Stabilization of cadmium and lead in portland cement paste using a synthetic seawater leachant

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Abstract
The seawater leachability of cadmium and lead sludges solidified in portland cement paste was investigated. The leachates of the wastes containing cadmium and lead were analyzed for metal contents using flame atomic absorption spectrophotometry (AAS) as well as pH and alkalinity. A control sample containing only portland cement paste was also leached to serve as a comparison. Results show that the total cadmium leached from the stabilized waste over a 50‐day period was about 1.0% of the total cadmium added to the paste. Lead was not detected in the leachate, however, this could be due to physical and analytical interferences associated with lead. The pH and alkalinity data show that initially, hydroxide is leached from the waste and calcium carbonates are formed on the particle surfaces. After a 50‐day period, the pH and alkalinity of the waste leachates approach that of the seawater leachant.The microstructure of the waste samples was investigated using scanning electron microscopy (SEM), energy dispersive X‐ray analysis (EDXA) and powder X‐ray diffraction (XRD). The presence of lead did not appear to have an effect on the hydrated structure of the portland cement paste, whereas cadmium seemed to create a more porous microstructure with abundant colloidal ettringite (a calcium sulfoaluminate compound often found in hydrated portland cement paste). Results show that the fixation of cadmium in portland cement paste accelerates sulfate attack on the paste by seawater, which in turn leads to destructive cracking of the paste. It appears that cadmium release is related to the physical destruction of the portland cement paste.

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