Improvement in fluidity loss of magnesia phosphate cement by incorporating polycarboxylate superplasticizer

Effect of polycarboxylate superplasticizer (PCE) on fluidity of magnesia phosphate cement (MPC) paste was investigated. The fluidity and fluidity loss were assessed with mini slump. Hydration process of MPC in the presence of PCE was analyzed with setting time, hydration heat, X-ray diffraction (XRD), scanning electron microscope (SEM), and conductivity. Adsorption behavior of PCE and borax in MgO suspension was studied with total organic carbon analyzer, inductive coupled plasma emission spectrometer, and X-ray photoelectron spectroscopy. The results show that PCE has almost no effect on plasticizing MPC paste, but can obviously increase the 30 min fluidity. The ineffectiveness of dispersion of PCE is because borax in MPC can significantly obstruct adsorption of PCE as well as invalid PCE adsorbed on the surface of MgO particles. The improvement in 30 min fluidity is due to the retarding effect of PCE on hydration of MPC, which can be illustrated from the results of setting time, hydration heat, XRD and SEM. The main reason for retardation is because PCE can enhance the stability of magnesium-based borate layer and can also hinder the formation and growth of struvite by adsorption. Such results would be expected to provide guidance on the design of new type of retarder for MPC and also offer useful experience for controlling the fluidity of MPC paste over time in practical engineering.