# Effect of Bentonite Addition on the Properties of Fly Ash as a Material for Landfill Sealing Layers
## 3 Results and Discussion  
### Hydraulic Conductivity  
In Figure 7, the test results of hydraulic conductivity of fly ash and its mixtures with bentonite are shown. Charts are plotted as a relationship of flow velocity to hydraulic gradient. As can be observed in Figure 7, the value of flow velocity decreased when the higher load was attached in case of every sample. At a higher hydraulic gradient, the flow velocity increased, as expected and reported for clay and other materials like sand-bentonite mixture in Reference [48]. At the lowest hydraulic gradient, the flow of water is often irregular. It might be caused by the occurrence of an initial gradient \(i_{0}\) = 4-8 which, when exceeded, will result in reliable results of hydraulic conductivity. The initial gradients were not observed in previous fly ash studies in the literature. Relationships presented in Figure 7 can be described by exponential curves according to [PERSON]'s recommendations [43; 44], especially in the case of the FA+15%B mixture, while others follow the linear relationship. According to the presence of the initial gradient, the \(i_{1}\) can be calculated, and its value varies from 12 to 24. Figure 7: Flow velocity at a hydraulic gradient under variable effective stresses for the samples of: (**a**) fly ash, (**b**) fly ash with 5% of bentonite, (**c**) fly ash with 10% of bentonite, and (**d**) fly ash with 15% of bentonite. Figure 8 presents the dependency of hydraulic conductivity on effective stress tested at hydraulic gradient \(i\) = 32. It is a gradient value close to one recommended in constant gradient method \(i\) = 30 [49] for testing materials intended to be used in sealing layers. In the case of samples tested at \(i\) = 32 and \(\sigma_{\mathrm{v}}\)\({}^{\prime}\) = 400 kPa it has been concluded, that 5% of bentonite addition causes a decrease of hydraulic conductivity value almost twofold, 10% also twofold and 15% almost three times, compared to fly ash. Figure 8 shows the dependence of hydraulic conductivity on the value of effective stress and its largest decrease in the case of fly ash, where a decrease in the value of \(k\) from \(4.7\times 10^{-9}\) to \(3.1\times 10^{-9}\) m/s was observed. It can be seen that hydraulic conductivity decreases with increasing bentonite content. The values range from \(3.1\times 10^{-9}\) m/s to \(4.7\times 10^{-9}\) m/s for 0% bentonite, from \(1.7\times 10^{-9}\) m/s to \(2.6\times 10^{-9}\) m/s for 5% bentonite, from \(1.6\times 10^{-9}\) m/s to \(2.2\times 10^{-9}\) m/s for 10% bentonite and from \(1\times 10^{-9}\) m/s to \(1.2\times 10^{-9}\) m/s for 15% of bentonite content in fly ash-bentonite mixtures. Figure 9 presents hydraulic conductivity for bentonite addition contents tested under different effective stresses at hydraulic gradient \(i\) = 32. From the figure, it can be seen that the hydraulic conductivity of fly ash decreased with increasing bentonite content. It also can be observed that the variation in the \(k\) value under load is smaller when the bentonite content is higher in the mixture. As mentioned above, hydraulic conductivity test conditions are indispensable when describing the permeability characteristics of the material. Present results show saturated hydraulic conductivity. The saturation degree of the sample affects the value of hydraulic conductivity. The author conducted the test in the consolidometer for the partially saturated sample, without a former saturation process, at the initial degree of saturation. The volume of water inflow to the partially saturated sample was different from the volume of water outflow from the sample, therefore the flow velocity cannot be reliably assessed. As the water flows, the sample degree of saturation increases, but it is not certain that the back pressure value used to test partially saturated sample is sufficient to fully saturate the sample. It is shown that the condition of full and partial saturation is also important in determining compression and settlement characteristics of the compacted fly ash [50].