What is the reason for adding superplasticizer to concrete? What are the functions of superplasticizer?

Superplasticizer admixtures are concrete admixtures used in the field of building materials. They can be classified according to various standards such as appearance, superplasticizer capacity, composition materials, and chemical composition. The agent called superplasticizer can reduce the amount of cement water, improve the plasticity of concrete, and reduce the shrinkage of concrete. The action mechanisms of superplasticizer include dispersion, lubrication, steric hindrance and relief. There are many types of superplasticizer formulations. What is the reason for adding superplasticizer to concrete? What are the functions of superplasticizer?

What is a superplasticizer? superplasticizer admixtures are concrete admixtures that can reduce the amount of mixing water while almost maintaining the collapse of concrete. Most of them are anionic surfactants, including lignin sulfonate, naphthalene sulfonate formaldehyde polymer, etc. It can disperse the cement particles after adding the concrete mixture, improve operability, reduce unit water consumption, and enhance the fluidity of the concrete mixture. Or reduce the cement consumption per unit and save cement.

What are the types of superplasticizers?

  1. According to material composition

Lignosulfonate, polycyclic aromatic hydrocarbon salt, water-soluble resin sulfonate, naphthalene series superplasticizer, aliphatic superplasticizer, amino superplasticizer, polycarboxylic acid superplasticizer, etc.

  1. Divide by appearance

It is divided into manual and powder. The homemade content is generally 20%, 40% (also called mother liquor) 60%, and the powder content is generally 98%.

  1. Composed by chemical composition

Lignosulfonate superplasticizer, naphthalene series superplasticizer, melamine series superplasticizer, sulfamate series superplasticizer, fatty acid series superplasticizer, polycarboxylic acid series superplasticizer.

  1. As water decreases and capabilities increase

General superplasticizer (also called plasticizers, the shrinkage rate is more than 8%, represented by lignin sulfonates), high-efficiency superplasticizer (also called superplasticizers, the shrinkage rate is more than 14%, including melamine, sulfamate, aliphatic, naphthalene, etc.) and high-performance superplasticizer (above 25)

What are the functions of superplasticizer?

  1. Reduce the shrinkage rate of concrete solidification and prevent concrete components from cracking. Improve cold resistance, beneficial to winter projects.
  2. Without changing the strength of concrete and the proportion of different raw materials (except cement), reducing the amount of cement and mixing a concrete superplasticizerwith a cement quality of 0.2% and 0.5% can save more than 15% to 30% of the cement amount.
  3. Mixing high-efficiency concrete superplasticizercan more than double the life of concrete even if the normal service life of the building is more than doubled.
  4. If the proportion of various raw materials (excluding water) and the collapse degree of concrete are not changed and the amount of water used is reduced, the strength of concrete can be improved. The crude steel and post-strength are more than 60% and 20% higher than concrete without superplasticizerrespectively. By reducing water, it is possible to cast high-strength concrete labeled C100.
  5. Without changing the proportion of various basic materials, the rheology and plasticity of concrete can be improved so that concrete construction can be constructed without gravity, pumping, and vibration, increasing the construction speed and reducing construction energy consumption.

What is the mechanism of action of superplasticizer?

  1. Lubricating effect

Due to the strong hydrophilic polarity of the superplasticizer, the adsorption film of the superplasticizer on the surface of the cement particles can form a stable solvated water film with water molecules. This water film has a good lubrication effect and can effectively reduce the friction between cement particles. Sliding resistance further improves concrete fluidity.

  1. Dispersion

After the concrete is mixed with water, under the hydration of the cement particles, the cement fragments provide a double-layer structure that forms a solvated water movie. The surface of the concrete particles has a contrary charge, which triggers bonding between the cement fragments and triggers the cement slurry to form flocculation. Structure 10%~30% of the mixed water is surrounded by cement particles and cannot participate in free flow and lubrication, thus affecting the concrete.

Because after adding the superplasticizer, the superplasticizer molecules are directionally adsorbed on the surface of the cement particles. The surfaces of the cement particles have the same charge (usually negative charge) to form electrostatic repulsion, causing the cement particles to disperse each other, disintegrate the agglomerated structure, and release part of the superplasticizer. Packing water to participate in the flow can effectively increase the fluidity of the concrete mixture.

  1. Graft copolymer branched chain sustained release

New superplasticizer, such as polycarboxylate superplasticizer, have a spatial positional resistance effect on the molecules of the superplasticizer during the preparation process, and the cement can be slowly cut in a hydrated, highly alkaline environment, thus releasing dispersed Polycarboxylic acid improves the dispersion effect of cement particles and controls slump loss.

  1. Steric hindrance

In the framework of the superplasticizer representative, the hydrophilic arm or legs extend right into the aqueous remedy, forming a hydrophilic three-dimensional adsorption layer of a specific density externally of the adsorbed cement particles. As cement particles approach, the adsorbed layers begin to overlap. That is to say, steric hindrance will occur between cement particles. The more overlap, the more excellent the steric resistance, the greater the interference with the cohesion between cement particles, and the slump of concrete will remain in good condition.

In the past, the superplasticizer used in handmade concrete were generally ordinary superplasticizer such as pulp waste liquid, calcium lignin, and molasses. The shrinkage rate was not high, generally 5% to 10%. With the improvement of hydraulic concrete quality requirements, the reduction rate of superplasticizer has increased. The quality requirements for aqueous agents are also getting higher and higher. Naphthalene high-efficiency superplasticizer which is widely used in large hydropower projects such as the Peat Three Gorges, has a shrinkage rate of 20% to 30%. It is mainly used to prepare high-strength and high-flow concrete or concrete that requires a significant reduction in water consumption. High-efficiency superplasticizer has a significant effect on improving cement usage efficiency and can save about 20% of cement usage.


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