High-Phosphogypsum Bricks: Microstructural, Mechanical, and Radiological Optimization According to Clay Type for Circular Construction

This study explores the valorization of phosphogypsum (PG) in clay-based composites for sustainable production of bricks, addressing key factors such as drying behavior, mechanical performance, microstructure, porosity, and radiological safety. The analysis of drying kinetics reveals a rapid mass loss during the first 120 min, with intermediate PG–clay ratios (50:50) promoting optimal pore connectivity. Mechanical testing of fired composites indicates that green clay–PG (50:50) has the greatest performance, with a Young’s modulus over 2000 MPa and a maximum load capacity of 4000 N. Additionally, porosity decreases from ∼40% in the unfired condition to ∼10% after firing. Microstructural analysis confirms well-connected sintering necks and a cohesive clay–PG network. Radiological assessment indicates radium-equivalent activity of ≈50 Bq/kg for green clay composites, safely below the regulatory limit of 370 Bq/kg, while yellow PG formulations approach 257.6 Bq/kg. A multi-criteria optimization identifies the PG50–G50 (green clay) formulation as optimal, providing a balanced combination of mechanical strength, structural integrity, low porosity, and radiological safety. These findings indicate that fired PG–clay composites represent a viable route for producing high-performance, eco-friendly construction materials, supporting waste valorization, circular economy concepts, and advancing sustainable building practices.