Foundry Sand in Sustainable Construction: A Systematic Review of Environmental Performance, Contamination Risks, and Regulatory Frameworks

Abstract

The significant expansion of the construction sector and corresponding depletion of natural sand resources have intensified the search for sustainable alternatives, with waste foundry sand (WFS) emerging as a promising candidate. This systematic review evaluates the environmental performance and engineering feasibility of using WFS as a substitute for natural sand in construction. A PRISMA-guided search identified 152 peer-reviewed studies published between 2001 and 2024, which were categorized into four thematic areas: material characterization, construction applications, environmental impacts, and regulatory frameworks. The findings indicate that substituting 10–30% of natural sand with WFS in concrete and asphalt can deliver compressive strength within ±5% of control mixes and reduce water absorption by 5–15% at optimal replacement levels. However, contamination risks remain a concern, as chromium and copper concentrations in raw WFS have been reported at up to 931 mg/kg and 3318 mg/kg, respectively. To address these risks and ensure responsible reuse, a six-stage framework is proposed in this study, comprising end-of-waste classification, contaminant assessment, material preprocessing, certification, and regulatory monitoring. A comprehensive decision tree is also presented to guide the feasibility assessment of WFS reuse based on contaminant levels and material performance.

1. Introduction

The rapid expansion of the construction industry, which now consumes approximately 30–40 billion tons of concrete annually [1], has placed increasing stress on global natural sand reserves and highlights the need for alternative, sustainable fine aggregates. In tandem, the metal casting industry produces substantial quantities of WFS, a byproduct that presents both environmental challenges and opportunities for resource recovery [2,3].

The WFS is a uniformly graded, silica-rich byproduct generated from the repeated use of natural sand in foundry molds and cores for metal casting operations [4,5]. Within foundries, sand can typically be recycled eight to ten times, after which its physical and chemical properties deteriorate to the point that it must be discarded as waste [4,6]. Globally, the annual generation of WFS is estimated at around 100 million tons, with significant contributions from the United States, India, South Africa, and other major industrial economies [7]. For example, South Africa alone produces approximately 500,000 tons of WFS per year [8]. Improper disposal of WFS, primarily via landfill, presents environmental risks such as groundwater contamination, soil degradation, and excessive landfill use [9].

The composition of WFS is primarily determined by the foundry process and binder systems employed. Greensand systems, which are commonly used in iron and steel foundries, typically consist of 85–95% silica sand, 4–10% bentonite clay, 2–5% water, and 2–10% carbonaceous materials, while chemically bonded sands contain 93–99% silica sand and 1–3% synthetic binders [4]. As sand is recycled, WFS accumulates residual binders, metal oxides such as aluminum oxide (Al₂O₃) and iron oxide (Fe₂O₃), and trace heavy metals including chromium and copper, with reported concentrations reaching up to 931 mg/kg and 3318 mg/kg, respectively [10]. The physical characteristics of WFS typically include a specific gravity of 2.4 to 2.9 [5,11,12], fineness modulus in the range of 2.3 to 3.1 [12,13], and generally higher water absorption than that of natural sand (0.38–4.15%) [5,12], necessitating detailed characterization before reuse.

A primary environmental challenge arises from the classification of WFS as either non-hazardous or hazardous material, depending on its source and potential contaminant content. Non-hazardous WFS is most commonly produced by iron, steel, and aluminum foundries, while hazardous WFS originates mainly from leaded brass and copper foundries and may contain elevated levels of toxic metals and organics [14,15]. Most WFS leachates contain low concentrations of hazardous substances, but higher levels of chromium, copper, or organic compounds such as polycyclic aromatic hydrocarbons have been reported in certain cases [11].

A considerable body of research shows that WFS can be beneficially reused in construction applications, particularly as a partial replacement for natural sand in concrete and mortar production. Replacing 10% to 30% of natural sand with WFS has been shown to maintain or even enhance the compressive strength, density, and durability of concretes and mortars, provided that contaminant levels are properly managed [13,16,17]. Nevertheless, challenges remain due to the variability in WFS composition, potential presence of toxic contaminants, and inconsistent regulatory frameworks governing its reuse [18,19].

Current regulatory practices and national policies vary widely, emphasizing the need for harmonized standards, systematic risk assessments, and alignment with circular economy objectives to enable the safe, large-scale adoption of WFS in construction. Effective regulation and innovative processing methods can promote the sustainable use of WFS, providing both economic and environmental advantages [7,11].

This review examines the global evidence base for WFS reuse in construction, outlining its composition, categories, recycling practices, effects on mortar and concrete, and international production trends, as well as technical, environmental, and policy barriers. It proposes a multi-phase, evidence-driven framework for safe and scalable deployment, aiming to contribute substantively to the development of a circular materials economy in the construction sector.

2. Methods

A structured literature review was conducted in accordance with the PRISMA 2020 [20] guidelines to ensure transparency, reproducibility, and comprehensive coverage of relevant studies on WFS reuse in construction (Figure 1). The search included articles, book chapters, and conference proceedings published between 2001 and 2024 in the English language. The following Boolean string was used in the title and abstract fields to identify relevant publications to capture a comprehensive range of terms referring to the reuse of WFS in environmentally responsible construction contexts: (“waste foundry sand” OR “used foundry sand” OR “spent foundry sand” OR “reclaimed foundry sand” OR “foundry sand waste”) AND (“sustainable construction” OR “green concrete” OR “circular economy” OR “construction reuse” OR “eco-friendly building”).

An initial search identified 516 records from databases, while two additional records were found via other sources (one from a website and one from an organization). Prior to screening, records were removed for duplicates (n = 43) and ineligibility (n = 146), leaving 327 records for title and abstract screening. During this screening phase, 111 records were excluded based on title and abstract relevance.

A total of 216 reports were sought for retrieval, with 8 reports not retrievable. Thus, 208 records were assessed for eligibility. Of these, reports were further excluded for reasons such as lack of relevance (n = 29), inadequate coverage (n = 33), insufficient keyword appearance (n = 24), and failure to meet inclusion criteria (n = 9). The two records from non-database sources were also assessed, and one was included after eligibility review. Ultimately, a total of 112 studies were included in the final synthesis.

3. Research Status and Emerging Directions

3.1. Sources and Compositions of WFS

3.1.1. Sources

Figure 2 shows the metal casting process where natural sand is used to make molds. Typically, WFS refers to the waste fraction of sand that results from the metal casting industry after natural sand is used for producing molds and cores that enable the precise shaping of molten metals into specified designs [8,21]. The process begins with sand being shaped into molds through pattern making, followed by mold assembly, and then molten metal is poured into the mold. During the casting process, molding sands are recycled and reused multiple times. However, after many production cycles, sand grains start to break down due to heat and mechanical abrasion, a process illustrated by the separation of discarded sand during shakeout. This necessitates the continuous addition of new sand to the system. When sand is no longer suitable for manufacturing, it is removed and disposed of at foundry landfills or off-site landfills. Foundries typically generate large quantities of WFS, which can either be recycled for non-foundry applications or disposed of in landfills [22].

Foundry sands can be classified as non-hazardous and hazardous; non-hazardous are mostly from industries that produce iron, steel, and aluminum-based materials, while hazardous are from industries that produce lead, brass, and bronze. Due to their toxicity, hazardous foundry sands can contaminate the atmosphere or leach into the sand with phenol and inorganic elements such as lead, chromium, cadmium, zinc, and iron. The figure also shows inspection and quality control steps, including visual inspection, X-ray, ultrasonic testing, and final inspections to ensure mold and casting quality before finishing. Previous research has indicated that the leachate from foundry sands contains low concentrations of hazardous substances [19,23,24,25]. However, others identified a measurable proportion of hazardous substances like hazardous polyaromatic hydrocarbons [26,27,28], a chemical resulting from the incomplete combustion of carbonaceous substances.

3.1.2. Composition

The chemical and physical properties of WFS are shaped by the specific molding techniques and industrial sectors where it is utilized, with variations depending on the type of binder used (clay-bonded or chemically bonded) [2]. The type of materials produced at foundries also influences the composition of WFS, as ferrous metal foundries generate sand with higher iron oxide content, while non-ferrous metal foundries produce sands with distinct chemical profiles [29]. Repeated usage in casting processes introduces metal residues and other contaminants, further altering WFS properties over time [30]. Recognizing these variations is essential for evaluating the potential reuse of WFS in construction, since understanding its specific characteristics helps in developing effective strategies for repurposing this material.

Chemical Composition

The chemical composition of waste foundry sand (WFS) is predominantly defined by foundry process type, binder system, and sand-recycling intensity, all of which influence the concentration of hazardous substances and suitability for construction reuse. Greensands, mainly used in ferrous foundries, generally contain 75–95% silica (SiO₂), 2–9% aluminum oxide (Al₂O₃), and 1–7% iron oxide (Fe₂O₃) [4,18,21,31]. With repeated reuse, WFS accumulates residual binders and heavy metals, including chromium (Cr) up to 931 mg/kg and copper (Cu) up to 3318 mg/kg, substantially higher than natural sand or soil baselines [9,10,21]. Zinc (Zn) can range from 18–100 mg/kg [5,32], while mercury (Hg) and cadmium (Cd) are typically below 1 mg/kg [9,33].

Chemically bonded foundry sands, common in non-ferrous casting, tend to display 80–90% silica, 5–12% aluminum oxide, and 1–6% iron oxide [18], and often show higher levels of organics and metals related to specific binder chemistries and alloys. These sands often have polycyclic aromatic hydrocarbons (PAHs), such as naphthalene, at levels of 2–10 mg/kg [9,34].

Chromite and specialty sands used for alloy and stainless-steel production show much lower silica content but are rich in aluminum and iron oxides; chromium commonly exceeds 100 mg/kg and may reach 931 mg/kg [9], while copper and zinc are also elevated [35]. In contrast, natural sands generally have 95–97% SiO₂ and much lower harmful metals (Cr, Cu, Zn < 10 mg/kg; PAHs < 1 mg/kg) [5,21]. Across all WFS types, elevated loss on ignition (LOI) values—up to 12%—signal residual binders, in contrast to the much lower values found in clean sands [36].

Because of these composition differences, WFS often contains several times higher regulated metals and organics than construction-grade sands. This underlines the requirement for site-specific chemical and leachate testing before reuse to ensure compliance with environmental safety standards [4,18,34]. Variability may be high even within one region or foundry, so ongoing monitoring and batch-specific analysis are necessary for all beneficial reuse scenarios [37,38,39].

Physical Properties

The physical properties of waste foundry sand (WFS) significantly influence its performance in construction applications, particularly in concrete and geotechnical mixtures. Figure 3 presents a heatmap summarizing the key physical characteristics of WFS samples reported across multiple studies, including specific gravity, bulk density, fineness modulus, water absorption, particle content passing the 75 µm sieve, moisture content, and clay lump content. The values were column-normalized to enable visual comparison across studies, and original measurement values were annotated to preserve quantitative accuracy.

Figure 3 illustrates the physical properties of waste foundry sand (WFS) samples obtained from various research studies [4,5,16,40,41,42,43,44,45,46,47], with only [4,5,16,41,45] plotted for clarity. The heatmap uses a color scale to represent the normalized values of each property across different datasets, allowing for quick visual comparison while accounting for variations in measurement scales. The properties evaluated include specific gravity, which affects the density and strength of constructed materials; bulk density, influencing compaction and stability; fineness modulus, related to the particle size distribution and workability; and water absorption, indicating the sand’s capacity to absorb moisture, which can impact the mixture’s curing and durability.

Figure 3. Physical properties of WFS samples reported in various published studies [4,5,16,41,45].

Figure 3. Physical properties of WFS samples reported in various published studies [4,5,16,41,45].

Additionally, the analysis considers other key attributes such as the percentage of particles passing through a 75 µm sieve, which reflects the fineness of the sand, along with moisture content, a crucial factor for handling and processing, and clay lumps, which can adversely affect the binding and strength of construction materials if present in higher amounts. The numeric values within each cell correspond to the original measured data, providing precise information despite the normalized color coding.

From the comparison, it was observed that the majority of WFS samples fall within or close to the limits recommended by ASTM C33 for fine aggregates, which specify specific gravity values, typically between 2.4 and 2.9, and fineness modulus ranging from approximately 2.3 to 3.1. Such conformity suggests that much of the WFS is suitable for reuse in standard construction applications, such as concrete production. However, some samples showed deviations, particularly in parameters like water absorption or particle size distribution, which could influence performance in specific uses. Furthermore, several studies reported low clay lump content (<1%) and moisture levels (<2%), indicating these samples’ suitability for reuse without additional processing. These attributes are essential because high clay content can impair the bonding in concrete, and excessive moisture can lead to handling issues and inconsistent mix quality. Overall, this comparison underscores the potential of WFS as a sustainable fine aggregate, provided its properties are carefully evaluated and matched to project requirements.

Figure 4 displays the particle size distribution (PSD) curves for various waste foundry sands (WFSs) and reference sands, overlaid with the ASTM C33 gradation limits [5,40,44,47]. Each curve illustrates the cumulative percentage of particles finer than a given sieve size, with vertical dashed lines marking the D10 and D60 values indicators of the particle sizes at which 10% and 60% of the sample are finer, respectively. These D-values are extracted and plotted to evaluate the gradation uniformity, which directly influences key properties such as packing density, permeability, and workability in various construction applications. For clarity D-values for only two samples are shown.

It highlights the variation in particle size distribution among different WFS samples. Greensands (WFS01–WFS03) exhibit finer gradation profiles, with a higher percentage of smaller particles, aligning closely with the upper and lower gradation limits. Conversely, chemically bonded sands (WFS04, WFS05, and WFS_AI) tend to have coarser distributions, with some D60 values exceeding 0.7 mm, indicating the presence of larger particles that could affect the mixture’s consistency and strength. Benchmark materials such as Ottawa F65 and Base Sand are included for comparison, demonstrating closer conformity to ASTM gradation limits and serving as references for optimal performance.

This analysis underscores the broad variability observed in WFS properties, emphasizing the necessity for site-specific characterization prior to reuse. While many samples demonstrate acceptable gradation profiles, outliers, particularly those with excessive fines, abnormal absorption, or coarser distributions must be identified and addressed through processing, such as screening or blending, or adjustments in mix design.

Figure 4 highlights that all sands plot near the ASTM C33 envelope at the course–intermediate sieves (≈1.18–0.425 mm), with most divergence appearing toward the fine tail (0.30–0.075 mm). The WFS curves span from relatively uniform/coarse gradations (e.g., the steep WFS04 curve that drops early and remains low at 0.075 mm, tending toward the lower bound) to broader/finer. The reference sands (Ottawa F65, Base Sand, SCFS) bracket the WFS behavior, and several WFS track closely to these references across much of the range, indicating broadly comparable grading. The representative D10 and D60 markers illustrate that some WFS possess a coarser effective size and wider spread than Ottawa F65, while others are finer; together this implies variability in permeability and packing potential across the WFS set. Practically, the majority of WFS meet or closely approach the C33 limits over the sieves that most influence workability, with only tail-end departures that could be corrected by minor blending (for coarse outliers) or washing/air classification (for fine-rich mixes).

3.2. Challenges and Environmental Impacts of WFS Disposal

WFS disposal presents significant environmental, economic, and regulatory challenges. In South Africa, an estimated 500,000 tons of WFS are produced annually, with around 300,000 tons generated in the Gauteng Province alone [5]. This disposal occupies valuable landfill space and contributes to soil, water, and air pollution, exacerbating environmental degradation. The production and disposal processes also generate greenhouse gas emissions, further impacting climate change [34,48]. WFS releases hazardous substances, including phenolic resins and heavy metals, into the environment, thereby posing risks to both ecosystems and human health. Chifflard et al. [17] evaluated the chemical composition of WFS mixed with cement in proportions of 1%, 3%, and 5%. The results showed that the samples with 1% cement slightly exceeded permissible limits for polycyclic aromatic hydrocarbons (PAHs), while those with higher cement content met the regulatory standards. The disposal of this sand in landfills not only incurs high costs but also contributes to environmental degradation. In some regions, the leaching of contaminants from WFS has led to the contamination of local water disposal supplies, affecting agricultural productivity and posing health risks to nearby communities [49].

3.3. WFS in Sustainable Construction Materials

The construction industry is a major consumer of natural resources and a significant contributor to environmental degradation. Therefore, the need for sustainable construction materials and methods is pressing. For example, in South Africa, the demand for sustainable building materials has grown as the country seeks to balance development with environmental conservation [5]. The reuse of WFS cannot only reduce the environmental impact of sand disposal but also offer an alternative to the demand for natural sand used as construction material [17]. By integrating WFS into construction applications, the depletion of natural sand resources can be mitigated while also lowering construction costs and improving the competitiveness of the foundry industry through the reduction of disposal expenses [50,51].

3.3.1. Research Progress on the Reuse of WFS

Research to date shows that waste foundry sand (WFS) can serve as a partial fine aggregate replacement in concrete, mortar, and asphalt mixtures [17,52]. In concrete, most studies converge on an optimum replacement window of 20–30% by mass; within this range, compressive strength either matches or slightly exceeds the control, and durability indicators such as chloride diffusion, sulfate expansion, and freeze–thaw scaling improve because the fine, silica-rich grains of WFS densify the cementitious matrix. Bhardwaj and Kumar [11] also caution that higher dosages raise water demand and can reduce strength, as less paste remains to coat the enlarged surface area. Complementary experiments by Iloh [8] confirmed that mixes containing 30% WFS at w/c = 0.40 and 70% WFS at w/c = 0.60 maintained or slightly improved 28-day compressive strength, whereas 100% replacement caused approximately 10% strength loss and poorer permeability indices.

WFS has also been investigated for use in mortar and asphalt mixtures. Several studies report that modest WFS additions can raise mortar performance by refining particle packing; mixes containing up to about 30% WFS often show higher compressive strength and lower porosity, benefits that translate into improved durability of masonry units [53]. For instance, Sgarlata et al. [54] showed that mortar mixes with WFS exhibited better compressive strength and reduced porosity, critical factors for the performance and durability of mortar. In hot-mix asphalt, WFS is commonly used as a mineral filler or partial fine aggregate replacement. Experiments have demonstrated that incorporating roughly 10–15% WFS can increase Marshall stability and resistance to permanent deformation, thereby extending pavement life under traffic loading [55,56]. Campelo et al. [57] further showed that asphalt mixtures containing WFS maintained these mechanical advantages under a wide range of temperature and moisture conditions, suggesting suitability across diverse climates. A microscopic study by Paulo P.O.L. Dyer et al. [58] supports these findings; WFS particles resemble commercial synthetic sands petrographically, exhibit a low alkali–silica-reactivity risk, and consist mainly of 46–52% silica with ferrous oxides and carbonaceous matter that do not disrupt the asphalt matrix. Overall, the literature indicates that WFS can be a viable supplemental material in mortar and asphalt, provided the replacement level remains within the experimentally validated limits (≈30% for mortar and ≤15% for asphalt) and mix designs are adjusted to account for the fine, high-surface-area character of the sand.

Various studies have also explored several other alternatives to WFS disposal. Examples are the use of WFS in construction materials such as flowable fill, aerated concrete, and controlled low-strength material. These applications can utilize a percentage of greensand, alkaline phenolic, and resin shell sands as fine aggregate replacements, although the presence of residual resin particles may affect the strength of the final product [49]. Other alternatives are summarized in

Table 1. The sustainable alternatives to WFS disposal.

Application	Considerations and Benefits	Applicable Foundry Sand Types	References
Aerated Concrete and Controlled Low-Strength Concrete	Up to the optimum of 30% of replacement of WFS increases the compressive and flexural strength, and up to 70% gives less but acceptable strength.	Greensand, alkaline phenolic, and resin shell sands.	[59]
Agriculture	It can be mixed with soils for agricultural benefits. Requires further testing for crop-specific suitability.	Greensand.	[22]
Hot Rolled Asphalt	The greensand clay content reduces bitumen bleed, and it has been proven successful in the UK and overseas. It is used as a partial replacement (50%).	Alkaline phenolic and resin shell sands.	[60]
Concrete Block Making	Suitable for both low-density (aerated) and dense blocks. Phenol leaching risks may require process adjustments.	Greensand and resin shell sands.	[61]
Road Base Construction	Unbound courses may risk contaminant leaching; testing is required to ensure environmental safety.	Most chemically bonded sands, including greensand.	[62]
Brick Manufacturing	Effective as an aggregate filler. Iron spotting may affect surface continuity but can be desirable for unique effects.	Greensand and chemically bonded sands.	[54]
Cement Production	Used in cement manufacturing. Strength reductions due to residual resin particles may occur.	Greensand, alkaline phenolic, and resin shell sands.	[63]
Soil Stabilization	Used to improve soil strength and durability in geotechnical applications like embankments and retaining walls.	Greensand and chemically bonded sands.	[64]
Landfill Cover Material	Acts as an alternative cover for landfills, reducing the cost of sourcing natural materials.	Spent greensand and resin shell sands.	[64]
Ceramic Production	Suitable as a filler material in ceramic products, improving durability and thermal resistance.	Greensand and chemically bonded sands.	[54]
Geopolymer Concrete	Used as a replacement aggregate in geopolymer concrete, enhancing mechanical properties and sustainability.	Greensand and resin shell sands.	[65]
Artificial Reefs	Incorporated into concrete structures for creating artificial reefs, providing marine habitat restoration.	Chemically bonded sands.	[66]
Potting Media and Topsoil Mix	Used as a component in horticulture for improving soil aeration and drainage.	Greensand (after leachate testing).	[67]
Pipe Bedding Material	Acts as a cushioning material for pipes, reducing reliance on natural aggregates.	Greensand and resin shell sands.	[68]
Paving and Tiles	Incorporated into concrete tiles and interlocking pavers, enhancing strength and aesthetics.	Greensand and resin shell sands.	[69]
Refractory Products	Utilized in refractory brick or lining production for high-temperature industrial applications.	Chemically bonded sands with high silica content.	[66]
Synthetic Aggregate Production	Processed into lightweight synthetic aggregates for construction, reducing the need for natural aggregate mining.	Chemically bonded sands.	[63]

.

Table 2. Recommended optimal WFS content by construction application.

Construction Material	Optimal WFS Content (% by Mass of Fine Aggregate)	Key Performance Criteria	Contamination Risk/Notes	References
Structural concrete	10–30%	Maintains compressive strength and water absorption within ±5% of control; ensures workability and durability	Typically meets leaching limits for Cr, Cu, Zn at ≤30%; periodic tests advised	[17]
Mortar	10–20%	Comparable workability and flexural strength with optimized ratios	Organic content and fines must be controlled	[17,70]
Aerated concrete	10–15%	Required density, strength, and insulation properties were upheld	Strict monitoring of heavy metal leaching is required	[17]
Asphalt mixtures	Up to 15%	Maintains moisture resistance, rutting, and strength	Higher contents may increase stripping and moisture sensitivity	[71]
Flowable fill/controlled low-strength material	Up to 100%	Flow and strength criteria are easily met	Leaching below EPA regulatory limits for most WFS sources	[68]
Road embankment/fill	Up to 100%	Meets compaction, shear strength, and compressibility standards	Ensure contaminants are below soil reuse thresholds	[68]
Brick/block production	10–30%	Preserves compressive strength, durability, and density	Fines and organic/metal content may affect firing/curing	[70]

summarizes the optimal WFS content for various construction materials, balancing performance and contamination risks. Generally, substitution levels range from 10–30% for concrete and mortar, maintaining mechanical strength and environmental safety, while non-structural applications like flowable fill tolerate up to 100% WFS. These guidelines aid practitioners in selecting safe and effective WFS incorporation rates tailored to the intended use.

3.3.2. Techno-Economic Analysis of WFS and Its Use in Concrete

The integration of WFS as a substitute for natural sand in concrete not only offers promising environmental benefits but also presents considerable economic opportunities and challenges. From a technical perspective, the feasibility of WFS use is underpinned by extensive research showing that replacing 10–30% of natural sand with WFS in concrete can achieve comparable compressive strength, workability, and durability relative to conventional mixes, provided contaminants are controlled and mix designs are optimized. Recent studies even show that substitution levels up to 80% may be possible under certain conditions, although at higher dosages, issues such as increased water demand or variability in mechanical properties must be managed through appropriate processing and quality assurance measures [5,11].

Economically, WFS presents a unique advantage, as it is often available at little or no cost to concrete producers. Foundries typically seek to reduce their landfill expenses, sometimes exceeding USD 40 per ton, by offering WFS to construction industries either free of charge or at reduced prices. This direct cost saving is especially compelling in regions where natural sand scarcity drives up aggregate prices [18]. Further, replacing conventional sand with WFS can lower raw material costs for concrete production by 10–25%, given local availability and modest processing requirements [63].

However, several economic and logistical factors complicate the overall cost structure. While untreated WFS incurs minimal material expenses, it regularly requires processing such as screening, washing, or even chemical or microbial treatment to comply with contaminant thresholds and performance specifications. These treatment processes can raise the effective cost of WFS to 5–15 USD per ton, narrowing the price gap with natural sand but often remaining competitive, particularly when accompanied by savings on disposal fees [18,63]. Transport costs are another key determinant of economic viability; if WFS must be hauled over long distances, delivery expenses, generally 8–15 USD per ton per 100 km, can quickly erode the economic benefits compared to locally sourced sand [18].

In addition to direct cost considerations, reusing WFS in concrete yields a range of external economic and environmental benefits. By diverting waste from landfills, the construction and foundry sectors collectively avoid landfill fees and reduce environmental impacts linked to waste decomposition and leachate generation. Reduced reliance on natural sand extraction can help mitigate the ecological and regulatory costs associated with river or pit mining, contributing to a more sustainable and resilient aggregate supply chain [18,40].

Despite these advantages, widespread adoption of WFS in concrete is hindered by market conservatism, regulatory uncertainties, and the need for regular testing to ensure compliance with engineering and environmental standards. In particular, the requirement for frequent contaminant screening and batch-to-batch quality assurance adds to operational costs, while the absence of harmonized standards for WFS reuse can limit market confidence and project approvals [18,72]. To overcome these limitations, industry stakeholders are encouraged to invest in standardized preprocessing and testing infrastructure, foster collaboration between foundries and concrete producers to minimize logistical costs, and advocate for regulatory frameworks that recognize the safety and sustainability of properly managed WFS.

Taken altogether, techno-economic evaluations confirm that WFS can be a cost-effective and environmentally preferable partial replacement for natural sand in concrete, especially in regions where foundries and construction activity are co-located. Realizing these benefits on a scale depends on effectively managing the costs of processing, transport, and quality assurance, as well as addressing the institutional and regulatory barriers that currently limit broader implementation.

3.3.3. Environmental and Durability Impacts of WFS in Sustainable Construction

Environmental Performance: CO₂ Emissions and Resource Conservation

According to the study of Cioli et al. [34], WFS is a byproduct of metal casting, typically carrying low direct CO₂ emissions, which are about 3.5 kg CO₂ per ton of WFS for handling and reprocessing, according to life cycle assessments. This does not include emissions from the casting process itself, which are attributed to metal production. When WFS is used to replace virgin sand in concrete, the environmental gains are substantial; extracting, washing, and transporting natural sand emits 10–15 kg CO₂ per ton, so each ton of WFS reused in its place directly avoids these emissions. For instance, industry-wide adoption in the U.S. could prevent up to 20,000 tons of CO₂ emissions annually. At the product level, replacement of 30% natural sand with WFS in concrete typically achieves 3–5 kg CO₂ savings per ton of finished concrete.

Long-Term Durability and Environmental Safety

With respect to structural performance, concrete containing 10–30% WFS as fine aggregate demonstrates comparable or slightly improved freeze–thaw resistance and long-term strength retention, so long as the WFS is well-processed to manage porosity and limit contaminants [5,11,73]. Some studies show that WFS-modified concrete retains over 90% of its initial compressive strength after 300 freeze–thaw cycles, performance on par with traditional sand concrete [73]. However, exceeding 40% replacement may increase water absorption and reduce durability, likely due to higher matrix porosity [11,31].

Potential environmental risks are mainly associated with heavy metal accumulation and leaching. Laboratory leaching and weathering simulation tests consistently indicate that when WFS is encapsulated in concrete or mortar, leachate concentrations of metals such as chromium and copper remain below regulatory limits, even after accelerated aging or cyclic moisture exposure [18,25,34]. The cementitious matrix effectively immobilizes heavy metals, reducing the potential for environmental harm [13]. Nevertheless, due to limited long-term field data, ongoing leachate monitoring remains advisable, especially for large or high-WFS-content projects.

Advanced Treatment and Quantitative Assessment of Waste Foundry Sand for Construction

The inherent variability and potential for contamination in WFS require effective pre-treatment to ensure it can be safely and efficiently incorporated into concrete and mortar.

Table 3. Quantitative comparison of treatment methods.

Treatment Method	Description	Efficiency	Direct Cost	By-Product Management	Scalability	References
Physical	Sieving, washing, and mechanical separation to remove fines, binders, and clay	30–60% (binders, organics)	2–8 USD/ton	Fines, no hazardous waste	Highly scalable	[5,31]
Chemical	Acid/alkali washing (often using industrial effluent acids)	Up to 95% (metals, organics)	5–15 USD/ton	Liquid effluents require neutralization	Industrial-scale effluent management is required	[18,74]
Microbial/Biological	Composting, fungi/bacterial inoculation for degradation and immobilization	90–95% (metals, organics)	3–10 USD/ton	Compost/sand, minimal hazardous residue	Pilot/demonstration scale, site-specific	[24,75]
Thermal (combined/advanced)	Controlled heating for binder/organic removal and sterilization	High (usually used in combination)	Higher (site/process-specific)	Volatilized organics, sterilized sand	Used as an integrated or advanced step	[76]

compares WFS treatment methods for construction applications and demonstrates that each technology offers distinct advantages and considerations in terms of cost, contaminant removal efficiency, by-product management, and scalability. Physical treatments, including sieving and washing, are highly scalable and economical, often serving as essential precursors for further processing. These methods moderately reduce organic contaminants and fines but are generally inadequate for controlling high levels of heavy metals.

Chemical treatments utilizing acid or alkaline washing excel in removing heavy metal and organic pollutants, achieving up to 95% contaminant reduction. However, their application generates hazardous liquid effluents that require appropriate neutralization and disposal infrastructure, leading to moderately higher operational costs and potential regulatory challenges. Microbial or biological approaches such as composting and bioaugmentation attain high removal rates for both metals and organics through naturally occurring microbial activity, with minimal hazardous by-product generation. These methods remain cost-effective, though they are typically limited to pilot or demonstration scale due to a lack of standardized large-scale protocols and certain regulatory constraints. Advanced or thermal methods are often deployed as part of integrated treatment strategies, relying on controlled heating or multi-stage processes to achieve rigorous contaminant standards, albeit at greater energy and operational expense.

Selecting the most appropriate treatment strategy for WFS is fundamentally context-dependent, governed by the contamination profile, local regulatory requirements, available infrastructure, and target end-use in construction. Integrated protocols that combine physical, chemical, and/or biological steps are often preferred to balance safety, technical performance, and cost efficiency. Ultimately, the successful reuse of WFS in construction hinges on adopting treatment options that align with sustainability principles, regulatory compliance, and practical considerations inherent to industrial settings.

3.3.4. Challenges of WFS Reuse

Despite the promising potential of WFS in construction materials, several challenges limit its widespread adoption. A major concern is the significant variability in the chemical composition of WFS [11,13,17,41,77,78,79]. This inconsistency makes it essential to analyze each WFS sample before reuse or disposal to ensure safety and performance standards are met. Additionally, the presence of contaminants such as heavy metals and polyaromatic hydrocarbons in WFS poses environmental and health risks [19]. These contaminants necessitate thorough testing and treatment before WFS can be safely used, adding to the overall cost and complexity of its utilization. Khan and Maharani (2024) [74] proposed a solution for mitigating heavy metals in WFS through chemical reclamation using acidic industrial effluents, as illustrated in Figure 5. Their study demonstrated that metal oxides and impurities could be effectively removed through optimized chemical processes. By employing industrial effluent acids, the reclaimed sand exhibited improved properties, meeting key foundry standards such as reduced clay content, enhanced grain fineness, and minimized loss on ignition. These findings highlight chemical reclamation as a cost-effective and sustainable approach to improving the performance and reuse potential of WFS. The study promotes sustainability by reducing waste and reusing foundry sand, but its reliance on industrial effluent acids raises concerns about environmental impacts and long-term viability.

Another challenge is the lack of standardized guidelines and regulations for the use of WFS in construction [72]. Without clear standards, it is challenging for industry stakeholders to adopt WFS, resulting in hesitancy and limited confidence in adoption. Moreover, the transportation and processing of WFS can be logistically challenging and expensive, particularly for foundries located far from construction sites [4]. These logistical issues can negate the cost benefits of using WFS, making it less attractive compared to traditional materials. Addressing these challenges through research, regulation, and infrastructure development is crucial for the successful integration of WFS into sustainable construction practices.

3.4. Binders for WFS Stabilization: Green vs. Non-Green

Research has demonstrated that the use of binders in mortar and concrete significantly enhances the physical properties of the mix [80,81,82,83,84,85,86,87,88,89]. Therefore, similar improvements are expected when WFS is used as a full or partial replacement for natural sand in concrete or mortar. As in conventional concrete, the binder in WFS-based mixtures chemically reacts with water to bond aggregates together. This effect was visually confirmed by Kahaf et al. [90] through scanning electron microscopy (SEM), where fly ash geopolymer (FAG) was observed surrounding WFS particles and filling voids in the matrix, thus enhancing the mixture’s density and strength (Figure 6).

As depicted in Figure 6, WFS particles are surrounded by FAG geopolymer and poles filled by the FAG polymer, which can increase the bond and strength of the mix [90]. The microstructural improvements observed suggest a corresponding enhancement in mechanical properties at the macro scale. To validate this, Zhang et al. [73] tested the flexural and compressive strengths of WFS stabilized with different binders, following the European Standard SFS-EN 196-1 used for mechanical properties of mortar [91]. Their findings revealed significant variability in flexural and compressive strength across different binders, primarily attributed to differences in binder composition (Figure 7a,b). In a subsequent experiment, the binders were mixed with WFS and a 75% proportion of crushed concrete. After 28 days, compressive strength tests showed that WFS stabilized with Ecolan achieved the highest strength, followed by quick cement. Bio-ash and fly ash-stabilized samples demonstrated slightly lower but comparable strengths. An interesting observation was that, while cement-stabilized mixes maintained almost constant strength after 28 days, mixes stabilized with other binders continued gaining strength over time (Figure 7c).

Binders for WFS stabilization can generally be categorized into two groups: green binders and non-green binders. Green binders offer significant environmental benefits due to their biodegradability, low toxicity, and reliance on renewable or agricultural waste sources [10]. However, they often face limitations such as lower thermal stability, limited regions, and variability in performance [92,93,94]. In contrast, non-green binders offer superior mechanical and thermal properties, ensuring strong and durable molds. Still, their use poses serious environmental and health hazards due to toxic emissions and non-biodegradability [95]. Research suggests combining green and non-green binders could optimize performance while minimizing environmental impact. While green binders align with sustainable practices and are gaining global interest, they require further optimization to match the efficiency and consistency of non-green alternatives in industrial applications.

3.5. Environmental Benefits and Associated Challenges of WFS Reuse

Research confirmed that recycled WFS provides both engineering and environmental benefits, making it a viable material for sustainable construction applications like subgrade fills, bricks, asphalt aggregates, and pipe bedding [2,68,96,97,98]. Geotechnical tests show that WFS meets the requirements for non-structural fill applications, such as road embankments, with acceptable strength and durability. Additionally, leachate analysis confirmed that metal concentrations are within regulatory limits, indicating minimal environmental risks when used as construction material. The high silica content in WFS enhances its hardness, making it comparable to natural aggregates, whereas the use of WFS in place of quarry sand helps conserve natural resources and reduces landfill disposal [68]. Despite these benefits, there are challenges regarding the potential for leaching toxic substances due to contaminants from the casting process. Arulrajah et al. [68] emphasized the importance of leachate testing for each new source of WFS to ensure environmental safety. Future research should focus on enhancing the integration of WFS with other recycled materials to optimize its performance across diverse applications and conducting field trials to validate its behavior under real-world conditions.

Recycling WFS offers substantial environmental benefits by reducing landfill waste and minimizing the ecological strain of sand mining. Its high silica content and consistent quality make it a valuable alternative to virgin sand, significantly lowering the demand for natural aggregates while preserving natural resources. By incorporating WFS into construction applications such as road bases, asphalt, and concrete, energy use and carbon emissions from traditional sand extraction and processing are also reduced. For instance, current recycling efforts, as noted by the U.S. EPA, save thousands of tons of CO₂ emissions annually and conserve substantial energy resources, showcasing the positive environmental impact of these practices [64,99]. Additionally, WFS immobilizes harmful substances when integrated into concrete, thereby mitigating leaching risks and ensuring its safety for both structural and non-structural applications, while promoting circular economy practices by transforming industrial waste into valuable materials [64].

Table 4. Environmental benefits and challenges of WFS in sustainable construction.

Aspect	Environmental Benefits	Challenges Associated with Benefits	References
Landfill and Resource Conservation	- Reduces landfill waste and conserves natural resources by substituting virgin sand. - Alleviates environmental strain caused by excessive sand mining.	- Requires effective collection and sorting to ensure quality for reuse. - Improperly treated WFS can still pose contamination risks if disposed of carelessly.	[62,64,68,99]
Energy and GHG Emission Reduction	- Lowers energy use and greenhouse gas emissions associated with traditional sand mining and transportation.	- Transporting WFS to distant construction sites can offset emission savings if not locally sourced.	[64,68]
Circular Economy and Material Reuse	- Promotes circular economy practices by transforming industrial by-products into construction resources. - Waste minimization.	- Ensuring consistent quality across different WFS batches can be challenging due to variability in industrial processes.	[62]
Safe Application	- Immobilizes harmful substances within concrete, minimizing leachate risks and ensuring safety for structural and non-structural uses.	- Leachate testing is required for each new source of WFS to confirm safety and compliance with environmental standards.	[65,68]
Economic and Sustainability Impact	- Reduces disposal costs and transportation emissions when WFS is sourced locally. - Increases the economic viability of sustainable construction by lowering material costs.	- Processing costs, especially for removing contaminants or optimizing performance, can limit its economic advantage.	[62,64]
Contaminants and Leachate	- Most WFS leachates, including heavy metals like cadmium, lead, and mercury, are below detection limits in controlled environments, reducing risks to groundwater contamination. - Provides opportunities for safe reuse in construction.	Some WFS, particularly from copper-based foundries, may have higher risks due to variability in contaminant levels, necessitating site-specific leachate management.	[19,25]
Concrete and Road Construction Applications	- Substituting 10–20% of natural sand with WFS in concrete offers economic advantages while maintaining acceptable mechanical properties. - WFS can replace clay in embankments, providing sufficient shear strength and erosion resistance.	- Limited substitution levels (10–20%) restrict broader adoption in concrete. - High-performance embankment applications require rigorous property testing.	[15]

summarizes various aspects of WFS applications, including their benefits and the challenges of implementation, as addressed in the literature. Variability in composition, stemming from differing binding agents and industrial processes, impacts the uniformity and mechanical performance of construction materials. Clay-bonded greensand may compromise workability and strength, while chemically bonded sands may introduce residual contaminants, necessitating rigorous testing and treatment. Moreover, economic barriers, including transportation and processing costs, restrict widespread adoption. Addressing these issues will require standardized treatment protocols, improvements in material consistency, and the development of local recycling infrastructures to ensure WFS is both environmentally sustainable and economically viable [64]. By overcoming these obstacles, WFS could become a cornerstone of sustainable construction practices globally.

3.6. Regulatory Frameworks and Standards of WFS Reuse

The attempt to establish a standard framework for WFS management and reuse in asphalt mix was established by Dyer et al. [56]. This framework, presented in Figure 8, suggests the involvement of various sectors to manage waste from production to the road construction site. Dyer et al.’s [56] management system, while structured and multidisciplinary, presents several inefficiencies and gaps that limit its effectiveness.

Though intended to ensure thorough oversight, the division into five separate management sectors introduces unnecessary bureaucracy that may slow down decision-making and operational efficiency. Additionally, reliance on periodic reports and letters of intent for communication lacks real-time oversight, making the system less adaptive to potential transportation and on-site operations challenges. Furthermore, while the study emphasizes a holistic approach, it does not clearly outline the long-term environmental impact assessment of WFS in asphalt paving, which is crucial for ensuring sustainability and regulatory compliance. The system also depends heavily on a single foundry for WFS flow control, which could pose scalability challenges if multiple sources with varying WFS characteristics are introduced.

The regulatory landscape governing the reuse of WFS varies considerably across regions, with national and local authorities establishing distinct frameworks. In France, specific leaching thresholds have been established for WFS applications in road construction, with allowable contaminant concentrations determined based on the thickness of the applied layer [2]. At the European Union (EU) level, WFS is classified within a broader regulatory framework that differentiates between inert and hazardous waste categories, imposing restrictions according to material composition and associated environmental risks. In the United States, regulatory approaches toward WFS reuse vary at the state level.

Generally, the use of WFS in manufacturing applications such as bricks, asphalt, and concrete is subject to minimal restrictions, while applications involving structural fill are more strictly regulated due to potential leachate and contamination concerns. To promote safe and standardized reuse practices, the U.S. Environmental Protection Agency (USEPA) has developed a comprehensive toolkit to guide program development, environmental monitoring, and regulatory compliance [34].

Despite the existence of various regulatory frameworks, inconsistencies and gaps continue to impede the broader adoption of WFS in construction applications. As outlined in

Table 5. Key considerations for WFS reuse regulations and implementation.

Aspect	Key Points	Quantitative Specifications	Implications	Available Tools/Regulations
End-of-Waste (EoW) Criteria	Establishes legal classification for WFS as a secondary raw material rather than waste.	EU Directives 2008/98/EC and 2018/851/EC outline reuse conditions.	Provides a clear legal pathway for WFS utilization.	Directives and EoW Codes
Ecotoxicity Testing	Evaluates the toxic effects of WFS on aquatic and terrestrial organisms.	Acceptable WFS toxicity levels based on Vibrio fischeri and Daphnia magna bioassays.	Ensures safe environmental reuse of WFS in construction and agriculture.	Microtox, bioassay kits
Toxicity Characteristic Leaching Procedure (TCLP)	Assesses potential leaching of hazardous substances from WFS.	Acceptable Chromium (Cr) leaching limit: 114 mg/kg post-treatment.	Helps prevent groundwater contamination.	TCLP equipment, ICP-MS
Risk Assessment for Reuse	Evaluates exposure pathways, including inhalation, soil contamination, and water impact.	Defines regulatory thresholds for sub-base roads, potting media, and manufactured soils.	Ensures compliance with environmental and health standards.	USEPA protocols for construction reuse
Environmental Benefits Assessment	Reduces virgin raw material consumption and landfill waste.	CO2 emissions reduction.	Supports circular economy initiatives.	Life Cycle Assessment (LCA) tools (SimaPro, GaBi)
Life Cycle Assessment (LCA)	Evaluates the environmental impact of WFS across its reuse lifecycle.	Reduces landfill demand and saves disposal costs.	Identifies environmental hotspots and improvements.	SimaPro, GaBi
Mechanical Tests	Defines acceptable compressive, flexural, and tensile strength for WFS-based materials.	Compressive strength changes by substitution in concrete.	Ensures suitability for structural applications.	Strength-testing equipment
Chemical Tests	Evaluates silica content and heavy metal presence in WFS.	Silica (SiO2) content: 81.9–95.1%; heavy metal limits set by regulations.	Ensures material safety and suitability for reuse.	XRD, SEM, ICP-MS
Physical Tests	Determines particle size, density, and thermal resistance.	84–93% of particles smaller than 100 μm; specific gravity: 2.35–2.60.	Influences workability and performance in construction materials.	Particle size analyzers (like sieve analysis)
Durability Tests	Measures long-term performance of WFS in various applications.	Water absorption and resistance to sulphate attack.	Ensures longevity of WFS-based materials in real-world conditions.	Durability testing equipment
Economic Benefits	Evaluates cost savings from WFS reuse.	Cost savings in material procurement for construction projects.	Encourages industry adoption and financial viability.	Cost-benefit analysis tools
Regulatory Challenges	Variability in regulations across regions.	No global standard; country-specific thresholds for leachate limits.	Standardized regulations would facilitate broader adoption.	Regional compliance tools

, key considerations include defining end-of-waste (EoW) criteria, implementing ecotoxicity testing, adhering to standardized leaching protocols such as the Toxicity Characteristic Leaching Procedure (TCLP), and evaluating mechanical and chemical properties for material suitability. Although some jurisdictions have established clearer pathways for WFS recycling, many regions still lack comprehensive regulatory support. Greater harmonization of international guidelines would facilitate broader acceptance and ensure that WFS is repurposed effectively within sustainable construction practices.

Based on available gaps, this study proposes a comprehensive, multi-phase regulatory framework for WFS reuse, as illustrated in Figure 9. The framework is designed to ensure safe, standardized, and scalable integration of WFS into construction and related sectors. It begins with end-of-waste classification, which provides the legal foundation by reclassifying WFS as a secondary raw material based on quality requirements, traceability, user safety, and economic use. This feeds into comprehensive material characterization, where physical and chemical properties, material variability, and long-term behavior are thoroughly assessed. In the third phase, Environmental Safety Assessment, laboratory and field evaluations are conducted to determine the leaching behavior of hazardous substances and potential risks to human and ecological health.

Next, the standardized treatment and preprocessing phase establishes technical protocols such as drying, mixing, and grading to meet reuse specifications. This is followed by a certification and labelling process that ensures compliance, promotes transparency, and facilitates market acceptance of WFS-derived products. The final stage, regulatory oversight and monitoring, introduces ongoing inspection systems and compliance mechanisms to ensure long-term adherence to safety and environmental standards. By integrating legal, scientific, and operational considerations, the framework not only fills critical gaps observed in previous approaches but also supports circular economy initiatives by enabling the safe and effective reuse of WFS in various construction applications.

3.7. Application of WFS in the Built Environment

The physical and chemical properties of waste foundry sand (WFS) make it suitable for various applications in the built environment. Studies have highlighted that WFS is primarily composed of uniformly graded silica sand, which exhibits high thermal stability, fine particle size distribution, and low permeability [2,8,100]. Martins et al. [32] reported that WFS particles can present a round shape with uniform grading and some voids in between. This morphology allows particles to pack more efficiently, achieving higher packing density and improved compaction. The spherical geometry enhances mixture workability by facilitating better flow and reducing internal friction, while the reduced angularity lowers water demand in cementitious applications, potentially improving both strength and durability. However, the smoother surface and reduced mechanical interlocking of rounded particles may slightly diminish interfacial bonding strength in certain applications.

In contrast, the research of Iloh [8], through the SEM micrograph presented in Figure 10a for chemically bonded WFS and Figure 10b for green WFS, reveals that greensand retains its round shape, but chemically bound sands present a different morphology, with predominantly angular particles, sharp edges, and irregular outlines. The surface texture is rough, with visible asperities and micro-protrusions that can enhance mechanical interlocking within cementitious matrices, potentially improving interfacial transition zone (ITZ) strength [8]. This combination of angularity, roughness, and grading variability can increase water demand and reduce workability compared with the rounded WFS reported by Martins et al. [32], but may also provide higher bond strength and improved mechanical performance if mix design adjustments are made. The presence of residual binder films, inherent to chemically bonded sands, could further influence hydration reactions and durability outcomes. These contrasting morphologies highlight the importance of tailoring WFS processing and mixture proportioning to optimize its performance in different construction applications.

Uniform grading of WFS enhances the packing density in concrete and mortar [11], and it improves compressive strength and durability when WFS is used as a partial substitute for natural sand to the optimum fraction. Additionally, the high silica content, which typically exceeds 85%, contributes to chemical stability in concrete, reducing expansion and the likelihood of cracking over time. The low permeability of WFS makes it desirable for applications in paving blocks, bricks, and precast elements, where reduced water absorption is beneficial for long-term performance [17]. In geotechnical and civil engineering applications, the low plasticity and non-reactive nature make WFS an effective material for soil stabilization [101]. When blended with expansive soils, WFS helps reduce shrink-swell behavior, improving the overall stability of the soil [102]. The angular particle shape of WFS enhances shear strength, making it valuable for embankments, backfill materials, and road sub-base layers. Furthermore, its low compressibility makes WFS an excellent candidate for landfill cover systems and embankment fill, preventing excessive settlement over time and contributing to the stability of civil infrastructure projects [103].

Environmental applications of WFS leverage its permeability and high binding capacity. Kumar et al. [47] studied the behavior of Indian waste foundry sand for geotechnical applications by measuring permeability and relative density at various WFS proportions. The results showed that as the density decreases, the proportion of WFS increases, as shown in Figure 11.

Kumar et al. [46] prove WFS as a suitable material for erosion control mats and barriers, effectively preventing soil erosion in vulnerable areas. WFS also demonstrates potential as a filtration medium in wastewater treatment systems, where its fine particle distribution aids in removing suspended particles and contaminants from industrial effluents [104]. Additionally, the chemical stability of WFS allows for its use in land reclamation projects and as a capping material for contaminated sites, reducing the risk of leachate formation and groundwater contamination [18]. Another promising application for WFS is the manufacture of artificial aggregates. The silica-rich composition allows WFS to undergo sintering; therefore, it can be used in producing lightweight artificial aggregates that can be used in concrete [105]. These aggregates offer enhanced insulation properties and are valuable in the production of energy-efficient building materials. Through advanced thermal processes, WFS can also be converted into ceramic aggregates, which are used in high-strength concrete and specialized construction applications [3]. Figure 12 shows various applications of WFS highlighted in multiple literature.

To complement the proposed regulatory framework, a structured decision-making model is presented in Figure 13, offering a practical tool for assessing the feasibility of WFS reuse in construction applications. This stepwise decision guides stakeholders through a logical screening process, starting from evaluating WFS leachate compliance with environmental standards. If the material fails at any stage, it is deemed unsuitable for reuse. The model ensures that only WFS that meets critical environmental and engineering criteria advances to application. The subsequent screening steps include assessing heavy metal concentrations, particularly chromium, copper, and lead, and verifying the effectiveness and cost-efficiency of any chemical or microbial treatment applied to the WFS.

Beyond environmental considerations, the final decision hinges on whether the treated WFS satisfies mechanical and chemical performance requirements for specific construction applications such as bricks, subgrade stabilization, or concrete. Parameters such as compressive strength, water absorption, and silica content are vital at this stage. If all conditions are met, the WFS is classified as “Feasible for Reuse.” Otherwise, it is filtered out to prevent environmental risk or structural underperformance. This model provides a simple screening mechanism that supports responsible reuse decisions and reinforces the practical implementation of the regulatory framework.

3.8. Hypothetical Application Scenario: Framework and Decision Tree Validation

To validate the feasibility and operability of the proposed six-phase regulatory framework and decision tree model,

Table 6. Stepwise application of the six-phase framework and decision tree model for WFS reuse in South African road construction.

Framework Phase	Action in a Hypothetical Scenario	Outcome/Decision Point	References
1. End-of-Waste Classification	Foundry applies to the classification of non-hazardous WFS and documenting source, traceability, and origin.	WFS provisionally approved for potential secondary use.	[5]
2. Comprehensive Characterization	Lab analysis: specific gravity 2.5; fineness modulus 2.6; Cr 15 mg/kg, Cu 12 mg/kg—compliant with limits.	WFS was judged technically compatible; minor pre-treatment was recommended.	[5,47]
3. Environmental Risk Assessment	TCLP and ecotoxicity tests on WFS-concrete confirm all values below thresholds.	Passed for downstream use in construction; move to preprocessing.	[24,34]
4. Standardized Treatment and Processing	Sand is sieved and washed to optimize gradation and reduce LOI; record-keeping is implemented.	Physical and chemical properties conforming to the mix design; batch ready for certification.	[5,11]
5. Certification and Labelling	An independent certified lab issues a compliance certificate and batch label.	The WFS batch was authorized for use as 30% replacement sand in a road project.	[4]
6. Regulatory Oversight and Monitoring	Ongoing site audits and leachate spot-checks by the construction quality team and regulatory authorities.	Compliance is regularly verified; non-conforming material is excluded/remediated.	[34]
Decision Tree Checkpoints	At each phase, the decision tree ensures material failing criteria are halted, remediated, or redirected to landfill.	Only fully compliant WFS enters the construction stream, eliminating environmental or technical risks.	[5,18]

presents a hypothetical scenario of WFS reuse in a Gauteng (South Africa) road construction project. This scenario outlines each phase described in the framework and mirrors regulatory and technical steps recommended by published literature.

Comparative Analysis of WFS Treatment and Reuse Policies: Europe, U.S., and South Africa

Comprehending regulatory differences across major regions is critical for the responsible, large-scale implementation of WFS reuse.

Table 7. Comparison of institutional approaches to WFS reuse by region.

Aspect	Europe (EU and UK)	United States	South Africa
Regulatory classification	Generally, WFS is classified as non-hazardous if contaminant thresholds (EU Waste Directives) are met; individual member states may apply stricter criteria [14,34].	It varies by state; some states classify it as an industrial by-product, with eligibility for beneficial reuse based on leachate and metals testing [106].	Classified as hazardous by default unless proven otherwise, the SA Waste Management Act enforces stricter controls [107].
Risk assessment and testing	Per EU directives, characterization is mandatory for heavy metals, organics, and leaching; ecotoxicity assessment is also required (yet methods are still evolving) [33,36].	US EPA and USDA risk assessments provide national benchmarks; most states require TCLP or similar leachate and metals tests, with variable organics testing [106].	Leachate, total metals content, and acid potential are tested; classification is generally stricter than EU/US, raising disposal costs [108].
Reuse approval mechanisms	National and local permits; some countries (e.g., Finland, UK) are piloting ‘end-of-waste’ reclassification for geo-construction uses [109]	Combination of general permits, exemptions for qualified uses, and notification/approval schemes.	It requires a Waste Management License for storage and use outside the foundry; end-use in construction is possible but hindered by strict origin-based classification [110].
Permissible reuse pathways	Geo-construction, green infrastructure, compost, cement, and asphalt. Some countries allow landfill mining and restoration of previously disposed WFS [109].	Manufacturing, structural fill, road bases, manufactured soils, and landfill daily cover. Agricultural reuse is allowed if pollutant levels are low [111].	Primarily limited to internal reuse or licensed projects; off-site reuse for construction requires full compliance and licensing, seldom permitted for direct land application [110].
Examples of policy innovations	The Finnish MARA decree includes WFS in geo-construction, and pilot composting projects have produced national implementation guidelines [112].	States like Illinois and Indiana exempt compliant WFS from some industrial waste regulations, and the EPA supports silica-based WFS for soil and road uses [106,111].	Several reclamation initiatives respond to local pollution concerns, but broader regulatory harmonization with international practice is lacking [107,110].
Key barriers/challenges	Harmonization of ecotoxicity protocols; member state variability. Some hesitation due to public perception and a lack of pan-European standards [34].	State-by-state differences, lack of federal uniformity, and inconsistent organics regulation make it complex to approve higher-risk applications [111].	High administrative burden; hazardous default classification raises costs and reduces incentive for proposed reuse; limited sector-specific standards [108,110].

provides a comparative overview that highlights institutional approaches, approval mechanisms, and key policy distinctions between the European Union (EU), the United States (U.S.), and South Africa.

3.9. Challenges and Recommendations for Future Research

The long-term goals of environmental strategies for WFS should focus on promoting its beneficial reuse as a substitute for natural resources, thereby conserving raw materials, reducing landfill dependency, and enhancing sustainability [34]. However, several knowledge gaps hinder the reuse of WFS, limiting its full environmental potential.

3.9.1. Key Knowledge Gaps Restricting WFS Reuse

Despite the promising potential of WFS in sustainable construction, several critical knowledge gaps continue to hinder its widespread adoption and reuse. One of the most significant gaps is the incomplete characterization of WFS. Comprehensive data regarding its physical, chemical, and toxicological properties remain limited, particularly in relation to contaminants such as heavy metals and polyaromatic hydrocarbons [18]. This lack of thorough analysis raises concerns about the long-term environmental and structural impacts of using WFS in construction projects. Additionally, there is limited understanding of the dynamics of contaminants derived from WFS. The environmental behavior and interactions of organic and metallic compounds within ecosystems are poorly understood [63]. These uncertainties pose challenges in predicting the potential risks associated with WFS reuse, necessitating further research into how contaminants migrate and react under various environmental conditions.

The absence of well-defined legislative and management frameworks further restricts the reuse of WFS. The lack of standardized guidelines and cohesive regulations undermines stakeholder confidence and deters industries from adopting WFS as a viable material [34]. Establishing clear, harmonized environmental regulations and developing standardized protocols for WFS reuse are essential steps to encourage greater industry participation and ensure the safe, sustainable application of WFS in construction and other sectors. Addressing these knowledge gaps through collaborative research and policy development will play a pivotal role in unlocking the full potential of WFS, fostering innovation, and supporting circular economy objectives.

3.9.2. Priority Areas for Future Research

One critical challenge lies in understanding how the foundry process affects WFS composition, including the influence of organic binders and additives on contamination levels [8]. While international research indicates that metal contaminants are generally within acceptable limits, organic contaminants require further investigation. Comprehensive characterization of inorganic and organic elements should be a key focus of future studies. Also, research in areas of comprehensive strength and other types of tests, like acoustics (finished products), fire tests, and other structural application tests of WFS, is needed.

3.9.3. Strategies for Enhancing WFS Reuse

Enhancing the reuse of WFS requires a combination of industry-driven initiatives and regulatory support. Foundries must take a proactive role in promoting WFS reuse by providing detailed and standardized product information [72]. Data sheets should include metrics such as particle size distribution, clay and sand content, compaction curves, and concentrations of metals and organic contaminants. To maintain a high-quality WFS waste stream, foundries must improve management practices to prevent contamination during production and disposal [96]. Regular quality control, including sampling and analysis, should be implemented to ensure consistency. Larger foundries could adopt advanced online monitoring technologies, while smaller facilities may benefit from simpler, cost-effective solutions designed for their needs [10]. By enhancing quality control measures and fostering greater collaboration across the industry, WFS can become a valuable contributor to sustainable construction and circular economy initiatives.

3.9.4. Research Focus Areas in Sustainable Construction

Research into WFS applications in sustainable construction remains limited, particularly in its applicability in built environments. Future investigations should emphasize several critical areas to unlock the full potential of WFS in sustainable building practices. A key focus area is contaminant leaching, which necessitates studying the behavior of heavy metals and organic compounds in WFS-blended materials under environmental conditions. Factors such as rainfall, heat, and freeze–thaw cycles can influence leaching patterns, affecting the long-term environmental performance of these materials. Comprehensive assessments are required to ensure that WFS-based construction materials do not pose risks to ecosystems or public health over time.

Understanding the human health impacts associated with WFS use in construction is essential. Evaluating the risks posed by airborne particulate matter generated during WFS handling and application will help address respiratory and other health concerns, particularly in large-scale construction projects. Additionally, assessing the long-term exposure effects of residual contaminants in WFS-based materials will ensure that their application does not compromise worker or community safety. Another critical aspect involves studying the performance and durability of WFS-enhanced construction materials. Comparative analyses between WFS-enhanced concrete and asphalt and their traditional counterparts can provide valuable insights into strength, durability, and thermal resistance. Research should also investigate the potential of WFS in subgrades, road embankments, and structural fills, emphasizing environmental safety and cost-effectiveness to encourage broader adoption.

One emerging avenue of interest is the microbial treatment of WFS to enhance its performance. Research has shown that microbial-treated WFS can improve the compressive strength, durability, and environmental resistance of concrete, making it a viable and sustainable alternative [75]. Promoting microbial treatment aligns with circular economy principles by reducing landfill waste, conserving natural resources, and minimizing the environmental impact of sand extraction [3,24]. Addressing these focus areas through rigorous research and collaboration between researchers, policymakers, and industry stakeholders will be essential to optimizing the use of WFS. By expanding the application of WFS across various sectors, the construction industry can make a significant contribution to sustainable development and circular economic initiatives.

4. Conclusions

This systematic review demonstrates that waste foundry sand has substantial potential as a sustainable alternative to natural sand in construction, offering both engineering viability and environmental benefits. A synthesis from 152 peer-reviewed studies shows that incorporating WFS at substitution levels between 10% and 30% can achieve comparable or improved mechanical properties relative to conventional concrete and water absorption reductions. Environmental gains are similarly significant, with reported reductions of 40% in CO₂ emissions and a substantial portion in landfill usage when WFS is reused in place of virgin aggregates. However, the review also highlights persistent challenges, including the presence of hazardous contaminants such as chromium (up to 931 mg/kg) and copper (up to 3318 mg/kg) in raw WFS, which pose risks to human health and ecosystems if unmanaged. Treatment approaches such as microbial or chemical remediation have shown promising results yet lack standardization and scalability across regions. Additionally, logistical and economic constraints, as well as the lack of harmonized legal pathways for WFS classification and reuse, continue to hinder broader implementation.

To bridge these gaps, this study proposes a six-stage regulatory framework grounded in quantitative testing protocols and performance benchmarks. This framework includes the following: (1) end-of-waste classification, (2) comprehensive material characterization, (3) environmental risk assessment, (4) standardized treatment and preprocessing, (5) certification and labeling, and (6) ongoing regulatory oversight. Each phase integrates measurable thresholds for heavy metals and physical performance to ensure scientific rigor and compliance with environmental and construction safety standards. Given the wide variability in WFS properties and the inconsistencies across existing regional standards, the findings underscore the urgent need for a globally harmonized and data-driven regulatory framework. Future research should prioritize multi-country policy alignment, cost-benefit analysis of treatment technologies, and the development of real-time monitoring systems to validate WFS safety and performance in situ. By embedding quantitative thresholds into policy and practice, WFS can be effectively transformed from an industrial byproduct into a mainstream, certified input for sustainable infrastructure, contributing meaningfully to circular economy goals and environmental resilience.