Hydraulic conductivity of CFBC fly ash-based geopolymer: Toward sustainable liner materials

The availability of natural soils with hydraulic conductivity below 1 × 10 − 9 m/s for liner systems is declining, while industrial waste such as circulating fluidized bed combustion (CFBC) fly ash remains underutilized. Conventional geosynthetic clay liners (GCL) are also prone to degradation in aggressive environments, particularly acid mine drainage (AMD). This study explores the valorization of CFBC fly ash as a single precursor in geopolymer paste production, focusing on hydraulic conductivity and initial AMD interaction for sustainable liner applications. CFBC fly ash contents ranging from 55 to 75% by total geopolymer weight were investigated, and their physical, mechanical, and hydraulic properties were evaluated. The mixture containing 60% CFBC fly ash exhibited optimal performance, achieving a compressive strength of 12.3 MPa, an ultrasonic pulse velocity of 2163 m/s, and a hydraulic conductivity of 3.8 × 10 − 11 m/s . This formulation was further tested by immersion in AMD (pH 2.65) for 28 days with 7-day replacement cycles, showing no visible deterioration and only minor changes in the Si–O–T bond, as confirmed by FTIR. These results highlight the potential of CFBC fly ash-based geopolymers as sustainable alternatives to natural soil and GCL, combining low hydraulic conductivity with preliminary stability under acidic exposure.