Reactivity of lime and related oxides. XIII. Sulphating of hydrated lime

Samples of quicklime and hydrated lime have been sulphated by sulphuric acid and by sodium sulphate solutions at 22°. Rates of sulphating, changes in surface area and phase composition and crystallite size have been correlated with the surface properties of the initial limes. The products have been compared with calcium sulphate dihydrate directly precipitated from solution by double decomposition, and subsequently aged. In acid sulphating, the gypsum from the quicklime has approximately the same specific surface (and average crystallite size) as that directly precipitated from 0.5 M‐calcium nitrate and N‐sulphuric acid. The more active intermediate hydrated lime dissolves more readily and does not impede the sulphating so seriously as the less active initial hydrated lime samples. The latter give products with larger specific surface (smaller average crystallite size) from the more concentrated acid solutions, where the rapid separation of gypsum ultimately impedes the sulphating of the remaining lime. Sulphating with 0.5 M‐soldium sulphate produces increasing alkalinity until about 16% of both the quicklime and the hydrated lime have been converted to gypsum. The sulphate initially separating, probably as a gel, impedes the reaction. The surface decreases until the gypsum begins to crystallise, but its crystallinity remains inferior to that of the acid‐sulphated lime of much lower specific surface.