Description of gypsum retarder Citric Acid
Gypsum retarder is mainly used for plaster building materials such as plastering plaster, bonding plaster, gypsum putty, gypsum products, etc. It is mainly used to reduce the setting speed of gypsum.
Traits of gypsum retarder citric acid
Gypsum retarder citric acid is white powder and easily soluble in water.
The mechanism of citric acid on the retardation of gypsum
The retarding effect of citric acid, when a small amount (below 0.1%) is added, can retard the setting and slightly reduce the strength; when the amount is increased (above 0.2%), it will seriously affect the dihydrate gypsum The habit of crystals prevents the crystals from growing into and connecting to each other. Even if hydration is completed, the gypsum slurry cannot be coagulated.
The influence of citric acid on the hydration of building gypsum
(1) Citric acid has a strong retarding effect and is an efficient retarder for building gypsum. When the content is not more than 0.1%, the setting time of the gypsum slurry is prolonged, the heat of hydration becomes slow, the early hydration rate is greatly reduced, but the final hydration rate is not affected, and the strength of the gypsum hardened body is slightly reduced.
(2) Citric acid and calcium ions form a stable calcium citrate complex. The stability of the complex is greatly affected by the pH value, so under different pH values, the retardation effect of citric acid is significantly different. Generally, the retarding effect is best when the pH value is 8-10.
(3) Citric acid is adsorbed on the newly formed crystal embryo through complexation, which reduces its surface energy, increases its nucleation barrier, reduces the number of crystal nuclei, and reduces the overlap of ions on the crystal. The combination rate allows the crystals to have sufficient time and space to develop and grow, and the crystal size is coarsened. When the content is above 0.1%, the strength of the hardened gypsum body will be significantly reduced.
Usage of gypsum retarder
(1) The content of gypsum is 0.03%-0.5%.
(2) As the content of gypsum retarder increases, the setting time is prolonged.
(3) As the temperature increases, the content of gypsum retarder needs to be increased appropriately.
(4) Due to the different types and properties of gypsum, the effect of gypsum retarder may be different, so determine the dosage before use.
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