Accordingly, it is highly possible to assume that the selected processing conditions (pressure, temperature and especially such highly caustic conditions causing alkaline metal catalysis) enhanced the dissolution of the particle contacts resulting in the formation of a gel which under stress aided a porosity reduction via viscous flow, akin HHP studies on glass [19]. However, additional theoretical and experimental work is certainly necessary to fully understand the exact densification mechanism. As it is known, the equipment used for cold sintering is basically considered as a partially open system whereas HHP apparatus is a closed chamber to maintain the hydrothermal conditions affecting the densification rate. Still, both processes give similar densification trends and offer analogous mechano-chemical contributions [1,23].