Environmental impact and mechanical improvement of microbially induced calcium carbonate precipitation-treated coal fly ash–soil mixture

Reuse of coal fly ash (CFA) as construction materials is one of the sustainable methods for mitigating its environmental impacts. However, the potential leaching of heavy metals and the limited effect on mechanical improvement impede the reuse of CFA. The objective of this research was to investigate the effects of environmental impact and mechanical improvement of the microbially induced calcium carbonate (CaCO₃) precipitation (MICP) process on the CFA–soil mixture. Two resources of CFA, named CFA₁ and CFA₂, were used. CFA₁ is classified as off-specification fly ash, and CFA₂ is classified as class F-type fly ash. Experimental results showed that the unconfined compressive stress (UCS) of MICP-treated soil increased significantly with the addition of CFA. The MICP-treated 3% CFA₁–soil mixture obtained the highest UCS of 2200 kPa, which was much higher than that of the MICP-treated soil (798 kPa). The maximum UCS of the MICP-treated 3% CFA₂–soil mixture was 1398 kPa. X-ray diffraction analysis indicated that the main mineral crystal pattern present in the MICP-treated samples was calcite. Scanning electron microscopy images revealed that the formed calcium carbonate worked as a bridge to connect soil particles together, attached to the surface of soil or located in the pore side of soil particles. In addition, the batch leaching test indicated that cadmium (Cd), copper (Cu), magnesium (Mg), nickel (Ni) and zinc (Zn) were detected in the MICP-treated soil. With the addition of CFA, there was no metal exceeding the maximum contaminant level from the MICP-treated CFA₁–soil mixture.