Effect of CO2 on leaching from a cement-stabilized MSWI fly ash

A cement-stabilized MSWI fly ash was leached according to the diffusion test NEN 7345. The influence of carbonation during leaching was tested by bubbling N₂, air or CO₂ through the leachant. The original test, where no gas is bubbled, was used as a reference. The periodically renewed leachant was analysed for its ionic concentrations. Porosity measurements, XRD and scanning electron microscopy (SEM) analysis were performed.For some metals, leaching is seriously underestimated when precipitation on the tank walls is not included. Thus, the sum of the dissolved portion and the precipitated portion has to be taken into account. Diffusion tests with N₂ and air bubbling gave results that were not significantly different from those obtained in the reference test. Bubbling of pure CO₂, however, influenced leaching profoundly. This was mainly due to the lowering of pH. The reaction of hydroxides to carbonates—another effect of CO₂ bubbling—had a negligible effect on leaching.