The application of coal mining waste to the production of construction ceramics: radiological and mechanical aspects

1. INTRODUCTION

Waste accompanies all human activities, especially in industrial ones. One of the most environmentally burdensome industries is hard coal mining due to the generation of large amounts of waste. It is estimated that around 60 million tons of mining waste are generated annually across the world (11. Murmu, A.L.; Patel, A. (2018) Towards sustainable bricks production: An overview. Constr. Build. Mater. 165, 112-125. https://doi.org/10.1016/j.conbuildmat.2018.01.038.
).

Inappropriate management of this waste may cause the degradation of the natural environment. The waste from hard coal mining accumulated in landfills is a considerable burden for the local natural environment. The scale and extent of this phenomenon depends on many factors, including the amount and type of the waste, as well as its chemical composition and the content of heavy metals, the concentration of natural radionuclides, radon exhalation and other physical properties (22. Puertas, F.; Suárez-Navarro, J.A.; Alonso, M.M.; Gasco, C. (2021) NORM waste, cements, and concretes. A review. Mater. Construcc. 71 [244], e259. https://doi.org/10.3989/mc.2021.13520.
).

In addition to the aspects related to environmental protection, the economic aspect is also important which is related to the process of storing the waste and the resulting financial burden.

Some of the waste, due to its properties, can be used in the construction industry (33. Subashi De Silva, G.H.M.J.; Hansamali, E. (2019) Eco-friendly fired clay bricks incorporated with porcelain ceramic sludge. Constr. Build. Mater. 228, 116754 https://doi.org/10.1016/j.conbuildmat.2019.116754.
-44. Kusiorowski, R.; Zaremba, T.; Piotrowski, J. (2014) The potential use of cement-asbestos waste in the ceramic masses destined for sintered wall clay brick manufacture. Ceram. Int. 40, 11995-12002, https://doi.org/10.1016/j.ceramint.2014.04.037.
), e.g. in the production of construction ceramics (5-85. Luo, L.; Li, K.; Fu, W.; Liu, C.; Yang, S. (2020) Preparation, characteristics and mechanisms of the composite sintered bricks produced from shale, sewage sludge, coal gangue powder and iron ore tailings. Constr. Build. Mater. 232, 117250. https://doi.org/10.1016/j.conbuildmat.2019.117250.
6. Lemougna, P.N.; Yliniemi, J.; Ismailov, A.; Levanen, E.; Tanskanen, P.; Kinnunen, P.; Roning, J.; Illikainen, M. (2019) Recycling lithium mine tailings in the production of low temperature (700-900 ^oC) ceramics: Effect of ladle slag and sodium compounds on the processing and final properties. Constr. Build. Mater. 221, 332-344, https://doi.org/10.1016/j.conbuildmat.2019.06.078.
7. Stolboushkin, A.Y.; Ivanov, A.I.; Fomina, O.A.(2016) Use of coal-mining and processing wastes in production of bricks and fuel for their burning. Procedia Eng. 150, 1496 - 1502. https://doi.org/10.1016/j.proeng.2016.07.089.
8. Kusin, F.M.; Munirah, S.N.; Hasan, S.; Hassim, M.A.; Molahid, V.L.M. (2020) Mineral carbonation of sedimentary mine waste for carbon sequestration and potential reutilization as cementitious material. Environ. Sci. Pollut. Res. 27, 12767-12780. https://doi.org/10.1007/s11356-020-07877-3.
). For instance the waste form glass wool industry can be used to porous ceramics production (99. Lemougna, P.N.; Yliniemi, J.; Adediran, A.; Luukkonen, T.; Transkanen, P.; Finnilä, M.; Illikainen M. (2021) Synthesis and characterization of porous ceramics from spodumene tailings and waste glass wool. Ceram. Int. 47 [23], 33286-33297. https://doi.org/10.1016/j.ceramint.2021.08.231.
-1010. Adediran, A.; Lemougna, P.N.; Yliniemi, J.; Transkanen, P.; Kinnunen, P.; Roning, J.; Illikainen, M. (2021) Recycling glass wool as a fluxing agent in the production of clay- and waste-based ceramics. J. Clean. Prod. 289, 125673. https://doi.org/10.1016/j.jclepro.2020.125673.
). The following mining wastes may be used in the construction ceramics industry (1111. Rubin, J.A. (2019) The influence of material and technological factors on radon exhalation from concrete with a cement matrix. Monograph. Publishing house of the Silesian University of Technology. Gliwice, 2019.
):

Shale is one of the components of gangue obtained in the extraction of hard coal and it is stored in dumps. The coal extraction and separation process does not have ideal efficiency and therefore a certain amount of coal is stored in dumps along with waste rocks. This situation can lead to self-ignition which will initiate thermal activity. High temperatures may change some physical properties of shale. Additionally, a dump with shale poses a potential risk to the environment due to the possibility of sulfides leaching, which contaminate groundwater and soil. Thermally active dumps emit significant amounts of CO and CO₂ (12-1412. Gawor, L. (2014) Coal mining waste dumps as secondary deposits - examples from the Upper Silesian Coal Basin and the Lublin Coal Basin. Geol. Geophys. Environ. 40 [3], 285-289. https://doi.org/10.7494/geol.2014.40.3.285.
13. Abramowicz, A.; Rahmonov, O.; Chybiorz, R. (2021) Environmental management and landscape transformation on self-heating coal-waste dumps in the Upper Silesian Coal Basin. Land. 10, 23. https://doi.org/10.3390/land10010023.
14. Szczepanska-Plewa, J.; Stefaniak, S.; Twardowska, I. (2010) Coal mining waste management and its impact on the groundwater chemical status exemplified in the Upper Silesia Coal Basin (Poland). Pol. Geol. Inst. Spec. Pap. 441, 157-166.
).

Mining waste can be used in ceramic production as a component of ceramic masses (1111. Rubin, J.A. (2019) The influence of material and technological factors on radon exhalation from concrete with a cement matrix. Monograph. Publishing house of the Silesian University of Technology. Gliwice, 2019.
, 1515. Fecko, P.; Tora, B.; Tod, M. (2013) Coal waste: handling, pollution impacts and utilization. The coal handbook: towards cleaner production: Volume 2: Coal Utilisation.
, 1616. Murzyn, P.; Pyzalski, M. (2018) Characteristis and assessment of the suitability of coal-rich mining waste in the production of ceramic building materials. Multidisc. J. Waste Resour. Resid. 1, 30-37. https://doi.org/10.26403/detritus/2018.20.
). Self-burned shale - waste from mining industry, due to its chemical and mineralogical composition, is similar to brick clay. It’s physical properties are also similar to brick clay. It can be classified as sintering materials and can therefore be useful for the production of ceramic products (1111. Rubin, J.A. (2019) The influence of material and technological factors on radon exhalation from concrete with a cement matrix. Monograph. Publishing house of the Silesian University of Technology. Gliwice, 2019.
).

A significant problem with the proposed mining waste management is the varied content of natural radionuclides, e.g. ²²⁶Ra, ²³²Th and ⁴⁰K, present in investigated wastes. The research (1111. Rubin, J.A. (2019) The influence of material and technological factors on radon exhalation from concrete with a cement matrix. Monograph. Publishing house of the Silesian University of Technology. Gliwice, 2019.
) carried out on 24 samples of shale collected from 13 dumps in the area of the Upper Silesian Coal Basin show the variability of the concentration of the above radionuclides: ²²⁶Ra (52.7 - 367 Bq·kg^-1), ²³²Th (3.5 - 106 Bq·kg^-1), ⁴⁰K (241 - 892 Bq·kg^-1).

The permissible limit for the concentration of natural radionuclides in construction materials which reach the trading standards set by the European Union is regulated by the EURATOM 2013/59 Directive (1717. Council Directive 2013/59/Euratom of 5 December 2013 laying down basic safety standards for protection against the dangers arising from exposure to ionising radiation, and repealing Directives 89/618/Euratom, 90/641/Euratom, 96/29/Euratom, 97/43/Euratom and 2003/122/Euratom.
). The maximum concentration of radium ²²⁶Ra, thorium ²³²Th and potassium ⁴⁰K is determined by the following formula (Equation [1]):

where C_Ra, C_Th and C_K mean the activity concentration of ²²⁶Ra, ²³²Th and ⁴⁰K, respectively (expressed in Bq·kg^-1).

The expression is used to ensure that annual effective dose for people, does not exceed 1 mSv if such material is used to construction the model room, according to the following scenario:

Another aspect related to natural radioactivity is the exhalation of radon ²²²Rn from ceramic blocks produced from mining waste. Radon is a noble gas, a product of radioactive decay of ²²⁶Ra. It can migrate in the pore space and get out of the block (exhalation). However, the exhalation rate depends not only on the concentration of the parent isotope - ²²⁶Ra, but also on the porosity and permeability of the material. Directive EURATOM 2013/59 (1717. Council Directive 2013/59/Euratom of 5 December 2013 laying down basic safety standards for protection against the dangers arising from exposure to ionising radiation, and repealing Directives 89/618/Euratom, 90/641/Euratom, 96/29/Euratom, 97/43/Euratom and 2003/122/Euratom.
) introduces the reference level of radon concentration in air in buildings of 300 Bq·m^-3. The construction materials should not result in the increase of radon concentration in rooms above this limit.

In the scope of this study, the possibility of using shale from one of the Upper Silesian Coal Basin dumps as the basic raw material in the production of construction ceramics was investigated. Three ceramic samples (cylindrical bricks) were prepared. The samples were sintered in three various temperatures in order to verify the influence of this parameter to mechanical and radiological properties. Subsequently, the basic mechanical properties of sintered bricks were tested. The radioactivity content and radon exhalation rate were also examined.

2. MATERIALS AND METHODS

2.1 Study area

 ⌅

This work focuses on mining waste generated in the Upper Silesian Coal Basin, Poland (Figure 1). This is an area where the industry has been dominated by coal mining for almost 200 years. The management of the generated waste is a very important issue for this region. Approximately, 30 million tons of waste from hard coal mining is generated in Poland annually and 600 million tons was deposited in Poland - accumulated in landfills, dumps and sediment ponds (1616. Murzyn, P.; Pyzalski, M. (2018) Characteristis and assessment of the suitability of coal-rich mining waste in the production of ceramic building materials. Multidisc. J. Waste Resour. Resid. 1, 30-37. https://doi.org/10.26403/detritus/2018.20.
).

Figure 1.  The location of the investigated mining area - the Upper Silesian Coal Basin, Poland.

2.4 Radiological characterization

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2.4.1 Gamma-ray spectrometry

 ⌅

The activity concentration of the following radionuclides ²³⁸U, ²³⁵U, ²²⁶Ra, ²²⁸Ra, ²²⁸Th, ²¹⁰Pb and ⁴⁰K was determined using a gamma-ray spectrometry equipped with a hyper purity germanium detector (HPGe) with relative efficiency of 35%, cooled by liquid nitrogen (LN₂). The energy calibration was performed using a multi-gamma source which contains isotopes (²²⁶Ra, ²¹⁴Bi, ²¹⁴Pb, ²¹⁰Pb, ²²⁸Ac, ²¹²Pb ²⁰⁸Tl), emitting gamma lines in a wide range (46 - 2600 keV). Specialized software, LABSOCS by Canberra, was applied to perform efficiency calibration. It relays on the Monte Carlo simulation model which is based on the characteristics of detector and sample (chemical composition, dimensions, etc). This approach enabled the non-destructive measurement of the samples which were also investigated in radon exhalation experiments. The samples were tested in their original shape - a cylinder of height 8 cm and 8 cm in diameter (the dimensions could be affected by contraction which occurred after brick firing). The samples were covered with radon leakage proof aluminum foil in order to reach radioactive secular equilibrium between radium and radon progeny. The mathematical model has been validated by the use of certified reference materials (CRM) containing natural radionuclides - RGU-1, RGTh-1 and RGK-1 provided by the International Atomic Energy Agency (IAEA). The LabSOCS calculates the density correction factor and the same model can be applied to samples with different density. The software calculates all the correction factors related to cascade coincidence summing (CCS). The following energy lines were used in activity concentration determination: ²³⁸U was determined via ²³⁴Th (63 keV) and ^234mPa (1001 keV); ²³⁵U was determined directly (144 and 163 keV) and additionally via line interfering with ²²⁶Ra (186 keV); ²²⁶Ra was determined via radon progeny ²¹⁴Bi (609, 1120 and 1765 keV) and ²¹⁴Pb (295 and 352 keV) after a 3-week waiting period in order to reach secular equilibrium between radium and radon progeny; ²²⁸Ra was determined via ²²⁸Ac (911 keV); ²²⁸Th was determined via ²¹²Pb (239 keV) and ²⁰⁸Tl (583 keV) - the appropriate correction for series branching was applied; ²¹⁰Pb and ⁴⁰K were determined directly based on 47 and 1461 keV, respectively.

Based on the results of the gamma-ray spectrometry, the activity index I was calculated according to Equation [1]. Activity index was introduced in the report of the European Commission - Radiation protection 112 and Directive EURATOM 2013/59. One element in Equation [1] - ²³²Th cannot be determined by gamma-ray spectrometry directly due to the lack of significant gamma lines in the spectra. However, in the case of secular radioactive equilibrium, ²³²Th may be determined via its progeny - e.g. ²²⁸Ac or ²¹²Pb. In such a situation, the progeny isotopes concentration may be assigned to the primordial isotope.

2.4.2 Radon exhalation rate determination

 ⌅

The radon ²²²Rn exhalation rate of the tested samples was determined. The measurement of the radon exhalation rate was performed using the closed chamber method (CCM). This method is commonly used for the measurement of the radon exhalation rate of building materials. A similar method was used in investigations performed by Leonardi et al. (1919. Leonardi, F.; Bonczyk, M.; Nuccetelli, C.; Wysocka, M.; Michalik, B.; Ampollini, M.; Tonnarini, S.; Rubin, J.; Niedbalska, K.; Trevisi, R. (2018) A study on natural radioactivity and radon exhalation rate in building materials containing NORM residues: preliminary results. Constr. Build. Mater. 173, 172-179. https://doi.org/10.1016/j.conbuildmat.2018.03.254.
). A cylindrical exhalation chamber of 60 dm³ was used. The thickness of the chamber’s steel walls is 5 mm. Active monitor of radon concentration - AlphaGuard equipped with an open ionization chamber and the tested sample was placed inside the chamber, closed and sealed. Effective chamber volume when taking into account the presence of AlphaGuard and sample is 55.5 dm³. In the experiments, the samples, previously measured with gamma-ray spectrometry, were tested. The measurement time was about 7 days per sample. A 1-hour period in diffusion mode (without pumping) was applied. After the experiment, the results were transferred from the device to a PC and analyzed.

In CCM, radon concentration in the accumulation chamber is related to the radon exhalation rate of the sample enclosed in it. In particular, radon concentration built up C(t) inside the chamber is modeled by two-dimensional diffusion theory, according to Sahoo (2020. Sahoo, B.K.; Mayya, Y.S. (2010) Two dimensional diffusion theory of trace gas emission in soil chamber for flux measurements. Agric. For. Meteorol. 150, 1211-1224. https://doi.org/10.1016/j.agrformet.2010.05.009.
) and can be expressed by the following formula:

$C\left(t\right)=C_0+C_m(1-e^{-\lambda t})$  [3]

$C_m=\frac{E\bullet S}{\lambda \bullet V_{eff}}$  [4]

Where,

E = radon exhalation rate (Bq m^-2 h^-1)

S = surface of sample exposed in the chamber (m²)

V_eff = volume of accumulation chamber - [volume of AlphaGuard + volume of the sample] (m³)

λ = effective decay constant of radon (h^-1)

C_o, C_m = radon concentration in the accumulation chamber at time=0 and the maximum value (Bq·m^-3).

Starting from equation 3-4, the data was plotted and fitted in MS Excel using an exponential function and Solver algorithm (see results shown in Figure 2). The parameter achieved from the best fit and used to calculate the radon exhalation rate according to eq. 3 is C_m. Then, E is calculated according to Equation [5]:

$E=C_m\frac{\lambda \bullet V_{eff}}{S}$  [5]

Figure 2.  Growth of radon concentration in the exhalation chamber - sample CL_1000.

3. RESULTS AND DISCUSSION

4. CONCLUSIONS

In the scope of this work, ceramic bricks were prepared using mining waste - shale collected from dumps. The physical and chemical properties of raw materials and prepared ceramic elements were tested. Tested properties of bricks, such as density, porosity and compressive strength, indicate significant similarity to conventional construction ceramics. However, the influence of burning temperature on the physical properties of bricks, such as compressive strength, is noticeable. The closest similarity to conventional ceramics is observed in the case of the bricks burned at 1100^oC. The lack of alkali elements in used shale is probable reason of poor compressive strength of bricks prepared in temperature below 1100^oC. Prepared bricks can be classified to the 20th class of compressive strength according to EN 771-1:2011+A1:2015-10 Requirements for masonry elements. Part 1: Ceramic masonry elements. Also, the highest values of other functional parameters were observed in the case of the highest burning temperature. Using a waste or clay containing more alkali elements can improve mechanical parameters of bricks fired in temp. 900-1000^oC.

The leachability of some elements should be tested, however the authors did not have capabilities to perform such experiment. Nevertheless, the result of leaching experiment could help to understand how the elements are bounded in the brick structure.

The burning temperature did not influence radionuclide content in bricks. The reduction of radionuclide concentration during the firing of bricks was not observed. There was no difference (in the range of measurement uncertainty) among the bricks prepared at each temperature. In each case, the activity index I did not exceed the limit value. As a result, there is no limitation in the use of such bricks in residential construction. Despite the relatively high concentration of ²²⁶Ra and moderate permeability, the measured radon exhalation rate E is very low. This may be caused due to the formation of a tight shell around the radium-containing grains during the burning process. Such a shell reduces radon migration to pore space (emanation) and results in low exhalation rate. However, this explanation requires separate research.

Due to the advantageous physical and chemical parameters of ceramics made from mining waste, the recommendation of this method waste recycling is justified.