Abstract
The large-scale accumulation of phosphogypsum (PG) and phosphate marl (YM) remains a major environmental challenge in phosphate-producing regions, while their potential as alternative ceramic raw materials is still insufficiently explored. This study investigates the combined utilization of these two industrial residues for manufacturing ceramic wall tiles through statistical process optimization. A Doehlert experimental design coupled with response surface methodology was employed to evaluate the influence of PG content (0–50 wt.%), firing temperature (900–1100 °C), and liquid-to-solid ratio (0.20–0.60) on bulk density, bending strength, and water absorption. Predictive models with high statistical significance were established and used for multi-response optimization. The optimum formulation consisted of 30 wt.% PG, fired at 1046 °C with a liquid-to-solid ratio of 0.44, producing tiles with a bulk density of 2.11 g ·cm⁻³, water absorption of 5.8%, and bending strength of 15.34 MPa. Based on water absorption, the optimized tiles fall within the BIIa range defined by ISO 13006, although their bending strength remains below the requirement for full compliance with this class. Phase evolution indicated extensive transformation of the initial calcium sulfate-bearing phases and the formation of a ceramic matrix containing mainly diopside and anorthite. Microstructural observations revealed substantial liquid-phase sintering and a relatively dense matrix with limited residual open porosity, partly sealed by a Ca–Mg–silicate glassy phase. This microstructural development accounts for the measured mechanical performance and reduced water uptake. Toxicity Characteristic Leaching Procedure tests showed that the concentrations of the measured elements released from the raw materials remained below the applicable US EPA thresholds under the tested conditions. These findings demonstrate that phosphate marl and phosphogypsum can be jointly converted into ceramic wall-tile bodies at a relatively moderate firing temperature. The proposed approach provides a promising valorization pathway for two abundant phosphate-industry by-products while reducing reliance on conventional ceramic raw materials.