2Mg(OH)2•MgCl2•2H2O and 2Mg(OH)2•MgCl2•4H2O, Two High Temperature Phases of the Magnesia Cement System

In the course of investigations relating to magnesia cement the basic magnesium salt hydrates 2Mg(OH)2 center dot MgCl2 center dot 2H2O (2-1-2 phase) and 2Mg(OH)2 center dot MgCl2 center dot 4H2O (2-1-4 phase) were found as stable phases in the system MgO-MgCl2-H2O at a temperature of T = 120 degrees C and were characterized by thermal analysis, Raman spectroscopy and X-ray powder diffraction. The crystal structures of the 2-1-2 and 2-1-4 phases were determined from high resolution laboratory X-ray powder diffraction data.Both phases crystallize in space group C2/m with Z = 2 (2-1-2: a = 15.3285(3) angstrom, b = 3.2208(5) angstrom, c = 8.1096(2) angstrom, beta = 91.99(1)degrees, V = 400.13(2) angstrom 3; 2-1-4: a = 15.1264 angstrom, b = 3.1715(2) angstrom, c = 10.5230(1) angstrom, beta = 101.55(1)degrees, V = 494.60(1) angstrom 3). The main building unit of both crystal structures is made up by infinite triple chains of edge-linked distorted MgO6 octahedra. In case of the 2-1-4 phase, the parallel stacks are in addition separated by interstitial one dimensional zig-zag chains of disordered Cl ions and H2O molecules.