Under the condition that the slump of concrete is basically the same, the admixture for mixing water can be reduced. According to its water reducing rate, it can be divided into common superplasticizer, superplasticizer, and high performance superplasticizer.
The main functions of superplasticizer:
Increase liquidity. When the water consumption and water-cement ratio are constant, the slump of concrete can increase 100~200mm, and does not affect the strength of concrete.
Improve the strength of concrete. Under the condition of keeping the fluidity and the amount of cement unchanged, the water consumption of mixing can be reduced by 10% to 40%, thus reducing the water-cement ratio and improving the strength of concrete.
Save cement. Under the condition of keeping the fluidity and water-cement ratio unchanged, the amount of mixing water and the amount of cement can be reduced at the same time.
Improve the durability of concrete.
At present, it is generally believed that the water-reducing effect of superplasticizer is mainly due to the adsorption, dispersion, wetting and lubrication of superplasticizer. In the process of mixing cement with water, cement minerals contain components with different electric charges, and the mutual attraction of positive and negative charges will lead to the flocculation structure of concrete. the flocculation structure may also be formed because the cement particles collide and attract each other at some edges and corners due to the thermal movement of cement particles in the solution. Because the flocculation structure wraps part of the mixed water and reduces the fluidity, the effect of superplasticizer is shown in the following three aspects.
1. The superplasticizer is a surface active substance, and its molecule is composed of hydrophilic group and hydrophobic group. The hydrophobic group is directionally adsorbed to the surface of cement particles, and the hydrophilic group points to the aqueous solution, so that the surface of cement particles has the same electric charge. The repulsive force separates the cement particles, releases free ions of flocculation structure, and increases fluidity.
2. The hydrophilic group absorbs a large number of polar water molecules, increases the thickness of solvated water film on the surface of cement particles, plays the role of lubrication and improves workability.
3. The superplasticizer reduces the surface tension, and the cement particles are more easily wetted, which makes the hydration more sufficient, so as to improve the strength of concrete.
Now, it is generally accepted that there are three theories about the action mechanism of superplasticizer: electrostatic repulsion theory, steric hindrance effect theory and reactive polymer slow release theory.
Electrostatic repulsion theory:
Most of the superplasticizers are anionic surfactants. Because the cement particles have a positive charge (Ca2+) on the surface at the initial stage of hydration, the anions SO3- and COO- in the superplasticizer molecules will be adsorbed on the cement particles, forming an adsorption double layer, and the cement particles close to each other will be affected by the electrostatic repulsion and van der Waals gravity at the same time. With the increase of the absolute value of potential, the repulsion between the particles is gradually dominated, thus preventing the condensation between the particles. at the same time, the electrostatic repulsion can also release the water wrapped in the cement particles, making the system in a good and stable dispersion state. With the progress of hydration, the water reduction dose adsorbed on the surface of cement particles decreases, the absolute value of potential decreases, and the system is unstable, resulting in condensation. This theory is mainly applicable to naphthalene series, melamine series and modified wood calcium series and other commonly used superplasticizers.
Steric hindrance effect theory:
This theory is mainly applicable to polycarboxylate superplasticizer. When the thickness of the adsorption layer on the surface of the cement particles increases, it is beneficial for the cement particles to disperse and release the free water between the cement particles. The existence of a large number of comb-shaped side chains-(CH2-CH2-O) n-in the polymer molecules of polycarboxylic superplasticizers makes the molecular structure comb-shaped and forms a large adsorption layer on the surface of cement particles. The water-reducing effect is mainly caused by the steric hindrance effect caused by macromolecular chains and their branched chains. After more and longer branched chains 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 lasting dispersion effect.
Theory of slow release of reactive polymers:
This theory is similar to the steric hindrance effect theory, and is mainly used in the study of water reduction and slump protection of polycarboxylic acid series high performance superplasticizer. When the polymerization molecule of polycarboxylate superplasticizer contains ester or anhydride, it is a kind of non-water-soluble reactive polymer fine particles. In the alkaline environment of concrete, the carboxyl group is negatively charged by the adsorption of cement particles, which makes the electrostatic repulsion between cement particles play a secondary supplementary role, increase the dispersion of cement particles again, delay cement setting, and prevent the slump loss of concrete.
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