Influencing Mechanism of Magnesium-to-Phosphate Ratio on the Rheology and Microstructure Development of 3D-Printed Magnesium Phosphate Cement at Early Hydration

The growing application of magnesium phosphate cement (MPC) has increased the demand for understanding its rheological properties to achieve the workability that meets various working conditions (e.g., 3D printing). This paper studied the effects of magnesium to phosphate (M/P) mole ratio on rheological properties and microstructure development of MPC by conducting rheological tests (dynamic shear stress test and structural build-up test), combined with the analysis of mineral phase production and water and pore distribution. The synergy of the bridging effect and the colloidal interaction on the yield stress of MPC with different M/P mole ratios has been clarified. Results show that as the M/P ratio increased, the plastic viscosity increased while the yield stress initially decreased before rising again during the initial hydration process. The governing mechanism on the rheological properties of MPC varies with different M/P ratios. At low M/P ratio, large amount of intermediate hydration product, schertelite, was formed in MPC, enhancing the bridging effect at the early hydration process. At high M/P ratio, more unreacted MgO particles flocculated to build a denser microstructure due to the strong colloidal force, thus causing the rise in the yield stress of MPC.