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What is the main mechanism of action of concrete admixture?

Release time:

2022-08-30

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What is the main mechanism of action of concrete admixture?

Due to the use of concrete admixtures, the large fluidity of concrete has been applied. In self-compacting concrete and high-strength concrete, thickeners are added to improve the performance of underwater concrete. Due to the addition of retarders, the setting time of cement can be prolonged, which may reduce slump loss and prolong the operation time. Thanks to antifreeze, the freezing point of the solution is reduced, or the deformation of ice crystal structure will not cause freezing damage. Construction is possible at negative temperatures.

1. Concrete admixture

Using the adsorption of cement particles to concrete admixtures and the dispersion of concrete admixtures to cement, the flocculation structure is formed after the cement is added to the cement slurry. When the concrete admixture molecules are adsorbed by the slurry on the cement particles, a diffused electric double layer is formed on the surface of the slurry, which becomes a polar molecule or molecular group, and the hydrophobic end is adsorbed on the surface of the cement particles and the hydrophilic end is toward the aqueous solution, forming a monolayer or multi-molecular layer adsorption film. This reduces the surface tension of the water, releasing the water molecules in the floc.

At the same time, due to the directional adsorption of the surfactant, the cement particles have the same charge outward, thereby generating a repulsive force. Therefore, the cement slurry forms an unstable suspension state. The surface of cement particles is lubricated, and the polar hydrophilic end of concrete admixtures is oriented towards the aqueous solution, which is mostly associated with water molecules in the form of hydrogen bonds. In addition, the hydrogen bonds between water molecules form a layer of water film on the surface of cement particles to prevent direct contact between cement particles and play a lubricating role.

2. Air entrainment agent

Because of its surface activity, it can be adsorbed on the water-air interface, significantly reducing the surface tension of water, so that the aqueous solution is easy to form many new surfaces (I. e., water is easy to produce bubbles under stirring); at the same time, the entrainer molecules are oriented on the bubbles, forming a monolayer adsorption film, making the liquid film firm and not easy to break. After the generation of bubbles in the solution, the surface energy increases due to the large expansion of the interface between the two phases. For any system, there is a tendency that the free energy automatically tends to be small to maintain the stability of the system. The surface energy of the gas-liquid interface is as low as possible to generate stable bubbles.

3. Retarder

The early hydration process of Portland cement is divided into four stages, namely, the initial reaction stage, the hydration reaction occurs immediately after the cement is mixed with water, C3S generates calcium silicate hydrate and releases Ca(OH)2;C3A mineral dissolves in water and reacts rapidly with dissolved gypsum to form ettringite, which adheres to the surface of cement particles to form a thin film coating. In the static stage, the plasticity of the cement paste is basically unchanged because the film-like coating formed in the initial reaction stage prevents the further hydration of cement and water. During the setting period, about 6 hours to 8 hours after the cement is mixed with water, the cement setting phenomenon occurs.

When the coating layer on the surface of cement particles is broken, it continues to hydrate, resulting in a setting period; hardening period, after the setting period, into the hardening period, then the hydration rate of cement is slow, but still continues, hydration continues to fill the pores, the strength continues to improve.

Key words:

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