Components of oxysalt bonded cements

The three major types of oxysalt bonded AB cement are the zinc oxychloride, the magnesium chloride and the magnesium oxysulphate cements. The bases employed, therefore, are either zinc oxide or magnesium oxide, both of which readily undergo hydration in aqueous solution, behaving as M(OH)2 species and acting as a source of hydroxyl ions. They are thus both clearly bases in the Bronsted-Lowry sense. 

By contrast, the acidity of the metal salts used in these cements has a less clear origin. All of the salts dissolve quite readily in water and give rise to free ions, of which the metal ions are acids in the Lewis sense. These ions form donor-acceptor complexes with a variety of other molecules, including water, so that the species which exists in aqueous solution is a well-characterized hexaquo ion, either Mg(OH2)g or Zn(OH2)g. However, zinc chloride at least has a ternary rather than binary relationship with water and quite readily forms mixtures of Zn0-HCl-H20 (Sorrell, 1977). Hence it is quite probable that in aqueous solution the metal salts involved in forming oxysalt cements dissolve to generate a certain amount of mineral acid, which means that these aqueous solutions function as acids in the Bronsted-Lowry sense. 

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