AbstractCalcium oxide samples have been hydrated at a series of temperatures between 20° and 95°. The crystallite size of the hydroxide produced is determined more by the rate of hydration than by the rate of ageing; the latter factor becomes more important where there is localised heating of the oxide particles during the hydration. Precipitation of calcium hydroxide from solution by double decomposition also gives initially samples of smaller crystallite size at the higher temperatures. In parallel with the hydration of the oxide, the increased rate of production of primary hydroxide crystallites is more important than their increased rate of ageing at the higher temperatures.These results are applied to study in more detail the expansion of slower hydrating limes which causes unsoundness in cements and ‘lime‐blowing’ in