1.Definition of retarder
Retarder is an additive that reduces the hydration rate and heat of hydration of gypsum and prolongs the setting time.
2.Classification of retarder
At present, there are mainly three kinds of gypsum retarders in common use.
① organic acids and their soluble salts: hydroxycarboxylic acid and its salt, citric acid, tartaric acid and its salt, sucrose.
② Inorganic salts: zinc salt, phosphate.
③ Protein retarder: osteoprotegerin.
3. Mechanism of retarding agents
In the gypsum retarding agent retarding mechanism research, has not yet achieved a consistent opinion.
It is generally believed that gypsum retardation has the following mechanisms:
Reduce the solubility of hemihydrate gypsum;
Slowing down the dissolution rate of hemihydrate gypsum;
Adsorption of retardation ions to the surface of gypsum dihydrate crystals and binding of these ions into the crystal lattice;
Formation of complexes that limit the diffusion of ions into the vicinity of the gypsum dihydrate crystals.
These mechanisms are summarized as: the adsorption theory, the complex salt generation theory, the precipitation theory, and the controlled crystallization generation theory.