We present a two-step procedure for treating contaminated liquid waste generated during the processing of uranium, specifically residual uranium-contaminated liquid after precipitation. After precipitating U(IV) as sodium diuranate under alkaline conditions, the resulting solution was solidified by immobilising it in an alkali-activated material. Static leaching tests indicated excellent material stability in water. Exposing the material to aggressive chemical conditions (nitric acid) resulted in slow and incomplete dissolution of uranium (and of structural elements) from a thin superficial layer with further passivation. Characterisation of the solid phase was performed to assess the stability of the alkali-activated material under the tested conditions.