The chemistry of cements formed between zinc oxide and aqueous zinc chloride

A series of cements has been prepared from zinc oxide powder and aqueous zi nc chloride, using solutions corresponding to concentrations of 20%, 30%, 4 0%, 50% and 60% end a ratio of ZnO powder to zinc chloride solution of 1 : 1. As with cements of the zinc oxide/zinc nitrate system, these ZnO/ZnCl2 c ements were found to be weak in compression (not exceeding 10 MPa) with str ength rising with increasing concentration of ZnCl2. The pH change as the r eaction proceeded was monitored and generally showed a rapid increase, foll owed by a slight decrease, and a subsequent slower increase. This is assume d to arise because the doubly charged aquo-zinc cation, Zn(H2O)(n)(2+) (n = 4 or 6) behaves as a weak acid, due to so-called salt hydrolysis: Zn(H2O)( n)(2+) + H2O reversible arrow ZnOH(H2O)((n-1))(+) + H3O+ and reacts to form a salt, thus setting up a classic weak acid/salt buffer system. Finally, c ements were stored in water for 1 month, and were generally found to increa se in mass during the first week, with the greatest increase occurring in t he cement made from 20% ZnCl2 solution. All cements lost mass between 1 wee k and ? month, showing them to be sparingly soluble at room temperature. (C ) 1998 Chapman & Hall.