Time-resolved porosity changes at cement-clay interfaces derived from neutron imaging

Interfaces between cement and clay are common in engineered barrier systems for the underground disposal of radioactive waste. The geochemical contrast between clay and cement leads to alterations of both materials following dissolution and precipitation reactions. Porosity is a key parameter for, and a key outcome of, such reactive transport. Here we report on the evolution of porosity within small samples made from ordinary portland cement and Na montmorillonite. Water contents, a proxy for porosity in saturated samples, were derived from neutron imaging. Specific calibration procedures were developed to account for multiple scattering effects. An increase in cement porosity was observed propagating similar to 2 mm away from the interface within 600 days, and a considerable decrease in clay porosity within similar to 2 mm. Our experiments provide unique information on the dynamics of the processes up to nearly two years. The data set provides valuable input to better constrain reactive transport models.