Valorization of alkali-activated fly ash-slag claddings to enhance the mechanical and leaching properties of phosphogypsum-based cold-bonded aggregates

This research investigates the efficacy of two cladding techniques, namely soaking in alkali-activated fly ash-slag (AAFS) slurry (AS) and two-step pelletization using AAFS powder (AP), in improving the mechanical and leaching properties of phosphogypsum-based cold-bonded aggregates (PCBAs). Findings demonstrate that both AS and AP claddings effectively decrease water absorption from ∼7.5% to < 6.3%, while simultaneously enhancing strength and carbonation resistance. Additionally, these cladding methods notably reduce ions leaching, particularly sulfate, by over 86%. The enhanced leaching resistance achieved through AS is attributed to chemical immobilization via ettringite formation, whereas AP primarily relies on physical obstruction through an impermeable dense shell. The reduced sulfate leaching makes PCBAs compatible with Portland cement, resulting in an enhanced 90-day compressive strength of PCBAs-based concrete from 32.6 to 45.7 MPa (for AS) and 59.6 MPa (for AP). This study expands the utilization of cost-effective and low-carbon PCBAs for promoting a greener build environment.