Investigation on macroscopic properties, leachability and microstructures of surface reinforced phosphogypsum-based cold-bonded aggregates

A trade-off between phosphogypsum (PG) content and mechanical performance is often encountered when recycling PG in the form of the phosphogypsum-based cold-bonded aggregates (PCBAs) for practical engineering. Surface reinforcement is an effective strategy to make the PCBAs qualified materials without too much loss in the PG content. This article proposes a core-shell structure with a PG-rich core and a strong shell which optimizes the recycling efficiency. Then, physical properties, mechanical properties (single pellet strength and cylinder compressive strength) and leaching properties of PCBAs, with varied core-shell designs, are tested. Besides, detailed analyses on their microstructures are made via XRD, TG, LF-NMR and X-CT technologies. Thickness of the shell is considered a critical design parameter, and an optimized value is suggested. For PCBAs with 8-mm diameter, shell with a 1.5-mm thickness performs best. The proposed core-shell structure and the systematic assessment methods are expected generally applicable to variously sized PCBAs for an overall performance optimization.