Application of Defoamer in Concrete

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

Types of Air Bubbles in Concrete

Concrete contains a lot of air bubbles before vibrating, mainly including air bubbles introduced during concrete mixing, transportation and blanking, air bubbles introduced by water reducing agent and Microscopic air bubbles introduced by an air-entraining agent.

(1) Air bubbles introduced during concrete mixing, transportation and blanking. This type of bubble diameter is large, unevenly distributed and extremely unstable, and it is easy to aggregate into larger bubble diameter bubbles and easily burst, so they are called unstable bubbles. The unstable air bubbles introduced by mechanical stirring will have adverse effects on the fluidity of concrete, as well as on the mechanical properties and durability of concrete after hardening.

(2) Air bubbles introduced by water reducing agent. The water reducer can introduce a certain amount of air bubbles. Due to the same electrical repulsion, these air bubbles are located between the cement particles like ball bearings to disperse the cement particles, thereby increasing the sliding effect between the cement particles. However, these bubbles are uneven in size, irregular in shape and unstable. With the progress of transportation and vibration, they tend to aggregate and merge into large bubbles, and finally overflow to the concrete surface to form apparent bubbles, resulting in honeycomb pitting defects .

(3) Air bubbles introduced by the air-entraining agent. The air-entraining agent can form many fine air bubbles with a size between (20-200)um and uniform distribution inside the concrete. The liquid film on the surface of this kind of bubbles is relatively firm. From a thermodynamic point of view, the electrokinetic potential of the liquid film is relatively high, which can prevent the coalescence of the bubbles, and the bubbles are relatively stable and difficult to burst. It is essentially different from the air bubbles introduced by the water reducing agent, and is beneficial to the impermeability and other durability of the concrete.

The defoamer helps to remove large air bubbles in the concrete. On the one hand, adding defoamer can eliminate air bubbles between concrete and formwork to a certain extent, effectively prevent or eliminate the generation of honeycomb and pockmarks on the concrete surface, and make the concrete surface have higher smoothness and gloss. On the other hand, the defoamer can eliminate a large number of air bubbles inside the concrete, reduce the air content and internal porosity of the concrete, and improve the mechanical properties and durability of the concrete. The defoamer in concrete mainly eliminates the air bubbles introduced by the water reducer. Therefore, in engineering, it is often compounded with a defoamer in the polycarboxylate water reducer to solve the problem of large air entrainment of the polycarboxylate water reducer. .

Compound of defoamer and polycarboxylate water reducer

Because the polycarboxylate water reducer mother liquor has a large air content, high surface activity, and good foam retention, when it is directly used in concrete, it will cause air content in the concrete High, apparent air bubbles and low strength and other adverse effects, so it is necessary to compound an appropriate amount of defoamer to eliminate large air bubbles in concrete. The basic performance testing of the combination of defoamer and polycarboxylate water reducer generally includes the compatibility of defoamer and water reducer and the effect of defoamer on concrete performance.

(1) Compatibility of defoamer and water reducer

The difficulty in compounding the defoamer with the polycarboxylate water reducer is the compatibility with the water reducer. The compatibility of the defoamer and the water reducer can be evaluated by testing the dissolution state of the defoamer in the polycarboxylate water reducer. The solubility of the defoamer in the polycarboxylate water reducer is good and long If there is no stratification over time, the compatibility is good, and it can be compounded with the water reducing agent; while the defoamer with poor compatibility cannot be compounded with the water reducing agent, and can only be added to the concrete alone. Adding defoamer and polycarboxylate water reducer to cement paste can also be used to evaluate the performance of defoamer and polycarboxylate water reducer by testing the initial fluidity of cement paste and the loss of fluidity over time. compatibility. The defoamer with good compatibility with the polycarboxylate water reducer should be a defoamer that has no obvious adverse effects on the initial fluidity of the cement slurry and the loss of fluidity over time.

(2) Effect of defoaming agent on concrete performance The effect of defoaming agent on concrete performance is manifested in two aspects: the working performance of concrete and the mechanical properties after hardening performance. Generally, the impact of defoamers on concrete properties is evaluated by testing the slump and slump loss, air content and strength of concrete. It can greatly reduce the air content of concrete, has little effect on concrete slump and slump loss, and has obvious effects on the strength of concrete.��High action defoamer works well.