Solubility Equilibria in the System Mg(OH)₂–MgCl₂–H₂O from 298 to 393 K

Abstract

The solubility equilibria of the ternary system Mg­(OH)2–MgCl2–H2O were determined at temperatures from 298 to 393 K applying equilibration periods of up to 3.5 years. As a result, four thermodynamically stable magnesium chloride hydroxide hydrates (Sorel phases) exist in the ternary system within the investigated temperature range. These are the 3-1-8 phase [3Mg­(OH)2·MgCl2·8H2O], the 9-1-4 phase [9Mg­(OH)2·MgCl2·4H2O], the 2-1-4 phase [2Mg­(OH)2·MgCl2·4H2O], and the 2-1-2 phase [2Mg­(OH)2·MgCl2·2H2O]. The also known 5-1-8 phase [5Mg­(OH)2·MgCl2·8H2O] was found to be metastable in the solid–liquid system. With this work, a reliable solubility data set is now available, for example, to prove the long-term stability of magnesia building materials in the presence of salt-bearing media, a challenging demand on the material in a special application as a barrier construction material in salt formation.

Recent development in magnesium oxychloride cement

“…Phase 5 was also proposed to exist as a metastable phase at room temperature, before converting to the stable phase 3 . This was confirmed by Melanie, who studied the solubility equilibria of the ternary system Mg(OH) 2 ‐MgCl 2 ‐H 2 O at temperatures from 298 to 393 K applying equilibration periods of up to 3.5 years. The adjustment of equilibrium was realized by isothermal saturation method.…”

“…The adjustment of equilibrium was realized by isothermal saturation method. From the results, four thermodynamic stable MgCl 2 hydroxide hydrates were found, which were phases 3‐1‐8, 9‐1‐4, 2‐1‐4, and 2‐1‐2 . Phase 3 exists as a stable phase at a lower temperature (298 to <373 K), and phases 9‐1‐4, 2‐1‐4, and 2‐1‐2 are stable at high temperature .…”

“…33,35,36 This was confirmed by Melanie 37 , who studied the solubility equilibria of the ternary system Mg(OH) 2 -MgCl 2 -H 2 O at temperatures from 298 to 393 K applying equilibration periods of up to 3.5 years. The adjustment of equilibrium was realized by isothermal saturation method 37 . From the results, four thermodynamic stable MgCl 2 hydroxide hydrates were found, which were phases 3-1-8, 9-1-4, 2-1-4, and 2-1-2.…”

“…From the results, four thermodynamic stable MgCl 2 hydroxide hydrates were found, which were phases 3-1-8, 9-1-4, 2-1-4, and 2-1-2. 37 Phase 3 exists as a stable phase at a lower temperature (298 to <373 K), and phases 9-1-4, 2-1-4, and 2-1-2 are stable at high temperature. 37 Phase 5 was found to be metastable, because it was observed in the crystallization field during the formation of phase 3 from supersaturation.…”

On verdigris, part II: synthesis of the 2-1-5 phase, Cu3(CH3COO)4(OH)2·5H2O, by long-term crystallisation from aqueous solution at room temperature

“…These were carried out at room temperature since lower temperatures usually lead to phases with higher water contents although the formation, transformation and crystallisation of hydroxide salts can take years. 26,27 This procedure led to the successful synthesis of the 2-1-5 phase. The thermal behaviour, as well as the spectral and magnetic properties of this phase, was investigated and the crystal structure was solved from laboratory XRPD data.…”