Application of defoaming agents in concrete

Types of bubbles in concrete

Before concrete is vibrated, there are a large number of bubbles inside it, mainly including air bubbles introduced during concrete mixing, transportation and blanking, bubbles introduced by water-reducing agents, and tiny bubbles introduced by air-entraining agents.

(1) Air bubbles introduced during concrete mixing, transportation and blanking. This type of bubble has a large diameter, uneven distribution and is extremely unstable. It is easy to aggregate into bubbles with a larger diameter and burst easily, so it is called unstable bubble. This unstable bubble introduced by mechanical mixing will have a negative impact 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-reducing agent can introduce a certain amount of bubbles. Due to the same electrical repulsion, these bubbles are between the cement particles like ball bearings to disperse the cement particles, thus increasing the sliding effect between the cement particles. However, these bubbles are uneven in size, irregular in shape and unstable. As transportation and vibration proceed, they tend to aggregate and merge with each other to form large bubbles, which eventually overflow outwards and reach the surface of the concrete to form apparent bubbles, resulting in honeycomb pitted surface defects.

(3) Bubbles introduced by air-entraining agents. The air-entraining agent can form many fine bubbles with a size between (20-200)um and evenly distributed inside the concrete. The liquid film on the surface of this type of bubble is relatively strong. From a thermodynamic point of view, that is, the liquid film has a high electrokinetic potential, which can prevent the bubbles from coalescing. The bubbles are relatively stable and difficult to burst. It is essentially different from the bubbles introduced by water-reducing agents, and is beneficial to the impermeability and other durability of concrete.

Defoaming agents help eliminate large air bubbles in concrete. On the one hand, adding defoaming agent can eliminate the bubbles between the concrete and the formwork to a certain extent, effectively prevent or eliminate the generation of honeycombs and pitted surfaces on the concrete surface, and make the surface of the concrete have higher smoothness and gloss. On the other hand, defoaming agents can eliminate a large number of bubbles inside concrete, reduce the air content and internal porosity of concrete, and improve the mechanical properties and durability of concrete. Defoaming agents in concrete mainly eliminate bubbles introduced by water-reducing agents. Therefore, in engineering, defoaming agents are often compounded with polycarboxylic acid-based water-reducing agents to solve the problem of large air entrainment of polycarboxylic acid-based water-reducing agents. .