Effect of Glass Cullet Content on the Mechanical and Compaction Behavior of Cement-Bound Granular Mixtures for Road Base/Subbase Applications

Featured Application

The results of this study can be directly applied to the design of sustainable road base and subbase layers using cement-bound granular mixtures (CBGMs) incorporating recycled glass cullet. Mixtures containing 20% glass cullet achieved the highest compressive strength (9.4 MPa), while those with 30% ensured maximum waste utilization with acceptable mechanical performance. The findings provide practical guidance for balancing strength and environmental benefits, supporting the implementation of circular economy principles and the use of locally available glass waste in road construction under Central European climatic conditions.

Abstract

The growing accumulation of glass waste and the limited availability of natural aggregates present major challenges for sustainable road construction. This study aimed to evaluate the influence of the glass cullet content (GC) in the range of 0–30% on the mechanical and compaction properties of cement-bound granular mixtures (CBGM 31.5 mm, R_c class C_5/6) intended for the road base and subbase layers. Laboratory tests were carried out to analyze the effect of GC on the optimum moisture content (OMC), the maximum dry density (ρ_d,max), and the compressive strength after 7 and 28 days (R₇, R₂₈). The results showed a systematic decrease in OMC and ρ_d,max with increasing GC content, by approximately 18% and 2.8%, respectively, for the mixture containing 30% glass. All CBGM mixtures met the strength requirements for class C_5/6 (R_c = 6–10 MPa), with the highest value of R28 obtained for the mixture containing 20% GC (9.4 MPa), representing a 24% increase compared to the reference mix. The relationship between GC content and compressive strength was best described by a second-degree polynomial function (R² = 0.60–0.65), indicating an optimum within the 10–20% range. Strength enhancement was attributed to synergistic effects of physical mechanisms (filler effect and improved particle packing) and chemical activity (pozzolanic reactivity of fine glass fractions). The 30% GC mixture provided the minimum required strength while achieving the highest level of waste utilization and environmental benefit. Therefore, the optimal GC content should be determined as a balance between mechanical performance and sustainable use of secondary materials in the temperate climatic conditions of Central Europe.

1. Introduction

The development of sustainable pavement materials has become a priority in modern road engineering due to the growing scarcity of natural aggregates and the need to reduce CO₂ emissions related to construction. Among various solutions, the use of secondary raw materials such as glass waste offers both environmental and economic benefits, supporting the principles of the circular economy and sustainable infrastructure design.

1.1. Background and Classification of Cement-Bound Granular Mixtures

Hydraulically bound mixtures (HBMs) represent a fundamental group of construction materials used in the load-bearing layers, base courses, and frost protection layers of road pavements. One of their key types is cement-bound granular mixtures (CBGMs), whose properties, composition, and classification requirements are defined by the EN 14227-1 [1] standard, while the procedures for strength testing are specified in EN 13286-41 [2]. In road engineering practice, the most commonly used CBGMs for base courses are classes C_5/6–C_8/10, which correspond to compressive strengths in the range of 6–10 MPa after 28 days. Meeting these requirements is the fundamental criterion for the acceptance of mixtures in road structures, both in terms of durability and load-bearing capacity. Recent studies have demonstrated that recycled materials such as reclaimed asphalt pavement (RAP) can successfully replace natural aggregates in cement-bound granular mixtures. Majer and Budziński [3] reported that CBGM incorporating RAP achieved compressive strengths below 10 MPa, corresponding to classes C_5/6–C_8/10, which are suitable for road base and subbase layers. These results confirm the potential of secondary raw materials in hydraulically bound mixtures and provide a background for extending this approach to other recycled components such as glass cullet.

1.2. Use of Glass Waste in Pavement Materials

In the United States, research on the use of glass cullet in asphalt pavements, known as Glasphalt, began in the 1970s. Several states, including New York, Florida, and California, conducted experimental applications of asphalt mixtures containing glass in the base, binder, and even surface layers. Based on years of experience, the American Association of State Highway and Transportation Officials (AASHTO) and the Federal Highway Administration (FHWA) developed technical specifications and guidelines recommending the use of glass in amounts up to 15% of the aggregate mass, in fine fractions passing the 4.75 mm sieve. This limitation helps to reduce the risk of segregation, cracking, and surface damage [4,5,6]. Although initial studies focused on asphalt mixtures, the knowledge gained provided a foundation for further research on the use of glass cullet in hydraulically bound materials, such as cement-stabilized mixtures, where analogous physical and rheological mechanisms, friction, packing, and adhesion, play a significant role [7].

More recently, studies by Arulrajah et al. [8] and Bilgen and Altuntaş [9] have extended this approach to cement- and lime-stabilized pavement layers containing recycled glass, demonstrating that moderate glass contents (up to 20%) can improve stiffness and compaction, while excessive dosages lead to loss of structural integrity. Other works have also investigated hybrid systems combining recycled glass with supplementary binders and alkali-activated or geopolymeric matrices for pavement and soil stabilization applications. These studies have reported promising mechanical and durability performance under laboratory conditions, as the use of finely ground glass or glass aggregate can contribute to the formation of N–A–S–H and C–A–S–H gels, improving matrix densification and reducing the carbon footprint of base and subbase materials [10,11,12].

Building upon these developments, further investigations have focused on alkali-activated and geopolymer binders specifically designed for road base and subbase applications. Xiao et al. [13] demonstrated that glass-powder-based geopolymer binders incorporating recycled glass aggregate achieved compressive and resilient moduli comparable to cement-treated bases, while also exhibiting improved freeze–thaw resistance and lower CO₂ emissions. Similarly, Kalatehjari et al. [14] confirmed the potential of alkali-activated waste glass for sustainable stabilization of weak road subgrades, providing a viable low-carbon alternative to Portland cement. In a comprehensive review, Zafar et al. [15] highlighted that glass-based alkali-activated binders (GBAACs) and alkali-activated waste glass (AAWG) can be successfully applied in pavement and foundation layers, offering both mechanical efficiency and environmental benefits. Complementary studies and reviews by Ogundana and Afolalu [16] and Indraratna et al. [17] emphasized the growing implementation of recycled glass in transportation infrastructure and road construction. Together, these works demonstrate that the integration of glass aggregate and glass powder into cement-treated and geopolymer-bound pavement materials represents a feasible strategy for enhancing sustainability in road-base design while maintaining required mechanical performance.

1.3. Environmental Significance and Circular Economy Context

From an environmental perspective, the management of glass waste plays a significant role in the material balance of a circular economy. Although glass is fully recyclable, it is often left in landfills due to contamination, heterogeneous composition, and the high costs of segregation. According to life cycle assessment (LCA) analyses, the use of glass cullet in road construction can substantially reduce the consumption of natural aggregates and the CO₂ emissions associated with their extraction and transportation, similar to other recycling-based technologies [18,19,20,21,22,23]. In addition, the utilization of locally available glass waste in hydraulically bound materials such as CBGM contributes to lowering the overall environmental footprint throughout the pavement life cycle. Incorporation of these secondary raw materials into road construction technologies represents a practical implementation of the principles of circular economy by minimizing waste generation, reducing the exploitation of primary resources, and decreasing carbon dioxide emissions [24].

The verification of these effects under temperate climatic conditions is essential for the practical implementation of CBGM with glass waste in European road construction.

1.4. Review of Previous Research on Glass in Cementitious Mixtures

In recent years, there has been a growing interest in the use of glass waste material (GWM) as a substitute for natural aggregates or as a micro filler in cement-bound materials. Studies by Arnold et al. [25] and Arabani et al. [26] demonstrated that a moderate amount of glass in cement-stabilized mixtures can maintain or even enhance the stiffness and bearing capacity of the base layers. Más-López et al. [27] extended the scope of the analysis by investigating the use of glass waste in soil stabilization and confirmed that its addition improves mechanical properties and reduces cement consumption in soil–glass–cement mixtures. Similarly, Ismail and Al-Hashmi [28] found that concrete containing 20% crushed glass as a partial replacement for fine aggregate exhibited approximately 4% higher compressive strength after 28 days compared to the reference mixture, further confirming the beneficial effect of a moderate glass content in cementitious materials. Czapik et al. [29] analyzed the influence of waste glass used both as fine aggregate and as a micro filler in cement mortars. Their findings showed that the inclusion of fine glass powder improved the microstructure and compressive strength of the material due to the pozzolanic activity of the glass particles.

An analysis of previous research indicates that the optimal glass content in cement-based mixtures typically ranges between 15% and 20%, beyond which the observed benefits gradually diminish. Siddika et al. [30] reported that numerous studies confirm a reduction in strength when the glass content exceeds approximately 20%. Ahmed et al. [31], who investigated cement concrete in which glass cullet replaced natural sand, found a similar trend; the compressive strength increased up to 20% glass content, after which further addition resulted in a decrease.

In cement-based materials, the mechanism of glass action is two-fold:

• The filler effect, in which fine glass particles fill the pores within the cement matrix, improving packing density and stress transfer [9,32];
• The pozzolanic reactivity of fine fractions (<75 µm), which react with calcium hydroxide to form additional C–S–H hydration products [32,33,34].

As demonstrated by Li et al. [32] and Zhao et al. [35], both the microfilling effect and the pozzolanic reactivity of fine glass fractions can lead to increased strength and reduced structural porosity, even despite a slight decrease in bulk density. Bilgen and Altuntaş [9] confirmed that the optimal glass content in cement and lime-stabilized materials usually falls within the range of 10–20%, while higher contents result in the loss of matrix continuity and reduced layer load capacity. These findings are consistent with the observations of Perera et al. [22] and Pacheco-Torres et al. [36], who indicated that properly selected proportions of glass waste can positively affect both the mechanical properties and the durability of hydraulically bound base layers.

Studies [25,26] also make a significant contribution to the development of sustainable road materials, which is also made by studies [37,38], which indicate that the incorporation of recycled materials and fibers into CBGM mixtures improves their mechanical properties and resistance to cracking. Nowak et al. [39] confirmed the feasibility of using recycled glass waste residues in the production of mineral aggregates, while Kurpiska et al. [40] demonstrated the effectiveness of lightweight glass aggregates in frost-protective layers. On a global scale, research by Wen et al. [41] shows that the synergistic combination of glass aggregates and glass powder in cementitious materials provides a beneficial strengthening effect on the ITZ structure.

Although numerous studies have investigated the use of glass in concrete and other cementitious materials, relatively limited research has focused on its influence on the properties of hydraulically bound road mixtures such as cement-bound granular mixtures (CBGMs). It should be noted that most existing studies on the incorporation of glass cullet into cement-stabilized road materials have considered glass contents that do not exceed 20–25% by aggregate mass. Only a few studies have extended this range to 30%, reporting, however, experimental challenges and a decline in the structural performance of the layer at higher glass contents. Arulrajah et al. [8] examined unbound mixtures containing glass waste material used in subbase and subgrade layers under warm Australian climatic conditions. Their findings indicated that when the glass content reached 30%, the stability of the material deteriorated compared with mixtures incorporating lower proportions of glass. Further investigations of recycled concrete aggregate (RCA) and glass cullet stabilized with 3% General Blend (GB) cement—a medium-setting cement type with medium setting commonly used in Australian road base applications, revealed similar trends [7]. The results showed that strength development persisted up to approximately 20–25% glass content; however, a further increase to 30% resulted in a reduction in both unconfined compressive strength (UCS ≈ 6–9 MPa) and stiffness modulus, corresponding to the CBGM classes C_5/6–C_8/10. Bilgen and Altuntaş [9] investigated cement and lime-stabilized mixtures that incorporate glass powder intended for the base layers. Their experimental program included mixtures containing 0–25% glass powder. The results showed that increasing the glass content to around 20% enhanced both compaction and mechanical strength (CBR and UCS), while at 25% these parameters began to decline. Microstructural analyses (SEM and XRD) confirmed that fine glass particles contributed to the formation of hydration products and the densification of the cement–lime matrix.

In parallel with conventional hydraulic binders, recent research has focused on the development of alkali-activated and geopolymer systems utilizing industrial by-products such as fly ash, slag, and glass waste. These binders exhibit reduced carbon footprints and favorable mechanical and durability properties, offering a sustainable alternative to Portland cement in pavement and structural applications [11,12,30].

In addition to mechanical performance, durability aspects such as alkali–silica reactivity (ASR) and freeze–thaw (F-T) resistance are crucial when incorporating glass into cementitious mixtures. The risk of ASR increases with higher glass content due to amorphous silica, but it can be effectively mitigated by fine grinding (<100 µm) and by using supplementary cementitious materials (SCMs) such as fly ash or slag [42,43,44]. Finely ground glass powder also enhances matrix densification through pozzolanic reactions [45]. At the microstructural scale, recent investigations have clarified the mechanism of ASR inhibition. Xiao et al. [46] demonstrated, using boron as a tracer, that in hyperalkaline environments the formation of surface precipitates on reactive silicate phases can substantially inhibit their dissolution, thereby mitigating the progression of alkali–silica reaction (ASR). Moreover, studies have shown that adequate particle grading and air entrainment can maintain or even improve the freeze–thaw durability of glass-modified concretes [47,48]. These mechanisms should be further validated for CBGM under temperate climatic conditions, where cyclic freezing and thawing may intensify ASR-related degradation.

1.5. Research Gap and Climatic Limitations of Previous Studies

It should be noted that most of the aforementioned studies of road materials were conducted under warm or subtropical climatic conditions—specifically in southern Australia [7,8] and northern Iran [26]—with laboratory tests performed at temperatures of 20 to effects of 25 °C, without exposure to frost or moisture. As a result, the mechanical and structural relationships identified in those studies may not fully reflect the behavior of CBGM mixtures under Central European climatic conditions, which are characterised by significant temperature fluctuations and numerous freeze–thaw cycles. These factors affect not only the integrity and continuity of the cementitious matrix but also the interfacial transition zone (ITZ) between glass particles and the cement paste, leading to local thermal stresses, microcracking, and variations in bond strength. Consequently, even mixtures with similar compositions may exhibit substantially different mechanical properties depending on the prevailing climatic conditions. Therefore, it should be recognized that the influence of higher glass content (more than 25%) in cement-stabilized road materials is still not sufficiently understood, and its effects require verification under both laboratory and field conditions. The present study addresses this research gap by analysing CBGM mixtures designed for a temperate–cool climate typical of Central Europe.

1.6. Scope, Objectives and Novelty of the Present Study

This study experimentally evaluates the influence of glass cullet (GC) content in the 0/5.6 mm fraction on the mechanical behavior of cement-bound granular mixes (CBGM 0/31.5 mm) of strength class C_5/6. The investigation includes the evaluation of compressive strength after 7 and 28 days according to the EN 13286-41 standard [2], as well as the relationship between density and strength.

The main objective of the study is to identify the optimal range of GC content (approximately 10–20%) in which the mixtures meet the requirements of the CBGM C_5/6 strength class while exhibiting improved microstructural characteristics. The tested mixtures are suitable for use in the subbase layer of pavement structures designed for heavy and very heavy traffic (7.30 × 10⁶ < ESAL₁₀₀ ≤ 52.00 × 10⁶), as well as in base layers intended for medium traffic conditions (0.50 × 10⁶ < ESAL₁₀₀ ≤ 7.30 × 10⁶).

The novelty of this study, compared to previous research, lies in the use of a wider range of glass cullet content in the base layers (up to 30%), as well as the application of regression modelling to quantitatively describe the nonlinear relationships between the content of glass waste material (GWM) and strength parameters. Unlike most previous studies focused on cement concrete or mortars, this research focuses on road construction materials of the type CBGM, for which it has been demonstrated that a moderate addition of glass cullet allows compliance with the C_5/6 R_c class requirements while simultaneously reducing bulk density. In addition to evaluating the mechanical properties, the study also analyzed the influence of the glass cullet content on the compaction parameters, including optimum moisture content (OMC) and the maximum dry density (ρ_d,max). Furthermore, the feasibility of applying up to 30% glass cullet was verified to determine whether this level allows achieving maximum environmental and economic benefits while maintaining the minimum required mechanical performance of CBGM mixtures. The results obtained provide a basis for further applied research and planned verification of laboratory observations on a real scale through the construction of a test section. This study also represents an attempt to assess the potential of the investigated waste material in the context of sustainable road construction.

The present study constitutes the first stage of a wider research program aimed at developing and validating sustainable CBGM mixtures that incorporate glass waste. This phase was designed as a pilot laboratory investigation focusing on the key classification parameters required by EN 14227-1 [1] and the national guidelines WT-5:2010 [49], namely compressive strength and compaction characteristics. In subsequent stages, the research will be extended to include a broader set of mechanical and durability tests, as well as field verification through the construction of a full-scale trial pavement section to confirm the laboratory findings under real traffic and environmental conditions.

Based on the identified research gap, the subsequent sections of this paper present the materials, methodology, and experimental results that verify the influence of glass cullet content on the mechanical and compaction behavior of CBGM. The findings of this pilot phase provide an initial framework for the development of design recommendations for hydraulically bound mixtures containing waste glass under Central European climatic conditions, and may also serve as a reference for similar applications in other regions of Central and Eastern Europe, where comparable climatic and economic factors affect pavement construction and rehabilitation practices. However, the verification of the material’s long-term performance of the material under variable temperature and moisture conditions requires further research, including extended durability tests and field validation.

2. Materials and Methods

2.1. Experimental Plan

Four hydraulically bound mixtures, composed of controlled graded aggregate and cement, were subjected to analysis. The constituent materials were mixed to ensure the production of a homogeneous mixture. To evaluate the effect of glass waste materials (GWMs) on the physical and mechanical properties of the cement-bound granular mixture (CBGM), the following mixture variants were prepared for testing:

• M:1_ref—reference CBGM mixture without glass waste material addition;
• M:2_10—CBGM mixture containing 10% glass cullet (GC) by mass;
• M:3_20—CBGM mixture containing 20% glass cullet (GC) by mass;
• M:4_30—CBGM mixture containing 30% glass cullet (GC) by mass.

The percentage of GC was assumed to be in relation to the total mass of the dry mineral mixture (sand, aggregate, glass cullet). CBGM mixtures were designed to meet the requirements of the WT-5 2010 national guidelines [1], corresponding to the R_c class C_5/6 according to EN 14227-1 standard [2]. The compressive strength, determined after 7 and 28 days in appropriately prepared specimens of the bound mixtures, was adopted as the main parameter to assess the influence of GWM on the properties of CBGM.

Within the scope of the laboratory research program, the following standardized tests were performed, necessary for the proper design of the CBGM mixtures and for the comparative evaluation of their physical and mechanical properties:

• Sieve analysis of glass and aggregates according to EN 933-1 standard [50];
• Determination of the particle density and water absorption for glass according to the EN 1097-6 standard [51];
• Determination of the maximum dry density and the optimum moisture content of CBGM mixtures using the Proctor compaction method according to EN 13286-2 [52];
• Compressive strength test according to the EN 13286-41 standard [2].

The chemical composition of the cement and the limestone aggregate used in this study was verified based on supplier data sheets and corresponds to standard materials commonly used in CBGM production.

2.2. Properties of Raw Materials

2.2.1. Mineral Mixture

The following types of aggregates, which comply with the requirements of EN 13242+A1 [53], were used for the preparation of the CBDM mineral mixtures investigated:

• Fine aggregate—natural sand with a particle size distribution of 0/2 mm;
• Coarse aggregate—aggregate mixture with a particle size distribution of 0/31.5 mm.

Aggregates were obtained from mines located in the Świętokrzyskie region of Poland. The fine aggregate consisted of natural yellow washed sand (Figure 1a). The results of the sand sieve analysis according to the EN 933-1 standard [50] are presented in Figure 1b and are summarized in

Table 1. Grading of natural sand used in the CBGM mineral mixture.

Sieve Aperture Size (mm)	Percentage of Material Retained 100 × Ri/M1 (% by Mass)	Cumulative % Retained (% by Mass)	Reduced Fractions (%)
63	0.0	100.0	0.5
45	0.0	100.0
31.5	0.0	100.0
22.4	0.0	100.0
16	0.0	100.0
11.2	0.0	100.0
8	0.0	100.0
5.6	0.0	100.0
4	0.0	100.0
2	0.5	99.5
1	1.9	97.6	95.6
0.5	21.6	76.0
0.25	42.1	33.9
0.125	26.5	7.4
0.063	3.5	3.9
<0.063	3.9		3.9
Sum	100.0

. The particle size distribution was determined by dry sieving through standard sieves ranging from 0.063 to 63 mm, and the cumulative passing curves were used to verify compliance with the CBGM grading envelope.

The maximum particle size of the natural sand is 2 mm. The particulate matter content (<0.063 mm) is 3.9%.

Table 2. Basic properties of natural sand used in the CBGM mineral mixture.

Property	Test Method	Performance Properties/ Category
Aggregate sizes (d/D)	EN 933-1 [50]	0/2
Grading	EN 933-1 [50]	GA85
Fines content	EN 933-1 [50]	f3
Apparent particle density ρa (Mg/m3)	EN 1097-6 [51]	2.650
Total sulfur	EN 1744-1 [54]	≤1.0

presents a summary of the geometric and physical characteristics of the natural sand used in the CBDM mineral mixtures.

The aggregate mixture, with a nominal particle size range of 0/31.5 mm, was composed of limestone. The results of the aggregate sieve analysis according to the EN 933-1 standard [50] are presented in Figure 2b and are summarized in

Table 3. Grading of coarse aggregate used in the CBGM mineral mixture.

Sieve Aperture Size (mm)	Percentage of Material Retained 100 × Ri/M1 (% by Mass)	Cumulative % Retained (% by Mass)	Reduced Fractions (%)
63	0.0	100.0	80.1
45	0.0	100.0
31.5	5.2	94.8
22.4	14.3	80.5
16	15.5	65.0
11.2	15.8	49.2
8	9.0	40.2
5.6	8.4	31.8
4	6.9	24.9
2	5.0	19.9
1	2.0	17.9	9.9
0.5	3.5	14.4
0.25	2.5	11.9
0.125	0.9	11.0
0.063	1.0	10.0
<0.063	10.0		10.0
Sum	100.0

.

Sieve analysis shows that the coarse aggregate mixture is characterized by continuous grain size with a coarsest grain size of 31.5 mm and a dust content of 10%.

Table 4. Geometric and physical properties of the coarse aggregates used in the CBGM mineral mixture with respect to EN 13242+A1 [53].

Property	Test Method	Performance Properties/Category
Aggregate sizes (d/D)	EN 933-1 [50]	0/31.5
Grading	EN 933-1 [50]	GA85
Tolerance of grading	EN 933-1 [50]	GTA10
Flakiness index	EN 933-3 [55]	Fl35
Grain density:	EN 1097-6 [51]
Apparent particle density ρa (Mg/m3)		0/4–2.62 ± 0.03 4/16–2.66 ± 0.03 16/31.5–2.59 ± 0.03
Oven-dried particle density ρrd (Mg/m3)		0/4–2.57 ± 0.02 4/16–2.59 ± 0.02 16/31.5–2.55 ± 0.02
Saturated and surface-dried particle density ρssd (Mg/m3)		0/4–2.59 ± 0.02 4/16–2.62 ± 0.02 16/31.5–2.56 ± 0.02
Fines content	EN 933-1 [50]	f4
Water absorption	EN 1097-6 [51]	WA241
Resistance to fragmentation, Los Angeles test method	EN 1097-2 [56]	LA30
Freeze–thaw resistance	EN 1367-1 [57]	F2
Resistance to wear	EN 1097-1 [58]	MDE15

presents a summary of the geometric and physical characteristics of the coarse aggregate used in CBDM mineral mixtures.

The results of the sieve analysis and the geometric and physical properties of the aggregates tested, as declared by the producers (road aggregate quarries), confirm compliance with the requirements specified in the national guideline [49] or aggregates intended for CBGM 0/31.5 mixtures of strength class C_5/6. Mineral mixtures exhibited a continuous classification (G_A85 according to the EN 933-1 standard [50]), which is beneficial for the packing of the particle skeleton, resulting in higher maximum dry density and reduced porosity after the mixture hardened. The moderate content of fine particles (<0.063 mm) in the applied aggregates, corresponding to categories f₃ and f₄ according to EN 13242+A1 [53], is advantageous for the design of the CBGM mix, as it provides adequate cohesion and filling of the voids between particles, thus promoting proper compaction and structural stability of the mixture.

These materials can be used for the construction of the subbase layer of pavement structures designed for heavy and very heavy traffic (7.30 × 10⁶ < ESAL₁₀₀ ≤ 52.00 × 10⁶), corresponding to the number of equivalent 100 kN standard axles per traffic lane over the design period, typical of national, main and motorway routes with a high proportion of heavy vehicles. Aggregates analysed can also be applied in base layers intended for medium traffic conditions (0.50 × 10⁶ < ESAL₁₀₀ ≤ 7.30 × 10⁶), characteristic of provincial and regional roads with moderate heavy vehicle loading. These findings confirm the suitability of the analysed aggregates for use in the laboratory investigations presented in this paper.

2.2.2. Cement

The composition of the CBGM mixture was based on Portland fly ash cement with high early strength CEM II/B-V according to the EN 197-1 [59] standard, about standard strength class 32.5 R, in accordance with the EN 196-1 [60] standard. The performance properties of the cement declared by the manufacturer are listed in

Table 5. Manufacturer’s declared properties of cement used in CBDM.

Basic Characteristics	Test Method	Performance Features
Ingredients and composition (% by mass)	EN 197-1 [59]	Main constituents: Portland cement clinker: 65–79 Fly ash siliceous: 21–35 Secondary constituents: 0–5
Compressive strength (MPa): Early Standard	EN 196-1 [60]	≥10.0 ≥32.5 and ≤52.5
Initial setting time (min)	EN 196-2 [61]	≥75
Soundness: Expansion (mm)	EN 196-3 [62]	≤10.0
Sulfate content (as SO3) (%)	EN 196-2 [61]	≤3.5
Chloride content (%)	EN 197-1 [59]	≤10.0

.

The selected cement type is suitable for CBGM applications due to its moderate heat of hydration, improved workability, and improved long-term strength development associated with the pozzolanic activity of fly ash. Furthermore, the use of CEM II/B-V contributes to a reduction in CO₂ emissions and improved durability of hydraulically bound mixtures by refinement of the pore structure and increased resistance to shrinkage and cracking.

2.2.3. Glass Waste Material (GWM)

Glass waste materials were used for the production of the cement-bound granular mixture (CBGM), with a representative sample shown in Figure 3a. The tests used glass cullet obtained from municipal waste collection, which, after appropriate crushing, served as a fine aggregate in the CBGM mixtures. Two groups of glass waste of different origins were used, classified in the European Waste Catalogue [63] under the following codes:

• 15 01 07—packaging glass (bottles, jars, etc.);
• 20 01 02—glass waste from the municipal sector, excluding packaging waste.

The recovered glass material was stockpiled by a road construction company and subsequently crushed using crushers to obtain products with particle size ranges of 0/5.6 and 0/8 mm. According to the product specification provided by the supplier, the glass cullet consisted entirely of soda–lime glass with an oxide composition presented in

Table 6. Chemical composition of glass waste material (GWM) used in the CBGM mixture.

Oxide Composition/ Substance	Content (%)	CAS No. [66]	EC No. [67]
SiO2	70–74	14808-60-7	215-684-8
Al2O3	0.5–2	1344-28-1	215-691-6
CaO	7–11	1305-78-8	215-138-9
MgO	3–5	1309-48-8	215-171-9
Na2O	13–15 (Na2O + K2O)	1313-59-3	215-208-9
K2O	13–15 (Na2O + K2O)	12136-45-7	235-227-6
Fe2O3	max. 0.1	1309-37-1	-
TiO2	max. 0.1	13463-67-7	236-675-5

. Furthermore, the tested glass was characterized by a Mohs hardness of 6 [64] and a softening point of approximately 750 °C [65].

The oxide composition of the glass waste material (Table 6) corresponds to that of typical soda–lime glass, dominated by SiO₂ and alkali oxides (Na₂O and K₂O), with small amounts of CaO and MgO. This composition ensures chemical stability of the soda–lime glass in the alkaline environment of cement hydration, preventing undesirable expansive reactions [68,69]. However, very fine amorphous silica fractions may exhibit limited pozzolanic reactivity under certain conditions, as reported in previous studies [33,70]. The presence of sodium and potassium oxides may also induce a fluxing effect that promotes matrix densification during the curing process [71,72]. Due to its predominantly amorphous silica structure and the absence of hazardous components, glass waste can be safely used as a fine aggregate substitute in CBGM mixtures, improving particle packing and contributing to the sustainable utilization of raw materials [73].

For glass cullet, a sieve analysis was performed in accordance with EN 933-1 [50] (method for aggregates), the result of which is shown in Figure 3b and listed in

Table 7. Grading of glass cullet used in CBGM mixtures.

Sieve Aperture Size (mm)	Percentage of Material Retained 100 × Ri/M1 (% by Mass)	Cumulative % Retained (% by Mass)	Reduced Fractions (%)
63	0.0	100.0	48.1
45	0.0	100.0
31.5	0.0	100.0
22.4	0.0	100.0
16	0.0	100.0
11.2	0.0	100.0
8	0.0	100.0
5.6	4.4	95.6
4	16.1	79.5
2	27.6	51.9
1	27.4	24.5	48.3
0.5	12.1	12.4
0.25	4.9	7.5
0.125	2.8	4.7
0.063	1.1	3.6
<0.063	3.6		3.6
Sum	100.0

.

Sieve analysis showed that the maximum particle size of the glass cullet was 5.6 mm, with 4.4% retained on the corresponding sieve. The dust fraction represented 3.6% of the total sample mass. The obtained grading can be classified as continuous, covering both fine and medium fractions, promoting favorable particle packing within the CBGM structure. This distribution facilitates optimal workability and compaction during specimen preparation [72,73]. Fine glass fractions fill the voids between larger mineral aggregate particles, reducing porosity and improving the cohesion and volumetric stability of the bound material [74,75]. These characteristics are essential for achieving a homogeneous and durable CBGM structure, particularly in base layers designed for heavy traffic conditions.

For the selected glass cullet used in the study, the particle density and water absorption were determined according to EN 1097-6 [51]. The particle density was measured using the pycnometer method for two size fractions of glass waste materials:

• 4–5.6 mm;
• 0.063–4 mm.

Figure 4 shows the pycnometer containing the glass cullet fraction of 4–5.6 mm after the soaking period. The mass of the analytical sample was determined by weighing the material in saturated surface-dry (SSD) conditions, followed by reweighing after oven drying to a constant mass.

Figure 5 shows the stages of drying of the glass waste material fraction 0.063–4 mm and the progression of the cone slump to the condition most similar to that recommended in EN 1097-6 [51]. Five phases of the cone slump were identified, illustrated in the photographs, and schematically represented as follows:

• Phase 1 (Figure 5a,b)—very moist glass cullet, retaining almost the complete shape of the metal mould;
• Phase 2 (Figure 5c,d)—moist glass cullet, partially retaining the mould shape; a slight cone slump is observed;
• Phase 3 (Figure 5e,f)—slightly less moist glass cullet, partial slump of the cone observed;
• Phase 4 (Figure 5g,h)—slightly moist glass cullet, significant cone slump observed;
• Phase 5 (Figure 5i,j)—saturated and surface dry glass cullet, almost completely collapsed; the peak is visible and the slopes are angular.

The particle densities (ρ_a, ρ_rd, and ρ_ssd) were determined using the pycnometer method according to EN 1097-6 [51]. The samples were immersed in water and weighed in saturated-surface-dry and oven-dry states.

Table 8. Results of the test of the density and water absorption of cullet according to EN 1097-6 standard [51].

Physical Property	Type of Material
	Glass Cullet, 4–5.6 mm	Glass Cullet, 0.063–4 mm
M1 mass (g)	1007.9	1002.3
M2 mass (g)	4099.9	4088.3
M3 mass (g)	3493.9	3493.9
M4 mass (g)	1006.8	999.6
Apparent particle density ρa (Mg/m3)	2.512	2.468
Oven-dried particle density ρrd (Mg/m3)	2.505	2.451
Saturated and surface-dried particle density ρssd (Mg/m3)	2.508	2.457
Water absorption after immersion for 24 h WA24 (%)	0.11	0.22

presents the results of the particle density and water absorption tests carried out for two fractions of glass cullet (GC). To determine these parameters, the following masses were measured:

• M₁—the mass of saturated and surface-dried GC in air, in grams (g), determined for cone fall conditions in the fifth phase (Figure 5i,j);
• M₂—the apparent mass of the pycnometer containing the saturated GC and water, in grammes (g);
• M₃—the mass of the pycnometer filled with water only, in grammes (g);
• M₄—the mass of the oven-dried test portion in air, in grams (g), determined for the sample under the condition shown in Figure 6.

Figure 6 shows the appearance of the glass cullet sample after drying in a laboratory oven. In this case, the apex is no longer visible and the surface outline is rounded.

The apparent particle density ρ_a of the coarser glass cullet fraction was 2.512 Mg/m³, while for the finer fraction it was slightly lower, 2.468 Mg/m³. The values of the oven-dried particle density (ρ_rd) and the saturated and surface-dried density (ρ_ssd) were also slightly lower for the finer glass cullet fraction. The apparent particle density values obtained fall within the typical range for soda–lime glass (2.45–2.55 Mg/m³) [76,77]. This density is also comparable to that of natural aggregates commonly used in CBGM mixtures (density difference < 5%), such as 0/31.5 mm crushed aggregate and sand, which promotes a uniform particle distribution within the cement-bound matrix and reduces the risk of segregation during mixing and compaction.

As a result of its smooth, nonporous surface, glass may slightly affect the adhesion between the cement paste and the aggregate particles, which should be considered during mix proportioning. The glass cullet exhibited very low water absorption, that is, 0.11% for the coarse fraction and 0.22% for the fine fraction, which practically confirms the generally recognized non-absorbent character of glass (absorption < 0.5%) [77]. These values do not indicate the actual absorption of water by the material, but reflect the fact that, according to EN 1097-6 [51] (test method for aggregates), a small amount of water was likely retained within microcracks or on the surface of irregularly shaped particles. This should therefore be regarded not as material absorption but as apparent surface absorption, resulting from the following:

• The roughness and irregular geometry of the glass particles;
• Incomplete drying of individual particles;
• The presence of microcracks generated during the crushing process.

As a result of its smooth, nonporous surface and very low water absorption, the glass cullet used in CBGM may reduce the amount of water retained on the particle surfaces, which in turn can affect the rheological behavior of the mixture and should be taken into account during the mix design process.

In summary, from the obtained test results, it can be concluded that glass cullet can be safely used as a fine aggregate in CBGM mixtures without the risk of excessive density differences or particle segregation.

2.3. Experimental Methods for the Design and Testing of CBGM Mixtures

2.3.1. Mix Design of CBGM Mixtures

The CBGM mix design procedure was based on laboratory tests carried out according to the requirements of national guidelines [49]. The composition of the mixtures was designed with respect to the compressive strength of the compacted specimens using the Proctor method according to the EN 13286-50 standard [78], using cylindrical moulds with a height-to-diameter ratio (H/D) of 0.8. Within the scope of the study, the optimum moisture content (OMC) and maximum dry density (ρ_d,max) were determined in EN 13286-2 standard [52] and EN 13286-50 standard [78]. The optimum moisture content (OMC) and maximum dry density (ρ_d,max) were determined using the modified Proctor test in a 150 mm × 120 mm cylindrical mould. Each sample was compacted in three layers by 56 blows of a 2.5 kg rammer dropped from 305 mm. The density–moisture relationship was plotted to identify OMC and ρ_d,max. The dry density (ρ_d) was calculated from the measured bulk density and the water content. The water content was determined by oven-drying the samples at 105 ± 2 °C to a constant mass.

These parameters were used to establish the amount of water required to achieve optimum compaction and proper hydration of the binder. It was assumed that the compressive strength R_c of the mixture, determined according to the EN 13286-41 standard [2], should be equal to or greater than the value required for the respective strength class according to the EN 14227-1 standard [1], that is, a minimum of 6 MPa after 28 days of curing of cylindrical specimens with H/D = 1 (0.8–1.21) for the strength class C_5/6. Cylindrical specimens (150 mm × 120 mm) were cured for 7 and 28 days and then tested under unconfined compression using a hydraulic testing press. The loading rate was maintained at 1 ± 0.1 MPa/s until failure, and the compressive strength was calculated as the ratio of the maximum load to the cross-sectional area.

2.3.2. Grading of the CBGM Mineral Mixtures

The particle size distribution curves of the mixtures were selected so that they fell within the grading envelope specified in the national guidelines [49] and according to the requirements of the EN 13242+A1 standard [53] for aggregates used in hydraulically bound mixtures. The grading design aimed to obtain a continuous particle size distribution curve, ensuring optimum compaction, minimum porosity, and suitable mechanical properties after binder hardening. Grading was determined by sieve analysis according to EN 933-1 [50], covering particle size fractions 0 to 31.5 mm.

Several variants of the mixtures were prepared in the laboratory, differing in the proportion of coarse and fine fractions, to assess the influence of the grain size distribution on the compactability and compressive strength of CBGM. The grading curves obtained were evaluated in terms of compliance with the recommended envelope for the mixtures intended for base and subbase layers. The final grading was selected to ensure high bulk density and uniform coating of aggregate particles by the cementitious matrix, which contributes to the achievement of the required CBGM strength class.

2.3.3. Water Content in CBGM Mixtures

To determine the water content in the mixtures, the required degree of compaction and the expected mechanical performance of the CBGM were taken into account. The water content was established based on the results of Proctor compaction tests performed according to EN 13286-2 [52], EN 13286-50 [78], and ASTM D1557 [79], in which the optimum moisture content (OMC) and the maximum dry density (ρ_d,max) were determined for each mixture tested.

The optimum moisture content was adopted as the reference point for determining the amount of water necessary to prepare the specimens with the maximum degree of compaction and proper hydration of the binder. The final water content in the mixture was determined with reference to the total mass of dry constituents, taking into account the aggregate grading, bulk density, and the required mixture workability, ensuring uniform compaction and preventing particle segregation.

2.3.4. Binder Content in CBGM Mixtures

The cement content in the mixture was determined based on the mix design procedure using the proportions of the proposed components and the results of laboratory tests. The amount of binder was selected considering the grading of the mineral mixture, the fine fraction content, the target compressive strength, and the workability and compactability requirements. A minimum binder content of 3% was adopted according to the EN 14227-1 standard [1] for mixtures containing aggregates with a maximum nominal size greater than 8.0 mm and up to 31.5 mm.

2.3.5. Preparation and Curing of CBGM Test Specimens

Cylindrical specimens compacted using the Proctor rammer were prepared according to EN 13286-50 [78]. A type B mould with dimensions of D = 150 mm and H = 120 mm was used. A light type A rammer with a mass of 2.5 kg, a base diameter D = 50 mm and drop height H = 305 mm was used. The specimens were compacted in three layers, each layer receiving 56 blows. In the Proctor test, the size was selected in relation to the maximum particle size of the mixture (the mould diameter of the mould should be at least four times greater than the largest aggregate size).

The specimens were stored for 14 days at (20 ± 2) °C and nearly 100% relative humidity. Saturation was performed under normal pressure with full immersion of the specimens in water. For the material within the mould, the determination of the optimum moisture content of the optimal mixture and its bulk and dry density was prepared using the modified Proctor method acc. EN 13286-50 standard [78] and ASTM D 1557 standard [79]. The curing periods prior to testing were 7 and 28 days.

2.3.6. Determination of Compressive Strength of Specimens

Compressive strength tests were carried out after 7-day and 28-day curing of compacted CBGM specimens using the Proctor method according to the EN 13286-50 standard [78], following the testing procedure specified in the EN 13286-41 standard [2]. The compressive strength was determined using a hydraulic testing press (Form Test Prüfsysteme, Riedlingen, Germany) with a maximum load capacity of 3000 kN. The precision of the press and the load indication system allowed loading and measurement with an accuracy of ±1%. The failure stress was recorded during unconfined uniaxial compression testing. After each test, the specimen failure pattern was visually assessed and classified according to the EN 13286-41 standard [2] (types A–C). Photographic documentation of the fractured specimens was prepared to compare the crack morphology and its conformity with the characteristic failure modes typical of hydraulically bound mixtures.

For each series of specimens, the mean compressive strength value and the standard deviation were calculated. The test results were considered valid when the difference between the results of the individual specimens did not exceed 15% of the mean value.

2.4. Statistical Analysis Methods

The statistical evaluation of the obtained results was carried out using Statistica 13.3 software (TIBCO Software Inc., Palo Alto, CA, USA) and Microsoft Excel 365. For each CBGM mixture, the compressive strength results after 7 and 28 days of curing (R₇ and R₂₈) were subjected to descriptive statistical analysis, including the calculation of the arithmetic mean, median, standard deviation (SD), and coefficient of variation (CoV). Standard statistical procedures were applied according to the ISO 2602 standard [80] and EN ISO 5725-2 [81].

To verify whether the data followed a normal distribution within individual groups, the Shapiro–Wilk test (SW–W) was applied, while the Brown–Forsythe test was used to assess the homogeneity of the variances. After confirming that both assumptions were met, a one-way analysis of variance (ANOVA) was performed to determine whether the mean compressive strength values differed significantly between the analysed CBGM mixtures.

The adopted level of significance was α = 0.05, and results with p ≤ 0.05 were considered statistically significant.

The relationship between glass cullet content (GC) and compressive strength was examined using second-degree polynomial regression models, developed separately for the 7-day and 28-day datasets. The adequacy of the models was evaluated on the coefficient of determination (R²), the F statistic and corresponding p-values.

Graphical data visualisations, including radar charts, box-and-whisker plots, and regression curves with 95% confidence intervals, were prepared using the Statistica and Excel environments.

All analyses were performed at a 95% confidence level, according to recognized procedures for statistical analysis in materials engineering.

3. Results

3.1. Design and Characterization of CBGM Mixtures

3.1.1. Composition of Dry Components in CBGM

Four cement-bound granular mixtures (CBGMs) were prepared for laboratory tests. The first mixture served as a reference sample (M:1_ref) and did not contain any glass waste material. It consisted of natural aggregate, natural sand, cement, and water. The remaining three mixtures included glass cullet (GC) used as a fine aggregate substitute. The granular composition of the CBGM mineral mixture was designed within the grading envelope specified by the national guidelines [49]. The particle size distribution curves of the mineral mixtures used for the CBGM design are shown in Figure 7, while the proportions of the individual mixtures are presented in

Table 9. Composition of dry constituents in the designed CBGM mineral mixtures.

Components (Material Type)	Composition of Mineral Mixture (% by Mass)
	M:1_ref	M:2_10	M:3_20	M:4_30
Limestone, 0/31.5 mm	75.0	70.0	65.0	60.0
Natural sand, 0/2 mm	25.0	20.0	15.0	10.0
Glass cullet, 0/5.6 mm	0.0	10.0	20.0	30.0

. Each mixture was formulated by partially replacing natural sand (0/2 mm) with finely crushed glass cullet 0/5.6 mm in quantities of 10%, 20%, and 30% by mass (relative to the total dry aggregate mass), with a proportional reduction in the limestone aggregate fraction (0/31.5 mm).

All CBGM mixtures exhibit continuous grading curves (Figure 7) that lie within the envelope bounded by the upper and lower limit grading curves, in accordance with the national guidelines for hydraulically bound mixtures [49]. The progressive increase in glass cullet 0/5.6 mm at the expense of both mineral constituents (0/2 mm sand and 0/31.5 mm aggregate) in mixtures M:2_10, M:3_20, and M:3_30 produces a systematic and physically consistent shift of the curves: a slight decrease in the very fine range (≤1.0 mm) and a pronounced increase in the intermediate range (approximately 4–11.2 mm). This indicates a concurrent reduction in the share of very fine and very coarse particles and an increase in the proportion of intermediate fractions, which is typical for the addition of glass cullet graded 0/5.6 mm. The trend of the grading curves confirms that GC was incorporated in a controlled manner, with grading continuity preserved. As a result, the modified mixtures retain the characteristics of well-graded materials that meet the requirements for CBGM 0/31.5.

Figure 8 presents photographs of the mineral mixtures containing glass cullet (GC) after mixing the components in the intended proportions.

The binder content used in this study was assumed to be 4% of the composition of dry ingredients used in CBGM, which was 1% higher than the minimum required by the EN 14227-1 [1] standard. Figure 9 shows the proportions of dry components for each of the mixtures tested, taking into account the cement content.

As shown in Figure 9, all CBGM mixtures were designed with a constant binder (cement) content, while the proportions of sand and coarse aggregate were gradually reduced and replaced with an equivalent amount of glass cullet (GC). This approach was intended to evaluate the influence of increasing glass content on the particle packing density, the compaction behavior, and the mechanical performance of the mixtures. The maintenance of the balance between the coarse aggregate and the fine constituents ensured a continuous grading curve and the proper workability of the mixtures during specimen preparation.

3.1.2. Optimum Moisture Content (OMC) and Maximum Dry Density

The determination of the optimum moisture content of the optimal mixture and its bulk density was prepared using the modified Proctor method acc. EN 13286-50 standard [78] and ASTM D 1557 standard [79]. Figure 10 shows a photograph of the preparation of specimens for density determination using the Proctor method in the laboratory.

For each mixture, a minimum of five specimens were tested. Based on the experimental results, compaction curves showing the relationship between moisture content (MC) and dry density (ρ_d) were determined for each mixture, as presented in Figure 11. From these curves, the optimum moisture content (OMC) and the maximum dry density (ρ_d,max) were identified. The calculated parameters corresponding to the optimum compaction conditions for each CBGM mixture are summarized in

Table 10. Optimum moisture content (OMC) and maximum dry density (ρ_d,max) of the CBGM mixtures.

Tested CBGM	Mold Weight	Mold Weight with Specimen	Mold Volume	Optimum Moisture Content	Bulk Density	MaximumDry Density ρd,max
	(g)	(g)	(mL)	(%)	(Mg/m3)	(Mg/m3)
M:1_ref	9643.2	14,629.9	2115.0	7.7	2.358	2.189
M:2_10	9643.2	14,552.7	2115.0	6.8	2.321	2.173
M:3_20	9643.2	14,512.1	2115.0	6.4	2.302	2.164
M:4_30	9643.2	14,452.7	2115.0	6.3	2.274	2.139

.

Based on the compaction curves showing the relationship between moisture content and dry density (Figure 11) and the data presented in Table 10, a systematic decrease in both optimum moisture content and density (including both bulk density and dry density) was observed with increasing glass cullet (GC) content in CBGM mixtures. The extreme parameter values obtained for the M:4_30 mixture indicate that at 30% GC in the CBGM mineral mixture:

• The optimum moisture content (OMC) decreased by approximately 1.4% (from 7.7% to 6.3%), i.e., by about 18% compared to the reference mixture M:1_ref;
• The maximum dry density (ρ_d,max) decreased by 0.050 Mg/m³ (from 2.189 to 2.139 Mg/m³), i.e., by 2.8% compared to M:1_ref;
• The bulk density decreased by 0.084 Mg/m³ (from 2.358 to 2.274 Mg/m³), that is, by approximately 3.6% compared to M:1_ref.

The added soda–lime glass cullet used in the mixtures was characterized by a particle density of 2.512 Mg/m³ for the 4/5.6 mm fraction and 2.468 Mg/m³ for the 0/4 mm fraction, which is lower than that of typical natural aggregates (approximately 2.65 Mg/m³). Consequently, partial replacement of the mineral fraction with glass cullet results in a moderate decrease in the maximum dry density (ρ_d,max) of CBGM mixtures with increasing glass content. The nonporous and smooth surface of glass particles, combined with their low water absorption capacity (negligible water absorption of 0.11% and 0.22% for the tested fractions), limits the amount of water retained on particle surfaces compared to rougher and more porous natural aggregates. As a result, the optimum moisture content (OMC) of CBGM mixtures decreases with increasing GC content, since a smaller portion of the mixing water is bound in the near-surface zone of the particles and a greater portion remains as free water, influencing the rheology and compaction behavior. This trend is reflected in the reduced OMC values with increasing amounts of GC in the mineral mixture. Additionally, the decrease in maximum dry density is also attributed to the shape and texture of the glass cullet particles, which are more angular after crushing than natural aggregates. This may slightly increase the intergranular void ratio under the same compaction energy, leading to a reduction in ρ_d,max.

Summarizing the results of the above analyses, it can be concluded that the decrease in OMC and ρ_d,max with increasing GC content is consistent with the physical properties of glass (lower particle density, negligible water absorption, and smooth surface texture) as well as with the particle packing mechanics under a given compaction energy according to the Proctor method. In the practical design of CBGM mixes, this implies that mixtures containing higher amounts of glass cullet require lower moisture content to achieve optimum compaction and may exhibit slightly lower ρ_d,max values. These factors should be taken into account when selecting compaction parameters during field implementation and when performing density control tests.

Based on the determined OMC, the framework composition of the CBGM mixtures, including water content, was established and is presented in

Table 11. Composition of CBGM mixtures with water content considered.

Components of the CBGM Mixtures	Composition of the CBMG Mixtures
	Quantity (kg/m3)				Percentage (% by Mass)
	M:1_ref	M:2_10	M:3_20	M:4_30	M:1_ref	M:2_10	M:3_20	M:4_30
Limestone, 0/31.5 mm	1463	1377	1275	1188	66.5	62.6	58.4	54.0
Natural sand, 0/2 mm	486	391	297	198	22.1	17.9	13.5	9.0
Glass cullet, 0/5.6 mm	0	198	396	594	0.0	9.0	18.0	27.0
CEM II/B-V cement	81	81	81	81	3.7	3.7	3.7	3.7
Water	169	150	141	139	7.7	6.8	6.4	6.3

.

The glass cullet content 0/5.6 mm in the CBGM mixtures increases from 0 to 594 kg/m³, gradually replacing natural aggregate (0/2 mm sand and 0/31.5 mm aggregate), which corresponds to 0, 9, 18, and 27% by mass relative to the total mass of all constituents of the mixture. The designed water to cement ratios (w/c) for the CBGM mixtures ranged from 1.7 to 2.1, which is typical for hydraulically bound mixtures of the class C_5/6 strength class according to EN 14227-1 standard [1]. These proportions ensure adequate density and workability while maintaining sufficient water for cement hydration and subsequent strength development. In contrast to conventional concrete, where the w/c ratio largely governs porosity and strength, the water content in CBGM is primarily selected to achieve optimum compaction (OMC). The lowest water demand was observed for the M:4_30 mixture, which required approximately 30 kg/m³ less water than the reference mixture. This reduction contributes not only to decreased water consumption but also to lower energy requirements during the construction and curing processes of the CBGM mixtures.

3.2. Compressive Strength of CBGM Mixtures: Experimental Results and Statistical Evaluation

3.2.1. Experimental Results of Compressive Strength Tests

All four mixtures were tested for their compressive strength (R₇ and R₂₈) according to EN 13286-41 [2]. For each mixture, six specimens (S1–S6) were prepared for testing. Independent sets of specimens were made for the 7-day (R₇) and 28-day (R₂₈) curing periods. An example of a specimen prepared from mixture M:2_10 during the compressive strength test is shown in Figure 12.

The compressive strength results of individual CBGM specimens (S1–S6) are presented on radar charts (Figure 13), separately for each mixture. The R₇ and R₂₈ results are shown in combined plots to illustrate the average strength gain at different curing stages. For clarity, the strength values (R₇ and R₂₈) for the individual specimens (S1–S6) were sorted in ascending order.

The radar charts for the individual CBGM mixtures indicate that all mixtures exhibited continued strength development over time, confirming continued cement hydration and matrix consolidation; however, the rate of increase depended on the glass cullet content (GC). It can be clearly observed that the mixture M:3_20 exhibited the highest 28-day compressive strength gain compared to its 7-day value, whereas M:4_30 showed the lowest increase. Mixtures M:1_ref and M:2_10 showed comparable strength gains. The spread of the compressive strengths among the 24 specimens was 2.7 MPa for R₇ and 3.8 MPa for R₂₈, indicating substantial variability in mechanical performance and a notable influence of GC.

Table 12. Descriptive statistics of the variables R₇ and R28 for CBGM and increase in R₂₈ strength compared to R₇ (median—median value, mean—mean value, SD—standard deviation of the sample, CoV—coefficient of variation, Δ—increase in compressive strength R₂₈ compared to R_7, %Δ—percentage increase in compressive strength R₂₈ relative to R₇).

Tested CBGM	R7				R28				∆	%∆
	Median	Mean	SD	CoV	Median	Mean	SD	CoV
	(MPa)	(MPa)	(MPa)	(%)	(MPa)	(MPa)	(MPa)	(%)	(MPa)	(%)
M:1_ref	6.5	6.5	0.26	4.0	7.5	7.6	0.42	5.5	1.1	16.9
M:2_10	6.9	6.8	0.38	5.4	7.9	7.9	0.45	5.7	1.1	16.2
M:3_20	7.6	7.5	0.25	3.3	9.6	9.4	0.47	5.0	1.9	25.3
M:4_30	5.6	5.6	0.36	6.4	6.2	6.3	0.21	3.3	0.7	12.5

presents the descriptive statistics of the compressive strength values after 7 and 28 days (R₇ and R₂₈) and the corresponding strength increase (Δ, %Δ) for the CBGM mixtures. The box-and-whisker plots (Figure 14) graphically reproduce the following statistics of the R₇ and R₂₈ variables for the CBGMs studied: minimum, first quartile (Q1), mean, median, third quartile (Q3) and maximum. Quartiles were determined taking into account the median.

A significant increase in mean compressive strength (%Δ = 25.3%, Δ = 1.9 MPa; Table 12) was obtained for the mixture containing 20% glass cullet. The reference mixture and the mixture with the lowest glass content (10%) exhibited moderate and comparable strength gains (%Δ = 16.9% and 16.2%, respectively; both corresponded to Δ = 1.1 MPa). The variant with 30% GC showed the smallest increase, i.e., %Δ = 12.5%, corresponding to an average gain of Δ = 0.7 MPa. These results indicate that a moderate addition of glass cullet (10–20%) promotes strength development—probably due to a more favorable grading and the potential pozzolanic activity of the fine glass fraction—while higher contents (≥30%) can hinder the binding efficiency within the cementitious matrix.

Based on the statistics summarized in Table 12 and depicted in Figure 14, the reference mixture M:1_ref and the 10% glass cullet mixture (M:2_10) exhibit the greatest similarity in the compressive strength (R₇ and R₂₈). Clear differences are observed for the remaining CBGM mixtures. The medians and distributions are consistent: M:3_20 plots the highest (R₇ and R₂₈: median = 7.6 MPa and 9.6 MPa; mean = 7.5 MPa and 7.4 MPa), while M:4_30 plots the lowest (R₇ and R₂₈: median = 5.6 MPa and 6.2 MPa; mean = 5.6 MPa and 6.3 MPa), with limited overlap of the interquartile ranges across mixtures. Standard deviations (SDs) of compressive strength ranged from 0.25 to 0.38 MPa for R₇ and 0.21–0.47 MPa for R₂₈, with the highest variability observed for the 28-day strength of M:3_20. The estimated coefficients of variation (CoVs) were low: 3.3–6.4% for R₇ and 3.3–5.7% for R₂₈, indicating good repeatability of the measurement and mixture uniformity. The box-and-whisker plots in Figure 14 show narrow interquartile ranges and no outliers, corroborating the low variability of the results.

Figure 15 presents the mean compressive strengths with standard deviations, referenced to the EN 14227-1 [1] minimum requirement for R₂₈ and the national guideline WT-5:2010 [49] upper bound for class C_5/6.

All tested CBGM mixtures met the compressive-strength requirement for class C_5/6 in accordance with EN 14227-1 [1] (minimum compressive strength value after 28 days ≥ 6 MPa). (28-day compressive strength ≥ 6 MPa). The highest mean strength was obtained for mixture M:3_20 (R₂₈ = 9.4 MPa), approaching the upper reference limit given in the national guideline WT-5:2010 [49] (10 MPa), whereas mixture M:4_30, at approximately 6.3 MPa, only slightly exceeded the minimum standard threshold. The range of mean compressive strength values between mixtures amounted to 3.1 MPa, which is considered technically relevant for CBGM of the C_5/6 strength class, reflecting the pronounced influence of glass content on mechanical performance.

Figure 15 illustrates the percentage changes in average compressive strength for the glass cullet mixtures (M:2_10, M:3_20, M:4_30) referenced to the non-modified mixture without glass cullet (M:1_ref). The results indicate that a 10% to 20% addition of glass cullet (M:2_10, M:3_20) exerts a beneficial modification effect, leading to an increased mean compressive strength at both the early (7 days) and later (28 days) ages. The difference in the strength between M:3_20 and M:1_ref is 1.8 MPa, corresponding to an increase of approximately 24%. However, at higher glass cullet content (30%, M:4_30), a reduction in the strength gain and in the final R₂₈ value is observed, which can be associated with reduced continuity of the cementitious matrix and limited paste–glass interfacial bonding relative to the reference mixture. For M:4_30, the average compressive strength was lower by approximately 1.3 MPa (17.1%) compared with M:1_ref.

3.2.2. Statistical Analysis and Model Fitting

To assess whether the differences in mean compressive strength between the groups (mixtures) observed in Table 12 and Figure 13, Figure 14 and Figure 15 are statistically significant, a one-way ANOVA was performed separately for R₇ and R₂₈ (independent samples). The normality assumption for the dependent variables was verified using the Shapiro–Wilk test (Figure 16) at a significance level of α = 0.05.

For all four CBGM groups, both for R₇ and R₂₈, the Shapiro–Wilk test returned p-values > 0.05, providing no evidence to reject the null hypothesis of normality for dependent variables.

For both variables, the assumption of homogeneity of variances was confirmed using the Brown–Forsythe test. The results are summarized in

Table 13. Results of the Brown-Forsythe variance uniformity test for R₇ and R₂₈ variables (SS Effect—sum of squares for effect; df Effect—degrees of freedom for effect; MS Effect—mean square for effect; SS Error—sum of squares for error; df Error—degrees of freedom for error, MS Error—mean square for error; F—F-statistic = MS Effect/MS Error; significance level α = 0.05).

Variable	SS Effect	df Effect	MS Effect	SS Error	df Error	MS Error	F	p-Value
R7	0.0617	3	0.0206	0.4117	20	0.0206	0.9987	0.4138 *
R28	0.1683	3	0.0561	1.3550	20	0.0678	0.8282	0.4938 *

* p-value > 0.05—fail to reject H₀ (variances are homogeneous).

.

Brown-Forsythe test yielded p-values of approximately 0.41 for R₇ and 0.49 for R₂₈, indicating that there was no evidence to reject the null hypothesis of equal variances between groups at α = 0.05. The sums of squares for the effect (SS Effect) were small relative to the error term (SS Error), implying that between-group variability accounted for only a minor portion of the total variance. The mean squares for the effect (MS Effect) and for the error (MS Error) were very similar—particularly for R₇—resulting in F statistics close to 1.

Given that the assumptions of variance homogeneity and normality were satisfied, a one-way analysis of variance (ANOVA) was applied for subsequent comparisons. The ANOVA results are reported in

Table 14. One-way ANOVA results for R₇ and R₂₈ (SS Effect—sum of squares for effect; df Effect—degrees of freedom for effect; MS Effect—mean square for effect; SS Error—sum of squares for error; df Error—degrees of freedom for error, MS Error—mean square for error; F—F-statistic = MS Effect/MS Error; significance level α = 0.05).

Variable	SS Effect	df Effect	MS Effect	SS Error	df Error	MS Error	F	p-Value
R7	12.1617	3	4.0539	1.9967	20	0.0998	40.6066	1.0749 × 10−8 *
R28	30.1950	3	10.0650	3.2433	20	0.1622	62.0658	2.6040 × 10−10 *

* p-value ≤ 0.05—reject H₀ (at least one group mean differs significantly).

.

A one-way analysis of variance (ANOVA) (Table 14) revealed significant differences between the means of the group (p < 0.05) for both R₇ and R₂₈. To identify which groups differed significantly, Tukey’s HSD post hoc multiple comparisons were performed. The results are summarized in

Table 15. Tukey’s HSD post hoc multiple comparisons for R₇ (M-mean value).

	p-Value
CBGM	{1} M = 6.48 MPa	{2} M = 6.80 MPa	{3} M = 7.53 MPa	{4} M = 5.55 MPa
M:1_ref {1}		0.3323 *	0.0002 **	0.0004 **
M:2_10 {2}	0.3323 *		0.0036 **	0.0002 **
M:3_20 {3}	0.0002 **	0.0036 **		0.0002 **
M:4_30 {4}	0.0004 **	0.0002 **	0.0002 **

Legend: —empty cells (symmetric values omitted); * p-value > 0.05—fail to reject H₀ (the means of the compared groups do not differ); ** p-value ≤ 0.05—reject H₀ (the means of the compared groups differ significantly).

for R₇ and

Table 16. Tukey’s HSD post hoc multiple comparisons for R₂₈ (M-mean value).

	p-Value
CBGM	{1} M = 7.58 MPa	{2} M = 7.93 MPa	{3} M = 9.43 MPa	{4} M = 6.28 MPa
M:1_ref {1}		0.4530 *	0.0002 **	0.0003 **
M:2_10 {2}	0.4530 *		0.0002 **	0.0002 **
M:3_20 {3}	0.0002 **	0.0002 **		0.0002 **
M:4_30 {4}	0.0003 **	0.0002 **	0.0002 **

Legend: —empty cells (symmetric values omitted); * p-value > 0.05—fail to reject H₀ (no significant difference between group means); ** p-value ≤ 0.05—reject H₀ (significant difference between group means).

for R₂₈.

As indicated by the p-values in Table 15 and Table 16, the difference in means between groups 1 and 2—that is, the reference mixture M:1_ref and the 10% glass cullet mixture M:2_10—was not statistically significant (p > 0.05). For all other pairwise group comparisons, the differences were statistically significant (p ≤ 0.05). These findings corroborate the trends observed in Figure 13, Figure 14 and Figure 15, that is,

• The mixture M:2_10 does not differ significantly from the reference mixture M:1_ref;
• The mixture M:3_20 exhibits significantly higher compressive strength than the other CBGM mixtures;
• The mixture M:4_30 exhibits significantly lower compressive strength than the other CBGM mixtures.

Based on engineering knowledge, the literature, the distributions in Figure 14 and Figure 15, and the statistical analyses, it was anticipated that the relationship between glass cullet content and compressive strength was expected to quantifiable. To visualize this effect at 7 and 28 days, regression analysis was performed. Linear regression did not produce satisfactory results (low correlation coefficients, 0.3 for R₇ and ≈0.2 for R₂₈; model not significant at α = 0.05). Among commonly used regression forms, the best fit was obtained with a second-degree polynomial model (Figure 17).

Table 17. Polynomial regression models (second degree) describing the relationship between the content of glass cullet content (GC) and the compressive strength (R_c) of CBGM mixtures after 7 and 28 days of curing (Std. error—standard error of the estimate; R²—coefficient of determination; F—Fisher’s F-statistic; p-value—probability of significance of the F-test significance; significance level α = 0.05).

Age	Regression Model (2nd Degree)	Std. Error	R2	F	p-Value
7	R7 = −0.006 × GC2 + 0.15 × GC + 6.33	0.49	0.65	19.58	1.6 × 10−5 *
28	R28 = −0.009 × GC2 + 0.24 × GC + 7.30	0.80	0.60	15.83	6.4 × 10−5 *

* p-value ≤ 0.05—reject H₀ (the regression model is statistically significant).

summarizes the fitted quadratic regression models that describe the influence of glass cullet content on the compressive strength at 7 and 28 days.

The determination coefficients obtained (R² = 0.65 and 0.60) demonstrate a satisfactory goodness of fit of the polynomial regression models to the experimental data, with corresponding correlation coefficients of R = 0.81 and R = 0.78, respectively. The standard error of estimate was 0.49 MPa for the R₇ model and 0.80 MPa for the R₂₈ model, confirming the reliability of the applied fitting functions. Both models and their individual terms were statistically significant (p < 0.005) and revealed a distinct nonlinear trend, with the maximum predicted compressive strength occurring at approximately 15% glass cullet content. Negative values of the quadratic terms indicate a decrease in strength at higher proportions of glass cullet.

The parabolic shape of the fitted curves suggests that a moderate glass cullet content (10–20%) enhances the mechanical performance of the CBGM mixtures, probably due to the improved particle size distribution and the potential pozzolanic reactivity of the fine glass fractions. Beyond this level, a decrease in compressive strength occurs, which can be attributed to discontinuities in the cementitious matrix and weakening of the bond between the glass particles and the cement binder. This behavior is consistent with previous reported findings for cement-based concretes [10,41].

According to the experimental results, the mixture containing 20% GC (M:3_20) exhibited an average compressive strength (R₂₈) approximately 0.93 MPa higher than the value predicted by the fitted regression model, while the mixture with 10% glass cullet (M:2_10) showed a strength of approximately 0.87 MPa lower. These findings indicate a more complex nature of the studied relationship and highlight the need to extend the investigation to include intermediate glass contents in CBGM mixtures, as well as to develop additional models capturing the influence of the waste glass (GC) on the mechanical properties of the composites. To validate the laboratory observations, the construction of a pilot test section under field conditions is recommended.

4. Discussion

In order to interpret the experimental results concerning the influence of glass cullet (GC) on the mechanical and compaction properties of CBGM mixtures in the context of existing knowledge, the obtained data were compared with previously published findings from other studies, focusing on their practical applicability in the construction of a road base/subbase layer.

4.1. Mechanical Properties of CBGM Mixtures

The results confirm that a moderate addition of glass cullet (approximately 20%) improves the mechanical performance of CBGM mixtures in class C_5/6. The 20% GC mixture achieved the highest compressive strength (9.4 MPa), approximately 24% higher than the reference mixture. Statistical analysis (ANOVA, Tukey’s HSD) confirmed significant differences between mixtures (p < 0.05), indicating that 20% GC significantly enhanced strength, while 30% GC caused a reduction.

This trend is consistent with previous studies that identified 15–20% as the optimal range for glass incorporation in cementitious and hydraulically bound materials [7,26]. The improvement can be attributed to the combined effect of fine glass particles filling voids between aggregates, which improves stress transfer, and to the possible pozzolanic reactivity of amorphous silica fractions (<75 µm) forming additional C–S–H phases within the matrix [33,70]. Although no microstructural analyses were performed in this study, such reactions are likely responsible for the strength gains observed at moderate glass contents.

At 30% GC, the compressive strength decreased toward the lower bound of class C_5/6, indicating matrix discontinuities and weak ITZ bonding between cement paste and glass surfaces. This threshold-type effect beyond ~25–30% GC is consistent with the findings of [7], who reported reductions in UCS and stiffness in cement-stabilized materials incorporating recycled glass. The observed nonlinear relationship (parabolic; R² ≈ 0.60–0.65) reflects the balance between microstructural densification at moderate glass content and loss of cohesion at excessive replacement levels.

The research conducted aligns with global efforts to develop sustainable material solutions for road construction. Recent reviews emphasize that a moderate incorporation of glass cullet (typically 10–20%) can enhance the mechanical and durability performance of hydraulically bound materials, while excessive amounts can cause a loss of cohesion and reduced strength due to increased matrix discontinuity and weak glass–cement interfaces [9,11]. The results obtained in this study confirm these observations for CBGM mixtures, highlighting the existence of an optimal glass content range and the need for further modelling and microstructural investigation to better understand the governing mechanisms.

It should be noted that this research was conducted as a preliminary study focusing on macroscopic mechanical and compaction behavior. Further microstructural analyses (XRD, FTIR, TG) are planned to confirm the presence and extent of pozzolanic reactions indicated by the observed strength development.

4.2. Compaction Parameters (OMC and Maximum Dry Density)

Modified Proctor tests showed a systematic decrease in both optimum moisture content (OMC) and maximum dry density (ρ_d,max) with increasing GC content. For 30% GC, the OMC decreased by ~14% and ρ_d,max by ~2.8% relative to the reference mixture, which is consistent with the lower specific gravity of soda–lime glass (~2.50 Mg/m³ versus ~2.65 Mg/m³ for natural aggregates) and its nonporous hydrophobic surface that limits water retention and affects compaction behavior.

Despite the reduction in bulk density, the 20% GC mixture reached the highest compressive strength, indicating that mechanical performance was governed more by microstructural densification and matrix bonding than by bulk density. Lower OMC values can be advantageous in practice by reducing water demand and improving moisture stability in temperate climates.

The results obtained show close agreement with those of Mirzahosseini and Riding [12], who linked them to lower particle density and water absorption of glass aggregates, which enhance particle packing and reduce intergranular friction within the matrix.

4.3. Comparison with Data from the Literature

Most existing research has focused on concrete or soil stabilization, while direct studies on CBGM remain limited. Arulrajah et al. [7] examined cement-stabilized blends of recycled concrete aggregate (RCA) and crushed glass for pavement base applications and reported a parabolic trend of unconfined compressive strength and modulus of rupture with respect to glass content, with optimum performance observed at approximately 20–25%. Similarly, Arabani et al. [26] investigated cement-stabilized glass–sand blends and confirmed that moderate incorporation of crushed glass improves strength while reducing optimum moisture content, though its material composition and application differ significantly from CBGM.

The trends reported in those studies are consistent with the present findings, indicating that moderate glass addition (20%) may promote beneficial pozzolanic reactions and modify the matrix structure, as observed by other authors. The present results are consistent with this trend, although no direct microstructural verification was performed in this study. The improvement observed at moderate GC levels is therefore interpreted mainly as a consequence of physical effects, particularly the filler effect and the inclusion of stiff, angular glass particles, whereas excessive glass content weakens the matrix and reduces strength. However, the quantitative dataset available in the literature for CBGM under temperate climatic conditions remains extremely limited. Therefore, the experimental results presented here provide new and comprehensive data on the mechanical and compaction behavior of cement-bound granular mixtures containing glass cullet, contributing to the development of sustainable pavement engineering solutions.

At the same time, when considering the long-term performance of CBGM mixtures containing glass cullet, potential durability issues such as alkali–silica reactivity (ASR) and freeze–thaw (F–T) resistance should be taken into account. ASR may occur due to the reaction between alkalis in the cement pore solution and amorphous silica in glass particles, leading to expansive gel formation and possible strength loss over time. However, this risk can be effectively reduced through fine grinding of the glass (<100 µm) and the use of supplementary cementitious materials (SCMs) such as fly ash or slag, which mitigate alkali availability and promote pozzolanic reactions [42,43,44,78]. Recent studies also show that well-graded glass fractions and partial air entrainment can maintain or even enhance F–T durability by improving matrix densification and limiting moisture ingress [46,47,81]. These aspects should be further investigated for CBGM under temperate climatic conditions to ensure long-term structural integrity.

4.4. Practical and Environmental Implications

The mixture containing 20% GC exhibited the highest compressive strength and overall mechanical performance, while the variant with 30% GC met the minimum requirements of the strength class C_5/6 while achieving maximum waste utilization and reducing the consumption of natural aggregates. This represents a practical compromise between strength parameters and environmental benefits.

The reduction in OMC indicates a lower demand for water and energy during construction and curing, while the incorporation of glass waste aligns with the principles of the circular economy. Based on the results obtained, mixtures containing 10–20% GC can be considered optimal in terms of mechanical properties, compaction parameters, and sustainable use of raw materials, while the addition of up to 30% GC can be considered the environmentally justified upper limit for pavement subbase applications under temperate climatic conditions.

4.5. Future Research Directions

The results obtained demonstrate that properly designed CBGM mixtures with a moderate addition of glass cullet can serve as a viable alternative to traditional materials and contribute to the increased use of secondary raw materials in road engineering, in line with principles of the circular economy. However, more research is required to refine the understanding of the mechanisms involved and validate the practical implementation potential under field conditions.

Future research should include the following:

• Testing of intermediate GC contents (e.g., every 2–5%) to more accurately capture the strength–composition relationship;
• Evaluation of CBGM mixtures in other R_c classes;
• Detailed XRF and microstructural analyses (SEM/EDS, XRD) to provide a more comprehensive characterization of the constituent materials and their interactions within the CBGM matrix;
• Microstructural analyses (SEM, µCT) to visualize interfacial transition zones and microcrack development;
• Determination of the resilient modulus to assess the cyclic behavior of CBGM mixtures and California Bearing Ratio (CBR) testing to evaluate bearing capacity in accordance with commonly used pavement design methods;
• Durability assessments including freeze–thaw, absorption, and permeability testing to evaluate long-term performance under Central European climatic conditions;
• Assessment of ASR susceptibility, mitigation strategies (e.g., SCMs, lithium-based admixtures), and the potential use of alkali-activated or geopolymer matrices to minimize expansion and improve chemical stability;
• Exploration of alkali-activated and geopolymer binders incorporating glass cullet as low-carbon alternatives for chemically stable pavement materials;
• Field validation through the construction and long-term monitoring of a full-scale pilot test section under real operating conditions.

These studies will provide a comprehensive understanding of the structural behavior, durability, and field applicability of cement-bound mixtures incorporating glass cullet, supporting their wider use in sustainable pavement engineering.

5. Conclusions

Based on laboratory investigations on the influence of the content of glass waste cullet (GC) on the mechanical and compaction behavior of cement-bound granular mixtures (CBGM 31.5 mm, defined in EN 14227-1 [1], R_c class C_5/6) intended for use in the subbase layer of pavement structures subjected to heavy and very heavy traffic loads, as well as in the base layer of pavements designed for medium traffic, the following conclusions have been formulated:

• The analysis of the compaction behavior showed a systematic decrease in both optimum moisture content (OMC) and maximum dry density (ρ_d,max) with increasing glass cullet (GC) content from 0 to 30%. The OMC decreased by about 18%, while ρ_d,max dropped by 0.050 Mg/m³(≈2.8%) compared with the reference mixture, which results from the lower specific gravity and smooth, nonporous surface of glass particles.
• All tested CBGM mixtures exhibited continued gain in compressive strength over time, though with varying growth rates. The highest increase in R₂₈ relative to R₇ was recorded for the mixture containing 20% GC (%Δ = 25.3%, Δ = 1.9 MPa), while the lowest increase was observed for the 30% GC mixture (%Δ = 12.5%, Δ = 0.7 MPa);
• Regardless of GC content, all mixtures met the minimum compressive strength requirements for class C_5/6 (R_c ≥ 6 MPa and ≤10 MPa). The highest mean compressive strength was achieved for the mixture containing 20% GC (9.4 MPa), while the lowest value was observed for 30% GC (6.3 MPa). The range of mean R₂₈ values between mixtures was 3.1 MPa, which is technically relevant but within acceptable limits for the CBGM C_5/6 classification;
• The results obtained indicate that the addition of 10–20% glass cullet exerts a beneficial modification effect, leading to higher mean compressive strength values at 7 and 28 days. The 28-day strength of the 20% GC mixture exceeded that of the reference mixture by approximately 1.8 MPa (≈24%). For 30% GC, the strength development efficiency decreased (−1.3 MPa, ≈−17% compared with the reference);
• Despite its lower bulk density (2.302 Mg/m³) compared to reference CBGM (2.358 Mg/m³), the 20% GC mixture exhibited a significant increase in the strength of R₇ and R28. This effect may be attributed to the microfilling action of fine glass particles and their partial pozzolanic activity, which should be verified in further studies;
• Statistical analysis (ANOVA, Tukey’s HSD) confirmed significant differences in compressive strength between mixtures (p < 0.05), indicating that 20% GC provides a statistically significant improvement, while 30% GC leads to a noticeable reduction;
• Polynomial regression (R² ≈ 0.60–0.65) showed a parabolic relationship between GC content and strength, with optimal performance at 10–20% GC; higher contents caused discontinuities in the matrix, suggesting the need for further microstructural investigation;
• The 30% GC mixture achieved the highest waste utilization but only met the minimum strength for class C_5/6, while 20% GC provided superior mechanical performance with slightly lower sustainability benefits. The optimal GC content should balance strength and environmental efficiency according to the pavement layer’s function.