(573f) Valorizing Phosphogypsum As a Set Retarder in Portland Cement: Investigation on Its Heavy Metals Release into Buffered Aqueous Media

Phosphogypsum is a by-product in phosphate fertilizers and phosphoric acid industries. It is valorized as a set retarder in Portland cement replacing the gypsum rocks. In this industry, influence of phosphogypsum contents on concrete and Portland cement properties was investigated. Phosphogypsum was mixed with Portland cement at different mass contents extended from 1% to 20% wt/wt. Setting time, specific area of Portland cement and response to compression strength were followed. Chemical characteristics and heavy metals release into buffered acidic and alkaline aqueous solutions were also investigated. Obtained results showed that less than 8% of phosphogypsum is the best mass content to enhance cement quality without altering significantly the mechanical properties. The highest compressive strength applied to concrete was recorded after 28 days under saturated humidity conditioning. Conversely, the increase of phosphogypsum content in cement over than 8% resulted in a decrease of mechanical strengths of the corresponding concrete. Metal leaching experiments were conducted on different mixture of concrete mortars immersed in buffered acid solutions. Leaching results have showed that highest concentration of chromium, zinc, copper and lead are transferred to aqueous media during the first three days of impregnation. Zinc has a particular behavior. It was entrapped in concrete mortars before being transferred into aqueous solutions. In addition, metal release was significantly enhanced when phosphogypsum contents increased in the cement