NMR studies of pore formation and water diffusion in self-hardening cut-off wall materials

Cut-off walls for the containment of polluted sites are vertical in-ground barriers of low hydraulic conductivity. To construct these barriers, self-hardening watery suspensions of a special cement-based hydraulic binder and a cement-stable bentonite are used. The formation of the pore structure during hardening of suspensions with different solid contents and the water self-diffusion in the resulting cut-off wall materials were studied by non-destructive 1H NMR techniques. It was found that an increased amount of hydrating solids in the suspension leads to a decrease in NMR relaxation times and self-diffusion coefficients of the pore water, indicating a reduction of the pore sizes and an enhancement of the diffusion resistance. The self-diffusion coefficients of the water in the hardened cut-off wall materials were determined to be about four orders of magnitude smaller than in bulk liquid water and two orders of magnitude smaller than in pure bentonite–water suspensions confirming the excellent diffusive resistance of the cut-off wall materials.