Rheology and Buildability of Sustainable 3D Printed Magnesium-Potassium-Phosphate-Cement Composites Incorporating MgO-SiO2-K2HPO4 (2024-08)¶

MKPCs with unique advantage of superior strength, excellent durability and high thixotropy provides alternative binder for 3D printing. The acid-base reaction between dead burnt MgO and K2HPO4 is intense and the retarding effect is limited in practical cases. This paper presents the investigation of MKPCs incorporating MgO-SiO2-K2HPO4 binders prepared with dead burnt MgO, soluble phosphate (K2HPO4•3 H2O) and silica fume (SF). The effects of magnesium-to-phosphate (M/P) mass ratio, SF and slag ratios on rheological properties and the printability were tailored and setting time reached to 50 min by increasing the alkaline environment of the solution. In addition, the pozzolanic activity of SF and slag were synergistically motivated and thus promoted the precipitation of K-struvite, MgSiO3 and C-(A)-S-H hydrates. Two-stage Athix values were calculated to describe the structuration rate. It is confirmed excellent thixotropy was dominated by early-age physical re-flocculation and later-age cement hydration. Although the structure deformation increased significantly as SF content increased, the beneficial effect of SF content on the increase of strength is significantly important than that of adverse effect on the buildability. Slag powder and fine aggregate were incorporated to make the binders more “printable”. A cylinder with 16 layers on optimum mixture was printed without deformation.