Modelling of long-term dynamic leaching tests applied to solidified/stabilised waste
Résumé
The paper aims at simulating the closed-system dynamic leaching of a cement-based monolith containing lead with the numerical reactive transport code HYTEC in a 3D-cylindrical geometry. The model considers, simultaneously, the chemical evolution of pore water, the progression of mineralogical alteration fronts, and the concomitant release of elements from the S/S waste. In good agreement with the experiment, element releases were found to be mainly controlled by either diffusion (Na, K, and, to a lesser extent, Cl), by surface dissolution (Ca, Si) or by a mixed evolution (Pb, SO4). All of the calculated mineralogical transformations take place in a thin layer beyond the monolith surface. Consequently, modelling of Ca, Si and SO4 releases was quite sensitive to the node size of the simulation grid and was improved by taking into account the increase of porosity and effective diffusion coefficient due to mineral dissolution in the leached layer. In agreement with experimental results, the deepest front corresponds under closed-system conditions to portlandite dissolution and calcium silicate hydrates CSH 1.8 transformation into CSH of lower Ca/Si ratio. A second, distinct and intermediate, front is made by ettringite dissolution. The network of CSH is globally preserved in the leached layer, complete dissolution occurring over a very small thickness only. Finally, hydrotalcite precipitation in the leached layer is expected by modelling due to pH drop.
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