Formation and reactivity of oxides and basic salts. II. Radio-isotope studies of some calcareous materials

Rates of calcium exchange between solid and solution in the ageing of hydrated lime and of calcium carbonate samples have been determined using calcium‐45 radio‐isotope. After a comparatively rapid calcium uptake of about 1 molecular layer at the hydrated lime surface, the exchange rate diminishes as the ions need to diffuse more deeply into the hydrated lime structure. Results for calcium carbonate samples of widely different specific surfaces are compatible with very slow ageing in water. Rates of gypsum precipitation from radioactive calcium chloride and nitrate solutions have been compared in neutral and acidic media and in the presence of albumen. Lower gypsum saturation ratios are established ultimately in neutral rather than acidic media, particularly in solutions containing no salts other than gypsum. Solid hydrated lime reacts rapidly with dilute sulphuric acid, aluminium and magnesium sulpnates, magnesium chloride or sea‐water, releasing calcium ions into solution. The radioactivities of the solutions decrease subsequently as gypsum separates out or calcium ions are adsorbed by the finely‐divided magnesium hydroxide precipitated. At higher magnesium salt concentrations, the calcium‐45 radioactivity does not initially increase so sharply. This is ascribed to basic salt formation preventing all of the lime from going quickly into solution.