Effect of early-hydration behavior on rheological properties of borax-admixed magnesium phosphate cement

Abstract Magnesium phosphate cement (MPC), as a new-fashioned cementitious material with doubly consensus superiorities of rapid-setting and ultra-high early strength, has a significant utilization potentiality in multi-extremely complicated engineering fields. Borax had been employed for prolonging the setting time of MPC as an unexceptionable retarder, which has been verified and approved practically. Nevertheless, the effect of early-hydration behavior on rheological properties (RPs) in borax-admixed MPC remains obscure. Consequently, we adopted rheometer to investigate the RPs of MPC with different borax contents, and then discussed the relationship between yield stress and plastic viscosity affected by hydration process. In addition, a series of microscopic methods were utilized for further exploring the influence of hydration process and products on the RPs of borax-admixed MPC. Results indicated that: 1) The yield stress and plastic viscosity of MPC dramatically decreased with the borax content, and the consequences induced by final curve-fitting are basically in accordance with Bingham model. 2) The Zeta potential exhibited that borax content had no significant effect on the continuous stability of the colloidal dispersion. Moreover, results of electrochemical impedance expressed that increasing the amount of borax enhanced the concentration of ions in MPC, which further induced reduced electrical impedance. 3) Low-field nuclear magnetic resonance and analysis of thermo-gravimetry and derivative thermo-gravimetry conveyed that the addition of borax had a distinct delayed coagulation on MPC, which was mainly attributed to the combination of B(OH4)- and Mg2+ for producing complexes during hydration and then constituted the retarder layer to delay the formation of struvite compared with normal hydration. 4) XRD results indicated that the rupture of retarder layer was the direct inducement of RPs enhancement after hydration reaction had been basically completed. All results verify the interaction between retarded hydration reaction of borax and enhanced RPs, which provide an important reference for the future repairing research of MPC.