Path-Dependent Hydration and Dehydration of CaCl₂

In heat storage research, CaCl₂ is one of the most often used salt hydrates due to its abundance, low costs, and the possibility of a high water vapor uptake. However, the phase transitions among the lower hydrates (0, 1/3, 1, 2) have been discussed since the discovery of CaCl₂·1/3H₂O (hereinafter called "tritohydrate") and the dependency of (de)-hydration steps on the reaction path. This research addresses the reasons behind this path dependency and the position of the tritohydrate in the phase diagram. With TGA experiments, we show that the hydration pathway is 0-1/3-2 and the dehydration pathway is 2-1-0 at low water vapor pressures and 2-1/3-0 at high vapor pressures. It is shown that poor kinetics of the 1/3-1 transition and the metastability of the tritohydrate cause this. We have resolved the crystalline structures of the tritohydrate and monohydrate, which makes clear that a 1/3-1 transition involves large rearrangements of the crystalline structure. Consequently, steps from 1/3-1 hydration and 1-1/3 dehydration will have a high activation energy, hampering these transitions.