Exploring potential of fly ash⇓bentonite mix as a liner material in waste containment systems under concept of sponge city

Many studies have been conducted to evaluate fly ash as an additive for the modification of hydraulic properties of soil. However, rarely are studies carried out that quantify physiochemical, strength, and hydraulic properties of fly ash–bentonite mix. Fly ash–bentonite mix may have the potential to be used as a landfill liner material instead of conventional sand in sand-bentonite landfills. Also, with the introduction of the new concept of sponge city, one of the aims of which is to minimize contamination and maximize the use of waste material, the exploration of fly ash–bentonite mix as a cover material in retention systems is highly crucial. The main objective of this study is to explore physicochemical, strength, and hydraulic properties as well as sorption characteristics of fly ash–bentonite mix. The sorption characteristics of the fly ash–bentonite mix for two heavy metals, lead ions (Pb⁺²) and nickel ions (Ni⁺²), were investigated. A criterion based on cation exchange capacity (CEC) and specific surface area (SSA) has been proposed. The study also developed correlations for estimating the CEC of soils with SSA and other soil parameters. Hydraulic conductivity decreases approximately four orders of magnitude with an increase in 30 % of bentonite. Unconfined compressive strength (UCS) is found to increase first (up to 40 % fly ash) and then reduces with further increase in fly ash. Furthermore, the curing time is also found to affect UCS significantly. The findings stated that the mix of 70 % or lower fly ash content (in fly ash–bentonite mix) is able to meet standard requirements for hydraulic conductivity and UCS. The Langmuir isotherm is found to better represent sorption behavior of fly ash–expansive soil, with nickel and lead as model contaminants. All the mixes considered in this study exhibited maximum uptake of Pb⁺². Sorption decreases very mildly for mixes that have some expansive soil content, whereas it reduces drastically for only fly ash content.