The positive role of phosphogypsum in dredged sediment solidified with alkali-activated slag

Generally, high-water-content of dredged sediment (DS) tends to suffer from inferior mechanical properties and obvious shrinkage after solidification, so finding solutions to this issue is helpful for promoting the recovery and recycling of DS. In this paper, in reference to natural gypsum (NG), phosphogypsum (PG) was incorporated into DS solidified with alkali-activated slag (AAS) system. The effect of PG (0 %-20 %) on the hydration process (0-168 h), mechanical properties (3 d, 7 d and 28 d) and autogenous shrinkage (0-7 d) of DS solidified with AAS was investigated. It is found that the addition of PG not only induces the generation of ettringite to compensate for shrinkage, but also accelerates the formation of C-A-S-H by providing active calcium to promote stiffness to resist shrinkage. This results in a reduction of autogenous shrinkage by 74.3 % and an increase of compressive strength by 28.5% when PG dosage is 15%. Compared with NG, the difference in 28d-compressive strength of PG group is not more than 7.34 % under equivalent dosages. The dissolved SO4 2-from PG could be adsorbed on CA-S-H and preserved in pore solution in the form of Na2SO4. The decrease in S/Si from 0.31 to 0.09 indicates stored SO42- could be released back into system to promote the further generation of ettringite. To obtain superior mechanical properties and volume stability, appropriate PG dosage is 10 %-15 %. Compared with the control group, it increases the content of ettringite and amorphous phase by 2.4 %-4.6 % and 3.3 %-3.7 %, respectively. This research not only provides theoretical support for DS solidified with AAS to realize efficient utilization of solid waste resources (i.e., DS, PG and slag), but also gives a new insight into solidification of other high-watercontent system, such as backfill mining, grouting materials and treatment of soft soil foundations.