BUILDING failures arising from the use of limes with a high content of magnesia have frequently been observed. These failures—such as spalling and blistering of plaster coats, disintegration of mortar joints and expansion of brickwork—are, according to Wells and others1,2, caused by the slow hydration of the magnesia in these ‘unsound’ limes, and may extend over periods up to several years. Complete hydration of these limes can be obtained by treatment with high-pressure steam; this treatment, however, which is extensively used by American lime manufacturers, is expensive both in equipment and operation, and a simpler one is desirable. Experimental work in this Division has been based on the principle underlying Sorel cement manufacture, in which inert basic salts of magnesia such as magnesium oxychloride or magnesium oxysulphate are formed by the reaction between magnesium oxide and magnesium chloride or magnesium sulphate.