Leaching of a Simulated ZnCl₂-Rich Radwaste Stabilized with Calcium Sulfoaluminate Cement: Experimental Investigation and First Attempt of Modeling

Abstract

This chapter reports the first leaching experiments performed on a simulated ZnCl₂-rich waste stabilized with calcium sulfoaluminate cement according to a standard test developed to understand and model the degradation processes of the cement paste. A simulated cemented waste form (cement paste prepared with a binder containing 20 % gypsum and a 0.5 mol/L ZnCl₂ mixing solution and submitted to a thermal cycle at early age simulating the temperature rise and fall occurring in a 200 L drum of cemented waste) was submitted to leaching by pure water (fixed pH of 7) for 3 months. The cumulative quantities of Ca²⁺, OH⁻, Cl⁻, and SO₄ ²⁻ ions in the leachates increased linearly versus the square root of time, showing that leaching was controlled by diffusion. Zinc was never detected, showing the excellent confining properties of the cement matrix. Examination of the solid sample at the end of the experiment revealed three zones: (1) the highly porous and friable surface layer, composed of aluminum hydroxide, perovskite, and probably C-(A)-S-H (thickness: 700 μm), (2) a less porous intermediate zone in which several precipitation and dissolution fronts occurred (thickness: 1,800 μm), and (3) the sound core. The hydrated phase evolution along depth was qualitatively reproduced with a model coupling transport by diffusion and chemical reactions.