Simulation of the properties of MgO-MgfCl2-H2O system by thermodynamic method

Magnesium oxychloride cement (MOC) has been investigated by many researchers who have studied its hydration products and properties. Previous works mainly concentrated on experimental studies. This study uses a thermodynamic approach to understand the formation conditions of two major hydration products of MOC, Mg-3(OH)(5)l center dot 4H(2)O (phase 5) and Mg-2(OH)(3)l center dot 4H(2)O (phase 3) at room temperature. The hydration reaction equilibrium of MOC phases has been predicted by using the geochemistry speciation code PHREEQC together with the extended 'Pitzer.dat' database. The results show that the formation of the hydration products are controlled by MgCl2 concentration, activity of H2O and pH values of the system. The equilibrium of solid phase diagram, the solubility of different hydration phases are consistent with those results obtained from experimental studies, which validates the thermodynamic model. The phase diagram provides qualitative insights on the synthesis of pure phase 3 and phase 5. For MgO-MgCl2-H2O system, the minimum MgCl2 concentration for phase 5 and phase 3 to form are 1.47 mol . kg(-1) and 2.25 mol .kg(-1), respectively. Increasing water volume can result in the transformation from phase 5 and phase 3 to brucite. Additionally, the stability analysis suggests that phase 5 prefers to formation at a higher condition of the a(Mg)2+, pH and a(H2O) as compared to the phase 3. Phosphate can significantly influence the hydration products composition of MOC during hydration but no new phosphate appears. (C) 2014 Elsevier Ltd. All rights reserved.