Valorization of Green Clay and Phosphogypsum in Sustainable Building Materials: A Local Solution for a Global Environmental Challenge

The industrial activities of the Phosphate Company in Tunisia generate massive amounts of phosphogypsum (PG), posing environmental challenges. This study investigates the eco-friendly valorization of Tunisian PG with local Gafsa green clay into sustainable construction materials, providing a scalable model for global PG management.Eco-materials were prepared with 50% PG and 50% clay (wt.%) at a 1:1 water-to-binder ratio, then cast without compaction into standard 4x4x16 cm^3 molds and fired at 1200°C. Compressive strength, radiological activity, and microstructural porosity were evaluated before and after firing. Porosity was quantified via optical microscopy and ImageJ software. At the National Center for Nuclear Sciences and Technologies (CNSTN), Ra-226, Th-232, and K-40 were monitored using high-resolution gamma spectrometry (HPGe), while U-238 activity was measured via alpha spectrometry.Firing the clay matrix at 1200°C induced significant structural and radiological changes. Mechanical characterization showed an exceptional consolidation of the material: Young's modulus increased from 81 to 2000 MPa, while the maximum load Fmax surged from 350 to 4000 N. ImageJ microstructural analysis explains this mechanical reinforcement by a advanced sintering process, with total porosity dropping from 27.7 % in the unfired state to 13.7 % after firing. Crucially, HPGe gamma spectrometry highlighted the environmental benefits of this thermal treatment: the radium equivalent index Ra{eq} was reduced from 148.5 Bq/kg to 50.3 Bq/kg. Remaining well below the international safety threshold of 370 Bq/kg, these results prove that high-temperature firing successfully locks radionuclides within the durable, densified clay matrix. This work validates the safety of recycling high-percentage Gafsa PG. Firing at 1200°C immobilizes radionuclides and optimizes durability, providing a global benchmark for PG valorization.