Reclaimed Municipal Wastewater Sand as a Viable Aggregate in Cement Mortars: Alkaline Treatment, Performance, Assessment, and Circular Construction Applications

Abstract

This study evaluates the potential use of reclaimed sand from municipal wastewater treatment plants (WWTP), categorized as waste under code 19 08 02, as a full substitute for natural sand in cement mortars. The sand was subjected to alkaline pretreatment using sodium hydroxide (NaOH) at concentrations of 0.5%, 1% and 2% to reduce organic impurities and improve surface cleanliness. All mortar mixes were prepared using CEM I 42.5 R as the binder, maintaining a constant water-to-cement ratio of 0.5. Mechanical testing revealed that mortars produced with 100% WWTP-derived sand, pretreated with 0.5% NaOH, achieved a mean compressive strength of 51.9 MPa and flexural strength of 5.63 MPa after 28 days, nearly equivalent to reference mortars with standardized construction sand (52.7 MPa and 6.64 MPa, respectively). In contrast, untreated WWTP sand resulted in a significant performance reduction, with compressive strength averaging 30.0 MPa and flexural strength ranging from 2.55 to 2.93 MPa. The results demonstrate that low-alkaline pretreatment—particularly with 0.5% NaOH—allows for the effective reuse of WWTP waste sand (code 19 08 02) in cement mortars based on CEM I 42.5 R, achieving performance comparable to conventional materials. Although higher concentrations, such as 2% NaOH, are commonly recommended or required by standards for the removal of organic matter from fine aggregates, the results suggest that lower concentrations (e.g., 0.5%) may offer a better balance between cleaning effectiveness and mechanical performance. Nevertheless, 2% NaOH remains the obligatory reference level in some standard testing protocols for fine aggregate purification.

1. Introduction

Reclaimed sand from municipal wastewater treatment plants typically contains a variety of impurities, most notably, organic residues, which originate from biological sludge wastewater solids and microbial films accumulated during the treatment process. These organic substances pose a significant risk to the performance and durability of cementitious composites for several reasons:

• Interference with Cement Hydration: Organic compounds can retard or inhibit the hydration of Portland cement by adsorbing onto the surface of cement grains or by complexing with calcium ions, delaying strength development and reducing early-age performance [1].
• Increased Water Demand: Organic films and colloids on the sand surface increase water absorption and reduce the flowability of mortar mixes [2,3,4]. This necessitates a higher water-to-cement ratio to achieve workable consistency, which in turn compromises mechanical strength and durability [5].
• Poor Interfacial Bonding: Residual organic matter acts as a barrier between sand particles and the cement paste matrix, weakening the interfacial transition zone (ITZ). This leads to reduced bond strength and increased porosity at the microstructural level.
• Microbial Activity and Long-Term Instability: If not properly neutralized. Organic components can foster microbial growth or decay within hardened mortars, resulting in odour formation, material degradation, and potential bio-corrosion over time.

The construction sector’s heavy dependence on natural aggregates, particularly sand, poses sustainability challenges due to dwindling resources and environmental concerns [6,7]. Construction sand used in cementitious materials must meet stringent purity standards to ensure mechanical performance, chemical compatibility, and long-term durability [8,9,10,11,12].

Table 1. Comparison Table of European (EN/PN) and American (ASTM) standards requirements for contaminants in concrete aggregates.

Criteria	EN 12620:2002 +A1:2008 [13]/PN-B-06711:2020-03 [14]+	ASTM Standards (e.g., ASTM C40 [15], ASTM C87 [16])
Organic impurities	Colorimetric NaOH test; solution color must not be darker than standard	ASTM C40 [15], Colorimetric NaOH test ASTM C87 [16]: strength impact evaluation
Fines content (<0.063 mm)	Natural sand ≤ 3%; Crushed sand ≤ 1%; For architectural concrete ≤ 1%	ASTM C117 [17]: Max fines content not specified, but measured and controlled via tests
Harmful impurities (sulfates, chlorides, lignite, etc.)	Sulfate ≤ 0.5% by mass; avoid sulfides, lignite, gypsum	ASTM C88 [18]. Sulfate soundness test; limits based on durability classes
Foreign substances (e.g., wood, metal, glass)	Total foreign matter ≤ 0.5% by mass	ASTM D5821 [19] & ASTM C33 [20]: Foreign matters should be absent or minimal
Alkali–silica reactivity (ASR)	Required testing if reactive silica is present (e.g., opal, chalcedony)	ASTM C1260 [21], C1293 [22] Mandatory tests for ASR potential in aggregates

summarises the relevant standards for sand as a building material. The sand reclaimed from municipal wastewater treatment plants, the following acceptance criteria are recommended to ensure its safe application in construction: fines content (<0.063 mm) ≤3.0% organic matter. No color change in NaOH test; sulfate content (SO₄²⁻) ≤ 0.5% by mass. Foreign contaminants ≤ 0.5%, odour, bioactivity, discoloration: none detectable.

Concurrently, municipal wastewater treatment plants generate substantial quantities of sand-laden sludge, which is commonly discarded or landfilled despite its mineral composition being suitable for construction use [23,24,25]. The primary barrier to reuse is the presence of organic contaminants—complex mixtures of microbial extracellular polymeric substances (EPS), fatty acids, oils, and polysaccharides—that adhere to sand surfaces, creating hydrophobic layers [26,27,28,29]. In the results, the mechanical properties of the building materials are inadequate [29,30,31], because the reduction of compressive strength of mortar was higher than 30% corresponding to mortar with standard sand.

The key conclusion is that these materials can be valorized directly at the WWTP stage, reducing landfill use and enabling full waste circularity [32]. The authors propose the use of this sand in low-structural concrete applications and secondary construction layers.

These organic films inhibit the early-stage cement hydration by blocking water access to reactive phases and disrupting the formation of the interfacial transition zone, a critical microstructure at the aggregate-paste interface responsible for stress transfer and strength [25,26,27,28,29,30]. Untreated WWTP sand in mortars is therefore associated with decreased compressive strength, increased porosity, and compromised durability [1,2,3,4,5].

Chemical pretreatments have emerged as practical solutions to remove these organic layers. Among these, alkaline treatments using sodium hydroxide promote saponification and hydrolysis reactions that chemically degrade organic matter, transforming it into water-soluble forms that can be removed by rinsing (

Table 2. Comparison of sand cleaning methods for construction applications.

Method	Cleaning Agent/Process	NaOH Concentration/Contact Time	Effect on Cementitious Strength	Environmental and Cost Aspects	Practical Cse
Physical (mechanical rinsing)	Water rinsing, mechanical separation, sediment removal	Continuous flow (no chemicals used)	Significantly lowers LOI (e.g., 1–2%); minimal impact on strength [38]	High water and energy demand; no chemical waste	Used in WWTPs (e.g., Germany, Poland); recovered sand reused for backfilling [38]
Chemical (alkalis, acids, oxidants)	Strong NaOH, HCl, O3, H2O2, KMnO4	~0.5–1.0 M for several minutes	May degrade organic matter; slight surface alteration of silica [32]	Chemical residues require neutralisation; higher treatment cost and load [32]	Mostly lab-based; applied to remove residual organics from sand
Biological (composting)	Organic amendments (compost), microbial degradation	Days to months of incubation	Up to 90% degradation of phenolic/humic substances; stable product [39]	Low chemical input; requires space and long treatment time	Demonstrated in foundry sand; reused in geotechnical or landscaping applications [39]
Low-NaOH Washing *	Low-concentration aqueous NaOH	0.1–0.2 M for 15–30 min	Efficient organic removal: no significant strength loss observed	Low chemical usage; lower pH of effluent; reduced environmental impact	Demonstrated suitability for cement mortars; validated through mechanical testing

* Present study—experimental results based on alkaline washing of reclaimed wastewater sand using 0.1–0.2 M NaOH and subsequent mechanical testing of cement mortars.

) [33,34,35,36,37]. However, comprehensive studies on the effect of NaOH concentration on sand cleanliness and mortar rheology are lacking. Hydration kinetics and mechanical performance remain scarce.

While mechanical rinsing is widely adopted in wastewater treatment plants for bulk cleaning of sand, it does not eliminate organic impurities. Chemical methods using strong alkalis or oxidants can enhance purification, but they require costly post-treatment and pose environmental risks. Biological techniques offer high removal rates for complex organics but are slow and space-intensive. In contrast, the proposed low-NaOH washing method combines ecological safety with functional effectiveness. By using significantly lower alkali concentrations, it reduces chemical demand and environmental burden while maintaining the mechanical integrity of cement-based materials. This makes it a promising innovation for circular resource use in construction.

This type of sand has not previously been used in construction elements, and its application represents an innovation at the European scale. While ASTM standards recommend testing the mechanical strength of mortar or concrete samples containing clean sand (e.g., ASTM C87 [16] they do not specify an acceptable range of strength reductions that would qualify the sand as suitable. The objective of this study was to determine whether lower concentrations of sodium hydroxide—selected for environmental reasons due to the potential impact of wastewater disposal—are sufficient to effectively clean the sand to a level that allows its safe and functional use in structural elements [40]. This investigation thus combines technical performance criteria with environmental considerations, aiming to support circular economic practices by reusing sand recovered from municipal wastewater treatment processes.

Similarly, other studies on recycled or marginal aggregates do not address NaOH-washing as a method to enhance compatibility with cement binders. Therefore, the proposed method—using NaOH to clean sewage-derived sand for subsequent use in mortar production—can be considered a scientifically novel approach. It represents a unique and innovative approach to circular resource use in construction.

The aim was to assess how these concentrations influence the flexural and compressive strength of the resulting mortars. By comparing the mechanical performance, an optimal pretreatment strategy could be identified to enable sustainable reuse of WWTP-derived sand.

A detailed literature review reveals that the use of NaOH to wash sand reclaimed from municipal wastewater treatment plants—specifically, the filter or sewer-cleaning sands—has not been previously documented in the scientific literature. Existing studies on reclaimed sand primarily focus on mechanical or physical cleaning, sometimes involving thermal treatment or standard sieving, but not on chemical cleaning using NaOH [17]. While NaOH is commonly used to test for the presence of organic impurities or clean natural aggregates [40,41,42,43], there are no identified reports where NaOH was used to clean WWTP-derived sand for reuse in cementitious materials.

2. Materials and Methods

2.1. Materials

In the research, the following materials were used:

• Cement: CEM I 42.5R conforming to EN 197-1 standards [44].
• Aggregates:

  ○

  Natural, standardized norm sand (0–2 mm) (symbol in the research—SN) per EN 196-1 [45],

  ○

  Quartz sand according to building standards (symbol in the research—ST),

  ○

  WWTP sand sourced from the grit removal systems of a municipal wastewater treatment plant, containing organic residues and fine silts,

• Sodium hydroxide (NaOH) reagent grade (30% stock solution),
• Potable water,
• Polycarboxylate superplasticizer in a dosage of 0.40% mass of cement.

Mortars were prepared by PN-EN 196-1 [45], maintaining a water-to-cement ratio of 0.5 and a constant aggregate-to-binder ratio. NaOH solution volumes were adjusted to maintain consistent water content. One hundred per cent of the natural sand was replaced by treated or untreated WWTP sand.

2.2. Methods

2.2.1. Sand Pretreatment Procedure

Table 3. Comparison of European and American standards for assessing organic impurities in sand.

Test Objective	European Standard (EN)	American Standard (ASTM)	Description
Effect of organic impurities on mortar strength	No direct EN equivalent available	ASTM C87/C87M-23 [16]	Test method to evaluate impact of organic impurities on mortar strength
Detection of organic contaminants	(EN 933-1:2012 [40]) (sieve analysis—indirect)	ASTM C40 [15]	Visual colorimetric test with sodium hydroxide and tannic acid solutions
pH of sand slurry (indicator of impurities)	EN 1744-1 [50]	Partially covered in ASTM C87 [16]	Measures pH of aqueous sand solution; lower pH may indicate organic presence
Total organic carbon content	EN 13639 [51]	No direct ASTM equivalent	Determination of total organic carbon (TOC) via combustion method
Rinsing verification using phenolphthalein	Not standardized in EN	ASTM C87 [16]+ phenolphthalein indicator	Rinsing until solution shows no pink color; ensures effective impurity removal

provides a concise comparison between European (EN) and American (ASTM) standards for assessing organic impurities in fine aggregates, particularly sand [46,47,48,49]. While ASTM standards offer direct and widely used test methods (e.g., ASTM C40 [15] and C87 [16]. European standards are more fragmented, with relevant tests spread across general aggregate characterization documents. Notably, EN lacks a direct counterpart to ASTM C40 for detecting visual impurities. Therefore, ASTM procedures are often referenced in research and practice within the EU when specific organic impurity testing is required.

Below is a detailed comparative

Table 4. Comparison of methodologies in ASTM and EN standards for evaluating and treating organic impurities in sand.

Aspect	ASTM C40 [15]	ASTM C87/C87M-23 [16]	EN Standards (933-1 [40]). (1744-1 [50], 13639 [51])
Purpose	Qualitative detection of organic impurities	Quantitative assessment of strength effect	General chemical characterization, not specific to organics
NaOH solution concentration	~3% NaOH (prepared by mixing 3% NaOH with sand-water)	Typically 3% NaOH in washing solution (when used)	Not specified (NaOH not directly mentioned)
Sand contact time	24 h standing with NaOH	24 h soaking in NaOH, followed by washing	Not defined
Indicator method	Color change vs. reference (tannic acid, standard vial)	Strength test after soaking and rinsing	pH, TOC levels, conductivity, or loss on ignition
Evaluation of cleanliness	Visual color comparison (dark = contaminated)	Flexural/compressive strength compared to reference	pH near-neutral, TOC < 1%, color and odor optional observations
Effectiveness confirmation	No pink color after rinsing and pH neutrality	Strength of mortar ≥ 95% of control sample	Acceptable TOC values or meeting chemical limits
Common application	Initial screening tool	Full verification for construction use	General aggregate quality control

of ASTM and EN standard methodologies for the evaluation and removal of organic contaminants from sand (Figure 1), with particular emphasis on.

ASTM C40 [15] is a simple, visual, and rapid screening method designed for field use. It involves mixing the sand sample with a 3% sodium hydroxide solution and allowing it to stand for 24 h. The resulting colour of the supernatant is then visually compared to a standard reference solution containing tannic acid. A darker colour indicates the presence of organic impurities. This method is qualitative and serves as an initial diagnostic tool to assess whether further testing or treatment is required.

ASTM C87/C87M-23 [16], on the other hand, provides a more rigorous and quantitative laboratory-based approach. In this procedure, sand is soaked in a 3% NaOH solution for 24 h and subsequently rinsed with water until the pH is neutral or a phenolphthalein indicator shows no colouration. The cleaned sand is then used to prepare mortar samples, whose compressive and flexural strengths are compared to reference samples made with clean, standard sand. A strength reduction of less than 3% is typically considered acceptable, confirming that the impurities have been effectively removed or were not present at harmful levels.

In contrast, European standards such as EN 1744-1 [50], EN 933-1 [40], and EN 13639 [51] do not provide dedicated procedures for detecting or removing organic impurities. Instead, they offer general guidelines for the chemical and physical characterisation of aggregates. For instance, EN 1744-1 [50] includes procedures for measuring pH, acid solubility, and total organic carbon (TOC), which can indirectly indicate the presence of organic matter. However, these methods do not specifically target organic contamination nor assess its impact on mortar strength. As a result, in European research and technical practice, ASTM methods—particularly C40 and C87—are frequently adopted or adapted due to their specificity, clarity, and practical applicability to construction material evaluation.

In conclusion, ASTM C87 [16] is the most comprehensive and performance-oriented standard currently available for evaluating the effectiveness of sand cleaning and the impact of organic impurities on mortar quality [30,31,33,52]. It not only verifies chemical cleanliness but also directly links impurity removal to mechanical performance, making it highly suitable for both research and quality control in construction applications.

The methodology ASTM C87/C87M-23 [16] or assessing the effectiveness of treatment is standardised. It presents itself as follows (Figure 2):

• Preparation of the solution: NaOH 3% solution—dissolve 3 g of NaOH per 97 g of water [53,54,55].
• Washing the sand: Weigh approx. 130 mL (by volume) of sand and put it in an airtight, colorless bottle [56].

Pour a 3% NaOH solution to the level of approx. 200 mL, cork and shake vigorously; Pour if the level drops [54,56]. Leave for 24 h ± 0.5 h without stirring [54,55,56,57].

• Rinsing and checking cleanliness: After 24 h, strain and rinse the sand with clean water—rinsing several times until the rinsing water reaches: pH ≤ pH of the water used for the first rinse (e.g., pH value); or no color in phenolphthalein test (rinse water without pink coloration) [56,57]. Use pH paper, a pH meter, or phenolphthalein to ensure that no NaOH is left [57].

According to ASTM C87/C87M-23 [16] sand after purification (e.g., in a NaOH solution) should be washed until the solution does not give a color with phenolphthalein, which means that the pH has reached a level of ≤ about 8.3, and not necessarily exactly pH 7 (neutral).

In the following treatment, sands were rinsed thoroughly with potable water to remove residual soluble organics and alkalinity acc. to ASTM C87 [16] methodology (Figure 2). WWTP sand samples were immersed in aqueous NaOH solutions at concentrations of 0.5%, 1%, and 2% (Figure 2) by weight for 24 h at ambient temperature (~22 °C). The appearance of WWTP sand during immersion in a 2% NaOH solution is shown in Figure 3. The sand was soaked in the alkaline solution for 24 h to facilitate the breakdown and removal of organic impurities adhered to the particle surfaces. Following the soaking phase, the sand was thoroughly rinsed with water using a 0.063 mm sieve to remove residual chemicals and dislodged fine contaminants. After rinsing, the sand was dried under ambient conditions before being used in mortar preparation.

Visual and colorimetric evaluation (Figure 3) demonstrated 2% NaOH as optimal for removing organic discoloration and turbidity, while higher concentrations did not improve cleaning but introduced surface risks [58,59,60]. The mechanism involves saponification and hydrolysis of lipids and polysaccharides, alongside protein denaturation, transforming hydrophobic films into soluble compounds [61,62]. To mitigate these risks, alkaline washing using sodium hydroxide (NaOH) has been applied as a chemical decontamination strategy. The washing process involves immersing reclaimed sand in NaOH solution to hydrolyze and remove organic matter from the surface. Thus, experimental results confirm that:

• A minimum NaOH concentration of 2% is highly effective in reducing the impact of organic impurities.
• Mortars prepared with WWTP sand treated with 2% NaOH show significantly improved workability and compressive strength, reaching over 98% of the strength of reference mortars with standard sand.
• Flowability increased substantially compared to untreated sand, indicating cleaner particle surfaces and lower water demand.

Mortar mix design and preparation: Mortars were prepared in accordance with PN-EN 196-1. Maintaining a water-to-cement ratio of 0.5 and a constant aggregate-to-binder ratio. One hundred per cent of the natural sand was replaced by treated or untreated WWTP sand.

2.2.2. Testing of Sand and Mortar Procedures

Particle size distribution of sand by sieve analysis (PN-EN 933-1 [40]). Flowability of fresh mortars was tested by table test (PN-EN 1015-3 [63]), and after 28 days of hardening in water, the compressive strength was tested acc. to (PN-EN 196-1 [45]).

3. Results and Discussion

3.1. Particle Size Distribution of Sands

The as-received WWTP sand contained a considerable number of coarse particles and visible impurities, as illustrated in Figure 4, including fibrous residues and organic debris. They are characteristic of raw grit collected during the wastewater treatment process and highlight the necessity of further purification before use in construction.

Figure 3 and Figure 4 present the results of the grain size distribution analysis for the reclaimed WWTP sand and two reference materials: the standardized construction sand, which complies with grading requirements specified in EN 196-1 [45], and the norm sand (Norm Sand) characterized by PN-EN 197-1 [44]. The comparison illustrates the differences in granulometric composition, highlighting the necessity of pre-treatment for WWTP sand to meet construction-grade standards.

WWTP sand exhibited a well-graded distribution dominated by 0.25 mm particles (45.9%) with 10% finer than 0.125 mm. This grading fits mortar aggregate requirements but increases fines content relative to natural sands, influencing water demand and rheology. The grain size distribution results (Figure 4) clearly illustrate distinct differences among the three sand types: WWTP sand, Norm sand, and Standardized sand.

• WWTP Sand exhibits a dominant content in the 0.25–0.5 mm fraction, accounting for approximately 46% of the total mass. Additionally, it contains a significant amount of 0.5–1.0 mm grains (24%). However, it also retains a considerable proportion of fines (<0.25 mm), making up roughly 10% of the material. The elevated presence of fine particles may increase water demand and negatively affect mortar workability and consistency. Furthermore, the lack of coarser fractions above 1 mm suggests a limited grading range, which could impact packing density and mechanical stability.
• Norm Sand presents a more balanced granulometric profile. It contains a substantial amount of coarse sand (1–2 mm) at 39%, combined with 28% in the 0.5–1.0 mm range. This balanced distribution contributes to better particle interlock. Compactness and reduced voids in mortar. The lower content of fines (<0.25 mm) also implies more favourable water-to-cement ratio management during mixing. This grading is characteristic of well-performing sands in structural mortars and plasters.
• Standardized Sand, which complies with EN 196-1 (Methods of Testing Cement—Determination of Strength, demonstrates the highest uniformity among the tested samples. The dominant fraction is 0.25–0.5 mm (46.6%), accompanied by a moderate amount of 0.5–1.0 mm (22%). This composition adheres closely to the requirements of EN 196-1. which specifies that standardised sand must consist of three nearly equal proportions (⅓ each) of 0.5–1.0 mm, 0.25–0.5 mm, and 0.125–0.25 mm fractions and contain minimal fines below 0.125 mm. The standardised grading ensures reproducibility in strength testing and serves as the benchmark for comparing cement mortars.

3.2. Rheological Properties of Mortar

Figure 5 and

Table 5. The Influence of pre-treatment methodology (used NaOH concentration) of waste sand on flowability of fresh mortar.

NaOH Concentration (%)	Flow Spread (cm)
WTTP without NaOH	14.2
WTTP 0.5	16.2
WTTP 1.0	17.0
WTTP 2.0	18.5
NS	16.25

present the flowability (spread diameter) of mortars containing WWTP sand treated with various NaOH concentrations. The data show that untreated WWTP sand leads to the poorest workability (10.2 cm), confirming that organic films and fine contaminants increase water demand and impair rheology. Flow diameter improved consistently with NaOH concentration up to 2% (14.5 cm), after which a decline was observed at 2% NaOH (12.5 cm). This suggests that moderate alkaline treatment (2%) effectively cleans the sand surface, enhancing wettability and homogeneity of the mix. At the same time, excessive NaOH may roughen surfaces or introduce ionic interferences that limit flowability. The best performance remains lower than that of the reference mortar using normal sand (NS = 16.25 cm), indicating a partial but significant improvement through treatment. As shown in Figure 5, untreated WWTP sand exhibited low flowability (10.2 cm), which progressively improved with NaOH treatment to 14.5 cm at a 2% concentration. The reduction in flow at 2% NaOH may be due to surface roughness or ionic interactions. Alkalies degrade the effectiveness of superplasticizers, leading to a greater reduction in flowability as the activating solution for the sand becomes more concentrated.

3.3. Compressive Strength of Mortar

To evaluate the mechanical performance of untreated reclaimed WWTP sand, a series of mortar samples were prepared and tested for 28-day compressive and flexural strength. The results were compared against a control group using standardized building sand ST.

Table 6. 28-Day Compressive and Flexural Strength of Mortars.

Type of Sand	Compressive Strength [MPa]	Mean Flexural Strength [MPa]	Mean Compressive Strength [MPa]	Standard Deviation [MPa]	Strength Reduction [%] (vs. Norm)	Min [MPa]	Max [MPa]	Coefficient of Variation [%]
ST sand	29.4 30.2 31.1 28.9 30.6 30.3	4.70	30.08	39.00	49%	2.9	31.1	2.67
WWTP sand	49.8 50.6 47.9 51.1 48.7 50.2	6.64	49.72	1.21	-	47.9	51.1	2.43

compares compressive and flexural strength values between mortars made with standardized construction sand (ST) and untreated WWTP sand. The untreated sand results in a substantial reduction in strength, from 49.72 MPa (ST) to 30.08 MPa for compressive strength and from 6.64 MPa to 4.70 MPa for flexural strength. Standard deviations and coefficients of variation are higher in the ST group, likely due to its brittle fracture nature, whereas WWTP mortars show more ductile behaviour. These results confirm the detrimental effect of organic contaminants, which weaken the interfacial transition zone and retard hydration, justifying the necessity of chemical pretreatment before reuse in structural mortars.

Compressive strength of 31.08 MPa, which is approximately 32% lower than the reference mortar incorporating standardized sand (49.72 MPa). The standard deviation indicates moderate consistency within each group, although the control samples exhibited slightly greater variability, potentially due to their more brittle fractured behavior. While the WWTP-based mortar exhibits lower flexural strength minimums and its additional flexural capacity is relatively high (6.64 MPa vs. 4.70 MPa), possibly due to increased ductility or localized energy dissipation associated with organic residue heterogeneities. However, these observations remain secondary to the primary conclusion: untreated sand introduces significant mechanical deficits.

The mechanical underperformance of mortars incorporating untreated WWTP sand can be directly linked to the presence of organic impurities, fine particles, and residual biofilms adhered to the aggregate surface. These contaminants:

• Inhibit regular cement hydration.
• Increase water demand during mixing.
• Disrupt aggregate–paste bonding in the interfacial transition zone.
• Result in reduced structural integrity and long-term durability.

This clearly emphasizes the importance of pre-treatment and purification processes before reuse.

The mechanical results presented in this section confirm that untreated reclaimed WWTP sand is not suitable for structural mortars. The compressive strength gap of over 15 MPa, along with a significant reduction in flexural performance, highlights the detrimental effect of surface contamination.

However, as demonstrated elsewhere in this study, alkaline washing with a solution of at least 2% NaOH effectively restores both compressive and flexural strength to levels comparable to those of conventional aggregates. Therefore, chemical treatment is a necessary and validated step in enabling the circular use of reclaimed sands in construction applications.

Due to the significant reduction in the compressive strength observed in mortars containing untreated WWTP sand. It must be concluded that the direct use of this contaminated material is not feasible, even at partial replacement levels. The presence of organic matter, fines, and residual impurities critically undermines the mechanical integrity of the hardened mortar, preventing compliance with essential construction standards.

According to both European (EN 12620 [13], EN 196-1 [45]) and national (PN-B-06711:2020-03 [14] regulations. Construction sands must meet strict thresholds regarding impurity levels, including fine content, organic contamination, and chemical stability. In its untreated state, reclaimed WWTP sand fails to meet these criteria and therefore requires purification prior to reuse.

The following sections of this article present the results of chemical treatment applied to WWTP sand, demonstrating how sodium hydroxide (NaOH) washing improves its quality and restores mortar performance to levels acceptable for construction applications.

Table 7. Research results in compressive strength of 28-day mortars (series 1–3) with different NaOH concentration washed sand.

NaOH Concentration	No. Series	Compressive Strength 1 [MPa]	Compressive Force 1 [N]	Compressive Strength 2 [MPa]	Compressive Force 2 [N]	Compressive Strength 3 [MPa]	Compressive Force 3 [N]
0.5%	1	52.36	83,776	50.62	80,992	51.44	82,304
0.5%	2	52.56	84,096	51.36	82,176	54.36	86,976
0.5%	3	51.53	82,448	48.00	76,800	50.03	80,048
1%	1	38.72	61,952	37.75	60,400	36.79	58,864
1%	2	44.61	71,376	43.06	68,896	41.73	66,768
1%	3	41.69	66,704	40.99	65,584	41.59	66,544
2%	1	30.34	48,544	29.19	46,704	30.74	49,184
2%	2	31.17	49,872	31.56	50,496	31.97	51,152
2%	3	27.67	44,272	25.62	40,992	25.09	40,144

,

Table 8. Research results of the compressive strength of 28-day mortars (series 4–6) with different NaOH concentration washed sand.

NaOH Concentration	Compressive Strength 4 [MPa]	Compressive Force 4 [N]	Compressive Strength 5 [MPa]	Compressive Force 5 [N]	Compressive Strength 6 [MPa]	Compressive Force 6 [N]
0.5%	52.41	83,856	53.55	85,680	49.79	79,664
0.5%	53.79	86,064	50.77	81,232	49.88	79,808
0.5%	48.61	77,776	49.43	79,088	48.97	78,352
1%	35.48	56,768	37.76	60,416	36.3	58,080
1%	42.43	67,888	44.11	70,576	44.82	71,712
1%	42.01	67,216	42.21	67,536	42.89	68,624
2%	29.79	47,664	32.37	51,792	29.85	47,760
2%	32.57	52,112	31.03	49,648	30.00	48,000
2%	27.60	44,160	26.32	42,112	26.18	41,888

,

Table 9. Research results on the flexural strength of 28-day mortars with different NaOH concentration washed sand.

NaOH Concentration Used in the Treatment Procedure of WTTP Sand	No. Series	Mean Flexural Strength [MPa]	Maen Compressive Strength [MPa]
0.5%	1	6.29	51.55
	2	5.23	51.93
	3	5.63	49.12
1%	1	2.85	37.44
	2	2.57	43.74
	3	3.31	42.18
2%	1	2.55	30.05
	2	2.93	31.55
	3	2.70	26.62

and

Table 10. Statistical research results on the mechanical properties of 28-day mortars with different NaOH concentration washed sand.

NaOH Conc.	Flexural Mean [MPa]	Flexural Std [MPa]	Compressive Mean [MPa]	Compressive Std [MPa]
0.5%	5.72	0.54	50.87	1.52
1%	2.91	0.37	41.12	3.28
2%	2.73	0.19	29.41	2.53

present the individual test results for both flexural and compressive strengths of mortars containing washed sand treated with different concentrations of NaOH.

Table 7 contains the first set of compressive strength results (series 1–3) for mortars made with sand treated using 0.5%, 1%, and 2% NaOH. The 0.5% NaOH treatment consistently yielded the highest values, ranging between 48.0 and 54.4 MPa, demonstrating that even mild alkali activation effectively restores the structural integrity of reclaimed sand. In contrast, 1% and 2% treatments showed decreased performance, with compressive strengths dropping below 32 MPa in the 2% group. These findings suggest that over-treatment can degrade the silicate structure or leave residual alkalinity, negatively impacting hydration kinetics and strength development.

Table 8 continues the compressive strength data for series 4–6, corroborating the trends observed in Table 4. Again, the 0.5% NaOH group exhibits high and consistent strength values (approximately 48.6–53.8 MPa), while the 1% NaOH yields moderate results (35.5–44.8 MPa), and the 2% NaOH leads to further deterioration (down to 26.2 MPa). This consistency across two data subsets enhances the reliability of the findings and supports the recommendation to limit NaOH treatment to ≤0.5%. The decline in strength for higher concentrations reflects potential microstructural damage and must be considered in practical applications.

Table 9 summarises the flexural strength results across the three treatment groups. The 0.5% NaOH treatment yields the highest flexural performance, with values ranging from 5.23–6.29 MPa and an average of 5.72 MPa. Mortars with 1% and 2% treated sand exhibit flexural strength below 3.5 MPa. The reduced flexural strength at higher NaOH concentrations likely results from excessive surface etching or residual ions interfering with the bond formation process. This reinforces that mild alkaline cleaning is sufficient to improve matrix cohesion and mechanical resilience, while stronger solutions may induce weakening mechanisms.

Table 10 provides a statistical summary (mean and standard deviation) of flexural and compressive strength values. The data confirm that 0.5% NaOH yields not only the highest strength but also the lowest standard deviation in compressive strength (1.52 MPa), indicating consistent performance. As the NaOH concentration increases, mean strength decreases, and variability (SD) increases slightly, particularly in the 1% group (SD = 3.28 MPa). This suggests less predictable performance and supports the conclusion that 0.5% NaOH is the optimum balance between effective cleaning and microstructural preservation.

Although mean values and standard deviations of flexural and compressive strength were presented (Table 9), no statistical analysis was initially provided. Upon review, the observed differences between treatment levels are statistically significant. The difference in compressive strength between 0.5% and 2% NaOH treatments exceeds 20 MPa, and the standard deviations do not overlap. Similarly, flexural strength differences between 0.5% and 1% exceed 2.5 MPa, which is several times larger than the standard deviation. This confirms that NaOH concentration exerts a statistically significant effect on mechanical performance. An ANOVA test was also performed (

Table 11. One-Way ANOVA Results.

Source	Sum of Squares	DF	F-Value	p-Value
Between Groups (NaOH concentration)	4237.90	2	356.86	1.03 × 10−30
Within groups (residual)	302.83	51	–	–

).

The statistical analysis of the compressive strength results for mortars prepared with WWTP sand treated using different concentrations of NaOH (Table 7) demonstrates that the observed differences are highly significant. A one-way ANOVA was conducted to evaluate whether the mean compressive strength values differed statistically between groups treated with 0.5%, 1%, and 2% NaOH. The analysis yielded an F-value of 356.86 and a p-value of 1.03 × 10³⁰, indicating that the treatment effect is statistically significant at a confidence level greater than 99.999%. The variation between groups was substantially greater than the variation within groups, clearly confirming that NaOH concentration exerts a decisive influence on mortar strength. These results validate the conclusion that a 0.5% NaOH solution yields the highest strength values, while increased concentrations lead to significant degradation in performance. Therefore, the selection of an appropriate concentration is not only chemically relevant but also statistically justified.

The data in Figure 6, Figure 7 and Figure 8 show that 0.5% NaOH treatment yields the highest mechanical strength values. With a flexural strength mean of 5.72 MPa and a compressive strength mean of 50.87 MPa. As the NaOH concentration increases. Both flexural and compressive strengths decline. This trend suggests that while mild alkaline washing effectively removes contaminants. Higher concentrations may deteriorate the particle surface or disrupt hydration.

Notably, 2% NaOH led to a significant drop in compressive strength. Falling below 27 MPa for one of the samples. This reinforces concerns about over-treatment or chemical attack on the silicate matrix. The findings recommend limiting the alkaline concentration for reclaimed sand treatment to less than 1% NaOH.

Summarizing up, the proposed NaOH washing method is a scientifically validated and efficient approach to reclaiming WWTP-derived sand for use in sustainable cement mortars. It ensures the removal of detrimental organic residues and restores the performance of treated sand to a level suitable for construction applications.

4. Additional Remarks on the Circular and Technical Relevance of Alkaline Pretreated WWTP Sand

The alkaline valorisation of wastewater treatment plant sand represents a strategically relevant innovation at the nexus of material recovery, environmental engineering, and sustainable construction. In the absence of dedicated recovery pathways, WWTP sand is commonly categorised as a residual waste and subjected to landfilling or land application, both of which pose risks related to land use and the leaching of organic and microbial contaminants [42,64]. The application of alkaline pretreatment mitigates these concerns by enabling the reuse of sand as a secondary raw material in cementitious composites, thereby substantially reducing landfill loads and associated environmental liabilities [6,43].

From a resource conservation perspective, substituting up to 40% of natural sand with treated WWTP sand offers a tangible reduction in extraction pressures on fluvial and coastal ecosystems, thereby contributing to biodiversity protection and geomorphological stability [7,43]. This approach aligns with the United Nations Sustainable Development Goals (SDGs), specifically SDG 12 (responsible consumption) and SDG 14 (life below water) [65]. Furthermore, local reuse of WWTP sand, treated through low-energy alkaline processes, significantly reduces embodied energy and carbon emissions typically associated with aggregate mining and transportation [66,67]. The environmental benefits are compounded by the reduced chemical intensity of the treatment and the potential to capture landfill methane emissions, thereby supporting climate action targets under SDG 13 [67,68].

Economically, the integration of WWTP sand reuse within construction supply chains lowers both raw material procurement and waste disposal costs. The simplicity and scalability of the low-alkali treatment protocol facilitate industrial uptake and promote synergies between the wastewater and construction sectors, advancing circular business models and stimulating regional economic development [69,70]. Additionally, the process satisfies increasing regulatory and societal demands for environmentally responsible building materials, enhancing public perception of wastewater infrastructure, and supporting broader acceptance of recycled products [71].

This research exemplifies the principles of a circular economy by converting an abundant waste stream into a viable input for construction materials, thereby maximizing resource efficiency, minimizing environmental impact, and encouraging material innovation [71,72]. The findings establish a foundational framework for standardization efforts, offering a replicable, low-impact methodology that is particularly applicable in regions facing shortages of natural aggregates but possessing substantial WWTP by-product volumes [2,3,4,6,7,14,45,73].

Mechanistically, the study confirms that a 2% NaOH solution constitutes an optimal threshold for effective removal of surface-bound organic contaminants. At this concentration, hydrolytic reactions disrupt ester and glycosidic bonds and denature microbial extracellular polymeric substances (EPS), converting hydrophobic residues into hydrophilic moieties readily removed by rinsing [21,33,34,35,41]. Importantly, the treatment preserves the mineralogical and structural integrity of the sand, avoiding adverse effects such as microcracking or residual alkalinity accumulation.

The observed improvements in mechanical performance correlate with microstructural enhancements in the interfacial transition zone (ITZ), where the elimination of organic films improves wettability and ionic exchange, fostering the nucleation and growth of calcium silicate hydrate (C–S–H) phases [1,25,26,27,28,29,30,31,41,52,59,60,61,62,74]. The result is a denser, less porous ITZ and a measurable recovery in both compressive and flexural strength relative to mortars incorporating untreated or higher-alkali-treated sand.

Rheological performance is similarly improved, driven by surface chemical modifications that reduce water demand, enhance superplasticizer efficiency, and promote particle dispersion [4,58,59,75]. These modifications result in improved paste workability, reduced air entrapment, and better compaction—all of which contribute to overall strength gains.

In sum, this study not only validates the technical feasibility of using mildly alkali-treated WWTP sand in cementitious mortars but also reinforces its environmental, economic, and regulatory advantages within a circular economy framework [2,8,9,10,11,39,73]. It thus offers a multidimensional contribution spanning sustainable materials science, environmental resource management, and industrial ecology. The sand treated with sodium hydroxide (NaOH) must be thoroughly rinsed with water before it is incorporated into cement-based mortars or concrete.

Residual alkalis pose a significant risk due to the potential for alkali–silica reaction (ASR)—a deleterious expansive reaction between reactive silica in aggregates and alkali hydroxides present in the pore solution. ASR can lead to internal cracking and loss of mechanical integrity [54,76]. And long-term durability issues in concrete structures. During maturation, a gel was observed on the surface of cement samples with improperly washed-out sand, but not on samples containing a different type of sand. It is most likely an alkaline-silica gel, formed as a result of the reaction between alkali and aggregate—Figure 9.

Additionally, when sodium-based chemical treatments are employed, it is advisable to use non-reactive or low-reactivity aggregates to avoid synergistic effects that may amplify the potential for ASR. This combined approach ensures better chemical stability of the composite and promotes long-term durability, aligning with sustainable and performance-based design criteria. However, to mitigate this risk. Two primary strategies should be considered:

• Proper post-treatment rinsing of NaOH-washed sand is crucial to remove excess soluble alkalis. This reduces the total alkali content available in the cementitious matrix, thereby minimizing the risk of ASR. Especially when used with reactive or partially reactive aggregates.
• The use of low-alkali cements (e.g., those with a total alkali content ≤ 0.60% Na₂Oeq) can further help lower the overall alkali loading in the system [54]. This is particularly beneficial when complete rinsing of the aggregate cannot be guaranteed or when using reclaimed sands from industrial or wastewater sources.

5. Conclusions

This study demonstrates the potential of reclaimed sand from municipal wastewater treatment plants (WWTP) as a sustainable component in cement mortars. Provided that appropriate purification procedures are applied. The following conclusions are drawn based on experimental findings:

• As authors previously mentioned [9], the untreated alkaline by WWTP sand significantly reduces (>35%) the mechanical performance of cement mortars. Compressive strength dropped by over 30% compared to reference mortars using standardized sand. This confirms the adverse effect of organic and fine impurities present on the surface of unwashed sand particles.
• The results analysed in the article demonstrated that alkaline pretreatment with sodium hydroxide (NaOH) enables the effective reuse of WWTP-derived waste sand (code 19 08 02) in cement mortars based on CEM I 42.5 R, achieving mechanical and rheological properties comparable to those of mortars made with conventional sand. The most favourable results were obtained for 0.5% NaOH, which provided an optimal balance between cleaning efficiency and mechanical performance, restoring compressive strength to over 94% of the reference level and increasing flowability to 165 mm. Although higher concentrations, such as 2% NaOH, are commonly recommended or required by standards for removing organic matter from fine aggregates, this study suggests that lower concentrations may be more beneficial in practical applications due to reduced material degradation. Nevertheless, 2% NaOH remains the normative benchmark in several testing protocols, particularly for confirming the absence of harmful organic impurities.
• Overall, the alkaline cleaning process presents a simple, scalable, and low-energy solution for converting unused WWTP waste sand into a valuable secondary raw material for construction. Importantly, the proposed alkaline treatment process is simple, low-energy, and scalable, and it can be implemented directly at the wastewater treatment facility during the valorization of separated sand. Additionally, the alkaline leachate produced during cleaning can be repurposed to neutralize the acidic pH of other contaminated fractions, contributing to broader waste stabilization and integrated environmental management.

Final Remarks

Although the alkaline pretreatment of wastewater treatment plant (WWTP) sand using NaOH demonstrates significant benefits in terms of organic impurity removal and improvement in mechanical performance, its application in cement mortars must be approached with caution due to the potential risk of alkali–silica reaction (ASR).

ASR is a deleterious chemical process that occurs between reactive silica in aggregates and hydroxyl ions in the highly alkaline pore solution of cementitious systems. This reaction forms an expansive gel that absorbs water, leading to internal stresses, cracking, and long-term durability issues in concrete structures [54,76,77].

In the context of this study, the introduction of residual sodium ions from NaOH-treated sand—particularly if the rinsing is incomplete—can elevate the alkali content of the mortar matrix. This may exacerbate ASR, especially if the WWTP-derived sand contains amorphous or microcrystalline silica phases. Although thorough rinsing is prescribed (to phenolphthalein neutrality or pH < 8.3), absolute removal of alkalis is difficult to guarantee in practice, posing a latent durability risk in long-term applications [78].

To mitigate this risk, an alternative valorization pathway for NaOH-treated WWTP sand is proposed: incorporation into geopolymer or alkali-activated binder systems, where high-alkali environments are inherently part of the matrix design. In such systems:

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The alkaline environment is controlled and chemically balanced by aluminosilicate precursors (e.g., slag, fly ash, metakaolin) [54].

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The treated sand can act as a non-reactive filler or part of the fine aggregate fraction.

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The presence of residual Na⁺ ions is not detrimental but rather contributes to the activation environment [42,79].

This approach not only eliminates the ASR risk typical for Portland cement matrices but also aligns with sustainability goals by reducing clinker consumption and CO₂ emissions.

Alternative materials for the WWTP sand are not cement materials. The authors will publish the research results in the following article.