Application of anionic surfactants in concrete industry

Most of the entraining agents are anionic surfactants. On the water vapor interface, the hydrophobic group is directively adsorbed to the air side; on the cement water interface, the cement or its hydration ions are adsorbed to the hydrophilic group phase; the hydrophobic group deviates from the cement and its hydration ions to form a hydrophobic adsorption layer, and tries to get close to the air surface. Due to the close proximity of ions to the air surface and the adsorption of air entraining agent molecules on the air-water interface, the surface tension of water will be significantly reduced, so that a large number of fine bubbles will be produced in the concrete mixing process. These bubbles have the same charge oriented adsorption layer, so they repel each other and can be evenly distributed. On the other hand, many anionic air entraining agents have calcium salt precipitation in cement aqueous solution with high calcium content, adsorbed on the bubble film, which can effectively prevent the bubble burst, and the introduced fine uniform foam can exist stably for a certain period of time.

It can be seen from the above mechanism that the interfacial activity of air entraining agent is similar to that of water reducing agent. The difference is that the interfacial activity of water reducing agent mainly occurs at the liquid-solid interface, while the interfacial activity of air entraining agent mainly occurs at the gas-liquid-solid interface. It is not difficult to see that the main role of air entraining agent is first to introduce bubbles, followed by dispersion and wetting.

When the dry mortar containing air entraining agent is mixed with water, because the air entraining agent can significantly reduce the surface tension and interface energy of water, many small closed bubbles are easily generated in the water solution during the mixing process, and the diameter of the bubbles is mostly below 200μm.

Air entraining agent introduces stable microbubbles into mortar by physical action. This reduces the density of wet mortar, better workability, and increases the yield of wet mortar. The air trapped in the mortar gives the concrete better thermal insulation, but it also reduces its strength. The amount of air entraining agent varies with the type of dry mortar and the type of air entraining agent, but the amount of air entraining agent is usually very low, generally only 0.002% to 0.01% of the cement quality, not more than 0.05% of the cement quality.

There are many factors affecting the gas content of mortar, including the characteristics and dosage of cement and admixture, water-cement ratio (water-binder ratio), air entraining agent variety and dosage, aggregate quality and particle distribution, mortar mixer type and capacity, mixing temperature, mixing consistency, air temperature and so on. Although the amount of air entraining agent is very small, the effect of air entraining agent on the performance of dry powder mortar is great. Its main functions are:

1 Improve the workability of dry mortar

The addition of air entraining agent can form a large number of small closed bubbles in the concrete mix. These micro bubbles are like balls, which reduce the friction resistance between the aggregate particles and increase the fluidity of the concrete mix. If the fluidity is maintained, water consumption can be reduced. At the same time, due to the uniform distribution of water on the surface of a large number of bubbles, the amount of water that can move freely is reduced, and the water secretion of wet mortar is reduced, while the water retention and cohesion are correspondingly improved.

2 Reduce the strength of dry mortar

Due to the existence of a large number of bubbles, the stressed area of mortar is reduced and the strength of concrete is reduced. However, air entraining agent has a certain water reducing effect (especially like air entraining water reducing agent, water reducing effect is more significant), and the reduction of water-cement ratio makes the strength compensated to a certain extent. However, the addition of air entraining agent will still reduce the strength of mortar, especially the compressive strength. Therefore, the content of air entraining agent should be strictly controlled, and the optimal addition amount can be determined by testing the gas content, construction performance and related strength of mortar. In addition, due to the existence of a large number of bubbles, the elastic deformation of mortar increases and the elastic modulus decreases, which is beneficial to improving the crack resistance of mortar.

3 Improve the impermeability and frost resistance of mortar

Air entraining agent can reduce the water yield of concrete mixture (general water yield can be reduced by 30%~40%). Therefore, the capillary of the secretion channel is also reduced. At the same time, the existence of a large number of closed microbubbles blocked or cut off the capillary seepage channels in the mortar, changed the pore structure of the mortar, and improved the permeability of the mortar. The bubble has a large elastic deformation capacity, which has a certain buffer effect on the expansion stress caused by water icing, so the freezing resistance of the mortar is improved, and the durability is also improved. In addition, the addition of air entraining agent will reduce the density of mortar, save materials, and increase the construction area.

It can be seen that the technical benefits brought by the incorporation of air entraining agents can be summarized as follows:

(1) The dispersive lubrication and water-reducing effects of air entrainment agent significantly improve the workability and workability of concrete;

(2) The ball bearing and lubrication of air entrainment agent bubbles improve the pumpability of concrete;

(3) Due to the surface adsorption of air entraining agent, the viscosity of cement slurry is increased, which can significantly reduce concrete bleeding and settlement shrinkage;

(4) As the water requirement of concrete mixed with air entraining agent is reduced, the capillary porosity is also significantly reduced, and the capillary channels in concrete are cut off by tiny bubbles, and the permeability resistance of concrete is improved;

(5) The appropriate amount of bubbles can buffer the expansion stress caused by freeze-thaw in concrete, and the freeze-thaw resistance of concrete can be increased several times;

(6) The corrosion resistance of concrete depends largely on its own permeability and compactness. Therefore, the corrosion resistance of concrete mixed with air entraining agent has also been significantly improved.