Abstract
In this study, the durability of phosphogypsum (PG)-based cemented backfill was investigated by drying-wetting cycles to explore deterioration of its strength and the release of impurities. The leachates in this test were composed of deionized water, 5% Na2SO4 solution, 5% NaCl solution, and a range of sulfuric acid solutions with pH values of 1.5, 3, and 5. After drying-wetting cycles, unconfined compressive strength (UCS), visual deterioration, porosity, microstructure and concentrations of phosphate and fluoride in the leachates were measured. The results showed that both saline and acidic solutions could lead to strength reduction of PG-based cemented backfill under different deterioration mechanisms. The mechanical damage of salinity was caused by micro-cracking and degradation of C–S–H. However, the H+ broke the backfill by dissolving hydration products, leaving the conjunctures between PG particles weakened. Furthermore, the environmental impact was investigated by measuring the concentration of phosphate and fluoride in the leachates. In acidic solutions, the release of phosphate and fluoride was greatly enhanced by H+. Compared to the great strength deterioration in saline leachates, the concentration of phosphate and fluoride were similar to that of deionized water, indicating that saline solutions had little impact on the release of hazardous impurities.
Highlights
A large number of cavities can be caused by the extraction of underground phosphate ore resources.Mining and industrial waste are often applied to fill the cavity of mines for the reduction of surface subsidence, increase of resource recovery and minimization of waste pollution [1,2,3,4,5,6]
When a PG-based backfill is refilled into cavities, the hardened structure of the backfill can provide early and cured strength [7,9]
PG-based backfill specimens with a curing age of 120 d were exposed to different solutions
Summary
A large number of cavities can be caused by the extraction of underground phosphate ore resources. Previous studies concentrate on the effect of raw materials (aggregate, binder, and water) on the cemented backfill process. Impurities can be stabilized by cementitious products, it is worth noting that cement-based solidified/stabilized impurities are vulnerable to external physical and chemical degradation processes [24,25]. Changes in the mechanical strength and pollutant contents in leachates were the main research themes. To this end, PG-based backfill specimens with a curing age of 120 d were exposed to different solutions (deionized water, 5% Na2 SO4 solution, 5% NaCl solution, and a range of acidic solutions with pH values of 1.5, 3, and 5). The pH fluctuation and the concentration of phosphate and fluoride in the leachates were measured and analyzed
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