Effect of adding polycarboxylate superplasticizer in gypsum


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2020-03-27

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The first generation of polycarboxylate superplasticizer is methacrylic acid/methyl alkenate copolymer, which is produced by the basic polymerization of methacrylic acid and methyl methacrylate, and the connection between its main chain and branched chain is an ester bond. The second generation of polycarboxylate superplasticizer is propylene ether copolymer, whose main characteristic is that there is an ether bond between the main chain and the branched chain, called polycarboxylate ether superplasticizer, which is a strictly alternating copolymer, because propenyl ether does not make the polymerization uniform.

The first generation of polycarboxylate superplasticizer is methacrylic acid/methyl alkenate copolymer, which is produced by the basic polymerization of methacrylic acid and methyl methacrylate, and the connection between its main chain and branched chain is an ester bond. The second generation of polycarboxylate superplasticizer is propylene ether copolymer, whose main characteristic is that there is an ether bond between the main chain and the branched chain, called polycarboxylate ether superplasticizer, which is a strictly alternating copolymer, because propenyl ether does not make the polymerization uniform. In recent years, a lot of research has been done on polycarboxylate ether superplasticizer, which has no esterification process, simple process and short production cycle; the capping group of raw materials contains unsaturated double bonds, which can be directly polymerized by one-step method; high concentration products can be produced.

When the dosage of polycarboxylic acid series superplasticizer is 0.2~0.3 of the mass of cementitious material, the water reduction rate can be as high as 25% ~ 45%. It is generally believed that polycarboxylic acid series superplasticizer has a comb-shaped structure. Through adsorption on cement particles or cement hydration products, three-dimensional hindrance effect is produced, which plays a role in dispersing and maintaining dispersion of cement. The adsorption characteristics of water reducing agent on the surface of gypsum particles and its adsorption-dispersion mechanism were studied, and it was concluded that the polycarboxylic superplasticizer was comb-like adsorption, the adsorption amount on the surface of gypsum was small, the electrostatic repulsion effect was weak, and its dispersion mainly came from the steric hindrance effect of the adsorption layer, and the dispersion produced by the steric hindrance effect was less affected by the hydration of gypsum, so it had good dispersion stability.

Cement has a coagulation promoting effect in gypsum, which will lead to the acceleration of gypsum setting time, and when the dosage exceeds 2%, it will have a significant impact on the early fluidity, and the fluidity will become worse with the increase of cement content. In order to reduce the effect of gypsum setting time on the fluidity of gypsum, an appropriate amount of gypsum retarder is added to gypsum because cement has an accelerating effect on gypsum. With the increase of cement content, the fluidity of gypsum increases. The incorporation of cement increases the alkalinity of the system, which makes the dissociation of water reducing agent in the system faster and more completely, and the water reducing effect is obviously enhanced. At the same time, the water demand of cement itself is lower than that of cement, which is equivalent to increasing the ratio of water to cement under the same water addition, and the fluidity will also increase slightly.

Polycarboxylate superplasticizer has excellent dispersion ability, and can greatly improve the fluidity of gypsum when the dosage is small. With the increase of dosage, the fluidity of gypsum increases greatly, polycarboxylate superplasticizer has a strong retarding effect, with the increase of dosage, the setting time increases greatly, the strong retarding effect of polycarboxylate superplasticizer, under the same water-paste ratio, the increase of dosage may lead to the deformation of gypsum crystal, the loosening of gypsum, and the flexural and compressive strength of gypsum decreases with the increase of dosage.

Polycarboxylate ether water reducing agent has a retarding effect on gypsum and reduces the strength of gypsum. Under the same dosage, adding cement and calcium oxide to gypsum can improve the fluidity of gypsum, because the water-paste ratio decreases, the density of gypsum increases, and the strength also increases; at the same time, due to the strengthening effect of cement hydration products on gypsum, the flexural and compressive strength of gypsum increases. With the increase of cement and calcium oxide content, the fluidity of gypsum increases, and the appropriate amount of cement can significantly improve the strength of gypsum.

When polycarboxylate ether superplasticizer is used in gypsum, adding appropriate amount of cement can not only increase the strength of gypsum, but also obtain a larger fluidity, and has little effect on the setting time of gypsum.

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