Classification of retarders and how do retarders achieve the retarding effect?
Keywords
Concrete retarder, concrete admixture, China amorphous calcium aluminate supplier, China polycarboxylate supplier
Retarder is an additive that prolongs the setting time of concrete, reduces the hydration rate and hydration heat of concrete.
There are many types of retarders, which can be divided into ordinary type and super retarder type according to the retarding time.
Ordinary retarders can generally only delay the setting of cement concrete for more than 10-15 hours;
Super retarders can make the setting time of cement concrete reach more than 70-80 hours.
If they are divided into inorganic and organic retarders according to their chemical composition.
Organic types include: hydroxycarboxylic acids and their salts, lignin sulfonates and their derivatives, sugars and carbohydrates, polyols and their derivatives, etc.
Inorganic types include: phosphates, metaphosphates, borates, fluorosilicates, and zinc, iron, copper salts, etc.
How do organic retarders produce their effects?
Most organic retarders are surface active and will produce adsorption at the solid-liquid interface; either through certain functional groups in their molecules, they react with free Ca2+ to form insoluble calcium salts that are adsorbed on the surface of mineral particles to achieve a retarding effect; or through hydrophilic groups, they adsorb a large number of water molecules to form a thicker water film layer, which plays a retarding effect by changing the structure formation process. Hydroxyl groups produce a retarding effect by controlling the concentration of Ca2+ in the liquid phase at the initial stage of cement hydration, that is, they form unstable complexes with free Ca2+ in the alkaline medium of cement hydration products. These complexes do not affect the later hydration of cement, and they decompose themselves as the hydration progresses. The reason why carboxyl or carboxylate groups slow down the hydration rate of cement is that they generate insoluble calcium salts with free Ca2+, and these calcium salts are precipitated on the surface of cement particles to slow down the hydration process. In the homologues of alcohols, the retarding effect gradually increases with the increase of the number of hydroxyl groups. Alcohol compounds form a stable solvated water film on the surface of cement particles to inhibit the hydration rate. The retarding mechanism of monosaccharides and oligosaccharides is the same as that of alcohols.
How do inorganic retarders work?
Most inorganic retarders can react with cement hydration products to form complex salts that precipitate on the surface of mineral particles to inhibit cement hydration.
Cement gel coagulation and development process:
It is related to the composition of cement minerals. For example, high cement activity leads to fast hydration and short setting time. Fine cement particles can also shorten the setting time.
