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Admixture dosage in concrete?

The principle of water reducing admixtures

The principle is mainly to use water reducer to modify the surface of concrete. Commonly used water reducing agents are anionic surface active substances. When the cement is mixed with water and set and hardened, due to the different charges of the cement minerals during the hydration process, electrostatic attraction is caused, or due to the collision and adsorption of certain edges and corners of cement particles, van der Waals force will form a flocculent structure.

A lot of mixing water is wrapped in these flocculent structures. The function of the water reducing agent is to release the enclosed water. The hydrophobic group of the water reducing agent is adsorbed on the surface of the cement particle in a direction, and the hydrophilic group points to the aqueous solution to form a monomolecular adsorption film.

Due to the directional adsorption, the cement particles have the same electric charge and generate electrostatic repulsion, so that the cement water system is in a stable suspension state. The flocculation structure formed at the initial stage of adding water to the cement is collapsed and free water is released, so as to achieve both water reduction and maintenance. The purpose of tradeability.

What are the superplasticizer?

Generally speaking, the naphthalene series is mainly explained by the electrostatic repulsion theory, while the polycarboxylic acid water reducer is mainly explained by steric hindrance. In addition to water reducing agents, there are some other concrete admixtures.

For example, the polycarboxylic acid slump retention agent is essentially a water reducing agent, but its branch can be hydrolyzed under alkaline conditions, similar to the sustained-release tablets in medicines, which can delay the release of the dispersion effect. So as to achieve the slump protection effect. There is also a rheology optimizer, which forms a three-dimensional network matrix structure to lock moisture through the attraction of various groups in the branched chain and prevents the segregation of concrete from bleeding.

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