A Study of Physicochemical Properties of Stockpile and Ponded Coal Ash

Citation: Šulc, R.; Šídlová, M.; Formáˇcek, P.; Snop, R.; Škvára, F.; Polonská, A. A Study of Physicochemical Properties of Stockpile and Ponded Coal Ash. Materials 2022,15, 3653. https:// doi.org/10.3390/ma15103653 Academic Editors: Alex Kondratiev and Dmitry Valeev Received: 28 February 2022 Accepted: 16 May 2022 Published: 20 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article A Study of Physicochemical Properties of Stockpile and Ponded Coal Ash Rostislav Šulc 1,* , Martina Šídlová2, Petr Formáˇcek 1, Roman Snop 3, František Škvára 2and Adéla Polonská2 1Department of Building Technology, Faculty of Civil Engineering, Czech Technical University in Prague, Jugoslávských Partyzán˚u 1580/3, 160 00 Prague, Czech Republic; [email protected] 2Institute of Glass and Ceramics, Faculty of Chemical Technology, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague, Czech Republic; [email protected] (M.Š.); [email protected] (F.Š.); [email protected] (A.P.) 3ˇ CEZ EnergetickéProdukty, s.r.o., Komenského 534, 253 01 Hostivice, Czech Republic; r[email protected] *Correspondence: [email protected]; Tel.: +420-220-444-243 Abstract: The article describes chemical and also selected physical properties of ponded high temperature fly ash (FA) and bottom ash (BA) from a Mˇelník lignite power plant located in the Czech Republic. The research was carried out on samples obtained from drills with a depth of up to 54 m and the age of the samples retrieved from the lowest layers of the stockpile dating back to 1960. At the same time, a comparison was made with fresh fly ash and fresh bottom ash obtained from the identical power plant. The study analyzed a total of 98 stockpile samples. The properties selected were studied across the entire stockpile, namely moisture content, specific density, specific surface, carbon content, elemental and phase composition, pH, electrical conductivity and leachability. SEM analyses were also performed. The performed measurements of chemical properties proved the chemical stability of the material even after several decades of storage in the stockpile. The largest changes are evident in the results of the analyses related to the leachability of SO 3 , Cl − and F − . In contrast, the pH does not change significantly, and the composition is pH neutral or alkaline. Regarding ponded BA, particle disintegration was noted depending on the increasing core borehole depth. Keywords: stockpile ash; ponded ash; recovery; CCPs; physicochemical properties; coal combustion 1. Introduction The European Coal Combustion Products Association (ECOBA) states that in 2016, 28 EU countries produced over 105 million tons of coal combustion products (CCPs). The Czech Republic, while being a relatively small country, still manages to produce around 13 million tons of CCPs per year, which is primarily caused by a high share of non-combustible components in the coal burned. CCPs include fly ash (FA), bottom ash (BA), boiler slag, FBC-ash, FGD-gypsum and SDA-products. Generally, fly ash is formed by burning pulverized coal in coal-fired power plants, most often at temperatures of 1100–1500 ◦C [1] . FA is transported with a flue gas and captured on electrostatic precipitators or fabric filters. FA particles are mostly spherical, but they can also be of irregular or angular shapes. During combustion, bottom ash is formed consisting of coarse granular particles and is collected at the bottom of coal furnaces [ 2 ]. The particles are porous, irregular and rough-textured [ 3 ]. FBC-ash, on the other hand, is formed during fluidized bed combustion at significantly lower temperatures not exceeding 950 ◦ C [ 4 ]. The particles of this type of ash are irregular and porous, and their morphology resembles the shape of the original coal grains. If limestone is additionally fed to the boiler to aid desulfurization, the resulting FBC-ash also contains anhydrite II and lime [ 5 ]. The valuable use of CCPs is primarily concentrated on the construction industry. The largest share of all CCPs produced consists of FA, which is subsequently employed in the widest range of applications, namely in the building and construction industry, civil engineering and for mine back filling [6,7]. Materials 2022,15, 3653. https://doi.org/10.3390/ma15103653 https://www.mdpi.com/journal/materials Materials 2022,15, 3653 2 of 15 Another use is offered by agriculture [ 8 , 9 ]. In addition, research is focused on alternative uses of FA such as the adsorption of SO 3 [ 10 ], NO x and Hg [ 11 ]. Bottom ash finds its application as a fine aggregate [ 12 ], highway material [ 13 ] or possibly for the production of bricks, fire-proof products, ceramics, etc. [ 14 ]. The use of FBC-ash is significantly more complicated compared to that of FA. However, it turns out that even this type of CCP may find a significant application in construction works [15,16]. A large part of the produced FA and BA has been deposited in stockpiles for decades, and their number is still increasing, despite the fact that today, not only in Europe, there is already a shortage of such materials on the market. With the closure of coal-fired power plants, this commodity will become increasingly inaccessible. It is therefore obvious that the reuse of fly ash from stockpiles is a pressing issue nowadays. In recent decades, analyses of fly ash have been carried out dealing with the ash deposited in stockpiles in the United Kingdom, the United States and, more recently, China. [ 1 ]. It turned out that each stockpile was unique, and an individual approach was needed for the efficient use of the extracted ash. It is evident that the research carried out in individual localities/countries will, in the future, contribute to a faster and more efficient use of the CCPs deposited worldwide. Originally, ash was stored by flowing it to ponds in a dilute slurry [ 1 ]. As the ash flowed, it sorted and stratified, resulting in a heterogenous body of material. Currently, ash is mostly stored dry. The stockpile ash deposited in this way may be relatively homogeneous, but it has also been found that the agglomeration of fly ash particles can occur, especially in fly ash with a high free lime content [ 17 ]. In addition, when stored in storage facilities, fly ash comes into contact with water that is either used to store the fly ash to prevent the production of dust or may come from rain. Rainwater can cause large leaks through the fly ash layers, which is also the reason why fly ash in the stockpile has high moisture even at depth. Robl et al. state that the agglomeration is greatest at a moisture content of 10–20%. Research of samples (wet stored fly ash) up to 18 months old [ 17 , 18 ] showed agglomeration of the samples with the presence of sulfate-based products on particles. The pozzolanic reactions were also noted. Sambor and Szymanek [ 19 ] analyzed almost 300 samples from the ash stockpile at a depth of 0–2.5 m exposed to the weather and found that the pH of the core borehole rose with its increasing depth, which resulted in alkaline reactions that promoted the leaching of chemical compounds into the stockpile. Several other works [ 20 , 21 ] indicate changes in the surface morphology of the deposited fly ash particles, namely the surface dissolution and the formation of new phases (secondary phases). Detailed research concerning ponded fly ash in the Czech Republic has not been carried out yet. The aim of this article is to present and compare the chemical, mineralogical and selected physical properties of deposited FA and ponded BA depending on the length and method of their storage, both coming from the locality Panskýles, which is a repository for CCPs from the power plant Mˇelník in the Czech Republic. The results of this work are intended to facilitate stockpile ash recovery. 2. Materials and Methods 2.1. Sampling All samples were collected from the Mˇelník power plant located in the Czech Republic and from the stockpile and pond from the locality Panskýles located next to the power plant. Stockpile Panskýles is approx. 2200 m long, 550 m wide and from 60 to 25 m deep. The samples consisted of fresh fly ash (labelled FA) and fresh bottom ash (labeled BA) produced by the power plant (consumption in 2020). Furthermore, samples of deposited ash (stockpile or ponded ash, labelled V3) and ponded bottom ash BA (labelled S1) were collected as core borehole. The stockpile has been in operation since the 1960s. Until the 1990s, the ash had been ponded into local lagoons corresponding to the V3 core borehole depth of approx. 25 m in the form of a hydromixure. Since the early 1990s, fresh fly ash (FA) was transported from the Mˇelník power plant to the stockpile in the dry state. FA to be deposited used to be mixed with bottom ash (BA), the latter being at the amount of 5 to 15%. The greatest depth of 54 m was reached by core borehole V3. Samples from Materials 2022,15, 3653 3 of 15 V3 core borehole depth exceeding 50 m contained subsoil. Furthermore, core borehole S1 was drilled in the segment of the stockpile, where only BA was ponded in the past. Core borehole S1 reached a depth of 44 m, with the last two meters, 43 m and 44 m, containing the limestone subsoil. All samples were collected by the Wirth B1A drilling rig. A total of 98 samples were acquired from both core boreholes V3 (54 samples) and S1 (44 samples), Figure 1. Approximately 10 kg of material were taken from each individual meter into a bucket and sealed so that no moisture escaped. The samples in the buckets were marked with a specific core borehole and depth, e.g., V3-5 m. Materials 2022, 15, x FOR PEER REVIEW 3 of 16 collected as core borehole. The stockpile has been in operation since the 1960s. Until the 1990s, the ash had been ponded into local lagoons corresponding to the V3 core borehole depth of approx. 25 m in the form of a hydromixure. Since the early 1990s, fresh fly ash (FA) was transported from the Mělník power plant to the stockpile in the dry state. FA to be deposited used to be mixed with bottom ash (BA), the latter being at the amount of 5 to 15%. The greatest depth of 54 m was reached by core borehole V3. Samples from V3 core borehole depth exceeding 50 m contained subsoil. Furthermore, core borehole S1 was drilled in the segment of the stockpile, where only BA was ponded in the past. Core borehole S1 reached a depth of 44 m, with the last two meters, 43 m and 44 m, containing the limestone subsoil. All samples were collected by the Wirth B1A drilling rig. A total of 98 samples were acquired from both core boreholes V3 (54 samples) and S1 (44 samples), Figure 1. Approximately 10 kg of material were taken from each individual meter into a bucket and sealed so that no moisture escaped. The samples in the buckets were marked with a specific core borehole and depth, e.g., V3-5 m. Figure 1. Location of core borehole V3 (50.4086175 N, 14.3813356 E) and S1 (50.4016497 N, 14.3977614 E) in stockpile Panský les, https://mapy.cz/ (accessed on 10 May 2022). 2.2. Methods Moisture content of all samples from core boreholes V3 and S1 and from fresh FA and fresh BA was identified by drying the samples to the constant weight at 105 °C. Three samples of 200 g of material were taken from each drilled meter. Loss on ignition (LOI) was performed at 950 ± 25 °C until reaching the constant weight, and each sample was tested 3 times. The elemental composition of all ashes was expressed in oxides and determined by X-ray fluorescence (XRF) analysis. All samples were homogenized in a laboratory vibrating mill before measurement. For XRF analysis, ARL 9400 XP sequential WD-XRF spectrometer was used (Switzerland). The data obtained was assessed by the software Uniquant 4. Measurement errors were below 1 wt.%. All values were recalculated with respect to loss on ignition. Diffraction patterns were collected at room temperature with an X’Pert 3 Powder θ-θ powder diffractometer (PANalytical, Almelo, The Netherlands) with parafocusing BraggBrentano geometry using Cu Kα radiation (λ = 1.5418 Å, Ni filter, generator setting: 40 kV, 30 mA). An ultrafast PIXCEL detector was employed to obtain XRD data over the angular range from 15 to 75 ° (2θ) with a step size of 0.013 ° 2θ and a counting time of 180 s/step. Figure 1. Location of core borehole V3 (50.4086175 N, 14.3813356 E) and S1 (50.4016497 N, 14.3977614 E) in stockpile Panskýles, https://mapy.cz/ (accessed on 10 May 2022). 2.2. Methods Moisture content of all samples from core boreholes V3 and S1 and from fresh FA and fresh BA was identified by drying the samples to the constant weight at 105 ◦ C. Three samples of 200 g of material were taken from each drilled meter. Loss on ignition (LOI) was performed at 950 ± 25 ◦ C until reaching the constant weight, and each sample was tested 3 times. The elemental composition of all ashes was expressed in oxides and determined by X-ray fluorescence (XRF) analysis. All samples were homogenized in a laboratory vibrating mill before measurement. For XRF analysis, ARL 9400 XP sequential WD-XRF spectrometer was used (Switzerland). The data obtained was assessed by the software Uniquant 4. Measurement errors were below 1 wt.%. All values were recalculated with respect to loss on ignition. Diffraction patterns were collected at room temperature with an X’Pert 3 Powder θ - θ powder diffractometer (PANalytical, Almelo, The Netherlands) with parafocusing BraggBrentano geometry using Cu K α radiation ( λ = 1.5418 Å, Ni filter, generator setting: 40 kV, 30 mA). An ultrafast PIXCEL detector was employed to obtain XRD data over the angular range from 15 to 75 ◦ (2 θ ) with a step size of 0.013 ◦ 2 θ and a counting time of 180 s/step. The fixed divergence slit was applied for the measurement. The back loading technique was utilized to eliminate preferred orientation. The data were evaluated with software package HighScore Plus V4.6 (PANalytical, Almelo, The Netherlands) and search-match was performed in PDF4 + database. Subsequently, the Rietveld method was applied to calculate mass content for present crystalline phases and amorphous phase using ZnO (10 wt. 10%) as an internal standard. Measurement errors were below 3 wt.%. SEM images were measured on a Hitachi S 4700 cold cathode scanning electron microscope (Tokyo, Japan) equipped with two secondary electron detectors and one reflected electron detector. Materials 2022,15, 3653 4 of 15 Leachability, pH and conductivity were determined from the analysis of extracts prepared by weighing 100 g of the dried sample with 1000 mL of distilled water, followed by homogenization for 24 h (EN 12457-4). The leachate was filtered through a 0.45 µ m filter paper by the vacuum filtration system. The pH of the extracts was determined according to ˇ CSN 10,523 using a pH meter 526 (WTW, GmbH & Co. KG, Weilheim, Germany) and conductivity according to ˇ CSN EN 27,888 using a Cond Level 1 instrument (WTW, Czech Rep.). Furthermore, the amounts of chlorides, fluorides and sulfates were determined on an ion chromatograph IC 930 Compact Flex (METROHM, Herisau, Switzerland) according to ˇ CSN EN 10304-1. Heavy metals were determined using a mass spectrometer ICPNexION 300X (PERKIN ELMER, Waltham, MA, USA) according to ˇ CSN EN 17294-1, 2. In addition, the amount of mercury was established according to ˇ CSN 75 7440 using a singlepurpose atomic absorption spectrometer AMA 254 (ALTEC, Chotˇeboˇr, Czech Republic). Measurement errors were below 1%. Physical properties were monitored at all core boreholes and depths and included particle size distribution (PSD), specific density ( ρ ) and specific surface (S) analyses. A laser-light scattering analyzer Bettersizer (Bettersize Instruments Ltd., Dandong, Liaoning, China) was used to determine the particle size distribution. Specific density ( ρ ) was determined by the pycnometric method, and the specific surface (S) of the samples was measured by air permeability Blain method ( ˇ CSN EN 196-6). Moisture content, PSD and loss on ignition analyses are presented in the article on all samples from core boreholes V3 and S1. For other analyses, representative samples were selected across the entire core borehole V3 and S1 to take into account all characteristics and material changes with increasing core borehole depth. 3. Results and Discussion 3.1. Moisture Content Determination of moisture in deposited ash V3 and bottom ash S1 is shown in the graph in Figure 2. The obtained results concerning moisture determination of deposited ash V from core borehole V3 are in the range of 17–47% at a depth of 3–50 m. At a core borehole depth of 1–2 m, the moisture value was 8%, resp. 11%, which was affected by the fact that sampling was carried out in the dry season of the year. In the case of bottom ash, the moisture values of the depths up to 24 m were higher, reaching about 30% in comparison with the V3 core borehole. In contrast, at the depth up to a maximum of 42 m, the moisture values were mostly lower in comparison with V3. An exception was the moisture of the sample from a depth of 38 m, where the highest value of 57.1% was recorded. The moisture content of the fresh ash analyzed was 6% while the bottom ash showed 23%. Materials 2022, 15, x FOR PEER REVIEW 5 of 16 Figure 2. Moisture Values for Samples from Core Boreholes V3 and S1. 3.2. Particle Size Distribution Figure 3 shows the mean particle size for deposited ash from core borehole V3 and ponded BA S1. The mean particle size for fresh FA was detected at 36.2 μm, and for fresh BA, it was 124.8 μm. Figure 3. Mean Particle Size of Deposited Ash from Core Borehole V3 and Bottom Ash from Core Borehole S1. The graphs show a relatively large variability in the mean particle size in core boreholes V3 and S1. The average values from all core borehole depths were 67 μm for core borehole V3 and 55 μm for S1. The highest value for core borehole V3 was recorded at the depth of 30 m, namely 156 μm, and the lowest value at the depth of 37 m, namely 13.1 0 10 20 30 40 50 60 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 [%] Depth [m] V3 S1 0 20 40 60 80 100 120 140 160 180 135791113151719212325272931333537394143454749 d50 [μm] Depth [m] V3 S1 Figure 2. Moisture Values for Samples from Core Boreholes V3 and S1. Materials 2022,15, 3653 5 of 15 3.2. Particle Size Distribution Figure 3shows the mean particle size for deposited ash from core borehole V3 and ponded BA S1. The mean particle size for fresh FA was detected at 36.2 µ m, and for fresh BA, it was 124.8 µm. Materials 2022, 15, x FOR PEER REVIEW 5 of 16 Figure 2. Moisture Values for Samples from Core Boreholes V3 and S1. 3.2. Particle Size Distribution Figure 3 shows the mean particle size for deposited ash from core borehole V3 and ponded BA S1. The mean particle size for fresh FA was detected at 36.2 μm, and for fresh BA, it was 124.8 μm. Figure 3. Mean Particle Size of Deposited Ash from Core Borehole V3 and Bottom Ash from Core Borehole S1. The graphs show a relatively large variability in the mean particle size in core boreholes V3 and S1. The average values from all core borehole depths were 67 μm for core borehole V3 and 55 μm for S1. The highest value for core borehole V3 was recorded at the depth of 30 m, namely 156 μm, and the lowest value at the depth of 37 m, namely 13.1 0 10 20 30 40 50 60 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 [%] Depth [m] V3 S1 0 20 40 60 80 100 120 140 160 180 135791113151719212325272931333537394143454749 d50 [μm] Depth [m] V3 S1 Figure 3. Mean Particle Size of Deposited Ash from Core Borehole V3 and Bottom Ash from Core Borehole S1. The graphs show a relatively large variability in the mean particle size in core boreholes V3 and S1. The average values from all core borehole depths were 67 µ m for core borehole V3 and 55 µ m for S1. The highest value for core borehole V3 was recorded at the depth of 30 m, namely 156 µ m, and the lowest value at the depth of 37 m, namely 13.1 µ m. No significant trend across the core borehole was observed in V3. In contrast, the ponded BA from core borehole S1 shows a decreasing character of the mean particle size, and a comparison with the value of the mean particle size of fresh BA shows a significant disintegration of the particles over time. The disintegration of ponded ash particles was highlighted in the work of Robl et al. [1]. 3.3. Specific Density and Specific Surface Specific density ( ρ ) and specific surface area (S) were measured across the entire core borehole depth. The values for selected core borehole depths V3 and S1 are provided in Table 1. Specific density values ranged from 2029 to 2501 kg/m3for core borehole V3 and 1912–2579 kg/m 3 for core borehole S1. The average measured values were 2070 kg/m 3 for core borehole V3 and 2222 kg/m 3 for core borehole S1, which, in comparison with the value of fresh FA of 2296 kg/m 3 and fresh BA being 2349 kg/m 3 , shows that fresh FA and BA have higher bulk density values on average compared to the deposited samples taken. The average value of the specific surface was 201 m 2 /kg for core borehole V3 and 180 m 2 /kg for core borehole S1. It can be seen here that the fresh samples had a higher specific surface area compared to the deposited samples, which is likely to be related to the agglomeration of the particles. 3.4. Carbon Content Measured as the LOI The amount of unburned carbon was determined by loss on ignition (LOI). Excessive amounts of carbon must be removed when using fly ash in cement, as it negatively affects the properties of the cement. Carbon is hydrophobic and, in addition, adversely affects the Materials 2022,15, 3653 6 of 15 properties of plasticizing and aerating additives. The loss on ignition was determined to be 2.1% for fresh fly ash and 5.7% for fresh BA. The graph in Figure 4shows the measured values of LOI for core boreholes V3 and S1. The carbon content of most samples did not exceed 5%, and according to ˇ CSN EN 450-1, it is considered as low carbon content FA. The increased amount of unburned carbon 7.3–16.1 wt.% in core borehole V3 in the depth of 26–29 m is probably related to the change of the ash deposition procedure. The original wet method of ponding caused carbon particles to leach to the surface of the stockpile. When changing to dry storage, this layer was then covered with stockpile ash, which was only sprinkled with water. Dry storage of deposited ash was used until the stockpile was recultivated. The high value of unburned carbon content is probably related to the increased amount of moisture in the depth of 26–29 m in V3 core borehole. Table 1. Values of Specific Density and Specific Surface for Selected Samples from Core Boreholes V3 and S1 and for Fresh Fly Ash (FA) and Fresh Bottom Ash (BA). Sample Specific Density [kg/m3] Specific Surface [m2/kg] FA fresh 2296 ±6 334 ±1 V3-4 2121 ±7 213 ±1 V3-8 2113 ±3 171 ±2 V3-12 2152 ±4 196 ±2 V3-16 2054 ±10 208 ±1 V3-20 2095 ±8 194 ±1 V3-25 217 ±5 381 ±0 V3-28 2332 ±4 112 ±1 V3-33 2167 ±7 145 ±1 V3-36 2204 ±9 138 ±1 V3-45 2305 ±4 247 ±1 BA fresh 2349 ±7 218 ±2 S1-3 2097 ±7 137 ±3 S1-6 2035 ±7 153 ±3 S1-10 2209 ±5 174 ±2 S1-16 2098 ±10 151 ±2 S1-22 2109 ±5 240 ±4 S1-26 2335 ±5 169 ±2 S1-32 2290 ±8 185 ±1 S1-38 2219 ±1 235 ±4 Materials 2022, 15, x FOR PEER REVIEW 7 of 16 depth of 26–29 m is probably related to the change of the ash deposition procedure. The original wet method of ponding caused carbon particles to leach to the surface of the stockpile. When changing to dry storage, this layer was then covered with stockpile ash, which was only sprinkled with water. Dry storage of deposited ash was used until the stockpile was recultivated. The high value of unburned carbon content is probably related to the increased amount of moisture in the depth of 26–29 m in V3 core borehole. Figure 4. Determination of Loss on Ignition from Core Boreholes V3 and S1. 3.5. Bulk Chemical Analysis XRF analysis was used to perform the elemental analyses of fresh FA, fresh BA and deposited ash from core boreholes V3 (1–50 m) and S1 (1–42 m). Selected results of the analyses are presented in Table 2. 0 2 4 6 8 10 12 14 16 18 13579111315171921232527293133353739414345474951 [%] Depth [m] V3 S1 Figure 4. Determination of Loss on Ignition from Core Boreholes V3 and S1. Materials 2022,15, 3653 7 of 15 3.5. Bulk Chemical Analysis XRF analysis was used to perform the elemental analyses of fresh FA, fresh BA and deposited ash from core boreholes V3 (1–50 m) and S1 (1–42 m). Selected results of the analyses are presented in Table 2. By comparing fresh FA and BA with the samples of deposited materials, it can be stated that no significant changes were found in the composition of both the major and minor elements. The results show that the samples are rich in silica (SiO 2 ), alumina (Al 2 O 3 ) and iron oxide (Fe 2 O 3 ). SiO 2 content ranges from 47.48 to 61.95 wt.% for V3 and 45.58–61.98 wt. % for S1. The content of Al 2 O 3 is in the range of 21.73–35.70 wt.% for V3 and 24.89–35.09 wt.% for S1. The Fe 2 O 3 content ranges from 3.77 to 12.05 wt.% for V3 and 4.22–16.94 wt.% for core borehole S1. With respect to the use of fly ash in concrete, from the point of view of the ˇ CSN EN 450-1 standard, it is required that the fly ash contains more than 70% of Al 2 O 3 , SiO2 and Fe 2 O 3 , and this requirement was met by all samples from core boreholes V3 and S1. The content of alkalis Na 2 O eq (=Na 2 O + 0.658 K 2 O) is in the range 0.99–1.98 wt.% for V3 and 0.99–2.23 wt.% for S1. The alkali values meet the requirements of ˇ CSN EN 450-1 for the total amount of alkalis, which must be less than 5 wt.%. Furthermore, the requirements for the maximum content of CaO, MgO and SO 3 were also satisfied. The amount of CaO ranged from 1.57 to 2.88 wt.% for V3 and 1.51 to 3.88 wt.% for S1, with the exception of sample S1 38 m, where a CaO value of 13.08 wt.% was recorded. The high CaO value is not directly related to ponded BA. CaO in the form of CaCO 3 , Ca (OH) 2 or CaO could be introduced into the stockpile as waste, or it could be introduced there in the form of a substrate for BA deposition in order to capture leached ions from the deposited material. The analyzed samples of fresh FA, BA and deposited ash are classified as siliceous ash (Class F) with a low CaO content < 5.1% and a content of primary oxides ( SiO2+ Al2O3+ Fe2O3 ) > 70%. The only dependence that can be observed was the changing sulfur content. The highest value detected in the fresh ash samples obtained was 0.56 wt.% of SO 3 . With an increasing depth of core borehole V3, this amount then decreases, which is probably related to leaching, with respect to the solubility of sulfates. At the same time, it is interesting to note that in the case of ponded BA, sulfur is present predominantly in the oxidation state of S −2 but occurring at a significantly lower concentration compared to the highest value detected in the sample of fresh BA. As for the ponded BA, its low sulfur content can be attributed to the wet deposition process, i.e., the faster dissolution of sulfates and the leakage through the bottom of the stockpile. From a chemical point of view, based on the results, both deposited ash and ponded BA can be classified as ash class F. Materials 2022,15, 3653 8 of 15 Table 2. Oxide Composition of Fresh FA, Bottom Ash (BA) and Selected Deposit Samples from Core Boreholes V3 and S1 in locality Panskýles by XRF analysis; measurement errors were below 1 wt.%. Sample Weight [%] SiO2Al2O3Fe2O3Na2O K2O CaO MgO TiO2P2O5SO3S−2V2O5Cr2O3MnO Co3O4NiO CuO ZnO SrO ZrO2Nb2O5BaO CeO2As2O3Others LOI FA fresh 47.19 34.29 7.52 0.47 1.19 1.86 0.97 3.10 0.26 0.54 0.00 0.08 0.02 0.05 0.01 0.01 0.02 0.03 0.05 0.06 0.02 0.08 0.01 0.01 0.04 2.10 V3-4 49.28 35.24 6.12 0.37 1.01 1.89 0.93 2.56 0.16 0.36 0.00 0.07 0.03 0.04 0.01 0.02 0.01 0.02 0.03 0.06 0.02 0.04 0.02 0.00 0.01 1.70 V3-8 48.27 34.75 6.98 0.44 0.99 1.95 0.87 2.42 0.15 0.40 0.00 0.07 0.03 0.05 0.01 0.01 0.01 0.02 0.03 0.05 0.01 0.05 0.00 0.00 0.08 2.35 V3-12 49.20 33.92 7.61 0.38 1.24 1.86 0.96 2.11 0.18 0.38 0.00 0.06 0.02 0.05 0.01 0.02 0.01 0.02 0.04 0.05 0.01 0.05 0.00 0.00 0.06 1.76 V3-16 50.19 33.66 6.56 0.46 1.42 1.89 1.00 2.17 0.18 0.37 0.00 0.07 0.02 0.04 0.01 0.01 0.01 0.02 0.04 0.04 0.01 0.06 0.00 0.01 0.05 1.68 V3-20 49.78 34.55 6.07 0.36 1.26 1.77 0.91 2.35 0.18 0.32 0.00 0.07 0.03 0.04 0.01 0.01 0.01 0.02 0.04 0.05 0.01 0.05 0.01 0.00 0.04 2.05 V3-25 49.30 33.09 5.74 0.44 1.60 2.78 1.09 1.98 0.21 0.13 0.00 0.07 0.02 0.04 0.01 0.01 0.02 0.04 0.04 0.04 0.01 0.06 0.01 0.01 0.06 3.19 V3-28 47.66 30.91 4.11 0.22 1.64 2.10 0.86 1.31 0.12 0.00 0.23 0.04 0.02 0.02 0.01 0.00 0.00 0.00 0.03 0.03 0.01 0.03 0.01 0.00 0.06 10.60 V3-33 49.75 32.15 6.20 0.40 1.36 2.29 1.09 2.23 0.21 0.21 0.00 0.06 0.02 0.05 0.01 0.01 0.01 0.01 0.04 0.05 0.01 0.08 0.00 0.00 0.04 3.71 V3-36 49.20 30.53 5.24 0.31 1.52 1.93 1.03 2.03 0.17 0.00 0.14 0.06 0.02 0.04 0.00 0.01 0.01 0.01 0.03 0.04 0.01 0.04 0.01 0.00 0.03 7.58 V3-45 50.94 31.66 6.72 0.42 1.44 1.92 1.10 2.74 0.20 0.00 0.02 0.07 0.02 0.05 0.01 0.01 0.01 0.03 0.04 0.06 0.02 0.07 0.02 0.01 0.04 2.38 BA fresh 44.25 32.23 8.12 0.30 0.91 1.57 0.67 5.12 0.28 0.00 0.29 0.09 0.02 0.11 0.01 0.00 0.02 0.01 0.05 0.08 0.03 0.07 0.03 0.00 0.04 5.70 S1-3 47.68 31.49 3.79 0.26 1.56 1.71 0.82 1.80 0.13 0.00 0.21 0.06 0.02 0.02 0.00 0.01 0.01 0.00 0.03 0.04 0.01 0.05 0.00 0.00 0.04 10.25 S1-6 51.51 33.74 5.53 0.26 1.53 1.64 0.95 1.81 0.13 0.00 0.06 0.06 0.02 0.03 0.01 0.00 0.01 0.01 0.03 0.04 0.01 0.04 0.01 0.00 0.04 2.54 S1-10 50.01 32.16 7.22 0.38 1.34 2.08 1.04 1.99 0.15 0.00 0.08 0.06 0.02 0.06 0.00 0.00 0.01 0.01 0.03 0.04 0.01 0.06 0.00 0.00 0.06 3.18 S1-16 52.11 31.89 6.64 0.38 1.31 2.23 0.97 2.52 0.18 0.00 0.01 0.07 0.03 0.05 0.01 0.01 0.01 0.01 0.04 0.05 0.02 0.05 0.01 0.00 0.04 1.37 S1-22 51.39 32.06 6.06 0.36 1.51 1.79 1.03 2.13 0.16 0.00 0.08 0.06 0.02 0.04 0.00 0.01 0.01 0.02 0.03 0.04 0.01 0.06 0.02 0.00 0.04 3.05 S1-26 51.44 31.38 6.60 0.39 1.42 1.90 1.06 2.47 0.18 0.00 0.05 0.06 0.02 0.05 0.00 0.00 0.01 0.01 0.04 0.05 0.01 0.05 0.02 0.00 0.05 2.72 S1-32 49.07 30.75 7.49 0.41 1.14 2.22 0.98 3.09 0.22 0.00 0.07 0.07 0.02 0.07 0.01 0.01 0.02 0.02 0.04 0.05 0.02 0.08 0.00 0.01 0.03 4.12 S1-38 41.88 25.25 6.15 0.25 1.01 11.96 0.90 2.75 0.18 0.70 0.00 0.07 0.02 0.06 0.01 0.01 0.01 0.02 0.05 0.05 0.01 0.06 0.00 0.01 0.04 8.57 Materials 2022,15, 3653 9 of 15 3.6. pH, Electrical Conductivity and TDS The pH analysis shows that fresh FA had a pH of 8.23; see Table 3. In the case of the deposited ash from core borehole V3, pH was between 6.87 and 7.90. Here, the lowest value corresponds to 16 m and the highest value is related to 28 m. For core borehole S1, pH was 7.38 for fresh BA, and in the case of the bottom ash from core borehole S1, pH varied between 6.92 to 7.68. An exception is constituted by sample S1 from the depth of 38 m, where the pH value 10.39 was recorded. This highest value of pH (10.39) is related to the high amount of CaO detected with XRF analysis. In sum, the entire stockpile V3 and S1 is pH neutral or alkaline. At the same time, no significant changes of pH were detected in the samples from the S1 core borehole compared to fresh bottom ash. Weak changes in pH were detected for the V3 core borehole between fresh FA and deposited FA. The changes in pH could have occurred due to long-term exposure of deposited ashes in the stockpile to rainwater, as rainwater is generally acidic. Acidic solutions can affect the mobility and leaching of elements in FA, as described by Dutta et al. [22]. The value of electrical conductivity (EC) for the fresh FA sample was 260 mS.m −1 , as shown in Table 3. The EC values for core boreholes V3 and S1 are significantly lower and range from 14.0 to 70.1 mS.m −1 for core borehole V3 and from 10.1 to 57.0 mS.m −1 for core borehole S1. Typically, high EC values in fresh fly ash indicate a higher content of ions in the leachate from the fly ash. In contrast, lower EC values indicate a significant ion loss in the stored material. This finding is an indicator of the ions leaching from deposited materials. These conclusions are consistent with the work of Bhattacharyya et al. [ 23 ] and Eze et al. [21], who studied weathered coal FA in South Africa. Table 3. Values of pH, Electrical Conductivity (EC) and Total Dissolved Solids (TDS) for Fresh FA, Bottom Ash BA and Samples from Core Boreholes V3 and S1; measurement errors were below 1%. pH EC [mS.m−1] TDS [mg.L−1] FA fresh 8.23 241.0 2220 V3-4 7.71 55.9 392 V3-8 7.41 61.1 411 V3-12 7.39 56.4 370 V3-16 6.84 70.1 442 V3-20 7.31 58.9 366 V3-25 7.54 45.1 288 V3-28 7.90 33.2 189 V3-33 7.81 22.6 123 V3-36 7.55 14.0 <100 V3-45 7.69 14.0 <100 BA fresh 7.38 241.0 2430 S1-3 6.96 16.8 111 S1-6 6.92 10.1 <100 S1-10 7.28 14.6 <100 S1-16 7.66 15.1 <100 S1-22 7.23 16.1 <100 S1-26 7.38 12.8 122 S1-32 7.68 13.7 <100 S1-38 10.39 56.7 388 Table 3further shows the values of the total dissolved solids (TDS) detected in leachates. The highest values were again identified for fresh FA and BA, namely 2220 mg/L and 2430 mg/L, respectively. The values for core borehole V3 range from 444 mg/L up to 100 mg/L, while, for core borehole S1, all the values are below 122 mg/l except for sample S1 from 38 m, displaying a value of 388 mg/L. The TDS results are analogous to the EC results. At the same time, the results clearly demonstrate that the elements leached to a