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Preparation of slow release and slump retaining polycarboxylate Superplasticizer

In the modern concrete construction industry, polycarboxylate superplasticizers (PCE) are indispensable high-performance admixtures, renowned for their high water-reducing rates, molecular designability, and low concrete shrinkage. However, common PCE exhibits poor slump retention under high temperatures and long-distance transportation conditions, which seriously affects the workability of fresh concrete and the quality of engineering construction. Adding traditional set retarders can only alleviate this problem temporarily and may cause excessive setting time and reduced strength if overused.

To address this industry pain point, research and preparation of slow release and slump retaining polycarboxylate superplasticizer has become a hot topic. This article focuses on a high-performance slow-release slump-retaining polycarboxylate superplasticizer synthesized with Ethylene Glycol Monovinyl Polyethylene Glycol Ether (EPEG) as the main macromonomer, systematically introducing its optimal synthesis process, performance characteristics, action mechanism, and practical engineering application effects, providing a professional reference for the selection and development of concrete admixtures.

Core Raw Materials & Synthesis Principle of Preparation of slow release and slump retaining polycarboxylate Superplasticizer

Key Raw Materials Selection

The synthesis of this slow-release slump-retaining PCE adopts a redox initiation system and free radical copolymerization reaction, with the following core raw materials (all industrial/analytical grade, easy to obtain in the market):

Main macromonomer: EPEG (relative molecular weight 3000), with high reactivity, 100% monomer conversion, and good compatibility with concrete raw materials.
Copolymer small monomers: Acrylic Acid (AA) provides carboxyl anionic groups for electrostatic repulsion; Hydroxyethyl Acrylate (HEA) introduces ester groups with slow-release hydrolysis properties, the core functional group for slump retention.
Auxiliary agents: Mercaptopropionic acid (MPA) as chain transfer agent to adjust molecular weight; Hydrogen peroxide (H₂O₂) + ascorbic acid (Vc) as redox initiator; Sodium hydroxide for pH neutralization adjustment.
Concrete test materials: P·O42.5 cement, Grade Ⅰ fly ash, river sand (fineness modulus 2.8), continuous gradation stone, etc.

Core Synthesis Mechanism

The key to the slow-release slump-retaining performance of this PCE lies in the ester groups introduced by HEA. In the alkaline environment of cement hydration, ester groups undergo slow, controlled hydrolysis, continuously releasing new carboxylate anions. These groups form new adsorption points on the surface of cement particles, maintaining the electrostatic repulsion and steric hindrance effect of cement particles for a long time, thus fundamentally solving the problem of concrete slump loss.

The whole synthesis process is carried out at room temperature, using a simple method with low energy consumption and green environmental protection.

Optimal Synthesis Process of Slow Release Slump Retaining PCE

The performance of polycarboxylate superplasticizer is closely related to the raw material ratio and the dosage of auxiliary agents. Through a large number of single-factor experiments, the influence of acid-ether ratio, ester-ether ratio, chain transfer agent dosage and initiator dosage on the dispersion and slump retention performance of the product was explored, and the optimal synthesis process parameters were determined as follows:

Molar ratio of monomers: (n(\text{EPEG}):n(\text{AA}):n(\text{HEA}) = 1:2.4:1.8).
Chain transfer agent dosage: MPA accounts for 0.65% of the EPEG macromonomer mass, which can adjust the molecular weight to the optimal range and ensure optimal dispersion of the product.
Initiator dosage: The total amount of redox initiator accounts for 0.3% of the EPEG macromonomer’s mass, ensuring a high polymerization rate and monomer conversion without excessive reduction in molecular weight.
Post-treatment: After the copolymerization reaction, adjust the pH to neutral with sodium hydroxide, then make up water to obtain the finished slow-release slump-retaining PCE (named PCS-SR).

Key Process Parameter Analysis

Acid-ether ratio (2.4): The appropriate carboxyl content ensures sufficient electrostatic repulsion, and the density of polyether long side chains is moderate to exert the best steric hindrance effect; an excessively high acid-ether ratio will reduce the steric hindrance and weaken the slump retention.
Ester-ether ratio (1.8): The optimal ester group content balances initial dispersion and slow-release performance; an excessively high ester-ether ratio can lead to excessive hydrolysis in the early stage and reduce the initial fluidity of cement paste.
Chain transfer agent (0.65%): Too low a dosage results in a too high molecular weight and poor expansion of the product on cement particles; too high a dosage results in a too low molecular weight and reduced steric hindrance.
Initiator (0.3%): Too low a dosage leads to a low polymerization rate and a low monomer conversion rate; too high a dosage leads to too rapid polymerization and reduced product dispersion performance.

Practical Application Value & Scope of slow release and slump retaining polycarboxylate Superplasticizer

The EPEG-based slow-release slump-retaining polycarboxylate superplasticizer synthesized in this research has the advantages of a simple synthesis process, low energy consumption, excellent performance, and wide adaptability, and has important practical application value in the concrete industry, especially suitable for the following engineering scenarios:

Long-distance transportation of ready-mixed concrete: Address slump loss during transportation from the mixing plant to the construction site and ensure concrete workability at the pouring site.
High-temperature construction engineering: In a summer high-temperature environment (above 30℃), the slow-release effect can effectively counteract accelerated cement hydration and prevent rapid slump loss in concrete.
Large-volume concrete pouring, such as bridge piers, dams, and high-rise building foundations, requires a long construction time and good slump retention, and the product can meet the requirements for continuous pouring.
High-performance concrete preparation: It offers both high water-reduction rates and excellent slump retention, improving concrete strength and durability and making it suitable for preparing high-strength, high-performance concrete.
Prestressed concrete engineering: The slow-release slump retention performance can prevent concrete from setting too quickly during prestress tensioning and ensure construction quality.

In addition, the product is well compatible with various cement types (Portland cement, composite cement) and mineral admixtures (fly ash, slag powder), and has no adverse effects on concrete’s setting time and durability, making it a green, high-performance concrete admixture.

Conclusion

Using EPEG-3000 as the main macromonomer, AA and HEA as copolymer monomers, a slow-release slump-retaining PCE with excellent performance can be synthesized by free-radical copolymerization under a redox-initiation system, with an optimal synthesis process that is clear and easy to industrialize.
The synthesized PCS-SR exhibits super-high cement paste fluidity retention (100% at 2h), ultra-low concrete slump loss (4.88% at 3h), and can significantly improve the compressive and flexural strength of cement mortar (20% and 16.8% increases at 28d, respectively).
The slow-release slump-retaining mechanism is that the ester groups in the molecular structure undergo slow hydrolysis in the alkaline cement environment, continuously releasing carboxyl groups and delaying early cement hydration by chelating Ca²⁺, thereby maintaining concrete fluidity in the long term.
The product has good compatibility with concrete raw materials, excellent comprehensive performance, and is far superior to the commercially available ordinary slump-retaining agent, with high practical application value.