Effect of the Adsorbing Behavior of Phosphate Retarders on Hydration of Cement Paste

Phosphate is usually considered as an excellent retarder in cement-based materials. It is often thought that the mechanism behind this excellent retarding effect is attributed to the precipitation of calcium-based phosphate or the formation of a complex on the surface of the cement particles. However, the generally accepted theory cannot convincingly explain the difference in retarding effect between polyphosphates and monophosphates. In this study, the adsorbing behavior, both adsorption amount and the composition and structure of the adsorption layer of three kinds of phosphates (i.e., trisodium phosphate, sodium tripolyphosphate, and sodium hexametaphosphate), were investigated to illustrate the mechanism behind the different retarding effects. Specifically, the retarding effect of phosphate was evaluated with the analysis of hydration products, hydration heat, and setting time. Inductive coupled plasma (ICP) emission spectrometer was performed to assess the adsorption amount of phosphate, whereas the thickness of the phosphate layer was obtained with X-ray photoelectron spectroscopy (XPS). The adsorption models were then proposed accordingly to illustrate the mechanism behind the different retarding effects. The results show that the retarding effect of phosphates is primarily decided by the thickness of the phosphate layer rather than the adsorption amount. The formation of an adsorption layer of monophosphate is because of the precipitation of calcium-based phosphate in the immediate vicinity of cement particles, whereas that for polyphosphates is attributable to the formation of a Ca-phosphate complex. The better retarding effect of polyphosphate than monophosphate is attributed to a thicker adsorption layer. (C) 2017 American Society of Civil Engineers.