Phosphogypsum as a component of calcium sulfoaluminate cement: Hazardous elements immobilization, radioactivity and performances

The CaSO4 center dot 2H(2)O content in phosphogypsum (PG) exceeds 90%, which renders it suitable to be used as a component of calcium sulfoaluminate (CSA) cement. In this study, PG was utilized as a component of CSA cement. The immobilization of soluble phosphorus (s-P2O5) and heavy metals in PG, the radioactivity and the performances of CSA cement were studied. Besides, a method to eliminate the adverse effect of s-P2O5 on CSA cement performances was proposed. Results demonstrate that the s-P2O5 and heavy metals can be effectively immobilized and the radionuclides content is within the acceptable level. The PO43- combines the Al3+ in the hydration system and forms insoluble AlPO4. It consumes Al3+ and lowers down the Al3+ concentration of the hydration system, which thereby lowers down the driving force for the precipitation of ettringite. Besides, due to the formation of AlPO4, the dissolution and hydration of C(4)A(3)(S) over bar is inhibited. The s-P2O5 obviously prolongs the setting time, lowers down the early compressive strength and leads to a relatively higher expansion rate. By the pretreatment with Ca(OH)(2) and Al-2(SO4)(3), the acidity of PG can be neutralized and the s-P2O5 can be effectively immobilized, which thereby eliminates the adverse effect of s-P2O5 on C(4)A(3)(S) over bar hydration and the performances of CSA cement. Therefore, PG can substitute dihydrate gypsum (DG) to be used as a component of CSA cement. (c) 2019 Elsevier Ltd. All rights reserved.