Evaluation and application of phosphogypsumclay-fired ceramics: optimal formulation for sustainable and high-performance building materials

Introduction: Phosphogypsum (PG), an industrial by-product, accumulates in Tunisia at a rate of 10 million tons annually, threatening soil and groundwater quality through acid infiltration. Converting this waste into fired construction bricks by blending it with local Douiret clay offers a sustainable waste management strategy. However, optimizing mechanical performance to meet rigorous building construction standards remains a critical challenge.

Methods: Five brick formulations with clay content ranging from 10% to 50% were fired at 1140°C. Pore microstructure was quantified using optical microscopy and processed via Otsu automatic thresholding. Mechanical properties, specifically maximum compressive strength (Rc​) and Young’s modulus (E), were determined through uniaxial compression tests.

Results: While porosity varied between 34.4% and 48.1%, mechanical performance peaked exceptionally at a 30% clay content. This specific formulation achieved a compressive strength (Rc​) of 27.2 MPa and a Young’s modulus (E) of 1.29 GPa, performing five to ten times better than other tested blends. With a resistance of 27.2 MPa, this formulation significantly exceeds Tunisian standard construction requirements typically ranging from 5 to 15 MPa for load-bearing bricks, positioning the material as a high-performance structural alternative. The weak correlation between porosity and strength (R^2<0.35) indicates that clay–PG interfacial bonding, rather than void fraction, is the primary driver of this mechanical robustness.

Conclusions: A 30% clay and 70% phosphogypsum ratio emerges as the optimal formulation, delivering mechanical performance that surpasses current industrial standards. These findings provide actionable guidance for addressing Tunisia's phosphogypsum crisis while advancing circular-economy practices within the construction materials sector.