| Abstract: | Reactions of lime and magnesia with magnesium chloride and sulphate have been studied in relation to industrial processes, namely, extraction of magnesium from sea-water and production of Sorel-type cements. Variations in phase composition, surface area and crystallite and aggregate sizes of the products have been correlated with experimental conditions. The solid limes react rapidly with the more dilute magnesium salt solutions, and the precipitated magnesium hydroxides are much more finely-divided. Basic magnesium salts initially formed are decomposed subsequently to hydroxide as the pH becomes stabilised at higher values. Greater concentrations of magnesium chloride give more extensive basic salt formation and prevent all of the lime going quickly into solution. Similar products are given with solid magnesium hydroxide and magnesium chloride and sulphate solutions. The Sorel-type cements obtained from magnesium oxide and the more concentrated chloride solutions set appreciably, even before loss of surface through final ageing. They show two distinct phases: (a) irregular grains of 10–50 μm diameter, and (b) acicular or tabular crystals up to 20μm long, mainly in radiating growths. By contrast, the magnesium oxysulphate crystals are not so elongated and the cements set with considerable loss of surface area, giving interlocking structures in which 20–50 μm-diameter grains are often ‘fused’ together as larger 400–500 μm aggregates. |
