# Effect of Bentonite Addition on the Properties of Fly Ash as a Material for Landfill Sealing Layers
## 1 Introduction  
Sealing layers are one of the most important components of municipal and industrial landfill construction. They provide protection for groundwater and soil against the seepage of harmful substances present in leakage from the waste. Landfill sealing layers can be distinguished into sealing the base and slopes, cover layer, and vertical cutoff walls. It is assumed that each landfill should have sealing barriers. Soils rich in clay minerals are generally used as compacted liner materials. Natural sealing layers should be characterized by tightness, the capability of self-sealing, ease of molding and resistance to filtration deformations [1; 2]. If there is no opportunity to place landfill construction in the area where geological barriers are present, artificial sealing layers should be made of compacted soils (which contain a significant quantity of clayey particles) or of other materials. The most important parameter of materials used for landfill liner construction is its hydraulic conductivity, which is defined as the velocity of flow of water per unit cross-sectional area under a unit hydraulic gradient. Hydraulic conductivity required by common regulations for landfill barriers has to be equal to or less than value of \(1\times 10^{-9}\) m/s [1; 3; 4; 5; 6]. These requirements for landfills of hazardous and non-hazardous waste are given by Council Directive 1999/31/EC [6]. According to Reference [6] hydraulic conductivity for sealing layers in landfill of inert waste should have the value of \(10^{-7}\) m/s or lower. In order to check the suitability of the material for the construction of landfill sealing layers, a series of tests should be carried out to determine chemical, physical, and mechanical properties. The application of natural and other materials should be verified with regard to the technical guidelines and local legal standards [2; 7]. Burning coal in power plants leads to the production of combustion by-products such as fly ash and bottom ash. The most common way of disposing of combustion by-products is landfilling. Test results of properties of fly ash presented in the literature [(8; 9; 10; 11; 12; 13; 14) indicate that it is a material that might be used instead of natural soils in construction engineering. Recycling combustion by-products is a good alternative to disposal and has economic and technical benefits as well. According to References [(15; 16)], unburned carbon presented in fly ash adsorb ions from the leachate and the alkaline nature of the fly ash helps to precipitate ions presented in leakage. [PERSON] et al. [(17)] mentioned that precipitates can block the pores of compacted fly ash. Also, the pozzolanic properties of fly ashes [(18)] affect hydraulic conductivity with time. That is why researchers test fly ash with different additions including lime [(19)], cement [(20)] or bentonite [(21; 22)]. Fly ash is a material where particles are predominately silt size, but it is much easier to compact than cohesive soils. However, bentonite clay is difficult to work in the field and prone to desiccation under dry conditions which leads to an increase in leakage rates [(23)]. The addition of bentonite to soil [(24; 25)], fly ash and fly ash-soil mixtures reduces its hydraulic conductivity as confirmed by test results [(26; 27; 28)]. As mentioned, the most important property in case of sealing layers is hydraulic conductivity of the material, however other properties of fly ash and its mixtures such as compressibility, tensile strength, shear strength are no less important, as presented in the literature [(29; 30)]. The objective of this study was to determine the impact of bentonite addition on the properties of compacted fly ash which is required in the construction of waste landfill liners. In present research, bentonite has been added in various, small and cost-effective percentages to investigate how the amount of the addition affects saturated hydraulic conductivity and other parameters of compacted fly ash.