Chloride binding of AFm in the presence of Na+, Ca2+ and Ba2+

Abstract

AFm (Ca4Al2(SO4)(OH)12·6H2O) played a significant role in chloride binding by exchanging its interlayer anion (SO42-) with Cl- to form FS (Ca4Al2Cl2(OH)12·4H2O) or KS (Ca4Al2(SO4)0.5Cl(OH)12·6H2O) in cement paste. Different cations including Na+, Ca2+ and Ba2+ might have different effects on the reaction among AFm, KS and FS. To explore the influence mechanism of the cation type on chloride binding of AFm, the effect of Na+, Ca2+ and Ba2+ on chloride binding of AFm was comparatively analyzed. AFm was added into NaCl, CaCl2 and BaCl2 solutions with different chloride concentrations including 0.1 mol/L, 0.3 mol/L, 0.5 mol/L and 1.0 mol/L. The chloride binding capacity was determined by Mohr method. X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscope and energy dispersive spectrometer (SEM-EDS) were used to investigate the phase compositions of AFm samples. Gibbs free energy minimization software (GEMS) could predict the final phase compositions in thermodynamic equilibrium. The results demonstrated that chloride binding capacity of AFm in NaCl, CaCl2 and BaCl2 solutions was declined in order: BaCl2 > CaCl2 > NaCl. The mechanism behind was that: AFm could produce KS and FS in NaCl, CaCl2 and BaCl2 solutions. Different from AFm in NaCl solution, gypsum was generated in CaCl2 solution and BaSO4 was formed in BaCl2 solution, which was attributed to Ca2+ and Ba2+ participating in the phase transformation reaction of AFm.