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Abstract: Sintering is an ubiquitous step in all ceramic processing to go from a porous body to a dense ceramic. We use extreme heat to drive this densification, but also high pressure or current. But dense ceramic materials in the Earth’s upper crust can be formed at mild pressures and nearly ambient temperature. This represents a tremendous reduction in energy for densification compared to technical ceramic sintering. Inspired by this geological phenomenon, we demonstrated that small amounts of water and pressures up to 500 MPa can densify calcium carbonate compacts up to 85 % relative density within only a few minutes. While the densification process in geological carbonates is presumably based on dissolution-diffusion-precipitation, the mechanisms underlying the densification of synthetic carbonate nanoparticles remain unknown. Even more so because other materials sintered using hydrothermal cold sintering shows different densification behaviour. We used a two-pronged approach to understand the mechanism behind this atypical densification: changing macroscale processing parameters, but also combining in situ high resolution synchrotron tomography with multi-scale indentation technique. Our findings provide a new perspective into the cold compaction of nanopowders with water and open promising routes for the manufacturing of CO2-based structural materials at mild processing conditions.

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