Effect of leaching conditions and curing time on the leaching of heavy metals in fly ash cement mortars

The main aspect in respect of environmental compatibility and impact of cement-based materials is the leaching of inorganic compounds when they are contact with environmental waters. The leaching of heavy metals Cr (total chromium and of Cr⁶⁺), Pb, Cd, Zn, Mn, Co, V, Ti, Ni and Cu in fly ash cement mortars has been investigated by tank leaching and shaken extraction test methods; comparisons are made between the leachability of heavy metals under different pH conditions and of metals from specimens at different curing ages. In addition, a pH controlled shaken 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 impact of pH variation of leachant on the leachability of heavy metals. The results of this study mainly indicate that: (a) heavy metals do leach from the tested cement mortars, but the amount leached is small; with increasing fly ash addition and water to binder ratio the leaching of heavy metals from cement mortars increases; (b) in a tank leaching test, the leaching of heavy metals from fly ash cement mortars is also a diffusion controlled process; compared with the tank leaching test, a shaken extraction test can extract more toxic heavy metals from hardened mortars; (c) the leachability of some heavy metals is greatly dependent on leachant pH: for example at pH around 7, leaching of Mn, Zn, total Cr (Cr for short in the following), Cr⁶⁺ and Pb will be much increased in comparison to that at pH over 12; (d) the leaching of Co, total Cr and Cr⁶⁺ from cement mortars is reduced with curing age, but no evident effect of curing age on the leaching of Cd, Ti, Ni, Mn, Zn and V has been observed; and (e) increased leaching of both Cr and Cr⁶⁺ will occur when cement mortars have been carbonated.