Fluoride immobilization and release in cemented PG backfill and its influence on the environment

Waste recycling must consider secondary pollution, which is affected by recycling methods. Cemented phosphogypsum (PG) backfill is a cost-effective method for PG recycling. However, due to dynamic environmental conditions, the impurity fluoride is challenging to stabilize. In this study, we investigated the immobilization and release of fluoride and its influence on backfill strength. The results showed that the fluoride was temporarily immobilized by PG. However, when the binder was mixed with PG to make the backfill, immobilized fluoride was re-released into the backfill slurry due to the increased pH caused by binder hydration. Therefore, simply converting fluoride into CaF precipitation cannot avoid the risk of fluoride exceeding the Chinese standard (GB8978-1996) (10 mg/L). Furthermore, fluoride deteriorated strength development by inhibiting binder hydration and weakening the backfill structure. The fluoride content in the slurry, rather than in PG, directly affected the backfill strength. Considering the recycling of PG as aggregate for backfill, fluoride should be removed in advance or immobilized in other low-solubility forms instead of CaF precipitation. These results were of great significance for the large-scale resource recycling and safety management of PG.