What is the mechanism of action of air-entraining agents in concrete?

Concrete air-entraining agent is a surfactant. When mixed into concrete, it can introduce a large number of evenly distributed and independently closed tiny bubbles during the mixing process. These tiny bubbles are different from the bubbles generated by introducing air during the concrete mixing process. The bubbles generated by introducing air through mechanical mixing are neither uniform nor stable. They can easily grow from small to large and escape during the mixing and vibrating process of concrete. If you want to To form stable, fine bubbles, air-entraining agents must be used.

The most important property of air-entraining agents is to reduce surface free energy (surface tension) and change the interface state of the system, thereby producing a series of effects such as wetting, emulsification, foaming, and solubilization. This is exactly the necessary condition for bubble formation. Because after bubbles are generated in the solution, the interface between the gas-liquid phase is greatly expanded, so that the surface energy also increases. For any system, there is a tendency for the free energy to automatically tend to the minimum before it is stable. Therefore, it is necessary to To produce stable bubbles, the surface energy of the gas-liquid interface must be low enough. The air-entraining agent can form positive adsorption at the gas-liquid interface. The molecules are directionally arranged, with the polar groups facing the water and the non-polar groups facing the air, which significantly reduces the surface energy of the gas-liquid interface and provides a basis for the formation of bubbles. necessary conditions.

Another condition for stable bubble generation is that the liquid film formed around the bubble should have a certain mechanical strength. When the air-entraining agent forms an adsorption layer, they are equivalent to forming a film layer. Due to the attraction between the air-entraining agent molecules, the film layer has a certain strength, coupled with the hydration of the air-entraining agent molecules, the formed film The layer is thicker, which makes it harder for the bubbles to coalesce.

Many air-entraining agents are ionic and have some electrical properties. Therefore, after the air-entraining agents are neatly arranged on both sides of the liquid film, their similar charges will repel each other, making it difficult for the bubbles to get closer, weakening the charge repulsion between the hydrophilic groups, increasing the density of the hydrophobic groups, and reducing the gas permeability. , extending the life of the foam. In addition, the bubble film adsorbing the air-entraining agent has a greater surface elasticity effect, which reduces the loss of the liquid mold, reduces the solubility of gas in the liquid film, and makes the bubbles more stable.

Usually, as the concentration of air-entraining agent increases, the surface tension will drop greatly and the foaming ability will also increase. However, when it reaches the highest point and the concentration is increased, the foaming ability will decrease instead. . In addition, there are a few air-entraining agents that reduce surface tension not too much, but their foaming ability is not small.

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