Impact of a Ni2+-influx on formation, stability, solubility and crystal structures of the magnesia cement phases 3-1-8 and 5-1-8 at 25 °C.

The impact of a Ni 2+ -influx on formation, stability, solubility and crystal structures of the 3-1-8 (3 Mg(OH) 2 ·MgCl 2 ·8 H 2 O) and the 5-1-8(5 Mg(OH) 2 · MgCl 2 · 8 H 2 O) binder phases of the magnesia cement system was studied by adding MgO, the 5-1-8 and the 3-1-8 phase to Ni 2+ -bearing 4.0 molal MgCl 2 -solutions at 25 °C and applying equilibration periods of more than 2 years (up to 800 days). Nickel was observed to enter the crystal structure of the 5-1-8 phase, yielding solid solutions among this magnesium chloride hydroxide hydrate and its Ni-analog, whereas no incorporation of Ni 2+ ions into the crystal structure of the 3-1-8 phase takes place. The presence of nickel in the access solution inhibits the transformation of the metastable 5-1-8 phase into the thermodynamically stable 3-1-8 phase and prolongs the duration of this process from a few days to more than 2 years. As the final product of the phase formation process a mixture of the 3-1-8 phase and the 2-1-4 Ni-phase (2 Ni(OH) 2 ·NiCl 2 ·4 H 2 O) could be identified. Analyses of the resulting liquid phases confirmed that almost the complete amount of dissolved nickel was removed from the solution by the addition of MgO, the 5-1-8 or the 3-1-8 phase. [ABSTRACT FROM AUTHOR]