Effect of borax and sodium tripolyphosphate on fluidity of gypsum paste plasticized by polycarboxylate superplasticizer

High fluidity gypsum-based materials have great potential for applying in grouting, self-leveling floors, and wall construction. Generally, high flowability can be obtained through adding superplasticizer and some salts that have retarding effect on hydration. In this study, influence of borax and sodium tripolyphosphate (STPP) on fluidity of gypsum paste plasticized by polycarboxylate superplasticizer (PCE) was investigated with total organic carbon analyzer, inductive coupled plasma emission spectrometer, dynamic light scattering (DLS), and X-ray photoelectron spectrometer. The fluidity results show that borax can increase the fluidity of the gypsum paste plasticized by PCE, while the change trend for STPP is inverse. The setting time and conductivity measurements show that both borax and STPP retard the hydration of gypsum paste, with contribution to increasing the fluidity. Adsorption results show that both STPP and borax can hinder PCE from adsorbing onto the surface, and thus the fluidity tends to decrease. DLS and conductivity results illustrate that the reaction of borax and STPP with Ca2+ in the immediate vicinity of the particles exerts negative effect on steric hindrance offered by the long side chain (i.e. polyethylene oxide, PEO). Furthermore, in PCE-borax system, retarding effect plays a dominant role in increasing the fluidity. However, in PCE-STPP system, the reduced adsorption amount as a result of competitive adsorption and the inactivated PEO caused by precipitation between STPP and Ca2+ play a dominant role in reducing the fluidity. The findings would be expected to provide guidance on the design of superplasticizer-retarder system for gypsum paste.