Near-Infrared Spectroscopic Analysis─Formation of Ca(OH)₂ and Ca(OD)₂ by Hydration of CaO with H₂O and D₂O

The dehydration behavior of Ca(OH) 2 was analyzed using near-infrared (NIR) and mid-infrared spectroscopy, with typical absorption bands attributed to the surface and interlayer hydroxyl groups of Ca(OH) 2 being observed at 7260 and 7083 cm –1 in the NIR region, respectively. The dehydration reaction of Ca(OH) 2 proceeded at approximately 470 °C, resulting in the disappearance of the interlayer hydroxyls of Ca(OH) 2 and the formation of isolated hydroxyls of the CaO surface (7231 cm –1 ). The hydration behavior of CaO with H 2 O was also investigated using NIR spectroscopy, revealing that upon contact with H 2 O vapor at 50 °C [relative humidity (RH), 55%], CaO was immediately hydrated to form the interlayer hydroxyls of Ca(OH) 2 after several minutes. Furthermore, since the difference between Ca(OH) 2 and Ca(OD) 2 could not be distinguished by X-ray diffraction measurements, the hydration behavior of CaO upon exposure to D 2 O vapor at room temperature was examined using NIR spectroscopy. This revealed the NIR absorption bands at 5357 and 5259 cm –1, corresponding to the surface and interlayer OD groups of Ca(OD) 2, respectively. However, it should be noted that since the dehydration of Ca(OH) 2 and the hydration of CaO were evaluated under atmospheric conditions, the formation of CaCO 3 due to CO 2 adsorption on the CaO surface could not be disregarded.