Water reducing agents have a series of important effects on the properties of cement slurry mainly through adsorption. The requirements for concrete raw materials are more stringent than those of traditional high-efficiency water-reducing agents such as naphthalene series and aliphatic ones. The current shortage of sand and gravel aggregates and the irregular production of commercial concrete make it easier than ever to introduce mud into concrete. This impact can be summed up in the following aspects: ① prolong the early stage of cement hydration induction; ② reduce the surface tension and system viscosity of the solution in the cement slurry; ③ introduce air into the system to form microbubbles, which is beneficial to fresh mixing. The flow of concrete; ④ Reduce the shear yield stress and plastic viscosity of the cement slurry; ⑤ Effectively slow down the hydration rate and heat release rate of cement minerals, and improve the volume stability of concrete: ⑥ Improve due to the significant water reduction effect The compactness of concrete, thereby improving the strength of concrete. In addition, the presence of water reducing agent may make the crystal particle size of hydration products smaller, which is beneficial to further improving the strength of concrete.
Water reducing agent mainly regulates cement hydration through adsorption on the surface of cement particles. In addition, the interaction between water-reducing agent molecules and ions in the solution may also affect the cement hydration process. Hekala believes that the water-reducing agent adsorbs on the surface of C3A to form an adsorption layer, which prevents the dissolution of cement minerals. The hydration rate of C3A in the early stage of hydration is lower than that of the blank system. Merlini studied the effect of water reducing agent on cement hydration. The results showed that the growth rate of ettringite phase in different hydration systems depends to a large extent on the admixture. The initial hydration system of cement after adding water reducing agent The amount of medium-crystalline vanadium is much lower than that of the blank system. It shows that on the one hand, the water-reducing agent is wrapped on the surface of the cement minerals by adsorption, thereby reducing the reaction rate in the early stage of cement hydration. On the other hand, the water-reducing agent molecules in the solution may also interact with the new ecological vanadium or vanadium in the solution. Ca2+ acts, thereby adversely affecting the growth of crystalline vanadium crystals. That is, the water-reducing agent molecules play the role of template agent in the growth process of wollastonite. Studies have shown that in addition to preventing the dissolution of cement minerals through adsorption, water-reducing agents can also affect the cement hydration process through the coordination of its carboxyl groups. For example, Puertas studied the effect of water reducing agent on cement hydration by measuring the heat of hydration of cement slurry. The results show that as the amount of water-reducing agent increases, the cement hydration induction period is significantly prolonged, while the exothermic shoulder peak of AFt transformed into AFm does not change much. He believes that the water-reducing agent can form a complex with Ca2+ in the liquid phase to reduce the Ca2+ concentration, thereby hindering the nucleation and growth of hydration products. Pourchet studied the effect of water-reducing agent on the hydration of C3S by examining the effect of water-reducing agent on the electrical conductivity of cement slurry. The results showed that the liquid phase of the cement slurry after adding water-reducing agent during the first two hours of the hydration reaction was The electrical conductivity basically does not increase or even decreases slightly, and the higher the density of carboxyl groups on the main chain of the superplasticizer, the lower the electrical conductivity of the slurry. However, the liquid phase conductivity of the blank system gradually increased to the maximum value with time during the first two hours of the hydration reaction. He believes that this is because the carboxyl functional group on the main chain of the water reducing agent can coordinate with Ca2+. As the density of carboxyl groups increases, the coordination effect of the water-reducing agent increases. Heikal also believes that the coordination of carboxyl groups is one of the main reasons for the decrease in electrical conductivity of cement paste.
In addition to regulating the hydration process of cement, water reducing agents can also affect the morphology of specific hydration products. When Plank synthesized hydration products through chemical methods, he found that the morphology of the hydration products with a positive Zeta potential changed significantly, such as high-sulfur calcium sulfoaluminate changing from an elongated rod-like structure to a compact needle-like structure; while the Zeta potential was The morphology of negative or close to 0 hydration products, such as potassium gypsum and colemanite, does not change significantly. Plank explained that on the one hand, the water-reducing agent accelerates the formation rate of the crystal nuclei of the hydration product; on the other hand, the adsorption of the water-reducing agent on specific crystal surfaces hinders the normal growth of the hydration product, and the zeta potential of the water-reducing agent is Hydration products that are negative or close to 0 have no obvious adsorption. Eusebio also discovered that water-reducing agents affect the crystal morphology of wollastonite. He believes that this is because the main chain of the water-reducing agent induces wollastonite to grow in different directions.
Based on the above research, the mechanism of water reducing agent regulating the cement hydration process can be summarized as follows: water reducing agent molecules are adsorbed on the surface of cement particles to form an adsorption layer, which leads to a reduction in the dissolution rate of cement minerals and makes the cement water The hydration quickly enters the hydration induction period; through the coordination of carboxyl groups, the ion concentration in the solution is reduced and the selective adsorption of the water-reducing agent on the hydration products delays the nucleation and growth of the hydration products, thus delaying the hydration acceleration period. form. In practical applications, different uses of concrete (such as precast concrete and pumped concrete) have different requirements for cement hydration properties, and the regulating effect of water-reducing agents on the cement hydration process can also adjust the molecular structure of the water-reducing agents. To control, this must be based on a deeper study of the control capabilities, characteristics and mechanisms of different functional groups in the molecular structures of different structures and types of superplasticizers on cement hydration.
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