Hydraulic conductivity of the polymer modified bentonite -sand- phosphogypsum (PMB-S-PG) mixture under drying-wetting and freezing-thawing cycles

Abstract

To improve the multi-path and multi-field exploitation of phosphogypsum (PG), a polymer modified bentonite-sand-phosphogypsum mixture is developed for application in anti-seepage of PG slag fields and roadbeds. In this research, the SEM tests, FSI tests, and hydraulic conductivity tests under different conditions are carried out. The PMB has a superior swell index (72 ml/2g) in water, but the PMB-S-PG0.5 mixture’s swell index is low (5.8 ml/2g) in water because the acidic PG. The SEM tests reveal that the PMB-S-PG0.5 mixture forms a dense honeycomb structure which can complicate the seepage channels and maintain the low hydraulic conductivity. Thus, the swell index is not an accurate indicator to hydraulic conductivity. The honeycomb structure in the PMB-S-PG0.5 mixture has elasticity, strength, and isotropy; it can effectively resist the effects of DW and FT cycles. During the DW cycles. The PMB-S-PG0.5 mixture has lower degree of crack development, superior crack self-healing properties and more tortuous seepage channels, resulting in lower hydraulic conductivity (4.14×10− 10 m/s) compared to the RB-S-PG0.5 mixture (6.90×10− 9 m/s) after 9 DW cycles. Due to the finer particles, the lower number of ice lenses, and the more seepage channels in PMB-S-PG0.5 mixture, the degree of ice nucleation in the PMB-S-PG0.5 mixture is lower than in the RB-S-PG0.5 mixture during the FT cycles. The PMB-S-PG0.5 mixture’s hydraulic conductivity was lower (9.72×10− 11 m/s) compared to the RB-S-PG0.5 mixture (3.26×10− 9 m/s) after 9 FT cycles. That is, the PMB-S-PG0.5 mixture is expected to be widely used to enable the resource reuse of PG.