Kinetics of Vapor‐Phase Hydration of Magnesium Oxide

Magnesium oxide prepared by calcining the carbonate at 1000°C was sintered at 1800°C and ground under methanol. Well‐sized 2 to 5 and 5 to 10 μ fractions were separated. After outgassing at 550°C in vacuum, the powders were hydrated at various temperatures to 98°C in a wide range of water vapor pressures, and the reaction was followed gravimetrically with a silica helix balance. At low temperatures and vapor pressures, where nucleation is exceedingly slow, the hydration of prenucleated samples was studied. The results are interpreted on the basis of a model in which an interface reaction controls the progress of the reaction. The reaction rate varies with the water vapor pressure p according to a function ( p / p * ‐ 1), where p 0.3 p sat. and appears to correspond to the equilibrium adsorption pressure. This suggests that the reaction depends on an adsorbed water vapor film which has free access to the reacting surface. The reaction rate constant varies with temperature according to an Arrhenius‐type equation, with an activation energy 16,100 cal per mole.