Blog

Study on performance of new carboxylic grafted polymer high efficiency slump retaining agent

In the era of high-performance green concrete, the extensive use of mineral micro-powders (fly ash, slag powder, limestone powder, etc.) has led to significant problems, including rapid concrete slump loss, segregation, and bleeding. The core cause is the strong adsorption of mineral micro-powders onto concrete admixtures, leading to a rapid decrease in the effective concentration of water-reducing agents at the surface of cement particles. To solve this industry pain point, the research and development of high-efficiency slump-retaining agents with strong anti-adsorption capacity and excellent slump-retention performance has become a key direction of concrete admixture innovation.

This article focuses on a new carboxylic acid-grafted copolymer high-efficiency slump-retaining agent synthesized with isopentenol polyoxyethylene ether (TPEG) as the core macromonomer. It systematically elaborates its synthesis process, key process parameter optimization, molecular weight characteristics, and concrete application effects.

Core Raw Materials and Synthesis Principle of new carboxylic grafted polymer high efficiency slump retaining agent

Key Raw Material Selection

This slump retaining agent is prepared by free radical direct polymerization with industrial-grade raw materials that are easy to obtain in the market, and the core raw material system is as follows:

Main macromonomer: Isopentenol polyoxyethylene ether (TPEG), which introduces hydrophilic polyether side chains to provide steric hindrance and improve compatibility with concrete.
Copolymer monomers: Acrylic acid (AA) provides anionic carboxyl groups for electrostatic repulsion; a self-made slump-retaining auxiliary (an ester compound with non-polar ester groups) is the core functional component for anti-adsorption and slow-release slump retention.
Initiator & chain transfer agent: Ammonium persulfate (APS) as a thermal initiator to trigger the polymerization reaction; mercaptoacetic acid (TGA) to control molecular weight and its distribution, and to avoid excessive molecular weight and cross-linking.
Neutralizer & test materials: Sodium hydroxide for pH neutralization adjustment; P·O42.5 cement, Grade Ⅱ fly ash, S95 slag powder for performance testing of cement paste and concrete.

Core Synthesis Mechanism & Process

The key to the excellent performance of this carboxylic grafted copolymer slump retaining agent lies in the non-polar ester groups introduced by slump retaining auxiliaries—it has high anti-adsorption capacity, which can resist the rapid adsorption of mineral micro-powders in concrete. In the alkaline environment of cement hydration, the ester groups react slowly and continuously release water-reducing components with a dispersion effect, thereby timely supplementing the effective concentration of water-reducing agents in the cement paste and preventing rapid decreases in concrete fluidity.

The synthesis process is simple and easy to industrialize, with the core steps:

Add TPEG, APS, and deionized water into a four-neck flask, pass nitrogen to remove oxygen, heat, and stir until the macromonomer is completely dissolved;
Raise the temperature to the optimal reaction temperature, then drop the TGA solution and the slump-retaining auxiliary solution simultaneously, and hold the temperature for 1h after dropping.
Cool the reaction product, then neutralize it with sodium hydroxide solution to obtain the carboxylic grafted copolymer, a high-efficiency slump-retaining agent (solid content 10%).

Optimization of Key Synthesis Process Parameters

The dispersion and slump retention performance of the carboxylic grafted copolymer slump retaining agent are closely related to the dosage of raw materials and polymerization reaction conditions. Through a large number of single-factor experiments, the effects of TPEG dosage, APS dosage, TGA dosage, reaction temperature, and slump-retaining auxiliary dosage on product performance were systematically explored, and the optimal synthesis process parameters were determined (using the 1h fluidity of cement paste as the key evaluation index).

1. TPEG Dosage: 0.07 mol

TPEG provides hydrophilic polyether side chains for the product, and its dosage directly affects the steric hindrance. When the TPEG dosage is 0.07 mol, the electrostatic repulsion of charged groups on the main chain and the steric hindrance effect of the side chains play a synergistic role to the maximum extent, and the 1h fluidity of the cement paste mixed with the slump retaining agent reaches 235 mm, with the best dispersion and slump retention performance. Excessive or insufficient TPEG dosage will disrupt the molecular balance and reduce slump retention capacity.

2. Initiator (APS) Dosage: 0.6% of TPEG mass

APS decomposes free radicals to trigger polymerization, and its dosage affects the polymerization rate and monomer conversion rate. When the APS dosage is 0.6% of the TPEG mass, the polymerization reaction proceeds at a moderate rate, the monomer conversion rate is high, and the 1h fluidity of the cement paste reaches 251 mm. Too much APS will cause rapid polymerization, while too little APS will lead to a low polymerization rate and insufficient effective components in the product.

3. Chain Transfer Agent (TGA) Dosage: 0.2% of TPEG mass

TGA is key to controlling the molecular weight and product distribution. It can avoid excessive molecular weight and wide distribution, and prevent cross-linking. When the TGA dosage is 0.2% of the TPEG mass, the product’s molecular weight is within the optimal range, and the 1h fluidity of the cement paste reaches 266 mm. Excessive TGA will reduce the molecular weight of the product and weaken its dispersion performance; insufficient TGA will lead to a molecular weight that is too high and poor fluidity of concrete.

4. Optimal Reaction Temperature: 65℃

Temperature has a crucial impact on the chain-initiation reaction in polymerization. At 65℃, the monomer activity and APS decomposition rate reach the optimal matching state: the monomer has high activity, the free radical generation rate is moderate, and the polymerization reaction proceeds uniformly. The 1h fluidity of the cement paste reaches 268 mm. Low temperature will reduce the polymerization rate and the effective components of the product; high temperature will cause the initiator to decompose too fast, which is not conducive to the stable progress of the reaction.

5. Slump Retaining Auxiliary Dosage: n(auxiliary):n(AA) = 4:1

The slump retaining auxiliary is the core component for anti-adsorption and slow-release slump retention. When the molar ratio of slump-retaining auxiliary to acrylic acid is 4:1, the product contains a moderate amount of non-polar ester groups, which maximizes anti-adsorption capacity and slow-release performance. The 1h fluidity of the cement paste reaches the maximum value of 272 mm, and the slump retention effect is the most significant.

Summary of optimal process conditions: AA dosage 3.6g, n(slump retaining auxiliary):n(AA)=4:1, TPEG dosage 0.07mol, APS dosage 0.6% of TPEG mass, TGA dosage 0.2% of TPEG mass, dropping time 3.5h, reaction temperature 65℃, heat preservation time 1h.

Molecular Weight and Its Distribution Characteristics

Molecular weight and its distribution are the key structural indices of the carboxylic grafted copolymer slump retaining agent, which directly determine its application performance in concrete:

Too small molecular weight: High air entrainment, poor water retention, easy bleeding of concrete, and extremely fast slump loss.
Too large molecular weight: Reduced dispersion effect, easy to become a flocculant, leading to concrete non-flowing, because the overlong or excessive branches reduce the adsorption capacity on the surface of cement particles.

The product synthesized under the optimal process conditions was tested by water-soluble gel permeation chromatography (GPC), and the key indices are as follows:

Number-average molecular weight (Mn): 8240
Weight-average molecular weight (Mw): 22246
Molecular weight distribution coefficient: 2.70

The test results show that the product has a narrow molecular weight distribution and a molecular weight within the optimal application range, which ensures excellent slump retention performance and compatibility with concrete, and avoids the adverse effects caused by either too large or too small molecular weight.

Practical Application Value and Engineering Application Scope

The new carboxylic acid-grafted copolymer high-efficiency slump-retaining agent offers the advantages of a simple synthesis process, low raw-material costs, excellent performance, and strong adaptability. It has important practical application value in the concrete industry, especially suitable for the following engineering scenarios with high requirements for concrete slump retention:

High-performance green concrete engineering: Concrete mixed with a large amount of mineral micro-powders (fly ash, slag powder, limestone powder, etc.) solves the problem of rapid slump loss by their strong adsorption.
High-temperature construction engineering: Summer high temperatures (above 30℃) accelerate cement hydration and lead to rapid loss of concrete fluidity.
Ready-mixed concrete long-distance transportation: It ensures that the concrete shows no obvious slump loss during transportation from the mixing plant to the construction site and maintains good workability at the pouring site.
Large-volume concrete pouring: Such as bridge piers, high-rise building foundations, dams, etc., which require long construction time and continuous pouring, and the concrete needs to maintain good fluidity for a long time.
Commercial concrete production: It is compounded with a polycarboxylate superplasticizer to improve overall performance and expand its application range.

Conclusion

Taking TPEG as the main macromonomer, AA and self-made slump retaining auxiliary as copolymer monomers, APS as initiator, and TGA as chain transfer agent, the carboxylic grafted copolymer high-efficiency slump retaining agent with excellent performance can be prepared by free radical direct polymerization, and the optimal synthesis process parameters are clear and easy to control.
The non-polar ester groups introduced by the slump-retaining auxiliary are the core of the product’s high anti-adsorption and slow-release slump-retention performance, which can resist the adsorption of mineral micro-powders and continuously supplement effective water-reducing components in the cement paste.
The product synthesized under the optimal process has a narrow molecular weight distribution and a molecular weight within the optimal application range, ensuring excellent dispersion and slump retention performance.
The product has the best application performance when combined with a polycarboxylate superplasticizer (7:3 ratio, 1.8% admixture). At high temperatures above 35℃, the concrete shows essentially no slump loss within 1h, good workability, and slightly improved compressive strength, which is far superior to single-doping with polycarboxylate superplasticizer.