Magnesium Oxychloride

Bond Type. Most bonded abrasive products are produced with either a vitreous (glass or ceramic) or a resinoid (usually phenoHc resin) bond. Bonding agents such as mbber, shellac, sodium siHcate, magnesium oxychloride, or metal are used for special appHcations. 

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). 

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. 

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... 

Above 100 °C, a different set of phases was stable in the simple magnesium oxychloride system (Demediuk, Cole Hueber, 1955 Cole Demediuk, 1955) a 2-form 2Mg(OH)2.MgCl2.4H20 and a 9-form 9Mg(OH)2.5H2O were found to occur. Apart from identifying these phases, these workers were not able to give details on the structures. 

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... 

Beaudoin, J. J. Ramachandran, V. S. (1975). Strength development in magnesium oxychloride and other cements. Cement and Concrete, 5 (6), 617-30. 

Cole, W. F. Demediuk, T. (1955). X-ray, thermal and dehydration studies on magnesium oxychlorides. Australian Journal of Chemistry, 8, 234-51. 

The use of magnesium oxychloride plasters for fireproofing is not recommended. Field experience has indicated that corrosion of the substrate steel occurs as the topcoat (over the fireproofing) weathers and moisture combines with the chloride present in the plaster to form hydrochloric acid. 

METLCAP is a chemical cement that encapsulates, stabilizes, and solidifies hazardous heavy metals in solid form, in slurry form, or in solution. The cement is composed of magnesium oxychloride, which forms when magnesium chloride and magnesium oxide, with water, are mixed together with the metals. The hardened cement product is insoluble and itself becomes a usable resource as cement or as fill material. The METLCAP technology is applicable as an in sitn or ex situ treatment or for high-pressure injection grouting and construction of slnrry walls. Currently, the process is patented and commercially available from Stark Encapsulation, Inc. 

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