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

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Small-angle neutron scattering was used to investigate structural evolution during the carbonation of Ca(OH)₂ and Mg(OH)₂ under humidified CO₂, using both H₂O and D₂O vapor. For Ca(OH)₂, carbonation led to a progressive increase in both nano- and meso-scale surface areas and a doubling of meso-scale diameter, indicating the development of a more complex, porous structure. In contrast, Mg(OH)₂ exhibited a reduction in meso-scale surface area and loss of fractal features, consistent with the formation of smoother, denser carbonate phases. When exposed to D₂O, Mg(OH)₂ formed increasingly smooth surfaces and developed D₂O films that grew with time and approached a saturation thickness of ≈1.9 nm. These results highlight distinct carbonation pathways: Ca(OH)₂ evolves toward greater structural complexity, while Mg(OH)₂ forms compact, hydrated carbonate layers, with D₂O enhancing water film stability and surface transformation. Unexpectedly, both SANS data and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) show that Mg(OH)₂ exhibits clear evidence of H-D exchange, confirming that surface hydroxyl groups are labile and accessible for interaction with D₂O.

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