Experimental study on the macroscopic and microscopic properties of cement paste backfill after treatment with carbon dioxide carbonize filling slurries during the mixing process

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Introducing CO2 during the mixing process of filling slurry for wet carbonation enables simultaneous carbonation and hydration reactions of the carbonated filling slurry (CSL). This study evaluates the microstructural evolution and strength development of carbonated cemented paste backfill (CCPB) under different carbonation times, and discusses the carbonation products and carbon sequestration of CCPB. The results reveal that introducing 99% pure CO2 at a flow rate of 1 L/min for 10 min into the filling slurry in a non-sealed container effectively optimizes the microscopic pore structure and mechanical properties of CCPB, achieving a carbon sequestration rate of 6.42%. Short-term carbonation leads to a continuous decrease in the T2 relaxation signal of CSL in the early hydration stage, enhancing pore structure by converting secondary pores to micropores. The main carbonation product is calcite crystalline CaCO3, which forms a dense carbonation layer on the surface of the tailings particles. However, prolonged carbonation results in the formation of microscopic pore encapsulation layer, hindering further CO2 diffusion, reducing carbonation efficiency, and deteriorating the mechanical properties of CCPB. With a carbonation time of 10 min, the UCS and EM of CCPB with different cement-to-tailings ratios show significant improvement, and the specimens exhibit lower fragmentation and fewer secondary cracks, maintaining better integrity after failure. In conclusion, the wet carbonation of filling slurry not only enhances the strength of the filling material but also achieves the reutilization of gaseous and solid wastes through carbon sequestration filling method, providing an efficient and eco-friendly solution for mine backfilling.