# Effect of Bentonite Addition on the Properties of Fly Ash as a Material for Landfill Sealing Layers
## 2 Materials and Methods  
### Materials  
Hydroraulic conductivity tests were carried out on fly ash (FA) and fly ash and bentonite mixtures (FA+B). Additionally, to provide a comparison, tests on sandy silty clay were conducted. Fly ash originated as a hard coal combustion by-product in Bialystok Heat and Power Station, was obtained from the dry disposal site. Commercially available bentonite powder was added to fly ash in the amount of 5 (FA+15%B), 10 (FA+10%B) and 15% (FA+15%B) by dry weight of the sample. The given percentage of the bentonite represents the mass of dry bentonite per mass of fly ash in a given sample. Cohesive soil properties were also investigated. Grain size distribution curves of tested materials are presented in Figure 1.  
According to the grain size distribution curves, it can be seen that fly ash corresponds to sandy silt saSi [(31)], whereas fly ash with different amount of bentonite addition is as follows: with 5%B to sandy silt (saSi), with 10%B to clayey silt with sand (sacSi), with 15%B to silty clay with sand (sasiCI) and bentonite to clay (Cl). Natural soil, which met requirements for cohesive soils that might be used as liner material [(7; 32)] was sandy silty clay (sasiCI), of which liquid limit (LL) = 39% and plasticity index (PI) = 24%. Scanning electron microscope (SEM) images of the tested materials are shown in Figures 2-4, and its chemical composition is shown in Table 1.  
Figure 1: Grain size distribution curves of tested materials. Figure 2: Scanning electron microscope (SEM) images of bentonite: (**a**) magnification of 540\(\times\), (**b**) magnification of 3700\(\times\). From Figures 2-4, the spherical shape of fly ash particles, the flocculated structure of irregular bentonite particles, and compact aggregates of particles in the case of the fly ash-bentonite mixture can be observed. In the micrograph of the mixture (Figure 4), the bentonite particles stuff and seal the voids between fly ash particles. During the contact with water, bentonite swells, so the pores are smaller. With higher bentonite content in the mixture, there are fewer pores. According to Table 1 and chemical compositions, the tested fly ash is classified as a Class F fly ash by ASTM C618 [33]. The results are summarized in Table 1. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Element** & **FA** & **B** & **Chemical Compound** & **FA** & **B** \\ \hline Si & 22.4\% & 31.1\% & SiO\({}_{2}\) & 43.3\% & 71.6\% \\ Al & 15.8\% & 7.7\% & Al\({}_{2}\)O\({}_{3}\) & 31.8\% & 18.4\% \\ Ca & 0.8\% & 1.9\% & C & 15.8\% & – \\ An & – & 1.5\% & K\({}_{2}\)O & 3.4\% & – \\ Mg & 1.0\% & 1.0\% & Fe\({}_{2}\)O\({}_{3}\) & 1.8\% & – \\ K & 2.5\% & – & MgO & 2.2\% & 2.6\% \\ Fe & 1.9\% & – & CaO & 1.1\% & 3.1\% \\ C & 3.5\% & – & TiO\({}_{2}\) & 0.6\% & – \\ Ti & 0.6\% & – & Na\({}_{2}\)O & – & 4.3\% \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of bentonite (B) and fly ash (FA). Figure 4: SEM images of fly ash-bentonite mixture—magnification of 2400\(\times\). Figure 3: SEM images of fly ash: (**a**) magnification of 1400\(\times\), (**b**) magnification of 3400\(\times\). similar to those presented in Ref [14]. Chemical composition of tested bentonite (Table 1) has higher content of silica and aluminum. Bentonite consists mainly of montmorillonite, which is a clay mineral. Tested bentonite chemical composition is similar to the results presented in Ref [34]. Values of specific soil density, optimum moisture content (OMC), maximum dry density (MDD) and specific surface area \(S_{\text{s}}\) of tested materials are given in Table 2.  
According to Table 2, specific densities of fly ash and fly ash-bentonite mixture are lower than for natural soil--the bentonite addition in a higher amount than 5% caused an increase of specific density. Lower specific gravity and the porous nature of the coal ash particles result in lower maximum dry density and higher optimal moisture content than natural soils.