Physico-mechanical, microstructural and thermal properties of foamed ceramics based on Algerian phosphate wastes

Phosphate mining activities generate large quantities of waste rock during the treatment process each year, posing significant environmental challenges. This study explores the valorisation of phosphate dry waste through phosphoric acid activation (10 M) to develop geopolymers. Five formulations were prepared by incorporating 0 to 70 wt.% phosphate dry waste into a metakaolin precursor, followed by curing for 28 days and sintering at 900 °C for 2 h to produce ceramic foams. After acid activation, quartz, muscovite, orthoclase, berlinite (AlPO4), and monetite (CaHPO4) phases are the main crystalline phases detected by XRD analysis. After the sintering process, monetite phase disappeared while quartz and berlinite persisted. Besides, new calcium-phosphate and magnesium-phosphate phases appeared. The addition of phosphate waste significantly influenced the microstructure, the crystalline phases and the properties of foamed ceramics. This involves a marked increase in total porosity from 34% to 65% leading to the decrease of compressive strength giving value of 2.06 MPa. Thermal conductivity of the samples incorporating 70 wt.% of phosphate waste obtained value of 0.09 W/m·K with 50% and 0.92 g/cm³ of water absorption and bulk density respectively. Importantly, the heavy metals leaching test shows that the porous ceramic samples have a leaching value lower than the international recommendations (USEPA). These findings indicate that incorporating phosphate dry waste offers a promising self-foamed geopolymers based foamed ceramics via acid activation with low-cost, environmentally friendly thermal insulation materials for sustainable construction.