Recycling of phosphogypsum and clay by-products from phosphate mines for sustainable alkali-activated construction materials

This work investigated the recycling the large quantities of phosphogypsum, generated by phosphoric acid plants, by exploring its potential use as a precursor in alkali-activated materials. The alkali-activation involved using phosphogypsum and calcined clays as precursors and an alkaline activator. The compressive strength of elaborated materials was evaluated after 28 days of curing. Additionally, various techniques were used to characterize the diverse samples. The results demonstrated the effectiveness of the alkaline activation process to induce the formation of Calcium Aluminum Silicate Hydrate and Sodium Aluminum Silicate Hydrate gels in the elaborated samples. The samples with 22.4% phosphogypsum, 11.28 M NaOH, and a liquid/solid ratio of 0.86 presented the maximum compressive strength of 27.3 MPa as they presented a more dense and less porous structure. The Toxicity Characteristic Leaching Procedure test showed no release of harmful substances. These findings suggest that phosphogypsum could be recycled in alkali-activation technology in the construction industry.