Foaming agent and foam stabilizing agent, as the core functional additives for the preparation of foam concrete, respectively assume the core functions of “efficient air entraining nucleation” and “long-term foam stabilizing and shaping”. Foaming agents achieve uniform air coverage and efficient bubble generation by reducing the surface tension at the gas-liquid interface; By thickening the liquid film, improving the interfacial viscoelasticity, and inhibiting the liquid film drainage and gas diffusion, the foam stabilizer can solve the problems of foam annexation, rupture, and sedimentation failure. The molecular structure characteristics, interfacial adsorption behavior, synergistic mechanism and paste matching law of the two directly determine the rheological properties of fresh foam paste, bubble uniformity and micro pore morphology of hardened matrix. At present, there is a lot of research on the application of single foaming systems in the industry, but there is still a lack of systematic reviews on the mechanisms, performance shortcomings, compatibility mechanisms, and cutting-edge modification technologies of different types of foaming agents and foam stabilizers. Therefore, this paper systematically combs the classification characteristics, action mechanism and performance laws of mainstream foaming agents and foam stabilizers for foam concrete, clarifies the micro dynamic mechanism of foam formation and instability, analyzes the structure effect relationship between the structure of additives and material properties, summarizes the composite synergistic modification technology, existing technical bottlenecks and future development trends, and provides theoretical support and technical reference for the formulation design and engineering application of high-performance, high stability, low volume weight foam concrete.
1 Basic theory of foam concrete foaming system and evolution mechanism of foam
The forming essence of foam concrete is a three-phase coupling evolution process of gas, liquid and solid, including four core stages of bubble nucleation, growth, stability, and curing. Foaming agent and foam stabilizer run through the whole process and dominate the evolution of bubble structure. The surface tension of pure water solution is extremely high, and the gas-liquid interface is extremely unstable after the introduction of gas. Bubbles will break and dissipate instantly, and a stable foam system cannot be formed. Foaming agents, as amphiphilic surface active substances, can spontaneously adsorb at gas-liquid interfaces, significantly reducing interfacial free energy and providing thermodynamic basis for bubble nucleation and stable growth; The foam stabilizing agent inhibits liquid membrane drainage, gas penetration and bubble merger from the dynamic level, prolongs the stability period of foam, ensures that the foam will not burst or collapse before the initial setting of cement hydration, and realizes the uniform shaping of micropore structure.
Classification, structural characteristics, and foaming performance mechanism of mainstream foaming agents
According to the source of raw materials, molecular structure and foaming principle, foam concrete foaming agents can be systematically divided into four categories: surfactant synthetic foaming agent, protein natural foaming agent, composite foaming agent, and new bio based foaming agent. There are significant differences in the interface characteristics, foaming times, bubble fineness, and paste adaptability of different systems, which are suitable for the preparation of foam concrete under different working conditions.
2.1 Synthetic surfactants foaming agents
Synthetic surfactant foaming agents are currently the most widely used foaming systems in industrial applications, mainly including anionic and nonionic types. Anionic foaming agents, such as sodium dodecylbenzenesulfonate, sodium fatty alcohol ether sulfate (AES), and sodium alpha olefin sulfonate (AOS), are typical representatives. The molecular hydrophilic groups have strong ionization and fast interfacial adsorption rate, which can quickly reduce the surface tension of aqueous solutions. They have high foaming ratio, fast foaming speed, and low cost, making them suitable for large-scale industrial production. This kind of foaming agent can quickly generate a large number of foam, with high initial bubble density and excellent molding efficiency. However, the single component foam liquid film has poor viscoelasticity, fast liquid discharge rate, short half-life of foam, weak stability, and the problem of bubble coarsening and collapse is easy to occur in the process of standing. When used alone, it is very easy to lead to uneven pore diameter of hardened concrete, large pore defects, and large strength dispersion.
2.2 Protein based natural foaming agent
Protein foaming agent is made from animal protein and plant protein by hydrolysis, modification and refining. It is the preferred foaming system for high-performance foam concrete. Its molecule is a long chain polypeptide structure, rich in amino, carboxyl and other polar groups, and has excellent amphiphilic structure. After adsorption at the gas-liquid interface, it can form a highly viscoelastic, high-strength and compact interfacial facial mask. Compared with synthetic surfactant, protein foaming agent has mild foaming, fine and uniform bubbles, significantly improved half-life of foam, strong anti drainage and anti annexation ability, and can effectively inhibit instability and collapse of foam.
In the environment of cement hydration with high alkalinity and high ionic strength, the protein foam has excellent structural stability and is not susceptible to electrolyte interference. The molded foam concrete has good micropore sealing, uniform pore diameter, controllable porosity, high strength, low water absorption and excellent thermal insulation performance of hardened products. But it has obvious shortcomings: high raw material costs, lower foaming ratio than synthetic foaming agents, slow foaming rate, some crude protein foaming agents have odors, and are prone to spoilage and mold during storage, which restricts large-scale low-cost applications. At present, the mainstream in the industry balances its foaming efficiency and stability performance through protein modification and compounding technology.
2.3 Composite foaming agent
Both single synthetic foaming agents and protein foaming agents have performance shortcomings, while composite foaming agents achieve complementary advantages through the synergistic mechanism of “anionic nonionic complex, protein surfactant complex”. Synthetic components ensure high foaming ratio and construction efficiency, protein or non-ionic components improve foam stability and paste adaptability, and effectively solve the problems of single system foam easy to collapse, uneven pore size, and poor paste adaptability. Composite foaming agents have the comprehensive advantages of high foaming efficiency, long steady-state life, uniform pore size, and controllable cost, and are currently the most widely used mainstream system in engineering applications. By adjusting the distribution ratio of the two groups, it can accurately adapt to the preparation requirements of foam concrete with different densities and strength grades, and the formula tolerance rate is significantly improved.
2.4 New bio based foaming agent
Biobased foaming agents have been a cutting-edge research direction in the field of green building materials in recent years, relying on microbial metabolism and natural biomass degradation to prepare functional foaming components. The foaming mechanism mainly consists of three stages: firstly, microbial proteases degrade natural proteins to generate soluble short peptides, which adsorb at the gas-liquid interface to reduce surface tension; Secondly, microorganisms metabolize carbohydrate substrates to generate CO ₂ gas, which assists in bubble nucleation and growth; Finally, the bioactive substance forms a stable adsorption layer at the interface to inhibit the instability of foam. Bio based foaming agent has the advantages of non-toxic, harmless, green, low-carbon, degradable, environmentally friendly, etc. The foam is delicate and stable, and the slurry compatibility is excellent. However, there are problems such as high preparation cost, low productivity, poor stability and controllability, and it is still in the laboratory research and pilot stage, and has not yet achieved large-scale industrial application.
Classification, foam stabilization mechanism, and performance characteristics of foam stabilizers
Foam stabilizing agent is the core additive to improve the steady performance of foam and optimize the pore structure of hardened concrete. It can significantly extend the half-life of foam and ensure the structural integrity of foam in the initial setting stage of cement hydration through multiple mechanisms such as thickening the liquid film, improving the interfacial viscoelasticity, improving the liquid viscosity, blocking gas diffusion, and inhibiting particle adsorption and foam breaking. According to the types of components and mechanisms of action, foam stabilizers can be divided into three categories: high molecular weight polysaccharides, polymer associated types, and inorganic nanosolid foam stabilizers. The mechanisms of action and performance advantages of each type of foam stabilizer differ significantly.
3.1 Polysaccharide polymer foam stabilizer
Polysaccharide foam stabilizers, represented by guar gum, xanthan gum, konjac gum, cellulose derivatives, and seaweed polysaccharides, are the most mature natural polymer foam stabilization systems. This type of water-soluble polymer can significantly increase liquid phase viscosity, increase liquid film drainage resistance, and delay gravity drainage and capillary drainage processes when dissolved in water; At the same time, polymer long chains can form multi-layer entangled adsorption films at the gas-liquid interface, greatly increasing the thickness of the liquid film and interface strength, reducing gas permeation rate, and effectively suppressing bubble coalescence and collapse.
The research shows that polysaccharide foam stabilizer can significantly reduce the 1h settling distance and bleeding rate of foam, significantly improve the uniformity of bubble size distribution, and effectively solve the problems of easy collapse and pore diameter coarsening of synthetic foaming agent foam. In the foam concrete paste, polysaccharide molecules can form a synergistic adsorption structure with cement particles and foaming agent molecules to improve the uniformity of the paste, reduce the defects of large pores and connected pores in the hardened matrix, and improve the compactness and mechanical properties of materials. However, excessive addition of polysaccharide foam stabilizers can lead to excessive viscosity, decreased flowability, increased construction resistance, and delayed cement hydration process, affecting early strength development. There is a clear balance between the addition threshold and performance.
3.2 Polymer based foam stabilizers
The associative polymer foam stabilizer is a new type of functionalized foam stabilizing material, with hydrophobic modified polyacrylamide and polyether based associative polymers as the core, possessing dual functions of temperature response and interface association. The molecular chain contains both hydrophilic main chains and hydrophobic side groups. In aqueous solution, it can undergo molecular association with surfactant foaming agents to form high-strength composite interfacial micelle structures; At the same time, in the process of cement hydration and temperature rise, the temperature response group can take place hydrophobic phase change, which can strengthen the interface association and further lock the foam structure.
This type of foam stabilizer does not significantly improve the viscosity of the base liquid phase, but can significantly improve the stability of the foam interface on the premise of ensuring the fluidity of slurry construction, and perfectly solve the problem of traditional polysaccharide foam stabilizer that “foam stability must reduce the fluidity”. The interface film formed by it has excellent elasticity and strong disturbance resistance, which can effectively resist the impact of cement particles, ion interference and temperature fluctuations. The steady life of foam has been greatly extended. It is suitable for the preparation of high-density, high fluidity and high-precision foam concrete, and is the core research direction of high-performance foam stabilizer.
3.3 Inorganic Nanosolid Foam Stabilizers
Inorganic nano foam stabilizers, represented by ultrafine powders such as nano silica, nano alumina, bentonite, and kaolin, rely on the interfacial adsorption and skeleton support of nanoparticles to achieve foam stability. Nanoparticles can firmly adsorb at the gas-liquid interface, forming a rigid inorganic protective layer that physically blocks gas diffusion and liquid film rupture; At the same time, nano powder can fill the micro defects of the liquid film, improve the density and mechanical strength of the interface, and greatly improve the anti disturbance and anti burst ability of foam.
Compared to organic foam stabilizers, inorganic nano foam stabilizers have excellent thermal stability, do not affect cement hydration, and have no organic residues. They can simultaneously achieve foam stabilization and matrix reinforcement. Nanoparticles filled in the hardened structure of foam pore wall can refine the micro pores of the matrix, improve the density and mechanical strength of the pore wall, and achieve the triple effect of foam stabilization, densification and enhancement. However, its weakness lies in that the nano powder is easy to agglomerate, and uneven dispersion will cause imbalance in the stability of local foam, leading to discrete pore size distribution, which needs to be optimized with the dispersion process.
4 Influence of foaming/foam stabilizing additives on macro properties of foam concrete
The type, dosage and mixing ratio of foaming agent and foam stabilizing agent directly control the pore structure parameters of foam concrete, and then determine the core macro properties of materials, such as dry density, compressive strength, thermal conductivity, water absorption, volume stability, etc. There is a significant structure effect relationship and performance balance law.
4.1 Regulation of pore structure parameters
The amount of foaming agent directly determines the amount of foam introduced and the porosity of the matrix: with the increase of the amount of foaming agent, the number of foam increases, the porosity of the matrix increases, and the dry density decreases; However, too high a content of foam will lead to excessive foam, thin liquid film, unstable structure, large number of bubbles merging and breaking, forming a large number of large pore size and connected pore defects, and the uniformity of pore structure will be greatly reduced. The addition of foam stabilizing agent can significantly optimize the pore size distribution, reduce the pore size dispersion, reduce the proportion of connected holes and defective holes, improve the closed porosity, and make foam concrete form a fine, uniform and regular microporous structure. The high-quality foam stabilization system can stably control the bubble pore size within the range of 0.1-1mm, with a closed pore rate of over 85%, providing a basic guarantee for material insulation, waterproofing, and mechanical properties.
4.2 Impact on Mechanical Properties
The mechanical properties of foam concrete are highly related to the integrity of pore structure and the density of pore wall. A moderate amount of foaming agent combined with foam stabilizer can form a uniform microporous structure, with uniform stress distribution and stable compressive and flexural strength; If the stability of foam is poor, the pore diameter is coarsened, and the number of defective holes is increased, the stress concentration is significant during the stress process, and the mechanical properties of the material are greatly reduced and the dispersion is increased. By optimizing the pore structure and improving the compactness of the pore wall, foam stabilizers can significantly enhance the strength of the matrix under the same dry density conditions, achieving a synergistic optimization of lightweight and high strength. However, excessive addition of foam stabilizers can cause the slurry to become viscous, hinder hydration, and result in uneven pore accumulation, ultimately leading to a decrease in mechanical properties.
4.3 Impact on Thermal and Durability Performance
Uniform closed microporous structure can effectively block air convection and heat conduction, giving foam concrete excellent thermal insulation performance. The foam concrete prepared by high-quality foam stabilizing system has fine pore diameter, good sealing performance, low thermal conductivity and excellent stability; The large pore and connected pore structure caused by the instability of foam will accelerate the heat convection transfer, leading to a significant deterioration of insulation performance. At the same time, closed micropores can effectively block water penetration, reduce the water absorption and moisture expansion rate of materials, and improve volume stability, frost resistance and anti-aging durability. On the contrary, the connected pores caused by the structural defects of foam will form a water vapor infiltration channel, causing the water absorption to soar and the durability to decline.
5. Progress in Composite Synergistic Foam Stabilization Mechanism and Modification Technology
Single foaming agent and foam stabilizing agent all have performance shortcomings, which can not meet the multiple demands of high foaming efficiency, high foam stability, high construction fluidity and high hardening performance at the same time. Multi component compound synergistic modification is the mainstream development trend of high-performance foam concrete additives.
5.1 Surfactant polymer complex system
Utilizing the synergistic advantages of high foaming efficiency of anionic surfactants and high foam stability of polysaccharide polymers, a binary complex system is constructed. Surfactants quickly realize bubble nucleation and growth, and macromolecular polysaccharides are adsorbed on the gas-liquid interface to thicken the liquid film and inhibit liquid drainage and foam breaking. The two molecules associate with each other to form a “flexible organic composite interface facial mask”, giving consideration to both foaming ratio and foam stability. The system has simple process, controllable cost and strong adaptability, which can significantly improve the collapse and uneven pore size of single synthetic foaming agent foam. It is the most mature composite system in engineering application.
5.2 Organic inorganic synergistic compound system
The high interface flexibility of organic foam stabilizer is combined with the high rigidity and high thermal stability of inorganic nano powder to achieve performance complementarity. Organic macromolecules ensure the stability of foam forming at low temperature and construction fluidity. Inorganic nanoparticles build a rigid interface skeleton, improve the foam’s ability to resist high temperature disturbance and particle impact, fill the micro pores of the pore wall, and strengthen the density and mechanical properties of the hardened matrix. This system can effectively solve the industry problem of “foam stabilization and strength balance”, and is suitable for the preparation of high strength, low bulk density and high durability foam concrete.
5.3 Temperature responsive intelligent foam stabilization system
The associative temperature responsive foam stabilizer developed based on the hydration and heating characteristics of cement can achieve intelligent control of “low-temperature foam assistance, high-temperature foam stabilization, and later enhancement”. During the construction phase at room temperature, the molecular binding effect is weak, the viscosity of the slurry is moderate, and the construction flowability is excellent; During the hydration and temperature rise stage, the temperature response group undergoes hydrophobic phase change, secondary association strengthens the interface facial mask structure, locks the foam morphology, avoids the thermal instability and collapse of foam caused by hydration and temperature rise, and precisely adapts to the performance control of foam concrete during the whole hydration and hardening process.
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Post time: Sep-09-2026