Solubility of Fe–ettringite (Ca6[Fe(OH)6]2(SO4)3 · 26H2O)

The solubility of Fe–ettringite (Ca6[Fe(OH)6]2(SO4)3 · 26H2O) was measured in a series of precipitation and dissolution experiments at 20 °C and at pH-values between 11.0 and 14.0 using synthesised material. A time-series study showed that equilibrium was reached within 180 days of ageing. After equilibrating, the solid phases were analysed by XRD and TGA while the aqueous solutions were analysed by ICP-OES (calcium, sulphur) and ICP-MS (iron). Fe–ettringite was found to be stable up to pH 13.0. At higher pH-values Fe–monosulphate (Ca4[Fe(OH)6]2(SO4) · 6H2O) and Fe–monocarbonate (Ca4[Fe(OH)6]2(CO3) · 6H2O) are formed. The solubilities of these hydrates at 25 °C are: Ca6[Fe(OH)6]2(SO4)3·26H2O⇆6Ca2++2Fe(OH)4−+3SO42-+4OH-+26H2O,logKS0=-44.0±0.7,Ca4[Fe(OH)6]2(SO4)·6H2O⇆4Ca2++2Fe(OH)4−+SO42-+4OH-+6H2O,logKS0=-33.2±0.5andCa4[Fe(OH)6]2(CO3)·6H2O⇆4Ca2++2Fe(OH)4−+CO32-+4OH-+6H2O,logKS0=-35.5±0.3.