Role of bentonite in improving the anti-seepage durability of fly ash-based geopolymer cutoff wall backfill under dry–wet cycles

Vertical cutoff walls have been widely employed for groundwater control and in situ containment of subsurface pollutants. The traditional cutoff wall materials are usually highly susceptible to dry–wet cycles due to precipitation, drought, and groundwater fluctuation, leading to cracking or even disintegration. This study adopted sodium bentonite (NaB) to enhance the anti-seepage durability of fly ash-based geopolymer cutoff wall backfill (GCWB) under dry–wet cycles. The influence of bentonite content on the workability, unconfined compressive strength, hydraulic conductivity and durability of GCWB was comprehensively examined. To explore the evolution mechanism of GCWB under dry–wet cycles, the microstructural characteristics were assessed by Mercury Intrusion Porosimetry and Scanning Electron Microscopy tests. Proper incorporation of bentonite can improve the water retention ability and enhance the integrity of matrix. The optimal material ratio with the lowest hydraulic conductivity was NaB-3% which can achieve 4.73 × 10^–11 m/s after cured for 28 days. The hydraulic conductivity of NaB-3% gradually increased to 5.00 × 10^–9 m/s after five cycles, which was 70.7% lower than that devoid of NaB, still meeting the commonly accepted limit (1 × 10^–8 m/s). Geopolymer has great advantage in reducing the hydraulic conductivity before dry–wet cycles; while, the expansion characteristics of bentonite play a dominant role in filling the pores developed during the cycles and refining the internal structure of matrix, greatly improving the durability of GCWB. This study proves that GCWB with NaB is a favorable cutoff wall material, especially in areas where dry–wet cycles may occur.