Stability of Bubble Film Machine

The essence of the air entrainment process of concrete is that under the action of external forces (such as mechanical mixing or mechanical mixing), the mechanical mixing energy of bubble membrane machine is partly converted into the surface free energy of concrete. As a result of air entrainment, the free energy of rod-concrete system increases, and the air entraining concrete becomes a thermodynamic unstable system. The increase of the free energy of the system is mainly due to the increase of the surface energy of the system due to the attraction of bubbles. Therefore, the size of C is equal to the increase of the surface free energy. The change of surface free energy of air-entrained concrete can be calculated by the following formula:

The role of air entraining agent is to increase the stability of air bubbles in concrete. To achieve this day, air entraining agent is from two aspects.

1. Reduce the free branding of the system and improve the stability of the air-entrained concrete system. The enrichment of air entraining agent on the gas-liquid interface reduces the surface tension of the gas-liquid surface, and further reduces the self-d3 energy U of the concrete system caused by the air entrainment process.

2. The dynamic barrier of bubble coalescence is added. The formation mechanism of dynamic barrier of bubble coalescence varies with the type of air entraining agent.

For ionic air entraining agent, the mechanism is mainly hidden by electrostatic repulsion. The first way is to concentrate the air entraining agent on the surface of the liquid film which forms bubbles. The polar groups of the adsorbed attractant molecules point to water and form a protective film. If the air entraining agent molecules are charged, the bubbles are charged as a result. In the process of bubble approaching each other under the action of mixing, bubbles are separated because of the influence of electrostatic repulsion. This effect is similar to that of emulsifier in maintaining the stability of the emulsion and preventing demulsification. In concrete without air entraining agent, small bubbles tend to merge into bubbles when bubbles are close to each other under the action of stirring. Then, under the dual action of buoyancy and stirring loosening, bubbles rise to the surface of concrete slurry, where bubbles break down and are consumed, and the air content of concrete slurry decreases.

For nonionic surfactants, the main mechanism is the stabilization of molecular hydration layer. The second method of bubble stabilization is related to the formation of hydrated layer on the surface of bubbles. The hydration layer may have several thicknesses of water molecules. Its function is to separate bubbles, stabilize the system and prevent flocculation. This action is thought to occur in the enrichment layer formed by nonionic surfactants. Because the enrichment layer of non-ionic surfactants can not charge the bubbles, the hydration layer is the only factor that can stabilize the bubbles, which ultimately leads to the poor air entrainment effect and larger bubble size of non-ionic air entraining agent compared with ionic air entraining agent.

In addition to the above bubble stabilization mechanism, Meielenz et al. also proposed the Insolu-ble Film Mechanism. That is, if the anionic air entraining agent and Ca''in concrete can form insoluble salt, the insoluble calcium salt will precipitate in the anionic air entraining agent enrichment layer on the surface of the bubble, forming a hard protective "shell" to protect the bubble from merging and disappearing.

It is well known that cationic and nonionic surfactants do not form insoluble calcium salt precipitation, but these two kinds of surfactants do have air-entraining effect. Moreover, anionic admixtures such as alkyl sulfonates, which can form soluble calcium salts with Cat'have good air entrainment effect. Therefore, the "insoluble film stabilization mechanism" may be important, but not necessary.