Immobilization of molybdenum by alternative cementitious binders and synthetic C-S-H: An experimental and numerical study

Excavation operations during construction produce millions of tons of soil sometimes with high leachable molybdenum (Mo) contents, that can lead to risks for both human health and the environment. It is therefore necessary to immobilize the Mo in excavated soils to reduce pollution and lower the costs of soil disposal. This paper studies the immobilization of Mo by three cementitious binders. To this end, one Ordinary Portland cement (OPC), one binder composed of 90% ground granulated blast furnace slag (GGBS) and 10% OPC, and one supersulfated GGBS binder were spiked with sodium molybdate at six different Mo concentrations from 0.005 wt% to 10 wt% before curing. In addition, to gain mechanistic insights, the capacity of synthetic calcium silicate hydrates (C-S-H) to immobilize Mo was studied. This study was completed by thermodynamic modeling to predict the immobilization of Mo at low Mo concentrations (95%) by the coprecipitation of powellite. Thermodynamic modeling was in good agreement with measured values when the equilibrium constant of powellite was modified to LogK = −7.2. This suggested that powellite is less stable in cementitious environments than would be expected from thermodynamic databases. Moreover, modeling showed that, for a solution at equilibrium with portlandite or C-S-H, the Mo concentration is limited to 1.7 mg/L by powellite precipitation. In contrast, for a solution saturated with respect to ettringite, the threshold concentration for powellite precipitation is 6.5 mg/L. [Display omitted] •Mo retention was studied in three cementitious binders and in synthetic C-S-H.•Supersulfated GGBS binder showed the highest capacity to immobilize Mo (>92%).•Powellite (CaMoO4) was identified as the main mechanism of Mo stabilization.•Limit concentrations were calculated for CaMoO4 precipitation in three solutions.•Modeling was performed for solutions at equilibrium with ettringite, CH and C-S-H.