Studies of Water Films and Carbonation via Neutron Scattering and Infrared Adsorption: In Situ Studies of Mg(OH) ₂ and Ca(OH) ₂

Small-angle neutron scattering (SANS) and polarization-modulation infrared reflection–absorption spectroscopy (PM-IRRAS) were used to investigate the coupled evolution of nanometer water films and carbonation products in Ca(OH)2 and Mg(OH)2 under humidified CO2. Quantitative SANS modeling demonstrates that subnanometer adsorbed films on Ca(OH)2 and thicker (≈1–2 nm) D2O films on Mg(OH)2 mediate ion transport and isotopic exchange at buried interfaces. Infrared spectra confirm H–D exchange on Mg(OH)2 but not on Ca(OH)2, revealing distinct surface accessibility. In situ, both hydroxides initially form amorphous hydrated carbonates─ACC and AMC─but diverge as Ca(OH)2 spontaneously dehydrates to CaCO3 at room temperature while Mg(OH)2 remains hydrated. This divergence reflects the higher hydration affinity of Mg2+ and links isotope-dependent water-film stability to the persistence of amorphous phases. Together, these findings show that interfacial water films dictate whether carbonation proceeds by porosity generation (Calcium) or densification (Magnesium), providing mechanistic insight into mineral carbonation, cement durability, and low-temperature CO2 alteration processes.