The decrease in relative density observed between 0 and 20 min or 40 min for CSP or HS, respectively, could be linked with the increase in pore size during the sintering.This phenomenon was already observed by Coble et al. [63] during the conventional sintering of ZnO. Kingery et al. reported that in conventional sintering, the pore growth occurs as a consequence of pore migration along with boundary migration, resulting in the coalescence of several pores [12,64]. Due to low sample volumes and high densities, mercury intrusion porosimetry analyses conducted on hydrothermally- and cold- sintered specimen did not allow to accurately study the evolution of the pore size distribution with the sintering time. However, after intrusion of mercury in different samples, it was noted that voids (cracks and pores) were mainly located at the rims of pellets (Supplementary Data, Fig. S 7).