Application of Defoamer in concrete
The workability of fresh concrete has a great influence on the pumpability, construction performance, mechanical properties and durability of hardened concrete, and the workability of fresh concrete depends largely on the air content of the concrete . In actual construction, the optimal gas content range of concrete should be determined during the trial matching process according to the different engineering parts and concrete functions, and effectively controlled, which will help ensure the quality of concrete construction.
The concrete contains a large number of air bubbles before vibrating, mainly including air bubbles introduced during concrete mixing, transportation and unloading, air bubbles introduced by water reducing agent, and micro bubbles introduced by air entraining agent.
(1) Air bubbles introduced during concrete mixing, transportation and unloading. This type of air bubble has a large diameter, is unevenly distributed and is extremely unstable. It is easy to aggregate into a larger bubble diameter and is easy to burst, so it is called an unstable bubble. Such unstable bubbles introduced by mechanical agitation will have an adverse effect on both the fluidity of the concrete and the mechanical properties and durability of the concrete after hardening.
(2) Air bubbles introduced by water reducing agent. The water reducing agent can introduce a certain amount of air bubbles. Due to the same electrical repulsion, these air bubbles are located between the cement particles and disperse the cement particles like a ball bearing, 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 often gather together to form large bubbles, and eventually overflow to the surface of the concrete to form apparent bubbles, resulting in honeycomb hemp defects. .
(3) Air bubbles introduced by the air-entraining agent. Air-entraining agent can form a lot of fine air bubbles with a uniform size between (20-200) um inside the concrete. The liquid film on the surface of such bubbles is relatively firm. From a thermodynamic point of view, the liquid film has a higher electromotive potential, which can prevent the coalescence of bubbles, and the bubbles are relatively stable and difficult to break. It is fundamentally different from air bubbles introduced by water reducing agents, and is beneficial to the impermeability and other durability of concrete.
The defoamer is good for removing large bubbles in concrete. Adding antifoaming agent can eliminate air bubbles between concrete and formwork to a certain extent, effectively prevent or eliminate the generation of honeycomb and hemp on the concrete surface, so that the surface of the concrete has higher flatness and gloss. On the other hand, the defoamer can largely eliminate the 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 reducing agent. Therefore, in the engineering, a polycarboxylic acid water reducing agent is often compounded with a defoaming agent to solve the problem of large air entrainment of the polycarboxylic acid water reducing agent .
Because the polycarboxylic acid-based water reducer mother liquid has a large gas content, high surface activity, and good foam retention, when it is directly used in concrete, it will cause adverse effects such as high gas content, many apparent bubbles and low strength, so it needs to be restored. Formulate an appropriate amount of defoamer to eliminate large bubbles in the concrete. The basic performance test of the defoamer and polycarboxylic acid-based water reducer generally includes the compatibility of the defoamer and the water reducer and the effect of the defoamer on the properties of the concrete.
(1) Compatibility of defoamer and water reducing agent
The difficulty in compounding the defoamer with the polycarboxylic acid-based water-reducing agent is the compatibility with the water-reducing agent. The compatibility of the defoamer and the water reducing agent can be evaluated by testing the dissolution state of the defoaming agent in the polycarboxylic acid water reducing agent. The defoaming agent has good solubility in the polycarboxylic acid water reducing agent and has a long If the time is not delaminated, the compatibility is good and can be mixed with the water reducing agent; while the defoaming agent with poor compatibility cannot be mixed with the water reducing agent and can only be added to the concrete alone. Defoaming agent and polycarboxylic acid-based water reducer are added to cement paste, and the initial fluidity and time loss of fluidity of cement paste can also be used to evaluate the defoamer and polycarboxylic acid-type water reducer. compatibility. The defoamer with good compatibility with the polycarboxylic acid-based water reducing agent should be an antifoaming agent that does not have a significant adverse effect on the initial fluidity and loss of fluidity of the cement paste over time.
(2) Effect of defoamer on concrete performance The influence of defoamer on concrete performance is manifested in two aspects: the working performance of concrete and the mechanical properties after hardening. In general, the influence of defoamers on the performance of concrete is evaluated by testing the slump and slump loss, air content and strength of concrete. The antifoaming agent, which can greatly reduce the air content of concrete, has a small effect on concrete slump and slump loss, and has a significant effect on improving the strength of concrete.