Synergistic use of brick and ceramic wastes with polyethylene glycol in high-performance fiber-reinforced composites

Abstract

The increasing demand for sustainable construction materials calls for innovative strategies that reduce natural resource consumption while maintaining or enhancing material performance. This study investigates the use of polyethylene glycol (PEG) as an internal curing agent in high-performance fiber-reinforced composites (HPFRC) incorporating brick waste sand (BWS) and ceramic waste powder (CWP). BWS and CWP were introduced as partial replacements for river sand (RS) and cement, respectively, to promote waste valorization and improve particle packing. PEG was employed to facilitate internal curing, enhancing hydration efficiency and overall matrix quality. Fresh and hardened properties of the developed HPFRC mixtures were evaluated through workability, compressive strength, flexural strength, tensile response, and water absorption tests. Microstructural analysis using scanning electron microscopy (SEM) and X-ray diffraction (XRD) was conducted to assess hydration products, matrix densification, and interfacial bonding. Results indicate that PEG improved workability by compensating for the reduced slump caused by the angular texture of BWS. CWP contributed to better particle packing and matrix refinement, producing a denser microstructure. The optimized mixture containing 1.5% PEG, 50% BWS, and 10% CWP achieved the best overall performance, with compressive strength of 63 MPa, tensile strength of approximately 7 MPa, flexural strength of 15.89 MPa, and water absorption of 1.15%. Microstructural observations confirmed improved matrix densification and interfacial bonding, demonstrating the synergistic effect of PEG-mediated internal curing and waste-derived materials. These findings validate the technical feasibility of incorporating PEG, BWS, and CWP in HPFRC, providing a practical approach for producing high-performance, resource-efficient composites with enhanced mechanical and durability characteristics.

Introduction

The rapid growth of the global population has led to a substantial increase in non-biodegradable waste materials, creating significant environmental and disposal challenges. Addressing these issues requires the implementation of waste hierarchy strategies that prioritize reduction, reuse, and recycling^1–3. In the construction sector, incorporating waste materials into cement-based composites has emerged as a practical approach to reduce reliance on natural resources and partially mitigate the environmental burden associated with cement production. This strategy is particularly relevant for high-performance fiber-reinforced composites (HPFRC), which typically contain high binder contents compared to conventional concrete⁴. Numerous studies have investigated the use of agricultural, industrial, and construction wastes as supplementary cementitious materials (SCMs) or aggregate replacements in cementitious systems^5–8.

Materials such as rice husk ash, cotton stalk ash, palm leaf ash, polyethylene terephthalate, crushed ceramics, steel slag, ferrosilicon, and recycled concrete aggregates have been successfully incorporated into conventional concrete mixtures^8,9. These materials may contribute to improved particle packing, micro-filler effects, or limited pozzolanic activity, potentially refining pore structure and enhancing matrix densification^10–14. Among construction wastes, crushed clay bricks and ceramic residues represent significant waste streams worldwide, with millions of tons generated annually^15–17. Their incorporation into cementitious composites provides an opportunity to reduce landfill accumulation and partially replace natural fine aggregates, supporting resource-efficient construction practices^18,19. Depending on their physical and chemical characteristics, such materials may influence workability, strength development, and microstructural refinement^10,11.

Curing conditions play a fundamental role in determining the mechanical and durability performance of cementitious materials²⁰. Conventional external curing methods, including immersion and water spraying, may face practical limitations such as uneven moisture distribution and dependency on environmental conditions²¹. In mixtures with relatively dense matrices and elevated binder content, internal moisture demand during early hydration can increase the risk of self-desiccation and autogenous shrinkage²². Insufficient internal water availability may result in microcracking, strength reduction, and compromised long-term performance^20,23.

Self-curing has gained increasing attention as an effective approach to overcome the limitations of conventional external curing, particularly in HPFRC with low water-to-binder ratios. In such systems, insufficient internal moisture restricts complete hydration, leading to autogenous shrinkage and microcracking. Therefore, internal curing has been proposed as a practical solution to maintain internal relative humidity and support continued hydration within the matrix^20,22. By supplying water internally, self-curing systems can effectively mitigate shrinkage-related effects, especially during the early stages of hydration^15,23. Among chemical-based internal curing agents, polyethylene glycol (PEG) has attracted research interest due to its water-soluble and moisture-retention characteristics²⁴. PEG interacts with water molecules through hydrogen bonding, reducing evaporation and helping to maintain internal humidity during hydration²⁰. This mechanism promotes sustained hydration and enhances the formation of calcium silicate hydrate, leading to a denser microstructure with reduced capillary porosity^16,20. Previous studies have reported notable improvements in strength development, reduced permeability, and enhanced durability when PEG is incorporated into cementitious materials, particularly in mixtures with higher water demand or low water-to-binder ratios^25–27.

Fiber reinforcement is commonly used to enhance post-cracking behavior and improve energy absorption in cementitious composites. Polypropylene fibers (PPF) are widely adopted due to their chemical stability, low density, and ability to improve crack resistance and impact performance^28–30. The interaction between fiber reinforcement, matrix densification, and curing conditions is critical for achieving stable post-peak response and controlled cracking behavior^11,31. Supplementary cementitious materials such as ground granulated blast furnace slag (GGBFS) further influence hydration products and pore structure refinement through secondary reactions with calcium hydroxide^32,33. However, mixtures incorporating SCMs often exhibit increased internal water demand, highlighting the importance of adequate curing strategies³⁴. Superplasticizers (SP) are typically included to enhance workability and ensure uniform fiber dispersion, thereby promoting improved mechanical performance^35,36.

Recent studies have explored combinations of internal curing agents, SCMs, and recycled aggregates in cementitious systems. For instance, Ogawa et al.³⁷ reported improved hydration and compressive strength in systems incorporating porous ceramic waste aggregates. Mousa et al.³⁸ observed enhanced mechanical performance when self-curing techniques were combined with silica fume. Amin et al.¹⁵ demonstrated strength improvements in high-strength concrete containing PEG and ceramic waste aggregates. These findings suggest that integrating waste materials with internal curing strategies may influence hydration behavior and mechanical response.

Within this sustainability-driven perspective, the partial replacement of river sand (RS) with brick waste sand (BWS) and cement with ceramic waste powder (CWP) offers a practical pathway to reduce the extraction of virgin raw materials while maintaining the mechanical strength and durability required in advanced cementitious composites^39,40. The inclusion of PPF further contributes to structural performance by enhancing crack control efficiency and improving post-cracking stability, thereby reducing long-term maintenance demands^30,33,41.

Accordingly, integrating internal curing agents such as PEG with recycled fine materials (BWS and CWP), supplementary cementitious materials (e.g., GGBFS), and chemical admixtures (SP and PPF) establishes a comprehensive framework for developing high-performance and environmentally responsible cementitious composites. Such integrated systems have demonstrated the potential to enhance compressive, tensile, and flexural behavior, while simultaneously lowering carbon emissions and extending service life, supporting the advancement of sustainable construction technologies^42,43.

Nevertheless, despite the extensive research on recycled materials and internal curing in conventional concrete, their integrated application in self-curing HPFRC remains limited. Previous studies have typically examined individual waste materials -such as ceramic waste, crushed bricks, or recycled aggregates- or have focused separately on PEG as an internal curing agent. However, the simultaneous incorporation of BWS and CWP within high-binder, fiber-reinforced systems has not been systematically investigated. In HPFRC matrices, where fiber–matrix interaction and binder composition play critical roles, the combined effects of recycled constituents and PEG-based internal curing may substantially influence hydration development, microstructural refinement, durability-related performance, and mechanical properties. Yet, comprehensive evaluations of these coupled interactions remain scarce. This highlights a clear research gap in understanding the synergistic behavior of these materials within self-curing HPFRC systems. In particular, the determination of optimal replacement levels that balance structural performance with sustainable material utilization is still unclear. Therefore, a systematic investigation is necessary to clarify the interactive effects of BWS, CWP, and PEG and to assess their feasibility as partial substitutes for natural sand and cement in self-curing HPFRC systems. The novelty of this study lies in the synergistic integration of PEG as an internal curing agent with BWS and CWP as sustainable waste-based materials within HPFRC, aiming to enhance hydration efficiency, microstructural densification, and overall mechanical and durability performance.

Materials and methods

Raw materials and mix proportion

In this experimental program, self-curing HPFRC were developed by integrating conventional constituents with recycled and functional materials to enhance mechanical performance and sustainability. OPC and GGBFS were used as primary binders to ensure adequate strength development and long-term durability. RS served as the reference fine aggregate.

To reduce reliance on natural resources and promote waste valorization, CWP and BWS were incorporated as partial replacements for binder and RS, respectively. The use of BWS as a replacement for RS is justified by its porous nature, which potentially contributes to internal curing, while CWP serves as a pozzolanic filler. PPF were introduced as discrete reinforcement to improve crack resistance and post-cracking behavior of the composite matrix. The fiber volume fraction was maintained constant at 3% by weight of binder (27.3 kg/m³), with a fiber length of 12 mm and a diameter of approximately 18–30 μm, resulting in a high aspect ratio to ensure effective bridging. PEG was employed as an internal curing agent to enhance hydration efficiency, mitigate autogenous shrinkage, and improve long-term durability. The dosage of PEG (1.0%, 1.5%, and 2.0%) was selected based on optimum ranges suggested in previous literature for self-curing composites to balance between hydration enhancement and workability. SP was added at a fixed dosage of 5 kg/m3 to maintain workability at a constant water-to-binder (w/b) ratio of 0.27. The mixture compositions investigated in this study are illustrated in Fig. 1. The chemical compositions of OPC, GGBFS, and CWP are presented in Table 1, while the physical and mechanical properties of PPF are summarized in Table 2.

Fig. 1.

Materials utilized in the study.

Table 1.

Chemical composition of binder materials.

Element	CaO	SiO2	Al2O3	Fe2O3	MgO	SO3	Na2O	LOI	K2O	ZnO	TiO2
OPC	62.7	20.2	6	3.3	2	2.2	0.01	1.7	----	----	----
GGBFS	43.1	32.8	13.4	0.45	5.5	1.9	0.4	0.8	0.3	1.04	0.8
CWP	6.10	60.40	18.47	7.62	0.477	0.241	2.32	----	1.972	0.126	1.12

Table 2.

The characteristics of PPF.

Length (mm)	Specific gravity (gm/cm3)	Elastic Modulus (GPa)	Tensile Strength (MPa)	Elongation (%)	Melting Point (C)
12	0.91	40–56	467–548	20–25	160

The binders used included OPC (CEM II 42.5), GGBFS, and CWP, with specific gravities of 3.15, 2.83, and 2.60, respectively. All materials were locally sourced in Egypt. OPC and GGBFS were supplied in pre-packaged powder form and used without further processing. CWP was produced by crushing ceramic tile waste using a Los Angeles abrasion machine to obtain fine particles. All binder materials were sieved through a No. 170 mesh to ensure adequate fineness for hydration and proper mixing consistency. Figure 2a presents the particle size distribution curves of OPC, GGBFS, and CWP, indicating that CWP exhibited the highest proportion of fine particles, followed by GGBFS and OPC.

Fig. 2.

Particle size distribution of (a) binder materials (b) fine aggregate materials.

For fine aggregates, RS and BWS were used, with specific gravities of 2.65 and 2.60, respectively. Both aggregates were sieved through a 4.75 mm sieve to satisfy the fineness requirements of the composite mixtures. BWS was utilized as a partial replacement for RS. The particle size distributions of RS and BWS are shown in Fig. 2b, confirming the relatively finer grading of BWS. The BWS, derived from crushed brick waste, was primarily characterized by its physical grading and absorption properties. Given its role as a fine aggregate replacement, it was considered a chemically inert filler in the hydration process, similar to natural river sand, and thus focused on its physical contribution to the matrix density and internal curing potential. SP used had a specific gravity of 1.08, while PPF had a specific gravity of 0.91.

The mixture designation system was established to clearly identify the effect of each variable. Control mixtures were prepared with identical base proportions and subjected to two curing regimes: air curing (C-A) and water curing (C-W).

Group 1 mixtures incorporated only PEG as an internal curing agent and were designated as P1, P1.5, and P2, corresponding to 1.0%, 1.5%, and 2.0% PEG by weight of OPC, respectively.

Group 2 mixtures included BWS as a partial replacement for RS, while maintaining a constant 1.5% PEG content. These mixtures were labeled P1.5B25, P1.5B50, P1.5B75, and P1.5B100, representing 25%, 50%, 75%, and 100% replacement of RS, respectively.

Group 3 mixtures incorporated CWP as a partial binder replacement in combination with 1.5% PEG and 50% BWS. These mixtures were coded as P1.5B50C10, P1.5B50C20, and P1.5B50C30, corresponding to 10%, 20%, and 30% replacement of OPC and GGBFS by CWP, respectively.

This systematic coding approach ensures clear identification of mixture variables and facilitates comparative evaluation of mechanical, durability, and microstructural performance. A detailed summary of all mixture proportions is provided in Table 3.

Table 3.

Mixes’ proportion of HPFRC (kg/m³).

Group	Mix code	RS	BWS	OPC	GGBFS	CWP	PPF	Water	PEG	SP
Control mixes	C-A	595	0	644	577	0	27.3	330	0	5
	C-W	595	0	644	577	0	27.3	330	0	5
1	P1	595	0	644	577	0	27.3	330	4.6	5
	P1.5	595	0	644	577	0	27.3	330	5.44	5
	P2	595	0	644	577	0	27.3	330	7.25	5
2	P1.5B25	446	137	644	577	0	27.3	330	5.44	5
	P1.5B50	297	273	644	577	0	27.3	330	5.44	5
	P1.5B75	149	409	644	577	0	27.3	330	5.44	5
	P1.5B100	0	545	644	577	0	27.3	330	5.44	5
3	P1.5B50C10	297	273	579	520	98	27.3	330	5.44	5
	P1.5B50C20	297	273	514	462	196	27.3	330	5.44	5
	P1.5B50C30	297	273	450	404	294	27.3	330	5.44	5

The experimental mixtures were proportioned with a constant water-to-binder (w/b) ratio of 0.27 and a total binder content of 1221 kg/m³. PPF were incorporated at a constant dosage of 27.3 kg/m³ to ensure consistency across all groups. The dosages of PEG (1%, 1.5%, and 2%) were selected based on well-established literature, indicating that these ranges effectively mitigate internal evaporation without compromising the integrity of the cementitious matrix. While the 1.0% PEG dosage yielded marginally higher compressive and flexural strengths, the 1.5% dosage was strategically selected as the baseline for Groups 2 and 3. This decision was primarily driven by the superior performance of 1.5% PEG in splitting tensile strength, which serves as a critical indicator of the internal curing efficiency. Specifically, this dosage appeared to optimize the hydration products within the matrix-fiber interfacial transition zone (ITZ), thereby enhancing the overall composite ductility and fiber-matrix bonding (factors that are paramount in HPFRC).

Mixing and Specimen Preparation

A total of twelve mixtures were developed and evaluated in this experimental program. The mixtures were categorized according to the type and level of modification introduced, including two control mixtures, three mixtures incorporating PEG, four mixtures containing BWS, and three mixtures integrating CWP. PEG was added as an internal curing agent at dosages of 1%, 1.5%, and 2% by weight of cement. BWS was used as a partial replacement for RS at levels of 25%, 50%, 75%, and 100% by volume. Similarly, CWP was incorporated as a partial binder replacement at 10%, 20%, and 30% by volume. The replacement levels for BWS and CWP were performed by volume to maintain the yield of the concrete constant. All remaining mixture constituents were maintained constant within each group to isolate the effect of the investigated variables. A detailed summary of mixture proportions is provided in Table 3, while the overall experimental matrix is illustrated in Fig. 3.

Fig. 3.

Flowchart of test matrix in this study.

All mixtures were prepared using a mechanical mixer to ensure adequate homogeneity. Initially, the dry components were blended for approximately 2 min to achieve uniform distribution. PEG and SP were pre-dissolved in the mixing water to facilitate proper dispersion within the matrix. The mixing water was then added gradually in two stages: half of the total amount was introduced first, followed by the remaining portion. Mixing continued for an additional 2 min until a consistent and workable mixture was obtained. The fresh composite mixtures were cast into standardized molds and compacted using a vibrating table to eliminate entrapped air. Specimens were demolded after 24 h. Crucially, to evaluate the self-curing efficiency, the PEG-modified mixtures were subjected to air-curing only (stored in laboratory air at 25 °C) without any external water application. Internal curing was sustained solely through the moisture-retention capacity of PEG and the potential internal reservoir effect of BWS. For comparison, the control mixture (C-W) was subjected to conventional water immersion curing at 25 °C to serve as a benchmark for maximum hydration potential. The structured experimental design enabled systematic evaluation of the individual and combined effects of PEG, BWS, and CWP on mechanical, durability, and microstructural properties, ensuring a clear interpretation of variable interactions within the developed composite system.

Test procedure

Workability

The workability of the fresh self-curing HPFRC was evaluated using the slump test in accordance with ASTM C143–10⁴⁴. A standard conical mold with a base diameter of 200 mm, a top diameter of 100 mm, and a height of 300 mm was used. The mold was placed on a rigid, level surface and filled with the fresh mixture in three equal layers. Each layer was compacted by rodding 25 times using a 16 mm diameter steel tamping rod to ensure proper consolidation. After filling and leveling the top surface, the mold was carefully lifted vertically, and the vertical subsidence of the specimen was measured to determine the slump value. The measured slump was used as an indicator of mixture consistency and flow characteristics.

Compressive strength

Compressive strength tests were conducted at curing ages of 7, 28, and 56 days in accordance with BS EN 12,390–3⁴⁵, using cube specimens with dimensions of 50 × 50 × 50 mm. The tests were performed using a calibrated compression testing machine, where load was applied continuously at a controlled rate until specimen failure. For each mixture and testing age, three specimens were tested, and the compressive strength was calculated as the average of the recorded values to ensure result reliability and minimize experimental variability.

Uniaxial tensile strength

The uniaxial tensile strength of the developed HPFRC was evaluated at 28 days in accordance with the Japan Society of Civil Engineers (JSCE) recommendations⁴⁶. This test method was adopted to characterize the direct tensile response and cracking behavior of the fiber-reinforced matrix under controlled loading conditions.

Dog-bone-shaped specimens were prepared to ensure stable stress transfer and minimize stress concentration at the gripping zones. Two specimens were tested for each mixture to obtain representative tensile performance. The specimen geometry was designed to promote uniform stress distribution within the central gauge region and to avoid premature failure outside the test Sect⁴⁷. Each specimen had a gauge length of 80 mm, a width of 30 mm, and a thickness of 13 mm in the reduced section, as shown in Fig. 4. Tensile load was applied under displacement-controlled conditions until failure, and the ultimate tensile strength was determined from the maximum recorded load divided by the corresponding cross-sectional area.

Fig. 4.

Uniaxial tensile testing of dog-bone specimens: (a) geometry, and (b) experimental setup (units: mm).

Flexural strength

The flexural behavior of HPFRC was evaluated using a three-point bending test on prism specimens with dimensions of 40 × 40 × 160 mm. The tests were conducted at 28 days of curing under displacement-controlled loading at a constant rate of 0.4 mm/min. For each mixture, three specimens were tested, and the reported flexural strength corresponds to the average value obtained, in accordance with the relevant standard procedure^48,49. The schematic representation of the test configuration and the experimental setup are presented in Figs. 5(a) and 5(b).

Fig. 5.

Three-point bending test: (a) Schematic illustration of the specimen geometry and testing setup, and (b) Prism specimens subjected to the three-point bending loading.

Water absorption

The durability performance of HPFRC was evaluated through water absorption testing in accordance with ASTM C642⁵⁰. This method was used to determine the water absorption capacity and apparent void content of hardened specimens. After 28 days of curing, the specimens were oven-dried at a temperature ranging between 100 and 110 °C until a constant mass was achieved. The specimens were then allowed to cool to ambient laboratory temperature (20–25 °C) prior to weighing. Before each mass measurement, surface moisture was carefully removed using a dry cloth to avoid measurement inaccuracies. Subsequently, the specimens were immersed in water until full saturation was reached. Saturation was considered complete when the difference in mass between two consecutive measurements did not exceed 0.5%. The water absorption (WA) was calculated using Eq. (1), based on the recorded dry and saturated masses.

1

Where:

WA is the absorption ratio.

is the saturated weight (g).

is the dry weight (g).

Microstructure characterization

Microstructural characterization was performed to examine the morphology and phase composition of the developed composite mixtures. After 28 days of curing, small fragments were carefully extracted from selected cube specimens using a rubber hammer to avoid inducing microcracking. Scanning electron microscopy (SEM) was employed to evaluate the microstructural features and matrix morphology. Prior to testing, the extracted samples were oven-dried and coated with a thin platinum layer to minimize surface charging and enhance image clarity during analysis⁵¹. In addition, X-ray diffraction (XRD) analysis was conducted to identify crystalline phases and assess changes in mineralogical composition. The measurements were carried out using a Rigaku MiniFlex 600-C diffractometer equipped with Cu Kα radiation and an X’celerator proportional detector. Scanning was performed over a 20 range of 10° − 60°, with a step size of 0.02° and a counting time of 0.5 s per step. For XRD preparation, specimens were oven-dried at 100 °C for 24 h, then finely ground and sieved to obtain particles smaller than 80 μm. This controlled grinding and sieving procedure ensured uniform particle size distribution, thereby enhancing measurement accuracy and improving the reliability of phase identification.

Results and Discussion

Workability

The slump values of the developed mixtures are presented in Fig. 6. The incorporation of PEG at dosages of 1.0%, 1.5%, and 2.0% resulted in increases in slump of 10%, 5%, and 5%, respectively, compared to the air-cured control mixture. This trend is consistent with the findings of Amin et al.^11,15. The improvement in workability can be attributed to the water-retention capacity of PEG, which reduces inter-particle friction and provides a plasticizing-like effect within the matrix. By lowering internal water evaporation and promoting better dispersion of fine particles, PEG contributes to a more cohesive and flowable mixture. The marginal difference between 1.5% and 2.0% PEG suggests that beyond a certain dosage, additional PEG does not significantly enhance flowability, likely due to saturation of the internal water-modifying effect. When BWS was used as a partial replacement for RS at levels of 25%, 50%, 75%, and 100%, a progressive reduction in slump was observed, despite maintaining 1.5% PEG in these mixtures. This reduction is primarily governed by the physical characteristics of BWS; specifically, its angular morphology and rough surface texture increase internal friction and mechanical interlocking between particles. Furthermore, the higher water absorption capacity of BWS siphons off a portion of the mixing water, elevating the effective water demand of the mixture and partially offsetting the lubricating benefits of PEG. In contrast, the incorporation of CWP at 10%, 20%, and 30% as a partial binder replacement (in mixtures containing 1.5% PEG and 50% BWS) resulted in an increase in slump of approximately 10%, in agreement with Lan et al.¹⁵. This enhancement can be associated with the fine particle size of CWP, which improves particle packing density and acts as a micro-filler within the matrix. The refined grading reduces inter-particle voids, releases entrapped water, and promotes smoother flow under shear. The synergistic interaction between the micro-filling effect of CWP and the water-retaining action of PEG further contributes to improved workability, effectively compensating for the frictional resistance introduced by BWS.

Fig. 6.

Measured slump values of HPFRC mixes.

The results indicate that while PEG enhances mixture consistency through internal moisture regulation, the aggregate characteristics of BWS dominate workability behavior at high replacement levels. Conversely, the incorporation of finely ground CWP can partially compensate for this reduction by improving particle packing and flow characteristics.

Compressive strength

The compressive strength results at 7, 28, and 56 days are summarized in Table 4 and illustrated in Fig. 7 (a, b, and c). The findings demonstrate that the incorporation of PEG, BWS, and CWP significantly influenced strength development, particularly at later curing ages. At 7 days, mixtures containing PEG exhibited slightly lower early-age strength compared to both air- and water-cured control specimens. The 1.0%, 1.5%, and 2.0% PEG mixtures achieved compressive strengths of 37 MPa, 32 MPa, and 27 MPa, respectively, compared to 40 MPa and 45 MPa for the air- and water-cured controls. This reduction can be attributed to the water-retention mechanism of PEG, which moderates the initial hydration kinetics by reducing rapid moisture consumption. While this effect slightly delays early strength gain, it promotes more uniform and sustained hydration over time.

Table 4.

Mechanical and durability characteristics of HPFRC mixes.

Group	Mix Code	Compressive Strength						Splitting tensile strength		Splitting Tensile Strength to Compressive Strength	Flexural strength		Flexural Strength to Compressive Strength	Water Absorption
		7 days		28 days		56 days		28 days		28 days	28 days		28 days	28 days
		Avg. (MPa)	SD	Avg. (MPa)	SD	Avg. (MPa)	SD	Avg. (MPa)	SD	%	Avg. (MPa)	SD	%	Avg. (MPa)	SD
Control Mixes	C-A	40	0.85	47	1.39	54	0.46	6.7	0.42	14%	12.80	2.19	27%	2.57	0.12
	C-W	45	1.01	49	2.55	50	1.13	6.6	0.32	13%	15.00	0.43	31%	1.67	0.15
1	P1	37	4.81	50	3.84	57	1.7	6.0	0.56	12%	15.18	0.00	30%	2.25	0.22
	P1.5	32	1.67	46	4.56	48	0.72	6.8	0.66	15%	12.56	2.15	27%	2.20	0.30
	P2.0	27	1.4	40	2.44	42	4.61	6.4	0.61	16%	11.81	2.58	30%	2.80	0.36
2	P1.5B25	34	2.12	49	3.11	54	5.66	7.2	0.56	15%	15.18	1.99	31%	1.25	0.13
	P1.5B50	40	1.44	49	1.74	61	1.41	7.7	0.07	16%	10.83	1.39	22%	1.47	0.32
	P1.5B75	48	1.7	55	4.03	66	1.7	6.6	0.05	12%	9.75	0.16	18%	1.81	0.20
	P1.5B100	42	4.46	51	0.23	60	4.16	7.0	0.54	14%	11.25	1.99	22%	1.57	0.29
3	P1.5B50C10	44	4.61	60	0	63	2.72	7.0	0.34	12%	15.89	2.19	26%	1.15	0.07
	P1.5B50C20	40	2.89	58	0.8	60	1.4	6.2	0.29	11%	9.66	1.14	17%	1.63	0.23
	P1.5B50C30	31	0.83	44	1.1	46	1.03	6.5	0.07	15%	10.97	0.00	25%	1.93	0.38
Avg: Average							SD: Standard Deviation

Fig. 7.

The compressive strength of HPFRC at 7, 28, and 56 days using: (a) PEG, (b) PEG and BWS, and (c) PEG, BWS and CWP.

At 28 and 56 days, the beneficial effect of internal curing became more evident. The 1.0% PEG mixture reached 50 MPa at 28 days, representing a 6.4% increase compared to the air-cured control (47 MPa). By 56 days, this mixture achieved 57 MPa, a 5.5% improvement over the C-A baseline (54 MPa). These results confirm that a moderate PEG dosage enhances long-term hydration efficiency and improves matrix densification, consistent with previous findings^52,53. The reduction observed at higher PEG contents suggests that excessive polymer addition may interfere with optimal binder packing or slightly reduce effective solid content. The incorporation of BWS in combination with 1.5% PEG further enhanced performance. Notably, the P1.5B75 mixture reached 55 MPa at 28 days and 66 MPa at 56 days, representing a significant strength increase of 17% and 22.2%, respectively, compared to the air-cured control. These improvements can be attributed to two primary mechanisms: (i) enhanced internal curing provided by PEG, ensuring continuous hydration, and (ii) the potential pozzolanic reactivity and micro-filler effect of finely ground brick particles, which contribute to additional C-S-H formation and pore refinement⁵⁴. The improved particle packing and ITZ densification likely contributed to the observed strength enhancement. When CWP was integrated into the system (Group 3), the synergistic effects were most pronounced at the 10% replacement level. The P1.5B50C10 mixture achieved the highest compressive strength among all tested self-curing mixes at 28 days, reaching 60 MPa—a 27.6% increase over the C-A control. Although strength development slowed at higher CWP contents (e.g., 44 MPa for P1.5B50C30 at 28 days), the moderate use of CWP effectively contributes to progressive hydration and microstructural refinement through secondary pozzolanic reactions. This behavior indicates that CWP contributes to progressive hydration and microstructural refinement at later ages through secondary pozzolanic reactions and filler effects. The densification of the binder matrix and reduction in pore connectivity are likely responsible for the enhanced long-term performance^55,56.

The results demonstrate that while PEG may slightly moderate early-age strength, its internal curing effect significantly enhances long-term compressive performance. The combined use of PEG, BWS, and moderate CWP replacement yields a synergistic improvement in matrix densification and hydration efficiency. When properly optimized, this integrated approach produces a high-strength and sustainable cementitious composite with superior long-term performance compared to conventional curing systems.

Uniaxial tensile stress-strain response

The uniaxial tensile stress–strain curves shown in Fig. 8 reveal distinct differences in cracking behavior and deformation capacity among the investigated mixtures. The air-cured control specimen (C-A) exhibited a nearly linear elastic response up to peak stress, followed by a rapid stress drop, indicating limited post-cracking resistance and a predominantly brittle failure mode. The absence of internal curing and matrix modification likely restricted crack distribution, resulting in localized fracture and reduced deformation capacity. The mixture containing 1.5% PEG (P1.5) displayed a more gradual stress development and an extended strain range prior to failure. This behavior suggests improved crack distribution and delayed localization, which can be attributed to enhanced hydration and matrix densification promoted by internal curing. By maintaining higher internal humidity during early hydration, PEG contributes to a more uniform microstructure, which supports more effective fiber bridging and stress redistribution after initial cracking. Further improvement was observed in the P1.5B50 mixture, incorporating 50% BWS in combination with 1.5% PEG. This mixture achieved higher tensile strength and exhibited a smoother post-peak response compared to the control and PEG-only mixtures. The improved performance can be associated with enhanced interfacial bonding between fibers and the surrounding matrix. The fine particles of BWS likely contributed to microstructural refinement and improved particle packing, strengthening the fiber–matrix ITZ and enabling more efficient stress transfer during crack propagation. The P1.5B50C10 mixture, which integrates 1.5% PEG, 50% BWS, and 10% CWP, demonstrated the highest tensile strength and a clearly defined strain-hardening region. The sustained increase in stress beyond the first cracking point indicates stable multiple cracking and effective fiber bridging action. The combined micro-filler and pozzolanic effects of CWP, together with the internal curing action of PEG, likely enhanced matrix compactness and reduced pore connectivity. This refined microstructure improves fiber anchorage and delays crack coalescence, thereby sustaining load-carrying capacity over a wider strain range.

Fig. 8.

The tensile stress–strain curves for selected mixes of HPFRC.

The progressive enhancement in tensile strength and deformation capacity across the mixtures highlights the synergistic interaction between internal curing and recycled constituents. The improved matrix densification, strengthened fiber–matrix interface, and enhanced hydration collectively contributed to more stable crack propagation and superior tensile performance^57,58.

Splitting tensile strength

Figure 9 presents the 28-day splitting tensile strength results for the investigated mixtures. The control specimens (C-A and C-B) recorded comparable strengths of 6.7 MPa and 6.6 MPa, indicating that conventional curing conditions did not produce significant differences in indirect tensile performance at this age. The incorporation of PEG alone resulted in minor variations in tensile strength. The mixtures P1.0, P1.5, and P2.0 achieved 6.0 MPa, 6.8 MPa, and 6.4 MPa, respectively. These results suggest that moderate PEG content (1.5%) maintains tensile capacity while providing internal curing benefits. Excessive PEG dosage (2.0%) did not yield additional improvement, which may be attributed to slight alterations in matrix continuity or binder packing at higher polymer content. The introduction of BWS significantly influenced tensile behavior, particularly at moderate replacement levels. The P1.5B25 and P1.5B50 mixtures reached 7.2 MPa and 7.7 MPa, respectively, representing noticeable improvement over the control. This enhancement can be attributed to improved particle packing density and microstructural refinement, which strengthen the ITZ and promote more uniform stress distribution under indirect tension. At higher replacement levels (75% and 100%), tensile strength values (6.6 MPa and 7.0 MPa) remained comparable to the control, indicating that even substantial substitution of natural sand did not compromise matrix integrity. The combined incorporation of BWS and CWP also demonstrated favorable performance. The P1.5B50C10 mixture maintained a tensile strength of 7.0 MPa, reflecting the beneficial micro-filler and secondary pozzolanic effects of low CWP content, which contribute to matrix densification and improved crack resistance. Increasing the CWP content to 20% and 30% resulted in slight reductions (6.2 MPa and 6.5 MPa), likely due to slower pozzolanic reactivity and partial dilution of the primary binder at early stages. Nevertheless, the values remained within an acceptable range, confirming the structural viability of these mixtures.

Fig. 9.

Splitting tensile strength of HPFRC mixes at 28 days.

The results indicate that an optimized combination of internal curing (1.5% PEG), moderate BWS replacement (around 50%), and limited CWP content (10%) provides the most favorable balance between sustainability and tensile performance. The observed trends are consistent with previous findings on HPFRC incorporating internal curing agents and recycled materials^59,60.

To better interpret the tensile performance of the developed mixes, the splitting tensile strength was normalized with respect to compressive strength. The obtained ratios, as presented in Table 4, range between 11% and 16%, with control mixes showing values around 13%–14%, while modified mixes incorporating fibers or supplementary materials reach approximately 15%–16%. This slight increase indicates an enhancement in tensile resistance relative to compressive strength, which can be attributed to improved crack-bridging and stress transfer mechanisms. Despite these improvements, the values remain within the typical range reported for high-performance concrete, confirming a consistent proportional relationship between tensile and compressive behavior.

Flexural strength

Figure 10 presents the 28-day flexural strength results of HPFRC mixtures, illustrating the influence of mix composition on bending performance. The results indicate that the incorporation of PEG, BWS, and CWP affects the flexural response depending on the replacement level and combination ratio. The control specimens (C-A and C-B) exhibited flexural strengths of 12.80 MPa and 15.00 MPa, respectively. The inclusion of 1.0% PEG (P1.0) slightly increased the flexural strength to 15.18 MPa, suggesting that internal curing contributed to improved hydration and matrix continuity. This trend is consistent with findings reported by Wu et al.²⁵, where internal curing agents enhanced bending performance through improved microstructural development.

Fig. 10.

Flexural strength of HPFRC mixes at 28 days.

Replacing RS with 25% BWS (P1.5B25) maintained a comparable flexural strength of 15.18 MPa, indicating that moderate incorporation of recycled brick particles does not adversely affect bending capacity. At higher BWS replacement levels (P1.5B50, P1.5B75, and P1.5B100), the flexural strengths ranged approximately between 9.75 and 11.25 MPa. Although slightly lower than the control mixes, these values remain within an acceptable structural range, demonstrating that substantial sand replacement with BWS can be achieved without severe deterioration in flexural performance⁶¹. The combined use of 50% BWS and 10% CWP (P1.5B50C10) resulted in the highest improvement, with an increase of 24.2% relative to the reference mixture (C-A). This enhancement may be attributed to the combined filler and pozzolanic effects of CWP, together with the internal curing action of PEG, leading to improved matrix densification and stress distribution under bending.

To evaluate the structural performance under bending and relate it to the fundamental compressive strength, the flexural strength was expressed as a ratio of compressive strength. The results, as shown in Table 4, show that this ratio varies between 17% and 31%, depending on the mix composition. Control mixes exhibit higher values (27%–31%), whereas modified mixes show more variation. This variation highlights the sensitivity of flexural behavior to changes in mixture constituents, particularly with the inclusion of fibers and supplementary cementitious materials. Nevertheless, flexural strength remains consistently higher than splitting tensile strength as a fraction of compressive strength, reflecting its closer representation of real structural performance and post-cracking behavior.

Durability evaluation

Water absorption

Figure 11 illustrates the water absorption results for the HPFRC mixes, highlighting variations based on the ratios of PEG, BWS, and CWP. For the control mixes, the water-cured mix (C-W) shows a 35% decrease in absorption compared to the air-cured mix (C-A), which is logically attributed to the more complete hydration process in the presence of water. In the first group, PEG concentrations of 1% and 1.5% led to a reduction in absorption by 13% and 14%, respectively, while the 2% dosage caused a slight 9% increase compared to the C-A mix, identifying 1.5% as the optimal PEG ratio for subsequent mixes. Furthermore, Group 3 demonstrated that CWS replacement levels of 25%, 50%, 75%, and 100% reduced absorption by 51%, 43%, 30%, and 39%, respectively, relative to C-A. Similarly, the final group showed that CWP concentrations of 10%, 20%, and 30% decreased absorption by 55%, 37%, and 25%, respectively; these improvements are due to the potent pozzolanic reaction of the CWP, which is entirely consistent with the previously observed compressive strength results⁶².

Fig. 11.

Water absorption for HPFRC mixes.

Microstructure analysis

Scanning electronic microscope (SEM)

The SEM images shown in Fig. 12 illustrate the microstructural characteristics of HPFRC mixtures with different modifications. The control mixture (C-A) exhibits a relatively heterogeneous and porous matrix, with visible micro-voids and partially un-hydrated cement particles. The interfacial regions appear less compact, indicating incomplete hydration and limited matrix densification. The incorporation of 1.5% PEG (P1.5) results in a comparatively denser matrix structure, with fewer visible voids and improved continuity of hydration products. This observation suggests that internal curing contributed to sustained hydration, leading to partial refinement of the pore structure and improved matrix integrity. In the P1.5B50 mixture, containing 50% BWS, angular brick particles are clearly embedded within the cementitious matrix. ITZs appear more compact compared to the control specimen. The improved packing effect and fine particles associated with BWS may contribute to a denser microstructure and enhanced mechanical interlocking within the composite. The P1.5B50C10 mixture, incorporating 50% BWS and 10% CWP, exhibits the most homogeneous and compact morphology among the examined samples. The matrix appears more uniformly filled with hydration products, and fewer continuous voids are observed. The presence of finer particles and potential secondary reaction products likely contributed to matrix refinement and improved internal structure. These observations confirm that the combined use of PEG, BWS, and CWP promotes a refined and cohesive microstructure, which aligns with the enhanced tensile strength and strain-hardening behavior observed in the mechanical results reported by Al-Fakih et al.⁵⁵. This confirms that the combined action of internal curing and pozzolanic reaction leads to enhanced formation of hydration products and matrix densification, demonstrating a clear synergistic effect.

Fig. 12.

SEM analysis of selected mixes of HPFRC.

X-ray diffraction (XRD)

The XRD patterns of HPFRC mixtures (C-A, P1.5, P1.5B50, and P1.5B50C10) are presented in Fig. 13. The diffraction peaks observed at approximately 20 ≈ 18°, 29°, 32°, 34°, 47°, 50°, and 54° are commonly associated with hydration-related phases such as portlandite (Ca(OH)₂), calcite (CaCO₃), and residual clinker phases (C₃S and C₂S). The broad hump in the 20–35° range corresponds mainly to amorphous calcium silicate hydrate (C–S–H).

Fig. 13.

XRD analysis of selected mixes of HPFRC.

The control specimen (C-A) shows relatively distinct portlandite peaks near 18° and 34°, indicating the presence of crystalline Ca(OH)₂ as a typical hydration product. With the addition of 1.5% PEG (P1.5), a slight reduction in portlandite peak intensity and a marginal broadening of the amorphous hump can be observed. This behavior suggests that PEG promotes sustained hydration through internal curing, leading to increased formation of amorphous hydration products rather than crystalline phases. For the P1.5B50 mixture, minor variations in peak intensity are detected, particularly within the 20–30° range, which may be associated with silica-rich components originating from the brick waste. The observed reduction in Ca(OH)₂ peak intensity may indicate partial consumption of portlandite due to interactions with reactive silica phases, contributing to additional formation of C–S–H gel. In the P1.5B50C10 mixture, the diffraction peaks appear slightly broader and less intense overall, indicating a relatively higher amorphous content. This can be attributed to the pozzolanic reactivity of ceramic waste powder, which consumes Ca(OH)₂ and forms additional calcium silicate hydrate and calcium aluminosilicate hydrate (C–A–S–H) phases. The reduced intensity of portlandite peaks further supports this interpretation. Moreover, the combined presence of PEG, BWS, and CWP enhances internal moisture availability and reactive surface area, resulting in a synergistic effect that promotes secondary hydration reactions and matrix densification.

It should be noted that XRD primarily identifies crystalline phases; therefore, detailed quantification of amorphous hydration products would require complementary analytical techniques. Nevertheless, the reduction in portlandite intensity and the increase in amorphous phases are consistent with SEM observations, which show a denser microstructure with fewer visible pores. These findings also correlate with the improved mechanical properties and reduced water absorption observed in the optimized mixture.

Conclusion

This study systematically investigated the development of HPFRC incorporating BWS, CWP, and PEG as a self-curing additive. The experimental results highlight the synergistic effects of these materials on mechanical, microstructural, and durability performance. The main conclusions are as follows:

• The addition of PEG improved internal curing and hydration continuity. An optimal dosage of 1% increased workability by 10% and enhanced compressive strength at 56 days, with gains of 5% and 14% compared to air- and water-cured samples, demonstrating its beneficial role in promoting hydration and early-age performance.

• Partial replacement of RS with BWS improved particle packing and ITZ densification, contributing to higher compressive and flexural strengths. The mixture containing 75% BWS achieved the most balanced mechanical performance, with a 56-day compressive strength increase of 38% relative to the PEG reference mix (1.5% PEG) and 22–32% compared to air- or water-cured samples.

• Incorporation of CWP as a supplementary cementitious material promoted pozzolanic reactions, refining the pore structure and enhancing durability. The hybrid mixture P1.5B50C10 (1.5% PEG, 50% BWS, 10% CWP) exhibited the best overall performance, achieving 28-day compressive strength improvements of 22–30% and flexural strength increases of 24–47% compared to other formulations.

• The optimized mixture P1.5B50C10 exhibited compressive, flexural, and tensile strengths of 63 MPa, 15.89 MPa, and 7 MPa, respectively, showing significant mechanical efficiency compared to the air-cured control (54 MPa, 12.80 MPa, and 6 MPa, respectively). These results indicate that the combined use of waste-based materials and PEG can effectively enhance load-bearing capacity under the tested conditions.

• Water absorption of the optimized mixture was the lowest (1.15%) compared to the air-cured control (2.57%), confirming that PEG and waste-based additives contribute to a denser, lower-permeability matrix.

• SEM and XRD analyses confirmed improved microstructural densification and pozzolanic reactivity. SEM images revealed a refined morphology, while XRD results indicated increased C–S–H formation and reduced portlandite peaks, reflecting enhanced hydration and matrix refinement.

• The combined use of BWS and CWP with PEG provides a practical approach to developing HPFRC with improved mechanical performance and durability. These findings demonstrate the potential for sustainable material reuse while maintaining high performance in fiber-reinforced cementitious composites.

Overall, the results demonstrate a strong synergistic interaction between PEG, BWS, and CWP, leading to simultaneous improvement in hydration, microstructure, mechanical performance, and durability.

Recommendations for future research

• Future research should evaluate the long-term durability of the developed HPFRC under aggressive environmental conditions, including chloride exposure, freeze–thaw cycles, and sulfate attack.

• Further investigation of shrinkage behavior and internal relative humidity is recommended to better quantify the effectiveness of PEG-based internal curing.

• Advanced characterization techniques, such as thermogravimetric analysis and fiber–matrix bond assessment, would provide deeper insight into hydration mechanisms and interfacial performance.

• Finally, incorporating life-cycle assessment (LCA) would allow quantitative evaluation of the environmental benefits associated with the use of recycled materials.

Author contributions

A.A.: Conceptualization, Methodology, Software, Writing- Original draft preparation, Investigation. O.Y.: Visualization, Investigation, Writing- Original draft preparation, Writing - Review & Editing, Supervision. M.M.E.: Data curation, Resources, Writing - Review & Editing, Supervision, Project administration. K.A.E.: Data curation, Resources, Writing - Review & Editing, Project administration. M.G.: Data curation, Resources, Writing - Review & Editing, Project administration. M.M.K.: Visualization, Investigation, Writing- Original draft preparation, Writing - Review & Editing, Supervision.

Data availability

All data generated or analyzed in this study are included in this manuscript.

Declarations

Competing interests

The authors declare no competing interests.