While questions remain on the densi

While questions remain on the densification mechanisms in UCS, we have clearly demonstrated that an LPS approach by itself is insufficient in providing an adequate description. Different densification mechanisms alternative to solution-precipitation should be proposed. In particular, the presence of the liquid has indeed an effect on the particle rearrangement bydecreasing the interparticle friction, thus improving the green density in the initial part of Stage II. Other effects are also expected to arise from the interaction of the ceramic particles with water, including: (i) formation of low-yield hydrated phases (as in the case of silica [47,48]), (ii) formation of protonated defects[7] which may facilitate plastic deformation [7] or grain boundary diffusion [6] (i.e. ZnO), and (iii) water-assisted recrystallisation which decreases the flow stress as reported in geological studies for NaCl(with water loads as low as 10 ppm) [45]. The densification mechanisms are therefore expected to strongly depend on the water-ceramic thermodynamic interaction and on the chemistry/atomic bonding of the material; thus, it is impossible to identify a general densification mechanism for UCS. However, the specific sintering mechanisms can be associated with the Coble’s creep-based hot-press sintering equation [29]: