The Application of Polycarboxylate Superplasticizer in High-performance Concrete

At present, polycarboxylate-based superplasticizer high performance water reducers are mainly used in high-strength concrete and self-compacting concrete in China. The freshly mixed concrete using polycarboxylate-based high-efficiency water reducers has good workability, is not sticky to the bottom, and is easier to be vibrated and compacted; the early strength increases rapidly, and the demolding strength is high. Polycarboxylate-based superplasticizer high-performance water reducers have good adaptability to the quality fluctuations of raw materials, especially the quality fluctuations of sand and gravel, and can obtain more stable concrete products. In precast concrete, the shrinkage rate of concrete using polycarboxylate-based superplasticizer high-performance water reducers is small, which can inhibit or delay the generation of surface cracks. Due to the good cohesion and water retention of the concrete, the concrete segregation and bleeding phenomena are greatly reduced, and the appearance quality of the structure is greatly improved.

1.The influence of adding polycarboxylate-based superplasticizer high performance water reducer on the construction performance of concrete 

The construction performance of concrete plays a crucial role in ensuring the quality of concrete projects, improving construction efficiency, and reducing labor intensity. The construction performance of ordinary concrete is mainly characterized by slump; while for high-performance concrete, due to the presence of more powder materials and less water usage, the entire system tends to be relatively viscous and difficult to be pumped; it takes a longer time for it to level by relying on its own weight; therefore, in addition to slump, extendibility and viscosity can better illustrate the construction performance of high-performance concrete.

The polycarboxylate-based superplasticizer high performance water reducer can achieve excellent dispersion effect even at a very low dosage. Moreover, the expansion degree of the concrete is large, and the slump tube backflow time of the concrete is significantly shorter than that of the concrete mixed with naphthalene-based water reducer. This indicates that the polycarboxylate-based superplasticizer high performance water reducer can significantly reduce the viscosity of concrete and improve the construction performance of concrete in concrete systems with a large amount of cementitious materials. 

Compared with naphthalene-based superplasticizer high range water-reducing agents, polycarboxylate-based polycarboxylate-based superplasticizer high performance water reducers can more effectively improve the performance of concrete, increase construction speed and quality. They can also more effectively reduce labor intensity and improve work efficiency.

2. The retention of workability of concrete mixed with polycarboxylate superplasticizer 

With the development and popularization of commercial concrete, the transportation time from mixing to vibration in the construction process of concrete is often required. Due to the increasingly congested urban traffic, people have higher and higher requirements for the slump retention of concrete. For high-performance concrete, not only should the slump retention be good, but also the expansion degree and the viscosity suitable for construction should be maintained well. That is, the construction performance of concrete should be maintained as one of the guarantees for the quality of the project.

The water-reducing effect of concrete admixtures depends on the dispersion and dispersion stability of cement particles. The dispersion stability of cement particles depends on the electrostatic repulsion and three-dimensional stability effect of the active agent adsorbed on the surface. According to the DLVO theory, the oxygen atoms of the ether bonds in cement react with water molecules in the aqueous solution to form strong hydrogen bonds, and a hydrophilic protective film is formed. The hydrophilic protective film provides dispersion stability. 

Traditional water-reducing agents adsorb onto the surface of cement particles in a rigid chain lying-down adsorption state. Without the three-dimensional stability effect of the water-reducing agents, the problem of concrete slump loss cannot be solved. However, the molecules of polycarboxylate polycarboxylate-based superplasticizer high performance water-reducing agents contain COOH and SO3 negative ions, which provide electrostatic repulsion. On the other hand, the ether bonds in the polycarboxylate polycarboxylate-based superplasticizer high performance water-reducing agent molecules react with water molecules to form strong hydrogen bonds, and form a hydrophilic protective film. The hydrophilic protective film provides dispersion stability. Therefore, compared with general water-reducing agents, polycarboxylate-based superplasticizer high performance water reducer can more effectively increase the workability and slump retention ability of concrete under low water-cement ratios.

3. The setting time of concrete mixed with polycarboxylate superplasticizer

The initial setting time of the concrete mixed with the polycarboxylate-based superplasticizer high performance water reducer is 8:50, the final setting time is 10:30, and the difference between the initial and final setting times is 1:40. The experimental results basically conform to the characteristic of the polycarboxylate-based high-performance water reducer that it has a slight retardation effect. The difference between the initial and final setting times is 25 minutes longer than that of the water reducer mixed with naphthalene. 

Compared with naphthalene-based superplasticizer high range water reducers, polycarboxylate-based superplasticizer high performance water reducers have a certain retardation effect on setting, and the difference between the initial and final setting times is relatively large. These characteristics are beneficial in concrete with a high dosage of cementitious materials, as they help to disperse the hydration heat of the concrete and avoid problems such as cracking caused by the concentration of hydration heat. The carboxyl groups in the molecular structure of polycarboxylate-based high-efficiency water reducers form complexes with calcium ions, delaying the time for calcium hydroxide to form crystals and reducing the formation of C-H-S gel. Delaying the cement hydration and reducing the temporary water requirement during hydration help to enhance their plasticizing effect.

4 Conclusion 

(1) The polycarboxylate-based superplasticizer high performance water reducers synthesized from raw materials such as polyethylene glycol monomethyl ether, acrylic acid, and allyl sulfonate has excellent dispersion and dispersion stability for cement. In the molecular structure of the polycarboxylic type water reducer, the higher the content of carboxyl groups, the stronger the retardation effect of the polycarboxylic water reducer/polycarboxylate-based superplasticizer high performance water reducers. 
(2) When defoaming agents are added to polycarboxylate-based superplasticizer high performance water reducers, it will not affect the dispersibility and stability of the water reducers in cement. The addition of defoaming agents can reduce the air content of concrete, which is very beneficial and necessary for improving the strength of concrete and enhancing the construction quality of concrete. 
(3) The polycarboxylate-based superplasticizer high performance water reducer has a comb-shaped molecular structure and exhibits stronger dispersion stability compared to traditional water reducers. This ensures more complete cement hydration and the strength of the concrete can be increased by more than 10% compared to the same mix ratio using naphthalene-based superplasticizer high range water reducers. 
(4) When using the polycarboxylate-based superplasticizer high performance water reducer with a dosage of 0.2%, the water reduction rate can reach over 25%. The experiment has proved that it is a high-efficiency water reducer with excellent comprehensive performance.