FORMATION MECHANISM ON PHASE 5 AND PHASE 3

From the viewpoint of crystallography and thermodynamics the formation and phase-transformation reaction of phase 5[Mg,(OH)5Cl·4H2O] and phase 3[Mg2(OH)3Cl·4H2O] as a major resultant for the hardening reaction of magnesium oxychloride cement have been systematically investigated. It is recognized that the formation of phase 5 and phase 3 is mainly controlled by the molar ratio of MgO, MgCl2, and H2O in the initial reaction mixture. Phase 5 and phase 3 are formed by the reaction between MgO or Mg2+ dissolved out from MgO and the aqueous solution of MgCl2. In the system of MgO-MgCl2-H2O both phase 5 and phase 3 are metastable phases, and under given conditions phase 5 may transform into phase 3 and the latter also into Mg(OH)2.