9Mg(OH)2·MgCl2·4H2O, a High Temperature Phase of the Magnesia Binder System

The metastable phase 9Mg(OH)2·MgCl2·4H2O (9-1-4 phase) was found at the extended metastable isotherm of Mg(OH)2 in the system MgO−MgCl2−H2O at 120 °C and occurs as intermediate binder phase during setting of magnesia cement due to temperature development of the setting reaction. The crystal structure of the 9-1-4 phase was solved from high resolution laboratory X-ray powder diffraction data in space group I2/m (C2/m) (a = 22.2832(3) Å, b = 3.13501(4) Å, c = 8.1316(2) Å, β = 97.753(1)°, V = 562.86(2) Å3, and Z = 1). Structural and characteristical relations of the phases in the system MgO−MgCl2−H2O can be derived, with which the development of the cement or concrete qualities becomes explainable.