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

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Retarders for concrete help to extend the initial setting time of concrete by as much as an hour. These additives are especially beneficial in warm weather conditions, where the fast hardening caused by elevated temperatures can be countered, providing additional time for mixing, transporting, and placement. Moreover, retarders also function effectively as water reducers.

This article will explore the mechanisms behind retardation, various types of retarders, their effects on concrete properties, and the numerous advantages they offer.

Mechanisms of Retardation

When a retarder is incorporated—either dissolved in the mixing water or applied to the surface—the hydration reactions of the concrete are temporarily disrupted. This interruption leads to a more extended dormant period. The specific mechanisms in play can vary based on the interaction between the type of retarder used and the cement type.

It’s critical to note that these retardation mechanisms are reversible; their effects will diminish after a set duration, allowing hydration to resume.

The retardation interaction can occur through four distinct mechanisms:

1. Adsorption

In this mechanism, the retarding admixture attaches to the surfaces of cement particles, creating a protective layer around them. This barrier slows down the access of water molecules to the unhydrated cement particles, thus delaying hydration.

This protective skin results in minimal hydration products contributing to the rigidity of the cement paste, allowing for a longer plastic state.

2. Nucleation

Upon mixing water with cement, calcium and hydroxyl ions are released. As the concentration of these ions reaches a critical level, hydration products such as C2S and CS begin to crystallize.

A retarding admixture can be incorporated into the cement, where it specifically adsorbs onto calcium hydroxide nuclei, hindering their growth until a certain level of supersaturation is attained.

3. Complexation

Initially, calcium ions released from the cement form complexes, which increases the solubility of the cement. In the presence of a retarding admixture, higher concentrations of ions such as Ca2+, OH-, Si, Al, and Fe accumulate within the aqueous phase of the cement paste, thus preventing calcium hydroxide precipitation and effectively retarding hydration.

4. Precipitation

This mechanism is similar to adsorption, but it involves the formation of insoluble derivatives of the retarder upon reacting with the alkaline solution. Consequently, the pH of the mixture exceeds 12 shortly after water contacts the cement.

These insoluble coatings that form around cement particles act as a diffusion barrier, limiting water access and thereby suppressing hydration.

Types of Retarders

Retarders can be divided into two categories based on their nature:

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

Concrete containing retarding admixtures generally exhibits lower initial compressive strength compared to similar mixtures that do not include these additives.

2. Workability and Rheological Values

Retarding admixtures can influence the workability of concrete slightly. They may produce an increase in initial slump ranging from 60-100mm.

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3. Slump Loss

Retarding admixtures are effective in minimizing slump loss, which consequently enhances initial workability.

4. Air Entrainment

While most retarding admixtures do not typically entrain air, certain types—particularly those based on hydroxycarboxylic acid—may decrease air content.

5. Freeze-Thaw Cycles

To improve the freeze-thaw resistance of concrete, air-entraining admixtures are often used. This is crucial when water in the concrete begins to freeze as the entrained air cells act as tiny chambers to accommodate the expansion of the freezing water.

6. Bleeding

The delay in the setting process means that concretes treated with retarders are more susceptible to bleeding.

7. Heat of Hydration

While retarding admixtures do not reduce the heat output of concrete, they postpone the peak temperature increase by a period equivalent to the retardation duration.

8. Volume Deformation

Including retarding admixtures has minimal impact on creep and drying shrinkage, but plastic shrinkage may see slight increases.

9. Durability

If properly cured, retarded concrete can be equally durable when compared to standard concrete.

Advantages of Retarder in Concrete

• Facilitates complex concrete placement or grouting.
• Creates special architectural surface finishes like exposed aggregate.
• Compensates for the rapid setting caused by high temperatures.
• Prevents cold joint formations during subsequent lifts.

Controlling concrete setting times

The chemistry of concrete is intricate, and the mix’s composition can significantly influence its behavior—affecting workability and setting times. Contractors often require concrete with prolonged set times, particularly when transported over long distances or during high temperatures.

The influence of temperature on concrete cannot be understated. An increase in temperature results in quicker setting times. Ready-mix producers frequently use set retarders (shortened to "retarders") in the mix to counteract temperature-induced setting acceleration. However, understanding which specific type of admixture to utilize and in what dosage is crucial.

Understanding the limits of set retarders

A prevalent misunderstanding is that retarders can control temperature fluctuations. Concrete’s heat gain can be attributed to either heat of hydration or ambient temperature increases. Conventional set retarders primarily slow the hydration process, allowing for better management of temperature gains due to the heat of hydration. Conversely, if the external temperature is warm, the concrete will become warm as well. Cooling methods typically include chilled water, ice, or liquid nitrogen. Although retarding admixtures do not cool down concrete, at high dosages, they can mitigate thermal acceleration, enabling proper placement of warmer concrete.

Additionally, set retarders cannot maintain a concrete slump indefinitely; their retarding effects diminish over time. While they effectively prevent normal slump loss during hydration, they have no impact on moisture loss within the concrete mix. To address such situations, utilizing set retarders alongside slump extenders is recommended. This combination offers further benefits such as:

• Enabling reliable extended set times for continuous mass concrete and tremie projects.
• Facilitating long hauls to outlying sites while maintaining slump.
• Allowing for longer truck discharge times.
• Lessening the necessity for portable batch plants at job sites.

Moving beyond traditional set retarders

Traditional set retarders create a barrier around hydrated products, temporarily halting further hydration. However, they typically have a narrow dosage range, which poses challenges in controlling concrete set times. Conventional retarders function effectively at low dosages, but at higher rates, set extension can become unpredictable, resulting in over-retardation. Often, after an initial delay, these retarders will lose efficacy quickly, leaving a short time frame for finishing work. This is where advanced hydration stabilizers like RECOVER® can play a pivotal role.

The RECOVER® concrete admixture stabilizes hydration through its organic chelating agents, which coat the hydrating cement grains. By preventing access to the necessary water and ions required for hydration, the hydration reaction is effectively paused, extending the set time and retaining mix slump, plasticity, air content, and stable temperature throughout the extended set period. By monitoring the crucial hydration events, this admixture grants more reliable control over the setting process.

Additionally, hydration stabilizers exhibit less sensitivity to cement type than traditional set retarders, promoting a more constant setting effect across various cement types.

Applications of RECOVER® Admixture

The RECOVER® admixture proves advantageous in various applications, including:

• Drill shaft concrete – Ideally, this concrete must maintain a minimum slump of 4 inches for the entire placement duration (ranging typically from 2 to 12 hours). In a bridge project in South Carolina, dosage rates varied between 4 to 10 ounces per 100 pounds of cement, maintaining a slump life between 4 to 8 hours. For drill shafts underwater, the ready-mix producer required 13 oz/cwt of RECOVER® admixture to ensure slump life exceeded 12 hours.
• Slip form concrete – Within 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, facilitating easy discharge with less manual intervention compared to traditional concrete mixes.
• Roller-compacted concrete – In Mississippi, implementing RECOVER® admixture enabled the production of zero-slump concrete with minimal effort during discharge from the truck mixer.
• Slab on grade – In the Florida panhandle, low doses (1.5 oz/cwt) of RECOVER® admixture combined with WRDA® 64 facilitated a four to five-hour set time even in high-temperature situations.
• Pervious concrete – Adhering to guidelines from the Florida Concrete Products Association, hydration control admixtures like RECOVER® are utilized at 6 oz/cwt of cement, enhancing discharge ease and minimizing mixer buildup during the seven-day curing period.
• Self-consolidating concrete – In this case, the RECOVER® admixture extends the slump flow life and lessens the ‘stickiness’ of the self-consolidating concrete mix.

Determining concrete admixture dosage

Hydration stabilizers have a predictable dosage response across various materials. Generally, when concrete temperatures hover around 70-75°F, advanced hydration stabilizers like the RECOVER® admixture can yield an approximate 30-minute delay for a 1 oz/cwt dosage. If the concrete is naturally setting quicker or slower, dosage adjustments will be necessary. Additionally, dosage can vary based on mix design, constituent materials, and other influencing elements.

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