Direct Ink Writing of Gypsum: Developing a Printable Gypsum Paste

Gypsum is a common material for statues, building, and casting molds (alloys and ceramics). Due to the incomplete hydration reaction between water and gypsum, it is difficult to print high-strength gypsum products using 3D printing techniques such as Binder Jetting. To enhance its strength, the hydration reaction should be completely performed, which could be fulfilled by direct ink writing (DIW). However, the reaction in the gypsum paste for DIW is so fast that less time is left for operating a 3D printer. In this work, a printable gypsum paste with a reasonable setting time was developed to print 3D gypsum structures via direct ink writing. A retarder and a thickener were introduced into the paste to prolong its setting time for operating and tailor its rheological property for printing, respectively. The setting time, expansion and rheological properties of the pastes were tested by the Vicat apparatus, consistometer and rheometer, respectively. The results show that citric acid (CA) is a suitable retarder, although decreases its compressive strength due to the directional grown gypsum crystals resulting from the selective adsorption of CA on gypsum powder, while hydroxypropyl methylcellulose (HPMC) is an acceptable thickener, which affects forming flocculation structure in the paste, resulting in higher viscosity and shear modulus.