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water reducing and retarding admixture

Using chemical admixtures in concrete construction is crucial for improving the performance of concrete and managing its performance at various stages of construction. Among these admixtures, water reducing and retarding admixture are crucial in improving workability and controlling setting time, especially under challenging conditions.

The purpose of using water reducing and retarding admixture

Water reducing and retarding admixture are chemical substances added to concrete during the mixing process to achieve two main objectives:

Reduce the required moisture content for specific processability.
Delays the setting time of concrete without affecting its final strength and durability.
By reducing the water-cement ratio, these additives help produce concrete with higher strength and better durability.
Delaying the solidification time gives them greater flexibility in transportation, placement, and precision processing, especially in hot weather conditions.

Working principle of water reducing agent and retarder

Water reduction mechanism

Electrostatic repulsion: Anionic surfactants adsorb onto cement particles, generating negative surface charges and dispersing clumped particles.
Stereoscopic obstacle: A water reducing agent based on polycarboxylate ester utilizes long polymer side chains to physically separate particles, reducing friction between particles and reducing up to 40% of water content.

Deceleration mechanism

Hydration inhibition: functional groups (such as hydroxyl and carboxyl) chelate calcium ions, delaying the formation of ettringite and C-S-H gel.
Pore solution modification: Sugar acids or phosphates alter the ion concentration in pore water, inhibiting early hydration reactions.

Types of water reducing and retarding admixture

Type	Primary Components	Water Reduction (%)	Retardation (Hours)	Typical Dosage (wt.% cement)
Lignosulfonate-based	Modified lignin derivatives	8-12	About 3	0.2–0.3
Naphthalene-based	Sulfonated naphthalene polymers	15-25	3-6	0.5–1.5
Polycarboxylate-based	Comb-type copolymers	20-40	4-8	0.5-1.0
Retarding-based	citric acid, tartaric acid, and gluconic acid	/	Adjustable (2–12)	Generally 0.3

What is the difference between water reducer and retarder?

The most significant difference lies in their main functions.

Water reducing agents are mainly used to reduce the water required in concrete or mortar mixtures while maintaining the required workability. By reducing the water-cement ratio, they improve concrete’s strength, durability, and impermeability.

In contrast, retarders are mainly used to slow down the hydration process of cement, thereby delaying the setting time of concrete. This allows more time to place, compact, and complete the concrete.

Mixed-use scenario of water reducer and retarder

1.Concrete pouring in hot weather

Mixing water reducer and retarder can reduce rapid slump loss and flash setting when the ambient temperature exceeds 35°C. For example, the Burj Khalifa building in Dubai can maintain operability for 6 hours even if the on-site temperature is 45°C.

2.Large-volume concrete structure

In the dam foundation, the combination of the two is used to control thermal cracking by delaying heat generation. The Three Gorges Dam project achieved a 12-hour delay window, reducing peak temperatures by 15°C.

3.Extension molding precast concrete

Mixing water reducer and retarder can accurately control the demolding time.
The water reduction effect makes it possible to produce high-strength prefabricated components with lower water-cement ratios, reducing the overall weight of the components while maintaining their strength. The retarding property allows for better control of the production process, as it gives workers more time to process and shape the concrete before it sets.

The benefits of using a mixture of water reducing and retarding admixture

Improved processability without increasing moisture content.
Enhanced ultimate strength and durability.
Better surface finish and lower porosity.
Has greater flexibility in placement and precision machining operations.
Minimize the risk of cold joints and surface cracking.
Save the overall cost of the project.

The challenge of using

1.compatibility

One of the main challenges in using water reducers and retarders is ensuring their compatibility with other concrete mixture components, including different types of cement, aggregates, and other admixtures.

2.dose control

The precise dosage control of these additives is crucial. Incorrect dosage may not achieve the expected effect and may also hurt the performance of concrete. Excessive use of water reducing agents can lead to excessive slump and segregation. In contrast, excessive use of retarders can cause unacceptable delays in setting time, which may affect construction progress. Therefore, strict quality control measures should be taken to ensure the correct dosage of additives.

3.Adverse effects on early power

Some water reducing and retarding admixture, especially those with strong retarding effects, can lead to a decrease in the early strength of concrete.

Conclusion

The excellent working performance of water reducing water reducing and retarding admixture makes them important in various building applications. Although their use presents compatibility and dose control challenges, they can be overcome through appropriate testing and quality control. With the continuous development of the construction industry and the demand for higher performance and more sustainable building materials, the role of water reducers and retarders will only become more important in the future.