Sorption Mechanisms, Long-Term Evolution, and Safety Implications of Cement-Based Materials Under Radioactive Waste Disposal Conditions

Cement-based materials are indispensable components in the engineered barrier systems of repositories for radioactive waste. They fulfill multiple critical roles, including structural support, chemical containment, and geochemical conditioning of the near-field environment, and thus constitute an indispensable component across the entire “near-surface to deep-geological ” disposal spectrum. This review presents a comprehensive synthesis of current scientific understanding regarding the behavior of cementitious materials in repository conditions, with an emphasis on radionuclide immobilization mechanisms, long-term degradation processes, and integration within multi-barrier systems. Key cement hydration products such as calcium(alumino)silicate hydrate(C-(A)-S-H), portlandite, ettringite(AFt), and layered double hydroxide(AFm) phases provide abundant sorption sites for cationic and anionic radionuclides through ion exchange, surface complexation, and incorporation mechanisms. Sorption is further modulated by the evolving pH, redox conditions, and porewater composition during cement degradation. Redox-sensitive radionuclides such as ⁹⁹Tc, ⁷⁹Se, and ^235,238U may be immobilized via reduction at steel-cement interfaces, while others interact with secondary phases such as carbonates, zeolites, or degraded aluminosilicates. A major focus is given to the influence of organic ligands, such as isosaccharinic acid(ISA), produced by cellulose degradation, and their strong ability to form soluble complexes with actinides and lanthanides. These interactions significantly reduce sorption efficiency, necessitating conservative treatment in safety assessments. The review also addresses advanced modeling strategies, including surface complexation models(SCMs), reactive transport modeling, and thermo-hydro-mechanical-chemical(THMC) simulations. These tools are essential for predicting long-term performance, supporting repository design, and informing regulatory frameworks. Integration with field data from underground research laboratories and validation through long-term experiments remain vital to reduce uncertainties. Alternative cement systems, such as low-pH cements, geopolymers, and magnesium-based binders, are explored as more sustainable and chemically compatible options for repository applications. While promising in terms of reduced pH and improved environmental profiles, these materials face challenges related to long-term durability, regulatory acceptance, and comprehensive radionuclide retention data. As cement interacts with bentonite buffers, steel containers, and host rocks, it generates complex interfacial zones characterized by mineral transformations, porosity changes, and redox shifts. Understanding these interfaces is crucial to predict the evolution of the multi-barrier system. The review synthesizes experimental insights and modeling approaches to capture these interactions and their implications for radionuclide transport. In conclusions, the paper outlines key challenges and research priorities, including validation of long-term behavior, refinement of thermodynamic databases, modeling of organics and microbial processes, and development of standardized protocols for new binders. By fostering interdisciplinary collaboration and integrating experimental, modeling, and regulatory perspectives, the scientific community can ensure that cementitious barriers remain reliable, adaptable, and effective components of safe radioactive waste isolation.