Wastewater Washed Mineral Waste and Sludge Ash Mixtures for Sustainable Construction Applications

Abstract

In the face of the raw materials crisis and environmental concerns, sustainable waste management has become a priority for current and future generations. Recycling waste from wastewater treatment plants in a closed loop protects natural resources, reduces landfill volumes, and lowers disposal costs. This paper presents the results of tests on the physical, filtration, and mechanical properties of mixtures of washed mineral waste (WMW) from grit chambers with fly ash from the thermal treatment of municipal sewage sludge (SSA) in a fluidized bed furnace. Additionally, radiological tests of the mixture components were conducted. Based on the conducted tests, the possibility of sustainable use in civil engineering, such as soil backfills and embankment construction materials, was assessed. The possibility of safely using waste materials in the indicated construction solutions was demonstrated for mixtures with dominant WMW content (90% and 70% by total weight). The waste mixtures correspond to poorly or medium-grade sands with a small amount of silt (uniformity coefficients of 3.33, 3.50, and 8.00). They are characterized by maximum dry densities of 1.542, 1.770, and 1.780 g/cm³; optimal moisture contents of 12.54, 12.86, and 20.25%; permeability coefficients of 0.08, 0.22, and 0.39 m/d; and internal friction angles of 38.4, 39.5, and 40.1°. The values of the determined parameters of some mixtures are similar to those of natural sands used as construction aggregates. All mixtures meet the geotechnical criteria for use in road embankments, below frost depth, and in flood embankment bodies. Mixtures with a 90% mass fraction of WMW were also approved for application as backfill for installation trenches. However, none of the mixtures met the hydraulic conductivity threshold required for the upper layers of embankments nor for backfill of abutments and retaining structures without the use of an additional binder (cement or lime), which is considered a prerequisite for these applications.

1. Introduction

The primary goal of proper wastewater treatment plant operation is to remove contaminants contained in wastewater, which generates various waste streams. However, increasing global water scarcity and the accompanying depletion of energy resources and natural resources have necessitated changes in treatment processes, resulting in the need for wastewater treatment plants to transform their economic model towards a closed loop [1,2,3]. Operations based on the circular economy paradigm maximize energy and resource recovery [4,5,6]. At the same time, they require wastewater treatment plants to be designed and operated as water resource recovery facilities (WRRFs) or biorefineries (WWBRs). The transformation of traditional wastewater treatment plants into designated facilities is also an important step towards achieving the Sustainable Development Goals (SDGs) related to wastewater treatment [7,8,9,10].

An example of a potential resource generated in wastewater treatment plants is sand, which is a component of mineral waste and fly ash from the thermal treatment of municipal sewage sludge. In wastewater treatment technology, the term mineral waste (MW) refers to a mixture of mineral pollutants, such as sand, gravel, and fine stone fractions, separated during mechanical treatment processes. The composition of this fraction may be additionally enriched with organic pollutants, including fruit pits, eggshell fragments, and wood, as well as components of anthropogenic origin, such as small fragments of glass, metal, and textiles [11]. Fly ash from the thermal treatment of municipal sewage sludge (SSA) is a fine-grained, inorganic waste material generated during the incineration of sewage sludge in specialized installations, most often fluidized-bed systems [12]. Both types of waste exhibit characteristics of anthropogenic soils, and the sand fraction in MW can reach 98% [13], while in SSA it can exceed 50% [14]. Assessing the potential use of MW and SSA in construction has been a matter of global interest for many years. This stems from the growing sand crisis [15,16,17,18], and therefore the urgent need to obtain alternative raw materials to commonly mined natural aggregates, thereby reducing pressure on geological deposits and extending the life of existing resources. Furthermore, waste disposal and storage pose a significant financial burden for wastewater treatment plants. Increasing awareness of the problem allows for more effective waste management and the possibility of deriving benefits from waste transformation into a product in the construction industry [19].

The properties of mineral waste largely depend on the pretreatment technology used in wastewater treatment plants, enabling a basic classification of this waste. It includes mineral waste (MW) and washed mineral waste (WMW). MW is untreated waste obtained directly from grit chambers or from special treatment plant facilities for receiving sludge from sewer manholes and streets. MW is characterized by a high organic and moisture content, which can lead to unpleasant odors [20,21]. WMW is produced during the additional separation and washing of mineral waste, a process used in wastewater treatment plants. These processes improve waste parameters by reducing or almost completely removing the content of organic substances, reducing waste hydration and odor [22,23,24]. In turn, the properties of SSA depend primarily on the composition of the wastewater supplied to the treatment plant, the sludge treatment technology, and the incineration process itself [25].

Table 1. Review: Research on sand reuse from mineral waste in wastewater treatment plants.

Country (Region) [Reference]	Waste Type Presented in the Reference	Proposed Reuse	Key Findings
Czech Republic (Moravia) [20]	Sewage sand (grit chamber)	Building material (no specific use indicated)	Varied content of organic substances (1.58–68.49%) and total solids (20.03–95.40%). Lack of uniform particle distribution.
Brazil (São Carlos) [26]	Residual sand (grit chamber)	Non-structural concrete elements, e.g., sidewalks and curbs (fine aggregate)	The waste washing and drying process reduced organic matter to 1%. Adequate strength was achieved using 70% of the residual sand.
Poland (Mazovia) [22]	Waste sand (flushing separator)	Concrete (aggregate), backfill excavations (soil additive)	Organic substance content (0.78–14.44%). The sand fraction content (85.70 to 94.80%).
China (Shenzhen) [27]	Sandy waste (grit chamber)	Building material (fine aggregate)	The waste has a low organic content and a high content of heavy metals and sand. The fine aggregates could be recovered through processes such as de-mixing, screening, and sand washing.
Poland (Cracow) [13]	Sand (flushing separator)	Concrete (fine aggregate)	Organic matter content (1.8%). The sand fraction content (98%). The leachability tests showed that this raw material has no environmental impact.
Poland (Bielsko-Biala, Gliwice) [28]	Desanding waste (flushing separator)	Construction aggregate	Organic matter content (2.17–15.91%). Sand fraction content (57.3–95.2%). Leachability levels of hazardous substances and heavy metals do not pose any potential harm to the natural environment.
Poland (Gliwice) [29]	Sand trap content (grit chamber); mixtures of stabilized municipal sewage sludge with sand trap content (2:1, 1:1, 1:2)	Soil-like material	Organic substance content (sand: 1.11%; mixtures: 6.63–13.49%). Heavy metal content after mineralization (sand: not assessed; mixtures: retained for the indicated use). Leachability of harmful substances and heavy metals (sand: retained; mixtures: exceeding permissible values). It was shown that the tested samples can only be used to create soil substrates after being washed or neutralized.
Poland [30]	The contents of sand from desanding (grit chamber)	Sub-base layers and other non-structural applications (aggregate)	The organic matter content of the sand (7.8%), and the sand fraction (90.4%). The leachability of harmful substances and heavy metals by the sand is maintained. The substitution of natural sand with waste sand in cement mortar formulations results in a significant reduction in mechanical performance (35% in compressive strength and 38% in flexural strength).
Poland [31]	Sand (grit removal systems)	Cement mortars (aggregate)	Alkaline pretreatment with sodium hydroxide (NaOH) enables the effective reuse of WWTP-derived waste sand in cement mortars, achieving mechanical and rheological properties comparable to those of mortars made with conventional sand. The most favorable results were obtained for 0.5% NaOH.
Poland [21]	Sand (flushing separator)	Concrete (fine aggregate); geopolymer material	Organic matter content (3.09%). Sand fraction content (94.89%). Uniform-grained sand. Potential use of sand after proper cleaning.
Poland [32]	Silica sand	Glass fiber reinforced plastic pipe	The review explores the origins and movement of silica sand within wastewater, and the technologies used for its recovery and purification, characterization techniques, applications in GRP pipe production, policy mechanisms that support circular flows, and areas for future research.
Poland (Warsaw) [33]	Washed mineral waste (flushing separator)	Backfill and road embankment material (soil)	Organic substance content (1.06%). Sand fraction content (94.17%). Uniform-grained and frost-unsusceptible soil. Geotechnical properties meet requirements for soil backfills. The application of a road embankment material needs to be proven through field tests (compaction index).
Poland (Warsaw) [23]	Washed mineral waste (flushing separator)	Flood embankment material (soil)	Organic substance content (1.01%). Sand fraction content (95.43%). The heavy metal content after mineralization was retained for road applications and, in some cases, for residential applications as well. Geotechnical properties meet requirements for flood embankment material. The application needs to be proven through field tests (compaction index).
Poland (Warsaw) [19]	Mixtures of washed mineral waste with municipal sewage sludge ash (90, 70, 50%)	Backfill, road and flood embankment material (soil)	Organic matter content (0.07–0.14%). Sand fraction content (75.90–94.96%). Selected mixtures with a dominant content of washed mineral waste (90 and 70%) retain acceptable heavy metal levels after mineralization and in water extracts. Their suitability for further use should be confirmed by geotechnical studies.

presents a detailed literature review on sand recovery from mineral waste generated in wastewater treatment plants. The results presented here focus primarily on laboratory-based industrial research. Therefore, their technology readiness level (TRL) can be assessed at TRL 3–5 (from analysis and initial proof-of-concept tests to component validation in a relevant environment).

The papers [20,21,26,27,28,29,30,31,32] present general and more specific possibilities of potential use of MW as construction materials, including a substitute for sand in the production of concrete, construction aggregate, or land reclamation material. Depending on the study, the scope of research on mineral waste included determining physical parameters (particle size distribution, specific or bulk density), mechanical parameters (compressive, flexural, and tensile strength), or chemical parameters (organic substance content, heavy metal content, and their leachability). Other studies [22,23,33] demonstrated the potential of using washed mineral waste as a sand substitute for concrete, soil backfill, or embankment construction materials in civil engineering. To assess the potential for incorporating WMW into earthworks and the safety of using recovered materials in the soil environment, the research focused on determining the waste’s geotechnical and chemical properties. These included physical parameters (granulometric composition, specific density, maximum dry density of the soil skeleton, optimal moisture content, and permeability coefficient) and mechanical parameters (internal friction angle, apparent cohesion, and oedometric compressibility modulus). Chemical determinations included the content of organic substances, calcium carbonate, and heavy metal concentrations in the waste and its aqueous extracts, enabling assessment of the level of metal immobilization in this waste. The WMW tests confirmed the effectiveness of the additional washing process used in wastewater treatment plants, resulting in an average organic content of 0.24–1.49%. Furthermore, the WMW contained over 95% of the sand fraction and was classified as uniformly graded medium sand based on its granulometric composition [23]. The results highlight the possibility of considering WMW as an alternative mineral raw material to natural sand. However, an undoubted disadvantage of washed mineral wastes regarding their grain size is the lack of fine-grained particles (grain size below 0.063 mm). Taking this into account, their use as soils in road and hydraulic engineering may require their compaction parameters to be improved by adding material with a dominant content of finer fractions, which will increase the uniformity coefficient of the target product [19].

Fly ash from the thermal treatment of municipal sewage sludge could be a suitable additive for washed mineral waste. Lynn et al. (2015) [34] and Wichowski et al. (2023) [14] suggest that the grain size of this material does not exceed 0.26 mm, indicating its suitability for use as a filler or fine aggregate. Studies published in the literature on the use of SSA in construction confirm its potential. The papers [35,36,37,38,39,40,41,42,43] focus on assessing the potential use of SSA to partially replace cement, as an aggregate in concrete production, an additive for soil stabilization, or as a component of bricks. To determine technological suitability, the basic chemical, physical, and mechanical properties that the material should meet were identified.

Stabilizing and improving the quality of uniformly graded soils in road projects is a widely used technology [44]. Creating mixtures from waste to produce a new product is one of the forms of recovering and recycling waste with particle sizes smaller than 10 mm, and at the same time, ensuring its reuse [45]. Such management of the two types of waste generated at the same wastewater treatment plant also has several organizational, economic, environmental, and technical benefits.

Despite promising empirical evidence, no standardization framework, engineering guidelines, or technical approvals have been established to sanction the use of WMW and SSA in geoengineering. This situation stems from multifaceted legal, economic, technological, environmental, and psychosocial conditions. A key problem is the physicochemical heterogeneity of the waste matrix, determined by the specific location and technological regime of a given treatment plant, which generates variable pollutant loads. Another significant barrier is the low level of environmental awareness, which—given the lack of educational tools—makes repositioning waste as a valuable secondary raw material more difficult. However, in light of European Union legal regulations regarding the End-of-Waste procedure, positive certification verification is a key factor. Submitting detailed qualitative analyses confirming that the WMW and SSA meet stringent performance parameters after a given treatment cycle enables their reclassification from waste to a fully functional secondary product approved for direct application. These requirements are determined by engineering and construction standards that define optimal geotechnical properties for fill and backfill materials, as well as by restrictive environmental protection regulations that impose maximum permissible limits on the concentrations of potentially toxic substances and their leachability [19].

The study aimed to comprehensively assess the potential of using mixtures of washed mineral waste and fly ash from the thermal treatment of municipal sewage sludge as alternative raw materials for the construction sector (as soil backfill and for road and hydro embankments). The analyses characterized key geotechnical properties, including physical, hydraulic, and mechanical parameters. A radiological assessment was also a key element of the verification process, allowing for the identification of potential environmental hazards. These studies determined the impact of fly ash addition on the behavior of washed mineral waste and confirmed compliance with construction standards and regulations. This, combined with an economic and environmental analysis, supports the consideration of the mixtures’ potential applications within the framework of sustainable development and the circular economy.

The presented research is innovative and establishes a new paradigm for waste stream management in wastewater treatment plant technological systems. The essence of this innovation is the circular economy concept, which represents a fundamental step in transforming the water and wastewater sector towards sustainable development. The recirculation and economic use of technological products generated in unit processes will reduce the volume of final waste while simultaneously limiting pressure on natural resource deposits. Implementing these solutions will deliver tangible benefits, including optimized operating costs for wastewater treatment plants. The holistic integration of technical, economic, and ecological dimensions gives the analyses a high degree of practical value, making it a crucial instrument for shaping the future of the circular economy in the water and wastewater industry.

2. Materials and Methods

2.1. Materials

Three mixtures were analyzed, consisting of two different types of waste collected from the largest wastewater treatment plant in Poland (2,100,000 population equivalent). These included WMW coded 19 12 09 (washed mineral waste, from grit chambers) and SSA coded 19 01 14 (fly ash, from a fluidized bed furnace) [46,47]. Both are classified as industrial waste [48] and non-hazardous waste [46,47,49,50]. A WMW sample (named P-2) of approximately 160 kg was collected in November from a temporary waste storage site located in an open-air area within the wastewater treatment plant and then dried in a laboratory dryer at 105–110 °C to constant weight (Figure 1). SSA samples (named PL1, PL2, and PL3) were collected in quantities of approximately 20, 40, and 60 kg, respectively, in January, September, and November of the same year, when the waste was transferred for disposal (Figure 2). The fly ash was in a natural powdery form, and its moisture content on the day of collection was determined to be zero.

To ensure sample representativeness and minimize the impact of local heterogeneity, a homogenization procedure was used for WMW and SSA waste. This procedure, based on mechanical mixing and the quartering method, enabled effective averaging of the composition of individual waste samples. Mixture samples prepared from these materials guarantee higher analytical reliability and reduce errors caused by material variability.

EU law allows the use of recycled materials as products. However, for them to cease being considered waste, they must undergo appropriate tests confirming that they meet technical standards and pose no environmental hazard. Assessment of the suitability of materials for engineering purposes (embankments, backfills) is primarily based on particle size analysis. The required soils are mineral, non-cohesive, sandy, and frost-unsusceptible, characterized by a uniformity coefficient greater than 3 for road embankments and greater than 5 for backfills of abutments and retaining structures. Although washed mineral waste is classified as mineral, non-cohesive, sandy, and frost-unsusceptible soil, it does not meet the particle size criterion [23,33]. The washing process in wastewater treatment plants removes fine-grained fractions, resulting in a uniformity coefficient below 3. To adapt WMW to the requirements for backfills and embankments, it was mixed with SSA. This additive has a significantly higher proportion of fine fractions, enabling correction of the grain distribution curve.

Six variants of the research mixtures were created by mixing WMW and SSA in varying weight proportions [19]. The formulations were developed experimentally, based on a literature review and laboratory tests of the specific characteristics of both wastes. The primary goal was to obtain environmentally friendly mixtures with technical properties that meet the uniformity coefficient requirement of construction standard. Two variants were analyzed: SSA from a single thermal cycle (Category A mixtures) and SSA from multiple combustion processes (Category B mixtures). This approach reflects real industrial conditions, where the material intended for further recycling may originate from a single batch or represent accumulated stock from different periods of the plant’s operation.

The environmental aspect of the practical use of mixtures requires controlling the organic substance content, which determines the durability of the structure. Determining heavy metal concentrations and their mobility is also crucial. Limit values for harmful substances are important when assessing the suitability of materials for soil and water environments [19]. Chemical analysis is a quick and cost-effective preliminary step, eliminating unsuitable samples before time-consuming tests (e.g., compactability, shear strength, or compressibility).

This paper expands on the preliminary analysis of the potential applications of the tested mixtures. The presented geotechnical test results introduce a novel perspective on the use of washed mineral waste, which has not been previously described in the literature.

Table 2. Proportions of WMW-SSA mixtures for further geotechnical tests.

Sample	Mass Ratios of Wastes (WMW/SSA)
	WMW (P-2)	SSA (PL1)	SSA (PL2)	SSA (PL3)
M1A	9	1	0	0
M1B	27	1	1	1
M2B	7	1	1	1

summarizes the optimal mass proportions of washed mineral waste and sewage sludge ash (WMW/SSA), selected based on successful completion of the first verification stage, which included the required grain size distribution and the preserved content of organic substances and heavy metals [19].

A summary of the suitability of the considered mixtures for construction applications is presented in Ref. [19]. The analysis shows that the tested mixtures are safe with respect to organic matter content. Still, their further use in construction is limited by environmental requirements regarding heavy metal concentrations and pH. The M1A mixture was identified as suitable for hydraulic and road applications (in flood embankment bodies of all structural classes, in excavations to frost depth, and in road embankments) to an embedment depth of 0–0.25 m below ground level. Furthermore, the M1B mixture was recommended for road applications (in excavations to frost depth and in road embankments), regardless of its embedment depth. An additional engineering advantage is the frost resistance of this mixture. Finally, the M2B mixture was limited to an embedment depth of 0–0.25 m below ground level in road construction (in excavations up to frost depth, in road embankments, and as backfill for bridge abutments and retaining structures). All analyzed mixtures (M1A, M1B, and M2B) can be used as backfill for installation trenches.

A diagram showing the resource recovery processes for WMW and SSA is shown in Figure 3.

2.2. Methods

Given the similarity of WMW and SSA to anthropogenic, non-cohesive soils, the soil testing methodology was adopted to determine the geotechnical parameters of WMW-SSA mixtures. Furthermore, due to the lack of widely available geotechnical parameters for SSA in the literature, this study also presents the characteristics of the collected SSA samples. The research cycle carried out was similar to that presented by Kostrzewa et al. (2025) [23]. Additionally, the test results were supplemented with radiological tests of the WMW-SSA mixture components.

Physical properties included determinations of granulometric composition (fraction content), specific density, values characterizing the limiting states of compaction (dry density of the soil skeleton corresponding to the loosest and densest possible arrangements of soil grains), maximum dry density, and optimal moisture content. The hydraulic conductivity determined filtration properties, while mechanical properties included the angle of internal friction, apparent cohesion, the oedometric primary compressibility modulus, the oedometric relaxation modulus, and the oedometric secondary compressibility modulus.

The dry particle size distribution of the WMW-SSA mixtures was assessed by sieve analysis on an electromechanical shaker, in accordance with the relevant standard [51]. A set of seven calibrated sieves with mesh sizes of 4, 2, 1, 0.5, 0.25, 0.125, and 0.063 mm; a collecting pan; and laboratory scales were used for the analysis. The test was performed on samples weighing approximately 300 g. A detailed test procedure is presented by Andriulaityte et al. (2024) [52]. The results were used to prepare a grain size distribution curve of the tested material.

The specific density of the SSA and WMW-SSA mixtures in their dry state was determined using a pycnometer, according to the methodology described by Pisarczyk (2022) [53], based on a modified procedure outlined in the relevant standard [54]. The test was performed on samples weighing approximately 50 g. The obtained results were used to calculate indirect geotechnical physical parameters, such as void ratio, porosity, and degree of saturation after compaction.

The quantities characterizing the limiting states of compaction include dry density, which corresponds to the loosest and densest possible arrangements of soil grains. The dry density of SSA and WMW-SSA mixtures in a dry state was tested using a standard cylinder with a volume of approximately 500 cm³, a piston, and a vibrating fork, in accordance with the relevant standard [55].

The optimal moisture content and maximum dry density of SSA and WMW-SSA mixtures were determined in a Proctor apparatus in a 1 dm³ cylinder, using a standard compaction energy of 0.59 J/cm³, in accordance with the provisions of the relevant standard [55]. A detailed test procedure is presented by the authors of [56,57]. The obtained results were used to prepare compacted samples for further filtration and mechanical tests.

Hydraulic conductivity tests for SSA and WMW-SSA mixtures in the compacted state were conducted under uniaxial strain conditions using a hydraulic apparatus in a 1 dm³ cylinder. The tests were conducted in accordance with the relevant standard [58], with vertical loading, upward flow, and hydraulic gradients of 0.4, 0.6, and 3.0. The first two values fall within the hydraulic gradient range (0.3–0.8) recommended by Wiłun (2013) [59] for determining the sand permeability coefficient. Additionally, as summarized by Dąbska (2021) [60], the dominant filtration deformation in soils with a uniformity coefficient below 10 (treated as non-suffusion) is static liquefaction. The permissible hydraulic gradient value for such soils for vertical upward filtration should be 0.4. In turn, a hydraulic gradient of approximately 0.6 was already adopted when determining the permeability coefficient of washed mineral waste [23]. The latest value of the adopted hydraulic gradient results from the recommendations for determining the hydraulic conductivity of fly ash-based barrier material [61]. Laboratory test results were compared with the hydraulic conductivity values determined based on Slichter’s empirical formula [62], due to the range of applicability (0.01 mm ≤ D₁₀ ≤ 5 mm), for both SSA and WMW-SSA mixtures. The empirical calculations used the previously determined parameters for the SSA and WMW-SSA mixtures, including the effective diameter (D₁₀) and porosity (n).

The shear strength of the SSA and WMW-SSA mixtures in the compacted state was determined using a direct shear apparatus, in accordance with the relevant standard [63]. The apparatus used in the study had a square cross-section of 80 mm × 80 mm. Shear strength tests were conducted at five normal stress levels (12.5, 25, 50, 100, and 200 kPa). A detailed test procedure is presented by Zabielska-Adamska (2020) [64].

The compressibility of the SSA and WMW-SSA mixtures in the compacted state was measured in oedometers in accordance with the relevant standard [65]. The samples were subjected to gradual load changes, with each subsequent load value being twice as large during loading or twice as small during unloading. The load steps corresponded to 12.5, 25, 50, 100, and 200 kPa, typical of shear tests. In both cases, load changes were introduced only after the sample height had stabilized. A detailed test procedure is presented by Vukićević et al. (2019) [66].

Radiological characterization of sewage sludge ash (SSA) and washed mineral waste (reference sand) was performed using two complementary gamma-ray measurement systems integrated within a single interpretive framework. A NaI(Tl) spectrometric system (HERMES-GSU) provided activity-based quantification in Bq·kg⁻¹, while a high-resolution LaBr₃(Ce) spectrometer (BRAD) was used exclusively for quality assurance/quality control (QA/QC), mineralogical context, and qualitative geochemical verification (%K, U, Th). Where required, BRAD elemental outputs were converted to activity units using standard conversion factors—1% K ≈ 313 Bq·kg⁻¹ K-40; 1 ppm U ≈ 12.35 Bq·kg⁻¹ Ra-226; 1 ppm Th ≈ 4.06 Bq·kg⁻¹ Th-232—assuming secular equilibrium within the U-238 decay series.

The nuclides of interest were K-40, Ra-226, Th-232, and Cs-137. Ra-226 and Th-232 were quantified via their short-lived progeny (Pb-214/Bi-214 for Ra-226; Ac-228/Tl-208 for Th-232), while K-40 and Cs-137 were determined from their characteristic gamma-ray lines at 1460.8 keV and 661.7 keV, respectively. All materials were measured in semi-infinite geometry using approximately 300 g of homogenized bulk material to minimize geometric variability and ensure reproducible attenuation and solid angle conditions across both instruments.

For comparative assessment of external gamma radiation, two indices were calculated:

• Radium equivalent activity (Equation (1)):

$$R{a}_{eq}=A_{Ra}+1.43\hspace{0.17em}{A}_{Th}+0.077\hspace{0.17em}{A}_K$$

(1)

• Gamma activity index (Equation (2)):

$$I_{\gamma }=A_{Ra}/300+A_{Th}/200+A_K/3000$$

(2)

where A_Ra, A_Th, and A_K are activity concentrations (Bq·kg⁻¹).

For NaI(Tl) acquisitions of 20–30 min, combined standard uncertainties of ±(20–25)% were assigned as representative of strong photopeak’s (K-40, Cs-137) and ±(20–30)% for series-derived lines (Ra-226, Th-232), reflecting fitting statistics and efficiency uncertainties. Values below the minimum detectable concentration (MDC) are reported as < MDC. One anomalous Ra-226 value for PL2 was identified as an acquisition artifact caused by mis-windowing. The value was excluded and replaced by a repeat measurement that yielded a physically consistent result. Efficiency calibration was performed using a certified K-40 reference material, supplemented by energy window calibration on natural background lines of the U-238 and Th-232 series. Repeated background measurements were used to quantify the instrumental contribution to uncertainty. Reported uncertainties combine counting statistics and efficiency model components.

The tests were performed in at least 3 repetitions, except for the shear strength tests, which were performed on 15 samples for each WMW-SSA mixture and 9 to 11 samples for SSA.

3. Results and Discussion

3.1. Physical and Permeability Parameters

3.1.1. Granulometric Composition (Fraction Content)

Table 3. Summary of sieve analysis results for WMW-SSA mixtures.

No.	Fraction Size [mm]	Percentage Content [%]
	Name	M1A	M1B	M2B
1.	>4.0	0.82	2.07	1.83
2.	2.0–4.0	2.49	2.99	2.11
3.	1.0–2.0	7.71	7.97	6.18
4.	0.5–1.0	27.65	27.32	21.31
5.	0.25–0.5	42.86	40.00	31.48
6.	0.125–0.25	10.48	10.93	12.93
7.	0.063–0.125	3.11	3.58	9.54
8.	<0.063	4.88	5.14	14.62

presents the results of sieve analysis, illustrating the division of WMW-SSA mixtures into various granulometric fractions. Based on the obtained results, grain size distribution curves were created (Figure 4), and the most crucial physical parameters of the granulometric composition of WMW-SSA mixtures were determined (

Table 4. Granulometric composition of selected WMW-SSA mixtures and their components (WMW and SSA).

No.	Parameter	Unit	WMW-SSA			WMW 1	SSA 2
			M1A	M1B	M2B	P-2	PL1	PL2	PL3
1.	Gravel fraction	%	3.31	5.06	3.94	4.45	0.00	0.00	0.00
2.	Sand fraction	%	91.81	89.80	81.44	95.43	53.31	58.26	53.40
3.	Silt + Clay fraction	%	4.88	5.14	14.62	0.12	46.69	41.74	46.60
4.	Effective diameter D60	mm	0.490	0.500	0.400	0.550	0.081	0.098	0.080
5.	Effective diameter D50	mm	0.410	0.410	0.340	0.480	0.068	0.075	0.068
6.	Effective diameter D30	mm	0.300	0.310	0.180	0.350	0.040	0.045	0.040
7.	Effective diameter D10	mm	0.140	0.150	0.050	0.250	0.015	0.015	0.015
8.	Uniformity coefficient CU	-	3.50	3.33	8.00	2.20	5.40	6.53	5.33
9.	Curvature coefficient CC	-	1.31	1.28	1.62	0.89	1.32	1.38	1.33

¹ The research results come from the paper [23]. ² The research results come from the paper [19].

). Additionally, individual results for the waste components (WMW and SSA) constituting the mixture were imported into Figure 4 and Table 4 for comparative purposes.

The gravel fraction of individual WMW-SSA mixtures did not exceed the total content of the silt and clay fractions, reaching 3.31%, 5.06%, and 3.94%, respectively. The share of fine-grained fractions (<0.063 mm) was 4.88, 5.14, and 14.62% for WMW-SSA mixtures, in which 10, 10, and 30% SSA content was assumed, respectively. The sand fraction dominated in the mixtures, especially particles with a grain size of 0.25–0.50 mm, indicating medium sand (0.20 < mSa ≤ 0.63 mm). The total share of WMW in WMW-SSA mixtures, including the gravel fraction, did not significantly affect the classification of the mixtures by grain size, compared with the results presented in the previous paper [19]. As noted earlier, according to the geotechnical classification PN-EN ISO 14688 [67,68], WMW-SSA mixtures are coarse-grained soils (sands) in which the 0.063–2 mm fraction constitutes over 50% by mass. Increasing the SSA share increases the content of fine-grained fractions (silt and clay) at the expense of the sand fraction, thereby increasing the grain size uniformity coefficient. The observed benefit of stabilizing WMW with SSA results from improved grain size distribution, allowing for the transformation of uniformly graded medium sands (WMW) towards poorly (mixtures M1A, M1B) or medium (mixture M2B)-grade medium sands with low fine-grained fractions content.

3.1.2. Specific Density

Table 5. Specific density of WMW-SSA mixtures and their components (WMW and SSA).

Parameter	Unit	WMW-SSA			WMW 1	SSA
		M1A	M1B	M2B	P-2	PL1	PL2	PL3
Specific density ρs	g/cm3	2.539	2.533	2.460	2.620	2.093	2.114	2.285

¹ The research results come from the paper [23].

presents the results of the specific density of WMW-SSA mixtures and their components (WMW and SSA).

The specific density of natural mineral soils results from the density of the minerals that comprise them. The value of this parameter for WMW may be reduced by the formation of layers of organic material covering the mineral particles in the wastewater [69], and, in the case of SSA, by thermal processes occurring in fluidized bed furnaces. SSA has a lower specific density than WMW. These values are relatively low, indicating that SSA is a lightweight material, significantly lighter than natural mineral soils with a similar grain size (fine sands). According to Morman-Wątor & Pilecka (2024) [70], the specific density of quartz sands is 2.65 g/cm³.

The WMW-SSA mixtures (M1A and M1B) with a dominant WMW content (90%) have similar specific density values. In contrast, the specific density of the M2B mixture, containing 70% WMW and 30% SSA, is slightly lower than that of the other WMW-SSA mixtures as the SSA percentage increases. The specific density of the mixtures falls within the range of their constituent components (2.093–2.620 g/cm³).

The range of specific density of WMW-SSA mixtures corresponds to sands and humus silts (2.30 ÷ 2.64 g/cm³) [71], while that of SSA corresponds to the range indicated for organic soils (muds: 1.40–2.60 g/cm³), as well as various types of waste, which include mining waste (1.96–2.59 g/cm³), combustion waste (also known as energy waste: 1.59–2.74 g/cm³) and municipal waste (1.70–2.50 g/cm³) [53].

3.1.3. Quantities Characterizing the Limiting States of Compaction

Table 6. Quantities characterizing the compaction limits of WMW-SSA mixtures and their components (WMW and SSA).

No.	Parameter	Unit	WMW-SSA			WMW 1	SSA
			M1A	M1B	M2B	P-2	PL1	PL2	PL3
1.	Dry density corresponds to the state of the loosest possible composition of soil grains ρdmin	g/cm3	1.592	1.608	1.319	1.570	0.382	0.723	0.756
2.	Maximum void ratio emax	-	0.595	0.575	0.865	0.669	4.479	1.924	2.022
3.	Maximum porosity nmax	-	0.373	0.365	0.464	0.401	0.817	0.658	0.669
4.	Dry density corresponds to the state of the densest possible composition of soil grains ρdmax	g/cm3	1.890	1.898	1.653	1.850	0.506	0.992	1.050
5.	Minimum void ratio emin	-	0.343	0.335	0.488	0.416	3.136	1.131	1.176
6.	Minimum porosity nmin	-	0.256	0.251	0.328	0.294	0.758	0.531	0.540

¹ The research results come from the paper [23].

presents the test results for the parameters characterizing the compaction limits of WMW-SSA mixtures and their components (WMW and SSA). These include the minimum and maximum dry density of the skeleton. Additionally, based on these values and the specific density results (Table 5), the minimum and maximum porosity indices and the porosity of the test materials were calculated.

WMW-SSA mixtures (M1A and M1B) with a WMW content of 90% are characterized by similar values of the parameters indicated in Table 6. Both the minimum and maximum dry density of the WMW-SSA mixture (M2B) decreased due to the increase in the percentage of SSA, which is directly reflected in the values of the void ratio and porosity. The range of dry density values of the sands with a specific density of the sands’ void ratio of 2.65 g⁄cm³ and a range of their void ratios of 0.3–1.0 is 1.33–2.04 g⁄cm³ [23]. The obtained results (Table 6) confirm that the WMW-SSA mixtures correspond to the sands in terms of void ratio values as well as dry density of the sands (M1A and M1B). The M2B mixture has a minimum dry density slightly below the indicated range.

SSA achieved significantly lower values of minimum and maximum dry density compared to WMW, which in turn corresponds to higher values of the void ratio and porosity. SSA has a lower dry density than natural soils due to its porous particle structure. The observed variations in the minimum and maximum parameters may be due to irregularities in particle shape and degree of sharpness. However, these considerations should be considered limited, as the contribution of grain shape features was not addressed in this study and was assessed only visually from SEM microstructural images [19].

3.1.4. Maximum Dry Density and Optimal Moisture Content

Table 7. Maximum dry density and the optimal moisture content of the WMW-SSA mixtures and their components (WMW and SSA).

No.	Parameter	Unit	WMW-SSA			WMW 1	SSA
			M1A	M1B	M2B	P-2	PL1	PL2	PL3
1.	Maximum dry density ρds	g/cm3	1.780	1.770	1.542	1.790	0.861	0.912	1.001
2.	Optimum moisture content wopt	%	12.86	12.54	20.25	11.95	65.45	60.80	52.40
3.	Degree of saturation after compaction Sr	-	0.768	0.738	0.838	0.680	0.959	0.977	0.935

¹ The research results come from the paper [23].

presents the test results for the maximum dry density and the optimal moisture content of the WMW-SSA mixtures and their components (WMW and SSA). Additionally, based on these values and the specific density results (Table 5), the degree of saturation of the test materials at maximum compaction was calculated.

The parameters characterizing compactability, i.e., maximum dry density of the skeleton and optimal moisture content, of the WMW-SSA mixtures (M1A and M1B) with a dominant WMW content (90%) showed little variation. Similarly, the value of the obtained degree of saturation at maximum compaction. In contrast, the M2B mixture had a lower maximum dry density of the skeleton and a higher optimal moisture content, as well as a higher degree of saturation, compared to the M1A and M1B mixtures. This is due to the higher proportion of lighter fine-grained fractions resulting from the addition of water-binding SSA to the material.

The test results obtained for the WMW-SSA mixtures (M1A and M1B) allow for them to be classified as sands, with a maximum dry density of the soil skeleton (1.65–2.10 g/cm³) and optimal moisture content (8.0–13.5%). The degree of saturation of the mixtures slightly exceeds the range of values for sands (0.4–0.7), indicating till (0.7–0.8) or cohesive soils (above 0.7). For the M2B mixture, the parameter values characterizing compactability are similar to those for cohesive soils (clays) [53].

SSA was characterized by significantly lower values of the maximum dry density of the soil skeleton compared to the WMW, as well as much higher values of optimal moisture content and degree of saturation.

3.1.5. Hydraulic Conductivity

Table 8. Hydraulic conductivity of WMW-SSA mixtures and their components (WMW and SSA).

No.	Parameter	Unit	WMW-SSA			WMW 1	SSA
			M1A	M1B	M2B	P-2	PL1	PL2	PL3
1.	Hydraulic conductivity (in apparatus) k10	m/d	0.22	0.39	0.08	7.71	0.07	0.07	0.07
2.	Hydraulic conductivity (Slichter formula) k10 2	m/d	0.85/2.92	0.92/3.12	0.25/0.76	4.31/11.97	0.35/0.44	0.11/0.22	0.12/0.23

¹ The research results come from the paper [23]. ² The results were obtained for the n_min/n_max porosity values presented in Table 6.

presents the results of the hydraulic conductivity tests for WMW-SSA mixtures and their components (WMW and SSA). Additionally, based on the effective diameter (Table 4, no. 7) and porosity (Table 6, no. 3 and 6), the hydraulic conductivity was calculated using Slichter’s empirical formula. According to the findings of Kostrzewa et al. (2025) [23], the hydraulic conductivity values determined using Slichter’s formula based on porosity limits showed the highest agreement with test results for WMW.

The hydraulic conductivity of the WMW-SSA mixtures varied, in contrast to the SSA, for which the results were of the same order of magnitude and practically identical. Compared to WMW, the hydraulic conductivity of the WMW-SSA mixtures was almost 20 and 35 times lower for the M1B and M1A mixtures, respectively, and almost 100 times lower for the M2B mixture.

According to the classification [71], the M1A and M1B mixtures can be characterized as poorly permeable (silty sands, loamy sands) with a hydraulic conductivity of 10⁻⁵–10⁻⁶ m/s, and the M2B mixture and SSA as semi-permeable (loams, sandy loams) with a hydraulic conductivity of 10⁻⁶–10⁻⁸ m/s. However, with reference to the classification [59], the M1A and M1B mixtures can be characterized as sands (silty or humus) with a hydraulic conductivity of 10⁻³–10⁻⁴ cm/s, and the M2B mixture and SSA as silts, sandy silts, or sandy loams with a hydraulic conductivity of 10⁻⁴–10⁻⁶ cm/s.

The hydraulic conductivity calculated from Slichter’s empirical formula was used only to estimate the approximate hydraulic conductivity of the tested SSA and WMW-SSA mixtures. However, for all SSA and WMW-SSA mixtures, the hydraulic conductivity determined through laboratory tests was several times lower than the minimum hydraulic conductivity calculated from Slichter’s empirical formula. It should be noted that the empirical formula assumes an ideal pore shape, while the actual structure can drastically affect permeability.

3.2. Mechanical Parameters

3.2.1. Internal Friction Angle and Apparent Cohesion

The direct shear test results showing the shear strength of the WMW-SSA mixtures and their components (WMW and SSA) are presented in Figure 5. Based on the linear approximation equations, the internal friction angle (Φ) and apparent cohesion (c_p) were calculated and presented in

Table 9. Summary of direct shear test results for WMW-SSA mixtures and their components (WMW and SSA).

Sample	Linear Equation	Coefficient of Determination (R2)	Number of Measurements (n)	Internal Friction Angle (Φ) [°]	Apparent Cohesion (cp) [kPa]
WMW-SSA (M1A)	τ = 0.8218∙σ + 11.45	0.9818	15	39.5	11.45
WMW-SSA (M1B)	τ = 0.7878∙σ + 19.21	0.9780	15	38.4	19.21
WMW-SSA (M2B)	τ = 0.8395∙σ + 19.90	0.9855	15	40.1	19.90
WMW (P-2) 1	τ = 0.6893∙σ + 14.98	0.9745	17	34.4	14.98
SSA (PL1)	τ = 0.9718∙σ + 15.66	0.9893	10	44.1	15.66
SSA (PL2)	τ = 1.0545∙σ + 15.39	0.9873	11	46.4	15.39
SSA (PL3)	τ = 0.7173∙σ + 25.62	0.9419	9	35.5	25.62

¹ The research results come from the paper [23].

.

Shear strength was relatively high. The angle of internal friction values between the WMW-SSA mixtures were characterized by little variation. Apparent cohesion was comparable among the mixtures in which all three SSA additives were used and almost twice as high as the apparent cohesion in the M1A mixture, which used only a single SSA (PL1). For the first two SSA samples (PL1 and PL2), comparable values of internal friction angle and apparent cohesion were obtained. The exception was SSA (PL3), for which a lower internal friction angle and higher apparent cohesion were obtained. The internal friction angle and apparent cohesion values did not show any specific relationship with the SSA content in the mixtures.

In terms of the internal friction angle values, the WMW-SSA mixture can be classified as coarse and medium sands (37–39°), or even gravels and tills (40–42°), in a very dense state. The internal friction angle values of SSA (PL1 and PL2) exceed those of gravels and tills in a very dense state, while SSA (PL3) can be classified as fine and silty sands in a dense state (33–36°) [59].

With respect to apparent cohesion, only in the case of the M1A mixture was compliance with the expected value (approximately 11 kPa) observed for medium sands in a very dense state and with a degree of saturation of 0.768 at maximum compaction [72]. In the remaining mixtures and SSA, the cohesion value can be related to the values assigned to low-cohesion soils, which include sandy loams, sandy silts, or silts in a hard plastic (20–28 kPa) or plastic (12–20 kPa) state [59].

3.2.2. Oedometric Compressibility Modulus

The oedometric values of the primary compressibility, unstressing, and secondary compressibility modulus of the WMW-SSA mixtures and their components (WMW and SSA), depending on the applied load, are presented in Figure 6, Figure 7 and Figure 8, respectively.

WMW-SSA mixtures exhibit higher compressibility than WMW, resulting in lower oedometric compressibility modulus. This is a result of using SSA as an additive in the mixtures, which can be explained by the sharp edges and porous surface texture of the ash particles. SSA also exhibits compressibility similar to WMW-SSA mixtures, although significantly higher, and consequently lower values of compressibility parameters (oedometric compressibility modulus).

Both SSA and WMW-SSA mixtures were characterized by primary and secondary oedometric compressibility modulus values that were predominantly within the range for fine and silty sands in a loose state, not exceeding 30 and 50 MPa, respectively. Although from a construction perspective, a reduction in oedometric compressibility modulus, which indicates a greater ability to reduce volume under load, is usually unfavorable, as it leads to larger and potentially uneven settlements, it should be recognized that oedometer tests are considered unreliable. Optimal results are obtained from test loads applied to constructed structures [53]. The lack of correlation between settlement determined under laboratory conditions and actual settlement is due to several factors, including pozzolanic properties and the hardening of compacted materials. Short-term laboratory tests do not account for the increase in sample rigidity that occurs under long-term loading in actual conditions.

3.3. Radiological Parameters

The activity concentrations of K-40, Ra-226, Th-232, and Cs-137 for WMW (P-2) and SSA samples (PL1–PL3) are summarized in

Table 10. Activity concentrations (Bq·kg⁻¹), radiological indices and uncertainty ranges for WMW (P 2) and SSA samples (PL1–PL3).

Sample	K-40	Ra-226	Th-232	Cs-137	Raeq	Iγ
	Bq·kg−1					-
WMW (P-2)	420 ± 105	278 ± 70	92 ± 20	<46	442	1.53
SSA (PL1)	740 ± 185	<40	<30	46 ± 15	57	0.25
SSA (PL2)	2500 ± 625	80 ± 50	62 ± 20	102 ± 25	≈330	≈1.13
SSA (PL3)	812 ± 200	840 ± 200	97 ± 25	116 ± 30	1041	3.56

. Marked variability is observed, reflecting differences in mineral fraction content and the degree of inorganic enrichment.

K-40 spans moderate (PL1, PL3) to high levels (PL2), consistent with alkali enrichment during thermal processing; P-2 displays an intermediate, typical silicate aggregate value. Ra-226 is negligible in PL1, corrected to 80 ± 50 Bq·kg⁻¹ in PL2, markedly elevated in PL3 (840 ± 200 Bq·kg⁻¹), and moderate in P-2 (278 ± 70 Bq·kg⁻¹). Th-232 is measurable in PL2-PL3 and P-2, with P-2 reflecting a thorium-bearing heavy mineral signature; PL1 remains below detection. Cs-137 is low across SSA and undetected in WMW. PL1, PL2 and P-2 fall within activity ranges reported for many natural geomaterials, whereas PL3 exhibits elevated Ra-226 and Th-232 characteristic of higher-activity NORM, not typical of common sands or gravels.

Because both instruments were used complementarily, not competitively, the radiological picture is internally consistent: (i) NaI(Tl) supplies activity concentrations in Bq·kg⁻¹ for the four target nuclides; (ii) LaBr₃(Ce—used in the background—stabilized line selection and confirmed mineralogical trends (K, U, Th), enabling coherent, single-unit reporting in Table 10. Elevated K-40 in PL2 mirrors high K content; enhanced Ra-226 and Th-232 in PL3 agree with stronger U/Th series signatures; P-2’s Th-232 points to natural heavy mineral enrichment rather than contamination.

Ra_eq integrates the contributions of K-40, Ra-226 and Th-232. PL1 is very low (57 Bq·kg⁻¹), PL2 is moderately (≈330 Bq·kg⁻¹) driven by K-40 and Th-232 with corrected Ra-226, whereas PL3 reaches 1041 Bq·kg⁻¹ due to its high Ra-226. P-2 is intermediate (442 Bq·kg⁻¹), consistent with thorium-rich sands.

I_γ follows the same hierarchy: PL1 (0.25) ≪ PL2 (≈1.13) < P-2 (1.53) ≪ PL3 (3.56). Although PL3 and P-2 present elevated indices, they remain within the broad ranges reported for NORM bearing geomaterials used outside residential indoor environments.

The calculated gamma activity index (Iγ) may be compared with the screening approach described in the European Commission Radiation Protection 112 report [73] and Council Directive 2013/59/Euratom [74], where Iγ = 1 is commonly adopted as a reference level for materials intended for bulk indoor use. In the present study, PL1 remains below this reference level, whereas PL2, PL3 and P-2 exceed it to varying degrees. However, these criteria were developed primarily for indoor building materials and are not directly applicable to open-air hydrotechnical or geotechnical structures characterized by substantially lower occupancy and greater geometric dispersion of gamma radiation. At present, no dedicated regulatory screening levels are defined for open-air hydrotechnical or geotechnical structures within the European radiological protection framework. Consequently, the reference values established for indoor building materials are used here solely as a conservative benchmark, rather than as directly applicable compliance criteria. For hydraulic engineering applications, exposure conditions correspond to open-air environments, where external gamma radiation is subject to substantial geometric dispersion. In outdoor hydrotechnical settings, effective gamma dose is substantially reduced due to occupancy factors, distance, and geometric attenuation, consistent with EC and UNSCEAR exposure models. This remains a qualitative radiological assessment rather than a full dose reconstruction. Consequently, even the elevated Iγ value observed for PL3 remains practically comparable to gamma fields associated with natural rock aggregates used in revetments, embankments, and armor layers.

From a radiological perspective, the investigations of SSA and WMW samples exhibit activity levels characteristic of materials containing naturally occurring radionuclides, although with notable variability between individual ashes. PL1, PL2 and P-2 fall within the broad activity ranges reported for natural geomaterials used in earthworks, whereas PL3 displays elevated Ra-226 and Th-232 activities that result in radium-equivalent and gamma index values exceeding the criteria typically applied to indoor building materials. These elevated values do not, however, preclude its use in engineering practice.

In open-air applications such as hydrotechnical structures, embankments, or earthworks, external gamma radiation is strongly mitigated by distance, limited occupancy times, and the absence of enclosed geometries. Under such exposure conditions, even the higher-activity SSA sample (PL3) does not pose a significant radiological concern under the expected exposure conditions, if it is used within appropriate application-specific constraints and with consideration of relevant exposure pathways. The radiological indicators obtained in this study provide the technical basis for a broader interdisciplinary assessment presented later, in which radiological behavior is assessed in conjunction with physicochemical and environmental parameters to determine overall engineering suitability.

The radiological characterization presented here, and the independent physicochemical investigation reported elsewhere were conducted on the same primary material set: washed mineral waste P-2 and sewage sludge ashes PL1, PL2, and PL3. The Kostrzewa et al. (2026) [19] study provides exhaustive information on granulometry, microstructure, elemental composition, bulk density, organic matter content, and heavy metal mobility for these materials. The present work complements those findings by quantifying natural radionuclides (K-40, Ra-226, Th-232) and anthropogenic Cs-137. The integration of these independent datasets enables a deeper understanding of how mineralogical architecture, chemical composition, and particle size distribution modulate radiological signatures, with direct implications for the environmental safety and engineering suitability of the materials.

A consistent relationship emerges between the mineralogical characteristics identified in the Kostrzewa et al. (2026) [19] study and the radiological behavior observed here. Sample PL2, which exhibits a bimodal grain size distribution dominated by fine ash particles (~110 µm), alongside pronounced agglomeration and enriched aluminosilicate phases, was shown to contain a higher proportion of K-bearing mineral glass. These features coincide with its markedly elevated K-40 activity. Potassium is hosted primarily within amorphous aluminosilicate matrices formed during thermal sludge treatment; therefore, the enhanced abundance of such phases in PL2 explains its radiological distinctiveness.

A similar coherence is evident in the behavior of radionuclides associated with the U and Th series. As shown in the independent physicochemical characterization reported by Kostrzewa et al. (2026) [19], PL3 displays a more crystalline microstructure, higher heavy metal content, and a greater abundance of Fe- and Al-bearing phases than PL1 or PL2. These mineralogical traits are consistent with an increased presence of U- and Th-bearing phases. This aligns with the radiological finding that PL3 demonstrates the highest activities of Ra-226 and Th-232 among all SSA samples. In contrast, PL1, which contains fewer fine fractions and displays lower degrees of geochemical enrichment, shows radionuclide activities below detection limits for Ra-226 and Th-232. The alignment between mineralogical indicators and gamma spectrometric results provides cross-validation of both analytical approaches (

Table 11. Integrated Material Characteristics: Physicochemical vs Radiological Behavior.

Sample	Dominant Microstructural/Chemical Features	Key Granulometric Traits	Heavy Metal Enrichment Pattern	Radiological Signature (K-40, Ra-226, Th-232, Cs-137)	Interpretive Integration (Mechanistic Link)
WMW (P-2)	Quartz-dominated medium sand; low organic matter; simple mineralogy	95.4% sand; negligible fines	Very low HM content except natural Ni/Co	Moderate K-40 and Th-232; no U-series elevation	Natural geogenic signature matches quartz-rich sands; absence of fine reactive phases prevents radionuclide accumulation
SSA (PL1)	Fine sand with high silt; simple unimodal distribution; low Fe/Al enrichment; limited porous glassy phases	Fine fraction significant but mineralogically simple	Lowest HM concentrations among SSA	Very low Ra-226 and Th-232; moderate K-40; Cs-137 detectable but low	Minimal fine-phase mineral enrichment explains weak U/Th signatures; moderate K-bearing silicates yield mid-range K-40
SSA (PL2)	Bimodal fine fraction; strong agglomeration; abundant aluminosilicate glass; elevated K-rich phases	Fine + mid-size clusters; high surface area	Moderate HM enrichment	Highest K-40 across SSA; moderate Ra-226/Th-232	K-40 peak results from high K-bearing phases in fine ash; agglomeration increases gamma yield stability
SSA (PL3)	Crystalline SSA; high Fe/Al phases; porosity and microcrystals typical of U/Th-bearing matrices	Fine fraction and agglomerates present	Highest HM concentrations (Zn, Pb, Cu, Ni, Co)	Highest Ra-226 and Th-232; moderate K-40	Crystalline Fe-/Al-rich phases stabilize U/Th progeny; HM enrichment correlates with enriched radionuclides

).

Kostrzewa et al. (2026) [19] documents clear differences in particle size composition among the SSA samples: PL1 and PL3 are classified as fine sands with substantial silt fractions, whereas PL2 exhibits a more complex bimodal distribution. Finer particles—particularly in the <200 µm fraction—may preferentially host radionuclides, especially those associated with the U/Th decay series, due to their higher specific surface area and greater capacity for binding metals and trace elements. This mechanism is well reflected in the radiological pattern: PL2, with its fine rich and agglomerated structure, shows the highest K-40 activity; PL3, characterized by its crystalline yet fine-dominated matrix, exhibits elevated Ra-226; and PL1, with a simpler fine fraction profile, remains radiologically the least enriched. These relationships underscore the central role of granulometry in modulating radionuclide distribution within ash-based materials.

The washed mineral waste P-2 presents an instructive contrast. Its composition is dominated by quartz-rich medium sand (more than 95% of the sand fraction) and shows a minimal presence of fine-grained or chemically enriched phases. As expected, its radiological activity corresponds to that of a natural silicate aggregate, with moderate K-40 and Th-232 values and no evidence of U-series enrichment. The harmony between granulometric simplicity and radiological neutrality further strengthens the overall interpretive framework (

Table 12. Mechanistic Interpretation Matrix: Mineralogical and Physicochemical Controls on Radiological Behavior.

Mechanistic Domain	Empirical Observation (from Full Dataset)	Mechanistic Interpretation	Radiological Expression	Supported by
Aluminosilicate glass phases (SSA)	PL2 and PL3 contain abundant amorphous and semi-crystalline aluminosilicates, with higher K, Al, Si proportions	Glassy ash matrices incorporate alkali metals (K), and selectively trap U/Th daughter products in micro-porous domains	Elevated K-40 (PL2); elevated Ra-226 and Th-232 (PL3)	SEM/EDS + radiological data
Fine-fraction dominance	SSA samples contain 40–47% silt/superfine ash; PL2 has bimodal 8 µm + 110 µm distribution; PL3 fine fraction strongly present	Fine particles have high surface area, sorption capacity, and high affinity for radionuclides and trace metals	Amplified gamma activity in PL2 (K-40) and PL3 (Ra/Th series)	Particle size distribution + gamma spectroscopy
Crystalline Fe/Al-rich microphases	PL3 contains more crystalline domains and Fe oxides (color, microstructure)	Fe/Al phases support incorporation of U/Th decay-chain microphases, stabilizing their presence	Highest Ra-226 and Th-232 in PL3	SEM + radionuclide fingerprints
Heavy metal enrichment as proxy for geochemical reactivity	PL3 shows highest Zn, Pb, Cu, Ni, Co; PL2 moderate; PL1 low; P-2 minimal	HM enrichment correlates with reactive phases capable of concentrating U/Th daughter isotopes	PL3 > PL2 > PL1 radiologically (Ra, Th series)	ICP/FAAS chemical analysis + radiology
Ash alkalinity (pH 8.9–9.4)	SSA eluates show elevated pH; mixtures even higher depending on SSA fraction	High pH suppresses solubility and mobility of U/Th radionuclides and heavy metals	Radiological stability (no detectable mobility)	Leaching tests + radiological immobility
Quartz-dominated mineral waste (P-2)	P-2 shows 95.4% sand, simple mineralogy, minimal fine fraction, no reactive phases	Lack of reactive silicate or oxide domains reduces radionuclide accumulation capacity	Moderate K-40, natural Th-232, no U-series anomalies	Mineralogy + gamma spectra

).

The Kostrzewa et al. (2026) [19] study identifies PL3 as the chemically most enriched SSA sample, showing the highest concentrations of Zn, Pb, Cu, Ni, and Co among the ash materials, while PL2 and PL1 follow a decreasing trend in elemental enrichment. A similar ordering is observed in the radiological dataset, suggesting that both heavy metal enrichment and radionuclide levels respond to the distribution of reactive mineral phases. P-2 again occupies a separate position: it is chemically stable and shows low heavy metal concentrations (except for naturally occurring Ni and Co peaks in extracts), paralleling its moderate radiological signature. This reinforces the classification of P-2 as a natural-like mineral material.

The combined datasets indicate consistently low environmental mobility of contaminants in both WMW and SSA, regardless of leaching conditions. According to the comprehensive material analysis presented by Kostrzewa et al. (2026) [19], heavy metals are immobilized at levels approaching 100% across all materials, regardless of leaching agent, with extract concentrations far below regulatory thresholds. This high immobilization coincides with alkaline pH conditions (7.4–9.4), which promote the formation of insoluble metal hydroxides and phosphates. These same alkaline conditions suppress the mobility of radionuclides by maintaining U/Th-series elements in structurally bound, insoluble forms.

Because hydraulic structures are open-air environments, external gamma radiation is the dominant radiological pathway. Here, radiological indices (Ra_eq, I_γ) span the range observed in natural materials, including elevated NORM cases, indicating no meaningful radiological hazard under the expected exposure conditions. The combination of high chemical immobilization, minimal radionuclide mobility, and stable microstructure provides a strong basis for assessing the environmental safety of these materials in hydrotechnical and geotechnical applications.

The radiological and physicochemical evidence demonstrates a consistent internal logic across both studies. Samples with high chemical and mineralogical enrichment (notably PL3) also exhibit elevated radionuclide levels, while samples with low enrichment (PL1, P-2) display correspondingly weak radiological signatures. The results reveal a direct, mechanistically plausible link between ash microstructure, particle size distribution, elemental composition, and radiological behavior. Moreover, the negligible leachability of both heavy metals and radionuclides underscores the long-term environmental stability of these materials. This integrative perspective indicates that SSA and WMW, when properly characterized and used within appropriate engineering constraints, can be safely incorporated into hydraulic embankments, earthworks, and related geotechnical structures. The conclusion regarding negligible radiological risk applies specifically to open-air hydrotechnical and geotechnical applications, including embankments, revetments, earthworks, and backfill layers, where the materials are not used in enclosed residential environments. Under such conditions, exposure is limited by low occupancy times, geometric attenuation, partial shielding by overlying materials, and the absence of long-term indoor habitation. Consequently, external gamma exposure remains substantially lower than in scenarios considered by indoor building-material regulations. The negligible radiological risk conclusion is therefore valid for exposure scenarios involving intermittent occupational presence or incidental public contact, such as construction, maintenance, or inspection activities, and does not apply to conditions involving long-term or continuous residential occupancy.

3.4. Criteria for Selecting Soils for Use in Road and Hydro Engineering Structures

The potential of WMW-SSA mixtures for construction applications was assessed. The analysis included verification of the mixtures’ applicability as backfill for trenches, bridge abutments, and retaining structures, as well as materials for the construction of road embankments and flood embankment bodies. The selection criteria were based on applicable standards, guidelines, legal provisions, and the literature. The criteria are derived from construction standards (road, installation, and water and land improvement), technical specifications for earthworks, regulations for hydro structures, and procedures for assessing the technical condition of flood embankments. International technical recommendations define best practices for the construction of small earth dams, the design of flood embankments, and the development of roads and bridges. These criteria are presented in papers [19,23,33,64].

Table 13. Technical requirements for soil in earthworks (trenches, backfills, embankments)—own study based on [19,23,33,64].

Purpose	Requirement
Trench to frost depth 1 [19,33]	Frost-unsusceptible soil	Particle content (≤0.075 mm): ≤15% Particle content (≤0.02 mm): ≤3%
	Doubtful soil 2	Particle content (≤0.075 mm): 15–30% Particle content (≤0.02 mm): ≤3–10%
Installation backfill at least 0.3 m above the top of the pipe or its casing [19]	Sandy soil	Grain size ≤ 20 mm
	Appropriate value of the compaction index	≥1.00 (at a depth of 0.00–1.20 m) 0.97 2 (at a depth of more than 1.20 m)
Protective layer of trench backfill [19]	Non-rocky, free of clods and stones, mineral, non-cohesive, and fine- to medium-grained soil
	Appropriate value of the compaction index	≥1.00 (to the ground surface or the required elevation)
Backfill of bridge abutment and retaining structure 3; upper backfill layer 4 [19,33]	Sand (at least medium-grained) Non-cohesive soil 4 Frost-unsusceptible soil 4	Uniformity coefficient: ≥5
	Hydraulic coefficient	≥10−5 m/s ≥6 × 10−5 m/s 4
	Appropriate value of the compaction index	≥1.00 5
Road embankment 1,6; upper embankment layer in the frost depth 4 [19,33,64]	Appropriate soil Non-cohesive soil 4 Frost-unsusceptible soil 4	Grain diameter ≤ 200 mm Fraction of sand and gravel: ≥35% Particle content (<0.075 mm): ≤75% Uniformity coefficient: ≥3 7 Uniformity coefficient: ≥5 4
	Organic substances content	≤10%
	Maximum dry density of the skeleton	≥1.0 g/cm3
	Hydraulic coefficient 4	≥6 × 10−5 m/s 4
	Internal friction angle	≥20°
	Frozen soil and soil containing branches, roots, stumps, wood, and plastic waste, as well as materials that may spontaneously combust and those contaminated with harmful chemicals, should be avoided.
Flood embankment of all classes 8 [19,23]	Mineral or anthropogenic soil (sand)	Clay content < 30%
	Organic substances content	<2–3%
	Chemically uncontaminated materials	No assessment criteria or test method are specified
	Moisture content before compaction (non-cohesive soil)	>0.7 × wopt
	Appropriate value of the compaction index (non-cohesive soil—fine or medium sand)	≥0.97 (for Class I–II structures) ≥0.95 (for Class III–IV structures)
	Utilizing local soil is highly recommended, as they allow for the structure design and ensure its safety. Frozen soil and soil containing waste, debris, plant residues, tree stumps, and other contaminants of uncontrolled quality should be avoided.

¹ The required moisture content of the compacted material, the compaction procedure, and the thickness of the layers should be determined experimentally during testing compaction with the equipment used. ² The application requires improving its properties with an additional binder (cement or lime). ³ Backfill based on ash mixtures or artificial aggregates is permitted, provided that it is embedded in a dry or water-insulated location. ⁴ If the soil is unsuitable, the upper layer must be stabilized with a binder (cement or lime). ⁵ Except for the slopes of the cones at the wings and the frontal slopes of the openwork abutments and embedded in the embankment, where the compaction index should be at least 0.95. ⁶ For the lower layer of an embankment below the frost depth, medium sand. Ash mixtures are permitted, provided that they are embedded in dry or water-insulated locations. For the upper layer of embankment at the frost depth, medium sand. The condition for using ash mixtures is the additional improvement of their properties with binders (cement or lime). ⁷ Conditional acceptance of soil with a lower uniformity coefficient is possible, subject to preliminary field compaction trials that verify the target compaction can be successfully achieved. ⁸ Poland’s primary hydro-engineering facilities are classified in the Regulation [75]. The classification of a structure is defined by its form, operational purpose, and physical dimensions, as well as the specific design parameters dictated by its magnitude and practical significance.

summarizes the requirements for soils for use in trenches, backfills, and road and flood embankments.

The M1A and M1B mixtures meet the criteria for frost-unsusceptible soils, allowing for them to be embedded directly into trenches up to frost depth. However, the addition of gravel to the M2B mixtures altered its grain size, classifying it as a doubtful soil that requires reinforcement with an additional binder before use.

All WMW-SSA mixtures meet the key material criteria for soil type and grain size, making them suitable for backfilling installation trenches (at a minimum depth of 0.3 m above the pipe or its casing) and for protective layers of trench backfill.

The use of WMW-SSA mixtures as backfill for bridge abutments and retaining structures is currently not possible. The main obstacles are insufficient hydraulic conductivity and the fact that the M2B mixture—despite its appropriate uniformity coefficient—is not a frost-unsusceptible soil. Until the M2B mixture containing the binder additive is retested, these mixtures do not meet the technical requirements for this type of structure.

All WMW-SSA mixtures meet the material standards for road embankments (grain size, maximum dry density, and internal friction angle). WMW-SSA mixtures are suitable for embedding in embankments without modification, provided they are located in an area isolated from water and below the frost depth. Their use in the upper zone of the embankment, up to the frost depth, requires prior binder treatment.

All WMW-SSA mixtures meet the criteria regarding grain size and soil type intended for embedding into flood embankment bodies.

Preliminary verification of WMW-SSA mixtures confirmed their compliance with the standards regarding the permissible content of organic substances for road and flood embankments [19].

The current assessment of the suitability and properties of the proposed WMW-SSA mixtures is preliminary and heavily limited by laboratory conditions. It is based on assumptions and models that, despite their detail, do not fully account for the complexity and variability of the actual construction environment. All selected solutions and WMW-SSA mixture formulations require rigorous verification under the target application conditions. The most important parameter to be assessed will be the compaction index, which will confirm the suitability of the tested mixtures.

3.5. Economic and Environmental Impact

The wastewater treatment process, regulated by European Union directives, generates waste streams, including washed mineral waste and fly ash from the thermal treatment of municipal sewage sludge. According to Eurostat data [76], from 2014 to 2022, the amount of waste falling into the mineral waste category from the waste treatment and stabilized waste category in Poland increased by almost 40% (from 1,097,778 to 1,529,819 tons). In turn, the amount of sewage sludge generated during the same period increased by approximately 5% (from 556,000 to 580,660 tons) [77].

Current mineral waste management methods, focused primarily on landfill and disposal, do not meet modern standards. Extreme rates for mineral waste disposal, reaching approximately EUR 500/ton (Poland, as of 2023), generate enormous operating costs for wastewater treatment plants. With plants with a capacity of 780,000 people equivalent producing over 3000 tons of this waste annually, this expenditure is becoming a serious budgetary problem [13,19]. Available data also indicate that mineral waste landfill costs were approximately USD 70/ton (Brazil, as of 2014) [26].

With respect to sewage sludge generated annually, the following management methods are specified (Poland, as of 2022): in agriculture (17.52%); for land reclamation (2.87%); for the cultivation of plants intended for compost production (2.40%); and as thermally transformed (25.19%), landfilled (6.79%) and other (not directly indicated): 45.23% [48]. On a European scale, costs allocated to sewage sludge management range, on average, from 160 to 310 EUR/ton of dry matter, which is a direct result of national conditions and methods of final disposal [78]. However, there are deviations from the indicated range. In Poland (as of 2014), sewage sludge management costs reached EUR 75/ton (agriculture), EUR 125/ton (landfill), EUR 150/ton (composting), EUR 375/ton (co-incineration), and EUR 438/ton (mono-incineration) of dry matter [79]. Since these processes account for 20% to 65% of wastewater treatment plants’ total operating expenses, sludge management optimization has become a key economic and ecological challenge. This provides a strong impetus for the scientific community to develop technologies and solutions that minimize sludge production [78].

Therefore, embedding recycled materials from wastewater treatment plants, such as washed mineral waste or fly ash from the thermal treatment of municipal sewage sludge, into construction is a current and promising research direction. Transforming waste into a raw material that can be directly used in construction solutions reduces waste volume, as reflected in the space occupied in landfills, and lowers operating costs for wastewater treatment plants. This approach also reduces the costs of purchasing sand directly by municipal water and sewage companies when implementing their own network renovation projects. This, for the market price of natural sand (Poland, as of 2023), is approximately 10 EUR/ton of material, with transportation costs of approximately 2.8 EUR per 1 km of travel in a 20-ton vehicle [80].

The use of waste materials in construction products reduces demand for traditional aggregates, thereby lowering environmental impact and reducing the need to exploit mines. Replacing fossil aggregates and energy-intensive cement production with recycled materials reduces greenhouse gas emissions. This has been confirmed by using sewage sludge ash as a concrete component and recycled sand in cement mortar [43,80]. It is important to emphasize that WMW and SSA are permanently generated byproducts of the wastewater treatment process. Their production is continuous and, unlike conventional aggregate extraction or recycling procedures, does not require additional refining treatments or mechanical disintegration of the material structure (e.g., crushing). Activities in this area support the circular economy strategy, enabling the achievement of the Sustainable Development Goals (SDGs) by combining technological efficiency with environmental protection. The transformation of waste management in cities implements the assumptions of SDG 11. Furthermore, improving the environmental performance of industrial processes (SDG 12) and reducing harmful emissions (SDG 13) demonstrates that implementing the results of experiments on the practical use of wastewater treatment plant waste in construction will deliver measurable environmental benefits.

3.6. Scaling Resource Recovery Technologies

The use of waste-based materials in construction, such as WMW or SSA, is a promising step towards a circular economy. However, despite the wide range of benefits of sustainable recycling, it is important to remember that the properties of these wastes can vary and depend on factors such as the size and technology of the wastewater treatment plant, the origin of the wastewater, the condition of the infrastructure, and the season. This raises crucial questions about their long-term impact on structural performance, the environment, and human health.

In accordance with legal provisions, each batch of waste considered for construction use after the treatment cycle must be treated as unique and subjected to qualitative analysis. Current research on the properties of the WMW-SSA mixtures focused on laboratory tests. These constituted industrial research that can be classified as Technology Readiness Level (TRL) 4 [81]. At this level, the individual components (waste) were tested and then combined to verify their compatibility and exhibit the crucial properties in a controlled environment. Key elements of validation in this context included verifying that the mixtures met construction standards and environmental regulations regarding the content of harmful substances, and that their components did not present a radiological hazard, confirming that these mixtures correspond to conventional construction materials and pose no threat to the environment or human health. As a result of the analysis, for some of the construction applications indicated in the paper, it will be necessary to adjust the mixture formulation by improving its properties with an additional binder (cement or lime).

The next step in technology scaling (up to TRL 5) involves moving from laboratory samples to validation in a relevant environment. The essence of the research is to prepare a semi-finished product and test it under conditions that simulate its final operating environment (e.g., subjecting samples to temperature fluctuations, additional dynamic loads, or appropriate chemical factors found in the natural environment). The scale of operation can be limited to the laboratory or presented on a small model (trench or embankment) of proportional dimensions. The goal is to verify whether the material concept functions and maintains its properties. A significant step, reinforcing the main research objective, could also be the development of numerical models that reflect the behavior of the mixtures over a longer period of time.

The next and particularly crucial step in scaling up the technology is to conduct engineering tests during a prototype demonstration in an environment that simulates relevant construction conditions (TRL 6). Pilot-scale field tests on the compaction field (in trenches or embankments) over a longer period of operation will allow for the determination of actual compactability, load-bearing capacity, and resistance to deformation under static and dynamic loads, the susceptibility of the mixtures to technological processes (e.g., workability of the mixtures on site), as well as monitoring the impact of the embedded material on the environment and human health. These activities constitute industrial research aimed at verifying whether the material’s behavior is consistent with the project assumptions and previous laboratory models. These activities could serve as the basis for developing guidelines for optimal material embedding and maintenance methods, and the measurements and analyses conducted during the field tests will support further experimental development (TRL 7–9).

4. Conclusions

In an era of resource scarcity and climate change, recycling is a key element of the circular economy. This study examines the feasibility of sustainable use of mixtures of washed mineral waste and fly ash from the thermal treatment of municipal sewage sludge at Poland’s largest wastewater treatment plant. Mixtures WMW-SSA exhibit properties comparable to those of natural soils. The addition of SSA increased the uniformity coefficient (by 51–263%), the optimal moisture content (by 5–69%), and the internal friction angle (by 12–17%), while simultaneously reducing the specific density (by 3–6%), maximum dry density (by 1–14%), hydraulic conductivity (by 95–99%), and oedometric compressibility and unstressing modulus (by 0–66%) compared to washed mineral waste.

Radiological characterization of the SSA and WMW components revealed activity levels governed primarily by mineralogical composition and particle size distribution, consistent with the intrinsic heterogeneity of thermally processed anthropogenic materials rather than atypical contamination. Whereas PL1, PL2, and WMW (P-2) fall within activity ranges commonly reported for natural geomaterials used in earthworks, PL3 exhibits elevated Ra-226 and Th-232 concentrations yielding Ra_eq and I_γ values that exceed reference criteria established for indoor building materials under EC Radiation Protection 112 and Council Directive 2013/59/Euratom. No dedicated regulatory screening levels currently exist for open-air hydrotechnical or geotechnical applications; accordingly, those indoor reference values serve here as a conservative benchmark only. Under the open-air exposure conditions relevant to the intended applications—characterized by geometric attenuation, low occupancy, and the absence of enclosed residential environments—external gamma radiation constitutes the dominant pathway and remains substantially reduced relative to indoor scenarios. Consequently, none of the investigated materials is expected to present a significant radiological constraint when used as embankment fill, earthwork material, or installation trench backfill under the conditions described in this study.

The WMW-SSA mixtures met the technical requirements for use as trench backfill materials up to the frost depth (only the mixtures with a dominant WMW content: M1A and M1B), as backfill for installation trenches, and as protective materials for trench backfill. The mixtures’ properties were also confirmed to be compatible with soil properties used as materials for constructing road embankments in conditions isolated from water and below frost depth, and in flood embankment bodies. A critical exception to the use of the WMW-SSA mixtures concerned the backfilling of bridge abutments and retaining structures. This resulted from the failure of all WMW-SSA mixtures to meet the minimum hydraulic conductivity threshold (10⁻⁵ m/s) and the failure of the M1A and M1B mixtures to meet the uniformity coefficient threshold (≥5). A similar situation occurred in the upper layers of the road embankment at the frost depth (required hydraulic conductivity: 6 × 10⁻⁵ m/s, and uniformity coefficient ≥ 5). In other cases, where the M2B mixture is embedded in the frost depth or used as the upper layer of backfill for bridge abutments and retaining structures, it is necessary to stabilize the mixture with an additional binder (cement or lime) before use. A similar situation applies to the use of WMW-SSA mixtures in the upper layers of the road embankment at frost depth.

The most practical and immediate result of the circular economy is the use of M1A and M1B mixtures as backfill for installation trenches, which directly supports the operational needs of water and sewage utilities. This use will successfully replace natural sand, reducing waste volume and disposal costs.

WMW-SSA mixtures require further research and development (R&D), primarily focused on industrial research (TRL 5–6). Subsequent stages of mixture development should include laboratory testing using cement or lime additions, as well as the construction of test benches in simulated and relevant environments. In the context of sustainable development, defining the technical and economic characteristics of recovered resources and estimating the impact of their recycling on carbon footprint reduction are also crucial. These activities constitute key steps in the process of certifying wastes as valuable construction materials. They can also initiate work to establish standards that promote the principles of sustainable management of resources generated in wastewater treatment plants.