Dense gypsum ceramics prepared by room-temperature cold sintering with greatly improved mechanical properties

Dense gypsum (CaSO4·2H2O) ceramics were successfully fabricated by a simple room-temperature cold sintering process with 5 wt% water. The relative density of gypsum ceramics increased from 89.6% to 96.8% with increasing the applied uniaxial pressure from 100 to 400 MPa during cold sintering. The relative density changed slightly for higher pressure, and microcracks were observed as well as abnormal grain growth. Both the compressive and flexural strengths reached the peaks at 98.5 MPa and 26.5 MPa for the uniaxial pressure of 400 MPa, which were improved by 2.6 and 2.0 times, respectively comparing with the bulk gypsum prepared by traditional method from α-plaster. Furthermore, the dry-pressed gypsum compacts were very fragile, and had relative densities 5–12 % lower than the cold-sintered ceramics, indicating that the slight solubility of gypsum in water (0.2 g/100 g) played a critical role in the densification, microstructural evolution and greatly improved mechanical properties of cold-sintered gypsum ceramics.