Reactivity of lime and related oxides. II. Sorption of water vapour on calcium oxide

Calcium oxide samples differing widely in surface activity (specific surface 1 to 100 m.2g.−1) have been hydrated by exposure to water vapour at room temperature. Although the rates of hydration differ considerably, each sample takes up in chemical combination a stoicheiometric quantity of water but, in further uptake of water by the newly formed calcium hydroxide by physical adsorption, the isotherm gives no ‘stepwise’ evidence of formation of the alleged hemi-hydrate, Ca(OH)2,0.5H2O. Changes in surface area and lattice structure during hydration indicate that the most active limes become aged (agglomerated) rapidly in the presence of water vapour, and must be hydrated at very low relative pressures to maintain the same number of crystallites; limes of moderate activity can be hydrated without ageing; the least active limes undergo hydration only when the number of crystallites increases, the necessary fission of the original crystallites being accompanied by slow rates of hydration. Progressive changes in surface area during hydration are related to the volumes of the oxide and hydroxide on the basis that the reaction proceeds inwards from the outside of each particle by an advancing interface mechanism.