Hydration pathways of CaCl2 inside matrices with different pore sizes

Abstract

For use as a heat storage material, CaCl₂ is often impregnated into porous materials. This is done to stabilize the salt against conglomeration and its dissolution due to the low deliquescence relative humidity. However, CaCl₂ has overlapping temperature and water vapor pressure conditions for its trito- and monohydrate, which are kinetically hindered against each other creating path-dependent (de-)hydration steps. These pathways may change under the influence of confinement. These changes can influence the temperature output for heat batteries using CaCl₂ composites and could make the taken pathways for hydration and dehydration either more complex or simpler than the pure salt. So, in this research, the hydration and dehydration steps of CaCl₂ inside different clays (Vermiculite, Halloysite, and Sepiolite) and silica gels were investigated with respect to their transformations compared to the bulk salt. Therefore, the kinetic phase transition onsets were determined with isobaric TGA measurements together with PXRD in situ experiments to confirm or identify the crystalline phases. This showed that inside pores, CaCl₂ forms the monohydrate rather than the tritohydrate. The decrease of pore diameter leads to easier formation of monohydrate over tritohydrate. This trend can be explained by the crystal structures of the hydrates and their unit cell volumes considering that larger crystals are difficult to form in the limited space inside the pore systems. This change in phase transition steps influences the transition temperatures, which affects its application for heat storage.