Polycarboxylate superplasticizers (PCE) have been favored by researchers and engineers since their inception in the late 1980s due to their low dosage, high water reduction rate (not less than 25%), good reinforcing effect, strong molecular adjustability, and the fact that no formaldehyde is used in the synthesis process.
Challenge 1: The Mechanism of PCE's Action – The flowability of cement paste incorporating PCE is significantly improved. Researchers generally believe that PCE adsorbs onto the surface of cement particles. The carboxylic acid groups on the main chain of PCE molecules adsorbed on the surfaces of adjacent cement particles disperse the particles through the principle of like charge repulsion. In addition, the side chains of PCE molecules can provide steric hindrance, thus giving PCE excellent dispersibility. However, how exactly is PCE adsorbed onto the surface of cement particles? Is it a result of electrostatic interaction or driven by entropy increase? This remains inconclusive.
Challenge 2: The Intercalation Effect of PCE – PCE may intercalate in certain hydration products. What is the mechanism of PCE intercalation?
Challenge 3: Early-Strength PCE – Research results on early-strength PCE have been obtained, and a small number of early-strength PCE products have begun to appear on the market. However, doubts remain regarding the mechanism of action of early-strength PCE.
Problem 4: Accelerating PCE – Currently, most common accelerators on the market are aluminum sulfate-based liquid alkali-free accelerators. While these accelerators have a relatively small impact on the later-stage strength of concrete, their compatibility with cementitious materials is not ideal.
Problem 5: Shrinkage-reducing PCE – After pouring, cement concrete undergoes a series of shrinkages, such as plastic drying shrinkage, chemical shrinkage, autogenous shrinkage, and hardening drying shrinkage, leading to shrinkage cracks that severely affect the mechanical properties and durability of the structure. Numerous experiments have shown that incorporating effective shrinkage-reducing agents can reduce the plastic drying shrinkage, autogenous shrinkage, and hardening drying shrinkage rates of concrete by more than 40%, thus reducing the probability of severe cracks in concrete structures. Regarding the mechanism of shrinkage-reducing agents, the generally accepted conclusions are: (1) the incorporation of shrinkage-reducing agents significantly reduces the surface tension of the liquid phase inside the concrete; (2) the incorporation of shrinkage-reducing agents helps to slow down the evaporation rate of moisture inside the concrete; and (3) the incorporation of shrinkage-reducing agents changes the pore structure of the slurry inside the concrete. Regardless of the mechanism of shrinkage-reducing agents, their large dosage (1%–3%) affects the enthusiasm of the engineering community for their use. Because aqueous solutions of PCE also have low surface tension, there is high hope for the successful development and production of shrinkage-reducing PCE. However, to date, no large-scale shrinkage-reducing PCE products have emerged.
Problem 6: The Adaptability of PCE – Current PCE products struggle to meet the requirements when dealing with concrete raw materials from different regions. Even with experienced compounding techniques, those promoting and applying PCE sometimes find themselves at a loss when faced with diverse raw materials and other admixtures across the country.
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