Thermodynamical analysis of the effects of modifiers and carbonation on the phase assemblages of magnesium oxychloride cement

Abstract

The influences of modifier and carbonation on the phase assemblage of magnesium oxychloride cement (MOC) are little concerned in the literature. A thermodynamic modeling approach is employed to explore the influencing mechanisms of two commonly used modifiers (i.e. FeSO4 and KH2PO4) and phase assemblage of MOC under natural and accelerated carbonation. And quantitatively XRD test of the modified or carbonated MOC samples is used to verify the reliability of thermodynamic modeling results. The results indicate that the addition of FeSO4·7H2O has no effect on the phase assemblage of MOC, but can improve the formation of 5 Mg(OH)2·MgCl2·8H2O (phase 5), whereas KH2PO4 inhibits the formation of phase 5. FeSO4·7H2O and KH2PO4 raise the concentration of Mg2+ in pore solution, hence enhancing the stability of phases in the matrix. The carbonate product (i.e. chlorartinite) typically identified in MOC is metastable. The carbonation of MOC may result in the following evolution of Mg-carbonates: chlorartinite → MgCO3·MgCl2·7H2O → nesquehonite → hydromagnesite.