The Role of Adding Polycarboxylate Superplasticizer to Gypsum

　　The first-generation polycarboxylate-based high-range water-reducing admixture is a methacrylic acid/methyl methacrylate copolymer, which is synthesized via the basic polymerization of methacrylic acid and methyl methacrylate, with ester linkages connecting the main chain and side chains. The second-generation polycarboxylate-based high-range water-reducing admixture is an acrylate ether copolymer, characterized by the presence of an ether linkage between the main chain and the side chains; this type is referred to as a polycarboxylate ether high-range water-reducing admixture and is a strictly alternating copolymer, since acrylate ethers do not promote uniform polymerization. In recent years, extensive research has been conducted in China on polycarboxylate ether high-range water-reducing admixtures, which feature a no-esterification process, simple processing, and a short production cycle. Moreover, the end-group functionalization of the raw materials contains unsaturated double bonds, enabling direct one-step polymerization and the production of high-concentration products.

　　When used at dosages of 0.2% to 0.3% by mass of the cementitious materials, polycarboxylate-based high-efficiency water reducers can achieve water-reducing rates as high as 25% to 45%. It is generally believed that these water reducers possess a comb-like molecular structure; upon adsorption onto cement particles or cement hydration products, they generate steric hindrance, thereby dispersing the cement and maintaining its dispersion. Studies on the adsorption characteristics of water reducers on gypsum particle surfaces and on their adsorption–dispersion mechanisms have shown that polycarboxylate-based high-efficiency water reducers exhibit comb-shaped adsorption, with relatively low adsorption amounts on gypsum surfaces and weak electrostatic repulsion. Their dispersing effect primarily arises from the steric hindrance provided by the adsorbed layer, and this steric hindrance is only slightly affected by gypsum hydration, resulting in excellent dispersion stability.

　　Cement has a setting-accelerating effect on gypsum, which shortens the gypsum setting time. When the cement content exceeds 2%, it significantly impairs early fluidity, causing fluidity to deteriorate as the cement dosage increases. Given cement’s setting-accelerating effect on gypsum, an appropriate amount of gypsum retarder is added to mitigate the impact of shortened setting time on gypsum fluidity. As the cement content rises, gypsum fluidity also increases; moreover, the addition of cement elevates the alkalinity of the system, promoting faster and more complete dissociation of the water-reducing admixture, thereby markedly enhancing its water-reducing performance. At the same time, because cement itself has a lower water demand than gypsum, maintaining the same water-to-cement ratio effectively increases the water-to-paste ratio, which also leads to a slight increase in fluidity.

　　Polycarboxylate superplasticizers exhibit excellent dispersing ability and can significantly enhance the flow of gypsum even at low dosages. As the dosage increases, the flow of gypsum rises markedly; however, these superplasticizers also possess a strong retarding effect on setting, causing the setting time to increase substantially with increasing dosage. Moreover, under the same water-to-cement ratio, higher dosages of polycarboxylate superplasticizers may lead to deformation of gypsum crystals, resulting in a more porous hardened paste and a corresponding decline in both flexural and compressive strength as the dosage increases.

　　Polycarboxylate ether superplasticizers exert a retarding effect on gypsum and simultaneously reduce its strength. At the same dosage, the addition of cement or calcium oxide to gypsum both enhances its flowability; as the water-to-cement ratio decreases, gypsum density increases, leading to higher strength. Moreover, the strengthening effect of cement hydration products on gypsum results in increased flexural and compressive strengths. As the dosages of cement and calcium oxide increase, gypsum flowability improves, and an appropriate amount of cement can significantly enhance gypsum strength.

　　When polycarboxylate ether water-reducing agents are used in gypsum, the addition of an appropriate amount of cement not only enhances the strength of the gypsum but also results in greater workability, while exerting only a minor influence on the setting time.