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

Abstract

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.