Effect of Sodium Tripolyphosphate on Clay Tolerance of Polycarboxylate Superplasticizer

Abstract

High fluidity concrete has been widely used in modern civil engineering project to ensure that the highly efficient construction process can be achieved. Generally, the fluidity can be obtained with the incorporation of superplasticizer system composed of polymers and retarders. Sodium tripolyphosphate (STPP), a commonly used retarder in cement-based material, can obviously increase the dispersion of polycarboxylate superplasticizer (PCE), and this has been widely employed in real concrete. However, the effect of STPP on clay tolerance of PCE has not been confirmed until now, and it is still uncertain whether STPP in PCE system has positive or negative effect on workability of fresh concrete with poor-quality aggregates containing montmorillonite (Mt) which is one of the most harmful clay minerals. In this study, the effect of STPP on fluidity of cement-Mt paste with PCE has been investigated. The fluidity was tested with mini slump to assess the clay tolerance of PCE. The adsorption amount of PCE and STPP in Mt suspension was tested with total organic carbon analyzer and inductive coupled plasma emission spectrometer to characterize the adsorption behavior. The interlayer spacing was evaluated with X-ray Diffractometer and the structure of intercalated Mt by PCE was characterized with Fourier-transform infrared spectrometer and thermo gravimetric analyzer, and these results were used to illustrate the effect of STPP on intercalation of PCE. Finally, the dispersion model was proposed to reveal the mechanism behind. The results show that STPP can disperse the Mt particles and increase adsorption amount of PCE in Mt suspension, thereby accelerating rather than hindering the intercalation of PCE into Mt. The fluidity of cement-Mt paste with PCE-STPP system is depended on the added dosage of STPP: with the dosage less than 0.20%, the contribution of STPP to dispersing the cement particles predominates, hence increasing the fluidity of the paste and improving the clay tolerance; while with the dosage more than 0.20%, the acceleration of the intercalation of PCE resulting from STPP becomes predominant, thereby reducing the fluidity of the paste. This result can provide useful experience for the improvement in clay tolerance of PCE in real concrete with poor aggregate containing Mt.