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This study reports the sintering of zinc oxide (ZnO) through the comparison between the hydrothermal sintering (HS) and the cold sintering process (CSP) operating in closed and open conditions, respectively. Sintering was performed at 155 ± 5°C applying a pressure of 320 MPa, and during different holding times (0 min, 20 min, 40 min and 80 min). Whatever the low sintering process used, ceramics characteristics are almost similar in terms of relative densities and ZnO structure. However, several differences such as the nature of stabilized phases, grain sizes and quantities of residual molecules in the densified pellets, were characterized and explained. The formation of zinc acetate "bridges" was observed ex situ in hydrothermally sintered samples. A detailed ReaxFF molecular dynamics simulation was performed to help understand the formation mechanisms of zinc acetate "bridges" and compare the chemical activities between HS and CSP. Closed systemsEarly investigations on low temperature densification of materials were conducted on closed systems. In the early 70′s, D.M. Roy et al. prepared, at Penn State University, cement pastes with excellent mechanical properties and almost no porosity by hot pressing Cementous powders and water [1,2]. In the mid-70′s, further studies were made by S. Sōmiya et al. on reactive hydrothermal sintering to obtain densified oxides, starting from metal powders at temperatures higher than 900°C [3,4]. In combining both these approaches, N. Yamasaki, K. Yanagisawa et al. from Kochi University developed an apparatus for "hydrothermal hot pressing", based in uniaxial pressure of a mixture of powder and solvent in a system sealed with PTFE (Teflon ™) gaskets [5]. This technique led to the sintering of several oxides such as SiO 2 , TiO 2 , hydroxyapatite, CaCO 3 , Ca 3 Co 4 O 9 , zeolites,

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