Mechanical stability criterion, triple-phase condition, and pressure differences of matter condensed in a porous matrix

In contrast to the triple-point condition of bulk material, condensed matte r in porous media can coexist stably as liquid, solid, and vapor over a wid e temperature range. The necessary conditions are found by a thermodynamic approach starting with the potential which reflects the grand canonical dis tribution and is characterized by heat and matter exchange. The other param eters are volume and surface. Therefore, it is designated the free mechanic al potential. General expressions for mechanical stability are given. On co ndensation and melting the nonwetting phases vanish. These are thermodynami cally stable phase transitions. For the opposing effects evaporation and fu sion, an energy barrier must be transgressed either by nucleation or by int rusion as discussed here. These are metastable states. Phase transitions ar e the conditions which limit the triple-phase region. Within this region hi gh negative pressures are generated in the unfrozen liquid independent of t he pore size where it exists. The findings are applied to water in the disp erse matrix of hardened cement paste. They are the basis for "micro ice len s pumping." (C) 2001 Academic Press.