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Generated hazardous or toxic waste posses a serious threat if dumped into ponds or low lying areas which leads to contamination, this necessitates the effective landfill liner system. Mainly compacted clayey soils are used as an engineered barrier. Recently, composite materials have gained popularity as landfill liner materials, including the use of waste materials amended with low permeable soils. Though, studies on the composite optimum mix and its corresponding thickness are very scarce. Here, we evaluated the unconfined compressive strength and hydraulic conductivity of fly ash-bentonite composites. Efforts were also made to determine the thickness of landfill liner composite using a finite difference method (i.e. MATLAB). The results reveal that composite consists of 30% bentonite and 70% fly ash is suitable for landfill liner, which meets strength and permeability criteria. Numerical simulation for five major contaminants shows that the composite plays a crucial role in reducing the leaching of heavy metals and suggests an optimum thickness in the range of 126-154 cm. Overall, the findings of the study indicate that fly ash-bentonite composite can be used to solve real-life challenges in a sustainable way. Landfill liner is a low-permeable barrier, which acts as a partition between the waste and its surrounding environment. Well constructed landfills are more secure than open dumping 1. The fundamental factor influencing the nature of compacted clay liners is their low permeabilities, which should be as less as 1.0 × 10-7 cm/s suggested by RCRA (Subtitle D). To avoid contamination of groundwater (due to permeation of leachate), generally clayey soils are compacted to achieve desired permeability. Sand-bentonite composites are also used as engineered barriers or liners to prevent leaching of contaminants 2. Other than hydraulic conductivity, strength also needs to be considered in assessing landfill liner material. Earlier studies and environmental guidelines proposed a minimum unconfined compressive strength (UCS) of 0.20 MPa to utilize as a landfill liner material 3. Few studies reported that waste-bentonite composites satisfy the strength criteria when a correct mix proportion used 3,4. The utilization of wastes like fly ash solves the problems associated with waste management and also saves the extensive exploitation of natural materials 5,6. Fly ash and bentonite can replace the sand-bentonite liners because of lack of available natural sand and an increase in the cost, which is widely used in other construction activities. Fly ash is known for its pozzolanic reactions, and that solidifies and gains strength over time when cured. Therefore, the curing period prompts an increase in strength and reduction of hydraulic conductivity. Recent studies have examined the amendment of fly ash to the soil, found the enhanced geotechnical properties such as cation exchange capacity, shear strength, and hydraulic conductivity 3,7,8. Problems in landfill lining system of a waste containment facilities can be escalate...

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.

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