Interaction of cemented paste backfill (CPB) and circumneutral mine water during column experiments

Cemented paste backfill (CPB) is used as a support and filler material in underground mines. In underground conditions, before mine closure and flooding, surface and groundwaters with variable quality are in contact with CPB material, which may result in the leaching of harmful substances from the CPB. In this study, we used actual mine water from a gold mine to flush CPB in a laboratory test to simulate underground conditions and the environmental influence of CPB on groundwaters more realistically than before.Different CPB materials were treated with mine water in columns for nine months. The columns were drained and filled with new mine water batches four times during the experiment to simulate intermittent flushing underground. The dissolved elemental content, major cations, pH and oxidation-reduction potential (ORP) of the waters were measured, and the chemical results were analysed by performing geochemical modelling with the widely used geochemical code PHREEQC. X-ray tomography was also used to examine the internal structure of the CPB specimens.The results demonstrated that calcium and sulphate present in CPB parent materials leached into the mine water, which might cause a deterioration of CPB. In contrast, the concentrations of dissolved As, Sb and Ni decreased in the mine water when it was in contact with CPB materials. However, arsenic started to be re-released from the materials at the end of the nine-month experimental period. The geochemical modelling suggests that the removal of metals/metalloids from the mine water was driven by the surface complexation reactions on originally present and/or freshly precipitated metal (hydr)oxide minerals. A longer and modified testing period is needed to thoroughly examine the conditions under which these adsorbed elements start to release over time.