Blog

Ｒesearch on synthesis and performances of polycarboxylate slump retaining agent

Polycarboxylate superplasticisers have become mainstream in the concrete admixture market due to their high water-reducing rate, good slump retention, and environmental protection characteristics. However, with the shortage of sand and gravel resources and the uneven quality of raw materials in construction projects, the problem of rapid concrete slump loss has become increasingly prominent. Traditional retarding agents can only alleviate this problem to a certain extent and may even prolong the concrete setting time.

A high-performance polycarboxylate slump-retaining agent is the key to solving this engineering problem. This article will detail the synthesis process, performance optimisation, and practical application effects of a polycarboxylate slump-retaining agent, supported by professional experimental research results, to provide technical reference for concrete construction and admixture research.

What is a polycarboxylate slump retaining agent?

Polycarboxylate slump retaining agent is a modified polycarboxylate concrete admixture developed on the basis of polyether-type polycarboxylate superplasticizers through molecular structure design. Its core design idea is to protect part of the carboxyl groups in the polycarboxylate molecule. These protected groups have strong anti-adsorption capacity and will not adsorb on cement particles at the initial stage of cement hydration.

In the alkaline environment formed by cement hydration, the protected groups undergo hydrolysis, releasing carboxyl groups that exert a sustained water-reducing and dispersing effect. This “delayed release” mechanism ensures that the concrete maintains good workability for a certain period, effectively controlling slump loss, and can be used alone or compounded with ordinary polycarboxylate superplasticisers, offering strong application flexibility.

Key Synthesis Process of Polycarboxylate Slump Retaining Agent

The polycarboxylate slump retaining agent is synthesised by aqueous solution free radical polymerisation with isopentenol polyoxyethylene ether (TPEG), acrylic acid (AA), and slump retaining auxiliary small monomer (BTZ) as the main raw materials. The whole synthesis process is simple and environmentally friendly, without producing waste liquid or waste gas, which aligns with the development requirements for green building materials. The specific synthesis steps are as follows:

Add TPEG and bottom water to a four-necked flask equipped with a stirrer, thermometer, and peristaltic pump, and heat and stir in a water bath until completely dissolved.
Prepare material A by mixing acrylic acid, slump-retaining auxiliary and water, and material B by mixing chain transfer agent, initiator and water.
When the copolymerization system is heated to the set temperature, add material A and material B at a constant rate (material B is added for an additional 30 minutes beyond material A).
After the dropping is completed, keep the temperature for 0.5~1h. When the reactant is cooled to below 40℃, adjust the pH to neutral with sodium hydroxide to obtain the finished polycarboxylate slump retaining agent.

Performance Characteristics of Polycarboxylate Slump Retaining Agent

Based on the optimal synthesis process, the prepared polycarboxylate slump retaining agent (BGM) exhibits excellent comprehensive performance in cement paste fluidity tests, concrete performance tests, and compatibility tests, and its performance is comparable to, or even better than, that of commercially available slump retaining agents.

Basic Water Reduction Performance

The polycarboxylate slump-retaining agent exhibits a water-reduction rate which increases with increasing solid content dosage. When the solid dosage is 0.18%, the water reduction rate can reach 20.0%. Although its water reduction rate is slightly lower than that of ordinary polycarboxylate superplasticisers (CGM) under the same dosage, it can achieve the same water reduction effect by appropriately increasing the dosage (about 1.2 times the dosage of ordinary superplasticisers).

More importantly, the combined use of BGM and CGM shows a synergistic effect, and the water reduction rate can reach 26.8% at a dosage of 0.09% + 0.09%, which is close to the water reduction rate of a single CGM at 0.18%.

Excellent Slump Retention Performance

The core advantage of the polycarboxylate slump-retaining agent is its outstanding slump-retention performance. The concrete performance test shows that:

When only an ordinary polycarboxylate superplasticiser is used, the concrete has a large initial slump but severe slump loss, and the 1h slump is significantly reduced.
When BGM is compounded with CGM, as the replacement amount of BGM increases, the initial slump of concrete decreases slightly, but the slump loss is significantly reduced. When the ratio of CGM:BGM is 7:3, the 1h slump of concrete is even higher than the initial slump, showing a “negative loss” phenomenon.
When BGM is used alone (1.2 times the dosage), the initial slump of concrete can reach the level of single CGM, and the 1h slump shows significant negative growth, with excellent slump retention, and the concrete has good wrapping properties on sand and gravel without bleeding or segregation.

Good Compatibility with Cement

Compatibility with different types of cement is an important indicator of the performance of concrete admixtures. The slump-retaining agent was tested with four common cements (Lianshi, Conch, Wannianqing, Hongshi) at a compound ratio of 70% CGM + 30% BGM and a dosage of 0.18%.

The results show that the concrete has a small slump loss over time for all four cements, the initial slump is about 200~210mm, and the 2h slump is still maintained at about 170~180mm. It has good compatibility with various types of Portland cement, addressing the poor adaptability of traditional admixtures to different cements.

No Negative Impact on Concrete Strength

While improving slump retention, the polycarboxylate slump-retaining agent does not reduce concrete’s mechanical strength and even has a slight improvement effect. The concrete compressive strength test shows that:

The 3d, 7d and 28d compressive strengths of concrete compounded with CGM and BGM are basically the same as or slightly higher than those of concrete with single CGM, with the 28d compressive strength reaching about 35MPa.
When BGM is used alone, the 28-day compressive strength of concrete can reach 36.4MPa, which is higher than that of the single CGM group.
Compared with commercially available slump retaining agents (BTJ1, BTJ2), the concrete strength prepared with BGM is significantly higher, and the 28-day compressive strength is about 3~4MPa higher, showing better mechanical performance advantages.

Practical Application of Polycarboxylate Slump Retaining Agent

Application Scenarios

The polycarboxylate slump-retaining agent is especially suitable for construction projects with high requirements for concrete workability and long construction time, such as large-scale infrastructure projects (bridges, highways, high-speed rail), high-rise building construction, and precast concrete component production. It can effectively address slump loss and construction difficulties caused by the long transportation time of commercial concrete and complex on-site conditions.

Application Methods

Compound use (recommended): Compound with ordinary polycarboxylate superplasticisers at a ratio of 7:3~8:2, with a total solid dosage of 0.15%~0.18%. This method can balance the water-reducing effect and slump retention performance of concrete, and offers the best economic benefit.
Single use: When the construction project has extremely high requirements for slump retention, it can be used alone, with the solid dosage increased to 0.18%~0.20%, to ensure the long-term workability of concrete.

Application Advantages

Improve construction efficiency: Effectively control concrete slump loss, avoid the need for on-site secondary mixing, and accelerate construction progress.
Enhance concrete quality: Improve the wrapping property of concrete on aggregate, reduce bleeding and segregation, and improve the compactness and durability of concrete.
Strong adaptability: It is compatible with various types of cement and uneven-quality sand and gravel resources, and is suitable for a wide range of construction environments.
Green and environmental protection: The synthesis process does not use formaldehyde and does not produce liquid or gas waste, which aligns with the national green building development policy.

Conclusion

The polycarboxylate slump retaining agent synthesised with TPEG, AA, and BTZ2 as main raw materials via aqueous solution free-radical polymerisation has the advantages of excellent slump retention performance, good cement compatibility, a certain water-reduction rate, and no negative impact on concrete strength.

In practical engineering, the combined use of this slump-retaining agent with ordinary polycarboxylate superplasticisers can exert a synergistic effect, ensuring water reduction in concrete and effectively controlling slump loss, thereby solving the long-standing engineering problem of rapid slump loss under uneven raw material quality conditions.