Influencing mechanism of mineral admixtures on rheological properties of fresh magnesium phosphate cement

•Bingham model can describe the rheological behavior of MPC pastes with fly ash and silica fume.•Admixtures increase the yield stress, and their effects on the plastic viscosity are different.•Hydration is the dominant factor for rheological properties of MPC, followed by surface charge and separation distance of particles. Magnesium phosphate cement (MPC) has been used as grouting materials with unique advantages of quick setting and high early strength. The rheological properties (RPs) of MPC, being vitally crucial for site operation, need to be studied further. This study added fly ash (FA) and silica fume (SF) into MPC to prepare MPC-FA and MPC-SF pastes, and the RPs were measured. The influencing mechanisms of FA and SF on the RPs were studied by measuring the zeta potential, hydration heat, 1H nuclear magnetic resonance (NMR) relaxation signal, water film thickness, and mineral changes. Experimental results indicate that the rheological parameters can also be analyzed by using the Bingham model. FA and SF increase the yield stress (τ0), but they have different effects on plastic viscosity (η). The η value increases remarkably with an increased FA content, and it decreases slightly when increasing the SF content. The mechanisms of FA and SF on RPs of MPC can be explained from three aspects, i.e., hydration process, water film thickness and zeta potential. FA accelerates the hydration and makes the inflection points of 1H NMR relaxation signal sharply ahead of schedule. The decrease in free water reduces the water film thickness among solid particles correspondingly. Likewise, the presence of FA decreases the zeta potential value significantly. Hydration is the most important factor for the rheological behavior of MPC paste, followed by the separation distance of the particles and followed by the surface charge.