Formulations of bentonite-cement composites are explored as a basis for creating a range of structured, semipermeable layers adapted to in situ capping of contaminated underwater sediments. In addition to being mechanically stable, such composites allow for control of cap microstructure and transport properties through the fine-tuning of postdepositional hydration processes in the cap. Microscopic studies combined with flowability and setting tests were used to establish relationships between the structural and mechanical properties of the composites. Apparent diffusion coefficients for transport through the caps were measured and corresponding porosity values were calculated for the composites formed under different hydration conditions. Analysis of calculated porosity values combined with microscopy data indicates that the particle packing density plays a crucial role in the development of cap's microstructural features and retardation capacity. © Mary Ann Liebert, Inc.