Influence of composition, compaction and wetting drying cycles on the leachates from two hydraulically bound coal mine wastes

• Wetting-drying raised the porosity of hydraulically bound coal mine waste (HBCMW) • HBCMW compacted poorer showed higher concentration of most analytes in leachate. • HBCMW richer in sulphide-bearing minerals showed higher SO 4 2- level in leachate. • Elevated Ca level in leachate is likely to be from the cement used in HBCMW mixture. • Concentrations of heavy and transition metals were < 5 μg/g in leachate from HBCMW. Coal mine waste (CMW) poses many environmental challenges globally. They are mostly stored in coal tips or utilised as raw materials in low value landscaping applications within the mine site or in local proximity. To widen and valorise its use, there is a growing interest in its inclusion in higher value hydraulically bound construction materials, however, a key limitation currently holding back this potential relates to the very sparse knowledge available about the possible environmental impacts of such materials. To bridge this gap, an experimental programme has been carried out to investigate the impacts of composition, compaction and wetting–drying cycles of hydraulically bound materials employing JANK and DTEK (two CMWs collected from Poland and Ukraine) as aggregates. In particular, the hydraulically bound coal mine wastes (HBCMW) were subjected to two laboratory-simulated wetting–drying cycles to assess the chemistry of the leachates released. The results suggest that higher concentrations of sulphide-bearing minerals in DTEK compared to JANK have caused consistently higher sulphate anion (SO 4 2- ) concentrations in the leachates generated by hydraulically bound DTEK. Cementation of CMW particles has also led to elevated Ca concentrations in the leachates, due to interaction with the cement paste used as the binder in the initial mix. In addition, the leachates from the more loosely compacted JANK samples exhibit higher concentrations of most measured analytes, including heavy and transition metals, than the leachates from the more densely compacted JANK samples, possibly due to the former’s higher porosity exposing larger surface areas for leaching. The two laboratory-simulated wetting–drying cycles were found to increase the porosities of all HBCMW samples.