ZINC OXYCHLORIDE CEMENT

In the cements of this type a number of phases are known to be present. For example, in the zinc oxychloride cement two discrete phases, corresponding to the composition ZnO. ZnCl. H O in the ratios 4 1 5 and 1 1 2 respectively, are known to occur (Sorrell, 1977). Similarly, in the magnesium oxychloride cement, phases corresponding to Mg(OH)a. MgClj. HjO in the ratios 5 1 8 and 3 1 8 have been shown to exist and have been studied by X-ray diffractometry (Sorrell Armstrong, 1976). 

The precise structural role played by the water molecules in these cements is not clear. In the zinc oxychloride cement, water is known to be thermally labile. The 1 1 2 phase will lose half of its constituent water at about 230 °C, and the 4 1 5 phase will lose water at approximately 160 C to yield a mixture of zinc oxide and the 1 1 2 phase. Water clearly occurs in these cements as discrete molecules, which presumably coordinate to the metal ions in the cements in the way described previously. However, the possible complexities of structure for these systems, which may include chlorine atoms in bridging positions between pairs of metal atoms, make it impossible to suggest with any degree of confidence which chemical species or what structural units are likely to be present in such cements. One is left with the rather inadequate chemical descriptions of the phases used in even the relatively recent original literature on these materials, from which no clear information on the role of water can be deduced. 

Sorrell, C. A. (1977). Suggested chemistry of zinc oxychloride cements. Journal of the American Ceramic Society, 60, 217-20. 

These cements were the earliest of the oxysalt bonded cements to be prepared (Sorel, 1855) and their chemistry has been the subject of numerous investigations over the years. There are considerable difficulties associated with such investigations. Not only does the cement contain a complex mixture of different crystalline precipitates but it is unaffected by boiling water and dissolves only slowly in strong acids. Consequently separation or analysis of any of the phases which may be present is difficult. Nonetheless, as early as 1925 at least 17 crystalline compounds were claimed to occur in the zinc oxychloride cement (Mellor, 1925). 

Sorrell s study of zinc oxychloride cement (1977), in addition to making an important contribution to our understanding of the nature of this material, also highlighted a more general feature of the chemistry of zinc... 

Another class of AB cement, the oxychloride cements of zinc and magnesium, are also formulated in aqueous solution and retain substantial amounts of water on setting (Sorrell Armstrong, 1976 Sorrell, 1977). 

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. 

In very dilute HCl solutions, specifically those with a pH above 5-48, the 4 1 5 phase was found to be insoluble. By contrast, addition of concentrated HCl to the 4 1 5 phase was shown to lead to formation of the 1 1 2 phase (Sorrell, 1977). Below 35wt% HCl, the 4 1 5 phase was found to dissolve congruently. Since the 1 1 2 phase was also found to dissolve congruently in hydrochloric acid solutions with concentrations above 23 wt %, it follows that there is a range of concentrations over which both phases are soluble in aqueous HCl. This behaviour explains why the zinc oxychlorides have proved to be unsatisfactory in attempts to use them as dental cements. The preparation of such cements from concentrated aqueous solutions of ZnClj results in the formation either of the 1 1 2 phase alone or of mixtures of the 4 1 5 and 1 1 2 phases, neither of which is stable in the presence of water. Preparing dental cements from less concentrated solutions also results in the formation of mixed phases, unless the bulk composition has excessive amounts of ZnO present. In these latter cases the cement stability is acceptable but it lacks both a workable consistency and a reasonable working time. 

A number of other studies of AB cements have used X-ray diffraction. For example, Sorrell (1977) and Sorrell Armstrong (1976) employed the technique in the study of oxychloride cements formed in aqueous solution by interaction of oxides and chlorides of either zinc or magnesium. Individual phases were identified, again using Cu K radiation, this time comparing results with those previously obtained for pure compounds. Results from these two studies are described in detail in Sections 7.2 and 7.3 respectively. 

Oxychloride and oxysulfate cements are another class of acid-base cements. These are formed by reaction of a metal oxide such as that of magnesium oxide with a chloride or sulfate of a metal in the presence of water. Magnesium and zinc based oxychloride cements have been developed fully. 

For high-temperature applications, sauereisen cement (Omega CC cement) and zinc oxychloride (dental cement) are useful irreversible cements. Sauereisen cement is made by suspending ceramic powders in sodium silicate solution ( water glass ). This cement sets very hard and withstands temperatures up to 1000°C. Zinc oxychloride is made by mixing calcined zinc oxide powder with concentrated zinc chloride solution. One can also use a ceramic putty (Omega CC high-temperature cement), which must be cured at 180°C and is then serviceable up to 850°C. 

Other fast-setting cements not mentioned so far include alkali-activated slag cement (see section 8.5), alkali-activated fly ash binder (see section 9.1), geopolymeric cement (see section 15.7.1), zinc phosphate cement (see section 12.4), magnesium oxychloride and oxysulfate cement (see section 15.1), and alkali silicate binder(see section 15.3). 

The Arrhenius definition is not suitable for AB cements for several reasons. It cannot be applied to zinc oxide eugenol cements, for these are non-aqueous, nor to the metal oxychloride and oxysulphate cements, where the acid component is not a protonic acid. Indeed, the theory is, strictly speaking, not applicable at all to AB cements where the base is not a water-soluble hydroxide but either an insoluble oxide or a silicate. 

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