Sustainable Valorization of Spent Garnet Wastes in Construction Eco-Materials: Validation Stage of Performance Assessment

Abstract

Spent garnet (SG) wastes are generated in significant quantities by several industrial activities, including abrasive waterjet cutting (AWJ), abrasive blasting, and filtration and powdered media applications. These wastes represent a promising secondary raw material for the production of sustainable construction materials, particularly green mortars and concretes, through their partial replacement of natural sand in cementitious systems. Such applications are relevant to both hydraulically setting inorganic binders (cement-based materials) and alkali-activated cementitious materials (AACMs). The valorization of SG wastes offers multiple benefits, notably a dual environmental advantage: reducing the consumption of natural aggregates and diverting industrial waste from disposal by integrating it into a new life cycle as a value-added by-product. Additional potential advantages include reduced production costs and possible improvements in the overall performance of mortars and concretes. Despite these benefits, the use of SG as an aggregate replacement remains insufficiently explored, with existing studies providing only preliminary and fragmented evidence of its feasibility. This paper presents an overview of a comprehensive four-year research program investigating SG wastes derived from single-cycle AWJ processes and their incorporation into conventional mortars as partial fine aggregate replacement in cement-based construction composites. The validation stage of the performance assessment expands the range of SG sources by including new sampling from the original suppliers, enabling verification of the repeatability and reproducibility of earlier findings. A broad set of physical, mechanical, and durability properties—particularly resistance to freeze–thaw cycles—is evaluated to achieve a robust and comprehensive material characterization. These results are further correlated with chemical and microstructural analyses, providing critical insights to support the technological transfer of SG-based construction materials to industrial applications with reduced carbon footprint.

1. Introduction

Abrasive Water-Jet (AWJ) machining has become a cornerstone in precision cutting of metals, composites, and natural stone due to its capacity for high material removal rates with minimal thermal damage and edged distortion. Central to the process is the entrainment of hard abrasive particles—most commonly garnets, but also including other mineral particulates—into a pressurized water jet stream to facilitate cutting action. The consumption of abrasive material during AWJ operations is significant, with reports estimating abrasive costs as high as 60% of total operational expenses in hard material cutting applications [1,2,3,4,5]. Consequently, large volumes of spent abrasive waste—typically a mix of fragmented abrasive grains, fines, and waterborne residues—accumulate as industrial waste. The resulting garnet grains and other mineral fractions (ceramics, composites, stone, marble and glass, etc. depending on the processed material) are generally collected in the vicinity of the cutting device as a slurry, namely a mixture of water and fragmented, degraded garnet particles with a broad range of sizes. Garnet slurries are periodically extracted at irregular intervals and, upon natural drying, accumulate into waste piles, further generating landfills of spent garnets (SG). They generate environmental pollution and associated waste management costs, but can also represent a potential secondary resource for the construction industry [6,7].

The term “Garnet” typically refers to a group of complex silicate minerals characterized by diverse chemical compositions but similar crystal lattice structures [8]. These silicate minerals adhere to a general chemical formula of A₃B₂(SiO₄)₃, where A and B represent divalent and trivalent metal cations, respectively, such as calcium, magnesium, aluminum, iron, or manganese [9,10]. This compositional variability gives rise to a diverse range of garnet species, each exhibiting distinct physical and optical properties. Although garnets are most commonly red or reddish-pink, they can also occur in shades of green, orange, yellow, or black [10,11,12,13,14]. The distinctive characteristics of garnets—such as high grain hardness, sharp angular edges, and specific particle size distributions—make them highly suitable as an abrasive medium in abrasive waterjet (AWJ) cutting processes. These properties ensure a consistent abrasive flow, prevent blockages in the focusing tube, and ultimately enhance cutting quality, speed, and precision [15].

The leading Garnet-producing countries are Australia, China, India, and the United States (U.S.), with annual outputs of approximately 150,000, 470,000, 700,000, and 54,000 metric tons, respectively [16]. According to Mineral Commodity Summaries 2024 [17], in addition to those in the United States, major garnet deposits exist in Australia, China, Czech Republic, India, Pakistan, and South Africa (for foreign and domestic markets). Deposits in Russia and Turkey also have been mined, primarily for internal markets, but production data were not reported. Additional garnet resources are in Canada, Chile, Spain, Thailand, and Ukraine, but with small mining operations. According to [17], the 2024 U.S. garnet production of 80,000 metric tons was estimated to be 8% of total global garnet production (980,000 metric tons), recording an increase by 12% compared with production in 2023. The 2024 estimated US domestic amount of refined garnet sold or used decreased by 15% compared with 2023, but this represents a particular exception to the overall increasing trend.

Garnets, mostly from blast cleaning and water-jet-assisted cutting, can be recycled multiple times without significant degradation of their quality. But this recycling operations rarely actually happens [18]. For instance, in 2013, Malaysia’s shipbuilding industry imported over 2000 tons of garnet, most of which ultimately ended up in landfills. Disposing of such waste through landfilling is costly—due to transportation and tipping fees—and environmentally problematic, potentially leading to biodiversity loss, groundwater pollution, and other issues [19,20].

In recent years, researchers have explored the use of spent garnet (SG) as a substitute for fine aggregate in mortar and concrete production [20,21,22]. Global concrete consumption reaches approximately 25 billion tons annually, posing significant environmental challenges, including the depletion of natural resources and the high environmental footprint of cement production [23]. The concrete industry also consumes billions of tons of aggregates each year to meet growing demand. Currently, the majority of mineral additions (e.g., slag, fly ash, limestone filler), which are by-products or waste from industrial processes, are used to reduce cement consumption, as a potential substitute for it, while also aiming to improve certain physical–mechanical properties. The possibility of using mineral additions—whether inert or with low hydraulic or pozzolanic character—as aggregate substitutes has not received sufficient attention, even though aggregate constitutes approximately 70% of the total mass of mortar or concrete and represents a depletable natural resource sourced from river beds (pit aggregate) and natural quarries (quarry aggregate) [20,24]. Intensive aggregate extraction disrupts river ecosystems by lowering water tables, increasing salinity, accelerating soil erosion, and causing broader environmental damage, impacting long-term ecological balance and quality of life, particularly for communities near river systems [20,25,26,27]. In this context, it appears critical to investigate alternative materials, especially industrial waste products, as substitutes for conventional aggregates. Consequently, Garnet sand wastes, (SGs), resulting from material processing via AWJ (Abrasive Water Jet cutting) technology, are seen as a potential substitute for aggregate (fine fraction, sand) as a mineral addition in mortar and concrete compositions [20,21,22]. Waste garnet generated from abrasive water jet (AWJ) cutting is typically collected as slurry and stored in containers that allow excess water to evaporate. In practice, these deposits are often maintained without specific protective measures, which may result in contamination by organic matter, fine dust, or clays, as observed for some SG sources investigated in this study. Although waste garnet is not inherently hazardous to the natural environment, inadequate storage can alter its physical properties and affect its suitability for reuse. For effective valorization as a fine aggregate in cement-based materials, storage under dry, covered, and ventilated conditions, along with moisture control and determination of the saturated surface dry (SSD) state for accurate water correction, is recommended.

Existing studies have evaluated the SG suitability in conventional cement-based composites [24,28,29], concrete mixes with diverse mineral additions [30], high-strength concretes [31,32] including with high temperature exposure [33,34], lightweight composites [27,34] and mortar mixes [35,36]. Baeră et al. also considered cement-based mortars as possible construction materials for SG waste integration [6,7,12,20,37,38,39]. Alkali-activated cementitious materials (AACMs), such as geopolymers mixes developed by partial aggregate substitution by the means of garnet wastes, were studied by Muttashar et al. [25,40,41,42], Lăzărescu et al. [43], Danish et al. [44,45] and others [46,47,48]. The effect of spent garnet as a replacement for natural sand in 3D printed mortar was also investigated [49,50]. At the same time, garnet tailings were also considered as possible additions in concrete [51], mainly based on their latent hydraulic potential, for binding system improvement. The standard experimental approach typically involves developing a reference mortar or concrete mix, followed by testing replacement levels of 25%, 50%, 75%, and even 100% of the fine aggregate with spent garnet materials, generally derived from AWJ or blasting processes, followed by comparative evaluation in terms of physical, mechanical and durability performance of the developed composites. These studies confirm the potential of spent garnet as a viable substitute, providing valuable supplementary data and serving as a foundation for further research. Nevertheless, the majority of these investigations remain preliminary, often limited to mechanical strength properties, and fail to systematically address a broader range of influencing factors, including different waste sources and various sampling periods, curing regimes, mixture proportions, and other critical parameters.

The present study provides an overview of a four-year research program investigating spent garnet (SG) waste from single-cycle abrasive waterjet (AWJ) cutting processes and its incorporation into conventional mortars. It integrates key experimental procedures, data analysis, and targeted validation aspects. Accordingly, a structured validation procedure is proposed, based on clear criteria of repeatability, reliability, and performance consistency, and tailored to the experimental and theoretical activities associated with the use of SG as a partial substitute for natural aggregate in cementitious compositions.

The primary objective is to establish a coherent and robust experimental and methodological framework for validating the incorporation of mineral waste-derived additions, such as spent garnet (SG), as partial substitutes in cementitious materials (mortars and concretes). This validation is achieved through a systematic assessment of the physical, mechanical, and durability properties in both fresh and hardened states.

The proposed validation protocol establishes a robust and reproducible framework for defining the applicability domains of spent garnet (SG)-based cementitious materials. To achieve this objective, an extensive experimental program was conducted, encompassing evaluations in both fresh and hardened states. This included determinations of physical and mechanical properties (density, workability, water absorption, flexural and compressive strength at various ages), durability performance (resistance to freeze–thaw cycles), and microstructural characteristics (via X-ray diffraction (XRD) and scanning electron microscopy (SEM)). The program aimed to validate the potential of spent garnet (SG) waste derived from abrasive waterjet (AWJ) processes as a partial aggregate substitute in sustainable mortar production. Ultimately, it facilitates confident technological transfer to industrial applications, supporting the development of low-carbon concrete and eco-friendly precast construction products.

2. Synthetic Assessment of Single-Cycle AWJ Romanian Spent Garnet Waste Utilization as Sand Replacement in Cementitious Materials

The investigation into the incorporation of garnet waste (SG) into cementitious materials forms part of the research project PN 23 35 04 01 within the Nucleu Programme of the National Research, Development and Innovation Plan 2022–2027 (project acronym: ECODIGICONS), grounded in the valorization of industrial by-products and residues through their innovative integration into construction materials, in alignment with European Commission strategies [5,52].

Preliminary studies were focused on determining de basic compatibility of the SG addition to the cementitious matrix as partial replacement of the natural sand and evaluated various substitution levels ranging from 10% to 50% [37,38,39]. Analysis of these initial results and discussions with the industry representatives indicated that low substitution rates (below 25%) are economically unjustified. While the 10% replacement level offers limited practical significance from an economic perspective, it served as a valuable reference point for elucidating performance trends across incremental substitution rates [37,38,39]. In contrast, medium substitution levels (30–50%) exhibited satisfactory performance in both fresh and hardened states, with mechanical strengths comparable and even superior to the developed reference mixes, warranting further investigation [7]. The initial sampling source was extended to several other sources, and different sampling were also performed from the same source, according to the study dynamics [7,37,38,39]. Accordingly, the present validation study focuses on these medium-level substitutions (30% and 50% relative to the reference).

The experimental procedure began with the preparation of two reference mortar mixes (R1 and R2), formulated using standard raw materials: natural sand (S, 0/4 fraction), cement (C), and water (W), with a predefined S/C ratio of 3. The primary distinction between R1 and R2 was the water content, which resulted in different water-to-cement (W/C) ratios, 0.61 and 0.56, respectively [37,38,39].

Table 1. Mix proportions of the considered mortar mixtures: References and SG mixtures [39].

Components	C	S 0/4	SG	W/C	S/C
Mixtures
R1	1	3	-	-	0.61
R2	1	3	-	-	0.56
SG, 25%	1	2.25	0.75	-	0.56
SG, 50%	1	1.50	1.50	-	0.56
SG, 10%	1	2.70	0.30	-	0.56
SG, 20%	1	2.40	0.60	-	0.56
SG 30%	1	2.10	0.90	-	0.56

presents the mix proportions of all mixtures evaluated in the preliminary study (expressed relative to cement content, C = 1.0), including the reference mortars (R1 and R2) and the spent garnet (SG)-modified mortars. Subsequent SG-modified mixes were developed by progressively replacing the natural sand (S) with spent garnet (SG) on a mass basis, in accordance with substitution levels reported in the literature for the valorization of garnet waste in mortar and concrete production [25,28,31,32,33,34,35,36,40,41,42]. Initial replacements targeted 25% and 50%, with the intention of extending to 75% and full (100%) substitution [37]. Unexpected drying-related issues were observed at the 50% substitution level (SG 50%), leading to difficulties during demolding from metallic molds. Consequently, the substitution strategy was adjusted to lower increments, starting at 10% and progressing in 10% steps (i.e., 20% and 30%), while maintaining the same W/C ratio, reduced from 0.61 (R1) to 0.56 (R2) (Table 1). This modification ensured comparable drying behavior and ease of demolding across the revised series.

Exact annual consumption figures for garnet abrasive in individual AWJ cutting facilities in the Romania, specifically Wester region, are not publicly documented. However, based on typical European industry benchmarks, small-to-medium size waterjet shops (1–3 cutting heads, 1–2 shifts) generally use 15–40 tons of garnet per year per machine, while larger specialized subcontractors may consume 100–300 tons or more annually, depending on operational intensity and workpiece complexity [53].

Evaluation of fresh-state properties revealed enhanced cohesiveness with increasing SG content, while maintaining satisfactory workability and improved flowability. These characteristics suggested a potential reduction in water demand; however, water content was kept constant across all mixes to preserve the W/C ratio, ensuring valid comparisons of mechanical performance both against the references and among the SG-modified mortars [7,37,38,39].

The viability of the proposed substitution is confirmed if the SG material is inert and safe from the perspective of its chemical composition. The technical data sheet for virgin garnet [54] generating the SG waste for the study confirms its ecological profile in compliance with Regulation (EC) No 1272/2008 [55] and Regulation (EC) No 1907/2006 [56]. Given its origin as naturally mined almandine ore processed solely through physical methods to produce a high-quality abrasive, the material presents no harmful environmental impact. Recent investigations into the valorization of garnet waste in concrete and mortar applications have demonstrated that virgin garnet consistently complies with regulatory thresholds for heavy metal content, specifically arsenic (As), barium (Ba), cadmium (Cd), total chromium (Cr), lead (Pb), selenium (Se), silver (Ag), zinc (Zn), and copper (Cu) [20,21,22,27,30,33,40,41,42,43]. Leaching tests [49] for heavy metals (Cu, Zn, Pb, Cd, Ni, Fe, Hg, Cr) showed concentrations below detection limits. XRD analysis indicates that iron—present in the major garnet components (ilmenite, almandine, and andradite)—is the element most likely to be released. Its concentration remaining below the detection limit further demonstrates the stability of spent garnet (SG). To conclude, in compliance with EN 1744-3 [57], spent garnet (SG) additions demonstrate satisfactory leaching behavior and can thus be effectively utilized as a fine aggregate replacement for natural sand in concrete and mortar formulations. However, the heavy metal profile of spent garnet (SG) is predominantly influenced by the substrate material processed during abrasive waterjet (AWJ) cutting. Consequently, a thorough chemical characterization of the resulting SG waste constitutes a critical parameter that must be rigorously assessed and verified to lie within acceptable limits before its incorporation as a partial fine-aggregate replacement in mortar or concrete formulations.

Preliminary Romanian investigations within project PN 23 35 04 01 into mortars incorporating spent garnet (SG) derived from single-cycle abrasive waterjet (AWJ) processes as a partial replacement for natural fine aggregate (0/4 sand), have yielded promising results. The addition exhibits good compatibility with the cementitious matrix and contributes to enhanced overall mechanical performance. In summary, the valorization of single-cycle AWJ-derived spent garnet in mortar and concrete formulations appears feasible, with partial substitution levels ranging from 10% to 50% of the sand content demonstrating viable outcomes.

3. Validation Methodology and Performance Criteria for the Incorporation of Spent Garnet as Aggregate Substitute in Cement-Based Mortars

Previously mentioned international studies have demonstrated the potential of spent garnet waste as a partial aggregate substitute in cementitious or geopolymer compositions. However, these studies do not provide procedural validation and behavioral performance patterns for critical properties of the proposed mineral additions and compositions (achieved via multiple samplings from the same sources at different times and targeted experimental evaluation). Also, they are not approaching a customized experimental validation targeting a specific application domain (like repair and finishing mortars, etc.), offering scattered preliminary conclusions in the topic but far from a practical technological transfer.

Within project PN 23 35 04 01, preliminary tests and comparative evaluations of fresh- and hardened-state performance identified the 30% and 50% spent garnet (SG) compositions as prototype SG mortars. Subsequently, a structured Validation Procedure was developed for preliminary conclusions regarding the valorization of mineral additions as partial substitutes for natural aggregate in cementitious compositions. The primary objective is to provide a robust experimental, methodological, and performance-criteria framework that validates the use of mineral waste-derived additions (e.g., spent garnet—SG) as partial replacements in cementitious materials (mortars and concretes). This is accomplished through comprehensive assessment of physical, chemical, microstructural, mechanical, and durability properties in both fresh and hardened states, thereby facilitating the delineation of specific application domains and supporting technological transfer to industrial practice.

The proposed general validation criteria:

(a)

Repeatability: Consistent results across successive SG batches collected at different times, from the same source and different sources.

(b)

Reliability: Values confirmed by standardized methods (EN, ASTM) and comparison with reference (cement-sand control) mixes.

(c)

Performance Pattern: Clear trends in mechanical and durability properties relative to substitution level and waste characteristics.

(d)

Robustness: Correlation between experimental data and theoretical modeling to rule out uncontrolled variations.

The proposed Validation Procedure (VP) has the character of a preliminary validation and is open to necessary improvements (modifications and additions). The evaluation criteria were established through consultations with academic researchers as well as technologists and industrial engineers, in order to ensure methodological robustness and relevance for practical industrial implementation and it currently includes the following stages:

(A) Extended Compatibility Assessment:

• Objective: Verify SG applicability from multiple sources (e.g., SG A, SG, B) as partial sand replacement.
• Method: Characterize the raw SG material via relevant parameters (chemical composition, Particle Size Distribution, PSD); prepare standardized mortars and evaluate key parameters (mechanical strength at relevant ages, density).
• Criterion: Values comparable to reference (approx. ±20% variability).

(B) Core Validation:

• Objective: Test repeatability (robustness) with same-source samples collected at different times (e.g., SG A-1, SG A-2, SG A-3, several months apart).
• Method: Standardized mortars evaluated via compressive and flexural strength at relevant ages: 7, 28, 112 days.
• Criterion: Similar performance trends (stable pattern), even if absolute values show minor batch-to-batch variation.

(C) Durability Assessment:

• Objective: Assess freeze–thaw resistance for relevant SG types
• Method: Exposure to 50 freeze–thaw cycles followed by post-exposure testing (mass loss, residual strength), adapted from SR 3518:2009 [58] measure post-exposure mass loss/residual strength.
• Criterion: Residual strength > 50% of control; no major degradation or significant mass loss.

Validation is deemed successful if:

• Results are repeatable across different batches and sources of the same addition.
• Mechanical and durability properties show a predictable pattern comparable to references.
• Experimental data confirm preliminary conclusions and support industrial-scale application recommendations.

The proposed validation methodology was applied to single-cycle AWJ spent garnet waste from Timisoara’s local industry (western Romania). The study incorporated SG material from two distinct suppliers (sources A and B), with four separate sampling periods considered for source A and one for source B.

4. Materials and Methods

4.1. Materials

The mortar reference (R) and the SG substitution mortars were produced with locally available raw materials:

-

Portland Cement, CEM II/A-LL 42.5 R (C), provided by a local supplier;

-

Natural sand (S 0/4), granular class 0/4 (Figure 1a), provided by a local supplier;

-

Spent Garnet waste (SG), sampled from by two distinct local sources A and B, generating the SG A and SG B waste (Figure 1b–f);

-

Water (tap water);

-

Free additives mixtures were produced for the study.

The current study considers spent garnet (SG) waste sourced from two local suppliers in western Romania. Source A is the National R&D Institute for Welding and Material Testing (ISIM Timisoara), which generates single cycle AWJ SG waste for research purposes [5] and industry-related requirements. The A source provided samples SG A-1 (collected in April 2022), SG A-2 (collected in August 2023), SG A-3 (collected in December 2023), SG A-4 (collected in April 2025). The considered intervals among sampling, exceeding several months and even one year was deemed relevant for assessing the long-term consistency and stability of the material’s properties and composition. The B source is a local company specialized in AWJ cutting operations for a large variety of materials (metal, ceramic and glass, plastic, etc.), providing several samples as well, but only one is currently considered (SG B-1, sampled mid 2023). Figure 1 presents the aspect of natural sand (S 0/4) and of the SG wastes generated from the tow sources, A and B.

The experimental program, structured to address the three stages of the proposed Validation Procedure, is outlined in Figure 2. It encompasses detailed protocols for the physical, chemical, and mineralogical characterization of the natural fine aggregate (S 0/4) and the inert spent garnet (SG) addition, thereby providing a preliminary identification assessment and facilitating the current compositional integration phase. Five distinct spent garnet waste types (SG A-1, SG A-2, SG A-3, SG A-4, and SG B-1) were incorporated into conventional mortars as partial replacements (30% and 50% by mass) for the natural aggregate (S 0/4). These modified mortars were evaluated—relative to the reference mortar (R)—in terms of physical, mechanical, durability, and mineralogical properties, in full alignment with the implementation stages of the Validation Procedure. The primary objectives were to substantiate the proposed valorization approach, delineate specific application domains, and pave the way for subsequent technological transfer to industry.

4.2. Methods for Microstructure Properties Evaluation of SG Waste and Natural Sand S 0/4

4.2.1. Methodology for SEM Analysis of SG Waste and Natural Sand S 0/4

The morphological features and elemental composition of the samples were examined using a scanning electron microscope equipped with an energy-dispersive X-ray detector (Inspect S + EDAX, FEI, The Netherlands). Imaging was performed in low-vacuum mode to preserve the surface characteristics of the samples.

The chemical composition was determined by energy-dispersive X-ray spectroscopy (EDX). The EDX analysis of SG-A was conducted within IMCEMC Timisoara using a different methodology compared to SG-B*, which was EDS evaluated by ISIM Timisoara laboratory. Consequently, the detected carbon in the SG-A spectra is attributed to the sample preparation environment, specifically the use of double-sided carbon adhesive tape (99.99% purity) for mounting the specimens.

4.2.2. Methodology for XRD Analysis of SG Waste and Natural Sand S 0/4

The morphological features and elemental composition of the samples were examined using a scanning electron microscope equipped with an energy-dispersive X-ray detector (Inspect S + EDAX, FEI, The Netherlands). Imaging was performed in low-vacuum mode to preserve the surface characteristics of the samples (Figure 2). X-ray diffraction patterns were recorded using an X’Pert PRO MPD diffractometer (PANalytical, Almelo, The Netherlands) equipped with Cu–Kα radiation (λ = 1.5406 Å). Data were collected over a 2θ range of 10–80° under standard operating conditions.

4.2.3. Methodology for FT-IR Analysis of SG Waste and Natural Sand S 0/4

Fourier-transform infrared (FT-IR) spectroscopy was employed as a diagnostic technique to identify organic and inorganic compounds. The vibrational modes of the spent garnet and granite samples were characterized using an FT-IR spectrophotometer. ATR-FT-IR spectra were acquired at room temperature in the 4800–400 cm⁻¹ region using a Bruker Vertex 70 spectrometer (Bruker Optik GmbH, Rosenheim, Germany) fitted with a Platinium ATR accessory (Bruker Diamond A225/Q.1).

4.3. Methods for Evaluation of Aggregate Particle Size Distribution (PSD)

The particle size distribution (PSD) of the aggregates was determined by dry sieving according to EN 933-1 [59] (Tests for geometrical properties of aggregates—Part 1: Determination of particle size distribution—Sieving method). All materials were oven-dried to constant mass prior to testing to eliminate moisture effects on results. Each test used a 200 g representative sample. Sieving was conducted using a mechanical shaker with standard sieves ranging from 2 mm to 50 μm aperture size, plus a receiving pan for the finest fraction. Cumulative grading curves were then constructed from the mass retained on each sieve. PSD analysis was conducted on the conventional fine sand (0/4 fraction, S) and on all spent garnet (SG) types: SG A-1, SG A-2, SG A-3, SG A-4, and SG B-1. Additionally, PSD determinations were carried out for the combined S+SG mixtures at the selected substitution levels of 30% and 50%.

4.4. Mortar Mix Design and Specimen Preparation

4.4.1. Mortar Mix Design

The present study evaluated five distinct spent garnet (SG) wastes: SG A-1, SG A-2, SG A-3, SG A-4 (from the same supplier but sampled at different times), and SG B-1 (from a separate supplier in a single collection), as detailed in Section 4.1. This selection ensured coverage of both the Extended Compatibility Assessment (Stage A) and Core Validation (Stage B) within the proposed Validation Procedure. The reference (R) was produced as a conventional mortar with A/C (Aggregate/Cement) = 3. The SG waste was incorporated into mortar mix at substitution levels of 30% and 50% (by mass of natural fine aggregate), levels previously identified as efficient [7,37,38,39]. Mix proportions for all compositions are summarized in

Table 2. Mix proportions of references and SG mixtures (kg/m³).

Components	Cement (kg/m3)	S 0/4 (kg/m3)	SG (kg/m3)	Water (kg/m3)	W/C -
Mixtures
R	516	1549	-	271	0.56
SG 30%	516	1084	465	271	0.56
SG 50%	516	774.5	774.5	271	0.56

and align with the data provided in Table 1.

4.4.2. Specimen Preparation

The mixing technology adopted was specifically designed for preliminary small-volume castings, with a maximum mixture volume of 1.3 L. Mixing was carried out using a paddle mixer with a total capacity of 4.75 L. The mixer operated at rotational speeds of 140 and 280 rpm, corresponding to revolution speeds of 61.5 and 123 rpm, respectively, in accordance with the requirements of EN 196-1 [60].

The technological sequences and their order were established following the specifications of EN 196-1 [60] to ensure consistency and repeatability of the specimen preparation process. Following mixing, prismatic mortar specimens with dimensions of 40 × 40 × 160 mm were cast in accordance with EN 196-1 [60]. Immediately after casting, the molds were covered to prevent moisture loss and stored under controlled laboratory conditions. After 24 h, the specimens were removed from the molds and immersed in water at a temperature of (20 ± 1) °C. The specimens were maintained under these conditions until the designated testing ages were reached. For each mixture, three specimens were prepared and tested in accordance with the specified, relevant standardized procedures, and the results reported represent the average of the measured values.

4.5. Methodology for Evaluation of Fresh State Properties of SG Mortars

The fresh state properties of mortar mixtures provide essential information regarding their workability and cast-in-place facile procedure, with direct influence on the material hardened state performance. The fresh state behavior of the reference and SG-modified mortars was systematically evaluated to assess the effect of SG incorporation in terms of visual aspect, consistency, fresh state density and workable life of fresh mortar. Supplementary material of the paper provides complete information regarding the testing methodology and the obtained results.

4.5.1. Evaluation Method of Fresh State Aspect and Mortar Workability

The consistency of the fresh mortar was determined by measuring the flow diameter using the standardized flow table method, in accordance with EN 1015-3 [61], for both the reference mixture and the SG-modified mortars at substitution levels of 30% and 50%. The fresh state aspect was visually evaluated and practical observations were considered.

4.5.2. Evaluation Method of Fresh State Density

The bulk density of fresh mortar was determined in accordance with EN 1015-6 [62]. The test was performed by weighing the mortar immediately after mixing, placed into a container of known and predefined volume, and calculating the bulk density as the ratio between mass and volume. In this study, a container with a volume of 1 L was used for the determination of the bulk density of fresh mortar.

4.5.3. Evaluation Method of Workable Life of Fresh Mortars

The workable life of the SG mortar mixtures was determined in accordance with EN 1015-9 [63], using Method B. This determination was performed only for SG mortar mixtures with a substitution percentage of 30%, identified as optimum substitution percentage. The workable life was defined as the elapsed time, expressed in minutes, required for the mortar consistency to differ by 30 mm relative to the reference consistency measured 10 min after mixing. Consistency measurements were subsequently performed at 15-min intervals, in accordance with EN 1015-3 [61].

4.6. Methods for Evaluation of Hardened State Properties of Mortars

4.6.1. Evaluation Method of Dry Bulk Density and Water Absorption of Hardened Mortars

In accordance with EN 1015-10 [64], the dry bulk density and water absorption of hardened mortars were evaluated for the reference mixture (R) as well as for the SG-modified compositions with substitution levels of 30% and 50%.

For all investigated mortar compositions, after 28 days, the specimens were dried at a controlled temperature until a constant mass was reached, thereby obtaining the dry mas m_s,dry. Subsequently, each specimen was immersed in water, with care taken to eliminate trapped air bubbles, to determine the apparent immersed mass m_s,i. Afterwards, the specimens were weighed in air after removing excess surface water using a damp material, thus obtaining the saturated mass m_s,sat The volume of the specimen, V_s, was determined by hydrostatic weighing, based on the difference between the mass of the saturated specimen measured in air and its apparent mass when immersed in water, and calculated using the density of water, ρ_w, corresponding to the test temperature, (20 ± 2) °C. The dry bulk density ρ_dry, was then calculated Equation (1) as the ratio between the dry mass and the volume of the specimen determined using Equation (2), following the procedure described in EN 1015-10 [64].

$${\rho }_{dry}={\displaystyle \frac{m_{s,dry}}{V_s}}[\mathrm{kg}/{\mathrm{m}}^3]$$

(1)

$$V_s={\displaystyle \frac{{m_{s,sat}-m}_{s,i}}{{\rho }_w}}[{\mathrm{m}}^3]$$

(2)

Based on these values, the water absorption (W%) of the specimens could also be assessed. Water absorption (W%) was calculated using Equation (3) in accordance with EN 1015-10 [64] as the relative increase in mass between the saturated and dry states of the specimens.

$$W\%={\displaystyle \frac{{m_{s,sat}-m}_{s,dry}}{m_{s,dry}}}\times 100 [\%]$$

(3)

4.6.2. Method for Evaluation of Water Absorption Coefficient Due to Capillary Action of Hardened Mortar

The water absorption coefficient due to capillary action of the hardened mortar was determined in accordance with EN 1015-18 [65]. After 28 days after casting, the samples were dried in a temperature-controlled environment until a constant mass was reached. The sides of each sample were sealed with paraffin to ensure that water penetration occurred only through the bottom surface. After the mortar prisms were split to obtain 40 × 40 × 80 mm test specimens, they were partially immersed in water, usually to a depth of a few millimeters (5–10 mm), and the increase in mass over time was recorded continuously. The increase in mass due to water absorption was measured at predefined time intervals: M₁ at 10 min and M₂ at 90 min. The capillary absorption coefficient (C) was determined using Equation (4), as specified by EN 1015-18 [65].

$$\mathrm{C}=0.1(M_2-M_1\left)\right[\mathrm{k}\mathrm{g}/{\mathrm{m}}^2{\mathrm{m}\mathrm{i}\mathrm{n}}^{0.5}]$$

(4)

4.6.3. Methods for Evaluation of Flexural and Compressive Strength

The flexural strength and the compressive strength of mortar specimens was determined in accordance with the standardized procedures specified in EN 196-1 [60] and EN 1015-11 [66]. Flexural strength testing was performed on prismatic samples measuring 40 × 40 × 160 mm, loaded in a three-point bending test, followed by the compressive test conducted on the resulting halves prisms. The flexural loading rate was 50 ± 10 N/s, (smooth and continuous increase until fracture) and the compressive loading rate was: 2400 ± 200 N/s (smooth and continuous increase over the entire load application until fracture), corresponding to a stress rate of approximately 0.5 MPa/s on the 40 × 40 mm loaded surface. All specimens were cured under continuous water immersion at a controlled temperature of (21 ± 2) °C until the designated testing ages, following demolding 24 h after casting. The tests were performed at three representative ages, namely early age (7 days), regular age (28 days), and late age (112 days), to evaluate the evolution of mechanical performance over time.

The flexural strength was calculated based on the maximum load recorded during testing ($F_f$), in accordance with Equation (5).

$$R_f={\displaystyle \frac{3F_{f}l}{2b^3}}[\mathrm{MPa}]$$

(5)

The compressive strength was calculated as the ratio between the maximum applied load (F) and the loaded cross-sectional area (A), (Equation (6)).

$$R_c={\displaystyle \frac{F}{A}}[\mathrm{MPa}]$$

(6)

Mechanical performance—specifically flexural strength (determined via three-point bending) and compressive strength—constitutes a critical parameter that is systematically evaluated throughout all three stages of the validation procedure. In the Extended Compatibility Assessment (Stage A), these strengths are measured at specified curing ages (e.g., 7, 28, and 112 days) to establish the compatibility of spent garnet (SG) from diverse sources with the cementitious matrix. During the Core Validation stage (Stage B), the same properties are assessed to confirm the repeatability and consistency of performance trends in samples from the same source but collected at different times. Finally, in the Durability Assessment (Stage C), residual flexural and compressive strengths are quantified after exposure to 50 freeze–thaw cycles (procedure adapted from SR 3518:2009 [58]), thereby providing a direct measure of long-term resistance under cyclic freezing and thawing conditions. This rigorous and consistent emphasis on mechanical strength serves as the primary metric for the robust validation of SG waste valorization as a partial fine-aggregate substitute.

4.7. Methods for Microstructure Properties Evaluation of SG Mortars

Microstructural characterization of the SG-modified mortars—including scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and Fourier-transform infrared (FT-IR) spectroscopy—was performed using the identical methods and equipment described in Section 4.2 for the raw spent garnet (SG) waste. These techniques enabled direct comparison of microstructural features and phase composition between the raw additions and the hardened cementitious matrix. The same analytical procedures were applied to specimens subjected to freeze–thaw exposure, allowing assessment of durability performance through identification of microstructural alterations (e.g., microcracking, phase changes, or degradation products) induced by the 50 freeze–thaw cycles. For the present stage of the research, microstructural evaluations were conducted selectively on the SG A-1 30% and SG A-2 30% mortar mixes, following a preliminary triage process. This targeted selection allowed focused investigation of representative compositions exhibiting promising performance from earlier assessments, while optimizing resource allocation for detailed SEM-EDX, XRD, and FT-IR analyses.

4.8. Methods for Durability Performance Evaluation of SG Mortars

The durability performance of the SG mortars was evaluated by assessing their resistance to freeze–thaw cycling after exposure to 50 cycles. The present study focuses on medium-level SG substitutions (30% and 50%), selected following preliminary screening. The 10% substitution level, while economically unviable, provided valuable guidance during compositional design and initial performance evaluation.

The freeze–thaw resistance was determined in accordance with the Romanian standard SR 3518:2009 [58], employing a destructive method to measure the loss in flexural and compressive strength. Testing was conducted on 40 × 40 × 160 mm prismatic specimens at 28 days of age. Subsequently, test specimens (TS) underwent 50 freeze–thaw cycles, while control specimens (CS) were immersed at (20 ± 2) °C.

The conditioning regime for freeze–thaw-exposed specimens comprised:

Freezing phase: Specimens held for 4 h in a freezing chamber at (−17 ± 2) °C.

Thawing phase: Specimens held for 20 h in a water bath or climatic chamber at (>90% relative humidity) and (20 ± 5) °C.

The evaluation encompassed the following aspects:

• Strength loss: Residual flexural and compressive strengths of TS compared to CS.

The strength loss is calculated using the relation (7):

$$\mathrm{ŋ}={\displaystyle \frac{R_m-R_t}{R_m}}\times 100[\%]$$

(7)

where

• ŋ—Strength loss with respect to the considered Reference;
• Rm—Strength (tensile/compressive) of the control specimens, in (MPa);
• Rt—Strength (tensile/compressive) of the specimens subjected to freeze–thaw cycles, in (MPa).

• Visual aspect: Comparative examination of control specimens versus freeze–thaw-exposed specimens for signs of degradation, including cracks, fissures, delamination, edge damage, material detachment, scaling, etc.
• Mass loss: Successive specimen weighing to detect significant mass loss as an indicator of degradation induced by freeze–thaw cycling.

Specific measurements were performed on saturated specimens (consistent with the condition induced by the exposure regime) at the following relevant time points:

-

t₀b: at 28 days from casting, before exposure to freeze–thaw cycles;

-

t₀a: at 28 days from casting, after exposure to the cycles;

-

t₁₀b: at 28 days from casting plus completion of 10 cycles, before those cycles;

-

t₁₀a: at 28 days from casting plus completion of 10 cycles, after those cycles;

-

t₅₀b: at 28 days from casting plus completion of 50 cycles, before those cycles;

-

t₅₀a: at 28 days from casting plus completion of 50 cycles, after those cycles, etc.

• Microstructural evaluation: SEM with EDX, XRD, and FT-IR analyses (using the same methods and equipment as described in Section 4.2) applied to post-exposure specimens to identify possible alterations such as microcracking or phase modifications.

For the current research stage, durability evaluations—following preliminary triage based on mechanical performance—were performed on SG A-1 (30% and 50%), SG A-2 (30% and 50%), and SG B-1 (30% and 50%) mortar mixes. This included strength loss visual aspect and mass loss. Microstructural analyses (SEM-EDX, XRD, FT-IR) were selectively applied only to SG A-1 30% and SG A-2 30% mixes, prioritizing the most representative compositions.

5. Results and Discussions

5.1. Microstructure Properties of SG Waste and Natural Sand S 0/4

5.1.1. SEM Analysis of SG Waste and Natural Sand S 0/4

The SEM images in Figure 3 show that GS materials are characterized many, small sheet-shape particles with many pores, in accordance with previous studies [47].

The chemical composition was determined by energy-dispersive X-ray spectroscopy (EDX). The EDX analysis of SG-A was conducted within IMCEMC Timisoara using a different methodology compared to SG-B*, which was EDS evaluated by ISIM Timisoara laboratory. Consequently, the detected carbon in the SG-A spectra is attributed to the sample preparation environment, specifically the use of double-sided carbon adhesive tape (99.99% purity) for mounting the specimens. The EDX results are presented in

Table 3. Chemical analysis via EDX spectra of SG wastes.

Element	S 0/4	SG A-1	SG A-2	SG A-3	SG A-4	SG B-1 *
O	46.2	37.6	36.2	36.4	37	40.05
Fe	1.2	22.2	23.2	21.5	23.8	24.57
C	17.3	14.1	13.6	13.7	13.9
Si	19.4	12.9	11.8	13.3	11.7	16.49
Al	9.4	7.3	5.9	8.8	7.6	10.03
Mg	0	3.1	2.1	1.9	1.9	3.9
Ca	2.4	1.6	4.7	1.6	1.6	1.56
Ti	0	0.8	1.1	1.7	2.5	1.26
Mn	0	0.5	0	1.1	0	0.86
Na	4					0.79
Cu			1.1			0.5
K	46.2	37.6	36.2	36.4	37	40.05
Total	99.9	100.1	99.7	100.0	100.0	100.0

* EDS analysis performed in 2023 by ISIM Timisoara laboratory, with distinct methodology.

. The semi-quantitative analysis highlighted the presence of iron in the garnet material (SG) at mass percentages of approximately 20%, in accordance with the typical garnet chemical composition [39,40,41], which decreased significantly in the final product. Furthermore, copper was identified in the SG A-2 sample, most likely as a contamination resulting from the processing stage. However, Cu concentration after incorporation into cement mortar decreased substantially as identified in SG A-2 mortars, demonstrating the capacity of the cementitious matrix to effectively immobilize and confine heavy metals [35].

5.1.2. XRD Analysis of SG Waste and Natural Sand S 0/4

X-ray diffraction patterns were recorded using an X’Pert PRO MPD diffractometer (PANalytical, Almelo, The Netherlands) equipped with Cu–Kα radiation (λ = 1.5406 Å).

Data were collected over a 2θ range of 10–80° under standard operating conditions. The XRD patterns obtained for samples SG A-1, SG A-2, SG A-3, SG A-4, and the reference sample S 0/4 are presented in Figure 3. and reveal a garnet-dominated mineral assemblage for the SG materials. In SG A-1 sample, the main peaks correspond to almandine (Fe₃Al₂Si₃O₁₂), accompanied by minor spessartine, pyrope, and ilmenite. Almandine reflections are observed at ~18°, 25°, 29°, 33°, and 41° 2θ, consistent with its cubic crystal structure [67]. The presence of multiple garnet end-members suggests medium- to high-grade metamorphic conditions, consistent with previous studies of garnet-bearing metamorphic rocks [67,68]. Sample S 0/4, in contrast, exhibits strong diffraction peaks corresponding to plagioclase feldspars (albite and anorthite) and quartz (SiO₂), with reflections near 22°, 27°, and 56° 2θ for feldspars and 26.6° 2θ for quartz, indicating a felsic, plagioclase-rich lithology [69].

Figure 4 presents the XRD patterns of aggregates: Natural sand 0/4 (S 0/4), SG A-1, SG A-2, SG A-3, and SG A-4.

5.1.3. FT-IR Analysis of SG Waste and Natural Sand S 0/4

The results of the FT-IR test on the GS A garnet samples and natural sand S 0/4 are presented in Figure 5. The FT-IR spectra obtained for the garnet waste samples (SG A-1, AG A-2, SG A-3 and SG A-4) and natural sand S 0/4 reveal characteristic absorption bands associated with silicate minerals, confirming the aluminosilicate nature of these materials. In all spectra, strong and well-defined absorption peaks appear in the region between 870 and 986 cm⁻¹. These bands are assigned to the stretching vibrations of Si–O and Al–O–Si bonds within the tetrahedral silicate framework typical of garnet minerals such as almandine or pyrope [70,71]. The intensity and position of these bands indicate that the main crystalline phase of the studied materials consists of Fe–Mg–Al silicates [70]. The sample S 0/4, in contrast, exhibits a dominant band at approximately 1043 cm⁻¹, corresponding to the asymmetric stretching of Si–O–Si bonds characteristic of quartz or other free silica phases [72]. This suggests that S 0/4 contains a higher proportion of quartz usual for regular natural aggregates. The differences in this spectral region indicate a slight mineralogical variation between the recycled garnet samples SG and the reference material S 0/4 [1,3]. In the mid-infrared region, bands observed between 1520 and 1530 cm⁻¹ in most samples can be attributed either to deformation vibrations of hydroxyl groups (O–H) or to C=O stretching modes, reflecting minor amounts of carbonate or organic contamination [13]. Notably, the SG A-4 sample presents an additional absorption at around 1703 cm⁻¹, characteristic of the carbonyl (C=O) stretching vibration, suggesting the presence of organic compounds such as oils, lubricants, or resin residues [13]. The weak absorption bands in the 2130–2170 cm⁻¹ region, common to all samples, are associated with carbon dioxide molecules adsorbed onto the particle surfaces [13]. These features are typical for materials exposed to atmospheric CO₂ or subjected to mild carbonation at the surface level. In the high-wavenumber region, all spectra display broad but distinct absorption bands between 3730 and 3850 cm⁻¹, corresponding to the stretching vibrations of hydroxyl (O–H) groups [13]. The relatively consistent intensity of these O–H bands across all samples suggests a comparable degree of surface hydration or weathering. Overall, the FT-IR results confirm that the studied garnet waste materials are primarily composed of aluminosilicate phases with variable amounts of adsorbed CO₂ and H₂O. Samples SG A-1, SG A-2, and SG A-3 exhibit nearly identical spectral features, reflecting a stable and homogeneous silicate composition typical of used garnet abrasives [70,71]. The spectrum of SG-A-4 indicates additional organic and carbonate species, attributed to industrial contamination, while S 0/4 differs by its enhanced quartz signature [13,72]. The presence of hydroxyl bands at high wavenumbers further supports the occurrence of superficial hydration and partial oxidation at the surface of the garnet grains.

In summary, FT-IR spectroscopy provides clear evidence that the garnet waste samples maintain their fundamental silicate structure, while slight variations among spectra reveal differences in contamination, hydration, and surface chemistry resulting from industrial application and environmental exposure [13,70,71,72].

5.2. Particle Size Distribution (PSD) of Aggregates

PSD analysis was conducted on the conventional fine sand (S 0/4) and on all spent garnet (SG) types: SG A-1, SG A-2, SG A-3, SG A-4, and SG B-1, as well as for each mortar mixture containing the proposed substitution levels: S + SG 30% and S + SG 50%. The PSD curves are presented in Figure 6.

The particle size distributions of the SG materials differ markedly from that of the conventional natural sand 0/4. The SG materials are characterized by a very fine and narrow grading, with the majority of particles concentrated in the fine sand to very fine sand range (<0.25 mm) and a significant proportion below 0.10 mm. Their PSD curves are steep, indicating uniform grading and a limited spread of particle sizes. In contrast, the natural sand 0/4 exhibits a broader and coarser particle size distribution, extending from fine sand to coarse sand fractions (up to 4 mm). All SG A-series (A-1 to A-4) and SG B-1 curves are very close to each other, indicating similar grading envelopes and comparable production or processing methods, which leads to constancy when considering customized cement-base materials, applicability domain. The SG curve is flatter and shifted to larger diameters, reflecting a moderately well-graded material with a relatively low fines content. This grading favors high permeability and low water demand but results in less efficient particle packing.

From a cement-based materials perspective, the SG materials alone behave more like micro-aggregates or fillers rather than conventional fine aggregates. Their high surface area and fine size make them unsuitable as a full replacement for natural sand but potentially valuable as a grading modifier or filler component. A 30% substitution level yields a more continuous grading, increasing the fine fraction without eliminating the coarse sand skeleton. This modification is expected to enhance particle packing and cohesion in cement-based materials while maintaining acceptable workability for concrete applications. At a 50% substitution level, the dominance of fine particles is likely to increase water demand and sensitivity to mixture proportions. Such blends are therefore more suitable for fine-grained cementitious systems, including mortars, screeds, and rendering materials, where higher fines content is advantageous and mix design adjustments can be readily implemented.

5.3. Fresh State Properties of SG Mortars

The results regarding the fresh state properties of SG and R mortar mixtures are further presented.

5.3.1. Fresh State Aspect and Mortar Workability

The experimental results reveal that all mortar mixtures exhibited a homogeneous appearance in the fresh state. Compared to the reference mixes (R), the SG-modified mortars demonstrated enhanced cohesiveness and a creamier texture (Figure 7). This improvement intensified with increasing sand replacement levels and can be attributed to the higher content of fine particles in the spent garnet (SG) material, as observed in the preliminary investigations.

The results of the consistency determinations using the flow table method are graphically presented in Figure 8. Supplementary material of the paper provides complete information regarding the testing methodology and the obtained results.

5.3.2. Fresh State Density

The results of the bulk density of fresh mortar are graphically presented in Figure 9. Supplementary material of the paper provides complete information regarding the testing methodology and the obtained results.

Fresh-state evaluations—consistency and density—confirm the viability of substituting natural sand (S 0/4) in conventional mortars with spent garnet (SG) waste. SG incorporation improves workability and increases density due to superior particle packing, as predicted from granulometric analysis. This trend is consistent across all SG-modified mixes, with greater benefits at 50% substitution, reinforcing the reliability of SG valorization in construction materials. The S + SG B-1 (50%) composition shows the largest flow increase (+40 mm), indicating a strong positive effect on workability. SG 1 and SG 3 remain stable regardless of substitution level. SG A-2 approaches reference values, showing moderate improvement.

Reference mortar (R) exhibits stable fresh density (~2140–2150 kg/m Density increases with higher substitution levels, aligning with improved consistency and granulometric predictions.

5.3.3. Workable Life of Fresh Mortar

The results regarding the workable life of the SG mortar mixtures, determined for SG A 30% mortars, are presented in Figure 10 and demonstrate that, even at a fixed substitution level, the nature of the SG material plays a decisive role in governing the workable life of cement-based mortars, emphasizing the necessity of material-specific mix design optimization when SG is used as a sand replacement.

5.4. Hardened State Properties

5.4.1. Dry Bulk Density and Water Absorption of Hardened Mortars

The results for dry bulk density and water absorption of the SG mortar mixtures are presented in Figure 11a,b. Supplementary material of the paper provides complete information regarding the obtained results.

The incorporation of SG as partial replacement of natural sand leads to a systematic increase in the apparent dry density of the mortars compared to the reference mix (R). While R exhibits a dry density of 2040 kg/m³, all SG-modified mortars show higher values, ranging from 2170 to 2190 kg/m³ at a substitution level of 30% and from 2210 to 2280 kg/m³ at 50%. At 30% SG substitution, dry density increases by approximately 6–7% relative to the reference, with limited variability among SG types. At 50% substitution, the density increase becomes more pronounced (≈8–12%), with SG A-1–50% reaching the maximum value of 2280 kg/m³. The increase in dry density with increasing SG content reflects the improved packing density of the granular skeleton due to the finer particle size distribution of SG, which enhances pore filling between coarser sand grains. This effect is more pronounced at 50% substitution, where the contribution of SG dominates the aggregate system and leads to denser microstructures.

Water absorption values remain within a narrow range for all mixtures (10.0–10.8%). Compared to R (10.4%), several SG-containing mixes exhibit slightly lower absorption, whereas others show marginally higher values, particularly at 50% substitution. Despite the increased density, water absorption does not decrease proportionally and remains relatively stable across all mixtures. Slight increases in absorption observed in some 50% SG mixes may be attributed to the higher specific surface area of SG particles, which promotes water retention within finer capillary pores.

From a practical standpoint, the simultaneous increase in dry density and maintenance of moderate water absorption values suggests that SG incorporation does not compromise the durability-related properties of the mortars. On the contrary, the denser matrix associated with SG-containing mixes may contribute to improved mechanical performance, provided that mix design parameters are adjusted to account for the slightly higher water demand at higher substitution levels.

5.4.2. Water Absorption Coefficient Due to Capillary Action of Hardened Mortar

The results water absorption coefficient due to capillary action of hardened mortar of the SG mortar mixtures are presented in Figure 12.

The systematic increase in the capillary water absorption coefficient following SG incorporation indicates a modification of the pore structure toward higher capillary continuity, despite the previously observed increase in dry density. This apparent contradiction suggests that SG promotes a refinement of pore size distribution rather than a simple reduction in total porosity, leading to a greater proportion of connected fine capillary pores that facilitate water uptake by capillary suction. The progressive increase in capillary absorption with higher SG content highlights the role of the finer SG fraction and its higher specific surface area, which enhances capillary forces and water transport within the hardened matrix. Differences among SG types reflect variations in particle morphology and grading, with SG7 consistently exhibiting the highest coefficients, pointing to a less favorable pore network configuration in terms of water transport resistance. From a durability perspective, the elevated capillary absorption coefficients observed in SG-containing mortars suggest a potentially higher susceptibility to moisture ingress, particularly at high replacement levels. However, the values remain within a moderate range, indicating that SG substitution up to 50% may be acceptable for applications where moisture exposure is controlled or where additional mitigation strategies (e.g., reduced water-to-binder ratio or supplementary cementitious materials) are employed.

5.4.3. Flexural and Compressive Strength

Flexural (determined via three-point bending) and compressive performance represents critical parameters systematically motorized throughout all three stages of the validation procedure. Supplementary material of the paper provides complete information regarding the obtained results and specific comparative assessments supporting the SG integration hypothesis.

• Extended Compatibility Assessment (Stage A): This stage evaluates flexural and compressive strengths at specified curing ages (e.g., 7, 28, and 112 days) to confirm the compatibility of spent garnet (SG) from diverse sources with the cementitious matrix. Consistency of the hypothesis is verified by comparing results across SG batches collected at different times from the same source and from different sources. To this end, two distinct SG suppliers were considered, with four separate sampling periods from Source A. The mechanical test results presented in Figure 13 and Figure 14 prove the SG applicability from multiple sources (e.g., SG A, SG, B) as partial sand replacement, fully satisfying the predefined performance criterion, namely mechanical property values (flexural and compressive strengths) comparable to the reference mortar, within an acceptable variability range of approximately ±20% (Figure 15).
• Core Validation stage (Stage B): This stage assesses flexural and compressive strengths at specified curing ages (e.g., 7, 28, and 112 days) to verify the repeatability and consistency of performance trends across samples from the same source but collected at different times. Repeatability was evaluated using Source A (samplings 1 and 2) and Source B (sampling 1), with mortars prepared at 30% and 50% SG substitution levels.

The results and specific comparative assessments are presented in Figure 16.

The results from Core Validation (Stage B) fully satisfy the predefined performance criterion, namely the demonstration of similar performance trends (stable pattern) across batches, despite minor variations in absolute strength values.

All tested mortars exhibited predominantly brittle failure under both flexural and compressive loading, irrespective of SG content and curing age. The reference mixture confirmed a stable baseline response, characterized by abrupt fracture without significant post-peak deformation. Mortars containing 30% SG showed the most favorable performance, with increased flexural and compressive strengths accompanied by a more homogeneous crack pattern, suggesting improved stress transfer and matrix continuity. At 50% SG substitution, compressive strength remained significantly enhanced (approximately +30–40% relative to the reference), while flexural strength gains were less pronounced, indicating a shift in failure control toward the interfacial transition zone (ITZ). Nevertheless, no transition toward quasi-ductile behavior was observed, and fracture remained governed by brittle mechanisms for all mixtures and testing ages (7, 28, and 112 days).

The 30% SG compositions appear optimal, enhancing both flexural and compressive strengths. This improvement is attributed to a superior particle size distribution (promoting denser particle packing and reduced porosity) and the mineralogical contribution of SG (high Fe and Al content, elevated hardness). In contrast, 50% SG compositions provide short-term gains (7–28 days) but offer no clear long-term benefits (112 days) and may even substantially reduce strength. This is likely due to the high specific surface area of SG particles, which increases water demand and compromises matrix compactness.

• Durability Assessment (Stage C): This is fully covered by 5.5 Durability Performance of SG mortars.

5.5. Durability Performance of SG Mortars

Durability evaluations (post-preliminary triage) were conducted on SG A-1 (30%, 50%), SG A-2 (30%, 50%), and SG B-1 (30%, 50%) mortars, but strength loss (residual flexural/compressive after 50 freeze–thaw cycles), mass loss (via successive weighing), and visual degradation (cracks, scaling, edge damage, detachment) were assessed only for SG A-1, SG A-2, and SG B-1 compositions. Microstructural analyses (SEM-EDX, XRD, FT-IR) were limited to SG A-1 30% and SG A-2 30% mixes.

The evaluation encompassed the following aspects:

Strength loss: Residual flexural and compressive strengths

The results are presented in

Table 4. SG 30% and SG 50% mortars—Mechanical strength loss after the 50 freeze–thaw cycles: Flexural Strength and Compressive Strength.

Mixtures Re	Rm [MPa]	Rt [MPa]	ŋ1 [%]	ŋ2 [%]	ŋ3 [%]
			Reference for ŋ Calculation
			Rm (W Cond)	Rm = R (W Cond)	Rm = R (FT Cond)
Flexural strength
R	7.8	2.5	68.3	68.3	0.0
SG A-1—30%	7.8	7.1	9.1	9.6	−185.1 *
SG A-2—30%	7.9	3.1	60.5	60.2	−25.5 *
SG B-1—30%	8.2	3.4	58.8	56.6	−37.0 *
R	7.6	1.2	83.9	83.9	0.0
SG A-1—50%	8.1	4.2	48.3	44.6	−244.9 *
SG A-2—50%	8.0	7.4	7.6	2.8	−505.2 *
SG B-1—50%	8.2	2.7	67.1	64.7	−120.1 *
Compressive strength
R	43.8	29.1	33.7	33.7	0.0
SG A-1—30%	40.0	41.0	−2.5 *	6.4	−41.1 *
SG A-2—30%	40.4	29.8	26.3	32.0	−2.5 *
SG B-1—30%	42.7	31.2	27.0	28.9	−7.3 *
R	41.8	20.8	50.3	50.3	0.0
SG A-1—50%	42.5	39.9	6.2	4.7	−91.7 *
SG A-2—50%	42.7	40.1	6.2	4.1	−92.8 *
SG B-1—50%	40.2	29.4	26.9	29.7	−41.4 *

* Negative values of ŋ indicate strength gain with respect to the considered reference.

(flexural strength loss and compressive strength loss) for the mixes developed with 30% SG and 50% SG substitution. Comparative analysis of mechanical strength loss after 50 freeze–thaw cycles (flexural and compressive strength) is illustrated in Figure 17.

The comparative evaluation of strength loss (η) is performed using three relevant reference conditions:

-

η₁ [%]: Calculated relative to the individual compositional reference (Rm)—specimens of the corresponding mix conditioned by water immersion (W) throughout the 50 freeze–thaw cycles.

-

η₂ [%]: Calculated relative to the reference mortar R2 conditioned by water immersion (W).

-

η₃ [%]: Calculated relative to the reference mortar R2 subjected to freeze–thaw cycling (FT).

Multi-Criteria Freeze–Thaw Performance Analysis of SG Mortars (30% and 50%)

I. Analysis Based on Strength Loss Coefficient η₁ [%] (Reference: Individual compositional water-conditioned specimens (W))

Reference Mortar R (Primary benchmark): Flexural strength shows high losses (68.3–83.9%), confirming vulnerability to the aggressive freeze–thaw regime. Compressive strength shows moderate losses (33.7–50.2%).

30% SG Mortars: Flexural strength: SG A-1-30% shows minimal loss (9.1%); SG A-2-30% and SG B-1-30% exhibit losses of ~60% and ~58%, respectively—comparable to or slightly better than R. SG A-1-30% clearly excels. Compressive strength: SG A-1-30% gains strength (–2.5%, likely due to continued hydration); SG A-2-30% and SG B-1-30% show intermediate losses (18–30%), better than R. SG A-1-30% superior.

50% SG Mortars: Flexural strength: SG A-2-50% exhibits minimal loss (7.6%); SG A-1-50% and SG B-1-50% moderate (~48–65%); SG A-2-50% and SG A-1-50% standout. Compressive strength: SG A-1-50% and SG A-2-50% show minimal (~6%); SG B-1-50% moderate (~21–27%). SG A-1/A-2-50% most robust.

II. Analysis Based on Strength Loss Coefficient η₃ [%] (Reference: R subjected to freeze–thaw (FT)—preferred for direct comparison under identical severe conditions, highlighting relative durability benefits of SG substitution)

30% SG Mortars: Flexural Strength: All evaluated mortars outperform R-FT (η₃ negative, up to –185% for SG A-1-30%); SG A-1-30% dominant, followed by SG B-1-30% (−37%) and SG A-2-30% (−25%). Compressive Strength (30% SG): All outperform R-FT (η₃ −2.5% to −41%); SG A-1-30% leads.

50% SG Mortars: Flexural strength: All outperform R-FT (η₃ −80% to −505%); SG A-2-50% exceptional (>6× residual strength), followed by SG A-1-50%. All outperform R-FT (η₃ −41% to −92.8%); near-complete retention for SG A-1/A-2-50%.

General Conclusions (η₃ Basis):

Evaluated SG mortars consistently exhibit superior freeze–thaw resistance relative to R under identical exposure, despite absolute losses versus water-conditioned references. Regarding both, flexural and compressive strength, all SG mortars show consistent lower losses than E-FT. The Top performers are SG A-1 and SG A-2 (both levels), with 50% substitution (especially SG A-1/A-2-50%) often outperforming 30%. SG B-1 also demonstrates notable benefits.

Regarding the validation pattern of the proposed protocol, it can be stated that superior behavior correlates with balanced grading and denser pore structure in top-performing SG sources, enhancing overall durability.

Visual aspect:

Visual analysis of the SG mortars, both SG 30% and SG 50% confirms the preservation of material integrity for all evaluated specimens—both control and test—with no evidence of degradation features such as cracks, fissures, delamination, corner or edge damage, or similar defects. Relevant visual aspects of the specimens are included in the Supplementary material.

Mass loss:

Mass variation up to 50 cycles showed favorable behavior for both control specimens (R, SG 30%, SG 50%) and freeze–thaw-exposed specimens (R FT, SG 30% FT, SG 50% FT). Figure 18 presents the mass recording at relevant ages and demonstrate stable prismatic specimen masses throughout cycling, with no significant losses.

5.6. Microstructure Properties of SG Mortars

SEM-EDX, XRD, and FT-IR analyses—employing the same methods and equipment described in Section 4.2—were conducted on SG mortar specimens after completion of the freeze–thaw regime. This included both control specimens (CS), conditioned in water throughout the 50 cycles, and test specimens (FT), subjected to the 50 freeze–thaw cycles. The post-exposure microstructural evaluation aimed to detect potential alterations (e.g., microcracking or phase modifications) and enable comparative analysis between the two conditioning regimes. Microstructural analyses were performed on SG A-1 30% (CS, FT), SG A-2 30% (CS, FT) and reference R (CS, FT) mixes.

5.6.1. SEM Analysis of SG Mortars

Figure 19 shows the SEM imaging on SG A-1 30% (CS, FT), SG A-2 30% (CS, FT) and reference R (CS, FT) mixes. SEM analysis revealed the presence of calcium silicate hydrate (C-S-H) and ettringite on the specimen surfaces. Mortars incorporating garnet residue exhibited comparable crystalline structures, indicating that the use of spent garnet as aggregate does not adversely affect the cement hydration reactions.

Comparative SEM analysis of freeze–thaw (FT) exposed specimens reveal clear microstructural alterations compared to water-conditioned controls. The reference mortar (R-FT; Figure 19, images c and d) exhibits the most pronounced degradation features—such as increased microcracking, surface erosion, and pore coarsening—which align closely with its substantial mechanical strength losses (Table 4, Figure 17). In contrast, SG-modified mortars display milder deterioration, consistent with their superior residual strengths. Across all specimens, hydration products—primarily calcium silicate hydrate (C-S-H)—and secondary calcium carbonate deposits are observed, suggesting potential self-healing mechanisms that may partially mitigate damage induced by cyclic freezing and thawing [72]. These findings further support the enhanced durability conferred by SG incorporation.

The chemical composition was determined by energy-dispersive X-ray spectroscopy (EDX). The results are presented in

Table 5. Chemical analysis via EDX spectra of SG mortars, R and SG A 30%.

Element	R-W	R-FT	SG A-1 30%—W	SG A-1 30%—FT	SG A-2 30%—W	SG A-2 30%—FT
O	49.9	45.7	43.3	42.8	51.2	41.6
Fe	0.6	0.8	1.5	2.7	1.7	4.0
C	18.8	17.2	16.4	16.3	19.3	15.8
Si	7.3	9.5	19.1	9.6	4.8	6.0
S	0.0	0.0	0.0	0.2	0.0	0.7
Mg	0.4	0.0	0.0	0.0	0.5	0.5
Ca	20.6	22.1	16.2	18.6	18.8	27.2
Mn	0.0	0.0	0.0	0.4	0.0	0.0
Mo	0.0	1.9	0.0	0.0	0.0	0.0
Na	0.0	0.0	0.0	1.5	0.0	0.0
Br	2.3	2.8	3.5	7.3	3.0	4.2
K	0.0	0.0	0.0	0.7	0.0	0.0
Ti	0.0	0.0	0.0	0.0	0.8	0.0
Total	99.9	100.0	100.0	100.1	100.1	100.0

and they are confirming the decrease in copper concentration, initially present in SG A-2 (Table 3), demonstrating the capacity of the cementitious matrix to effectively immobilize and confine heavy metals [35].

5.6.2. XRD Analysis of SG Mortars

Figure 20 presents the XRD patterns of SG mortars: SG A-1 30% (CS, FT), SG A-2 30% (CS, FT) and reference R (CS, FT) mixes.

The XRD patterns of R FT and R W are primarily characterized by calcite (CaCO₃) and quartz, although minor albite is also detected in R FT. Specifically, the characteristic calcite peaks at approximately 29.4°, 39°, 43°, and 47° 2θ confirm its presence, while quartz is identified at ~26.6° 2θ [68]. Taken together, these observations suggest a carbonate-rich matrix, which is likely derived from recrystallized limestones or calc-silicate facies. In contrast, SG A-1 30%-W exhibits quartz as the major phase, accompanied by minor amounts of calcite, anorthite, and almandine. Similarly, SG A-1 30%-FT shows a comparable mineral assemblage; however, the garnet content is more pronounced. The coexistence of silicate and carbonate phases in these samples implies metamorphic overprinting within mixed lithologies [67,68,69]. Furthermore, the diffraction patterns of SG A-2 30%-W and SG A-1 30%-FT reveal a more complex assemblage, comprising calcite, quartz, anorthite, almandine, and andradite, with albite also present. Notably, the simultaneous occurrence of multiple garnet end-members (almandine and andradite) together with different feldspar varieties indicates that metasomatic processes likely occurred, possibly involving Ca-rich fluid infiltration and differential recrystallization of plagioclases [6,67].

5.6.3. FT-IR Analysis of SG Mortars

The results of the FT-IR test on SG mortars and Reference are presented in Figure 21. FT-IR spectroscopy provides clear evidence that the garnet waste samples maintain their fundamental silicate structure, while slight variations among spectra reveal differences in contamination, hydration, and surface chemistry resulting from industrial application and environmental exposure [13,70,71,73,74,75].

The microstructural observations provide a coherent explanation for the mechanical and durability performance of the SG-containing mortars. SEM analysis performed on the reference and SG 30% mixtures revealed a dense hydration matrix dominated by well-developed C–S–H gel and finely distributed ettringite, with SG particles well embedded in the cement paste. Compared to the reference mortar, the SG 30% mixture exhibited a more compact interfacial transition zone (ITZ), promoting improved stress transfer and explaining the observed increases in flexural and compressive strengths. This densification, however, is associated with a redistribution of pore sizes rather than a reduction in total porosity. The refinement of the microstructure and ITZ leads to enhanced capillary pore connectivity, which accounts for the increased total water absorption and, more distinctly, the higher capillary water absorption coefficients measured for SG-containing mortars. Although direct microstructural evidence is not available for the SG 50% mixtures, their performance suggests that similar mechanisms persist at higher replacement levels, with ITZ improvement reaching a threshold beyond which capillary continuity increases without proportional gains in flexural strength. These results support a schematic interpretation in which SG incorporation enhances mechanical performance at moderate replacement levels while moderately increasing moisture transport due to refined but more continuous capillary pathways.

Similar microstructural benefits—denser interfacial transition zones, increased formation of secondary C-S-H, pore refinement, and reduced microcracking after environmental exposure—have been reported in mortars and concretes containing spent garnet [20,24,26] as well as analogous fine hard mineral wastes such as crushed granite fines [76], high-purity quartz powder [77], and fine recycled glass powder [78]. These studies consistently observed improved cohesion, enhanced secondary hydration products, and better resistance to freeze–thaw-induced damage when fine, inert, angular particles were used as partial sand replacement, which aligns closely with the present findings on spent garnet (SG) mortars. The notably reduced extent of microcracking in SG specimens after 50 freeze–thaw cycles is particularly consistent with the enhanced durability and microstructural stability documented for granite fines [76] and spent garnet in high-strength matrices [20,26].

6. Conclusions and Future Research Directions

This work presents a comprehensive and long-term investigation providing highly relevant evidence for the valorization of spent garnet (SG) waste from single-cycle abrasive waterjet (AWJ) cutting as a sustainable partial substitute for natural fine sand in cement-based mortars. The study spans more than four years of experimental research, incorporates multiple SG sources with different physical characteristics, and addresses material behavior through an integrated program of physical, chemical, microstructural, mechanical, and durability evaluations. As such, it provides a robust and representative assessment of SG variability and performance consistency, significantly enhancing the relevance and generalizability of the findings. By incorporating material from multiple industrial sources [5,79,80,81] and repeated samplings over time, the research aims to capture real-world variability, smoothing the path towards practical applicability.

Based on the experimental results, the following observations can be highlighted:

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From a physical and granulometric standpoint, SG exhibits a significantly finer particle size distribution compared to conventional sand (0/4), resulting in improved packing density when incorporated into blended mixes. This effect was reflected in systematically increased dry densities for SG-containing mortars, particularly at 50% substitution, confirming the beneficial filler effect of SG on the granular skeleton.

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PSD, fresh-state consistency (flow table), and density evaluations confirmed that SG incorporation improves workability and increases both fresh and dry densities due to superior particle packing and finer granulometry.

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Chemical analyses (EDX, XRD, FT-IR) confirmed the inert nature of SG, with negligible heavy metal content (Cu was noticed in SG A-2 sample), successfully neutralized after incorporation into cement mortar.

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Microstructural analyses (EDX, XRD, FT-IR) showed the stable mineralogical composition (primarily almandine-rich garnet phases), ensuring environmental safety and regulatory compliance. Investigations on SG mortars revealed good integration of SG particles within the cementitious matrix, with abundant C-S-H formation and no disruptive phase alterations, with no evidence of weak interfacial zones and deleterious reactions or incompatibilities, confirming the previous findings of the international studies. The absence of adverse chemical interactions supports its suitability as a secondary raw material in cement-based systems. Post-freeze–thaw exposure, SG-modified mortars exhibited reduced microcracking and milder degradation compared to the reference, indicating enhanced microstructural stability and potential self-healing contributions from secondary carbonate deposits.

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In terms of mechanical performance, SG incorporation—particularly at 30% replacement—maintained or enhanced strength-related indicators (as discussed previously), confirming that the densified microstructure translates into structurally viable mortars, comparable to or superior to the reference. Higher replacement levels (50%) remained mechanically acceptable but exhibited more pronounced changes in transport-related properties.

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Durability Performance: Freeze–thaw resistance (50 cycles, adapted SR 3518:2009 [58]) showed markedly lower strength losses, negligible mass loss, and preserved visual integrity in SG mortars relative to the reference. Total water absorption and elevated capillary water absorption coefficient (probably due to finer interconnected pores) indicate that SG-containing mortars exhibit moderately increased moisture transport compared to the reference mix. However, these values remain within ranges commonly reported for mortars incorporating fine secondary aggregates. Importantly, no abrupt performance degradation was observed, suggesting acceptable durability for controlled-exposure applications.

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The structured three-stage validation procedure fully met its predefined criteria:

• (A) Extended Compatibility Assessment: Mechanical strengths from multiple SG sources and sampling periods remained within ±20% of the reference, confirming broad compatibility across heterogeneous industrial waste streams.
• (B) Core Validation: Consistent performance trends (stable patterns) were observed across temporally separated batches from the same sources, despite minor absolute variations, verifying repeatability and robustness.
• (C) Durability Assessment Residual strengths > 50% of controls, minimal mass loss, and limited visual/structural degradation validated superior freeze–thaw resistance.

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Sustainability and Circular Economy Benefits: By repurposing an industrial by-product traditionally destined for landfill, SG valorization reduces natural sand extraction, lowers cementitious material carbon footprint, and promotes resource circularity. The absence of significant environmental risks (inert chemistry) further supports eco-compatible construction practices.

Limitations and Future Research Directions:

Despite its comprehensive scope, the study is limited by its focus on mortar-scale applications and laboratory-controlled exposure conditions. Further research is needed to assess long-term durability under aggressive environments (e.g., chloride ingress, carbonation), upscale the approach to concrete formulations, and evaluate performance variability under industrial production conditions. Near-future work is already prepared to address pilot-scale demonstrations and performance-based standards alignment to further support industrial adoption and regulatory acceptance, to facilitate technological transfer to the construction and precast industries. Standardization of SG pre-treatment and quality control protocols is a key issue for consistent industrial adoption. In particular, a full Life Cycle Assessment (LCA) is required to quantitatively validate the environmental benefits of SG valorization, accounting for collection, processing, transport, and substitution scenarios.

The research is limited to spent garnet (SG) waste generated exclusively from single-cycle abrasive waterjet (AWJ) cutting processes. It does not encompass garnet residues from other common abrasive applications, such as abrasive blasting in shipbuilding or surface preparation, where the garnet is often multi-cycle and may accumulate chemical contaminants (e.g., heavy metals, paints, rust, or organic residues) from the blasted substrates. These alternative sources could introduce additional challenges related to chemical stability, leaching behavior, and regulatory compliance, potentially requiring dedicated pre-treatment or risk assessment protocols not addressed in the present work.

While the results obtained on mortars provide a solid foundation for the valorization of spent garnet waste, the transition to concrete mixtures will likely require additional considerations, including adjustments to aggregate grading, paste volume, and superplasticizer dosage to compensate for the higher water demand of finer SG particles. Future studies should therefore investigate the rheological behavior, mechanical performance under standard curing and accelerated aging conditions, and long-term durability of SG-modified concrete in order to fully assess its industrial applicability.

Overall, this study provides a scientifically rigorous, industrially relevant, and sustainability-driven framework for the valorization of waste garnet sand in cement-based materials, significantly contributing to the advancement of circular economy strategies in the construction sector.