Magnesium oxychloride phases

Dehua and Chuanmei (1999) consider that the formation of the magnesium oxychloride phases are produced in the following reaction sequence the MgO first dissolves in the magnesium chloride solution followed by the formation of the polynuclear complexes [Mgx(0H)J(H20)z]2x y. These complexes then further react with the chloride ion to form a continuous phase or hydrogel, which subsequently converts to the crystalline phases. 

Table 1. Peak Temperatures for Magnesium Oxychloride Phases...

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). 

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. 

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. 

Induction (nucleation) period. In the initial, dissolution stage the liquid phase became oversaturated with respect to the magnesium oxychloride hydrate phases. 

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) ... 

Deng, D., and Zhang, C.H. (1996) TTie effect of almninate minerals on the phases in magnesium oxychloride cement. Cement and Concrete Research 26,1203-1211. 

Dehua, D., and Chuanmei, Z., The Effect of Aluminate Minerals on the Phases in Magnesium Oxychloride Cement, Cement Conor. Res., 26 1203-1211 (1996)... 

Figure 22. Differential thermal analysis curves (DTA) and differential weight loss curves (DTG) of magnesium oxychlorides. The composition of the phases identified are indicated above the curves...

Chlorobutanoyl chloride Phosphorus tribromide Methyl magnesium chloride ALIQUAT (phase transfer catalyst) Phosphorus oxychloride Benzoyl chloride... 

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