Coupled THCM model of a heating and hydration concrete-bentonite column test

摘要

Radioactive waste disposal in deep geological repositories in clay formations envisage a compacted bentonite engineered barrier and a concrete liner. The alkaline conditions caused by the degradation of concrete could affect the performance of the engineered barrier. The geochemical interactions occurring at the concrete-bentonite interface (B-CI) for the non-isothermal unsaturated conditions prevailing at repository post-closure have been studied by CIEMAT with a heating and hydration concrete-bentonite column test. The column consists of a 3 cm thick concrete sample emplaced on top of a 7.15 cm block of compacted bentonite. The column was hydrated through the concrete at a constant pressure with a synthetic clay porewater while the bottom of the column was heated at 100 degrees C. Here we report a coupled thermo-hydro-chemical-mechanical (THCM) model of the column test, which lasted 1610 days. The model was solved with a THCM code, INVERSE-FADES-CORE. Experimental observations show calcite and brucite precipitation in the concrete near the hydration boundary, portlandite dissolution and calcite and ettringite precipitation in the concrete, calcite and sepiolite precipitation in the bentonite near the B-CI, calcite dissolution in the bentonite far from the B-CI and gypsum precipitation in the bentonite near the heater. Model results attest that advection is relevant during the first months of the test. Later, solute diffusion becomes the dominant transport mechanism. Calcite and brucite precipitate in the concrete near the hydration boundary because the concentrations of dissolved bicarbonate and magnesium in the hydration water are larger than the initial concentrations in the concrete porewater. Calcite and brucite precipitate in both sides of the B-CI. Sepiolite precipitates in the bentonite near the B-CI. The model predicts portlandite and C1.8SH dissolution in the concrete. Ettringite and C0.8SH precipitate near the hydration boundary while ettringite dissolve