Study On Effect Of Water Reducer On The Performance Of Building Gypsum And Its Action Mechanism

The strength of building gypsum depends largely on its water/gypsum ratio. its stoichiometric water requirement is about 18%,but in practice, a much higher water/gypsum is normally applied to reach the desired plastic consistency, which leads to the decrease of strength of hardened gypsum .An effective method to solve this problem is adding water-reducing agent ,which can produce plaster with high flowability and sufficient strength. However, because of loss of theoretical direction, concrete admixture is used blindly, leading to bad compatibility between admixture and gypsum, because gypsum is very different from concrete in many aspects. Therefore, a systemic admixture theory is in urgent need to provide the direction for the application of admixture in gypsum.In this study, relative theory such as the influence of admixture on the workability, flowability loss and strength, the mechanism of interaction between admixture and gypsum, the adsorption of admixture on gypsum particle, and the influence of adsorption on dispersion and some other properties of gypsum, the relationship between molecular structure and properties of admixture is systemically analyzed. Many advantage measure methods such as UV adsorption spectrophotometry, XPS spectrum, Zeta potential analysis are used to investigate the adsorption characteristics of admixture; mercury intrusion porosimetry (MIP) is used to study pore structure of gypsum; SEM is used to investigate the morphology of gypsum crystals.The results indicate that the potential for modifying the properties of gypsum plasters by use of water reducer. Among many concrete admixtures, FDN, SM, AN3000 is more effective in gypsum. By adding water reducer, the fluidity of plaster and strength of setting plaster is largely improved. The reason of rapid flowability-loss of gypsum plaster is closely related to its quick hydration and decrease of admixture concentration in liquid phase. AN3000 can keep workability for a long time because its particular adsorption conformation. The factors such as PH value, particle fineness, method of addition, temperature etc can all affect the adsorption of admixture, in which admixture milling with gypsum is the best method. The adsorption of admixture on gypsum keeps langmuir type â…  isotherms: with addition of admixture, adsorbed amount decreased, and then reached saturation at 0.7%(wt). The adsorbed amount isvery different to various admixtures, which largely depends on its molecular structure. The action mechanism of various admixtures is also different. The steric hindrance effect should be taken into account to explain the action mechanism of AN3000, whereas the electrostatic repulsion mechanism is predominating effect for FDN, SM. In general, Water reducer has an important influence on pore structure of hardened gypsum and minor influence on its micrograph. The increase of strength is a result of decrease of water used and porosity. Admixture will play an important role in application of high-performance gypsum-based materials in future.