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Foaming agent for concrete is exceptionally critical to the quality of foam concrete. An excellent foaming agent can make high-quality foam concrete. A high-quality foaming agent for concrete should have the following characteristics:
1. The more even the foam, the better, the bubble diameter should be the same size
The narrower the ideal pore size distribution of the pores, the better. That is, the pore diameters of the pores should be as consistent as possible, and the difference should not be too significant. This corresponding requirement requires that the foam should be uniform and not different in size. The bubble diameter of the foam cannot be the same, but it should be similar. The range of the bubble diameter is as small as possible, and the difference between the maximum bubble diameter and the minimum bubble diameter should not be too significant. As mentioned earlier, the pores formed by the foam are required to be uniform to avoid the concentration of compressive stress at the giant cells and reduce the compressive strength. If the size of the foam is uneven, the weight is concentrated in the giant bubble, which is very easy to cause it to become a weak link, and it will crack first when it is under pressure.
2. The higher the foam stability, the better, and the longer the foam stability time, the better
The foam with right balance has a tough liquid film and excellent mechanical strength and is not easy to be broken or excessively deformed under the extrusion of the slurry. Besides, it has self-water retention, the moisture on the liquid film is not easy to be lost under the action of gravity and surface tension, and can maintain the thickness and integrity of the foam liquid film for a long time so that the foam can remain for a long time without breaking.
(1) The stability of the foam has three effects on the pores: it can prevent most of the foam from disappearing and is fixed in the foam concrete after the initial setting of the slurry to form pores. If the stability of the foam is not excellent, most or a small part of the foam will burst after pouring, and the pores formed will be few, and the slurry will collapse even shortly after pouring, which is commonly known as the collapse mould, causing the pouring to fail completely.
(2) Foam with excellent stability, the slurry is not easy to deform too much under squeezing, and has a specific resistance to pressure to keep itself approximately spherical, which can eventually form spherical pores with good pore shape;
(3) Foam with excellent stability, the liquid film is not easy to rupture in the slurry body, and it is not easy to form the communication hole created by the gas communication after the rupture. Therefore, it eventually creates an ideal closed hole. The worse the foam stability, the fewer closed cells, and the more connected cells. Thus, the balance of the foam cannot be based on the fact that it does not collapse only after casting but should be found on the three indicators of non-collapse after casting, the resulting pores being approximately spherical, and no interconnection. Most people regard foam collapse as the measure of foam stability. This is a misunderstanding and the lowest standard of foam stability.
Foam stability can be measured by foam stabilization time when there is no standard testing instrument to determine its sinking distance. The foam stabilization time should meet the initial setting needs of the used gelling material. Because the slurry can only be fixed after the initial setting of the mud, and the shape of the foam is retained to make it become a pore.
Any cementitious material has an initial setting time, especially for the most widely used ordinary Portland cement, the initial setting is mostly later than 45min. If the foam stability is reduced, the cement and other cementing materials have not yet set, and the foam has burst, then the foam cannot form pores in the concrete. In general, the minimum requirements for foam stability should also be such that the foam stabilization time is longer than the initial setting time of the cementitious material by 10 to 20 minutes. Due to the inconsistent initial setting time of various gelling materials, the requirements for foam stabilization time are also different. In general, the foam stabilization time for the fast gelling material foam can be shorter, and the foam stabilization time for the slow gelling material foam should be as long as possible. Even if the temperature of the same kind of gelling material is different, the initial setting time is different, and the change is quite significant. For example, ordinary portland cement may initially set in less than 40 minutes in summer, but will not set in 80 minutes in cold winter below 5 ℃. Therefore, there is no uniform specific standard for the foam stabilization time, which should be determined according to the situation. To adapt the foam to the needs of various conditions of use, the longer the stabilization time, the better. Generally speaking, the foam stabilization time should meet the following requirements;
(1) When used in Portland cement and without setting accelerator, the stabilization time should be higher than 60min; ideally, it should be higher than 3h;
(2) When used in fast-setting cement such as sulfoaluminate cement, high alumina cement, iron aluminate cement, quick setting Portland cement, the stabilizing time should be higher than 30min, preferably higher than 60min;
(3) When used in magnesite cement, the foam stabilization time should be higher than 40min and preferably higher than 80min;
(4) When used for cementing materials mixed with a large amount of filler or fly ash and other active waste residues, the foam stabilization time should also be extended. The higher the amount of padding or active waste residue, the longer the foam stabilization time.
The specific foam stabilization time should be determined by a small test, based on the principle that the mould does not collapse after pouring, the pores are not connected after the formation, and the deformation is not excessive. The general policy is that the longer the bubble stabilization time, the better.
3. The larger the bubble diameter, the better, it should be more than 1mm
The requirements of pore size for pore size are the same, as are the criteria for bubble diameter. Many producers and some experts misled the pursuit of a foam pore size as small as 0.1mm. If the physical foaming diameter is large, the stability of the foam will be reduced, failing to produce foamed concrete. The same raw material and formula, the same equipment and process, only change the foam diameter, the compressive strength of 1mm foam diameter foam concrete is at least 20% higher than that of 0.1mm foam diameter foam concrete. Therefore, it is wrong to pursue a large bubble diameter of more than 0.1mm, which is a mistake in the production of foam concrete and should be corrected.
4. The lower the foam bleeding rate, the better, avoid using milky foam
It has been introduced that foams are divided into milky foams with less water and less foam and sponge foams with less water and water, according to the water content. Among them, milky foams are unqualified inferior foams and cannot be used.
After the foam is made, it gradually oozes out. Its bleeding includes two parts:
(1) Bleeding of foam liquid film. This part of the water is secreted from the liquid membrane, which is the water emitted by the liquid layer 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 the foam bursts, the higher the bleeding.
(2) The water between the bubbles. This part is the water that has not formed a bubble liquid film. The worse the foaming performance of the foaming agent and the foaming machine, the more water between foams that cannot create a foam liquid film. They are water that does not become bubbles.
When the foam's bleeding rate is very high, it will be as thin as the floating juice, and it will not pile up, and the amount of water in the foam is considerable. When this kind of foam is added to the cement slurry, the cement slurry becomes very thin, but the foam is very little, the volume of the sludge is small, and the foam concrete density produced is very high, which does not meet the technical requirements.
Therefore, milky foam cannot be used. The appearance of the foam that meets the technical specifications should be a sponge-like beautiful small bubble, piled up like white clouds, like bombed cotton, and will not flow randomly. The water content of this foam is minimal. This shows that the foam liquid membrane has slight bleeding, the foam is not easy to break, 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 a small bubble with a bubble diameter of less than 1 mm, not a giant balloon of a few millimetres or even tens of millimetres. Big bubbles are easy to pile up, like Baiyun, but because the bubble diameter is too large, it is also unqualified.
The low water permeability of the foam is mainly required to ensure the number of bubbles in the foam and the porosity of the foam concrete, that is, to ensure the foam concrete density.
5. Foam has no side effects on the gelled material
The cementing material is the primary source of the strength of the foam concrete. The addition of foam can not affect its gelation; that is, it does not produce a negative effect of gelation. This is not something that all kinds of bubbles can achieve.
In long-term tests, we have found that many foams hinder the cementation of cement, magnesite, and gypsum, which can reduce the strength of foamed concrete, and sometimes even the hardened concrete loses its power, like a piece of loose sand. The stability of these foams is excellent, and it does not disappear for a few hours, the appearance is like a sponge; the bleeding is deficient, the density is uniform. All aspects meet the technical requirements, but it is not compatible with the gelling material, so it cannot be used for production. This shows that the foam that is qualified in other aspects, such as affecting the strength, is ultimately unsatisfactory.
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