Synergistic solidification/stabilization mechanism of cadmium in phosphogypsum slag-based cementitious material

Abstract

The phosphogypsum slag-based cementitious material (PS), a type of low-carbon cementitious materials, showed its potential in solidifying heavy metals. This paper is aimed to investigate the efficiency of PS in immobilising heavy metal cadmium (Cd) and explore the solidification mechanism. In the study, the PS specimens doped with different Cd(NO3)2 content (0–1.0% by weight) were prepared and the solidification efficiency was determined by leaching test. Moreover, the mechanism of solidification of Cd in PS were explored by characterising hydration behaviour of Cd-substituted PS by XRD, TGA, NMR, FTIR and SEM-EDS. The results showed that the best performance was observed in Cd mixed PS with 50% slag, 45% phosphogypsum and 5% lime, which showed a compressive strength of 37.3 MPa and a metal leaching concentration of 0.134 µg/L. The ettringite and C-S-H were observed as the main hydration products of PS system. Moreover, it was found that Cd was bound to the phases of sulfoaluminate and silicate and substituted to Ca2+ in the crystal structure or Cd(OH)2 being trapped by C-S-H, the substitution and trapping of which lead to a shortening of the average chain length of C-S-H and delay the hydration process of the Cd-doped PS system.