Details about Concrete Retarders
All industries have been transformed by contemporary material chemistry, and the concrete industry is no exception. Concrete's properties may be changed significantly by adding a few chemicals, which is why its use is expanding daily.
Cement, sand, gravel, water, and concrete admixtures are the main ingredients of concrete, the most common building material. Concrete admixtures, also known as concrete additives, are crucial substances that alter one or more concrete qualities, including setting time, hardening, workability, strength, unit weight, and others, both in the plastic state and the hardened state.
A variety of admixtures, including concrete accelerators, concrete retarders, plasticizers, Superplasticizers, and air-entraining admixtures, can be used in concrete to change its characteristics. These admixtures are added to the concrete either before or during the mixing process.
It is crucial to remember that concrete retarders can be utilized to postpone the material's initial setting period by up to an hour.
As a result of the high temperatures in warm weather, these are typically used in order to reduce the speed at which concrete hardens, allowing for greater amounts of time to be spent mixing, transporting, and laying concrete. In addition to this, it is important to keep in mind that water reducers also function as retarders.
Concrete is mixed with concrete retarders to postpone the concrete's setting period. As you are aware, a chemical reaction between the water and cement in the concrete causes the setting or stiffening process.
When retarders are employed in the concrete, they temporarily slow down the chemical reaction known as hydration. As a result, the concrete stays in the plastic stage and is used for a longer period of time.
Concrete placement might occasionally take longer due to challenging circumstances or potential transit delays. Take ready-mix concrete as an example. Ready mix concrete is produced at a central batching facility, then it is frequently transported over vast distances to job sites, which might take a long time depending on the mode of transportation.
Mechanism of Concrete Retarders
Adding a retarder, dissolved in the mixing water or sprayed on the surface of the concrete, temporarily interrupts the hydration reactions, which creates a long dormant period. It's also important to realize that the mechanisms of retardation are temporary.
After a predictable period, the effects of the mechanisms disappear and the hydration continues.
There are four different ways in which the retardation process happens between retarder and cement.
Assimilation: In this form, a retarding additive is deposited onto the cement particles' surface. Around the cement particles, this layer of retarding admixture forms a protective skin. Cement particles are protected from getting hydrated by this diffusion barrier, which prevents water molecules from getting to the surface.
The nucleation process: A surface reaction occurs when calcium ions and hydroxyl ions are released from cement particles when water is added. The calcium hydroxide nuclei absorb a retarding additive that is added to the cement, which stops the development of the calcium hydroxide nuclei until a certain degree of supersaturation.
Complication: Complexes with calcium ions, produced by the cement grains, are created in the first few minutes. The development of such complexes increases the cement's solubility. The solution's buildup of calcium and hydroxyl ions prevents those ions from precipitating to produce calcium hydroxide.
Benefits of Concrete Retarders
A concrete retarder is advantageous in many ways, and a few of the most important benefits are discussed below:-
1. Retarding admixtures prolongs the time it takes for concrete to set while preserving its workability and cohesiveness.
2. Concrete retarder keeps concretely workable throughout the entire concrete placement process in major construction projects by preventing the creation of cold joints in subsequent lifts.
3. Concrete retarders can be used to lessen the dangers associated with long-distance transportation in hot weather and compensate for the accelerated effect of high temperature toward the first setting time.
4. By prolonging the setting time and enhancing the concrete's workability, the use of retarding admixtures increases the permeability of concrete.
5. In concrete where poor sand grading is inevitable, concrete retarders aid in reducing segregation and bleeding.
To learn more, watch the following video tutorial.
Video Source: Anime_Edu - Civil Engineering Videos
Drawbacks of Concrete Retarders
There are several disadvantages to using concrete retarders for construction that should be taken into account:-
1. Water reducers can be used with concrete retarders, although caution should be exercised during curing to prevent fractures and bleeding.
2. Depending on the water-to-cement ratio, the dose of retarding admixtures should be kept within certain bounds.
3. Gypsum should only be used as a retarding additive to delay setting time where suitable inspection and control are available.
4. When applied in varying amounts, concrete retarders may react differently with various cement kinds.
Knowing how to use concrete retarders properly is crucial for sustaining workability during hot weather concreting, challenging placement situations, extended transit, etc. However, it is strongly advised that test mixes with the real cement that will be used in construction be conducted before the retarder is employed.