Effect of sodium tripolyphosphate on adsorbing behavior of polycarboxylate superplasticizer

- The multiple-adsorption layer is responsible for the excellent retarding performance of STPP.
- Preferential adsorption of STPP in PC-STPP system was confirmed.
- The multiple-adsorption layer contains several layers of STPP inside and a PC layer outside.

Sodium tripolyphosphate (STPP) is usually employed as an auxiliary material in superplasticizer system, because the presence of STPP in superplasticizer system can significantly improve the workability of the fresh concrete. Generally, the retarding effect of STPP on cement hydration can obviously delay the formation of hydration products and cut down the consumption amount of free water and superplasticizer at the very beginning, which is accepted as the main reason for the enhanced dispersing ability of the superplasticizer system. However, the adsorbing behavior, not only including the adsorption amount but also the thickness of adsorption layer, is of great importance to the performance of polycarboxylate superplasticizer (PC). The aim of the paper is to explain why the presence of STPP can obviously enhance the initial dispersing ability of PC system from the point of view of adsorbing behavior. Specifically, adsorption amount of PC and STPP in PC-STPP system was tested with the Total Organic Carbon analyzer (TOC) and Inductive Coupled Plasma Emission Spectrometer (ICP); the thickness of adsorption layer was analyzed with X-ray photoelectron spectroscopy (XPS). The adsorption models were then proposed to reveal the relationship between the adsorbing behavior and the dispersing ability. The result shows that in STPP single system, the multiple-adsorption layer formed by STPP and Ca2+ are inferred to be the main reason for the excellent retarding effect. In PC-STPP system, adsorbing ability of STPP is considerably stronger than that of PC, which results in the preferential adsorption of STPP; the multiple-adsorption layer containing several layers of STPP inside and a PC layer outside is inferred responsible for the thicker adsorption layer than that of the single system PC. The results also suggest that the dispersing ability of the multicomponent superplasticizer system with the retarder is closely related to the thickness of adsorption layer rather than the superficial adsorption amount.