Magnesium oxychloride cements

The Chemistry and Technology of Magnesia, by Mark A. Shand Copyright 2006 John Wiley Sons, Inc. 

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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... 

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. 

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

The water of hydration and hydroxyl water associated with the MOC 5-form and 3-form are 44 and 49%, respectively. When heated to 297°C, the chemically bound water will be converted to steam with an energy requirement of about 1000 Btu per pound of water released. The MOC cement beneath the surface exposed to the heat will not be heated above this temperature until all of the water has been released and driven from the cement. Because of the high energy requirement for this process to occur, the insulative effect from the water of hydration is considerable and constitutes the principle means of insulation (Monde and Mayhan, 1973). Thermally decomposed MOC cement is primarily MgO and as such has a high reflectivity, which is also a significant factor in the overall insulative capability of magnesium oxychloride cement. It has been calculated that a 5-cm thickness of typical MOC cement with a density of 960 kg m-3, containing approximately 35% bound water and no fillers, requires over 6h for the nonheated face to reach a temperature of 1000°F (538°C). 

Magnesium oxychloride cement, also called Sorel cement, is obtained by mixing powdered magnesium oxide (calcined magnesia, MgO) with a concentrated solution of magnesimn chloride (MgClj). 

Hardened magnesium oxychloride cement is also susceptible to carbonation by the CO2 in the air. In this process the 3.1.8 phase gradually converts into a basic magnesium chlorocarbonate hydrate phase (de Castellar et al, 1996) ... 

Magnesinm oxychloride cement must not be combined with steel reinforcement or put in contact with steel surfaces, as chloride ions present in the pore solution and the low pH of the latter promote steel corrosion. However, magnesium oxychloride cement may be combined with glass fibers, cellulose fibers, wood chips, and a variety of other materials. Other drawbacks of this cement are poor dimensional stabihty and poor freeze-thaw resistance. 

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