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The influence of cement foaming agent on the quality of foamed concrete is extremely critical. A good cement foaming agent can make high-quality foamed concrete. How the various performance indicators of cement foaming agent affect the quality of foamed concrete.
The more uniform the foam, the better, and the diameter of the foam should be one size
The ideal pore size distribution of the pores is as narrow as possible, that is, the pore sizes of the pores are required to be as consistent as possible, and the difference should not be too large. This corresponds to the requirement that the foam should be uniform and not different in size. The bubble diameters of the foam cannot be exactly the same, but should be basically similar, the bubble diameter range should be as small as possible, and the difference between the maximum bubble diameter and the minimum bubble diameter should not be too large. As mentioned earlier, the pores formed by the foam are required to be uniform, which can avoid the concentration of compressive stress at the large pores and reduce the compressive strength. If the size of the foam is not uniform, the stress is concentrated in the large foam, which is very easy to cause it to become a weak link and be the first to crack when under pressure.
The higher the foam stability, the better, and the longer the foam stabilization time, the better
The foam with good stability has tough liquid film and good mechanical strength, and is not easy to burst or deform excessively under the extrusion of the slurry. In addition, it has self-water retention, the water 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 be stored for a long time without bursting.
1. The stability of the foam has three effects on the pores: most of the foam will not disappear, and it will be fixed in the foam concrete after the initial setting of the slurry to form the pores. If the foam stability is not good, most or a small part of the foam will burst after pouring, and there will be few pores formed.
2. Foam with good stability, the slurry is not easy to deform too much under extrusion, and has a certain resistance to pressure to keep itself approximately spherical, which can eventually form spherical pores with good hole shape;
3. Foam with good stability, the liquid film is not easy to rupture in the slurry, and it is not easy to form the communication hole formed by the collusion of the gas after the rupture.
Therefore, what it ends up with is the ideal closed pore. The less stable the foam, the fewer closed cells and the more open cells. Therefore, the stability of the foam can not be based on just not collapsing after pouring, but should be based on three indicators that do not collapse after pouring, the final formed pores are approximately spherical, and are not connected to each other. Most people take non-collapse after pouring as the measurement standard of foam stability, which is actually a misunderstanding and the minimum standard of foam stability.
Foam stability can be measured by foam stabilization time when there is no standard testing instrument to measure its sinking distance. The foam stabilization time should meet the needs of the initial setting of the cementitious material used. Because after the initial solidification of the slurry, the foam can be fixed and the shape of the foam can be retained to make it into a pore.
Any cementitious material has an initial setting time, especially the most widely used ordinary Portland cement, the initial setting is mostly later than 45min. If the foam stability is poor, the cement and other cementitious materials have not yet set, and the foam has burst, then the foam cannot 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 cementitious material by 10 to 20 minutes. Since the initial setting time of various cementitious materials is inconsistent, 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 cementitious material has different temperature, its initial setting time is different, and the change is quite large. For example, ordinary silicon cement may initially set in less than 40 minutes in summer, and will not initially set in 80 minutes in cold winter below 5 °C. Therefore, it is impossible to have a constant specific standard for the stabilization time of the foam, and it should be determined according to the situation. In order to make the foam adapt to the needs of various use conditions, it should be allowed to stabilize as long as possible. Roughly speaking, 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 greater than 60min; the ideal maximum is 3h;
2. When used in fast-setting cement such as sulfoaluminate cement, high alumina cement, iron aluminate cement, fast-setting Portland cement, etc., the stabilization time should be greater than 30min, preferably greater than 60min;
3. When used in magnesium cement, the foam stabilization time should be more than 40min, preferably more than 80min;
4. When used for cementitious materials mixed with a large amount of fillers or active waste residues such as fly ash, the foam stabilization time should also be extended. The larger the amount of filler 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 mold will not collapse after pouring, the pores will not be connected after the formation, and the excessive deformation will not be the principle. The big principle is that the longer the bubble stabilization time, the better.
The larger the bubble diameter, the better, and should be more than 1mm.
The requirements for the pores are the same for the pore size, and the requirements for the bubble diameter should also be the same. Many manufacturers and some experts are misleading to pursue the small pore size of the foam at 0.1mm. In fact, it is because of the physical foaming that if the foam diameter is large, the stability of the foam is poor and the foam concrete cannot be made. The same raw materials and formulations, the same equipment and process, just change the cell diameter. The compressive strength of foam concrete with a cell diameter of 1mm is at least 20% higher than that of foam concrete with a cell diameter of 0.1mm. Therefore, it is wrong to pursue a large bubble diameter above 0.1mm, which is a misunderstanding in the production of foamed concrete and should be corrected.
The lower the foam bleeding rate, the better, avoid milky foams
As mentioned earlier, foam is divided into milky foam with more water and less foam and sponge-like foam with more foam and less water, according to the size of water content. Among them, milky foam is unqualified and inferior foam and cannot be used.
After the foam is produced, it gradually oozes water. Its bleeding consists of two parts:
1. Bleeding of the foam liquid film. This part of the water is secreted from the liquid film, which is the water secreted by the liquid film under the combined action of gravity drainage, surface tension drainage, and liquid film destruction drainage. The thicker the liquid film of the foam, the greater the bleeding; the faster and more bubbles burst, the greater the bleeding.
2. Water between bubbles. This part is the water that does not form a liquid film of bubbles. The worse the foaming performance of the foaming agent and the foaming machine, the more water between the bubbles that cannot form a foam liquid film. They are actually water that doesn’t turn into bubbles.
When the bleeding rate of the foam is very high, it will be as thin as the floating juice, flowing chaotically, unable to pile up, and the water in the foam is very large. When this foam is added to the cement slurry, the cement slurry becomes very thin, and the foam is very small. The volume of the slurry is small, and the density of the foamed concrete produced is very high, which does not meet the technical requirements.
Therefore, milky foam is not acceptable. The appearance of the foam that meets the technical requirements should be sponge-like fine bubbles, piled up like white clouds, like bounced cotton, and will not flow chaotically. This foam has very little water content. This shows that its foam liquid film has little bleeding, the foam is not easy to burst, there is little water between the bubbles, and the amount of bubbles is large, and most of the volume is bubbles. It should be emphasized here that the spongy foam must be small bubbles with a diameter of less than 1mm, not large bubbles of several millimeters or even tens of millimeters. Large bubbles are easy to pile up and look like white clouds, but because the diameter of the bubbles is too large, they are also unqualified.
The low bleeding property of the foam is required 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.
Foam has no adverse effects on cementitious materials
The cementitious material is the main source of the strength of the foamed concrete, and the addition of the foam can not affect its cementitiousness, that is, it does not produce a negative effect of cementation. This point, not all kinds of bubbles can achieve.
In long-term experiments, we found that many foams hinder the cementation of cement, magnesite and gypsum, which can reduce the strength of foamed concrete. The stability of these foams is very good, it does not disappear for a few hours, the appearance is like a sponge, the bleeding is very low, the density is uniform, and it meets the technical requirements in all aspects, but it is not compatible with the cementitious 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 not technically acceptable.
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