Magnesium hydroxychloride

[7] These compounds are the primary components of matured magnesia cement, invented in 1867 by the French chemist Stanislas Sorel.

[8] In the late 19th century, several attempts were made to determine the composition of set Sorel's cement, but the results were not conclusive.

[14] In the system MgO – MgCl2 – H2O at about 23 °C, the completely liquid region has vertices at the following triple equilibrium points (as mass fractions, not molar fractions):[4] The other vertices are pure water, magnesium chloride hexahydrate, and the saturated Mg(OH)2 solution (0.0044 MgO + 0.9956 H2O by mass).

[4] The anhydrous forms decompose when heated above 450-500 °C by decomposition of the hydroxide and chloride anions, releasing water and hydrogen chloride and leaving a magnesium oxide residue, by the reactions:[7] Extended exposure of magnesium oxychlorides to water leaches out the soluble MgCl2, leaving hydrated brucite Mg(OH)2.

[15] On exposure to the atmosphere, the oxychlorides will slowly react with carbon dioxide CO2 from the air to form magnesium chlorocarbonates.

For best results, the magnesium oxide should have small particle size and large surface area.

[17] The protons then react with and dissolve the nearly insoluble oxide or hydroxide, by such reactions as[17] The ions [Mg(H2O)6]2+ and [Mg(OH)(H2O)5]+ in solution then combine into complex cations with multiple magnesium atoms, bridged by hydroxide anions and water molecules (magnesium aquohydroxo complexes), with general formula [Mgx(OH)y(H2O)z](2x−y)+.

The acidity provided by hydrolysis of the cations in solution dissolves this coating, and thus allows the process to run continuously until one of the reagents is exhausted.

[4] The difficulties of preparing the magnesium oxide and ensuring its full reaction can be avoided by using NaOH instead of MgO or Mg(OH)2, so that all reagents are solutions.

Stable phase 3 precipitates in a broader range of cases, namely when [Cl] is 6.48 ± 2.17 mol/L, [Mg] is 3.14 ± 1.12 mol/L, and the pH is 6.26 ± 0.14[14][19] A short note from 1872 reported the formation a solid with approximate formula 5MgO·MgCl2·13H2O, as a mass of fine needles, from a solution of magnesium ammonium chloride Mg[NH4]Cl3 with excess ammonia left standing for several months.

Phase diagram for the ternary system MgO – MgCl 2 – H 2 O at ~ 23 °C . [ 4 ] The stable oxychloride phases are P5 (Phase 5, 5:1:8) and P3 (phase 3, 3:1:8). The dark blue region is clear solution. The triple equilibrium points of the saturated solution are S1 (Sol: Mg(OH) 2 :P5) , S2 (Sol:P5:P3) , and S3 (Sol:P3: MgCl 2 ·6H 2 O) . The light blue region is the approximate range of compositions of homogeneous metastable gels.