In uranium mill tailings, oxides and oxi-hydroxides are responsible of about 70% of the radium immobilization, half being associated to amorphous forms (mainly iron and manganese oxides/oxi-hydroxides). With time, crystallization of these amorphous forms can occur, inducing a redistribution of radium between solids and solution. If the amount of mobile radium increases, the impact of these tailings on the environment may become significant. The aim of this study is to determine the amount of 226Ra released in solution during the crystallization process of hydrous ferric oxi-hydroxide (HFO) containing 226Ra. A reproducible co-precipitation procedure is developed. The transformation of Ra-HFO co-precipitate in crystallized forms (goethite, hematite, ...) is studied by ageing amorphous solids at 40$^{\circ}$C in different solutions. After co-precipitation, about 20% of the initial 226Ra present in solution is associated to amorphous HFO. At 40$^{\circ}$C, whatever the solution composition (MES or NaClO4), the solid evolution is very slow and, after 3-5 months, no significant amount of crystalline form is noticeable. Simultaneously, the amount of 226Ra present in solution is constant in MES and NaClO4 medium. In comparison, 226Ra sorption shows off inclusion of 226Ra in Ra-HFO precipitate.