Enhancing fly ash utilization in backfill materials treated with CO2 carbonation under ambient conditions

Abstract

The environmental concerns resulting from coal-fired power generation that produces large amounts of CO2 and fly ash are of great interest. To mitigate, this study aims to develop a novel carbonated CO2-fly ash-based backfill (CFBF) material under ambient conditions. The performance of CFBF was investigated for different fly ash–cement ratios and compared with non-CO2 reacted samples. The fresh CFBF slurry conformed to the Herschel-Bulkley model with shear thinning characteristics. After carbonation, the yield stress of the fresh slurry increased significantly by lowering fly ash ratio due to gel formation. The setting times were accelerated, resulting in approximately 40.6% of increased early strength. The final strength decreased when incorporating a lower fly ash ratio (50% and 60%), which was related to the existing heterogeneous pores caused by rapid fluid loss. The strength increased with fly ash content above 70% because additional C–S(A)–H and silica gels were characterized to precipitate on the grain surface, so the binding between particles increased. The C–S(A)–H gel was developed through the pozzolanic reaction, where CaCO3 was the prerequisite calcium source obtained in the CO2-fly ash reaction. Furthermore, the maximum CO2 uptake efficiency was 1.39 mg-CO2/g-CFBF. The CFBF material is feasible to co-dispose CO2 and fly ash in the mine goaf as negative carbon backfill materials, and simultaneously mitigates the strata movement and water lost in post-subsurface mining.

Keywords

• Fly ash utilization
• CO2 carbonation
• Ambient conditions
• Water conservative backfill mining
• Negative carbon backfill materials