Jun. 23, 2020
Most air-entraining agents are Anionic Surfactants. At the water vapor
interface, the hydrophobic base is directionally adsorbed toward the air. At the
cement-water interface, the cement or its hydrated ions are adsorbed to the
hydrophilic group, and the hydrophobic base is away from the cement. And its
hydrated ions, forming a hydrophobic adsorption layer, and trying to be close to
the air surface, due to the proximity of this ion to the air surface and the
adsorption of air-entraining molecules on the air-water interface, it will
significantly reduce the surface tension of the water and make the concrete
During the mixing process, a large number of fine bubbles are generated. These
bubbles have a directional adsorption layer with the same charge, so they repel
each other and can be evenly distributed; on the other hand, many anionic
air-entraining agents have calcium salts in cement solutions with high calcium
content Precipitation, adsorbed on the bubble film, can effectively prevent the
bubble from bursting, and the introduction of small and uniform bubbles can be
stable for a certain period of time.
It can be seen from the above mechanism that the interface activity of the air-entraining agent is similar to the water-reducing agent. The difference is that the interface activity of the water-reducing agent mainly occurs at the liquid-solid interface, while the interface activity of the air-entraining agent mainly occurs at the gas-liquid-solid interface. It is not difficult to see that the main function of the air-entraining agent is to introduce air bubbles first, followed by dispersion and wetting.
When the dry powder mortar containing the air-entraining agent is stirred with water, the air-entraining agent can significantly reduce the surface tension and interfacial energy of the water, so that the aqueous solution is prone to generate many small closed bubbles during the stirring process, and the bubble diameter is mostly below 200 μm.
The air-entraining agent introduces stable micro-bubbles into the mortar through physical action. This reduces the density of the wet mortar, improves workability, and increases the output of the wet mortar. The air remaining in the mortar makes the concrete have better thermal insulation performance, but also reduces the strength. The amount of air-entraining agent varies with the type of dry powder mortar and the type of air-entraining agent, but the amount of air-entraining agent is usually very low, generally only 0.002%~0.01% of the cement quality, and not more than 0.05% of the cement quality.
There are many factors that affect the gas content of mortar, including the characteristics and dosage of cement and admixture, water-cement ratio (water-to-cement ratio), variety and content of air-entraining agent, quality and particle distribution of aggregate, mortar mixer type and capacity , Mixing temperature, mixture consistency, temperature, etc. Although the amount of air-entraining agent is very small, the effect of air-entraining agent on the performance of dry powder mortar is very large. Its main functions are:
1.Improve the workability of dry powder mortar
The incorporation of air-entraining agent forms a large number of tiny closed bubbles in the concrete mixture. These micro-bubbles are like rolling balls, reducing the frictional resistance between aggregate particles and increasing the fluidity of the concrete mixture. If the fluidity is maintained, the water consumption can be reduced. At the same time, because the water is evenly distributed on the surface of a large number of bubbles, this reduces the amount of water that can move freely, so the bleeding of the wet mortar is reduced, and the water retention and cohesion are correspondingly improved.
2. Reduce the strength of dry powder mortar
Due to the existence of a large number of bubbles, the bearing area of the mortar is reduced and the strength of the concrete is reduced. But the air-entraining agent has a certain water-reducing effect (especially like the air-entraining water-reducing agent, the water-reducing effect is more significant), and the reduction of the water-cement ratio makes the strength compensated to some extent. However, the addition of air-entraining agent will still reduce the strength of the mortar, especially the compressive strength. Therefore, the amount of air-entraining agent should be strictly controlled, and the best addition can be determined by testing the gas content, construction performance and related strength of the mortar. In addition, due to the presence of a large number of bubbles, the elastic deformation of the mortar is increased, and the elastic modulus is reduced, which is beneficial to improve the crack resistance of the mortar.
3. Improve the impermeability and frost resistance of mortar
The air-entraining agent reduces the bleeding of the concrete mixture (generally the bleeding volume can be reduced by 30% to 40%). Therefore, the capillary of the bleeding channel is correspondingly reduced. At the same time, the presence of a large number of closed micro-bubbles blocked or blocked the capillary seepage channel in the mortar, changed the pore structure of the mortar, and made the mortar impermeability improved. The air bubble has a large elastic deformation capacity, and has a certain buffer effect on the expansion stress generated by the freezing of water, so the frost 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 the mortar, save materials, and increase the construction area.