Rheology control towards 3D printed magnesium potassium phosphate cement composites

The contradictory requirements in different processes of 3D printing hinder the application of the rapid hardening cementitious materials. This paper studied the printability of 3D printed magnesium potassium phosphate cement (MKPC) modified by silica fume (SF) through conducting the shear-rate-controlled and shear-stress-controlled rheological tests, aiming to better understand the role of SF for extrusion and structural build-up after deposition. The results show that the thixotropy of MKPC was much higher than that of other cementitious systems. The dynamic yield stress increased from 61.68 Pa to 303.38 Pa and the plastic viscosity decreased from 30.38 Pa s to 13.53 Pa s when SF content increased from 0 to 15%. The addition of SF decreased the negative high viscosity of MKPC related to the fast formation of reversible agglomeration in early age, making it an ideal viscosity reducer for extrusion process. SF is also an ideal plasticizer by increasing the static yield stress during the deposition stage. Finally, a stable hollow cuboid of 310 mm × 250 mm × 84 mm with 8 layers was printed.