The sponge effect of phosphogypsum-based cemented paste backfill in the atmospheric carbon capture: Roles of fluorides, phosphates, and alkalinity

The use of cement for solid waste solidification/stabilization is one of the most practised methods but is under scrutiny due to its substantial emission of greenhouse gases. However, the dual role of cement-immobilized solid waste may serve as a CO2 sink and promisingly reabsorb a great content of atmospheric CO2, which is hitherto unexplored. In this study, we detail the inherent potential sponge effect of phosphogypsum (PG) based cemented paste backfill (PCPB), finding that, at a high PG utilization rate, the PCPB application may produce 16.7 Mt/year of CO2 in China, whereas a reduction of 5.76 Mt/year could promisingly reach up when considering the future reabsorption. However, industrial applications are limited by the remained impurities and the maintenance of alkalinity. The results suggest that the phosphate impurities within PG have an adverse effect on CO2 uptake, which inhibits the precipitation of hydration products and inversely favors the formation of calcium-phosphate species that retard the dissolution of cement particles. By contrast, the fluorides imply an acceleration of the hydration reactions and accordingly hasten the carbonation process. Furthermore, geochemical modeling suggests that maintaining a basic pH condition of the system is another significant factor in promoting the CO2 capture capacity, of which a 35% increase in CO2 uptake can be acquired with a continuous low concentration NaOH supply, but this enhancement will require the widespread deployment of future validation. From the perspectives of CO2 balance, environmental requirements, and technological feasibility, PCPB is an effective way for in situ immobilizing PG with scalable potential. These new observations are expected to provide a deeper understanding and reliable guidance for the sustainable management of dumped PG and the zero emissions of the phosphorus fertilizer industry.