level of durability as comparable plain concrete if it is properly cured.
Benefits of Concrete Retarder
- Difficult grouting or concrete laying.
- Preventing cold joint development in subsequent lifts;
- Compensating for the accelerated effect of high temperature towards the initial set;
- Exposed aggregate finish as a special architectural surface finish.