Oxychloride cements

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). 

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. Armstrong, C. R. (1976). Reactions and equilibria in magnesium oxychloride cements. Journal of the American Ceramic Society, 59, 51. ... 

Harper, F. C. (1967). Effect of calcination temperature on the properties of magnesium oxides for use in magnesium oxychloride cements. Journal of Applied Chemistry, 17, 5-10. 

Matkovic, B., Popvic, S., Rogic, V., Zunic, T. Young, J. F. (1977). Reaction products in magnesium oxychloride cement pastes. System MgO-MgClj-HjO. Journal of the American Ceramic Society, 60, 504-7. 

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... 

Magnesium oxychloride cements are widely used for the fabrication of floors. They find application for this purpose because of their attractive appearance, which resembles marble, and also because of their acoustic and elastic properties and their resistance to the accumulation of static charge. They have also been used for plastering walls, both interior and exterior for exterior walls the cement often includes embedded stone aggregate (Sorrell Armstrong, 1976). However, there have been problems with this latter application, since the base cement has been found to be dimensionally unstable and, in certain circumstances, to release corrosive solutions and show poor weather resistance. 

The quality of magnesium oxychloride cements is highly dependent on the reactivity of the magnesium oxide used in their preparation. Typically, such oxides are prepared by calcination of the basic carbonate (Eubank, 1951 Harper, 1967), but their reactivity varies according to the conditions under which such calcination is carried out. As the reactivity alters so does the amount of oxide that can be incorporated into a cement relative to the amount of aqueous MgClj (Harper, 1967). 

Table 7.1. Compressive strengths of magnesium oxychloride cements made from basic carbonate Harper, 1967)...

There have been a number of studies aimed at understanding the chemistry of the curing and setting of magnesium oxychloride cements and at identifying the phases that are present in the final material. Investigations in the first half of the twentieth century revealed that cement formation in the MgO-MgCla-HaO system involves gel formation and crystallization of... 

The fact that the initial setting process for magnesium oxychloride cements takes place without observable formation of either the 5 1 8 or the 3 1 8 phase is important. It indicates that formation of an amorphous gel structure occurs as the first step, and that crystallization is a secondary event which takes place from what is effectively a supersaturated solution (Urwongse Sorrell, 1980a). This implies that crystallization is likely to be extremely dependent upon the precise conditions of cementition, including temperature, MgO reactivity, heat build-up during reaction and purity of the components in the original cement mixture. 

One method of overcoming some of the instability and loss in strength of oxychloride cements when exposed to water has been to modify them by impregnation with sulphur (Beaudoin, Ramachandran Feldman, 1977). The resulting material appears to be a composite in which the respective components complement each other. The magnesium oxychloride part has relatively poor resistance to water as initially formed, whereas the sulphur is difficult to wet and is completely insoluble in water. 

The mechanism by which sulphur has these observed effects is as follows. Immersion of native magnesium oxychloride cement in water brings about a slow dissolution which creates pores. When those pores are filled with sulphur, sites of possible stress concentration at points of contact between particles are modified. Similar effects occur when sulphur is used to impregnate hydraulic cements based on Portland cement and silica (Beaudoin, Ramachandran Feldman, 1977). 

Overall, these studies showed that sulphur could be used to impregnate magnesium oxychloride cements thereby yielding materials of superior... 

In a similar way there has been a passing reference to a cobalt oxychloride cement (Prosser et ai, 1986). No explicit details of the fabrication or chemical behaviour of this material were provided, but the ingredients were listed among series of acids and bases for forming cements as agents for the sustained release of trace elements to grazing animals. The implication of this paper was that cobalt oxide would function as the base... 

Beaudoin, J. J., Ramachandran, V. S. Feldman, R. F. (1977). Impregnation of magnesium oxychloride cement with sulphur. Ceramic Bulletin, 56, 424-7. 

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. 

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