Leaching of heavy metals in fly ash incorporated cement mortars under different leaching conditions

The main aspect to environmental compatibility and impact of cement-based materials is the leaching of inorganic compounds when they are contact with environmental waters. In this study, the leaching behaviors of heavy metals Cr, Pb Cd, Zn, Mn and Cu in cement mortars with fly ash addition have been investigated by both tank leaching and shaking extraction test methods. A comparison has been made between the leachability of heavy metals under different pH conditions. In addition, a pH controlled shaking extraction test method is proposed; an advantage of this method is that leachant pH remains nearly constant at 6.6+/-0.3 throughout the extraction process, leading to the diminution of the influence of pH variation of leachant on the leachability of heavy metals. The results of this study mainly indicate that: (1) heavy metals do leach from the tested cement mortars, but the leached amount is very small; with the increment of fly ash addition and water to binder ratio the leaching of heavy metals from cement mortars increases; (2) in a tank leaching test, the leaching of heavy metals from fly ash incorporated cement mortars is also a diffusion controlled process; compared with tank leaching test, shaking extraction test can extract more toxic heavy metals from hardened mortars; (3) the leachability of some heavy metals is greatly dependent on leachant pH, for example, at pH around 7 the leaching of Mn, Zn, Cr and Pb will be much increased in comparison to that at pH over 12; and (4) increased leaching of Cr will take place when cement mortars have been carbonated.