Foam stability refers to the ability of a foam to remain unbroken for a long period of time. The longer the bubble exists, the better the stability and the more difficult it is to break. The destruction process of the foam is mainly a process in which the liquid film separating the liquid is thinned from thick to cracked.
1 Foam stability factors
1.1 Thickness of the bubble film layer
If the thickness of the bubble liquid film is too small, the mechanical strength is low, the external force resistance is poor, and it is easy to be broken; the bubble liquid film is too thick, and the inter-bubble water is too much, resulting in excessively high foam bleeding, and the liquid film is too thick to accelerate the discharge. It is also easy to cause bubble breaking. Therefore, the thickness of the bubble film layer should be kept within a certain range.
1.2 Liquid surface tension
Lower liquid surface tension favors foam formation but does not guarantee foam stability. In general, the protein foaming agent has a high surface tension, and although the foaming ratio is low, the foam produced has high stability.
The strength of the bubble liquid film is a key factor determining the stability of the foam, and the strength of the liquid film depends on the robustness of the surface adsorbed film, its firmness is related to the surface viscosity, and the higher the surface viscosity, the longer the foam life is formed. On the other hand, the higher the viscosity of the liquid itself, the slower the liquid overflows from the liquid film, thereby enhancing foam stability.
1.4 Self-healing of surface tension
When the foam liquid film is subjected to an external force, the local thickness of the liquid film is thinned, and foaming is likely to occur. If a surfactant is used, when it is adsorbed on the surface of the liquid film, the surface can be expanded and shrunk to make the liquid film. Thickness recovery, strength recovery, self-healing of surface tension.
1.5 Surface charge
When the two surfaces of the bubble liquid film have the same kind of electric charge, they are mutually repelled, and the liquid film can be prevented from being squeezed and thinned to be broken.
1.6 Molecular structure of surfactant
The hydrophobic chain of the surfactant should have a moderate chain length to ensure that the adsorption film formed on the surface of the liquid film has a high viscosity; the hydrophilic group of the surfactant has a strong hydration ability, and the high flow in the foam liquid film can be achieved. The free water of the degree becomes the bound water of the less fluid, thereby increasing the viscosity of the liquid film.
2 The effect of foam stability on the properties of foam concrete
The stability of the foam directly determines the quality of the foamed concrete and affects the performance of the foamed concrete. The effects are mainly reflected in the following aspects:
2.1 The stability of the foam determines the pouring height and yield
After the foam is mixed into the slurry, the liquid film is subjected to the action of self-gravity drainage and surface tension drainage, and is also squeezed by the cement material. If the foam stability is poor, the foaming phenomenon is likely to occur in partial breakage. After the bubble, the remaining part will be subjected to a larger pressing force, which will cause a chain breakage phenomenon, which will eventually lead to the collapse of the mold, and the deposition of polycondensation under the cement material will result in uneven distribution of the finished foam concrete. .
2.2 Foam stability affects internal pore structure
The high-stability foam can prevent foaming in the early stage of the initial setting of the slurry, thereby forming spherical closed pores with fine pores and uniform distribution inside the concrete, and obtaining better durability and compressive strength.
2.3 Foam stability affects density
2.4 Foam stability affects slurry mixing
When the foam stability is poor, it is easy to break the foam and release moisture after being mechanically squeezed during the stirring process, so that the water ash is increased compared with the original design.
In some cases, in order to increase the stability of the foam, a foam stabilizer is often added to the foaming agent. The foam stabilizer refers to an additive which can prolong the life of the foam and is good for the retention of the foam, and is mainly a surfactant.