Abstract Dried and grated biological waste was immobilized with cement to generate specimens mimicking solidified phytoremediated radioactive waste. Stability of these solidified biological waste specimens was estimated during drastic long-term flooding, which is considered one of the typical environmental impacts affecting physical and mechanical characteristics of such specimens. For these investigations, biological waste generated during phytoremediation using the aquatic plant Veronica anagallis-aquatica was solidified and subsequently examined during flooding. After flooding with water of different compositions, mechanical and physical stability of the specimens was evaluated. Cementation of 3% dried biological waste resulted in a satisfying compressive strength of the resulting solidified material of more than 13 MPa. The highest value of hardness exceeded 25 MPa, and was obtained in samples cured in sea water or ground water due to mineral salts sealing the pores, whereas an insignificant decrease in durability was observed in those specimens cured in tap water. Maximum mass change caused by the water absorption during six months of curing was below 4.5% of the initial mass; this change was more pronounced for solidified waste immersed in sea water or ground water than for samples immersed in tap water. Retardation of material and status of the cement phases after flooding were investigated via FT-IR, XRD and SEM; the results point to the suitability of this cement type as powerful material for immobilization of biological radioactive waste, as manifested by acceptable durability and reasonable porosity during long immersion in different water compositions.