Post Date: 20,Jun,2022

3. The mechanism of action of superplasticizers

The mechanism of water reducing agent to improve the fluidity of concrete mixture mainly includes dispersing effect and lubricating effect. The water reducing agent is actually a surfactant, one end of the long molecular chain is easily soluble in water - hydrophilic group, and the other end is insoluble in water - hydrophobic group.

a. Dispersion: After the cement is mixed with water, due to the molecular attraction of the cement particles, the cement slurry forms a flocculation structure, so that 10% to 30% of the mixing water is wrapped in the cement particles and cannot participate in free flow and lubrication. effect, thereby affecting the fluidity of the concrete mixture. When the water reducing agent is added, because the water reducing agent molecules can be directionally adsorbed on the surface of the cement particles, the surface of the cement particles has the same charge (usually a negative charge), forming an electrostatic repulsion effect, which promotes the dispersion of the cement particles and the destruction of the flocculation structure. , release the wrapped part of the water and participate in the flow, thereby effectively increasing the fluidity of the concrete mixture.

b. Lubrication: the hydrophilic group in the superplasticizer is very polar, so the adsorption film of the superplasticizer on the surface of the cement particles can form a stable solvated water film with water molecules, and this water film has a good Lubrication can effectively reduce the sliding resistance between cement particles, thereby further improving the fluidity of concrete.

The effect of water reducer on concrete, etc.:

a. Set time. Superplasticizers generally have no retarding effect, and may even promote the hydration and hardening of cement. The retarded superplasticizer is a composite of superplasticizer and retarder. Under normal circumstances, in order to delay the hydration of cement and reduce the loss of slump, a certain amount of retarder is added to the water reducing agent.

b. Gas content. At present, the commonly used polycarboxylate water reducer has a certain air content, and the air content of the concrete should not be too high, otherwise the concrete strength will be greatly reduced.

c. Water retention.

Superplasticizers do not contribute much to reducing the bleeding of concrete, and may even increase the bleeding. Concrete bleeding increases when the dosage is excessive.