0 Introduction
Many studies have shown that a large number of poorly structured bubbles are the most important factors that reduce the working performance, mechanical properties and durability of cement-based materials. It is mainly reflected in the following three aspects.


(1) When the cement slurry foam is serious, the foam accumulates in the pump outlet to form an air cushion, making the pump run idling, and the construction is hindered [1,2];


(2) A large number of bubbles increase the volume of cement slurry, but these bubbles are easy to break, causing volume contraction and deformation;


(3) A large number of bubbles will lead to a decrease in the density of the cement slurry, reducing the mechanical properties and durability of the hardened cement slurry.


Air bubbles generally exist in the cement slurry in two forms. One is that when the buoyancy force of the bubble is greater than the resistance force generated by the cement slurry on the gas phase escape, the bubble floats up and gathers on the liquid surface, forming a continuous visible bubble. This kind of bubble is easy to produce honeycomb surface, resulting in apparent quality defects of cement-based materials. The other is that when the cement slurry has enough retarding effect on the gas phase escape, the bubbles are dispersed in the cement slurry discontinuously. Such bubbles can significantly reduce the density of cement-based materials and adversely affect the mechanical properties and durability of the materials [3].


Defoamer is an admixture that can help to reduce the large bubbles entrained or produced by concrete and mortar during mixing and construction. Defoamer is composed of substances that can reduce the surface tension of the liquid phase, and acts by inhibiting the formation of bubbles and breaking the bubble film. Defoaming agent is the most direct and effective means to eliminate harmful bubbles in cement-based materials [4]. Using defoaming agent to eliminate harmful large bubbles and optimize pore structure, the hardened cement-based material can achieve the requirements of flat surface, no honeycomb pockmarked surface, good homogeneity, good mechanical properties and durability.




1 Classification of defoamer
According to its main components, the defoamer can be divided into three types: non-silicon, silicon and silica-ether mixture. The non-silicon system mainly consists of polyether defoamer, others include higher alcohols, acids, hydrocarbons and esters, etc. Silicon defoamer is mainly composed of silicone oil or silica black silicone oil paste and modified silicone oil, which is the most widely used defoamer at present.


Defoamer has certain pertinence and specificity. The foaming mechanism and process conditions of each foaming system are different, and the defoamer required is different, but no matter what kind of foaming agent, it should have the following characteristics as a defoamer:


(1) Strong defoaming power, small dosage;


(2) The basic nature of the system will not be affected after accession;


(3) Low surface tension;


(4) Good balance with the surface;


(5) Good diffusivity and permeability;


(6) Good heat resistance, acid and alkali resistance;


(7) Stable chemical properties, strong oxidation resistance;


(8) Good gas solubility and permeability;


(9) Small solubility in a foaming solution;


(10) High physiological safety.




1.1 Polyether defoamer


Polyether defoamer was first put into production by Wyandott Company in the United States in 1954, and has developed rapidly, especially in the 1960s, with the development of polyether industry, polyether defoamer has developed most rapidly. In China, defoamer was also successfully developed and put into production in 1969, and was first used for antibiotic fermentation.


Polyether type defoamer mainly includes straight chain polyethers (such as polyoxyethylene and polyoxyethylene propylene, etc.), polyether derivatives whose end groups are alcohols, amines or esterified polyethers. Polyether prepared by ring-opening polymerization of ethylene oxide and propylene oxide is a water-soluble nonionic surfactant with excellent properties. When in contact with water, the oxygen atom in the ether bond can be bonded with the hydrogen atom in water, the molecular chain joint becomes a curved shape, and the hydrophilic group faces the outside of the molecule, so the polyether chain is easy to bond with water. When the temperature rises, the molecular movement is more intense, and the zigzag chain will become a zigzag shape and lose its bond with water. The temperature at which polyether molecules change from low temperature dissolved state to turbidity state is the turbidity point of polyether. Only when the temperature of the system exceeds the turbidity point, polyether defoamer can play the role of defoamer. By adjusting the proportion and molecular weight of ethylene oxide and propylene oxide, the turbidity point can be reduced, and the water solubility of the defoamer can be reduced, so that the defoamer can have good defoamer and defoamer ability at low temperature.


Polyether defoamer has the advantages of non-toxicity, convenient use, good dispersion, thermal stability and chemical stability, etc. The disadvantage is low defoamer breaking rate. Once a large number of bubbles are generated in the system, polyether defoamer cannot be effectively extinguished in time, and a certain amount of defoamer needs to be added [5].


1.2 Silicone defoamer


Pure silicone, such as dimethylsilicone oil, does not have a defoaming effect, but after emulsifying it, the surface tension is rapidly reduced, and the amount is very small, which can produce a strong foaming and foaming inhibition effect. Emulsified silica has become an important component of defoamer. Silicone defoamer in common use is prepared with silicone oil as the base component and suitable solvent, emulsifier and inorganic filler. As a defoamer, emulsified silicone has excellent defoaming ability, and its insolubility, physicochemical stability, physiological inertia and high and low temperature resistance are equal to one, which can be used in both water and non-water systems [6]. However, the emulsification problem is complex, if the emulsification is incomplete, the demulsification will seriously affect the use effect, the foam suppression ability is poor, and it is easy to produce silicon spots, so its application is limited to a certain extent.


1.3 siloether mixed defoamer


The siloether mixed defoamer consists of two forms: one is the combination of silicone oil and polyether by emulsifying method; The other is to connect polyether to the silicone oil chain by grafting method, that is, polyether modified polysiloxane defoamer. The preparation process of the former is relatively simple, but the emulsification problem is the key; The latter has the advantage of good stability, can be emulsified itself, without adding other additives, but its preparation process is relatively complex, the conditions are higher, and the cost is higher.


The siloether mixture, especially polyether modified polysiloxane defoamer, organically combines the advantages of silicone defoamer and polyether defoamer. It not only has the characteristics of polysiloxane defoamer, such as strong defoamer, low surface tension, low volatility, non-toxic, non-polluting and physiological inertia, but also has the characteristics of polyether defoamer, such as high temperature resistance and strong alkali resistance. It is a defoamer with excellent performance and broad application prospects [7]. In the siloxane copolymer molecule, the siloxane segment is an oleophilic group and the polyether segment is a hydrophilic group. The group of polyether chain segment has great influence on the performance of silane copolymer. Polyether chain energy saving provides hydrophilicity and foamability, while polyether chain energy saving provides hydrophobicity and permeability, which can effectively reduce surface tension. By adjusting the proportion of siloxane segments in the copolymer, the characteristics of silicone can be highlighted or weakened.




2. Action mechanism of defoamer
In cement-based materials, due to the low viscosity of the slurry, the bubble rises by buoyancy, and the liquid film of the bubble moving to the surface produces a pressure difference due to the action of gravity, the thickness of the film becomes thin, and the bubble finally bursts. This method of self-defoaming of cement-based materials alone cannot meet the comprehensive properties required by products such as self-compacting concrete or self-leveling mortar, and appropriate defoaming agents must be selected for further defoaming treatment.


The functions of defoamer include foam suppression and defoaming. Foam suppression means that when the system is added to the defoamer, its molecules are randomly distributed on the surface of the liquid, inhibiting the formation of elastic film, that is, inhibiting the generation of bubbles. Defoaming means that when the system produces a large number of bubbles, the defoaming agent is added, and its molecules are immediately dispersed on the surface of the foam, quickly spread out, and form a very thin double film layer, further diffusion, penetration, and layered invasion, thus replacing the thin wall of the original bubble film. Due to its low surface tension, it is easy to flow to the liquid with high surface tension, so that its liquid film is rapidly thinened, and the bubble is also subjected to the strong traction of the film layer with large surface tension around the bubble, resulting in the stress imbalance of the bubble, so as to defoamize. The defoamer molecules, which are insoluble in the system, then re-reach the surface of another bubble film, and so on [8].








Figure 1 Defoaming mechanism of defoaming agent




3 Application of defoamer in cement-based materials
Defoamer has been used in cement-based materials such as dry mix mortar and concrete. In the dry mixed mortar, the defoamer is mainly used for self-leveling mortar, which can eliminate the bubbles introduced by additives such as emulsion powder and water reducing agent in the mixing process, so as to make the hardened floor material smooth and flat, reduce the pores on the surface of the material, and improve the durability.


In concrete, defoamer mainly plays the role of eliminating large bubbles and optimizing pore structure. As we all know, the successful advent of water reducing agents has brought about a revolution in modern concrete, but water reducing agents are easy to produce bubbles in the concrete during use, especially polycarboxylic acid water reducing agents. Large bubbles will significantly reduce the mechanical properties and durability of concrete, it is necessary to use defoamer to eliminate these bubbles.


At present, the defoamer used in cement-based materials includes powder defoamer and liquid defoamer. Powder defoamer is mainly used in dry mixed mortar, and liquid defoamer is mainly used in concrete. At the same time, most water reducing agent manufacturers will mix defoamer in the water reducing agent produced to improve the bubble structure introduced when the water reducing agent is used.


3.1 Application of defoamer in dry mixed mortar


In the process of mixing dry mixed mortar with water, the air introduced is wrapped by wet mortar to form bubbles. At the same time, admixtures such as water reducing agent, redispersible emulsion powder and cellulose ether have hydrophilic groups, which can reduce the surface tension of the liquid, cause the accumulation of bubbles, and increase the stability of bubbles.


Mortar products (such as self-leveling mortar, waterproof mortar, etc.) with 3% to 6% microbubble gas content is the best. The uniform and stable small bubbles help to improve the mortar workability, enhance the mortar frost resistance and other durability. The increase of large bubbles will increase the voidage of mortar and reduce the mechanical properties of mortar. Especially for mortar products with waterproof requirements, it is necessary to suppress and defoaming the mortar.


3.1.1 Influence of defoamer on wet density of mortar


The defoamer can inhibit the formation of bubbles and destroy the formed bubbles in the mortar. Therefore, adding defoamer to mortar products can reduce the gas content of mortar and increase the wet density of mortar. As shown in Figure 2, the effect of different kinds of defoamer is different to some extent, but within a certain range, the wet density of mortar increases with the increase of the dosage of defoamer. When the content of defoamer reaches a certain value, the wet density of mortar tends to be stable.




FIG. 2 Influence of different types of defoamer on the wet density of self-leveling mortar




3.1.2 Influence of defoamer on fluidity of mortar


As shown in Figure 3, defoamer can improve the initial fluidity of mortar, and different types of defoamer have different improvement effects on the fluidity. Effective defoamer can not only make the bubble burst quickly, but also prevent the bubble regeneration for a long time.


FIG. 3 Flow promoting effect of different types of defoamer on self-leveling mortar


3.1.3 Influence of defoamer on mechanical properties of mortar


As shown in Figure 4, by eliminating harmful bubbles in the mortar, reducing the porosity of the mortar and improving the pore structure of the cement stone, the defoamer improves the compressive and flexural strength of the mortar, but it no longer increases after reaching a certain value.




FIG. 4 Influence of different types of defoamer on compressive strength and flexural strength of self-leveling mortar


3.2 Application of defoamer in concrete


The workability of freshly mixed concrete has a great influence on the pumpability, construction performance, mechanical properties and durability of concrete after hardening, and the workability of freshly mixed concrete depends largely on the gas content of concrete. In the actual construction, the optimum gas content range of concrete should be determined and effectively controlled in the process of trial allocation according to the different engineering parts and concrete functions, which is helpful to ensure the construction quality of concrete.


3.2.1 Types of bubbles in concrete


There are a lot of bubbles in concrete before vibration, including air bubbles introduced in concrete mixing, transportation and blanking, bubbles introduced by water reducing agent and tiny bubbles introduced by air entraining agent.


(1) Air bubbles introduced during concrete mixing, transportation and blanking. This kind of gas bubble diameter is large, the distribution is uneven and extremely unstable, it is easy to aggregate into a larger bubble diameter bubble, easy to burst, so it is called unstable bubble. This kind of unstable bubble introduced by mechanical mixing will have an adverse effect on both the fluidity of concrete and the mechanical properties and durability of concrete after hardening.


(2) Bubbles introduced by water reducer. The water-reducing agent can introduce a certain amount of bubbles, due to the same electrical repulsion, these bubbles are in the cement particles like ball bearings to disperse the cement particles, thereby increasing the sliding effect between the cement particles. However, these bubbles are not uniform in size, irregular in shape and unstable, and with the progress of transportation and vibration, they often converge with each other to become large bubbles, and eventually spill out to the surface of the concrete to form apparent bubbles, resulting in honeycomb surface defects.


(3) Bubbles introduced by air entraining agent. Air entraining agent can make the concrete inside the formation of many sizes between (20-200)um and uniform distribution of fine bubbles. The liquid film on the surface of such bubbles is relatively firm, from a thermodynamic point of view, that is, the electrokinetic potential of the liquid film is high, which can prevent the coalescence of the bubbles, and the bubbles are relatively stable and not easy to burst. It is fundamentally different from the bubbles introduced by water reducing agents, and is beneficial to the impermeability and other durability of concrete.


Defoamer is good for removing large bubbles in concrete. On the one hand, the addition of defoamer can eliminate the bubbles between the concrete and the formwork to a certain extent, effectively prevent or eliminate the generation of honeycomb and pockmarked surface of the concrete surface, so that the surface of the concrete has a high flatness and gloss. On the other hand, defoamer can eliminate a large number of bubbles in concrete, reduce the gas content and internal porosity of concrete, and improve the mechanical properties and durability of concrete.


The defoamer in concrete mainly eliminates the bubbles introduced by the superplasticizer. Therefore, the defoamer is often mixed in the polycarboxylic acid superplasticizer in engineering to solve the problem of large ventilation of the polycarboxylic acid superplasticizer.


3.2.2 Combination of defoamer and polycarboxylic acid superplasticizer


Since the polycarboxylic acid superplasticizer mother liquor has large gas content, high surface activity and good foam retention, it will cause adverse effects of high gas content, many apparent bubbles and low strength when it is directly used in concrete, so it is necessary to mix an appropriate amount of defoamer to eliminate large bubbles in concrete.


The basic performance test of the combination of defoamer and polycarboxylic acid superplasticizer generally includes the compatibility of defoamer and superplasticizer and the influence of defoamer on the performance of concrete.


(1) Compatibility of defoamer and water reducer


The difficulty of the combination of defoamer and polycarboxylic acid superplasticizer is the compatibility with the superplasticizer. The compatibility of defoamer and water reducer can be evaluated by testing the dissolution state of defoamer in polycarboxylic acid superplasticizer. If defoamer has good solubility in polycarboxylic acid superplasticizer and is not stratified for a long time, it has good compatibility and can be mixed with water reducer. The defoamer with poor compatibility can not be mixed with water reducing agent, and can only be added to the concrete alone. The compatibility of defoamer and polycarboxylic acid superplasticizer can also be evaluated by testing the initial fluidity and the loss of fluidity of cement paste by adding defoamer and polycarboxylic acid superplasticizer into cement paste. The defoamer with good compatibility with polycarboxylic acid superplasticizer should be the defoamer that has no obvious adverse effect on the initial fluidity and the time loss of fluidity of cement paste.


(2) The effect of defoamer on the performance of concrete


The effect of defoamer on concrete performance is manifested in two aspects: the working performance of concrete and the mechanical properties after hardening. The effect of defoamer on concrete performance is generally evaluated by measuring slump, slump loss, gas content and strength of concrete. The effect of defoamer which can greatly reduce the gas content of concrete, has little effect on slump and slump loss of concrete and has obvious effect on the strength of concrete is better.




4 Conclusion and prospect
Defoamer is used to eliminate harmful bubbles in cement-based materials and has wide application value in concrete and mortar.