Any cementing material, including cement and magnesite, has an initial setting time, especially for the most widely used ordinary portland cement. The initial setting is mostly later than 45 minutes. If the foaming agent has poor stability, cement and other cementitious materials have not yet set, and the foam has collapsed, the foam will not be able to form pores in the concrete. In general, the minimum requirement for foam stability is to make the foam stabilization time longer than the initial setting time of the gelling material by 10-20 minutes. Because the initial setting time of various gelling materials is not consistent, the requirements for foam stabilization time are also different. In general, the foam stabilization time for fast gelling materials can be shorter, and the foam stabilization time for slow gelling materials should be as long as possible. Even if the same kind of gelling material has a different temperature, its initial setting time is different, and the change is quite significant. For example, ordinary silicon cement may set in less than 40 minutes in summer.
But it will not set in 80 minutes in cold winter below 5 ° C. Therefore, it is impossible to have a uniform specific standard for the stabilization time of the foam, and it should be determined according to the situation. To adapt the foam to the needs of various conditions of use, it should be allowed to stabilize as long as possible. In general, the stabilization time of the foam should meet the following requirements;
1. When used in Portland cement without setting accelerator, the stabilization time should be more than 60min; the ideal maximum is more than 3h;
2. When used in fast-setting cement such as sulphoaluminate cement, high alumina cement, iron aluminate cement, fast setting Portland cement, the stabilization time should be more than 30min, preferably more than 60min;
3. When used in magnesium cement, the foam stabilization time should be higher than 40min, preferably higher than 80min;
4. When used for gelling materials doped with a large amount of filler or active waste residue such as fly ash, the foam stabilization time should also be extended. The more significant the amount of padding or active waste residue, the longer the foam stabilization time.
The specific foam stabilization time should be determined through a small test. The principle is that the mould does not collapse after pouring, the pores are not connected after formation, and the law of excessive deformation is not determined. The vital principle is that the longer the stabilization time, the better.
The larger the bubble diameter, the better. It should be above 1mm.
The pores have the same requirements for the pore size, as well as the requirements for the bubble diameter. Many producers and some experts have misled the pursuit of a fine foam with a pore size of 0.1mm. It is because physical foaming cannot make a foamed concrete if the foam diameter is large, and the stability of the foam is poor. The same raw materials and formula, the same equipment and technology, only change the foam diameter, the compressive strength of foam concrete with 1mm foam diameter is at least 20% higher than that of foam concrete with 0.1mm foam diameter. Therefore, it is wrong to pursue a small bubble diameter of more than 0.1mm, which is a misunderstanding in the production of foam concrete and should be corrected.
The lower the foam bleeding rate, the better, avoid using milky foam.
As mentioned earlier, foams are divided into milky foams with more water and less foam and creamy foams with more water and less foam. The milky foam is unqualified and inferior and cannot be used.
After the foam is confident, it gradually exudes water. Its bleeding includes two parts:
1. Bleeding of foam liquid membrane. This part of the water is secreted from the liquid film. It is the water that the liquid film emits under the combined action of gravity drainage, surface tension drainage, and fluid membrane destruction drainage. The thicker the liquid film of the foam, the higher the bleeding; the faster and more the foam collapses, the higher the bleeding.
2. Inter-bubble water between foams. This part is water that does not form a bubble liquid film. The worse the foaming performance of the foaming agent and the foaming machine, the more inter-bubble water cannot create a foam liquid film. They are water that does not become air bubbles.
When the bleeding rate of the foam is very high, it will be as thin as floating juice, flowing randomly and cannot be piled up, and the amount of water in the foam is large. When this foam is added to the cement slurry, the cement slurry becomes very thin. Still, the foam is minimal, the volume of the mud is small, and the density of the foamed concrete is very high, which is not technically desirable.
Therefore, milky foam cannot be used. The appearance of the foam that meets the technical requirements should be sponge-like fine vesicles, piled up like white clouds and like bouncing cotton, and will not flow randomly. The water content of this foam is minimal. This shows that its foam liquid membrane bleeds little, the foam is not comfortable to rupture, there is little water between the bubbles, and the amount of bubbles is significant, and most of the volume is bubbles. What needs to be emphasized here is that the sponge-like foam must be small bubbles with a bubble diameter of less than 1mm, not large bubbles of several millimetres or even tens of millimetres. Giant bubbles are easy to pile up, and they are like white clouds, but because the bubble diameter is too large, it is also unqualified.
The requirement for low water bleeding of the foam is mainly to ensure the number of bubbles in the foam and the porosity of the foamed concrete, that is, to ensure the density of the foamed concrete.