Retarders for concrete are used to delay the initial setting time of the concrete upto an hour. They are generally used in the hot weather condition to counter the rapid hardening due to high temperature, thus allowing the time for mixing, transporting and placing. Retarders also acts as water reducers.
In this article we discuss about the mechanism of retardation, types of retarders, effect of retarders on concrete properties and advantages.
Mechanisms of Retardation
Adding a retarder, dissolved in the mixing water or sprayed on the surface of the concrete, temporarily interrupts the hydration reactions, which creates a longer dormant period. The mechanisms that appear depend on the combination of the type of retarder and the type of cement.
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 way in which the retardation process happen in between retarder and cement. They are
1. Adsorption
In this type on the surface of the cement particles, a retarding admixture is adsorbed. This layer of retarding admixture creates a protective skin around the cement particles. Due to this diffusion barrier the water molecules are hindered to reach the surface of the unhydrated cement particles and the hydration is slowed down.
The result is that there is no considerable amount of hydration products to give rigidity to the cement paste so the paste remains plastic for a longer period.
2. Nucleation
When water is added to the cement, calcium ions and hydroxyl ions are released from the surface of the cement particles. When a critical value of the concentration of those ions is reached, the hydration products C2S and CS start to crystallize.
A retarding admixture, which is incorporated into the cement, is adsorbed by the calcium hydroxide nuclei, which prevents the growth of the calcium hydroxide nuclei until some level of supersaturation.
3. Complexation
During the first few minutes, some kind of complexes with calcium ions, released by the cement grains, are formed. The formation of those complexes causes an increased solubility of the cement.
During the hydration, in the presence of a retarding admixture, an increased concentration of Ca2+, OH-, Si, Al and Fe will occur in the aqueous phase of the cement paste. The accumulation of the calcium and hydroxyl ions in the solution prevent the precipitation of those ions to form calcium hydroxide. In that way, hydration is retarded.
4. Precipitation
Precipitation is nearly similar to adsorption but in the case of precipitation some kind of insoluble derivatives of retarder are formed by a reaction with the highly alkaline solution. Because of that, the pH of the solution rises over 12 after the first few minutes of the contact between water and cement.
The precipitation of protective coatings of these insoluble derivatives around the cement particles suppresses the cement hydration. The protective coating acts as a diffusion barrier so the water molecules can’t make a good contact with the cement particles
Types of Retarders
Retarders are divided into 2 category depending on the nature of the retarders , they are
1. Organic Retarders
- Lignosulphonates
- Hydroxycarboxylic acids and their salts
- Phosphonates
- Sugars
2. Inorganic or Chemical Retarders
- Phosphonates
- Borates
- Salts of Pb, Zn, Cu, As, Sb
Effects of Retarding Admixtures on Properties of Concrete
1. Strength
The initial compressive strength of a concrete to which retarding admixtures are added, is lower than the compressive strength of a similar concrete which was not treated with retarding admixtures.
2. Workability and Rheological Values
Retarding admixtures also have a small influence on the workability of concrete. They can cause an increase of initial slump by 60-100mm.
3. Slump Loss
Retarding admixtures have proven to be very effective in reducing slump loss and in this way increasing the initial workability.
4. Air Entrainment
Retarding admixtures do not normally entrain air but there are some types of retarders that do. Especially these based on hydroxycarboxylic acid may actually reduced air content.
5. Freeze-Thaw Cycles
Air entraining admixtures are often used to improve the freeze-thaw resistance of concrete. When the water in the concrete starts freezing, the air cells function as microscopic expansion chambers for the freezing water.
6. Bleeding
Since retarding admixtures delay the beginning of the setting process, retarded concretes are always more likely to bleed.
7. Heat of Hydration
Retarding admixtures do not reduce the heat output of concrete but delay the rise of the peak temperature by a time interval similar to the one by which the concrete was retarded.
8. Volume Deformation
Creep and drying shrinkage are not significantly affected by the inclusion of retarding admixtures but plastic shrinkage may be slightly increased.
9. Durability
If concrete is correctly cured, retarded concrete should be just as durable as equivalent plain concrete.
Advantages of Retarder in Concrete
- Complex concrete placement or grouting.
- Special architectural surface finish : exposed aggregate finish.
- Compensating the accelerating effect of high temperature towards the initial set.
- Preventing cold joint formation in successive lifts.
Read More: 15 Types of Admixtures Used in Concrete