Application of polycarboxylate Superplasticizer in gypsum

When the polycarboxylic acid-based high-efficiency superplasticizer(water redusing agent [https://www.longouchem.com/superplasticizer/]) is added in an amount of 0.2% to 0.3% of the mass of the cementitious material, the water-reducing rate can be as high as 25% to 45%. It is generally believed that the polycarboxylic acid-based high-efficiency water-reducing agent has a comb-shaped structure, which produces a steric hindrance effect by adsorbing on cement particles or cement hydration products, and plays a role in dispersing and maintaining the dispersion of cement.

The study of the adsorption characteristics of water-reducing agents on the surface of gypsum particles and their adsorption-dispersion mechanism has shown that the polycarboxylic acid-based high-efficiency water-reducing agent is a comb-shaped adsorption, with a small amount of adsorption on the gypsum surface and a weak electrostatic repulsion effect. Its dispersing effect mainly comes from the steric hindrance effect of the adsorption layer. The dispersibility produced by the steric hindrance effect is less affected by the hydration of gypsum, and thus has good dispersion stability.

Image: https://www.longouchem.com/uploads/polycarboxylate-Superplasticizer1.jpg

Cement has a setting-promoting effect in gypsum, which will accelerate the setting time of gypsum. When the dosage exceeds 2%, it will have a significant impact on the early fluidity, and the fluidity will deteriorate with the increase of cement dosage. Since cement has a setting-promoting effect on gypsum, in order to reduce the impact of gypsum setting time on gypsum fluidity, an appropriate amount of gypsum retarder is added to the gypsum.

The fluidity of gypsum increases with the increase of cement dosage; the addition of cement increases the alkalinity of the system, making the water reducer dissociate faster and more completely in the system, and the water-reducing effect is significantly enhanced; at the same time, since the water demand of cement itself is relatively low, it is equivalent to increasing the water-cement ratio under the same amount of water addition, which will also increase the fluidity slightly.Polycarboxylate water reducer has excellent dispersibility and can greatly improve the fluidity of gypsum at a relatively low dosage.

With the increase of dosage, the fluidity of gypsum increases significantly. Polycarboxylate water reducer has a strong retarding effect. With the increase of dosage, the setting time increases significantly. With the strong retarding effect of polycarboxylate water reducer, under the same water-to-cement ratio, the increase of dosage may cause the deformation of gypsum crystals and the loosening of gypsum. The flexural and compressive strengths of gypsum decrease with the increase of dosage.Polycarboxylate ether water-reducing agents slow the setting of gypsum and reduce its strength.

At the same dosage, adding cement or calcium oxide to gypsum improves its fluidity. This lowers the water-to-cement ratio, increases the density of the gypsum, and thus its strength. Furthermore, the reinforcing effect of cement hydration products on the gypsum increases its flexural and compressive strength. Increasing the amount of cement and calcium oxide increases the fluidity of the gypsum, and an appropriate amount of cement can significantly improve its strength.When using polycarboxylate ether water-reducing agents in gypsum, adding an appropriate amount of cement not only increases its strength but also provides greater fluidity with minimal impact on its setting time.

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