Influencing mechanism of sodium tripolyphosphate on the rheological properties of magnesium phosphate cement

Abstract Magnesium phosphate cement (MPC) has been applied in grouting engineering due to its features of quick setting, high early-strength, and perfect volume stability. The rheological properties (RPs) of MPC, being vitally important for site operation, need to be studied further. Sodium tripolyphosphate (STP) is one type of high-efficiency retarders used in MPC. This study investigated the effects of STP content on the RPs of MPC pastes, and the mechanisms were studied by characterizing the zeta potential, 1H NMR relaxation signal, water film thickness, mineral changes, and electrical impedance. Experimental results indicate that the plastic viscosity (η) increases gradually and the yield stress (τ0) decreases firstly and then increases with increasing the STP content. The influencing mechanisms of STP need to be distinguished by analyzing η and τ0 respectively. The water film thickness and the amount of intermediate hydration products within 15 min of hydration is directly related to the change in η. The porosity and distribution of free water obtained from 1H NMR in MPC slurry are the dominant factor for τ0, and followed by the diffused double layer. It is noteworthy that zeta potential is not an effective measure for studying the RPs of MPC. Results of electrical impedance also explain the mechanisms of STP retarding the hydration and improving the early-term strength of MPC.