Calcium carbide residue-modified phosphogypsum in one-part geopolymers: performance and mechanisms

The large-scale accumulation of phosphogypsum (PG) poses severe environmental risks due to acidic impurities that limit its utilisation in cementitious materials. This study investigates calcium carbide residue (CCR) pre-treatment to enhance PG incorporation into one-part geopolymer systems. Raw PG was pre-treated with CCR at dosages of 2 ∼ 8 wt% to neutralise acidic impurities, and then incorporated at 15 wt% into geopolymer matrices comprising fly ash, ground granulated blast furnace slag, and anhydrous sodium silicate. Results demonstrate that 6 wt% CCR pre-treatment reduced soluble phosphorus to near-baseline levels and achieved optimal 28-day compressive strength of 80.5 MPa, representing 15.5% improvement over untreated controls with 19.3% early-age enhancement. X-ray diffraction revealed enhanced ettringite formation (4.4% AFt) and sulphate phase conversion at optimal dosage. 29Si NMR spectroscopy showed that 6% CCR optimised silicate polymerisation by increasing Q1 sites to 20.7%, facilitating synergistic coexistence of C-(A)-S-H and N-(A)-S-H gels with reduced Ca/Si ratios (2.22). Scanning electron microscopy confirmed a densified microstructure with improved gel continuity. However, excessive CCR (8%) caused structural degradation due to elevated Ca/Si ratios (3.00) and impaired precursor dissolution. This work establishes CCR modification as a viable pathway for large-scale PG valorisation in sustainable construction materials.