Action Mechanism of Common Retarders

Action principle of retarders

1. Reduce the solubility of hemihydrate gypsum.

2. Slow down the dissolution rate of hemihydrate gypsum.

3. Retarding ions are adsorbed on the surface of dihydrate gypsum crystal, and these ions are combined into the lattice.

4. Form a complex to restrict the diffusion of ions near the dihydrate gypsum crystal.

Common retarders

1. Organic acids and their soluble salts: citric acid, tartaric acid, malic acid, etc.

2. Alkaline phosphate: sodium tripolyphosphate.

3. Proteins: bone glue, degraded proteins, polymerized amino acids.

4. Compounding of various retarders.

Retarding mechanism of citric acid

Citric acid is a ternary organic acid containing hydroxyl group, which ionizes H + and many kinds of acid ions under different conditions, which form different stable precipitates with calcium. Among them, the stability of the complex is the highest when the PH value is 8.5, so the setting time is the longest.

Retarding mechanism of sodium polyphosphate.

Sodium polyphosphate is a straight-chain trimer, which combines with calcium ions in the liquid phase to form insoluble calcium phosphate covering the surface of hemihydrate gypsum, which hinders its dissolution, thus greatly reducing supersaturation and reducing the nucleation probability of dihydrate gypsum crystal.

At the same time, the adsorption of sodium tripolyphosphate on the crystal nucleus surface of dihydrate gypsum reduces the surface energy of the crystal nucleus, increases the nucleation barrier, and prolongs the time for the crystal nucleus to reach the critical nucleation size, which is macroscopically characterized by the prolongation of the induction period and setting time of gypsum.

Retarding mechanism of proteins

It is found that macromolecular retarders (proteins) are mixed into gypsum-based materials, combine with calcium ions on the surface of dihydrate gypsum, cover the crystal nucleus surface of dihydrate gypsum, and reduce the crystal nucleus surface can inhibit the growth of crystal nucleus.

At the same time, due to the adsorption and protection of protein colloid, protein colloid inhibits the crystal growth of gypsum, but does not change the crystal morphology, and has little effect on the strength of gypsum.

Summary

1. Under the same dosage, the retarding effect of citric acid is the strongest. When the content is more than 0.2%, the retarding time suddenly increases sharply, and when the dosage is 0.3%, the retarding time reaches more than 8 hours, which almost achieves the effect of preventing gypsum from setting.

2. The content of sodium polyphosphate is 0.1%, and the retarding effect is relatively not obvious, and then the time increases suddenly with the increase of the amount of polyphosphate, which is difficult to control when in use.

3. There is a linear relationship between the retarding time and the content of protein retarders, which is beneficial to the operation control.

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