Optimizing the Mechanical Properties of Concrete Through Partial Replacement of Fine Aggregates with Wastewater Sludge

doi:10.21203/rs.3.rs-5225368/v1

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Optimizing the Mechanical Properties of Concrete Through Partial Replacement of Fine Aggregates with Wastewater Sludge

https://doi.org/10.21203/rs.3.rs-5225368/v1

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This study investigated the potential of using wastewater sludge (WWS) as a partial replacement for fine aggregates in concrete to optimize its mechanical properties, while mitigating environmental impacts. Wastewater sludge from three wastewater treatment plants in Polokwane, South Africa was used to replace sand at: 0, 5, 10, 15, and 20% by weight. The leaching of heavy metals, including calcium, silicon, aluminium, iron, and phosphorus, was evaluated by using a toxicity characteristic leaching procedure (TCLP) on samples submerged in iodine water for 28, 90, and 140 days. Scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), and energy-dispersive X-ray analysis (EDX) were employed to characterize the organic compositions of the sludge and sludge-based concrete. The results demonstrate that the incorporation of wastewater sludge significantly, reduced leachable heavy metals, with concentrations remaining within acceptable limits. Energy dispersive X-ray analysis revealed a substantial decrease in the metal content of the sludge-based concrete when compared with that of the original wastewater sludge. The surface morphology of the sludge-based concrete exhibited heterogeneous, crystalline, and rocky features, in contrast to the spongy and porous morphology of the sludge. Compressive strength tests showed that by replacing up to 5% of sand with wastewater sludge, maintained the required compressive strength of 25 MPa after 90 days of curing. These findings suggest that the partial replacement of fine aggregates with wastewater sludge in concrete can mitigate environmental pollution, while potentially optimizing the mechanical properties of the resulting material, thereby contributing to sustainable construction practices.

Wastewater sludge

Fine aggregates

Concrete

Heavy metal leaching

Mechanical properties

Environmental impact

Sustainable construction

The increasing demand for sustainable construction practices has generated significant interest in the utilization of industrial and municipal waste byproducts as building materials. One such byproduct, WWS, demonstrates considerable potential as a partial replacement for fine aggregates in concrete production. This approach addresses two critical challenges, viz: mitigating the environmental hazards associated with waste disposal and optimizing the mechanical properties of concrete. The incorporation of WWS into concrete formulations is consistent with the industry's objective of minimizing the environmental impact, while addressing the rising demand for high-performance, durable, and environmentally friendly construction materials. Research has highlighted the potential benefits of incorporating industrial waste, such as WWS into concrete. Odimegwu et al. (2021) emphasized the improvements in compressive strength and durability achieved by partially, replacing fine aggregates with alum sludge. Feitosa and Ferreira (2023) emphasized the positive effects of WWS on the compressive strength and water absorption properties of concrete. These studies illustrate the utility of WWS as a replacement for fine aggregates, thereby improving the mechanical performance without compromising the concrete integrity. In addition to its mechanical advantages, WWS offers a sustainable solution to the environmental challenges associated with waste management. Studies by Verde et al. (2022) and Rabie, El-Halim, and Rozaik (2019) have shown that WWS can optimize concrete properties, while reducing the environmental footprint of the construction industry. These results suggest that WWS, when appropriately treated and incorporated into concrete mixes, can alleviate waste disposal problems and reduce the dependence on natural resources, e.g., sand.

However, challenges remain in the optimization of the mechanical properties of concrete through sludge incorporation. The workability, cohesiveness, and compressive strength of the sludge-based concrete, must be carefully calibrated to meet the recommended performance standards. Dixit and Srivastava (2023) and Ghannam, Najm, and Vasconez (2016) reported that treated WWS can enhance the strength and durability of concrete, while maintaining its workability, thereby further supporting the feasibility of this approach. This study aims to build on existing research and investigate the effects of different sludge contents on the mechanical properties of concrete, with a particular focus on the compressive strength, durability, and water absorption. By studying different replacement ratios of fine aggregates by WWS, this study aims to optimize the balance between the mechanical performance and sustainability. The study contributes to the growing body of knowledge on sustainable concrete practices and provides insights into the potential for the large-scale application of WWS in the construction industry. The overarching objective of this research is to demonstrate that WWS, when utilized as a partial replacement for fine aggregates, can enhance the mechanical properties of concrete, while offering a sustainable alternative to traditional construction materials.

This study focused on three wastewater treatment plants (WWTPs), namely Mankweng, Polokwane, and Seshego, all in the Polokwane area, Limpopo Province, South Africa. According to the latest statistics in South Africa (STATS SA), Polokwane has a population of approximately 479,000, with 76% of households connected to water-based wastewater systems (Statistics South Africa 2016). The criteria for the selection of the wastewater treatment plants, were based on their design capacity of more than 5 mℓ per day, which is regulated in the Water Service Act 108 of 1997.

3.1 Materials

The materials used in this study, include potable tap water that complies with the South African National Standards (SANS) 241-1:2015, as well as Lafarge Builtcrete CEM II/BM (V-L) 42.5 N cement, which meets the specifications established in SANS 50197-1. Cement was sourced from the Builders Warehouse in Polokwane. Aggregates consisting of both 19-mm coarse and fine aggregates were supplied by the Rooiberg Stone Plant in Naboomspruit and Alpha Sand. The handling of WWS, which is classified as extremely hazardous, was conducted under stringent safety protocols, including the avoidance of direct contact with humans and inhalation. The personnel were equipped with respiratory dust masks. The collection and management of sludge adhered to the standards outlined in the SANS 5667-13:2007. Sludge was collected from the drying beds in the Mankweng, Polokwane, and Seshego wastewater treatment plants. The sludge conditions were evaluated prior to crushing. The sludge was oven-dried for 24 hours and spread on a clean, dry, non-absorbent canvas sheet before being crushed into a finer material. The sludge was crushed with a 50 mm steel hand tamper and sieved through a 5 mm sieve, as shown in Fig. 1. SANS 3001-AG1:2014 regarded aggregates passing through a 5 mm sieve, as fine aggregates. Foreign particles in the dry sludge were hand-picked. The crushed material was stored in a non-permeable plastic bag in a dry, cool environment to avoid moisture absorption.

3.2 Methods

Figure 2 presents a flowchart illustrating the concrete mix design process by using WWS as a partial replacement for sand aggregate. The diagram is divided into three main sections: Aggregates, Trial Mix, and Modified Mixture Design. In the Aggregates section, the specifications for coarse and fine aggregates comply with SANS 5845 and SANS 3001-AG10, respectively. The cement used met the standards of SANS 50197-1:2013 and the water quality met the requirements of SANS 241. The focus was on replacing sand with WWS at concentrations of 5, 10, 15, and 20%. These substitutions were experimentally evaluated to assess their performance and environmental impact. The Sample Mix section details the various mix designs tested, including a control sample without WWS and four additional mixes with sand replaced with 5%, 10%, 15%, and 20% WWS. Each experiment included a series of calculated and measured parameters, such as the water-cement ratio (SANS 10164-1:2006), cement content (SANS 50197-1:2013), and aggregate-cement ratio (SANS 1083). Important mechanical properties, such as the air content, elastic modulus, and energy absorption, are important and desirable characteristics of concretes. The WWS was characterized by using the SEM, EDX, and XRD to evaluate its composition and compatibility as a partial replacement for sand, which is crucial for understanding its impact on the final concrete mix.

The modified mix design process further incorporated WWS as a partial replacement for the fine aggregates. This section emphasizes the adjustments made to the standard mix design to accommodate the unique properties of the sludge. It evaluates the mechanical properties of the resulting concrete, including the compressive, flexural, and tensile strengths, as well as the durability and workability, assessed through slump tests. Chemical element and heavy metal leaching testing followed the SANS 11377 and EPA Method 1311 (TCLP) to ensure that the inclusion of WWS did not pose environmental risks associated with heavy metal contamination. Supported by the existing research, Hybská et al. (2017), Ansari et al. (2023) Yahyaei, Asadollahfardi, and Salehi (2020), reported that mixed interior maintenance is crucial for maintaining the optimal mechanical properties, while promoting environmental sustainability. Figure 2 summarizes a systematic, research-based approach for the development of a sustainable concrete mix by using WWS without compromising the resulting concrete’s structural integrity.

3.2.1 Comprehensive Methodology for Modified Concrete Mix Design Incorporating Wastewater Sludge as a Partial Sand Replacement

The flowchart in Fig. 2 presents a systematic methodology for incorporating WWS as a partial replacement for sand in concrete production. This process highlights a comprehensive approach that integrates material characterization, precise mix design, performance evaluation, and environmental impact assessments. It adheres to established standards to ensure that the mixture maintains, both its mechanical integrity and environmental sustainability.

3.2.1.1 Material Characterization and Standards Compliance

The coarse and fine aggregates are evaluated in strict accordance with the SANS 5845:2006 and SANS 3001:AG10 standards, respectively. Coarse aggregates are characterized by using parameters, such as particle size distribution, crushing value index, bulk density, and particle shape and texture to ensure that the mechanical stability of the mixture is not compromised. At the same time, fine aggregates are tested for grain size, bulk density, water absorption, and organic impurities, which are crucial factors that affect the workability and durability of the final mix. WWS is thoroughly, characterized by using advanced techniques, such as SEM, EDX, and XRD to analyze its morphology and chemical composition. This step is crucial for assessing the feasibility of using WWS as a partial sand replacement. In addition, cement (42.5N) was tested according to SANS 50197-1:2013 to ensure that it meets the required performance standards for strength, while the water used, complied with SANS 241, ensuring its quality in the mix.

3.2.1.2 Modified Mix Design and Optimization

This methodology places great emphasis on the calculation of the combined apparent relative density (RDT), which considers the relative densities of sand and WWS. This ensures that the material proportions are precisely, coordinated to achieve an optimal mix. Trial mixtures were carried out with 5%, 10%, 15% and 20% by weight of WWS replacement proportions. The cement mass (Mcf = Mc + δ) and water mass (Mwf = Mw + β) adjustments were calculated to maintain workability and achieve the desired mechanical properties. These adjustments followed the established guidelines of the Fulton's Concrete Technology (Addis 2009) and ensured a systematic material selection, dosage, and evaluation. Furthermore, Asadollahfardi and Mahdavi (2018) reported on the iterative process, which is required to maintain the workability and strength when developing sustainable concrete formulations. This iterative process, as shown in the flowchart, ensures that each component is calibrated to meet the required performance and durability standards, while considering the environmental impact through the leachate testing and the use of recycled materials.

Mcf

Adjusted (final) mass of cement.

Initial mass of cement.

Adjustment factor for cement mass as a percentage of the initial mass.

Mwf

Adjusted (final) water mass.

Initial water mass.

Adjustment factor for water mass as a percentage of the initial mass.

3.2.1.3 Performance and Structural Evaluation

The modified concrete mix went through rigorous testing, including slump tests, to evaluate its workability. Key structural parameters, such as compressive strength, density, porosity, and water absorption were measured to assess the physical and structural properties of the mix. Advanced characterization techniques, including SEM, EDX, and XRD, were employed to examine the microstructure and chemical composition, hence, ensuring that the incorporation of WWS did not compromise the concrete's performance. The curing process, which spanned over: 7, 14, 28, and 90 days, further guaranteed that the mix developed the required strength over time in accordance with established curing protocols.

3.2.1.4 Environmental Impact Assessment

The methodology placed significant emphasis on sustainability, with the leachate testing, conducted in accordance with SANS 11377:2009 and the Toxicity Characteristic Leaching Procedure (TCLP). This ensured that the concrete mix did not release harmful contaminants into the environment. The leachate testing procedures, combined with ICP-EPA Method 1311, assessed the potential environmental risks, associated with the use of WWS as a partial sand replacement, thereby ensuring that the concrete met the environmental regulatory standards. This comprehensive methodology provides a robust framework for incorporating WWS as a partial replacement for fine aggregates in concrete. This approach, includes a careful material characterization, optimized mix design, and performance evaluation, which aligned with the findings of Lamastra, Suciu, and Trevisan (2018) and Maddikeari et al. (2024). These studies highlighted the necessity of material characterization and adjustments to mixture components when introducing alternative materials, such as WWS, as well as the need to account for the distinct physical and chemical properties of WWS. This was reflected in the trial mix adjustments of the cement and water contents in this study.

The methodology illustrated in the accompanying flowchart successfully, integrated WWS as a sustainable alternative to traditional fine aggregates, thereby enabling the development of environmentally friendly concrete without scompromising the structural integrity.

3.2.2 Concrete Mix Design Procedure

This section describes the methodology employed to determine the optimal proportions of the concrete mix, considering the incorporation of the WWS.

3.2.2.1 Water and Cement Content Determination

The water-cement ratio (w/c) is a crucial factor that affects the workability, strength, and durability of concrete. For this study, a W/C ratio of 0.55 was chosen to achieve a balance between these properties, with the aim of achieving a slump of more than 75 mm to ensure adequate workability, as recommended by the Fulton's Concrete Technology (Addis 2009). This specific ratio makes it easy to maintain the desired workability during construction, while achieving the desired compressive strength and durability. To maintain this balance when incorporating different proportions of WWS, adjustments were made to the amounts of water and cement.

3.2.2.2 Coarse Aggregate Content

Accurate determination of coarse aggregate content is essential because it has a direct influence on the mechanical properties and durability of concrete. The coarse aggregate was characterized for its particle shape, nominal size, relative density, and compacted bulk density. These parameters were used to calculate the precise volume and the mass of the coarse aggregate required per cubic meter of concrete, thereby ensuring a uniform mix construction and even distribution within the concrete matrix.

3.2.2.3 Fine Aggregate Content

Fine aggregates, which are typically composed of sand, play a crucial role in determining the strength and stability of the concrete. In this study, both the quality and quantity of the fine aggregates employed, were carefully controlled. The relative and bulk densities of the sand were measured to determine the corresponding volume and mass per cubic metre of concrete. As the WWS partially replaced the fine aggregates, these measurements were essential for adjusting the mix design to maintain the desired strength and workability. The different sludge proportions required a precise adjustment of the sand content to achieve the desired properties in the final concrete mix.

3.2.2.4 Concrete Mix Proportions with Varying Dry Sludge Contents

This study investigated the effects of the partial replacement of fine aggregates with WWS at proportions of 5, 10, 15, and 20%. To accurately determine the required quantity of the fine aggregates, while maintaining the desired workability and strength of the concrete mix, sand and sludge were treated as a single combination of fine aggregates. Eq. 1 was used to calculate the total fine aggregate content, considering the relative densities of both materials.

RD_T = (P_s * RD_s) + (P_sl * RD_sl) (1)

where RD_T is the common relative density of the fine aggregate (sand and sludge), P_s and P_sl are the percentages of sand and sludge, respectively, in the fine aggregates and RD_s and RD_sl are the relative densities of sand and sludge, respectively, in the fine aggregate, which is consistent with the existing research that highlights the importance of adjusting the mix ratios when incorporating alternative materials, e.g., sludge. Franus, Barnat-Hunek, and Wdowin (2015) and Ramirez et al. (2017) highlighted the influence of sludge on aggregate properties, including density, porosity, and water absorption, highlighting the need for careful dosing. Similarly, Rabie, El-Halim, and Rozaik (2019) and Yao et al. (2022) reported on the influence of WWS on the compressive strength and workability of concrete, and highlighted the need for precise proportions of sand and sludge, based on their relative densities. Therefore, Eq. 1 allows a systematic approach to incorporate different sludge contents without compromising the concrete performance.

3.2.2.5 Modification of Sludge-Based Concrete Mix Design

The integration of WWS into concrete mixes, requires certain modifications to accommodate the unique properties of the material. In particular, the spongy nature of WWS increases the water requirement of the mixture. To maintain the desired workability and slump, while ensuring sufficient strength, both the water and cement content were adjusted. This process involved a series of test mixes to determine the optimal amounts of water and cement, based on the specific WWS content. The final mix design, incorporating these adjustments, is represented by Eq. 2 and Table 1, which calculates the required fine aggregate (sand and sludge) for one cubic meter of concrete:

M_s = RD_T * 1000 * \(\:\left(1-\:\frac{{\text{M}}_{\text{w}}}{{\text{D}}_{\text{w}}\:\text{*}\:1000}-\:\frac{{\text{M}}_{\text{c}}}{{\text{D}}_{\text{c}}\:\text{*}\:1000}-\:\frac{{\text{M}}_{\text{a}}}{{\text{D}}_{\text{a}}\:\text{*}\:1000}\right)\) (2)

where M_s, M_a, and M_c are the mass of the fine aggregate, coarse aggregate, and water per cubic meter in kg, respectively, and RD_T, D_a, and D_w are the relative densities, in kg/m³, of the fine aggregate, coarse aggregate, and water, respectively.

This iterative approach, which adjusts the water and cement contents, relative to the WWS inclusion, is consistent with the existing research on the incorporation of alternative materials into concrete. Wang et al. (2022) and Natarajan and Murugesan (2019) emphasized the importance of striking a balance between workability, strength, and sustainability when using materials, such as WWS or marble powder. Klus et al. (2019) and Ghannam, Najm, and Vasconez (2016) emphasized the need to modify the water and cement contents, to meet the slump requirements when incorporating sludge water. Tang et al. (2016) and Qasrawi (2016) highlighted the importance of the systematic adjustments in maintaining the workability and strength of alternative aggregates. This study builds on these findings and demonstrates a practical method for optimizing sludge-based concrete mixtures. Table 1 lists the concrete mix designs used to evaluate the effects of incorporating WWS as a partial replacement of sand. The study covered three different sites, viz: Mankweng, Polokwane, and Seshego, each containing a control mixture, and two modified mixtures (C1 and C2), characterized by the different proportions of WWS. A uniform control mix, across all the locations served as the reference point, for comparison purposes. It comprises of 1042 kg/m³ coarse aggregates, 364 kg/m³ cement, 751 kg/m³ sand, and 200 liters of water, excluding WWS. Modified mixtures C1 and C2 were formulated to evaluate the effects of replacing 5 wt % and 10 wt% sand, respectively, with WWS. To maintain the workability and strength during WWS incorporation, both the cement and water contents in the modified mixes were adjusted. For example, in the Mankweng site, C1a and C2a mixtures contained 35 kg/m³ and 67 kg/m³ WWS, respectively. These modifications resulted in a reduction in the sand content, whereas the cement content increased to 375 kg/m³ for C1a and 382 kg/m³ for C2a. The water content was adjusted to 210 liters for C1a and 218 liters for C2a. This trend of increasing cement and water contents with greater WWS inclusion, was consistently observed in the Polokwane and Seshego mixtures.

Table 1

Concrete mix proportions with varying WWS contents
Sample	Coarse Aggregates (kg/m³)	Cement (kg/m³)	Sand (kg/m³)	Dry sludge (kg/m³)	Water (l/m³)
Mankweng: Sample composition for one cubic of concrete
Control	1042	364	751	0	200
C1_a	1042	375	708	35	210
C2_a	1042	382	666	67	218
Polokwane: Sample composition for one cubic of concrete
Control	1042	364	751	0	200
C1_b	1042	375	731	37	200
C2_b	1042	382	694	69	205
Seshego: Sample composition for one cubic of concrete
Control	1042	364	751	0	200
C1_c	1042	375	674	34	222
C2_c	1042	382	605	60	240

C1 and C2 represent 5.0, and 10.0%, respectively, and partial sand replacement with sludge. The 15, and 20% partial replacements with sludge collapsed. The subscripts a, b, and c represent the sludges from Mankweng, Polokwane, and Seshego WWTPs, respectively.

3.2.2.6 Mixing of concrete aggregates

The concrete mixing process followed a systematic and methodical protocol to ensure uniformity and minimize material losses. Firstly, the dry ingredients, sludge, sand, and cement were thoroughly mixed until a uniform colour, indicated sufficient mixing. Coarse aggregates were then, incorporated and mixed until they were evenly distributed throughout the mixture. Water was then, added gradually and mixed until the desired texture and colour homogeneity were achieved. The concrete is considered suitable for casting, only after it reaches a satisfactory level of cohesiveness, workability, and slump. Following the casting process, the concrete cubes were compacted by using a vibrating table, following the standards specified in the SANS 5861-3:2006. To facilitate a proper curing, the labelled cubes were positioned on a non-vibrating surface in a temperature-controlled environment and covered with polyethylene bags to minimize moisture loss. The samples were cured for approximately, 24 hours to ensure adequate hydration of the cubes. For the curing process, they were immersed in drinking water and crushed after 7, 14, 28, and 90 days. The temperature of the water bath in which the concrete cubes were cured was maintained between 23 and 25°C. Firstly, the cubes experienced a 24-hour curing period at room temperature. The samples were then immersed in a temperature-controlled water bath for the remainder of the curing period. This comprehensive approach ensured the optimal development of the concrete properties.

3.2.3 Water Absorption Test

The water absorption test was performed by drying the six concrete samples at 100°C for 24 hours, followed by cooling for 5 hours. The mass of each dried sample was then, measured after cooling. The samples were then, immersed in water at 20°C for 24 hours. After the immersion, excess water was removed, and the mass was measured again to calculate the average water absorption. This procedure followed the standard methods for evaluating water absorption in concrete mixes, particularly those containing WWS as a partial replacement for fine aggregates.

Studies by Ramirez et al. (2017) and Jamshidi et al. (2012) have provided valuable insights into the influence of sludge on water absorption properties. Both studies suggested that a high sludge content, resulted in an increased absorption rate, which can negatively, affect the concrete strength. This highlighted the need for an accurate assessment through water absorption testing. Furthermore, (Suchorab et al. 2019) and (Klus et al. 2017) emphasized on the importance of evaluating moisture retention and durability in modified concrete formulations. Taken together, these studies highlighted the need for systematic water absorption testing of concretes, containing WWS to ensure that the performance standards for moisture management and durability are met.

3.2.4 Scanning electron microscope (SEM)

Small fragments of pulverized WWS, collected from a depth of 2–4 mm was prepared for morphological analysis by using the field-emission gun scanning electron microscopy (FEGSEM). A small sample of the sludge was then, attached to an aluminium stub with a double-sided carbon tape and coated with a thin layer of carbon by using a Quorum Q150T ES sputter coater to improve the conductivity under the electron beam. The morphologies of the sludge samples prepared, were examined by using a Zeiss 540 Ultra FEGSEM instrument at an accelerated voltage of 2–3 kV.

3.2.5 Energy dispersive X-ray (EDX)

The chemical composition of the WWS was further analyzed by using the energy-dispersive X-ray spectroscopy (EDX) technique. An Oxford EDX detector, integrated with a Zeiss 540 Ultra-Field Emission Scanning Electron Microscope (FEGSEM) was used for this analysis with an accelerating voltage of 15 kV.

3.2.6 X-ray Diffraction (XRD) analysis

The crystalline phases, present in the WWS, were analyzed by using the wide-angle X-ray diffraction. Small fragments of the sludge collected at a depth of between 2–4 mm, were oven-dried at 40°C for 48 h. The WWS samples were then, carbon-coated to minimize the charging effects. The XRD patterns were acquired by using a PANalytical X'Pert Pro diffractometer with CuK_α radiation (wavelength of 0.154 nm) at a voltage of 45 kV and a current of 40 mA.

3.2.7 Leaching of Heavy Metals from Sludge-Based Concrete

To evaluate the environmental safety of the sludge-based concrete, a comprehensive leaching study was conducted to assess the potential release of heavy metals. Concrete samples were prepared by using different proportions of WWS as a partial replacement for sand and then cured in iodine water. These samples were subjected to leaching tests at 28, 90, and 140 days intervals.

The Toxicity Characteristic Leaching Procedure (TCLP) was used to determine the concentrations of leached heavy metals. This widely accepted method, based on studies by Kumar, Gandhimathi, and Baskar (2016), and Marinoni et al. (2008), simulated the landfill conditions to assess the long-term leaching potential of the waste materials. Given the high acid neutralization capacity of the Mankweng, Polokwane, and Seshego WWS, the TCLP Extraction Fluid 2 was used, as recommended by Herselman and Moodley (2009).

According to the TCLP protocol, 100 g of dried sludge sieved to a particle size of less than 9.5 mm, was placed in an extraction vessel containing two liters of the TCLP solution. The sealed vessel was stirred at 30 rpm for between 18–20 h, at a controlled temperature of 23°C. After the extraction period was completed, the leachate was carefully filtered to analyze the heavy metal concentrations. This rigorous leaching assessment was based on an extensive research on the leaching behavior of heavy metals from concrete containing alternative materials. Shirazi and Marandi (2012) conducted a comparative study of heavy metal leaching from concrete and municipal WWS and provided valuable insights that are relevant to this study. Furthermore, Chen et al. (2023) investigated the chemical interactions that influence the leaching behavior, with particular emphasis on the role of chelating agents, e.g., ethylenediaminetetraacetic acid (EDTA), in leaching toxicity. Esmaeili and Heidarzadeh (2022) highlighted the critical importance of environmental safety assessments for concrete formulations by using alternative materials and demonstrated that the leaching levels in their study, remained within the acceptable limits set by the Environmental Protection Agency. The results of this leaching study, along with the findings from previous research, provide a comprehensive understanding of the potential environmental risks, associated with the use of sludge-based concrete. This information is essential for promoting sustainable and environmentally friendly use of this alternative building material.

4.1 Physical properties of wastewater sludges from various WWTPs

The physical characteristics of the sludges from the Mankweng, Polokwane, and Seshego WWTPs, are summarized in Table 2. To ascertain these properties, the SANS 5844:2014 and SANS 5845:2006 and visual analyses, were performed. The dust content compacted bulk density, relative density, moisture content, and pH were also determined. The Mankweng sludge dust level was determined 1.6%, which is within the allowable limit, whereas the Seshego and Polokwane sludges were above the 5% limit and had dust percentages of 6.8 and 10.9%, respectively. The high dust content can be reduced by ensuring that the crushing of the dry sludge is not excessive. The WWS can be used to partially, substitute the concrete components, e.g., sand, owing to the nature of the material (Mathye 2020). Table 2 shows the sludge samples with pH values, ranging from 5.9 and 6.2. According to the National Water Act 36 of 1998 (National Water Act 36 1998), acceptable sludge pH values, should range from 5.5 and 9.5. The Polokwane WWS had a pH of 5.9, which is still within the acceptable standards, despite being the lowest. This is attributed to the type of influent that may originate from the wet industries, such as the breweries and the agro-processing. Therefore, a brief analysis of WWS, should be conducted before it is incorporated into a concrete mixture.

Table 2

Physical properties of wastewater sludge from the Mankweng, Polokwane, and Seshego WWTPs in Limpopo, South Africa
WWTP	Dust Content (%)	Moisture Content (%)	Relative Density (kg/m³)	Bulk Density (kg/m³)	pH Value
Mankweng	1.6	4.4	1.39	432	6.2
Polokwane	10.9	5.2	1.23	316	5.9
Seshego	6.8	7.7	1.28	563	6.0

4.1.1 Dust content

The dust content of the sludge samples varied significantly, with the lowest being 1.6% from the Mankweng site and the highest being 10.9% from the Polokwane site. In the fine aggregate standards, such as the SANS 1083, which governs the quality of aggregates used in concrete, excessive dust content is generally, undesirable because it can reduce the workability and increase water demand in the mix. While the low dust content from the Mankweng site is closer to what is acceptable for fine aggregates, the higher dust content from the Polokwane site, may necessitate additional treatment or washing to reduce the number of fines aggregates before use. Excessive dust can lead to weak bonding in a concrete matrix and can reduce its overall strength.

4.1.2 Moisture Content and Bulk Density

The moisture content plays a crucial role in the incorporation of WWS into a concrete mix. The sludge samples had moisture content ranging from 4.4% (Mankweng) to 7.7% (Seshego). According to the fine aggregate standards, such as SANS 1200 G, the moisture content must be controlled to ensure consistent water-to-cement ratios and maintain the workability during mixing. The relatively low moisture content of the sludge samples is beneficial because it reduces the need for extensive drying before use. However, the moisture content should be accounted for, during the mix design to avoid an over-saturation, which can compromise the strength and durability of the concrete.

4.1.3 Relative Density and pH Value

The Mankweng sludge has a relative density of 1.39 kg/m³ and a pH of 6.2, indicating a high density and low chemical reactivity, therefore, making it suitable for durable and stable structural applications. The Polokwane sludge, with a relative density of 1.23 kg/m³ and a slightly acidic pH of 5.9, may require adjustments to the construction mixes to ensure the material integrity and prevent degradation. The Seshego sludge, with a relative density of 1.28 kg/m³ and a pH of 6.0, provides a balanced profile for robust applications, where slightly acidic conditions can be managed.

4.2 Particle size distribution of aggregates

4.2.1 Sieve analysis of fine aggregates (sand)

Table 2 presents the sieve analysis results for the fine aggregates, conducted according to the SANS 1083:2014 procedure. The grading curve, including the upper and lower permissible limits, demonstrates the fact that the fine aggregate conforms to all standard requirements, as outlined by Grieve (2009). Notably, 15.73% and 4.86% of the fine aggregates, passed through the 0.3 mm and 0.075 mm sieves, respectively. Table 2 further details the physical and chemical properties of the fine aggregate, including the dust content of 4.86%.

4.2.2 Sieve analysis of coarse aggregates

In the current study, the relative density was 2.7 and the compacted bulk density was 1545 kg/m³. About 0.14% of the coarse aggregates, passed through a 0.075 mm sieve. This is attributed to the low dust or clay content of the coarse aggregates, which led to low water requirements in the concrete mix. The coarse aggregate particle size was found to be within allowable limits.

4.2.3 Sieve analysis of wastewater sludge Mankweng, Polokwane and Seshego

The Sieve analysis of the WWS from the Mankweng, Polokwane, and Seshego sites, complied with the SANS 1083:2014 and SANS 3001:AG1 standards. This analysis indicated a diverse range of particulate constituents that are suitable for use in concrete as partial replacements for sand. The consistent adherence to the specified minimum and maximum gradation limits of the WWS samples, indicated their potentials as viable alternatives to the conventional sand and therefore, offers advantageous properties, such as favourable packing density and mechanical stability, which are crucial for construction projects. The gradation variations, reflect the diversity of the WWS sources and illustrate how regional differences, influence the aggregate properties, without affecting the overall quality of the material. These well-graded aggregates provide excellent load distribution and mechanical interlocking, which are essential for applications in load-bearing structures, foundations, road bases, and concrete mixtures. Although regions, such as Mankweng and Seshego, tend to produce coarser aggregates, this characteristic can improve the strength and durability of the concrete. With appropriate adjustments to the mix designs to accommodate the increased void content, while maintaining workability, the adaptability of these materials, suggests that the variability of WWS sources, provides a range of options for tailored applications and allows for the use of different material properties for specific construction requirements.

The use of WWS as an alternative to natural sand, is consistent with sustainable construction practices and this mitigates the environmental impacts, associated with the traditional sand extraction and transportation. This approach contributes significantly, to the environmental sustainability desires, by promoting the efficient use of locally available waste materials. The positive results of the sieve analysis, show that these materials can meet the stringent requirements of the modern construction applications and can represent an environmentally and economically sustainable option for the construction industry. This systematic use of sludge-derived aggregates, is supported by recent studies, for example by Areias et al. (2020), which emphasizes the particle size compatibility of sludge with natural sand, and by Smoczyński et al. (2019), which provides insights into the particle size distribution and treatment efficiency of different types of sludge, hence, highlighting the practical applications and environmental benefits of incorporating sludge into concrete formulations.

4.3 Scanning Electron Microscope of Wastewater Sludge

Figures 4, 5, and 6 show a series of scanning electron microscope images from Mankweng, Polokwane, and Seshego, respectively, illustrating the diverse morphologies of wastewater sludge (WWS) at different magnifications (100 kX, 50 kX, and 25 kX). These images provide insight into the intricate microstructure of WWS, and they reveal the key features that influence its potential application in the construction sector. At the highest magnification of 100 kX, the SEM images revealed a remarkable aspect of the WWS, i.e., a highly porous, spongy, and nanostructured surface as shown in Fig. 4a. This porous morphology significantly increases, the surface area of the material, which is crucial for the improvement of its pozzolanic reactivity. When finely ground, in the presence of moisture, the pozzolanic materials react with calcium hydroxide to form cementitious compounds. As shown in Fig. 5a, the increased surface area observed in the 100 kX images, allows for a stronger interaction between the WWS particles and the cement components, thereby resulting in a more robust pozzolanic response. This increased reactivity is critical for improving the strength and durability of the concrete. The results shown in Fig. 6a are consistent with those of Rusănescu, Rusănescu, and Constantin (2022) and Falk et al. (2020), who highlighted the significant influence of wastewater treatment processes, on the properties of the resulting materials and emphasized the importance of surface properties for pozzolanic reactions.

At the medium (50 kX) and low (25 kX) magnifications, the SEM images show a heterogeneous range of particle shapes and sizes within the WWS samples. This heterogeneity highlights the variability in the composition and the thermal processing conditions, inherent to the different wastewater treatment methods, as observed by Arvelakis and Frandsen (2005). This variability presents challenges and opportunities for WWSs to be used in construction projects. The heterogeneous nature of WWS, makes a consistent characterization and standardization difficult.

The prediction of the performance of WWS in construction applications, requires a thorough understanding of its particle-size distribution, morphology, and chemical composition. Detailed insights into the particle size distribution and surface area, provided by the SEM analysis, are crucial for the optimization of the use of WWS in construction. (Zhao et al. 2020) advocated for the advancements in materials science, to fully utilize WWS in construction environments, thereby emphasizing the importance of the understanding and the manipulation of these morphological features. Jamshidi et al. (2012) further substantiated this observation, highlighting how specific structural features, such as porosity and particle size distribution, positively influence material performance in construction, by enhancing the material’s pozzolanic activity. The SEM analysis of WWS, underscores the significant impact of its complex morphology on its potential applications in civil engineering. By understanding the relationship between microstructure and macro-performance, researchers and engineers can optimize the use of WWS in concrete and asphalt mixtures. Further research is however, required to develop standardized methods for characterizing and processing WWS to ensure consistent and reliable performance in construction applications.

4.4 Sludge-based concrete SEM Analysis

Figures 7 and 8 show the SEM images of the microstructural properties of concrete samples, modified by replacing fine aggregates with 5 and 10% weight fractions of WWS from the Mankweng, Seshego, and Polokwane sites, respectively. These images provide crucial insights into the morphology of the concrete mixes and linkages of the microstructural properties to the potential improvements in the mechanical performance. The microstructural observations at the 5% sludge replacement (Fig. 7a-c) by using the SEM image of the sludge from Mankweng (Fig. 7a), revealed an irregular texture, indicative of a non-uniform distribution of sludge particles within the cement matrix. This irregularity may compromise the integrity of the material by creating localized stress concentrations (Yang et al. 2019). In contrast, the Seshego sample (Fig. 7b), exhibited a finer and more uniform texture, hence, suggesting an enhanced integration between the sludge and cement, attributable to the superior mixing techniques or a more reactive sludge type. The Polokwane sample (Fig. 7c) shows a layered, fibrous structure, which despite its distinct appearance, may predispose the concrete to structural weaknesses and premature failure under mechanical stress (Franus, Barnat-Hunek, and Wdowin 2015). The microstructural observations at the 10% sludge replacement (Fig. 8a-c), show that by increasing the sludge concentration to 10% in the Mankweng sample (Fig. 8a), resulted in a more pronounced particle aggregation, potentially reducing the mix workability and mechanical strength of the cured concrete, owing to the formation of weak points within the matrix (Amin et al. 2022). In contrast, the Seshego sample (Fig. 8b), maintained a smooth texture, even with increased sludge content and exhibited visibly larger particle shapes, which can improve the ductility and load-bearing capacity of the concrete. In contrast, the increased sludge content in the Polokwane sample (Fig. 8c), increased the fibrous and layered textures, which increased the risk of delamination, and therefore, compromised the durability and strength of the material (Eshtiaghi et al. 2013). Comparative analysis and the implications for sustainable construction, are where the analysis elucidates the impact of increasing the sludge content on concrete microstructure, hence, emphasizing the significance of particle distribution and integration within the cement matrix. The consistent textural integrity, observed in the Seshego samples, suggests that the sludge sourced from this region, is more conducive to integration into concrete, therefore, enhancing the mechanical properties of the material, including the tensile strength and elasticity. The incorporation of WWS into concrete, represents not only a step towards sustainable construction, but also a complex interplay of material properties and engineering performance. The findings from the sludges from Mankweng, Seshego, and Polokwane, underscore the necessity for a meticulous selection and processing of the sludge types to optimize the structural and mechanical characteristics of concrete. Future research should concentrate on the establishment of detailed guidelines for sludge utilization in concrete to ensure a consistent quality and performance in sustainable construction applications, thereby enhancing both environmental benefits and material durability (Zari et al. 2023; Ni et al. 2015).

4.5 Energy dispersive X-ray (EDX) for wastewater sludge

The EDX analysis results for the Mankweng, Polokwane, and Seshego WWSs, show the different elemental compositions, which is crucial for the understanding of how variations in the elemental content, can affect the mechanical properties of concrete when these sludges are used as partial replacements for fine aggregates. The EDX analysis of the Mankweng WWS, is shown in Fig. 9. This figure provides substantial evidence for its potential utilization in concrete formulations, demonstrating significant levels of oxygen (47.9%) and silicon (16.4%), along with the noteworthy quantities of aluminium and calcium. These elements indicate the capacity of sludge to enhance the mechanical properties of concrete, owing to its pozzolanic activity. Specifically, the silicon present in the sludge, can react with the calcium hydroxide released during the cement hydration, to form additional calcium silicate hydrates (C-S-H), thereby improving the strength and durability of concrete. The presence of aluminium can also promote the formation of ettringite, a compound that can enhance the overall structural integrity of concrete. Consequently, the Mankweng sludge is positioned as a valuable supplementary cementitious material, thereby providing a sustainable approach for the incorporation of industrial waste into construction materials, while potentially, increasing the durability and mechanical strength of concrete structures (Lamastra et al., 2018). Figure 10 shows that the analysis of the Polokwane WWS, reveals a unique elemental composition beneficial for concrete applications, particularly its high silicon content (23.7%), hence suggesting the fact that an enhanced pozzolanic activity, is crucial for the formation of calcium silicate hydrates that strengthen and improve the durability of concrete (Zhang et al., 2020). Although sludge shows little calcium levels and contains trace elements, such as nickel and sodium, which could affect the hydration process and concrete stability, its rich aluminium content, also supports additional cementitious reactions, potentially boosting the structural integrity of concrete. Therefore, although the Polokwane sludge has promising properties as a supplementary cementitious material, careful mix design adjustments and monitoring of trace elements, are necessary to optimize its use in sustainable construction practices (Shulnikova et al. 2024).

The analysis of the Seshego WWS, presented in Fig. 11, shows that it has a significant potential as a sustainable additive in concrete formulations. Its composition, characterized by a remarkable abundance of oxygen and silicon, facilitates the pozzolanic reactions that are essential for improving the strength and durability of concrete, through the formation of calcium silicate hydrates (Lu et al. 2024). In addition, the presence of potassium and phosphorus in WWS, offers interesting opportunities, needed to influence the hydration process and improve the microstructural properties of the concrete. These elements can influence the pore structure and water retention, thereby increasing the resistance of the material to environmental detriments. However, due to the little calcium levels and the presence of phosphorus and potassium, careful consideration is required. The strategic optimization of the blend design, is critical for the effective utilization of the benefits of these elements, while mitigating the likely negative impacts on the long-term stability and performance of concretes (Zhang et al. 2020). A comparative analysis of the WWSs from Mankweng, Polokwane, and Seshego sites, revealed the different elemental compositions that significantly, influenced their potential applications in concrete formulations. The increased calcium content in the Mankweng WWS, is beneficial for the initial setting and the early strength development, therefore, making it suitable for rapid construction scenarios (Kowalski et al. 2018). In contrast, the increased silicon content in the Polokwane and Seshego WWSs, increases the long-term strength through pozzolanic reactions, which are critical for the concrete durability and structural integrity. However, trace elements, i.e., nickel in the Polokwane WWS and potassium and phosphorus in the Seshego WWS, require careful management to optimize the concrete performance and ensure its long-term stability (Ghannam, 2016; Zhang et al., 2020). Sustainable construction practices can be promoted, while effectively, using the industrial waste products by tailoring the concrete mix to the specific properties of the sludge. The elemental analysis underscores the importance of tailoring the concrete mix design, based on the specific chemical make-up of the sludge employed. The optimization of the replacement ratios and perhaps, the incorporation of additional chemical modifiers or admixtures, could maximize the beneficial properties of each type of sludge, aligning with the sustainable construction practices, while ensuring the structural integrity and durability of the resulting concrete. Further research should, however, explore the kinetic aspects of the pozzolanic reactions and the long-term effects of trace elements on concrete performance.

Table 3

summarizing the elemental compositions from the EDX analyses of the WWS from Mankweng, Polokwane, and Seshego
Element	Mankweng (Wt%)	Polokwane (Wt%)	Seshego (Wt%)
Oxygen (O)	47.9	43.1	48.1
Silicon (Si)	16.4	23.7	12.1
Aluminum (Aℓ)	8.4	12.1	3.8
Calcium (Ca)	7.8	3.2	3.2
Iron (Fe)	6.2	3.8	2.8
Sulfur (S)	5.1	1.3	0.7
Phosphorus (P)	4.6	2.7	2.4
Potassium (K)	1.6	3.4	2.4
Magnesium (Mg)	1.3	0.8	0.5
Titanium (Ti)	0.4	0.4	0.2
Chlorine (Ci)	0.2	0.1	0.1
Sodium (Na)	-	0.4	-
Nickel (Ni)	-	0.2	-

4.6 X-ray diffraction (XRD) spectra of wastewater sludge

The X-ray Diffraction (XRD) spectra of WWS from the Mankweng, Seshego, and Polokwane Wastewater Treatment Plants (WWTPs), provide significant insights into the mineralogical composition of these sludges. This composition can be directly, correlated with the findings from the energy-dispersive X-ray (EDX) analyses and the observed microstructures from the Scanning Electron Microscopy (SEM) images. Such correlations are essential for assessing the suitability of a sludge as a supplementary cementitious material (SCM) in concrete formulations. Figure 12 shows that the XRD spectrum of the Mankweng WWTP, exhibits prominent peaks for silica (SiO₂), which correspond to the high silicon content, as identified in the EDX analysis. The sharpness of these peaks suggests a crystalline form of silica, which is advantageous for pozzolanic activity because it can react with the calcium hydroxide in cement to form calcium silicate hydrates, thereby enhancing the strength and durability of concrete. Although the peaks for alumina (Aℓ₂O₃) and iron oxide (Fe₂O₃) were less intense, their presence supported the possibility of secondary reactions that could further strengthen the cement matrix, as indicated by the EDX data. Figure 13 depicts the spectrum for the Seshego WWTP, which displays a similar pattern with notable peaks for SiO₂, but with higher intensity and additional minor peaks when compared to the Mankweng sludge. This suggests a potentially increased reactivity, which may improve the hydraulic properties when incorporated into the concrete. The higher Fe₂O₃ content, evidenced by more pronounced peaks, could enhance the colouring and the possible photocatalytic properties of the concrete. Figure 14 illustrates that the Polokwane WWTP spectrum also revealed a significant SiO₂ content with sharp peaks, which is consistent with the strong pozzolanic activity potential. The peaks for Aℓ₂O₃ and Fe₂O₃ were more pronounced than those in the Mankweng WWTP, indicating a more complex mineral composition that may influence the final properties, such as the mechanical strength and environmental resistance of the concrete. The correlation between the EDX and SEM findings, indicates the fact that the high silicon content identified in the EDX analyses, aligns well with the intense SiO₂ peaks in all the XRD spectra, hence, reinforcing the potential for pozzolanic activity in the sludges. The presence of alumina and iron oxide, although less dominant, is significant for supporting secondary hydration reactions that can enhance the overall concrete performance. The SEM images provide microstructural insights, hence, suggesting that the presence of these minerals, contributes to the physical properties, such as texture and particle distribution of the WWTPs. For instance, the crystalline structures of SiO₂, may facilitate a better binding within the matrix, thereby enhancing the durability and the strength, observed in SEM analyses. In summary, the mineralogical composition, as revealed by XRD, along with the chemical analysis from the EDX and the microstructural insights from SEM, indicates that these sludges possess the potential to function as supplementary cementitious materials (SCMs). The presence of reactive silica and alumina can significantly, improve the mechanical properties, including its strength, durability, and resistance to chemical attack of concrete. However, the variability in composition, suggests that each sludge type may require specific considerations for optimal use in concrete formulations. This could involve the adjustment of the mix design to align with the chemical and physical properties of sludge for the desired concrete characteristics. Overall, the integration of these sludges into concrete, supports the sustainable construction goals by recycling waste materials and hence, potentially, reducing the environmental impact, associated with the conventional cement production. Further detailed studies on the kinetic aspects of these reactions and the long-term performance testing of concretes that incorporate these sludges, are recommended, to fully realize their potentials in construction applications.

4.7 Compressive strength of the concrete

Throughout the test, a w/c ratio of 0.55 was set and the partial substitution of sand content with WWS, was performed at 5, 10, 15, and 20%. The Mankweng, Polokwane, and Seshego WWS achieved a design strength of 25 MPa at 5% WWS content after 90 days of aging, as shown in Figs. 15, 16, and 17. The compressive strength tests reveal clear performance trends for concretes containing sludges from the Mankweng, Polokwane, and Seshego wastewater treatment plants. The understanding of these differences is crucial for the effective use of WWS as a partial replacement for fine aggregates in concrete thus, promoting sustainable construction practices. As expected, all the mixtures exhibited increased compressive strength with long curing times, indicating continued hydration and strength development over time. However, higher WWS replacements (15 and 20%) consistently, resulted in lower strength gains when compared with the control mix and those with lower sludge contents (5 and 10%). This trend was particularly, pronounced at later curing times of 28 and 90 days, hence, suggesting that excessive WWS incorporation may hinder long-term strength development, owing to the increased porosity and changes in the hydration kinetics. Similar results were reported by Bhattacharjee et al. (2015). Each WWS source, imparted unique properties to the concrete, thereby resulting in different performance profiles. As shown in Fig. 15, the Mankweng sludge, yielded a relatively consistent increase in strength, even at higher replacement levels. This positive trend suggests a favourable chemical composition that supports cement hydration and promotes pozzolanic reactions, thereby contributing to enhanced strength development. As shown in Fig. 16, the concrete incorporating Polokwane sludge, exhibited a more pronounced decline in strength with increasing sludge content. This observation suggests possible incompatibilities between the chemical composition or particle size distribution of the sludge and the cementitious matrix, which can potentially, affect the integrity and strength of the concrete; this was also confirmed by Shaikh and Shah (2023). Figure 17 shows the performance of the Seshego WWS, which is intermediate between that of the performances of the sludges from Mankweng and Polokwane, indicating an intermediate level of reactivity or compatibility with cement. This highlights the importance of considering the specific properties of each sludge source when designing concrete mixtures. These results highlight the urgent need for a tailored approach when incorporating WWS into concrete. Although the use of this industrial by-product is consistent with the goals of sustainable construction, by reducing the reliance on virgin aggregates and promoting the circularity of resources, the unique properties of the sludge, must be carefully considered to ensure an optimal concrete performance (Bamshad and Ramezanianpour 2024).

By optimizing the mix designs and exploring suitable strategies to mitigate the adverse effects of high sludge content, such as the adjustment of the water-cement ratio, the incorporation of chemical admixtures, or the implementation of advanced sludge pretreatment methods to improve compatibility with cement (Al-Tayeb, Daoor, and Zeyad 2020), one can better understand the kinetics of pozzolanic reactions in sludge-based concrete. This understanding can lead to optimized mixture designs that can improve the long-term force development mechanisms (Sanjuán and Argiz 2012). The evaluation of the long-term durability of sludge-based concrete, under different exposure conditions, is also important to ensure its suitability for various construction applications. By engaging with these research directions, it is possible to realize the full potential of WWS as a sustainable building material, thereby promoting both the environmental protection demands and robust structural performance.

4.8 Water absorption test

Increased WWS content in concrete, increased the water absorption and sludge porosity, which are the primary variables that influence long-term performance and durability of the resulting concrete. To achieve a good mechanical structure, the quantity of sludge input must be regulated (Chang et al. 2020). The SEM micrographs demonstrated that the fact that the porosity of the material, led to a great water absorption scenario. The water absorption increased as the sludge concentration increased as shown in Fig. 18. Although they were aged for 90 days by using various methods, the unit weights followed a similar pattern, based on the findings of this study. Water absorption tests reveal a significant relationship between WWS content and the porosity of the concrete. As the inclusion of WWS increased, the water absorption capacity of the concrete also increased, a finding that is corroborated by Chang et al. (2020) report. This correlation is particularly, evident in the concrete samples that were aged for 90 days, where the Scanning Electron Microscopy images clearly, demonstrate a direct link between high porosity and increased water absorption. Although the incorporation of WWS offers sustainability benefits, the management of its impact on water absorption, is crucial to the long-term performance of concrete. The control sample, which contained no WWS, exhibited the lowest water absorption (approximately 3%), indicating a denser and less permeable mixture. Conversely, the introduction of WWS, resulted in a proportional increase in the concrete’s water absorption, reaching a value of ~ 7%, at the highest WWS content (20%). The source of WWS, also plays a significant role. Concrete incorporating the Polokwane sludge, displayed the highest water absorption; this is likely due to the lower density and higher dust content of the sludge from this source. In contrast, the Seshego and Mankweng sludges, resulted in moderate water absorption rates (between 3.5-6%), making them potentially more suitable for applications that require low permeability. These findings underscore the importance of carefully, regulating the WWS content in concrete to balance its environmental benefits with those of the adequate mechanical integrity and durability.

4.9 Leaching Tests Confirm Environmental Viability of Sludge-Based Concrete

The leaching tests, conducted on sludge-based concrete, have yielded encouraging results, demonstrating its potential as a safe and sustainable alternative to conventional concrete. The analysis of heavy metal leaching, over extended periods (28, 90, and 140 days), has provided compelling evidence of its environmental compatibility. The heavy metal leaching remained below the hazardous limits, as shown in Table 4. Crucially, the concentrations of all tested heavy metals (Cd, Ni, Cr, Pb, Cu, As, Hg, and Zn), were well below the regulatory limits defined by environmental standards. This consistent trend across all the samples and the curing periods, underscores the safety of incorporating treated WWS into concrete production. Most heavy metals were either undetectable or present, only in trace amounts, even after 140 days, hence, indicating minimal leaching and long-term stability. Although slight increases in the Ni and Zn concentrations were observed over time in some samples, they remained within acceptable limits, demonstrating the ability of the material to effectively, immobilize potential contaminants. The stabilization of the chromium content, also suggests the formation of stable chemical complexes within the concrete matrix, thus increasing its long-term safety. These findings have significant implications for promoting sustainable construction practices. The low leaching rates confirmed the fact that appropriately treated WWS can be incorporated into concrete without posing significant environmental risks. This supports the safe reuse of industrial by-products, minimizes waste, and promotes resource circularity. This study has demonstrated the fact that sludge-based concrete can be engineered to meet both the structural requirements and stringent environmental regulations, thereby expanding its potential applications in the construction domain. Given the variability in WWS, ongoing research on mix design optimization is crucial to ensure a consistent performance and a long-term environmental safety. This includes the exploration of strategies for enhancing the mechanical properties and durability of materials without compromising their environmental profiles. By validating the environmental viability of sludge-based concrete, this study has provided valuable insights for the promotion of sustainable practices in the construction industry. It is envisaged that further exploration and refinement of this eco-friendly material will pave the way for a more resource-efficient and environmentally friendly environment.

Table 4

Consolidated Leaching Results of Toxic Metals from Sludge-Based Concrete at 28, 90, and 140-day test
Metal	Max Limit (mg/L)	28 Days Mankweng	28 Days Polokwane	28 Days Seshego	90 Days Mankweng	90 Days Polokwane	90 Days Seshego	140 Days Mankweng	140 Days Polokwane	140 Days Seshego
Cadmium	> 0.31	ND	ND	ND	ND	ND	ND	ND	ND	ND
Nickel	> 7.5	ND	0.04	ND	0.06	0.09	0.11	0.01	0.11	ND
Chromium	> 0.2	0.02	0.02	0.02	ND	ND	0.01	0.01	0.01	ND
Lead	> 1.2	ND	ND	ND	ND	ND	0.01	ND	ND	ND
Copper	> 1.3	0.04	0.06	ND	0.09	ND	ND	ND	ND	ND
Arsenic	> 3.8	ND	ND	ND	ND	ND	ND	0.01	0.01	0.02
Mercury	> 0.22	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001
Zinc	> 7	ND	ND	ND	0.01	0.04	0.08	0.01	0.02	0.01
AE: Acceptable exposure, ND: Not detected

This study highlights the potential of WWS as a partial replacement for fine aggregates in concrete to improve both sustainability and mechanical performance. Replacing up to 5% sand with WWS was found to maintain the required compressive strength of 25 MPa, while providing significant environmental benefits, including reducing natural resource depletion and minimizing landfill waste. Advanced characterization techniques have revealed the intrinsic properties of WWS, particularly its silicon and aluminium contents, which facilitated the pozzolanic reactions that improved the structural integrity and longevity of the concrete. The mechanical properties of the WWS-based concrete were optimized by carefully, adjusting the water-cement ratio and incorporating chemical admixtures. These modifications are essential for maintaining the workability and slump, which typically decreases as the sludge content increases. By fine-tuning the water-cement ratio, the mechanical properties, such as the compressive strength and durability, were retained despite the increased sludge inclusion. This optimization process ensured that the installation of the WWS did not compromise the structural integrity of the final product. Although the results demonstrated the suitability of WWS-based concrete for sustainable construction, it is important to emphasize the need for long-term durability studies.

This research examined samples up to 90 days; however, future investigations should explore the performance of WWS-based concrete over extended periods and under diverse environmental conditions, such as exposure to freeze-thaw cycles, chemical attack, and sustained loading. Such studies would provide a more comprehensive understanding of the long-term behavior and durability of WWS-based concrete in practical applications, further substantiating its sustainability properties. In terms of broader application implications, WWS-based concrete presents significant potential for large-scale implementation in infrastructural projects, particularly in regions where natural sand resources are scarce, or where waste management poses critical challenges. The use of WWS could be particularly, beneficial in the construction of non-structural elements, such as walkways and low-load-bearing capacity structures, where a slightly high porosity does not affect the overall performance. Furthermore, by comparing WWS-based concrete with the conventional concrete, in terms of environmental impact and performance, it was observed that WWS-based concrete significantly, reduced the carbon footprint in the construction industry. The scalability of this technology could facilitate the transition to a more circular use of resources in the construction sector, thereby contributing to the global movement towards sustainable construction practices. Wastewater sludge is a promising alternative to natural sand for concrete applications, offering an innovative solution that conserves natural resources and addresses waste management challenges. With further optimization of the mix designs and durability testing, WWS-based concrete has the potential to become a mainstream material in the construction industry, hence, fostering a transition to more sustainable and environmentally friendly construction practices.

Author Contributions

Kobe S Mojapelo: Conceptualization, Investigation, Methodology, Formal analysis, Validation, Writing Original Draft. Williams K Kupolati: Research supervisor, Validation, Formal analysis, Editorial review. Everardt A Burger: Research supervisor, Validation, Formal analysis, Editorial review. Julius M Ndambuki: Formal analysis, Editorial review. Emmanuel R Sadiku: Editorial support. Idowu D Ibrahim: Formal analysis, Editorial review

Funding statement

Financial support for this research was received from Tshwane University of Technology, SML projects, and Mocha Labs in Polokwane, South Africa.

Data availability statement

The data supporting these findings are included in the document.

Declaration of Competing Interest

The authors declare that they have no competing financial interests or personal relationships that could influence the work reported in this study.

Acknowledgments

The authors would like to thank the Tshwane University of Technology for providing a platform for this research. Special thanks to Mr. Roland Bisseck, Mr. Kgaugelo Mogashoa, and Mr. Rosina Maboka for their administrative and laboratory support.

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SANS 3001-AG1:2014: Particle size analysis of aggregates by sieving

SANS 241: Physical and chemical properties of potable water

SANS 50197-1: Composition, specifications, and conformity criteria for common cements

SANS 5844:2014: Particle and relative densities of aggregates

SANS 5845:2006: Bulk densities and voids content of aggregates

SANS 1083: Particle size distribution (grading)

SANS 3001-AG1: Fineness modulus (for sand)

SANS 3001-AG10: Aggregate crushing value (ACV)

SANS 3001-AG4: Aggregate impact value (AIV)

SANS 5845: Bulk density (kg/m³)

SANS 3001-AG4: Flakiness index

SANS 3001-AG4: Elongation index

SANS 3001-AG23: Water absorption

SANS 3001-AG21: Specific gravity (relative density)

SANS 3001-AG2: Los Angeles abrasion

SANS 3001-AG5: Moisture content

SANS 3001-AG11: Soundness (sodium or magnesium sulfate)

SANS 3001-AG7: Organic impurities in fine aggregates

SANS 3001-AG8: Deleterious materials

SANS 10164-1:2006: Water-cement ratio

SANS 50197-1:2013: Cement content (kg/m³)

SANS 1083: Aggregate-cement ratio

SANS 10164: Percentage of WWS replacement

SANS 5861-2: Air content

SANS 10162-1:2005: Modulus of elasticity

SANS 10164-2:2006 : Energy absorption

No competing interests reported.

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Optimizing the Mechanical Properties of Concrete Through Partial Replacement of Fine Aggregates with Wastewater Sludge

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Abstract

Figures

1. Introduction

2. Study focus area/s

3. Materials and method

3.1 Materials

3.2 Methods

3.2.1 Comprehensive Methodology for Modified Concrete Mix Design Incorporating Wastewater Sludge as a Partial Sand Replacement

3.2.1.1 Material Characterization and Standards Compliance

3.2.1.2 Modified Mix Design and Optimization

3.2.1.3 Performance and Structural Evaluation

3.2.1.4 Environmental Impact Assessment

3.2.2 Concrete Mix Design Procedure

3.2.2.1 Water and Cement Content Determination

3.2.2.2 Coarse Aggregate Content

3.2.2.3 Fine Aggregate Content

3.2.2.4 Concrete Mix Proportions with Varying Dry Sludge Contents

3.2.2.5 Modification of Sludge-Based Concrete Mix Design

3.2.2.6 Mixing of concrete aggregates

3.2.3 Water Absorption Test

3.2.4 Scanning electron microscope (SEM)

3.2.5 Energy dispersive X-ray (EDX)

3.2.6 X-ray Diffraction (XRD) analysis

3.2.7 Leaching of Heavy Metals from Sludge-Based Concrete

4. Results and discussion

4.1 Physical properties of wastewater sludges from various WWTPs

4.1.1 Dust content

4.1.2 Moisture Content and Bulk Density

4.1.3 Relative Density and pH Value

4.2 Particle size distribution of aggregates

4.2.1 Sieve analysis of fine aggregates (sand)

4.2.2 Sieve analysis of coarse aggregates

4.2.3 Sieve analysis of wastewater sludge Mankweng, Polokwane and Seshego

4.3 Scanning Electron Microscope of Wastewater Sludge

4.4 Sludge-based concrete SEM Analysis

4.5 Energy dispersive X-ray (EDX) for wastewater sludge

4.6 X-ray diffraction (XRD) spectra of wastewater sludge

4.7 Compressive strength of the concrete

4.8 Water absorption test

4.9 Leaching Tests Confirm Environmental Viability of Sludge-Based Concrete

5. Conclusion

Declarations

References

Further Readings

Additional Declarations

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