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6 major challenges to be solved in the research of polycarboxylate superplasticizers

Polycarboxylate superplasticizer (PCE) has been highly praised by researchers and engineering practitioners since its inception in the late 1980s for its low dosage, high water-reduction rate (not less than 25%), good reinforcement effect, strong molecular adjustability, and the absence of formaldehyde in the synthesis process. Especially after the promotion and application of PCE in high-speed rail and dam engineering in China at the beginning of the 21st century, a wave of PCE research was stirred up, which has not subsided for a long time.

Regarding the research on PCE, people focus on explaining its mechanism of action, further improving its performance, developing derivative products, and optimizing its application technology, to thoroughly solve the practical construction technology problems of cement concrete, improve the comprehensive performance of concrete (high workability, high strength, high toughness, and high durability), and maximize the goals of capital saving, waste utilization, energy conservation, and emission reduction. To dateTo date, although certain achievements have been made in PCE research, due to limited understanding of PCE, the problems solved are only the tip of the iceberg, and the research work remains very difficult.

6 major challenges to be solved in the research of polycarboxylate superplasticizers

Problem 1: Mechanism of PCE action

The flowability of cement slurry mixed with PCE will be significantly improved. Researchers generally believe that PCE adsorbs on the surface of cement particles, and the carboxylic acid groups on the main chain of PCE molecules adsorbed on adjacent cement particle surfaces produce dispersion through the principle of same-charge repulsion.

In addition, the side chains of PCE molecules can provide steric hindrance; thus, PCE has excellent dispersion performance. But how is PCE adsorbed on the surface of cement particles? There is still no consensus on whether this is due to electrostatic forces or an entropy-driven effect. In addition, the hydration induction period of cement slurry with PCE added is longer than that of cement slurry without PCE, which is manifested macroscopically as a delay in the setting time of the cement slurry.

So, how does PCE achieve retarding of cement slurry? Why does the hydration rate of cement paste mixed with PCE increase in the later stage?

Problem 2: The intercalation effect of PCE

Plank et al. suggested that PCE may intercalate into certain hydration products. So what is the mechanism of PCE intercalation? Some scholars believe that PCE molecules in the aqueous phase of cement slurry may provide sites for the nucleation of hydrated calcium silicate (C-S-H). Is this one reason for intercalation? In addition, mineral phases such as montmorillonite and bentonite also have a layered structure. Understanding the reasons for PCE intercalation can help develop clay-resistant PCE.

Problem 3: Early Strong PCE

There are some research results on early-strength polycarboxylate superplasticizers in academia, and a small number of early-strength PCE products have also begun to appear on the market. However, people still have doubts about the mechanism of action of early-strength PCE.

Many experimental results have shown that adding PCE can delay cement hydration to varying degrees, reduce accumulated hydration heat release in the early stage of cement slurry, and reduce CH generation in early-hardened cement slurry. The mechanism of action of early-strength PCE and whether this type of PCE indeed promotes cement slurry hydration are worth discussing.

Problem 4: Coagulation-promoting PCE

At present, the common types of accelerators on the market are mostly aluminum sulfate-type, liquid, alkali-free, and sulfate-type alkali-free accelerators. Although these accelerators have a small impact on the later strength of concrete, their adaptability to cementitious materials is not ideal. So, is there hope of developing a setting-promoting PCE that can promote rapid C-S-H nucleation in cement slurry in the early stage without affecting later strength development in hardened cement slurry?
The mechanism of action of a sulfate-type alkali-free PCE is to provide more soluble SO²^－₄ to the liquid phase, thereby promoting the formation of ettringite (AFt) in the cement slurry. Therefore, can a coagulation-promoting PCE be prepared by copolymerizing carboxylic acid monomers, non-ionic monomers, and large monomers with longer side chains that adsorb little on the surface of cement particles and chelate calcium ions, thereby promoting C-S-H nucleation?

Problem 5: Reduced PCE

After pouring cement concrete, a series of shrinkage processes occurs, such as plastic drying shrinkage, chemical shrinkage, self-shrinkage, and hardening-drying shrinkage, which leads to shrinkage cracks and seriously affects the mechanical properties and durability of the structure. Numerous experiments have shown that adding effective shrinkage-reducing agents can reduce the plastic drying shrinkage, self-shrinkageshrinkage, self-shrinkage, and hardening-drying shrinkage rates of concrete by more than 40%, thereby reducing the probability of severe cracks in concrete structures.

The widely recognized mechanism of action of shrinkage-reducing agents is that:
(1) the addition of shrinkage-reducing agents significantly reduces the surface tension of the liquid phase inside the concrete;
(2) the addition of shrinkage-reducing agents helps to slow down the evaporation rate of moisture inside concrete;
(3) the addition of shrinkage-reducing agents changes the pore structure of the internal slurry in concrete.

Regardless of the shrinkage-reducing agent’s mechanism of action, its high dosage (1% to 3%) affects its adoption in the engineering industry. Due to the low surface tension of PCE aqueous solution, people hope for the successful development and production of reduced PCE. However, to this day, no scaled-down PCE products are available.

Problem 6: Adaptability of PCE

Although polycarboxylate superplasticizers has a market share of over 80% in China’s water-reducing agent industry, some concrete production companies still reluctantly replace their polycarboxylate superplasticizers with naphthalene or aliphatic water-reducing agents. The reason is that current polycarboxylate superplasticizers products struggle to meet adaptability requirements when facing concrete raw materials from different regions.

PCE promoters can be ineffective when faced with different raw materials and additives across the country, even with experienced compounding techniques. These challenges mainly involve PCE adaptability. Establishing neural network models to assess the impact of different PCE types on various raw materials and additives may greatly benefit enterprises in polycarboxylate superplasticizers research and application.

Conclusion

In summary, although PCE has undergone more than 30 years of development since its birth in the 1980s, there are still many urgent problems to be solved in the research of PCE’s mechanism of action, the development of new PCE varieties, and the large-scale application of PCE.

The research on polycarboxylate superplasticizers is thriving! For a bright future for PCE, all relevant researchers should work together, strive for self-improvement, carry forward the past, and open up the future!