Recycling Bio-BasedWastes into Road-Base Binder: Mechanical, Leaching, and Radiological Implications

Citation: Peceño, B.; Hurtado- Bermudez, S.; Alonso-Fariñas, B.; Villa-Alfageme, M.; Más, J.L.; Leiva, C. Recycling Bio-Based Wastes into Road-Base Binder: Mechanical, Leaching, and Radiological Implications. Appl. Sci. 2023,13, 1644. https://doi.org/10.3390/ app13031644 Academic Editors: Joan Formosa Mitjans and Jessica GiróPaloma Received: 21 December 2022 Revised: 20 January 2023 Accepted: 25 January 2023 Published: 27 January 2023 Copyright: © 2023 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/). applied sciences Article Recycling Bio-Based Wastes into Road-Base Binder: Mechanical, Leaching, and Radiological Implications B. Peceño 1, S. Hurtado-Bermudez 2, B. Alonso-Fariñas 3, M. Villa-Alfageme 2, J. L. Más4and C. Leiva 3,* 1Escuela de Prevención de Riesgos y Medioambiente, Facultad de Ciencias del Mar, Universidad Católica del Norte, Larrondo 1281, Coquimbo, Chile 2Dpto. Física Aplicada II, ETSA, Universidad de Sevilla, Avda. Reina Mercedes 2, 41012 Seville, Spain 3 Dpto. Ingeniería Química y Ambiental, ETS-Ingeniería, University of Seville, Camino de los Descubrimientos s/n, Pabellón Plaza de América, 41092 Seville, Spain 4Dpto. Física Aplicada I, ETSI-Informática, Universidad de Sevilla, Avda. Reina Mercedes s/n, 41012 Seville, Spain *Correspondence: [email protected] Abstract: This work presents a physical, mechanical, durability, leaching, and radiological assay of three wastes (egg and scallop shells and olive pomace ash) as road-base binders. Two different waste/Portland-cement ratios (7.5/92.5 and 80/20) were studied. Density and compressive strength decreased when different wastes were added in every proportion. Additions of 7.5% of both shells reduce the density to about 2.5% and the compressive strength to 20%, while 80% reduces the density to 20% and the compressive strength to 90%, while the addition of biomass fly ash decreases the density and compressive strength in a higher proportion than shells. The durability against acid attack is increased when the three wastes are used, and this increase is higher when the waste dosage is increased (up to 15 times more when 80% biomass ash is used). With respect to leaching, scallop and eggshells can be used as a component of hydraulic road binder, but olive pomace ash presents leaching values higher than the limits of different regulations (Se, Pb, Ni, Mo, Cu, and As). From a radiological perspective, all road-base binders present an activity concentration index lower than 1, except when olive pomace ash was used, and the binders showed higher values of 40 K due to the high potassium content of fly ash. Keywords: scallop shell; eggshell; olive pomace ash; leaching; radioactivity; road-base binder 1. Introduction A road-base binder is a powder, made of a mix of different components, which is the basis for constructing hydraulically bound base mixtures for the treatment of materials for bases, sub-bases and capping layers, and earthworks in airports, roads, railways, and other infrastructures [ 1 ]. Portland cement, bituminous materials, lime, and industrial wastes such as slag, furnace dust, coal fly ash, and others (up to 10% by mass) are the most common materials used in road binders [ 2 ]. Although there is potential for the use of waste materials and the reduced consumption of high-value components such as cement [ 1 ], there are few investigations related to the incorporation of waste in road binders [ 2 ], especially the environmental (heavy metals and radioisotopes) assessment of the waste incorporated. Mainly, the incorporation of waste in hydraulic road binders has focused on wood ash [ 3 ], cellulose ash [ 2 ], and steel slag [ 4 ]. Therefore, at present, the technical and environmental feasibility of incorporating other wastes composed mainly of limestone, such as seashells and eggshells, is unknown. No previous studies have been extended to other types of ash, such as olive pomace ash. European standards EN 13282-2 [ 5 ] on additives to road-base binders indicate the characteristics that certain wastes must have to be used in this kind of application. One of them is that the waste cannot emit radiation or contain hazardous materials. Even so, Appl. Sci. 2023,13, 1644. https://doi.org/10.3390/app13031644 https://www.mdpi.com/journal/applsci Appl. Sci. 2023,13, 1644 2 of 15 the standards do not always specify the tests that must be performed or the maximum limits that cannot be exceeded, and that is what this study attempts to investigate in depth. Globally, population growth has caused an increase in the demand for manufactured food and, therefore, an increase in waste generation [ 6 ]. On the other hand, worldwide, construction demands more natural resources than any other sector [ 7 ]. To improve the environmental sustainability of construction and other sectors, different wastes, some of a biological nature, have been evaluated as substitutes for materials traditionally used in construction (cement, mortar, and concrete) [8]. For example, the global aquaculture industry produces between six and eight million tons per year of seashells from different sources (scallops, oysters, mussels, cockles, etc.) [ 9 ]. Approximately 60% of total production becomes waste [ 10 ] and only 25% of this is reused [ 11 ]. Furthermore, the disposal of seashell waste generates unpleasant odors and requires a large area of land for dumping [ 12 ]. The same environmental problems occur in the poultry industry, which generates more than 250,000 tons of eggshell waste per year [ 13 ]. Other biological wastes are generated in the agricultural sector, such as olive pomace. It is estimated that a ton of processed olives produces approximately 27% olive oil and 73% waste. In Andalusia (Spain) alone, three million tons/year of pomace are generated [ 14 ]. The improper management of olive pomace can cause major environmental problems due to its high phenol content [15]. Eggshells are useful in various construction materials, such as cement [ 16 ], mortar, concretes, alkali-activated binders, and bricks [ 17 ] (due to their high calcium carbonate content. Eggshell powder has shown good strength parameters when mixed with concrete, from previous studies shown below [ 18 ]. There are previous studies on the use of seashell waste in cement production [ 19 ], as a fine/coarse aggregate in concrete [ 20 – 22 ], and as fireproof materials [11]. Due to its high energy content and low fuel cost, olive pomace is used as biomass; approximately 30% is used to produce electric energy. However, the combustion of olive pomace produces more than 50,000 tons/year of ash [ 14 ]. These ashes have been used in cement manufacturing [ 23 ], bricks [ 24 ], fireproof materials [ 25 ], geopolymers [ 14 , 26 ], and mortar [27]. In this study, three different wastes of biological origins (eggshells, seashells from scallops, and ashes from biomass combustion (olive oil pomace)), have been used as raw materials in road binder. Although these three wastes are biological in origin, previous studies have declared the presence of heavy metals in eggshells [ 28 , 29 ] and seashells [ 30 ]. Olive oil pomace ash contains heavy metals (such as As and Pb) and a high content of potassium [ 25 ]. Therefore, these wastes could contain leachable heavy metals and radioisotopes, which, although present in low concentrations in the waste, make their use as construction materials unviable due to health and environmental risks. The research significance of this study is to assess and compare the physical, mechanical, and environmental properties of road binders when part of the cement is replaced by different bio-wastes (fly ash from olive pomace, eggshells, and seashells) into road binders in different proportions. 2. Materials and Methods 2.1. Tested Materials Portland cement (CP) type II 32.5 N according to European standards [ 31 ] was used. The cement particle size was less than 90 µ m Three different wastes have been analyzed: (1) Chilean–Peruvian scallop shells (Argopecten purpuritas) (SC) from the Chilean aquaculture industry were used. The material was previously crushed to obtain a particle size lower than 350 µ m. (2) Eggshell (E) was generated as solid waste generated from bakeries and was previously crushed to a particle size of less than 350 µ m. (3) Fly ash (OR) from olive pomace combustion in a power plant in southern Spain was used. The ash was sieved to obtain a maximum particle size of 350 µ m. The particle size distribution is shown in Appl. Sci. 2023,13, 1644 3 of 15 Figure 1. It was measured by using a Saturn DigiSizer II model. Egg and scallop shells present the highest differential percentages between 50 and 250 µ m and are similar for all the sizes of this interval, The average particle size of scallop shells is 98 µ m, as shown in Figure 1. Eggshells present a medium value (85 µ m) slightly lower than scallop shells. Fly ash from olive pomace combustion presents the highest differential percentages in the range between 20 and 150 µ m with a medium particle size of 44 µ m, but all of them are higher than Portland cement. Appl. Sci. 2023, 13, 1644 3 of 16 lower than 350 μm. (2) Eggshell (E) was generated as solid waste generated from bakeries and was previously crushed to a particle size of less than 350 μm. (3) Fly ash (OR) from olive pomace combustion in a power plant in southern Spain was used. The ash was sieved to obtain a maximum particle size of 350 μm. The particle size distribution is shown in Figure 1. It was measured by using a Saturn DigiSizer II model. Egg and scallop shells present the highest differential percentages between 50 and 250 μm and are similar for all the sizes of this interval, The average particle size of scallop shells is 98 μm, as shown in Figure 1. Eggshells present a medium value (85 μm) slightly lower than scallop shells. Fly ash from olive pomace combustion presents the highest differential percentages in the range between 20 and 150 μm with a medium particle size of 44 µm, but all of them are higher than Portland cement. Figure 1. Particle size distribution of scallop shells, eggshells, and fly ash from olive pomace combustion. The chemical composition of the materials was determined after chemical attack and dissolution at 750 °C (ASTM D 3682 [32]) using atomic absorption spectroscopy. Table 1 shows the main chemical components; the composition is expressed as oxides (but this does not imply that they are present in the material as oxides, either before or after calcination). The specific density was obtained according to ASTM D 854 [33]. Table 1. Main chemical components (% weight) of Portland cement, scallop shells, eggshells, and olive oil pomace ash. Note: CP: Portland cement; SC: scallop shells; E: eggshell; OR: fly ashes; LOI: loss on ignition. CP SC E OR SiO2 (%) 13.8 0.01 0.6 26.8 Al2O3 (%) 3.5 0.04 0.07 6.8 Fe2O3(%) 2.3 - 0.6 5.2 MnO (%) 0.1 0.04 - 5.6 MgO (%) 0.7 - 0.4 5.1 CaO (%) 59.3 54.0 62.4 16.4 Na2O (%) 0.1 0.5 - 1.5 Figure 1. Particle size distribution of scallop shells, eggshells, and fly ash from olive pomace combustion. The chemical composition of the materials was determined after chemical attack and dissolution at 750 ◦ C (ASTM D 3682 [ 32 ]) using atomic absorption spectroscopy. Table 1 shows the main chemical components; the composition is expressed as oxides (but this does not imply that they are present in the material as oxides, either before or after calcination). The specific density was obtained according to ASTM D 854 [33]. As shown in Table 1, the main component in the shells of eggs (E) and scallops (SC) was calcium carbonate [ 11 – 13 ]. The calcium carbonate decomposes at a temperature between 650 and 700 ◦ C, so when analyzed after its calcination, CaO appears and the LOI collects, which corresponds to CO 2 decomposed during the previous calcination. Additionally, the eggshells presented a little more SiO 2 , Fe 2 O 3 , and SO 3 . However, the scallop shells presented a greater amount of Na because, unlike eggshells, they are in contact with seawater. Olive oil pomace ash is mainly composed of silica and potassium, and the LOI value is due to the unburned matter present in the ash. According to EN 13282-2 [ 5 ], all wastes complied with the permitted concentration of SO3: less than 4%. The specific gravities of these wastes are lower than those of Portland cement (CP). Both types of shells present a similar specific density because they have a similar chemical composition. Appl. Sci. 2023,13, 1644 4 of 15 Table 1. Main chemical components (% weight) of Portland cement, scallop shells, eggshells, and olive oil pomace ash. Note: CP: Portland cement; SC: scallop shells; E: eggshell; OR: fly ashes; LOI: loss on ignition. CP SC E OR SiO2(%) 13.8 0.01 0.6 26.8 Al2O3(%) 3.5 0.04 0.07 6.8 Fe2O3(%) 2.3 - 0.6 5.2 MnO (%) 0.1 0.04 - 5.6 MgO (%) 0.7 - 0.4 5.1 CaO (%) 59.3 54.0 62.4 16.4 Na2O (%) 0.1 0.5 - 1.5 K2O (%) 0.5 - 0.2 22.5 TiO2(%) 0.2 - - - P2O5(%) 0.1 0.1 - - SO3(%) 1.7 0.3 1.3 - LOI (%) 15.5 43.2 34.4 9.4 Specific Density (g/cm3)3.19 2.63 2.64 2.73 2.2. Specimen Manufacturing Method Although additives greater than 10% are not recommended according to EN 13282- 2 [ 5 ], 7.5 and 80% replacements were carried out to obtain significant differences in the radiological study, keeping the water/solid ratio constant and equal to 0.35. The different compositions are shown in Table 2. Table 2. Tested compositions. Note CP: Portland cement; SC: scallop shells; E: eggshell; OR: biomass fly ash. Composition Cement (%) Waste (%) Water/Solid Ratio CP-100 100 - 0.35 CP-92.5-SC-7.5 92.5 7.5 CP-20-SC-80 20 80 CP-92.5-E-7.5 92.5 7.5 CP-20-E-80 20 80 CP-92.5-OR-7.5 92.5 7.5 CP-20-OR-80 20 80 Cement and waste were mixed for 4 min until a homogeneous mixture was obtained, for all the different compositions. After adding water to the mixture and mixing for 5 min, a workable mixture was obtained. Cylinder molds were filled once the mixture had reached the proper consistency. After 24 h, the material was demolded, then cured for another 27 days at 20 ◦C and 60% relative humidity. 2.3. Physical and Mechanical Properties The density was calculated in accordance with EN 1936 [ 34 ]. The compressive strength (CS) of the test specimens was determined based on the EN 196-1 [ 35 ] using a compression test machine (Tinus-Olsen). These tests were carried out on 4 × 4 × 4 cm prismatic specimens. Each result was obtained by testing three specimens. Appl. Sci. 2023,13, 1644 5 of 15 2.4. Durability: Resistance to Acid Attack In many applications under severe environmental conditions, the choice of a binder influenced the durability of finished materials, and in this work, mechanical resistance under a chemical attack (acid) has been studied. The resistance to acid attack (RA) of the hydraulic road binder has been measured according to previous studies [ 36 ]. After a 28-day curing period, some samples were immersed in water (reference specimens) and others in a 1 M sulfuric acid solution for 14 days. Three samples in water and three in acid have been tested for each composition. The compressive strength of the attacked specimens was measured (CS-acid) and reference specimens (CS-water) were compared according to the equation: RA =CS −acid CS −water (1) 2.5. Environmental Study European standards EN 13282-2 [ 5 ] indicate the characteristics that some wastes should meet to be used in road-base binders. One of them is that waste should not contain dangerous leaching substances or emit radiation. 2.5.1. Leaching Study The most common leaching test in Europe is EN-12457-4 [ 37 ], and it is used to classify wastes according to the EU Landfill Directive [ 38 ]. The test is very simple. For materials with particle size distributions less than 10 mm, it is based on single-stage leaching at a liquid/solid ratio of 10/1. The mixed liquid/solid was rotated for 24 h at 15 rpm. Deionized water was used in this study. An ICP spectrometer (Agilent Technologies, Madrid, Spain) from the Research, Technology, and Innovation Center at the University of Seville (CITIUS) was used. Two leaching tests for each leaching were carried out. In addition, this test is used in some European national and regional leaching regulations in order to assess the use of waste in construction applications. For instance, Portugal [ 39 ], Italy [ 40 ], and some regions of Spain (Catalonia [ 41 ] and Basque Country [ 42 ]) have established leaching limits according to this test’s results for the valorization of wastes as construction materials Metal analysis in leachates have been carried out through the Inductively Coupled Plasma technique. 2.5.2. Radiological Study Radioactive nuclides are found in varying amounts in traditional building materials and wastes [ 43 ]. Natural building materials and wastes contain various amounts of natural radioactive nuclides [ 43 ]. Many wastes could be considered NORM (naturally occurring radioactive material) [ 44 ], and their use is subject to the legal restrictions of international radiation protection safety standards, such as the European Directive 2013/59 [ 45 ]. These standards establish several parameters to characterize the final construction material containing wastes: activity concentration index (I), the absorbed dose (D, Gy/h), radium activity (Ra eq ), and effective dose (E, Sv/y). All these parameters are obtained by their activity concentration (in Bq/kg) of 232 Th and 238 U series radionuclides, and 40 K, which can be determined by gamma-ray spectrometry [8]. Two samples were tested for each composition. Cylinder specimens were positioned directly at the top of the detector endcap. A Canberra low-background high-purity germanium GR-6022 reverse electrode coaxial (XtRa) detector (60% relative efficiency) shielded by a 10 cm-thick high-purity lead served as the primary detector. The detector was connected to a DSA-1000 digital spectrum analyzer. The spectra were analyzed using Canberra Genie 2000 gamma software (version 3.2). All the samples were related to one day of sampling. The activity concentrations of the different gamma emitters were determined through the energies of 232 Th (911.2 keV of 228 Ac), 238 U (or its daughter 226 Ra via 351 keV emissions of 214Pb), and 40K (1460 keV). Appl. Sci. 2023,13, 1644 6 of 15 Additionally, a Low Energy Germanium (LEGe) GL-2020 detector was used for checking performance at low and moderate energies because it was fabricated with a thin side and front contact affording a better resolution than any other detector. The LEGe detector had an active area of 2000 mm 2 and thickness of 20 mm, and a 0.6 mm carbon epoxy window that improves ruggedness while maintaining good low-energy transmission was used. The detectors’ absolute efficiencies were calculated using Canberra LabSOCS software, which is based on the MCNP Monte Carlo code, considering the self-absorption of lowenergy gamma rays within the sample [ 46 ]. The minimum detectable activity (MDA) and the decision level of both detectors for 40 K, 210 Pb, 238 U, and 232 Th were calculated according to ISO 11929-4 [ 47 ]. Additionally, an alternative geometry was checked by placing a cylinder specimen viewed from the side (see Figure 2). The MDA values for this measuring geometry were compared with the geometry of a cylinder specimen viewed from the top. Appl. Sci. 2023, 13, 1644 6 of 16 day of sampling. The activity concentrations of the different gamma emitters were determined through the energies of 232Th (911.2 keV of 228Ac), 238U (or its daughter 226Ra via 351 keV emissions of 214Pb), and 40K (1460 keV). Additionally, a Low Energy Germanium (LEGe) GL-2020 detector was used for checking performance at low and moderate energies because it was fabricated with a thin side and front contact affording a better resolution than any other detector. The LEGe detector had an active area of 2000 mm2 and thickness of 20 mm, and a 0.6 mm carbon epoxy window that improves ruggedness while maintaining good low-energy transmission was used. The detectors’ absolute efficiencies were calculated using Canberra LabSOCS software, which is based on the MCNP Monte Carlo code, considering the self-absorption of low-energy gamma rays within the sample [46]. The minimum detectable activity (MDA) and the decision level of both detectors for 40K, 210Pb, 238U, and 232Th were calculated according to ISO 11929-4 [47]. Additionally, an alternative geometry was checked by placing a cylinder specimen viewed from the side (see Figure 2). The MDA values for this measuring geometry were compared with the geometry of a cylinder specimen viewed from the top. Figure 2. Gamma-ray spectrometry setup showing the measuring geometries with a cylinder specimen viewed from the top (left), and a cylinder specimen viewed from the side (right). Finally, the gamma-ray spectrometry analyses were validated based on several intercomparison programs [46,48]. The gamma radiation dose was calculated to assess the radiological hazard of the proposed hydraulic binders. Activity concentrations of 226Ra, 232Th, and 40K were converted into the absorbed gamma dose rate (D) using the conversion factors presented in the literature [49] at 1 m above ground (in mGy/h by Bq/kg): 𝐷(𝑛𝐺 ℎ)=0.462·𝑎𝑅𝑎−226 +0.604·𝑎𝑇ℎ−232 +0.147·𝑎𝑘−40 (2) Then, D can be converted into the annual effective dose indoors (E) according to Equation (3): Figure 2. Gamma-ray spectrometry setup showing the measuring geometries with a cylinder specimen viewed from the top (left), and a cylinder specimen viewed from the side (right). Finally, the gamma-ray spectrometry analyses were validated based on several intercomparison programs [46,48]. The gamma radiation dose was calculated to assess the radiological hazard of the proposed hydraulic binders. Activity concentrations of 226 Ra, 232 Th, and 40 K were converted into the absorbed gamma dose rate (D) using the conversion factors presented in the literature [49] at 1 m above ground (in mGy/h by Bq/kg): DnG h=0.462·aRa−226 +0.604·aTh−232 +0.147·ak−40 (2) Then, D can be converted into the annual effective dose indoors (E) according to Equation (3): EmSv y=0.8·10−6·Q·D·T(3) Appl. Sci. 2023,13, 1644 7 of 15 where (1) Q is a converting value of 0.7 Sv/Gy for gamma-ray environmental exposure; (2) the indoor occupancy factor of 0.8 indicates that 80% of the time was spent indoors; and (3) T is the time, equal to 1 year (8760 h.) [49]. On the other hand, the activity concentration index (I) is defined as: I=aRa−226 300 +aTh−232 200 +aK−40 3000 (4) where a x is the activity concentration of nuclide x in the construction material analyzed (Bq/kg). According to the European Commission Radiation Protection 112 technical guide [ 50 ], it was developed as a tool for screening construction materials. “I” gives an assessment of the gamma radiation dose that an individual may receive from construction materials. For bulk materials, if I ≤ 1.0; E is ≤ 1 mSv/y and the material could be used safely from a radiological point of view. Finally, radium activity (Ra eq ) determines the radiological hazards of 232 Th, 226 Ra, and 40 K distributed in construction materials. Ra eq is a weighted sum of the activity concentrations of the radionuclides considering that 259 Bq/kg of 232 Th and 4810 Bq/kg of 40K cause the same gamma dose due to 370 Bq/kg of 226Ra [51]. Raeq =(aRa−226)+ (1.43·aTh−232 +0.077·aK−40)(5) where axis the nuclide x activity concentration (Bq/kg). A safe construction material produces an external dose below 1.5 mGy/y, corresponding to a Raeq value lower than 370 Bq/kg. JASP R-based statistical software (version 0.14.1) was used for statistical analysis. Pearson’s correlation was calculated to consider the relationships between the physicochemical properties, radioactivity concentrations, and radiological parameters. Positive correlations are shown in blue, while negative correlations are shown in red on a correlation heat map. Correlations with p-values greater than 0.01 were considered insignificant, correlations with p-values less than 0.05 implied a 5% probability of random chance, p-values less than 0.01 implied a 1% probability, and p-values less than 0.001 implied a 0.1% probability of random chance. 3. Results and Discussion 3.1. Physico-Mechanical Properties of Road-Base Binder The density and CS of road-base binders decrease when different wastes are added in any dosage, as shown in Table 3. The density was reduced because 1) wastes present a higher particle size distribution than cement, which increases porosity between particles, and 2) wastes have lower specific gravity than cement and, consequently, a higher porosity within the particles (see Table 1). In this study, the specific gravity and particle size distribution of the egg and scallop shells are similar, producing densities very similar. When OR is used, the particle size is lower than both shells, and the specific gravity is like the other two wastes, producing a denser material. Also, the CS decreased when all wastes were added and when the proportion was increased. From a physical point of view, egg and scallop shells present a similar density, and, therefore, they have similar porosities, but they are higher than only Portland cement binders (CP-100). From a chemical point of view, scallop and eggshells are principally calcium carbonate and, therefore, they are an inactive addition because they do not have pozzolanic properties [ 52 ]. Both effects diminish the compressive strength when both wastes are added at a similar level. When OR is added, the compressive strength is reduced more than the shells’ addition due to the low reactivity of this kind of ash and because when OR is added, the compressive strength is reduced more than shells’ addition, due to the low reactivity of OR and because the number of principal hydration outputs of cement such as C-S-H and Ca(OH)2are diminished [53]. Appl. Sci. 2023,13, 1644 8 of 15 Regardless of the type of waste, the compositions made with 7.5% waste and CP meet the minimum mechanical requirements to be used as normal hardening road-base binders according to EN 13282-2 2. CP-92.5-OR-7.5 and CP-92.5-E-7.5 could be classified as N2 (compressive strength between 12.5 and 32.5 MPa), while CP-92.5-SC-7.5 and CP could be classified as N4 (compressive strength between 32.5 and 52.5 MPa). If the compressive strength is compared with road binder containing waste-paper ash in a similar dosage [ 54 ], the compressive strength is higher in the three wastes analyzed in this work, but lower when steel slag is used [55]. Table 3. Physical and mechanical properties Note: CP: Portland cement; SC: scallop shells; E: eggshell; OR: biomass fly ash. Composition Density (kg/m3)CS (MPa) CP-100 1857 ±22 42.6 ±1.2 CP-92.5-SC-7.5 1813 ±21 37.8 ±1.3 CP-20-SC-80 1449 ±17 2.5 ±0.3 CP-92.5-E-7.5 1800 ±25 33.2 ±0.9 CP-20-E-80 1507 ±18 2.6 ±0.3 CP-92.5-OR-7.5 1846 ±19 23.3 ±0.8 CP-20-OR-80 1651 ±11 1.4 ±0.2 3.2. Durability: Acid Resistance of Roa- Base Binders As shown in Figure 3, when the amount of waste in the binder is increased, the acid attack coefficient increases. For specimens CP-20-OR-80, CP-20-SC-80, and CP-20-E-80, the acid attack coefficient is greater than 1. Part of the Ca present in waste and cement in an acidic environment dissolves in water so that Ca 2+ reacts with SO 42− , producing CaSO 4· 2H 2 O. As shown in Figure 4, the acid attack caused the generation of a gypsum layer on the surface and inside the specimen, giving the specimens a white appearance. This formed gypsum has a higher compressive strength than the material before the acid attack [ 56 ]), so in cases of high waste dosage, it increases compression resistance. However, in materials with higher initial compressive strength and lower porosity (higher density), the gypsum induces higher volumetric expansions and leads to the spalling of the surface layers, producing a diminution after the attack [57]. Appl. Sci. 2023, 13, 1644 9 of 16 Figure 3. Resistance to the acid attack of hydraulic road binders. Note: CP: Portland cement; SC: scallop shells; E: eggshell; OR: biomass fly ash. Figure 4. Specimens (a) before, (b) after acid attack, and (c) CP-20-OR-80 after acid attack and compressive strength testing. 3.3. Leaching Results The construction materials must not pose a hazard to the safety, health, or hygiene, of occupants, neighbors, or workers. Moreover, they must not have a high impact on the environment during their useful life. Although, there are no specific regulations on the emissions of heavy metals in hydraulic road binder-containing wastes in Europe. Italy [39] and Portugal [40] have national regulations and Spain presents some regional regulations (Catalonia [41] and Basque Country [42]) for the recycling of the wastes in construction materials according to the European standard leaching test EN12457-4, the same one the European Union uses to limit the emissions of leaching heavy metals in waste landfills [38]. All use the same test but establish different limits for the same elements. The European Directive defines three waste categories: hazardous, non-hazardous, and inert wastes. Table 4 compares leaching data from the three wastes and Portland cement to the European Directive limits [38]. Regarding [38], OR can be considered a nonhazardous waste due to its As and Pb content being higher than the inert limit and lower than the non-hazardous limit. Scallop and eggshells can be considered inert wastes. In Figure 3. Resistance to the acid attack of hydraulic road binders. Note: CP: Portland cement; SC: scallop shells; E: eggshell; OR: biomass fly ash. Appl. Sci. 2023,13, 1644 9 of 15 Appl. Sci. 2023, 13, 1644 9 of 16 Figure 3. Resistance to the acid attack of hydraulic road binders. Note: CP: Portland cement; SC: scallop shells; E: eggshell; OR: biomass fly ash. Figure 4. Specimens (a) before, (b) after acid attack, and (c) CP-20-OR-80 after acid attack and compressive strength testing. 3.3. Leaching Results The construction materials must not pose a hazard to the safety, health, or hygiene, of occupants, neighbors, or workers. Moreover, they must not have a high impact on the environment during their useful life. Although, there are no specific regulations on the emissions of heavy metals in hydraulic road binder-containing wastes in Europe. Italy [39] and Portugal [40] have national regulations and Spain presents some regional regulations (Catalonia [41] and Basque Country [42]) for the recycling of the wastes in construction materials according to the European standard leaching test EN12457-4, the same one the European Union uses to limit the emissions of leaching heavy metals in waste landfills [38]. All use the same test but establish different limits for the same elements. The European Directive defines three waste categories: hazardous, non-hazardous, and inert wastes. Table 4 compares leaching data from the three wastes and Portland cement to the European Directive limits [38]. Regarding [38], OR can be considered a nonhazardous waste due to its As and Pb content being higher than the inert limit and lower than the non-hazardous limit. Scallop and eggshells can be considered inert wastes. In Figure 4. Specimens ( a ) before, ( b ) after acid attack, and ( c ) CP-20-OR-80 after acid attack and compressive strength testing. 3.3. Leaching Results The construction materials must not pose a hazard to the safety, health, or hygiene, of occupants, neighbors, or workers. Moreover, they must not have a high impact on the environment during their useful life. Although, there are no specific regulations on the emissions of heavy metals in hydraulic road binder-containing wastes in Europe. Italy [ 39 ] and Portugal [ 40 ] have national regulations and Spain presents some regional regulations (Catalonia [ 41 ] and Basque Country [ 42 ]) for the recycling of the wastes in construction materials according to the European standard leaching test EN12457-4, the same one the European Union uses to limit the emissions of leaching heavy metals in waste landfills [ 38 ]. All use the same test but establish different limits for the same elements. The European Directive defines three waste categories: hazardous, non-hazardous, and inert wastes. Table 4compares leaching data from the three wastes and Portland cement to the European Directive limits [ 38 ]. Regarding [ 38 ], OR can be considered a non-hazardous waste due to its As and Pb content being higher than the inert limit and lower than the non-hazardous limit. Scallop and eggshells can be considered inert wastes. In Portugal, waste recycling is allowed by the Portuguese Environment Agency [ 39 ] when limits for the inert waste of [ 38 ] are not exceeded in construction materials. Both shells could be reused, but not biomass fly ash. On the other hand, Italian Ministerial Decree 5 [ 40 ] establishes the maximum limits that waste must leach if it wants to be used in construction according to EN12457-4. Regarding the Italian Decree [ 40 ], Table 4shows that OR (and CP) cannot be used in construction materials in Italy due to high values of Se, Pb, Ni, Mo, Cu, and As. Scallop and eggshells meet all the requirements in Italian legislation. In Spain, there are no established national limits about the leaching emissions of hazardous heavy metals of construction materials containing wastes. However, for example in the case of Spain, there are regional laws that allow, based on the results of EN 12457- 4, the establishment of whether waste can be used in specific construction applications (pavements, roads), such as the Autonomous Government of Catalonia and Basque Country. These legislations establish different limits for the same compounds (and are different from the Italian and Portuguese norms). They even establish limits for compounds that are not limited in others (i.e., Hg, Pb, Sb, As, and Cu are limited in Catalonia and not in Basque Country and, V is limited in Basque Country and not in Catalonia). These limits can be observed in previous studies [ 58 ]. According to these limits. scallop and eggshells can be used but biomass fly ash exceeds the limits regarding Se, Mo, and Ni in both regulations and Pb, As, and Cu in Catalonia limits.