Reactivity of lime and related oxides. IX. Hydration of magnesium oxide

Magnesium oxide samples differing widely in surface activity (specific surface 10–180 m.2 g.−1) have been ‘dry’ hydrated by exposure to water vapour at 22° and ‘wet’ hydrated by liquid water at 22° and 95°. The rates of hydration differ considerably, the reaction generally proceeding inwards from the outside of each oxide crystallite by an advancing interface mechanism. Changes in surface area and crystal structure resemble those during the hydration of lime. The more active magnesias age (agglomerate) rapidly in both ‘dry’ and ‘wet’ hydrations, unless the relative water vapour pressures are low; even then, the very slow completion of the hydration ultimately produces considerable ageing. The less active magnesias ‘dry’ hydrate slowly like the less active limes, with splitting of the larger original crystallites increasing the total number present. ‘Wet’ hydration at 22° proceeds with some ageing of the hydroxide, even during the earlier stages and this becomes much greater later. At the higher hydration temperature, 95°, the product ages more rapidly, but the much greater hydration rate produces a time lag in the recrystallisation of the newly‐formed magnesium hydroxide to its normal structure. Later, expansion splits the aged material as the recrystallisation is completed, with the hydration becoming slow and irregular. Magnesias and limes prepared from their respective carbonates at 1000° therefore show similar hydration characteristics.