Phosphate's second life: Upcycling phosphogypsum and clay by-product through acid geopolymer technology

The present study explores the repurposing of large amounts of phosphoric acid by-products, specifically phosphogypsum (PG), by evaluating its effectiveness as a precursor in acid-activated geopolymer technology. Geopolymers were elaborated using PG and calcined clays, activated with phosphoric acid. The research focused on evaluating the mechanical properties and microstructure of these geopolymers after a 28-day curing period. X-ray diffraction (XRD) analysis revealed the formation of new crystalline phases, including brushite, newberite, and berlinite. Fourier-transform infrared (FTIR) spectroscopy confirmed the presence of P-O and Si-O-Al bonds, indicative of successful geopolymerization. Scanning electron microscopy (SEM) imaging demonstrated a dense, cohesive microstructure in optimized samples. Based on the response surface methodology, the study identified optimal conditions for maximum compressive strength: 28.83 % PG content, 14.39 mol/L phosphoric acid concentration, and a curing temperature of 72.45°C. This optimized formulation produced a dense, cohesive microstructure with a notable compressive strength of 21.46 MPa. The leaching test confirmed that no harmful substances were released from the geopolymer samples. These results suggest that PG can be effectively recycled through geopolymer technology for use in the construction industry. [Display omitted] •Phosphogypsum and red clays serve as precursors in acid-activated geopolymer synthesis.•Optimal mix achieves 21.46 MPa strength with 28.83 % phosphogypsum at 72.45°C curing.•Phosphoric acid activation at 14.39 mol/L concentration yields best mechanical properties.•Toxicity tests confirm geopolymer safety for construction industry applications.