Retarders for Concrete – Mechanism, Types and Effects on ...

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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.

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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.

Concrete chemistry is complicated, and the makeup of the mix can impact its behavior &#; from its workability to the set times. Often, contractors need concrete that has an extended set time when concrete is being transported over long distances or when it will be placed during high temperatures.

Temperature plays a role in almost every aspect of producing concrete. As concrete temperatures increase, concrete sets faster. Ready-mix producers often automatically add concrete admixtures called set retarders (&#;retarders&#; for short) to the mix to help offset this temperature-related set acceleration. However, the trick is knowing which type of concrete admixture to use and what dosage.

Understanding the limits of set retarders

A common misconception is that retarders can control temperature increases. Concrete gains temperature for one of two reasons: Heat of hydration or ambient temperature increases. Conventional set retarders typically slow the hydration process. This action provides a means of manipulating when temperature gains occur due to the heat of hydration. In contrast, if the ambient temperature is hot, the concrete will be hot. The common cooling options are either chilled water, ice, or liquid nitrogen. Although retarding admixtures do not cool concrete temperatures, at higher dosages, they can be used to offset thermal acceleration. This allows warmer concrete to be adequately placed.

In addition, set retarders cannot hold a concrete slump level indefinitely; at some point, the retarding effect will wear off. Retarders are useful for preventing normal slump loss that occurs during the hydration process, but they cannot help with slump loss due to moisture loss in the concrete mix. A solution for this type of situation is to use set retarders in conjunction with slump extenders. This provides additional benefits, such as:

• Enabling predictable extended set times for continuous placement on mass concrete and tremie projects
• Supporting long hauls to remote sites while preserving slump
• Allowing for extended truck discharge times
• Reducing the need for portable batch plants at the job site

Moving beyond traditional set retarders

Conventional set retarders form a barrier around hydrated products that temporarily stall further hydration from occurring. They typically have a narrow dosage range due to the unpredictability in controlling concrete set times. Conventional retarders work relatively well under low dosages, but when used at high dosage rates, the set extension can be unpredictable, and over-retardation can occur. Often, conventional retarders will hold hydration, then wear off somewhat quickly, giving a short window for finishing operations. This is where advanced hydration stabilizers are useful.

Solutions such as the RECOVER® concrete admixture provide advanced hydration stabilization. The organic chelating agents in the RECOVER® admixture coat the hydrating cement grains. The water and ions needed for further hydration are blocked to suppress cement surface activity. With the hydration reaction suspended, the set time is postponed, and the mix retains its slump, plasticity, air content, as well as a stable temperature for the duration of the set extension. By suppressing all major hydration events, the admixture provides more predictable control over the setting process.

Hydration stabilizers are also less cement-specific than traditional set retarders, which makes the setting effect more consistent across a broader range of cements.

The RECOVER® concrete admixture works well in a variety of applications, including:

• Drill shaft concrete &#; Typically, concrete for drill shafts must be able to maintain a minimum slump of 4&#; for the duration of the placement (typically from 2 to 12 hours). For one bridge project in South Carolina, the land-based drill shafts had a dosage rate ranging from 4 to 10 ounces per 100 pounds of cement, with a slump life is 4 to 8 hours. For the drill shafts located in the river, the ready-mix producer used 13 oz/cwt of RECOVER® admixture to achieve a slump life in excess of 12 hours.
• Slip form concrete &#; For a project in Alabama, a combination of 3 oz/cwt of WRDA® 64 water-reducing admixture and 3 oz/cwt of RECOVER® admixture was used for slip formed curb and gutter mixes and for slip form paving. The addition of the RECOVER® admixture allowed the ready-mix supplier to maintain the low slump required and increase the ease of placement. The mix design was easy to discharge from the truck mixer, requiring less handwork than traditional concrete mixes.
• Roller-compacted concrete &#; In Mississippi, RECOVER® admixture was used to produce zero-slump concrete that was discharged from the truck mixer with minimal difficulty.
• Slab on grade &#; In the Florida panhandle, low doses (1.5 oz/cwt) of RECOVER® admixture was used along with WRDA®64 admixture in large slab placements, providing a four to five-hour set time in high-temperature conditions.
• Pervious concrete &#; Florida Concrete Products Association guidelines for pervious concrete require the use of a hydration control admixture. In most applications for pervious concrete, RECOVER® admixture is used at 6 oz/cwt of cement. This allows for easy discharge of the mix, reduced buildup in the mixer, and increased hydration efficiency of the mix during the seven-day curing period.
• Self-consolidating concrete &#; RECOVER® admixture extends slump flow life and reduced &#;stickiness&#; of self-consolidating concrete.

Determining concrete admixture dosage

Hydration stabilizers provide a linear dosage response for a set of materials. As a rule of thumb, with concrete temperatures around 70-75° F, advanced hydration stabilizers such as the RECOVER® admixture typically provide roughly a 30-minute delay for a 1 oz/cwt dosage. However, if the concrete is naturally setting faster or slower, then the stabilizer dosage will have to be adjusted. Also, the dosage further depends on the mix design, materials, and other factors.

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