High-pH/low pH ordinary Portland cement mortars impacts on compacted bentonite surfaces: Application to clay barriers performance

The design of engineered barrier systems for high-level radioactive waste isolation requires the use of cement-based materials. Small-scale surface reactivity interface experiments were designed to perform a reference laboratory study. It simulated granitic groundwater infiltration through cement mortar/bentonite (CB) binary columns. The tests ran duplicated over 6 and 18 months and included low-pH, CEM I and CEM II ordinary Portland cement mortars in contact with bentonite. After the experiments, the materials were analysed using scanning electron microscopy attached to energy dispersive energy X-ray analysis (EDX), surface mineralogy using grazing incidence X-ray diffraction configurations, specific surface area and effluent chemistry. EDX chemical profiles measured from CB interface contact reproduced, in 100 mu m to 1 mm thickness, a characteristic geochemical Mg perturbation in the bentonite and calcite precipitation in cement materials. These processes are known to be developed in 10-15-years within < 5 mm of the interface. The comparison of these long, large scale experiments with the small scale experiments showed that the chemical perturbation thickness development rate will decrease over time. The limestone addition (CEM II) leaves less room for calcite precipitation, and ettringite became stabilized, which limited the extent of dissolution-precipitation processes. Then, the CEM II mortar had better resistance to perturbation.