Thermodynamics and kinetics of hydroxide ion formation in 12CaO·7Al ₄O ₃

We have examined the thermodynamics and kinetics of hydroxide (OH) ions that formed in cages of 12CaO·7Al ₂O ₃ (C12A7) with nanoporous structures. It is confirmed using thermogravimetric-evolved gas analyses (TG-EGA) that hydration in C12A7 is mediated by a reaction between an oxide (O ^2-) ion in the cage and an H ₂O molecule in the atmosphere to form two OH ^- ions in the cages. To simply and exactly quantify the OH ^- content from infrared absorption measurements of OH-stretching band, we propose a method combined with a thermodynamic analysis, allowing the simultaneous determination of the molar extinction coefficient of the OH-band, enthalpy, and entropy for the hydration. Hydration enthalpy in C12A7 is extremely high compared with other oxides and was enhanced by the marked instability of O ^2- ion in the cage. Consequently, high solubility of OH ^- ion is retained up to unusually high temperatures. Furthermore, we determined diffusion coefficients of species relevant to the hydration process and demonstrated that inward diffusion of OH ^- ions is the rate-determining process.