1. Introduction
The waste management system is a multifaceted structure whose operation should ensure sustainable development in three aspects: economic, environmental and social. The Waste Framework Directive 2008/98/WE sets out a clear hierarchy of waste management: reduce, reuse, recycle, recover and dispose [
1,
2]. Note that this hierarchy must be applied flexibly. Two criteria should always be taken into account, i.e., economic efficiency and the impact of the adopted method on the environment throughout the full life cycle. In 2016, the total waste generated in the EU-28 by all economic activities and households amounted to 2.5 billion tons [
2]. The municipal fraction accounts for 8% of the total waste stream generated annually in the EU-28. Solid municipal waste is a big problem in many countries, because it is visible and has a complex composition, many sources of origin, and is related to the consumption patterns in a given society. 47% of all solid municipal waste generated in the EU-28 is recycled or composted [
2]. However, waste management practices differ in different countries. In several EU countries, landfilling is still the basic method of municipal waste management. In countries such as Estonia, Luxembourg, France, Ireland, Slovenia, Italy, Great Britain, Lithuania and Poland, approximately 1/3 of municipal waste generated is stored, and more than 40% (except in Estonia) is recycled and subjected to thermal processing [
2].
Thermal processing is one of the elements of the current municipal waste management. Waste that cannot be recycled for various reasons is used for the production of electricity and heat. Additional advantages of thermal processing of municipal waste include: reduction of volume by 90% and weight of waste by 70% and destruction of pathogens dangerous for human health [
3,
4,
5]. As a result of the thermal processing of municipal waste, secondary waste (fly and bottom ash, slag, dust from the dust removal system, etc.) is generated, which requires further management due to the negative environmental impact. The chemical composition of ashes resulting from the thermal degradation of municipal waste differs significantly from the composition of ashes generated as a result of the combustion of ‘clean’ coal or biomass. Compounds of alkali metals and so-called acid compounds (sulfur, chlorine) present in the waste are emitted to the atmosphere in fly ash, while heavy metals, silicon or calcium deposited in bottom ash. Fly and bottom ash from thermal processing of municipal waste are often classified as hazardous waste [
3,
4,
5].
There are several ways to handle this type of waste, including storage of hazardous waste in the landfill, depositing in closed excavations of potassium salt, combustion in hazardous waste incineration plants and stabilization/solidification [
6,
7,
8]. Some types of secondary waste, e.g., slags, are used in construction. The development of technology aims at a wide use of mineral binders for the disposal of hazardous waste through the solidification process (immobilization). An analysis of the literature proves that the immobilization of dangerous method, in many regions should be used as the basic method of management of this type of wastes. Immobilization is recommended due to its relatively low cost, a possibility to transform many types of waste, which may be neutralized with this method (different chemical content) and the fact that solidified/stabilized waste do not pose any threat for the environment and people’s health and they may be used in the industry [
8]. According to the information written in the literature, the dangerous waste which most often undergoes a process of immobilization is industrial dust and sludge, gravels and ashes from the thermal process (including iron and steelworks processing, non-ferrous metal processing, municipal waste incineration plants, etc.). Solidification/stabilization technologies may be divided into 6 groups depending on the main components used and processes applied: cement-based, lime-based, thermoplastic processes, based on organic polymers, based on hermitization, and melting processes (grass). These groups can be applied in different ways and have different requirements for initial waste processing. However, all the groups aim at achieving physicochemical properties of the wastes such that the following goals of the process can be achieved: decreasing migration of the pollution into the environment, production of unified concrete matrixes that can be economically taken advantage of and facilitation of disposal, and transport of the wastes to the landfill. The literature sources mention that the stabilization of ash from municipal waste incineration plants, without preliminary processing, in concrete matrixes does not guarantee a decrease of leachability of chlorides and sulfates to the required permissible values. Furthermore, the content of chloride and sulfate salts may adversely affect the durability of concrete matrixes [
8,
9,
10]. The immobilization (solidification) process makes it possible to change the physical and chemical properties of the waste, as well as to reduce the solubility and leachability of substances harmful to the natural environment. To permanently immobilize waste in concrete, cement is commonly used, in accordance with standard requirements [
9,
10]. Solidification matrices for hazardous waste should have good physio-mechanical properties, which consequently affects the durability of composites over a long period of time. The durability of composites solidifying waste is important from an environmental point of view because it affects the leaching of contaminants into the aquatic or soil environment with which these matrices are in direct contact [
9,
10].
Concrete solidifying of hazardous waste stored in the environment may be exposed to various adverse environmental conditions (temperatures, precipitation, chemically aggressive groundwater and other aggressive liquids). The literature [
11,
12,
13,
14,
15,
16,
17,
18] contains little information on the impact of ashes from the municipal waste incineration plant on the properties of cement mortars as this is a novelty in the discussed problem. This article focuses on assessing the effectiveness of the process of immobilizing hazardous waste (ashes) by analyzing the leaching of contaminants (aggressive ions and heavy metals) from monolithic and crushed concretes. The purpose of the research performed was to design an environmentally friendly concrete mix and to use ash as an alternative aggregate in accordance with the idea of the Circular Economy. The performed tests demonstrate that the high resistance of the concrete makes it possible to use this type of waste for example for protecting waste landfill, where it may form a layer that separates the waste from the environment. Furthermore, the suggested solution will contribute to the decrease of greenhouse gases, CO
2 in particular, as the cement is replaced with waste. Moreover, the generated waste will be used in their place of origin which will eliminate the economic and environmental costs of international transport.
4. Results and Discussion
Table 2 presents the basic technical properties of the ashes tested. The total moisture of the tested ashes was low—below 1.5%. It should be added that the fly ash tested showed hygroscopic properties. In laboratory conditions, a water content increase of approx. 3% was observed within 7 days. The bulk density of the examined ashes was in the range (478–540) kg/m
3, which is lower than the density of fly ash from coal power plants (ok. 799 kg/m
3).
The specific surface area for the fly ash was 7454.77 cm2/g, and 150.9.0 cm2/g for the bottom ash, which classifies the tested ashes into a group of ashes with a developed specific surface area. This is due to their fine grain size and the occurrence of many porous, spherical grains with an extensive structure.
Figure 4 shows the results of loss on ignition (LOI) for the ashes tested. Ignition losses were determined by heating the tested ashes to constant mass in a muffle furnace at three temperatures: 600 °C, 815 °C and 950 °C in an oxidizing atmosphere.
In both cases, it was found that loss on ignition (LOI in 600 °C) met the criteria for admission of non-hazardous (LOI ≤ 8%) and inert waste (LOI ≤ 10%) at the landfill. In addition, ash loss on ignition was determined by ignited samples at 950 °C for an extended time of up to 1 h. This parameter is important because of the production of concrete blocks for the immobilization of contaminants. Fly ash due to the loss on ignition is divided into three categories: category A (LOI ≤ 5%), category B (LOI ≤ 7%), and category C (LOI ≤ 9%). Based on literature data, it is recommended to use category A ash for concrete, as high losses on ignition in ashes may result in deterioration of workability of the concrete mixture. Fly ash may be problematic because of its workability as its losses on ignition were above 20%.
Table 3 presents the chemical composition of the ashes tested. Total carbon in the tested ashes was below 7%. Particular attention should be paid to the content of organic carbon, as excessive amounts cause a decrease in the effectiveness of chemical admixtures, especially aeration agents, plasticizers and superplasticizers. In addition, the pozzolanic activity also decreases, affecting the unsightly appearance of the concrete surface and hindering the process of surface hardening of the concrete. The organic carbon content is a maximum of 3% of the ash mass. In the case of bottom ash, the total sulfur constitutes 1.41% of the waste mass, and for the fly ash, this value is 0.66%. The tested ashes had a high Ca content; in fly ash, this was at the level of 15.7%, while in the bottom ash, the value was 23.81%. Calcium content is associated with the waste gas processing system. Particular attention should be paid to the chlorine content in the ashes tested. The fly ash had a chlorine content of 7.22%, while the chlorine content in the bottom ash was 5.44%. Chlorine is an undesirable and environmentally troublesome element.
The heavy metal content in the fly and bottom ash formed during the process of thermal degradation of municipal waste had the following sequence: Zn > Pb > Cu > Cd. The tested ashes had a heavy metal content in the range presented in
Figure 5. Among the analyzed metals, the highest content was found for zinc, at 944.10 mg/kg in the case of bottom ash, and the lowest was found for cadmium, at 56.88 mg/kg in the case of fly ash.
Table 4 presents the leachability ranges of selected contaminants that may constitute an environmental problem.
High pH values for the tested ashes (pH > 9) may indicate high immobilization of heavy metals in the material. This is confirmed by the low concentrations of the studied metals in water extracts (
Table 5). The analyzed ashes can be an environmental problem due to their high salt content, mainly in the case of chloride and sulfate salts. The chloride leaching from both tested ashes exceeded the permissible levels for storage at hazardous waste landfills. The level of sulfate leaching was high but did not exceed the permissible standards.
The heavy metal content, i.e., barium, zinc, lead, copper, cadmium, chromium, cobalt, iron, manganese and nickel, was determined in the water extracts obtained from the ashes tested (
Table 5). From the fly ash, Ba was leached to the greatest extent, followed by Zn > Pb > Ni > Cu; the content of other metals (Cd, Cr, Co, Fe, Mn) was below the limit of quantification. Meanwhile, in the case of the bottom ash, Ba was leached the most, and then Ni > Cr > Pb > Cd, and the content of metals such as Zn, Cu, Co, Fe, Mn was below the limit of quantification. Due to the high content of barium, the ashes tested did not meet the criterion for being stored in landfills for non-hazardous and inert waste, or even for hazardous waste. The barium content exceedances were very high.
In parallel with the physicochemical research on ashes, concrete blocks with 30% waste added were designed and manufactured. The manufactured concrete blocks were matured for 28 days in laboratory conditions, after which water extracts were made of them. The leachability of contaminants from concrete with the addition of hazardous waste ashes may be related to the form in which this concrete occurs (monolithic or comminuted form), and this may translate into an environmental nuisance. In the case of monolithic forms, the level of leaching may mainly be determined by the process of release from the surface and diffusion.
In the case of crushed concrete, the leaching of contaminants determines the percolation process. This article presents the leachability of contaminants harmful to the environment for both forms of concrete. Both cases are analyzed, and the results obtained are presented later in the article.
Table 6 presents the leachability of selected contaminants from monolithic concrete that can potentially be a nuisance to the natural environment. High pH values (pH = 11) may indicate high immobilization of heavy metals and chloride and sulfate salts. The leachability of chloride and sulfate salts does not exceed the permissible levels for the storage of waste in landfills other than those for hazardous and inert waste. Based on this research and the results obtained, it can be determined that the ashes subjected to the immobilization process in the concrete matrix were not harmful to the environment. This is also confirmed by the low concentrations of tested metals in the water extracts (
Table 7).
The leachability of heavy metals from monolithic concrete forms after 28 days of maturation is presented in
Table 7.
The obtained concentrations of heavy metals in water extracts were compared with the permissible content for waste intended for storage in non-hazardous and inert landfills and hazardous waste. It is worth emphasizing that no content of heavy metals leached from monolithic concrete with 30% ash addition exceeded the permissible storage values for either considered case. The content of heavy metals in water extract from monolithic concretes was arranged in the following sequence: with the addition of fly ash, Ba > Pb > Cu; and with the addition of bottom ash, Ni > Pb.
In the course of the tests carried out, the crushed concrete was also leached.
Table 8 shows the content of selected contaminants that can be harmful to the environment in the water extracts from crushed concrete after 28 days of maturation.
High pH values (pH > 12) may indicate high immobilization of heavy metals, as well as chloride and sulfate salts. The leachability of chloride and sulfate salts in crushed concretes more than doubled in relation to monolithic concrete; this did not exceed the permissible levels of waste storage. This is also confirmed by the low concentrations of tested metals in water extracts (
Table 8).
Table 9 shows the content of heavy metals in water extracts of crushed concrete after 28 days of maturation. The obtained contents of heavy metals in water extracts were compared with the permissible contents for waste intended for storage in non-hazardous and inert landfills and hazardous waste [
12]. It is worth emphasizing that none of the contents of heavy metals have leached from crushed concrete with the addition of ashes exceed the permissible values for storage at a hazardous waste landfill [
12].
It should be noted that the crushed concrete with the addition of fly ash does not exceed the permissible values for storage at the landfill for non-hazardous and inert waste. Ba was marked by a high level of leaching, while in the case of crushed concrete with the addition of fly ash, it was 72.83 mg/kg; however, this content did not exceed the permissible values for storage in landfills for either considered case. Leachability more than three times higher was recorded for crushed concrete with the addition of bottom ash, the content of which was 241.93 mg/kg. Such a situation prevents the storage of this type of concrete in landfills other than those for hazardous and inert waste. The content of heavy metals in water extracts from crushed concrete was arranged in the following sequence: with the addition of fly ash, Ba > Ni > Pb > Cd; and with the addition of bottom ash, Ba > Ni > Fe > Pb.
The concentration of heavy metals in the water extracts obtained from the monolithic concrete forms after 28 days of maturation was lower than the concentration of heavy metals in the water extracts obtained from the crushed concrete. This may be due to the fact that subsequent crushing of concrete reveals subsequent surfaces from which heavy metals can be leached.
Figure 6 and
Figure 7 present the test results for the bending and compressive strength of concrete mortars with the admixture of fly ash and bottom ash. For comparison of the obtained test results, the same trial was performed for mortar with fly ash from the power plant. The reference concrete mortar was Portland cement CEM I 42.5 R.
Analysis of the results of the performed tests showed that the tested ashes form a component that fills the microstructure of the mortars without the properties of puzzolana, similar to powder, commonly used for mortars (
Figure 8).
The results of the presented tests prove that both fly ash and bottom can potentially be applied. Furthermore, hazardous waste that is a significant nuisance for the environment is being managed. It also has to be noted that the tested mortar did not undergo volume change and maintained its original dimensions. It can be concluded that the tested ashes, when used as a replacement for the cement, do not cause volume change of the cement binder, which is important from the point of view of the possibility of application.
The test results show unequivocally that concrete mortars with the admixture of ash (fly ash and bottom ash) are characterized by the same resistance as concrete mortars with the admixture of limestone powder, which are widely used in the construction industry. On the basis of the obtained test results, it can be concluded that the design concrete matrixes can be applied in similar technological circumstances. A search for a concrete matrix that is environmentally friendly and has good mechanical properties with the addition of ash from the thermal processing of municipal waste is still ongoing.
5. Conclusions
The obtained results confirm that the tested ashes, which come from the thermal degradation of municipal wastes, may be successfully neutralized in the concrete matrix. The test results show that the designed concrete matrix makes it possible to decrease the leaching of chloride and sulfate salts to the permissible values required by law. The obtained results give a basis for the use of the designed concrete matrix with the admixture of ashes for the construction of roads and other construction work in landfills. Particular attention should be given to the need for individual preparation of the concrete mix composition in accordance with the characteristics of the waste in order to achieve the most environmentally friendly matrix.
The research makes it possible to formulate the following conclusions. Solid ash does not meet the current legal requirements for storage at hazardous waste landfills. Storage in closed excavations of potassium salt mines is not possible in every European country. The tested ashes from the combustion of solid municipal waste contained a large amount of chlorine, which may affect the durability of the concrete. The tested ashes were characterized by high (0.1–10 g/kg) contents of zinc, lead and copper, and an average content (1–100 mg/kg) of cadmium in dry matter. Studies have shown that in cases of the improper preservation and/or processing of ash, there is a high risk of migration of aggressive ions such as chlorides, as well as sulfates and heavy metals, into the environment. The content of chloride ions in the ashes exceeded 3 times the permissible value of leaching of this parameter from waste intended for storage in hazardous landfills. The obtained results confirm that immobilization is an effective process for reducing the content of contaminants in the water extract. The content of chloride in monolithic and crushed concretes has been almost completely immobilized by the C-S-H phase. The degree of immobilization exceeds 98%. Also, the content of sulfate ions is immobilized at a similar level of 96% for both forms of the concrete matrix under consideration. It is possible to permanently immobilize heavy metals (Zn, Cu, Pb, Cd, Cr, Co, Fe, Mn, Ni) in a concrete matrix. The amount of metal ions that can be leached from concrete, both monolithic and crushed, is negligible during use.
The presented results are preliminary tests in a program aimed at limiting the leaching of contaminants from secondary waste generated in the process of thermal processing of the municipal fraction. In the next research steps, the designed concrete matrix with the addition of ashes should be tested in various environmental exposure classes according to PN EN 206, also to determine whether leachability parameters change after the concrete has remained in the given exposure classes.