Hydration–carbonation interactions in CO2-cured cemented phosphogypsum backfill for underground carbon storage

• Competition and synergy between hydration and carbonation are explored; • Binder content regulates the interaction between hydration and carbonation; • CO 2 uptake in backfill reaches 4.11% after 28 days; • Competitive consumption of Ca 2+ and OH⁻ affects strength development. Cemented phosphogypsum(PG) backfill has emerged as a sustainable approach for underground mining by combining solid waste utilization with CO 2 storage. This study investigates the interactive mechanisms between hydration and carbonation in PG-based backfill with binder contents of 10-30 wt% through multi-scale analyses. Results indicate that CO 2 curing significantly enhances carbon storage, achieving 4.11 wt% CO 2 uptake (41.1 kg CO 2 /t) after 28 days, which is approximately three times higher than that of air curing. Binder content is identified as the critical factor controlling the balance between hydration and carbonation. Below 20 wt%, competitive consumption of Ca 2+ and OH - inhibits hydration, reducing compressive strength by up to 50%. Around the 20 wt% threshold, carbonation initially improves strength, but excessive carbonate accumulation eventually leads to performance decline. Above 20 wt%, hydration and carbonation interact synergistically, resulting in both higher strength and CO 2 storage efficiency. These findings offer a mechanistic basis and practical design guidance for optimizing binder dosage in CO 2 -cured PG backfill systems, supporting the development of reliable and low-carbon mining operations within CCUS frameworks.