Bacterially induced CaCO3 precipitation for the enhancement of quality of coal gangue

The most critical shortcoming of gangue aggregate is its high water absorption and heavy metal, which seriously affects the utilization of coal gangue aggregates in concrete field. In this paper, an environmentally friendly bio-CaCO3 precipitation, induced by carbonate-precipitating bacteria, was applied to improve the performance of coal gangue aggregates. Two kinds of coal gangue (WG and BG) from different mining areas were used. The formation of bio-CaCO3 on/in the surface/pores of gangue aggregates prevented water penetration and therefore the water absorption was reduced. In addition, heavy metal ions in coal gangue aggregates may combine with carbonate ions, which improved the immobilization rate of heavy metals. The influence of bacterial concentration, immersion times and temperature on the bio-deposition efficiency was studied. The results showed that the best treatment strategy was one immersion at 20 °C, and the bacterial concentration was 108 CFU/ ml. When sufficient quantities of CaCO3 particles were precipitated in one immersion (2% for WG and 2.5% for BG), the water absorption of WG and BG was reduced by around 1.5 and 2.7 percentage points, respectively. The immobilization rate of all the heavy metals reached more than 40%, and the highest was 85%. The mortar prepared with coal gangue aggregates with optimized bio-treatment was superior to that made with none bio-treated ones. It was demonstrated that the bio-treatment could be an alternative way to improve the quality of coal gangue, which is beneficial for coal gangue waste recycling.