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2023-02-14

Why use concrete additives to make concrete stronger?

The mechanism of water reduction is that concrete first is the adsorption and dispersion of water reducer. After the admixture is added, the hydrophobic group of water reducer is directional adsorbed on the surface of cement particles, which is the same charge as the surface of cement. And under the action of isotropic electric repulsion, the floc structure formed by cement at the initial stage of adding water is dispersed and disintegrated, and the encapsulated cement molecules are released to achieve the effect of reducing water. The second is the wetting and lubricating effect of the admixture.
Commonly used water reducers include lignosulfonate water reducer, naphthalene type water reducer, melamine water reducer and polycarboxylate type water reducer, etc.
Under the condition that the concrete slump is basically the same, it can reduce the admixture of mixing water. According to the size of its water reducing rate, it can be divided into ordinary water reducing agent, high efficiency water reducing agent and high performance water reducing agent.

The main function of water reducing agent

  1. Increase liquidity. When the water consumption and water-cement ratio remain unchanged, the concrete slump can be increased by 100-200mm without affecting the strength of the concrete.
  2. Improve the strength of concrete. Under the condition of keeping the fluidity and the amount of cement unchanged, the mixing water consumption can be reduced by 10%~40%, thereby reducing the water-cement ratio and improving the concrete strength.
  3.  Save cement. Under the condition of keeping fluidity and water-cement ratio unchanged, it can reduce the amount of mixing water and reduce the amount of cement.
  4. Improve the durability of concrete.
Why use concrete additives to make concrete stronger

At present, it is generally believed that the water-reducing effect of the water-reducing agent is mainly due to the adsorption, dispersion, and wetting and lubricating effects of the water-reducing agent. In the process of mixing cement with water, because the cement minerals contain components with different charges, the mutual attraction of positive and negative charges will cause the concrete to produce a flocculation structure, and the flocculation structure may also be caused by the thermal movement of cement particles in the solution.

These edges and corners collide and attract each other to form. Since the flocculation structure wraps a part of the mixing water and reduces the fluidity, the effect of the water reducing agent is manifested in the following three aspects.

  1. The water reducing agent is a surface active substance, and its molecule is composed of two parts: a hydrophilic group and a hydrophobic group. The hydrophobic groups are oriented to adsorb on the surface of cement particles, and the hydrophilic groups point to the aqueous solution, so that the surface of the cement particles has the same charge. The repulsive force separates the cement particles, releases free ions in the flocculation structure, and increases fluidity.
  2. The hydrophilic group absorbs a large number of polar water molecules, increases the thickness of the solvated water film on the surface of cement particles, acts as a lubricant, and improves workability.
  3. The water reducing agent reduces the surface tension, and the cement particles are more easily wetted, so that the hydration is more sufficient, thereby improving the strength of the concrete.

Now, everyone generally accepts three theories on the mechanism of action of water reducing agents: electrostatic repulsion theory, steric hindrance effect theory, and slow release theory of reactive polymers.

Theory of steric hindrance effect

This theory is mainly applicable to polycarboxylate water reducers. When the thickness of the adsorption layer on the surface of cement particles increases, it is conducive to the dispersion of cement particles and the release of free water between cement particles. The presence of a large number of comb-shaped side chains – (CH2-CH2-O) n- in the polymer molecules of polycarboxylic acid-based water reducers makes the molecular structure take on a comb-like shape and adsorbs on the surface of cement particles to form a larger adsorption layer. Its water-reducing effect is mainly caused by the steric hindrance effect caused by the macromolecular chain and its branches. After the more and longer branch chains contained in it are adsorbed on the surface of cement particles, a thicker three-dimensional cladding can be formed on the surface of cement particles, so that the cement can achieve a better and longer-lasting dispersion effect.

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