Leaching Characteristics of Wood Biomass Fly Ash Cement Composites

: Wood biomass ash (WBA) represents an environmental and economic problem for energy producers, and there have been extensive studies of using WBA as a raw material in construction products. This study investigates the leaching characteristics of WBA cement composites based on testing results. Currently, 70% of WBA is landﬁlled, and the rest is primarily used in agriculture; therefore, this study also addresses the leaching as a component of environmental safety of these two primary WBA management approaches. An analysis of the leaching characteristics of WBA, monolithic, and crushed cement composites is performed by replacing 15% of the cement with WBA. The study was conducted using three WBA samples collected from di ﬀ erent power plants. Increased values of leaching from the WBA itself are indicative of potential issues that could occur in the case of its disposal. The study concluded that the WBA could be potentially environmentally acceptable as a raw material in the concrete industry because the heavy metals are stabilized within the cement matrix.


Introduction
Recently, in Croatia and other European Union (EU) countries, the promotion of the advantages and possibilities for electrical and thermal energy generation has resulted in a significant increase in the number of power plants that use biomass (particularly wood biomass) as a renewable energy source (RES). Worldwide, RES usage is the greatest in the electricity sector, where biomass represents the third-largest renewable source and generates 493 TWh worldwide, and in the heat generation sector where 95% of the renewable energy comes from biomass [1]. Biomass combustion generates ash, including wood biomass ash (WBA). WBA represents an environmental and economic problem for energy producers: the costs of disposal continue to increase [2,3], while the EU is attempting to limit its disposal at landfills through various directives [4,5]. According to Directive 2008/98/EC on waste [4] and the new Directive (EU) 2018/851 [5], the priority for WBA management is preventing it from being created, then recycling it, and, lastly, disposing of it. An application for WBA reuse must be identified because prevention is infeasible due to the expected increase in the use of biomass as an energy source and legislative measures decrease disposal options.
During wood biomass combustion in a power plant, two primary types of WBA are generated: (1) wood biomass bottom ash, which is ash collected from the bottom of the combustion chamber, and (2) wood biomass fly ash that can consist of relatively coarse fly ash collected from the cyclone or boilers and fine fly ash collected from electrostatic filters or baghouse filters [6]. Fly WBAs and evaluate a second life stage in which the material is used in a granular form and response of leaching to the change in surrounding environmental conditions). From available literature [6,58] a large variation in the properties of local WBAs is observed depending on several factors such as the type of biomass used, the combustion technology, the temperature of the combustion and the location of the WBA collection. These variation also refer to heavy metals content in WBA samples that can present possible environmental risk [59]. Therefore, it is recommended to focus not only on a good technical performance but also on the release of contaminants in multiple life cycles of mortars that include biomass fly ash [57], which is addressed by this paper. When using WBA as a raw material in cement composites and in construction products, it is essential to determine its environmental impacts [30], which result from potentially enhanced contaminant leaching. The environmental acceptability of WBA as a potentially new raw material in the cement and concrete industry is related to the ability of the cement matrix to bind the contamination elements (in this case heavy metals) physically and chemically within the hydrate structure which should be analyzed. Each construction product within the EU must comply with European legislation [60], which ensures that any construction product for which there is a harmonised standard is CE-marked and possesses reliable information concerning its performance. One of the basic requirements of The Regulation (EU) 305/2011 [60] is "hygiene, health, and the environment", which means that every construction product must prove that during its use it is safe for health and the environment. Currently, in practice, construction products are not often tested for leaching. Construction products can potentially negatively impact the local environment by releasing hazardous substances into the soil or underground, sea, or surface waters. Hazardous substance releases require assessment, and, for the leaching of the hazardous substances previously discussed into the environment, the Technical Committee CEN TC 351 has developed laboratory tests that use demineralised water as eluent [61].
The present study investigates the potential for WBA reuse or disposal by assessing leaching as a component of the environmental safety of (1) WBA disposal, (2) monolithic cement composites in a hardened state with 15% WBA as a cement replacement, and (3) crushed cement composites with 15% WBA as a cement replacement. Three WBA samples were obtained from different power plants that use different combustion technologies and use virgin wood as a biomass source. This case study serves as guidance for the implementation of the essential requirement "hygiene, health, and the environment" in the products' CE markings.

Materials
For the leaching test, wood biomass fly ash samples WBA1, WBA2, and WBA3 were selected and used to replace 15% of the cement in the cement composites; this proportion of WBA is considered acceptable because it does not degrade the composites' mechanical or durability properties [62]. Three samples were obtained from different plants, and their characteristics are provided in Table 1. A survey was performed during sampling, in which the respondents confirmed that they used virgin wood chips in their energy production [63].
The heavy metals content of each WBA and the cement are shown in Table 2 and discussed in detail in [6]. The heavy metals content in WBAs and cement (zinc, cadmium, chromium, nickel, lead, manganese, cobalt, barium, bismuth, strontium, and copper) was evaluated according to EN ISO 16968:2015 [64], while mercury contents were evaluated according to the standard ASTM D 6722-11 [65]. The WBA heavy metals concentrations that exceeded those for the cement are highlighted in bold italic. As clearly demonstrated, all the heavy metals concentrations of the chosen WBAs were higher than those of the cement except for Sr. Comparing the heavy metals concentrations of the studied WBAs with those of bottom WBAs [7,8,56,57,[66][67][68][69], the Cd and Co concentrations are visibly higher for all three samples of tested fly WBAs.

Specimen Preparation and Testing Methods
The leaching tests and sample labels are shown in Table 3. Labels F1, F2, and F3 represent the samples obtained from the eluates of WBA1, WBA2, and WBA3, respectively. Crushed mortar samples M1, M2, and M3 had 15% of their cement replaced with WBA1, WBA2, and WBA3, respectively, and monolithic samples M15-1, M15-2, and M15-3 had 15% of their cement replaced with WBA1, WBA2, and WBA3, respectively. REF was a monolithic sample containing only cement as a binder, while M0 designates the crushed mortar sample without WBA. The WBA leachate (F1, F2, and F3) and crushed cement mortar (M0, M1, M2, and M3) samples were tested according to the standard EN 124 57-2:2005 [71], wherein the liquid to solid phase ratio (L/S) was 10 for the granulated material that was exposed to the more direct action of water. The crushed mortar sample with 15% WBA was prepared according to the standard EN 196-1:2016 [72]. The composition of the mortar and storage of materials before usage followed the standard, which defines that the mass ratio of CEN components, standard sand, cement, and water is 3:1:0.5. The mortar contained 1350 ± 5 g of sand, 450 ± 2 g of cement, and 225 ± 1 g of water; 15% of the cement was replaced with WBA. The mortar was crushed and sieved to a particle size less than 4 mm to produce the crushed mortar samples.
Leaching testing per EN 12457-2:2005 [71] involved separating 20 g of WBA, placing it in a closed 250 mL bottle, and filling the bottle with 200 mL of solvent (deionised water) in order to attain a desired ratio of liquid/solid (L/S = 10). The glass bottle containing the sample-water mixture was rotated (mixed) for 24 h. After the mixing was complete, the mixture was allowed to settle for 15 ± 5 min. The eluate was then passed through a membrane filter with a pore diameter of 0.45 µm. Analysis of the heavy metals concentration using a spectrometer with excitation in inductively coupled plasma-ICP-OES (ICPE 9000, Shimadzu) was preceded by sample stabilisation with 65% nitric acid.
For determining the leaching characteristics of the monolithic cement paste samples, testing was performed per the CEN/TS 16637-2 standard (DSLT method) [73], wherein the release of substances to the environment is primarily controlled by diffusion. Construction products suitable for the DSLT include facades (brick, concrete, wood panels, glazing, and coatings), roof coverings, pavement samples (from concrete, stone, asphalt, and concrete slabs), foundations (concrete blocks and concrete), coastal protection, gravel railroad sections (track ballast), and glass tiles [74]. The DSLT involves periodic leaching solution replacement, as specified by the standard, to ensure a substantial difference in concentration between the solid and liquid phases. As compared to a leaching test for granular materials, the DSLT requires more time but provides information on the kinetic mobility of the harmful substances and complex leaching mechanisms [75]. For the monolithic sample leaching tests per CEN/TS 16637-2 [73], cement paste samples were prepared according to the EN 196-3:2016 [76] standard, see Table 4. After 3 days of ageing, leaching tests were performed on prism samples with dimensions of 4 × 4 × 16 cm. The leaching vessel was a closed polypropylene plastic container. The eluate sampling was performed at 0.25, 1, 2.25, 4, 9, 16, 36, and 64 days. Samples were submerged in demineralised and deionised water with a conductivity of less than 0.5 mS/m, such that the ratio of the volume of the liquid to the exposed mortar surface was L/S = 90 L/m 2 , i.e., there was 90 L of solvent (deionised and demineralised water) to 1 m 2 of sample surface area. The solvent volume was obtained using the following expression: where surface area of the tested sample in m 2 , and L/A must be equal to 80 ± 10 L/m 2 .
According to the standard, the distance between the solvent and the vessel bottom, i.e., the distance between the sample and the solvent surface, must be at least 20 mm. The solvent was removed from the vessel at specified times and replaced with the same amount of demineralised and deionised water until the end of the next period. The fluid removed was passed through a 0.45 µm filter paper.
The pH and electrical conductivity of the removed fluid and the mass of the monolithic DSLT sample were measured. The filtered fluid was prepared for heavy metals concentration testing using ICP-OES (acidification with nitric acid to below pH 2 before testing).
Leaching of the substance per unit of sample surface area is given by where period i-period time from 1 to n, r i -leaching value per unit surface area of the sample for the period i (i = 1 to 8), in mg/m 2 , V-solvent volume in L, and A-surface area of the tested sample in m 2 .
The leaching ratio is defined as the ratio of the leached metal concentration in the eluate to the total amount of metal measured in the tested material [77,78]: where c i = concentration of individual leached heavy metal (mg/kg) and c tot = content of individual heavy metal in WBA according to Table 2.

Discussion of the Application of WBA in Agriculture
Circular WBA management is ecologically and economically viable at the local level. Based on the conducted survey made under project "TAREC 2 Transformation of Wood Biomass Ash into Resilient Construction Composites", 61% of WBA is landfilled, while 38% is used in agriculture [63]. The heavy metals concentrations in WBA that are shown in Table 2 were compared with the limit values for heavy metals concentrations in some EU countries (Table 5) to evaluate the applicability of WBAs in agriculture. In Croatia, the end-of-waste criteria are specified by the "Ordinance on by-products and end-of-waste status" (OG 11//14) [23], through which WBAs can obtain agricultural product status in three categories, depending on the levels of heavy metals allowed. None of the tested WBAs meet the heavy metals limit criteria for use in agriculture: the Cd concentrations in samples WBA1 (3.1 mg/kg) and WBA3 (6 mg/kg), the Zn concentration in sample WBA1 (1850 mg/kg), and the Ni concentration in sample WBA2 (170.3 mg/kg) were higher than the limit values in Table 5, prohibiting their use in agriculture. Notably, the tested WBA samples met only the criteria for application to forestry in Finland. Producing WBA in one country and using it in another as part of its circular management must be economically viable.
As shown in Table 2, the heavy metal index (HMI) was determined as an overall evaluation of the WBAs' heavy metals contents. The HMI was proposed by the authors of [6], wherein the heavy metals contents were normalised to the limit values for the standard potentially toxic elements (Cd, Hg, Cr, Cu, Pb, Ni, and Zn) to evaluate the use of waste products in agriculture [77]. Sample WBA1 has greatest HMI (18.2); it was collected from a power plant that uses a grate combustor and pulverised fuel combustor as its combustion technology ( Table 1). The authors of [3] state that the temperatures in a grate combustor can reach 1000-1200 • C, while combustion temperatures in fluidised beds are somewhat lower (less than 900 • C); in pulverised fuel systems, the peak flame temperatures are commonly near 1600 • C, which is extremely high as compared to the other combustion systems. Because combustion temperature affects the chemical composition of the WBA, including the metals content (metals content increases with increasing temperature [33]), it could be expected that WBAs from power plants with pulverised fuel combustors cannot be used in agriculture. However, this should be investigated further.

Leaching from the WBA and Crushed Cement Composites with 15% WBA as Cement Replacement
When landfilling any waste, the appropriate procedures and criteria should be followed. In consideration that most WBA is currently disposed of as waste (70% in Europe and 61% in Croatia), the WBA leaching values were compared to the leaching value limits for the waste categories (inert, non-hazardous, and hazardous waste) prescribed in the "EU Directive 1999/31/EC" [78,79] and shown in Table 6.
Results of the heavy metals concentration testing of the WBA eluate samples (F1, F2, and F3) are given in Table 6 along with the results of the heavy metals concentration testing of the crushed mortar samples with 15% WBA used as a cement replacement (M0, M1, M2, and M3). These samples represent the end-stage of the construction material life cycle, disposal.
From Table 6, Zn and Cr had the highest concentrations of the WBA eluates (F1, F2, and F3). Significant Cr leaching was observed for all WBAs tested, especially for sample F1. By comparing the results for the heavy metals Pb, Ni, Zn, Cr, Cd, and Cu in the WBA eluates (F1, F2, and F3) with the values obtained for the crushed mortar samples (M1, M2, M3), when the WBA was incorporated into the cement mortar, significant decreases in the heavy metals concentrations were observed. This can be explained by the reduced heavy metals mobility, i.e., the incorporation of the heavy metals into the cement matrix [80], and confirms that the heavy metals were successfully encapsulated within the cement matrix. Based on the values obtained, assessments of the bonding of the heavy metals in the new environment (cement matrix) and environmental safety can be performed. The leaching ratio is a useful index for safety assessment: larger values for metal solidification, i.e., lower concentrations of leached metal, indicate a lower environmental impact and greater safety when using and disposing of the material. Figure 1 shows that the Cr leaching values were significantly high in the WBA eluates, but when the WBA was used in the cement composites, these values decreased substantially. The same trend was observed for the other metals (Cd, Cu, Pb, and Zn) except Ni, the concentration of which was 27.42% greater in the M2 sample than in the reference mixture. Large values for Cr leaching from the WBAs themselves were observed in [57], but the authors concluded, through the testing of crushed mortar, that the metals chemically bond in the mineral phase or are absorbed by the cement matrix, and the same was observed when coal ash was used in concrete. However, the testing of WBA eluates in [81] did not reveal Cr leaching or deviations from limit values.  WBA leaching values (F1, F2, and F3) were compared to the leaching limit values for certain waste categories (inert, non-hazardous, and hazardous wastes) prescribed in the EU Directive 1999/31/EC [78,79] and are shown in Table 6 along with the results of the heavy metals concentration testing of the crushed mortar samples with 15% WBA used as a cement replacement (M0, M1, M2, and M3). These samples represent the end-stage of the construction material life cycle, disposal. Comparing the WBA eluate results shown to the limit values for the individual waste categories (using the conversion factor L/S = 10 L/kg), none of the tested WBA eluates (F1, F2 and F3) was in the WBA leaching values (F1, F2, and F3) were compared to the leaching limit values for certain waste categories (inert, non-hazardous, and hazardous wastes) prescribed in the EU Directive 1999/31/EC [78,79] and are shown in Table 6 along with the results of the heavy metals concentration testing of the crushed mortar samples with 15% WBA used as a cement replacement (M0, M1, M2, and M3). These samples represent the end-stage of the construction material life cycle, disposal. Comparing the WBA eluate results shown to the limit values for the individual waste categories (using the conversion factor L/S = 10 L/kg), none of the tested WBA eluates (F1, F2 and F3) was in the inert waste category, based on at least one of their analysed parameters, respectively Cr values for F1, F2 and F3 were high and close to the limit for non-hazardous waste. For the case of disposal, the WBA leaching results obtained illustrate the vital importance of compliance with disposal procedures and the need for systematic control and testing to prevent local environmental pollution.

Leaching from the Monolithic Cement Samples
The leaching characteristics of WBA used in cement composites was assessed by the cumulative leaching of heavy metals during a 64 d testing period, with the results shown Figure 2 and expressed as mg per m 2 of the surface area. The cumulative testing values are provided in Table A1 (Appendix A).
Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 17 inert waste category, based on at least one of their analysed parameters, respectively Cr values for F1, F2 and F3 were high and close to the limit for non-hazardous waste. For the case of disposal, the WBA leaching results obtained illustrate the vital importance of compliance with disposal procedures and the need for systematic control and testing to prevent local environmental pollution.

Leaching from the Monolithic Cement Samples
The leaching characteristics of WBA used in cement composites was assessed by the cumulative leaching of heavy metals during a 64 d testing period, with the results shown Figure 2 and expressed as mg per m 2 of the surface area. The cumulative testing values are provided in Table A1. (Appendix A).  Table A1. The exception to this trend was Ni; in the 8th time step (at 64 d), its concentration slightly increased. The  Table A1. The exception to this trend was Ni; in the 8th time step (at 64 d), its concentration slightly increased.
The initial leaching that occurs on the outer surface of the monolithic sample causes the initially larger concentration values. Over time, the heavy metals leaching occurs due to the diffusion of the heavy metals ions from the cement matrix, which was observed after the 8th time step of the testing and agrees with [66]. In most cases, the heavy metals leaching became insignificant after long-term submersion [82]. The leaching values reducing over time indicates that the heavy metals leaching initially comes only from the surface layer of the sample. Significantly more time must elapse before the metal from the interior part of the sample is released to the external environment, due to slowed diffusion, the effect of the metals bonding to hydration products, and the reduced leaching of the majority of the heavy metals given the high pH value of the sample interior [83][84][85][86][87]. Stronger initial leaching of the heavy metals from monolithic samples containing WBA was observed by [66], and the authors suggested that the heavy metals leaching was probably caused by the diffusion of heavy metal ions from the cement matrix. Surface leaching, dissolution, and diffusion are the three dominant mechanisms for releasing heavy metals from the cement matrix [88]. The strong linear relationships observed between the cumulative leaching and time for Cr (R 2 = 0.99 for all mixtures) and Zn (R 2 = 0.92 − 0.97) indicate diffusion-controlled leaching. For Ni, delayed leaching in the initial phase [84] with subsequent diffusion-controlled leaching was observed.
Cumulative leaching values for mixtures M1-15, M2-15, and M3-15 for metals Cd, Cr, Cu, and Pb are equal to or less than those of the reference mixture, as shown in Table 7. The WBA mixtures' cumulative Zn leaching values were 11.06% (M1-15) to 34.3% (M2-15) higher than reference mixture, demonstrating the sensitivity of the Zn leaching to substituting the cement with WBA. The cumulative leaching values for Ni were less than the reference mixture except for sample M2-15, for which the value was 27.42% greater than the reference. Leaching is heavily dependent on the pH value of the solution, i.e., the leaching increases as the pH value of the solution decreases [39]. The mineralogical phases of the metal oxides present in the WBA affect their sensitivity to the presence of H+ ions in leaching. At lower pH values, there is an increase in heavy metals leaching [39]. During the testing, the pH values of the eluate were between 10.8 and 11.7. The demineralised water becomes highly alkaline when it encounters concrete, especially when, as in this case, there is no water flow [82]. In the absence of national legislation regarding limit values for leaching from monolithic products, the cumulative leaching values per sample surface area obtained for certain metals were compared to the Dutch limit values given in the guideline: "Soil Quality Degree" [66,84,89] (see Table 7). Despite the increased levels of some metals (Zn and Ni) observed for the mixtures containing WBA as a partial substitute for cement, the leaching of the cement composites with 15% WBA was significantly below the limit values provided in the Dutch guidelines [66,84,89], i.e., the tested values of heavy metals leaching are acceptable. These results concur with those of [22,57] when pure wood biomass was used for combustion.

Conclusions
This study analysed the impact of using WBA in cement composites regarding leaching characteristics by examining three cases: (1) the leaching of the WBA itself (WBA eluate), (2) the leaching of the crushed mortar that contained 15% WBA as a partial cement replacement, and (3) the leaching of the monolithic samples with 15% WBA as a partial cement replacement, per the proposed standard CEN/TS 16637-1.
Based on the testing, the following conclusions were reached: • As an overall quality measurement, the heavy metals index (HMI), indicates that the WBAs from power plants using a pulverised fuel combustor cannot be used in agriculture, although this should be further investigated.

•
By comparing the test values from the WBA leaching to the limit values for the individual waste categories, it was found that all of the examined WBAs failed to meet the criteria for categorisation as inert waste based on at least one analysed parameter.

•
The values for heavy metals leaching for the monolithic samples with 15% WBA as a substitute for cement are less than those for the reference sample, excepting Zn, but still lower than the specified criteria. All tested samples meet the criteria given in "Soil Quality Degree".

•
During various stages in the life cycle of cementitious building materials containing WBA, the heavy metals are stabilized in the cement matrix, and therefore WBA could be used in the construction sector following the basic construction requirement of "hygiene, health, and the environment".

Conflicts of Interest:
The authors declare no conflict of interest.