Thesis: Porous materials, especially the unsaturated ones, are complex systems in which several physicochemical parameters interact (eg relative humidity, PC, pore solution composition, geometry of the pore network). The precipitation of secondary phases inside and the associated changes (eg topology of the porous spaces) are important to understand for several applied topics: civil engineering, soil science or geology of deep wastes disposal. This experimental work was undertaken to better understand the mechanisms linking geochemical phase transitions and physicochemical properties of multiphasic porous media. The precipitation of salts in porous synthetic materials allowed us to identify two types of geochemistry-geomechanics coupling: the crystallization pressure (compression phenomenon, already known in the literature), and the capillary traction. These secondary precipitates are also responsible for a porous networks heterogenization which modifies the transfer functions. But we also show that the portions of liquid may be isolated by salts "corks" and thus develop new thermochemical properties. Ln particular, we have observed cavitation events in some of these occluded solutions which indicate that they underwent a metastable superheated state. Finally, differential extraction experiments showed that the solubility changes with the pore size, and a thermodynamic interpretation has been proposed. Some evidence that these phenomena may actually be active in natural processes were collected, and this extension to the natural environment must now be treated extensively.