New materials for thermochemical storage of concentrated solar heat are highly desirable for making this emerging technology competitive with the traditional sensible and latent heat storage. Keeping this in mind, we have prepared calcium hydroxide modified with potassium nitrate and studied its de-/rehydration dynamics by differential scanning calorimetry and thermogravimetry techniques. The following notable observations are described for the modified Ca(OH)2: (1) an acceleration of the dehydration and reduction of its temperature as compared with the pure hydroxide; (2) the temperature reduction depends on the KNO3 content Y and reaches 35 °C at Y = 5 wt%; (3) the addition of KNO3 only slightly reduces the dehydration heat which remains promising for heat storage applications. Fast rehydration of the doped CaO is observed at T = 290-360 °C and P(H2O) = 23-128 mbar, and its rate strongly depends on both temperature and pressure. De- and rehydrated products were studied by the BET analysis and IR-spectroscopy to elucidate possible ways for the salt to influence the Ca(OH)2 dehydration. The mechanism involving a chemical interaction between the salt and the hydroxide is discussed. The new material exhibits a large heat storage density, fast de-/rehydration and adjustable decomposition temperature, and may be considered as a promising candidate for thermochemical storage of concentrated solar energy.