Effect Of Retarder On The Performance Of Building Gypsum And Study Of Its Retarding Mechanism

Setting time of gypsum plaster is very short, usually only 6~30 min, and operation time is only 5-10 min, which cannot satisfy the practical plastering and construction of gypsum-based material. Selection of suitable retarding agent and its addition is particularly important for controlling the setting time of gypsum-based material in a certain rang, meeting the requirement of construction process. Though the retarder is widely used, the influence factor, the intrinsic reason that induces the strength drop and retarding mechanism are still studied superficially.hi this thesis, the setting time of various retarders had been tested, in which three retarders with good retarding action, citric acid, sodium tripolyphosphate, bone glue are respectively chosen from three types of retarders, hydroxyl carboxylic acid type, phosphate type and protein type, and the strength and setting time of gypsum added with them are determined. Effect of some factors, such as pH value, fineness of hemihydrate, type of gypsum on the retarding action of the retarders is also studied. Furthermore, By testing the supersaturation of liquid phase, hydration rate, hydration temperature as hydration of gypsum with the retarders, the effect of retarders on the hydration process is found out. The Scanning Electron Micrographs and the data of pore structure of gypsum and gypsum with the retarders also make us known the influence of retarders on the crystal morphology and pore structure, from which transformation of microstructure induced by retarders and the reason of which makes the strength drop are analyzed. Combined with the XPS spectrum, the mechanism of action with the retarders are finally proposed.Results of experiment show that the reason of strength decreasing of gypsum added with retarders is that the retarders reduce the supersaturation of liquid phase, and makes the crystal grain largen, and pore is followed to worsen, which lead to the strength drop to a large degree.Through the analyzing the macro-structure to micro-structure, the author considers that retarding mechanism of citric acid is that citric acid and Ca in the gypsum form the complexant ion, and hinder the crystallization center of dihydrate crystal bigger; that of sodium tripolyphosphate is that it combined with Ca, hinders the dissolve of hemihydrate. formation and growth of crystallization center; that of bone glue is the glue-protection and chemical absorption action of active group, which alsoretards the formation and growth of crystallization center. The action mechanism of retarders is a very complex problem and worth to be discussed deeply, and the molecular structure of resultant and the theory of chemical reaction need to be further investigated.