A sustainable landfill liner material: clay-fly ash geopolymers

Geopolymers offer a number of benefits, including high sorption capacity, sufficient durability, and substantial mechanical strength as well as low CO2 emission and limited drying shrinkage, which may make them sustainable candidates to be utilized as landfill liner materials. Hence, this research is aimed at evaluating how a clay-fly ash geopolymer can meet the requirements proposed for mineral liners so as to compensate for the scarcity of suitable local clay. In this study, clay-fly ash geopolymers are synthesized from the mixtures containing 60% fly ash to the total solid mass and then activated by 10 M NaOH solutions. Several experiments are conducted to assess the mechanical strength, permeability, durability, and sorption capacity of the proposed liner material. Results depict that geopolymerzation has led to a prominent alteration in clay structure, contributing to a non-plastic soil with lower swelling potential and desiccation cracking probability. Proven to enhance sorption capacity and resist freeze-thaw cycles, the clay-fly ash geopolymers are shown to satisfy the criteria of volume shrinkage < 4%, permeability ≤ 1×10−7 cm/s, unconfined compressive strength > 200 kPa, and plasticity index < 7–10. Microstructural analyses also corroborate that the binding agents result in the formation of coagulated particles covered by aluminosilicate gels, thus rendering a more sustainable material. Additionally, the sorption capacity of the clay-fly ash geopolymers exposed to freeze-thaw cycles also shows comparable values to the unexposed specimens.