Durability Analysis of Sludge Solidified with Soda Residue Subjected to Dry-Wet and Freeze-Thaw Cycles

Soda residue (SR), ground-granulated blast-furnace slag (GGBS), and quicklime (QL) were employed for solidifying sewage sludge, which is a technique for sustainable development by transforming industrial solid waste and sludge into engineering fill material such as backfill material in mine or abandoned caverns. The durability of solidified sludge against dry-wet and freeze-thaw cycles was investigated by unconfined compressive strength (UCS), X-ray diffraction, scanning electron microscopy, and nuclear magnetic resonance tests. The results demonstrated that the SR-GGBS-QL solidified sludge had good dry-wet and freeze-thaw durability. In general, UCS increased at first, but then it decreased to some degree with the increase in dry-wet and freeze-thaw cycles. The cycle number for peak UCS depended on the durability test types and SR content. The UCS values after seven dry-wet cycles or ten freeze-thaw cycles were higher than the initial values before cycles. The main products detected in SR-GGBS-QL solidified sludge were ettringite, hydroaluminite, and calcium silicate hydrate (C-S-H). In addition, some pollutants such as copper, arsenic, and chromium were stabilized. The weakening effect on the microstructure of the solidified sludge occurred after dry-wet or freeze-thaw cycles. However, the hydration reaction continued with the increase of dry-wet and freeze-thaw cycles, leading to an increase in the amount of hydration products (especially C-S-H) and compact microstructure. These contributed to high UCS values and good dry-wet and freeze-thaw durability. The strength and failure strain of solidified sludge still met the requirement of filling materials after dry-wet or freeze-thaw cycles.