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Leaching behavior of RM-FGD based backfilling grout under cyclic-pressure and wet-dry cycles induced by underground
Guiyong Liu1, Weizheng Liu2, Weihua Lu3
1School of Civil Engineering, Central South University, Changsha, 410075, China.
Abstract:
To investigate the leaching behavior of HMs from recycled industrial solid waste-based materials under cyclic-hydrostatic pressure and wet-dry cycling (C/C) caused by fluctuating groundwater tables in underground applications, a novel C/C environment simulation apparatus was designed. The temporal variations in HM leaching concentrations of red mud-flue gas desulfurization gypsum-based backfilling grout were compared under TCLP, C/C leaching, and constant-hydrostatic pressure leaching at 40, 80, 120, and 160 kPa. The effects of different leaching environments on the microstructure, mineral phase, and chemical characteristics of the grout were examined using SEM, MIP, FTIR, and XRD. Compared with the TCLP, C/C leaching under 160 kPa elevated the leachable concentrations of heavy metals. Specifically, the concentration of Pb rose from 1.6 ppb to 3.5 ppb, Cu from 18.9 ppb to 58.9 ppb, Cr from 25.6 ppb to 59.6 ppb, Cd from 0.43 ppb to 1.44 ppb, Mn from 0.25 ppm to 3.6 ppm, and As from 0.9 ppb to 48.2 ppb. The leaching concentrations of HMs showed strong correlations with those of structural elements (Fe, Na, S, and Si), especially with the Fe-S matrix. Combined with the chemical fractionation results, it indicates that C/C environment remobilizes part of the oxidizable fraction and a small amount of the reducible fraction of HMs. FTIR detected the penetration of leaching agent into the harden grout at a depth of 2 mm under C/C leaching at 160 kPa. MIP results revealed that C/C leaching significantly increased the total porosity and the proportion of macropores, demonstrating severe degradation of the hardened grout microstructure and enhanced leaching agent penetration. XRD results indicated obvious damage to the C(N)-(A)-S-H and AFt phases, while SEM images confirmed a substantial loss of surface compactness and integrity.
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