Effect of microbially induced calcium carbonate precipitation treatment on the solidification and stabilization of municipal solid waste incineration fly ash (MSWI FA) - Based materials incorporated with metakaolin

Abstract

Microbially induced calcium carbonate precipitation (MICP) has been considered as a potential treatment method for the solidification and stabilization of municipal solid waste incineration fly ash (MSWI-FA).The main obstacle for MICP treatment of MSWI-FA is the harsh environment which causes the bacteria fail to maintain their urease activity effectively, thus decreases the solidification effect and material properties. Currently, there is no research on blending metakaolin (MK) as a protective carrier for the bacteria into the MSWI-FA. The effect of the MICP process on the curing properties of MSWI FA-based cementing materials in the MK and MSWI-FA reaction system is largely unknown. In this study, different mixing ratios of MK were used to adjust the Ca/Si/Al ratio in the mixture, and the properties of the cementing material (MSWI-FA mixed with MK and water) and the MICP-treated material (MSWI-FA mixed with MK and bacterial solution) were investigated. This study contributes to find suitable additives to promote effect of MICP on the solidification of MSWI-FA and the improvement of material properties. The results showed when the mixing ratio of MSWI FA was 90 wt %, the MICP treatment was able to increase the compressive strength of the samples up to 0.99 Mp, and the compressive strength of samples reached 1.46 MPa, when the mixing ratio of MSWI FA was 80 wt %. Though the metakaolin did not show inhibitory effect on the urease activity, the compressive strength of the MICP-treated samples did not further show a significant increase when the mixture of MK was increased from 20 wt% to 30 wt%. Further investigation suggested that MICP activities of bacteria utilizing calcium sources could have an impact on the formation/deformation of calcium-containing hydration products in the reaction system, thus affecting the mechanical and chemical properties of MSWI based materials. MICP treatment is effective in the immobilization of certain heavy metals of MSWI FA, especially for Pb, Cd and Zn. This research shows the potential of using MICP to treat the MSWI fly ash, meanwhile, it is necessary to find suitable reaction system with the proper additives in order to further improve the properties of the MSWI FA based material in terms of mechanical performance.

Keywords: MSWI fly ash; Microbially induced calcium carbonate precipitation; Microstructures; Solidification; Stabilization.