Hydraulic and Swell–Shrink Characteristics of Clay and Recycled Zeolite Mixtures for Liner Construction in Sustainable Waste Landfill

This paper presents research considering hydraulic as well as swelling and shrinkage characteristics of potential recycled fine particle materials for compacted clay liner for sustainable landfills. Five locally available clay soils mixed with 10% (by mass) of NaP1 recycled zeolite were tested. The performed analysis was based on determined plasticity, cation exchange capacity, coefficient of saturated hydraulic conductivity after compaction, several shrinkage and swelling characteristics as well as, finally, saturated hydraulic conductivity after three cycles of drying and rewetting of tested specimens and the reference samples. The obtained results showed that addition of zeolite to clay soils allowed reduction in their saturated hydraulic conductivity to meet the required threshold (≤1 × 10−9 m/s) of sealing capabilities for compacted clay liner. On the other hand, an increase in plasticity, swelling, and in several cases in shrinkage, of the clay–zeolite mixture was observed. Finally, none of the tested mixtures was able to sustain its sealing capabilities after three cycles of drying and rewetting. Thus, the studied clayey soils mixed with sustainable recycled zeolite were assessed as promising materials for compacted liner construction. However, the liner should be operated carefully to avoid extensive dissication and cracking.


Introduction
Sustainable landfilling of residual waste (after recycling, reuse, and other manners of volume reduction) is the final step of the waste management disposal chain (e.g., [1][2][3]). Allen [4] defined sustainable landfilling as a concept of the safe disposal and subsequent degradation of waste within a landfill, with limited pressure on the environment and by the most financially efficient method. Sustainability of landfilling may be studied in most circles of sustainability but the environmental impacts of landfill (limiting or preventing the direct and indirect emissions) may be considered as crucial [5]. Thus, the sustainable landfill should pose zero (or minimal) risk to the natural environment during its operation and long after the closure of the landfill by preventing the pollutants being transported outside by applying the concept of landfill isolation. Natural earthen materials with very low permeability, supported by geosyntetic liners, geomembranes, geotextiles, etc. are commonly used in developed countries for top and bottom isolation liners, preventing water infiltration and leachate percolation through the top and bottom of the landfill. However, the application of sophisticated sealing materials is often limited in developing countries of low-and medium-income [1,3,[6][7][8][9]. The statutory requirements determining saturated hydraulic conductivity of mineral sealing of municipal waste landfill are similar in numerous countries, e.g., in the EU, Poland, Germany, and the USA the require that properties of soils, including their hydraulic conductivity. Moreover, it should also be clearly underlined that, according to several sources [55,56], mixtures of expansive clayey soils with different dozes of various zeolites may present higher expansiveness than the original specimens, which is in contrast to reports describing reduction in swelling characteristics due to dispersive clays stabilization by fine particle materials e.g., cement, fly ash, or stone dust [57,58] or some natural zeolites [51,[59][60][61].
This paper presents a study concerning the influence of NaP1 zeolite admixture on hydraulic as well as swelling and shrinkage characteristics of five clay soils locally available in the Eastern part of Poland, possible materials allowing construction of compacted clay liners for sustainable landfilling. The used zeolite was a sustainable material, produced locally from coal-burning fly ash. The produced clay-zeolite mixtures were tested according to the main hydraulic determinants of liner durability and sustainability, including their plasticity, saturated hydraulic conductivity after compaction, swelling and shrinkage characteristics, and saturated hydraulic conductivity after several cycles of drying and rewetting. In our opinion the obtained results may contribute to the actual knowledge considering the application of various clay substrates mixed with the sustainable waste materials, of different composition and plasticity, as locally available materials for compacted clay liners of sustainable landfills.

Materials and Methods
The performed research assessing the applicability of liners utilizing the compacted local clay soils mixed with the recycled zeolite material focused on the hydraulic conductivity after compaction with standard Proctor effort, swell and shrinkage characteristics, as well as the hydraulic conductivity of the tested specimens after three subsequent cycles of drying and wetting.

Specimens
The presented studies concerning hydraulic and swell-shrink properties of locally available clayey soils, possible to use as the compacted clay liners, mixed with NaP1 zeolite were conducted for specimens sampled in Gawlowka (G), Jelen (J), Lazek (L), Pawlow (P), and Zgorznica (Z), all located in Lublin Upland, the south-eastern part of Poland. Figure  1 presents samples of the tested clayey substrates and NaP1 zeolite. The specimens were sampled from the open pits of clay mines, from a depth up to approx. 1 m below ground level. Before mixing with zeolite and further testing, the clayey specimens were grounded to particle size lower than 5 mm. As the reference materials, samples of studied clays without admixture of zeolite were used (marked as G, J, L, P, and Z, according to sampling location). The locally produced NaP1 zeolite [62] was used as addition for improving the sealing capabilities of the sampled clays. The studied NaP1 zeolite was obtained using the patented installation for synthesis of zeolites [63] in the hydrothermal reaction of fly ash from hard coal combustion with sodium hydroxide (NaOH). Fly ash from the Jaworzno III Power Plant in Poland was used for zeolite synthesis. The hydrothermal synthesis was conducted using 20 kg of fly ash, 90 dm 3 3.3 M NaOH aq (12 kg of NaOH solid in 90 dm 3 of H 2 O). The synthesis lasted for 24 h at 80 • C [64]. Morphological characterization of zeolite NaP1 was performed using a Scanning Electron Microscope (SEM) Quanta 250 FEG by FEI, Hilsboro, OR, USA. Particle size analysis of the obtained zeolite was performed by a Mastersizer 3000 apparatus HYDRO EV add-on (Malvern Panalytical, Malvern, UK) using the laser diffraction technique with a measurement range of 0.01 µm to 2 mm. Chemical composition was determined using the semi-quantitative energy dispersive X-ray fluorescence (ED-XRF) method on Epsilon 3 (Panalytical, Eindhoven, The Netherlands) apparatus. The mineral phase composition of NaP1 zeolite was determined using the X-ray diffraction powder method (XRD) using a X'pert MPD X-ray diffractometer (Panalytical, Eindhoven, Netherlands) with a goniometer PW 3020 and X-ray source anode Cu (Kα) and a graphite monochromator. diffractometer (Panalytical, Eindhoven, Netherlands) with a goniometer PW 3020 and Xray source anode Cu (Kα) and a graphite monochromator.
The studied clay specimens were mixed with 10% (by mass) of the applied zeolite after drying both materials at 105 °C for 24 h [32,39,65]. The obtained specimens were labeled during the research as G + 10% NaP1, J + 10% NaP1, L + 10% NaP1, P + 10% NaP1, and Z + 10% NaP1, again in relation to sampling locations and zeolite admixture. The particle size distribution of the studied clay soils specimens was determined according to the PN-B-04481:1988 [66] and ASTM C136/C136M-19 standards [67] while the solid particle density was measured in a Le Chatelier flask by means of an air pycnometer, produced by Eijkelkamp, Giesbeek, The Netherlands. Qualitative mineralogical composition of the tested clay materials was determined by the X-ray diffraction (XRD) method using X'Pert APDm, by Panalytical, AlmeoCity, The Netherlands.
The Atterberg limits of the tested clay-zeolite mixtures and reference clay specimens were determined through standard procedures [68], while the plasticity index was calculated as the difference between the liquid and plastic limits. The gravimetric water content was obtained with the standard weight method [69]. The coefficient of saturated hydraulic conductivity of the tested materials under their natural conditions was measured in situ by the falling head field permeameter for fine-grained soils GeoN, Geo Nordic, Stockholm, Sweden. The coefficient of saturated hydraulic conductivity of the applied zeolite was determined under laboratory conditions by the falling head laboratory permeameter HM-5891A by Humboldt Mfg. Co., Elgin, IL, USA.
The cation exchange capacity (CEC) was calculated by summing the values of the sum of base exchangeable cations (BEC) and hydrolytic acidity (HA). Both these parameters were determined by the modified Kappen method. To determine BEC, the air-dried soil samples were treated with 0.1 mol/dm 3 HCl solution (w:v ratio of 1:5). To determine HA, the soil samples were treated with 1.0 mol/dm 3 CH3COONa (in the ratio w:v of 1:10). All the suspensions were shaken in the magnetic shaker for a half hour, then filtrated. The filtrates were titrated with 0.1 mol/dm 3 NaOH solution in a presence of phenolphthalein. Before determining BEC, the presence of carbonates in the samples was checked in the Scheibler apparatus.

Compaction and Hydraulic Conductivity Tests
The tested clay-zeolite mixtures and reference clay samples were compacted by the standard Proctor test with a 24.5 N rammer and a compactive effort of 600 kJ/m according to the PN-B-04481:1988 [66] and ASTM D698-12e2 [70] standards. The measurements were performed in the H-4145 Humboldt Mfg. Co., Elgin, IL, USA falling head permeameters [71] for compacted soils. The compaction was performed for several values of molding The studied clay specimens were mixed with 10% (by mass) of the applied zeolite after drying both materials at 105 • C for 24 h [32,39,65]. The obtained specimens were labeled during the research as G + 10% NaP1, J + 10% NaP1, L + 10% NaP1, P + 10% NaP1, and Z + 10% NaP1, again in relation to sampling locations and zeolite admixture.
The particle size distribution of the studied clay soils specimens was determined according to the PN-B-04481:1988 [66] and ASTM C136/C136M-19 standards [67] while the solid particle density was measured in a Le Chatelier flask by means of an air pycnometer, produced by Eijkelkamp, Giesbeek, The Netherlands. Qualitative mineralogical composition of the tested clay materials was determined by the X-ray diffraction (XRD) method using X'Pert APDm, by Panalytical, AlmeoCity, The Netherlands.
The Atterberg limits of the tested clay-zeolite mixtures and reference clay specimens were determined through standard procedures [68], while the plasticity index was calculated as the difference between the liquid and plastic limits. The gravimetric water content was obtained with the standard weight method [69]. The coefficient of saturated hydraulic conductivity of the tested materials under their natural conditions was measured in situ by the falling head field permeameter for fine-grained soils GeoN, Geo Nordic, Stockholm, Sweden. The coefficient of saturated hydraulic conductivity of the applied zeolite was determined under laboratory conditions by the falling head laboratory permeameter HM-5891A by Humboldt Mfg. Co., Elgin, IL, USA.
The cation exchange capacity (CEC) was calculated by summing the values of the sum of base exchangeable cations (BEC) and hydrolytic acidity (HA). Both these parameters were determined by the modified Kappen method. To determine BEC, the air-dried soil samples were treated with 0.1 mol/dm 3 HCl solution (w:v ratio of 1:5). To determine HA, the soil samples were treated with 1.0 mol/dm 3 CH 3 COONa (in the ratio w:v of 1:10). All the suspensions were shaken in the magnetic shaker for a half hour, then filtrated. The filtrates were titrated with 0.1 mol/dm 3 NaOH solution in a presence of phenolphthalein. Before determining BEC, the presence of carbonates in the samples was checked in the Scheibler apparatus.

Compaction and Hydraulic Conductivity Tests
The tested clay-zeolite mixtures and reference clay samples were compacted by the standard Proctor test with a 24.5 N rammer and a compactive effort of 600 kJ/m according to the PN-B-04481:1988 [66] and ASTM D698-12e2 [70] standards. The measurements were performed in the H-4145 Humboldt Mfg. Co., Elgin, IL, USA falling head permeameters [71] for compacted soils. The compaction was performed for several values of molding water content, from both wet and dry sides of the Proctor curve, selected according to results of the previous research [18]. Three molds were formed for the each applied initial water content. Figure 2 presents tested samples before compaction and after compaction inside the H-4145 permeameter mold. The samples prepared for molding gravimetric water content determination are also presented in Figure 2. Distilled water was used as a permeating liquid, based on numerous studies concerning applicability of various mineral materials for compacted liner construction [18,20,22,[24][25][26][27]29,[32][33][34].
Sustainability 2021, 13, 7301 5 of 21 water content, from both wet and dry sides of the Proctor curve, selected according to results of the previous research [18]. Three molds were formed for the each applied initial water content. Figure 2 presents tested samples before compaction and after compaction inside the H-4145 permeameter mold. The samples prepared for molding gravimetric water content determination are also presented in Figure 2. Distilled water was used as a permeating liquid, based on numerous studies concerning applicability of various mineral materials for compacted liner construction [18,20,22,[24][25][26][27]29,[32][33][34]. The obtained values of saturated hydraulic conductivity coefficients were subjected to the standard statistical procedures including Shapiro-Wilk normality tests and comparison of two dependent variables (Ks for clay-zeolite mixtures and reference samples) by nonparametric Wilcoxon test (due to observed distributions being different than normal).

Swell-Shrinkage Characteristics and Hydraulic Conductivity after Drying-Wetting Cycles
After the tests of saturated hydraulic conductivity, the swelling of all tested samples was measured directly inside the permeameters. The height of the sample for swelling determination was measured using a vernier caliper at 10 regularly distributed locations for each sample.
The percentage swelling index, SI (%), was calculated according to the following formula [18]: where: hs-height of the swelled sample (m); hi-initial height of the specimen before saturation (m). Next, to determine shrinkage characteristics, according to Peng et al. [72], Dörner et al. [40], Gerbhardt et al. [38], and Widomski et al. [18], the tested specimens were sampled in standard 100 cm 3 steel cylinders and dried in the oven at 105 °C. The dimensions of the studied cylindrical samples after shrinkage were measured by means of a vernier caliper, The obtained values of saturated hydraulic conductivity coefficients were subjected to the standard statistical procedures including Shapiro-Wilk normality tests and comparison of two dependent variables (K s for clay-zeolite mixtures and reference samples) by nonparametric Wilcoxon test (due to observed distributions being different than normal).

Swell-Shrinkage Characteristics and Hydraulic Conductivity after Drying-Wetting Cycles
After the tests of saturated hydraulic conductivity, the swelling of all tested samples was measured directly inside the permeameters. The height of the sample for swelling determination was measured using a vernier caliper at 10 regularly distributed locations for each sample.
The percentage swelling index, SI (%), was calculated according to the following formula [18]: where: h s -height of the swelled sample (m); h i -initial height of the specimen before saturation (m). Next, to determine shrinkage characteristics, according to Peng et al. [72], Dörner et al. [40], Gerbhardt et al. [38], and Widomski et al. [18], the tested specimens were sampled in standard 100 cm 3  were used to calculate two dimensionless shrinkage indicators, the geometry factor r s and the coefficient of linear extensibility COLE [73,74], according to the following formulas: where r s -dimensionless geometry factor; V d , z d -dry specimen volume (m 3 ) and height (m); V s , z s -saturated specimen volume (m 3 ) and height (m), where COLE-dimensionless coefficient of linear extensibility; V d -dry specimen volume (m 3 ); V s -saturated specimen volume (m 3 ).
The threshold values for r s and COLE factors allowing assessment of various types of deformation and shrinkage potential are presented after [38,75] in Table 1. The range of swelling and shrinkage potentials for each tested specimen (clay-zeolite mixtures and reference samples) and for the applied values of forming water contents was also determined as the differences between the dry bulk density after compaction and the dry bulk density measured after swelling and shrinkage [18,76,77].
Finally, to assess the long-term performance of tested specimens, under the unfavorable conditions of direct exposure of compacted clay liner to the atmospheric conditions, the coefficient of saturated hydraulic conductivity of the tested materials was determined after three consequent cycles of drying and rewetting. The experiment was performed on specimens sampled in the standard 100 cm 3 steel cylinders, one cylinder per one compaction mold. All the tested samples were air dried at room temperature, 20 ± 2 • C and slowly rewetted by capillary rise. After each drying-rewetting cycle, the coefficient of saturated hydraulic conductivity was determined for each sample using the falling head method in the laboratory parameter by IMUZ, Lublin, Poland.
After measurements, the obtained values of coefficients of saturated hydraulic conductivity for specimens after compaction and after three subsequent cycles of drying and rewetting were subjected to statistical analyses based on the standard methods of normality and nonparametric one-way ANOVA Kruskall-Wallis test (due to distributions being different than normal).

Basic Characteristics
The basic characteristics of NaP1 zeolite are presented in Figure 3. The particles of diameter 20-50 µm constitute approx. 54% of all particles. Two other fractions, 50-100 and 2-20 µm, present shares of 18 and 13%, respectively. The main chemical components of NaP1 are SiO 2 and Al 2 O 3 creating aluminosilicate framework of zeolite, the remaining important components identified are CaO and Na 2 O. The presence of the Na-P1 zeolite phase was confirmed by XRD analysis. The strongest d hkl reflexes for this phase were observed on the diffractogram of the tested sample ( Figure 3c). Additionally, Figure 3d Sustainability 2021, 13 was confirmed by XRD analysis. The strongest dhkl reflexes for this phase were observed on the diffractogram of the tested sample ( Figure 3c). Additionally, Figure 3d shows  The basic characteristics of the sampled clayey soils are presented in Table 2, while the determined Atterberg limits for all tested materials and reference samples are presented in Table 3. To better understand the influence of zeolite addition to the locally sampled clayey soils on their plasticity and swelling potential, the plasticity chart of all tested materials is presented in Figure 4. According to the plasticity chart, all the tested materials (the tested clay-zeolite mixtures and reference samples) may be recognized as clays of different plasticity and swelling potential. Specimens sampled in Gawlowka (G, G + 10% NaP1) and Jelen (J, J + 10% NaP1) presented low plasticity and potential swell, samples from Zgorznica (Z and Z + 10% NaP1) medium plasticity and potential swell, while materials obtained in Lazek (L, L + 10% NaP1) and Pawlow (P, P + 10% NaP1) were recognized as high plasticity clays of high swelling potential. As is visible in Table 3 and Figure  4, addition of 10% by mass zeolite to clayey specimens increased value of liquid and plastic limits and in some cases, as a result, reduced the plasticity index. Similar results were observed by Okeke et al. [25] for lateritic soils stabilized by various percentages of lime addition.  The basic characteristics of the sampled clayey soils are presented in Table 2, while the determined Atterberg limits for all tested materials and reference samples are presented in Table 3. To better understand the influence of zeolite addition to the locally sampled clayey soils on their plasticity and swelling potential, the plasticity chart of all tested materials is presented in Figure 4. According to the plasticity chart, all the tested materials (the tested clay-zeolite mixtures and reference samples) may be recognized as clays of different plasticity and swelling potential. Specimens sampled in Gawlowka (G, G + 10% NaP1) and Jelen (J, J + 10% NaP1) presented low plasticity and potential swell, samples from Zgorznica (Z and Z + 10% NaP1) medium plasticity and potential swell, while materials obtained in Lazek (L, L + 10% NaP1) and Pawlow (P, P + 10% NaP1) were recognized as high plasticity clays of high swelling potential. As is visible in Table 3 and Figure 4, addition of 10% by mass zeolite to clayey specimens increased value of liquid and plastic limits and in some cases, as a result, reduced the plasticity index. Similar results were observed by Okeke et al. [25] for lateritic soils stabilized by various percentages of lime addition.  5.81 × 10 −10 ± 9.9 × 10 −11 * (K + Ch) and (I + S) mean (kaolinite and chlorites) and (illites and smectities). 5.81 × 10 −10 ± 9.9 × 10 −11 * (K + Ch) and (I + S) mean (kaolinite and chlorites) and (illites and smectities).   Table 4 presents determined values of CEC for all tested samples. The CEC value determined for the tested NaP1 zeolite was equal to 104.65 ± 5.21 cmol(+)/kg. Admixture of NaP1 zeolite to the studied clayey substrates clearly resulted in increased cation exchange capacity of the obtained mixtures. The determined increase in CEC values over those determined for the reference samples started from 20% for silty clay materials sampled in Lazek to the level of 226% for sandy loam from Gawlowka. The increased cation exchange capacity may result in higher sorptive characteristics of the studied sustainable materials for waste landfill compacted liner, allowing grater interception of various chemical pollutants present in landfill leachate. The obtained results of CEC determination are in agreement with the previous observations, reported by e.g., Gholizadeh-Sarabi et al. [49].  Table 4 presents determined values of CEC for all tested samples. The CEC value determined for the tested NaP1 zeolite was equal to 104.65 ± 5.21 cmol(+)/kg. Admixture of NaP1 zeolite to the studied clayey substrates clearly resulted in increased cation exchange capacity of the obtained mixtures. The determined increase in CEC values over those determined for the reference samples started from 20% for silty clay materials sampled in Lazek to the level of 226% for sandy loam from Gawlowka. The increased cation exchange capacity may result in higher sorptive characteristics of the studied sustainable materials for waste landfill compacted liner, allowing grater interception of various chemical pollutants present in landfill leachate. The obtained results of CEC determination are in agreement with the previous observations, reported by e.g., Gholizadeh-Sarabi et al. [49].  Figure 5 shows the results of the Proctor test and measurements of saturated hydraulic conductivity for all applied clay-zeolite mixtures and the reference samples and for variable molding water contents. Figure 5 also presents determined values of bulk density after swelling and shrinkage for all tested samples and all applied molding water contents. In all tested cases the influence of 10% admixture of NaP1 zeolite to locally sampled clay substrates on Proctor destiny, swelling and shrinkage density, and on the values of the obtained coefficient of saturated hydraulic conductivity are visible. The discussed effects on NaP1 addition to locally available clays may be summarized generally as: decrease in Proctor bulk density as well as in density after swelling and shrinkage and, finally, decrease in coefficient of saturated hydraulic conductivity. Table 5 shows determined coefficients of saturated hydraulic conductivity for three selected values of initial moisture: w f (forming water content), w opt (optimal Proctor water content), w f < w opt (dry side of Proctor curve, left from the optimum), and w opt < w f < 1.2 w opt (wet side of the Proctor curve, right from the optimum), both for approx. 95% of determined Proctor density.  Figure 5. Results of Proctor test and saturated hydraulic measurements for various molding water content values: (a) Gawlowka reference specimen, (b) Gawlowka specimen + 10% NaP1, (c) Jelen reference specimen, (d) Jelen specimen + 10% NaP1, (e) Lazek reference specimen, (f) Lazek specimen + 10% NaP1, (g) Pawlow reference specimen, (h) Pawlow specimen + 10% NaP1, (i) Zgorznica reference specimen, and (j) Zgorznica specimen + 10% NaP1.
The results presented in Table 5 indicate the influence of NaP1 addition to locally sampled clayey specimens on the general decrease in the obtained values of coefficients of saturated hydraulic conductivity. In most cases, an increase in w opt was observed, followed by reduced saturated hydraulic conductivity. This issue is very important in case of Gawlowka, Jelen, and Zgorznica (reference samples which have low values of molding water content), it was impossible to achieve the statutory threshold of maximal allowed K s for compacted clay liner, i.e., 1.00 × 10 −9 m/s. Thus, taking into account the infiltration abilities of tested compacted reference clay specimens, the addition of NaP1 zeolite in most cases improved the sealing capabilities of studied liners using the light-textured soils, which is in agreement with [49]. On the other hand, in the case of Pawlow clay loam, see Table 2, formed at three tested initial water contents, the increase in the coefficient of saturated hydraulic conductivity was observed after NaP1 addition. A similar change was noted for silty clay sampled in Lazek and formed at w opt and w f < w opt. Thus, the performed research supports previous information considering the increase in saturated hydraulic conductivity of fine-particle heavy-textured soils mixed with zeolites [47,48,50,52,53]. However, in this case the noted increase in K s value did not cause the significant loss of sealing capabilities, all heavy-textured materials mixed with 10% of NaP1 zeolite showed value of coefficients of saturated hydraulic conductivity after compaction at the level of approx. 5.7 × 10 −11 -9.0 × 10 −10 m/s, lower than the required threshold for compacted clay liner. Thus, the tested clay-zeolite materials are fully applicable to compacted clay liner construction due to their very low permeability for water. The obtained results are comparable to values presented by Öncü and Bisel [51] for expansive soil, locally available in Turkey, mixed in a 0.5 ratio with zeolite. Additionally, application of recycled zeolite allows a comparable decrease in saturated hydraulic conductivity as reported for the other fine particle substrates mixed with local soils, including shale, coal gangue, fly ash, bentonite, etc. [22,26,27,29,30,32].
The performed statistical analyses, based on a nonparametric Wilcoxon test, as an alternative for the t-Student test, showed that in all studied cases the admixture of NaP1 zeolite to locally available clay soils resulted in statistically significant differences in median values of coefficients of saturated hydraulic conductivity in relation to the reference samples. Figure 6 presents values of determined swelling index, SI, for all tested mixtures and reference samples and for all applied initial compaction moisture values. It is visible that in all cases the mixtures containing 10% NaP1 zeolite showed higher values of SI, thus their deformation after saturation was greater. It is also worth noting that the value of SI decreased along the increase in the molding water content. Finally, the observed changes in SI for studied mixtures and reference samples were related to their plasticity index values (see Figure 4). The lowest values of swelling index were noted for low plasticity clay mixtures (and reference samples), i.e., sandy loam sampled in Gawlowka (G + 10% NaP1 and G) and clay loam sampled in Jelen (J + 10% NaP1 and J). Thus, the commonly reported [35][36][37][38] typical relation between plasticity and swelling of various clay specimens was also noted during this research. decreased along the increase in the molding water content. Finally, the observed changes in SI for studied mixtures and reference samples were related to their plasticity index values (see Figure 4). The lowest values of swelling index were noted for low plasticity clay mixtures (and reference samples), i.e., sandy loam sampled in Gawlowka (G + 10% NaP1 and G) and clay loam sampled in Jelen (J + 10% NaP1 and J). Thus, the commonly reported [35][36][37][38] typical relation between plasticity and swelling of various clay specimens was also noted during this research.  The changes in dimensionless r s factor describing type of samples deformation during shrinkage are presented in Figure 7. Generally, despite the changes resulting from 10% zeolite amendment, most of the samples containing NaP1 addition and reference specimens, in full range of the applied molding water content, remained in the range of predominantly vertical shrinkage. The only different behavior was observed in case of specimens sampled in Gawlowka (G + 10% NaP1 and G) for which addition of zeolite changed the shrinkage type from predominantly horizontal to vertical deformation for low values of molding water content applied. zeolite amendment, most of the samples containing NaP1 addition and reference specimens, in full range of the applied molding water content, remained in the range of predominantly vertical shrinkage. The only different behavior was observed in case of specimens sampled in Gawlowka (G + 10% NaP1 and G) for which addition of zeolite changed the shrinkage type from predominantly horizontal to vertical deformation for low values of molding water content applied. The type of shrinkage represented by all tested clay-NaPa1 mixtures and the reference samples may be assessed based on the calculated COLE values presented in Figure 8 for each applied molding water content. In three cases, the tested specimens, after mixing with zeolite, remain at the same level of shrinkage characteristics i.e., P and P + 10% NaP1 very high, J and J + 10% NaP1 as well as L and L + 10% NaP1 high and very high. In two other cases, addition of zeolite increased COLE values and changed shrinkage potential, i.e., for the Gawlowka specimen (from low for G to low-high for G + 10% NaP1) and Zgorznica (from high for Z to high-very high for Z + 10% NaP1). Again, it may be noted The type of shrinkage represented by all tested clay-NaPa1 mixtures and the reference samples may be assessed based on the calculated COLE values presented in Figure 8 for each applied molding water content. In three cases, the tested specimens, after mixing with zeolite, remain at the same level of shrinkage characteristics i.e., P and P + 10% NaP1 very high, J and J + 10% NaP1 as well as L and L + 10% NaP1 high and very high. In two other cases, addition of zeolite increased COLE values and changed shrinkage potential, i.e., for the Gawlowka specimen (from low for G to low-high for G + 10% NaP1) and Zgorznica (from high for Z to high-very high for Z + 10% NaP1). Again, it may be noted that type of shrinkage potential determined by COLE values may be related to plasticity and forming water content of studied clay specimens. that type of shrinkage potential determined by COLE values may be related to plasticity and forming water content of studied clay specimens.  Figure 9 shows the determined values of swelling and shrinkage potentials, in kg/dm 3 , calculated after Horn and Stępniewski [76], for all the tested specimens and all applied molding water content values.

Swelling and Shrinkage Characteristics
In all tested cases, the clay-NaP1 zeolite mixtures presented higher values of swelling potentials than the reference samples which are clearly related to their particle composition and the determined values of percentage swell index (see Figure 6). On the other hand, the clear increase in shrinkage potential after addition of NaP1 zeolite was noted only in two cases, for Gawlowka sandy loam and Zgorznica loam, which clearly corresponds to determined shrinkage potential described by the COLE indicator (see Figure 8).
To summarize the studied swelling and shrinkage characteristics of the tested clayzeolite mixtures and reference specimens the mean determined values of SI, COLE, and rs, together with the mean shrinkage potentials and deformation type are presented in Table 6. It is clearly visible that the highest mean shrinkage potentials (very high) were observed for mixtures and reference samples of the highest determined values of plasticity index and the highest content of fine particles (clay and clay + silt), i.e., Lazek 54.5%  Figure 9 shows the determined values of swelling and shrinkage potentials, in kg/dm 3 , calculated after Horn and Stępniewski [76], for all the tested specimens and all applied molding water content values.
In all tested cases, the clay-NaP1 zeolite mixtures presented higher values of swelling potentials than the reference samples which are clearly related to their particle composition and the determined values of percentage swell index (see Figure 6). On the other hand, the clear increase in shrinkage potential after addition of NaP1 zeolite was noted only in two cases, for Gawlowka sandy loam and Zgorznica loam, which clearly corresponds to determined shrinkage potential described by the COLE indicator (see Figure 8).
To summarize the studied swelling and shrinkage characteristics of the tested clayzeolite mixtures and reference specimens the mean determined values of SI, COLE, and r s , together with the mean shrinkage potentials and deformation type are presented in Table 6. It is clearly visible that the highest mean shrinkage potentials (very high) were observed for mixtures and reference samples of the highest determined values of plasticity index and the highest content of fine particles (clay and clay + silt), i.e., Lazek 54.5% and 95.5% as well as Pawlow 35% and 71%, respectively. On the other hand, the lowest shrinkage potential was observed for Gawlowka specimens, presenting the lowest plasticity index as well as clay and fine particles content, 19% and 26%, respectively.
shrinkage potential was observed for Gawlowka specimens, presenting the lowest plasticity index as well as clay and fine particles content, 19% and 26%, respectively.
The analyses of shrinkage characteristics of the studied light-textured clay-NaP1 zeolite mixtures showed agreement with literature reports suggesting that addition of fine particle zeolites to the natural clay specimens increases their expansiveness [55,56]. The contrary effect, the significant stabilization of clay soils by zeolite amendment, reported by [57][58][59][60][61] was not observed. The slight decrease of shrinkage potential value represented by COLE and shrinkage potential (Figures 8 and 9, respectively) was observed for clayzeolite mixtures based on specimens sampled in Jelen (clay loam), Lazek (silty clay), and Pawlow (clay loam).   The analyses of shrinkage characteristics of the studied light-textured clay-NaP1 zeolite mixtures showed agreement with literature reports suggesting that addition of fine particle zeolites to the natural clay specimens increases their expansiveness [55,56]. The contrary effect, the significant stabilization of clay soils by zeolite amendment, reported by [57][58][59][60][61] was not observed. The slight decrease of shrinkage potential value represented by COLE and shrinkage potential (Figures 8 and 9, respectively) was observed for clayzeolite mixtures based on specimens sampled in Jelen (clay loam), Lazek (silty clay), and Pawlow (clay loam).

Saturated Hydraulic Conductivity after Subsequent Cycles of Drying and Rewetting
The mean values of the determined coefficients of saturated hydraulic conductivity, for the tested specimens and reference samples, after compaction and three subsequent cycles of air drying and rewetting are presented in Figure 10. It is clearly visible that the first drying cycle results in a significant decrease in sealing capabilities of compacted clays due to increase in the measured values of K s . Generally, in all tested cases the value of K s after the first drying and rewetting experiment exceeds the European statutory threshold value (1.00 × 10 −9 m/s) by one to four orders of magnitude. In several cases, i.e., local clay substrates sampled in Jelen, Pawlow, and Zgorznica, admixture of NaP1 zeolite resulted in decreased, in relation to reference samples, mean coefficients of saturated hydraulic conductivity after one, two, and three cycles of drying and rewetting. The obtained results are in agreement with literature sources considering changes of saturated hydraulic conductivity of compacted clay soils after several cycles of drying and rewetting [18,20,[39][40][41][42][43][44][45]. The performed statistical analysis based on one-way Kruskal-Wallis ANOVA (due to distributions of variables being different than normal) showed statistically significant differences between determined Ks values for each tested specimens related to addition of NaP1 zeolite and dissication and rewetting of the samples.
Thus, it should be clearly underlined that compacted clay liners utilizing clay-zeolite The performed statistical analysis based on one-way Kruskal-Wallis ANOVA (due to distributions of variables being different than normal) showed statistically significant differences between determined K s values for each tested specimens related to addition of NaP1 zeolite and dissication and rewetting of the samples.
Thus, it should be clearly underlined that compacted clay liners utilizing clay-zeolite mixtures and clays alone should be operated carefully, limiting the possibility of earthen liners' exposure to the direct influence of atmospheric conditions resulting in liners desiccation, because increase in their hydraulic conductivity related to dissication cracking is an irreversible process.

Conclusions
The performed analyses considering the influence of 10% NaP1 zeolite admixture to locally available clay soils (the materials which allow the construction of compacted clay liners for the sustainable landfilling), specifically on their hydraulic as well as swelling and shrinkage characteristics allowed us to draw the following conclusions:

•
The application of NaP1 zeolite to the studied clayey soils positively affected their hydraulic characteristics after compaction allowing them to meet the required threshold of sealing capabilities for compacted clay liner, i.e., K s lower than 1 × 10 −9 m/s, in all studied cases for molding water content on both sides on Proctor curve. • Addition of NaP1 zeolite to local clay soils increased their plasticity, so, as it could be expected, swelling and shrinkage characteristics of the studied specimens were also alerted.

•
The observed swelling of clay-zeolite mixtures was higher in all studied cases, for all forming water contents, than observed for the reference samples.

•
In most studied cases addition of zeolite to locally available clays did not change the type of deformation after shrinkage, but for some cases, the increase in shrinkage potential type for light-textured samples was observed. • A clear increase in coefficient of saturated hydraulic conductivity and decrease in sealing capabilities of tested compacted clay-zeolite mixtures as well as reference samples, above the required threshold of 1 × 10 −9 m/s, after subsequent cycles of drying and rewetting of samples were observed. • In our opinion, according to the obtained results, the usage of zeolite, a sustainable recycled material, as an addition for improving the sealing capabilities of compacted clay liners of sustainable landfill may be useful in limiting the environmental and social impacts of the sustainable landfill.

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On the other hand, taking into account the increased expansiveness of studied clayzeolite mixtures as well as their limited capability to sustain the sealing capabilities after considerable dissication, each material planned for compacted clay liner construction should be carefully tested and the constructed liner should be properly operated, especially to avoid extensive shrinkage and possible dissication cracking. • Our research considering hydraulic and swell-shrinkage properties of locally available soils mixed with recycled zeolite should be continued in the future for different clay specimens, variable recycled zeolite addition, and for landfill leachate as the permeating liquid. Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.