Effect of accelerated carbonation on a Molybdenum-contaminated soil stabilized with magnesium oxide and limestone calcined clay cement

• Accelerated carbonation of stabilized soil with mixtures of MgO and LC3 up to 28 days. • Brucite carbonation leads to hydrated magnesium carbonates and potentially M-S-H • High Mo retention (>90%) maintained in MgO-containing treatments • Possible shift in main Mo retention minerals from brucite to HMCs and/or M-S-H A combination of magnesium oxide (MgO) and Limestone Calcined Clay Cement (LC3) has been previously identified as effective for the stabilization/solidification (S/S) of molybdenum (Mo) in a limestone tunnel sludge (Grand Paris express subway extension). However, the long-term performance of this S/S method is uncertain due to brucite's susceptibility to carbonation, the primary Mo-retaining phase. This study investigates the impact of accelerated carbonation on mineralogical transformations and Mo leaching behavior under controlled conditions (3% CO₂, 90% RH, 38°C) for up to 28 days. Mineralogical changes were monitored via XRD, TGA, and geochemical modeling. Leaching behavior was assessed using EN 12457-2 and US EPA 1313. Results show rapid carbonation of cementitious phases into calcite, with pH dropping from >12 to ≤10 within 6h, followed by brucite gradual reaction into poorly crystallized hydrated magnesium carbonates (HMCs) and potentially magnesium silicate hydrates (M-S-H). Mo leaching behavior was dependent on MgO content. In the formulations with 5 wt.% MgO, molybdenum release increased gradually but remained within acceptable limits, with retention attributed to uncarbonated brucite. In the 1 wt.% MgO + 3 wt.% LC3 formulation, Mo release initially spiked but stabilized at 0.03 mg/L after 28 days of carbonation, achieving over 90% leaching reduction and improved retention across a broader pH range (7-12). The durability of retention despite brucite carbonation suggests a shift in retention mechanisms from brucite sorption to possible immobilization by HMCs and/or M-S-H. Stabilized soils with MgO-LC3 mixture prove upon their efficacy in long-term Mo immobilization despite exposure to atmospheric CO 2 .