Hydraulic Conductivity, Microstructure, and Compositional Changes of Sand–Bentonite Backfill in Cutoff Walls Exposed to Organic Acids

Engineering Properties of Novel Vertical Cutoff Wall Backfills Composed of Alkali-Activated Slag, Polymer-Amended Bentonite and Sand

“…18 sieve (1 mm). The sand had a coefficient of uniformity (C u ) of 1.78 and a coefficient of curvature (C c ) of 1.00 [20]. The bentonite used was commercially available conventional sodium-activated calcium bentonite (CB) produced in Jianping, Liaoning Province, China, presented as a yellow-brown powder [20].…”

“…The sand had a coefficient of uniformity (C u ) of 1.78 and a coefficient of curvature (C c ) of 1.00 [20]. The bentonite used was commercially available conventional sodium-activated calcium bentonite (CB) produced in Jianping, Liaoning Province, China, presented as a yellow-brown powder [20]. The particle size of the CB was less than 0.075 mm, with a specific gravity of 2.71, a liquid limit of 335%, a plastic limit of 49%, a plasticity index of 286, a montmorillonite content of 76.2%, and a cation exchange capacity of 61.23 mmol/100 g. According to ASTM D2487 [30], the sand and CB were classified as poorly graded sandy soil (SP) and fat clay (CH), respectively.…”

“…Based on the k values of MSBS and MSBS-PAC backfills cured for 28 and 90 days in Figure 3, PAC amendment reduced the k of the backfill by about one order of magnitude. The structure of pores possesses a significant impact on the hydraulic conductivity of porous media [20,42]. According to Figures 2 and 6, the addition of PAC increased the void ratio, micropores volume, and cumulative porosity of the backfills and decreased the macropores volume in the backfill.…”

“…It is noted that exposure to high-valence metal ions including aluminum, magnesium, and calcium ions imposes a significant influence on the chemical stability of the montmorillonite in bentonite [14,16]. The high-valence metal ions released from MgO-activated GGBS in the backfill would replace the sodium ions initially adsorbed on bentonite particles, decreasing the DDL thickness of bentonite, and resulting in flocculation of bentonite particles and thus increasing the k of the backfill [8,16,20,23]. The addition of PAC to the backfill leads to tight bound of PAC polymer hydrogel to the bentonite particles and therefore forms a protective "thin coating" on the bentonite [28].…”

“…It is noted that the bentonite used in the ordinary MSBS backfill is conventional sodiumactivated calcium bentonite (CB), due to the lack of high-quality natural sodium bentonite (NaB) in China and India [19]. The hydraulic conductivity of ordinary SB backfills consisting of sand and CB dramatically increased when exposed to landfill leachate-impacted groundwater and heavy-metal-impacted groundwater [17,20]. Previous studies indicated that superabsorbent hydrogel additives with high water absorption capacity could reduce the hydraulic conductivity and enhance the pollutant containment of bentonite-based engineered barrier materials [21][22][23].…”