Bioprocessing‐inspired cold sintering of amorphous calcium carbonate

Abstract Cold sintering of dense ceramic materials at a significant lower temperature compared to conventional high‐temperature sintering has attracted broad interests in recent years. During biomineralization process, hydrated amorphous phases are often used as precursors by living organisms to produce biominerals under ambient conditions. However, the crystallization and densification mechanisms of amorphous phases under pressure have not been thoroughly explored. Here, we found that water played a critical role in controlling the crystallization pathways of amorphous calcium carbonate, leading to the formation of two hydrated crystalline phases, ikaite or monohydrocalcite (MHC), depending on the pressure, temperature, and external water content. Furthermore, ikaite and MHC were used as hydrated crystalline precursors for fabrication of anhydrous aragonite ceramic through in situ phase transformation and densification under pressure at low temperature (80°C). The hardness, Young's modulus, and flexural strength of the aragonite ceramic formed from MHC could reach 4.0 GPa, 70.0 GPa, and 55 MPa, respectively, which were comparable to those of nacre. Our study provides a new bioprocessing‐inspired cold sintering strategy for the fabrication of ceramics via metastable hydrated phases.