Sustainable high-performance concrete: harnessing recycled rubber and slag for strength and eco-friendliness

Abstract

This study proposes a sustainable high-performance concrete (HPC) incorporating industrial by-products and recycled materials, specifically granulated blast furnace slag and recycled rubber powder, as partial replacements for cement. A series of experimental tests were conducted to evaluate the mechanical performance of the modified concrete, including compressive, flexural, and Brazilian tensile strengths, along with workability characteristics. Results indicate that replacing cement with up to 30% slag resulted in less than 5–10% reduction in mechanical strength, while significantly improving workability. The inclusion of 10% recycled rubber powder enhanced ductility and fracture energy albeit with an associated reduction in compressive strength from 89 to 73 MPa. From an environmental perspective, carbon footprint analysis revealed that cement replacement led to substantial CO₂ emission reductions, with a 42% reduction at 30% slag and up to 37% reduction at 30% rubber content compared to conventional HPC. Life cycle cost analysis further demonstrated that the mix containing 30% slag achieved the highest strength-to-cost efficiency (0.22 MPa/$), highlighting its economic viability. Overall, the findings demonstrate that combining slag and recycled rubber offers an effective pathway toward environmentally friendly and cost-efficient HPC, suitable for sustainable construction applications where reduced carbon emissions, enhanced energy absorption, and adequate structural performance are required.

Introduction

As the construction industry faces increasing pressure to minimize its environmental impact, the quest for sustainable materials has never been more crucial^1–4. Concrete, the most widely used construction material globally, is a significant contributor to carbon emissions due to its high cement content, accounting for approximately 8% of global CO₂ emissions^5–9. Recent studies have shown that reducing cement content without compromising concrete quality is vital for creating environmentally friendly building materials^10–12.

By incorporating recycled rubber powder and slag into the concrete mix, this study investigates the potential for enhancing mechanical properties while simultaneously reducing the overall cement content. The integration of recycled rubber powder, derived from end-of-life tires, can significantly mitigate waste and enhance the material’s flexibility and toughness^13–16. Prior research has demonstrated that the addition of rubber can improve the energy absorption capacity and reduce crack propagation in concrete, leading to more resilient structures¹⁷.

Simultaneously, slag, a byproduct of the iron and steel industry, has gained attention for its potential to enhance the durability and strength of concrete when used as a partial replacement for cement^18–20. Numerous studies indicate that incorporating slag can improve the long-term performance of concrete due to its pozzolanic properties, which contribute to increased strength and resistance to certain environmental factors^21–23.

Several researchers have incorporated rubber waste HPC as a sustainable alternative, aiming to enhance its mechanical properties, improve durability, and reduce environmental impact through efficient waste management and material recycling. For example, Azevedo et al.²⁴ examined the durability and mechanical properties of HPC with rubber waste as a sand replacement. Their study demonstrated that the inclusion of fly ash and metakaolin minimized strength loss and significantly improved resistance to sulphuric acid. Results indicated that rubber waste up to 15% maintained high durability, while a mix of 45% fly ash and 15% metakaolin exhibited superior acid resistance.

Wang et al.²⁵ assessed the mechanical and durability properties of High-Performance Cementitious Composites containing rubber crumb. Their results showed that although compressive and flexural strength decreased with increased rubber content, steel fibers enhanced ductility. Strength loss was mitigated by hot water curing and rubber pre-treatment. A 40% rubber replacement demonstrated a better strength-to-density ratio, while chloride permeability remained comparable to the base material.

Ali et al.²⁶ explored the potential of recycled steel fibers as reinforcement in HPC, comparing them with manufactured steel fibers (MSF). Their findings revealed that recycled steel fibers retained 54–75% of MSF’s tensile strength and 61–77% of its flexural strength, while chloride permeability remained similar. Despite increased electrical conductivity due to metallic fibers, the corrosion risk of recycled steel fibers -reinforced HPC was classified as low.

The effects of replacing fine aggregate with tire rubber in HPC on mechanical and dynamic properties of HPC were investigated Singaravel et al.²⁷. Their study found that up to 10% rubber substitution resulted in minimal strength loss, while a 15% replacement significantly reduced strength. Despite this, rubber incorporation enhanced ductility and damping properties, benefiting dynamic performance. Attempts to mitigate strength reduction with fines and admixtures proved ineffective beyond the 15% threshold.

Ge et al.²⁸ evaluated the impact of rubber particles with different sizes (10–30 mesh) and contents (10–30%) as fine aggregate replacements in HPC. Their research indicated that a 10% replacement with 30-mesh rubber had the least adverse effect, reducing compressive and tensile strength by 18.19% and 5.56%, respectively. Rubber addition altered failure behavior from brittle to ductile, supporting its potential for sustainable, eco-friendly concrete applications.

An eco-friendly HPC by integrating tire rubber powder and waste wire to enhance sustainability was developed Mostafaei et al.¹⁷. Their study showed that substituting up to 40% of silica sand with rubber powder maintained sufficient compressive strength, while 3% waste wire reinforcement improved it to 106 MPa. Microstructural analysis revealed a weakened cement matrix at higher rubber contents, but the overall approach significantly reduced the material’s carbon footprint, promoting sustainable construction.

To further contextualize the findings of the current study within recent advances in sustainable concrete, several relevant investigations were incorporated. Mohanta and Murmu²⁹ assessed the use of Linz–Donawitz slag as an alternative coarse aggregate, demonstrating that steelmaking slags can effectively replace natural coarse aggregates with acceptable mechanical and durability performance, thus supporting the environmental viability of industrial by-products in structural concrete applications. Liu and Guo³⁰ examined ultra-high-performance concrete (UHPC) incorporating steel slag powder and aggregate, reporting improved paste fluidity and comparable nonevaporable water content to control mixes at later ages. Steel slag bonded well with hydration products, and up to 10% replacement maintained pore structure and strength. UHPC with steel slag aggregate further exhibited enhanced compressive strength. Moreover, Zhang et al.³¹ investigated steel slag as a partial cement replacement in UHPC, revealing slightly reduced early hydration and strength but minimal long-term strength loss due to unchanged pore structure and dense microstructure. Autogenous shrinkage remained similar, with minor reductions at higher slag levels. Life-cycle and leaching tests confirmed improved environmental performance.

Despite notable progress in using industrial by-products as sustainable concrete constituents, most existing studies focus on either slag or rubber individually, with limited work examining their combined influence using a comprehensive mechanical, microstructural, and sustainability perspective. Moreover, discrepancies remain regarding optimal replacement ratios and the potential trade-off between strength reduction and environmental benefit. Therefore, this study aims to develop a high-performance concrete (HPC) incorporating granulated blast furnace slag and recycled rubber powder, evaluating mechanical properties, fracture behavior, microstructural evolution, carbon footprint, and life-cycle cost. The novelty of this work lies in presenting an integrated experimental framework that identifies optimal replacement levels while quantifying sustainability gains. This approach establishes a research foundation for advancing low-carbon HPC design suitable for practical engineering applications.

Materials

For this research, several key materials were selected to formulate the HPC mixes. These materials include Ordinary Portland Cement (OPC), granulated blast furnace slag with a Blaine surface area of 4800 cm²/g, silica sand, and recycled rubber powder. To achieve a desirable workability and slump in the concrete mixtures, a polycarboxylate-based superplasticizer was incorporated into the formulation. This admixture is known for its ability to improve the flow characteristics of concrete while minimizing water content, thereby enhancing the strength and durability of the final product.

Cement

OPC serves as the primary binding agent in the concrete mix. It was selected for its availability and established performance in various construction applications. The chemical composition of OPC allows for effective hydration and strength development, providing a reliable baseline for comparisons in this study.

Slag

Granulated blast furnace slag, sourced from iron and steel production, is a byproduct with pozzolanic properties that enhance the durability and mechanical performance of concrete. The selected slag exhibited a Blaine fineness of 4800 cm²/g, ensuring a suitable surface area for reactivity when incorporated into concrete. The use of slag not only contributes to improved mechanical properties but also reduces the carbon footprint associated with cement production³².

Silica sand

Silica sand was chosen as the fine aggregate in this study. Known for its high purity and uniformity, silica sand enhances the workability, density, and mechanical strength of concrete. Its properties make it an ideal complement to cement and slag, facilitating optimal packing and bonding within the concrete matrix.

Recycled rubber powder

Recycled rubber powder, derived from ground used tires, was utilized as an alternative aggregate to enhance the concrete’s toughness and flexibility. Different percentages of rubber powder were incorporated into the concrete mix to evaluate its effects on mechanical properties. The use of recycled rubber not only addresses waste management issues but also aims to improve the energy absorption capacity of the final concrete product.

The particle size distribution of recycled rubber powder and slag are presented in Fig. 1. Rubber powder exhibited a wider particle size range, with more fine particles contributing to lower density and higher deformability in the matrix, whereas slag particles were relatively coarser and more uniformly graded.

Fig. 1.

Particle size distribution (PSD) of slag and recycled rubber powder used in this study.

The chemical and physical specifications of slag and cement are presented in Table 1.

Table 1.

Chemical and physical specifications of slag, and cement.

Chemical composition (% by weight)	Slag	Cement
Aluminum oxide (Al2O3)	15.5	5.72
Silicon dioxide (SiO2)	36.0	21.68
Iron oxide (Fe2O3)	1.15	3.28
Calcium oxide (CaO)	38.0	63.53
Sulfur trioxide (SO3)	0.5	1.62
Magnesium oxide (MgO)	8.7	1.75
K2O	0.5	0.52
Na2O	0.5	0.2
Cl	0.02	-
S2	1.3	-
Mn2O3	1.5	-
TiO2	1.4	-
Loss on ignition (LOI)	0.25	1.57
Density (gr/cm3)	2.75	3.15

Mixing design

The mix designs considered in the present study are listed in Table 2. The baseline selective mixing design for this study was developed based on previous research and established guidelines for HPC formulations. Initially, varying proportions of slag were tested, with 10%, 20%, 30%, 40% and 50% by weight of cement replaced by slag. This approach aimed to identify the optimal slag content that would yield the best mechanical properties without compromising workability. Following the initial evaluations, the mixing design which produced the most favorable mechanical performance was selected for further refinement.

Table 2.

The mixture proportions in the present study (kg/m³).

Designation	Cement	Slag	Silica sand	Rubber powder	W/B	SP
HPC	1100	–	1100	–	0.19	22
HPC (10%S)	990	110	1100	–	0.19	22
HPC (20%S)	880	220	1100	–	0.19	20
HPC (30%S)	770	330	1100	–	0.19	18
HPC (40%S)	660	440	1100	–	0.19	16
HPC (50%S)	550	550	1100	–	0.19	16
HPC (10%R)	693	330	1100	10	0.19	22
HPC (20%R)	616	330	1100	20	0.19	26
HPC (30%R)	539	330	1100	30	0.19	30

For this optimal mix, 10%, 20%, and 30% of the cement content was then replaced with recycled rubber powder. This step was crucial in exploring the potential benefits of rubber inclusion, such as improved ductility and reduced brittleness in the concrete. Each batch was mixed thoroughly to ensure uniform distribution of all components, allowing for subsequent testing and evaluation of resulting mechanical properties, including compressive strength, tensile strength, and flexural performance.

This systematic approach to material selection and mixing design not only adheres to functional performance criteria but also aligns with sustainability principles by reducing the overall cement content and reusing industrial byproducts and waste materials.

Test setup

To evaluate the mechanical properties of the developed HPC incorporating recycled rubber powder and slag, a series of standardized tests were conducted. These tests included compressive strength, flexural strength, and Brazilian tensile strength. Each test was performed on appropriately sized cylindrical and rectangular samples to ensure accurate and reliable data.

Density test

The density of the hardened concrete specimens was determined in accordance with ASTM C642³³, which is commonly used to evaluate the density, absorption, and void characteristics of hardened concrete. After 24 h of immersing in water to ensure full saturation the saturated surface-dry mass (M_ssd) was measured. Then, the specimens were oven-dried at 105–110 °C until a constant mass was achieved. The oven-dry mass was recorded as M_d. The apparent mass of each specimen while suspended in water (Mw) was also recorded. Using these measured masses, the density of the concrete was calculated based on the specimen volume derived from buoyancy principles, as specified in ASTM C642.

Compressive strength test

Compressive strength is one of the most critical parameters for assessing the performance of concrete. For this research, cylindrical samples with dimensions of 200 mm in height and 100 mm in diameter were prepared according to ASTM C39/C39M standards³⁴. The samples were cast in standard molds and cured for 28 days under controlled conditions to allow for proper hydration and strength gain (Fig. 2a).

Fig. 2.

Test setup; (a) Compressive strength; (b) Flexural strength; (c) Tensile strength.

Once the curing period was completed, the samples were subjected to compressive loading using a universal testing machine. The loading rate was maintained according to the guidelines specified in the standards to ensure accuracy. The maximum load applied was recorded, and the compressive strength was calculated based on the cross-sectional area of the cylinders. This test aimed to evaluate the overall strength of the concrete mixes, particularly in relation to varying proportions of slag and recycled rubber powder.

Flexural strength test

Flexural strength, or flexural strength, is essential for assessing the ability of concrete to resist flexural forces and tensile stresses that occur in structural applications. Rectangular beams measuring 30 mm in width, 70 mm in height, and 300 mm in length were prepared following the guidelines in ASTM C78/C78M³⁵. Similar to the cylindrical samples, these beams were also cured for 28 days before testing (Fig. 2b).

The flexural strength test was performed using a three-point loading setup on a universal testing machine. The beams were placed on two supports, and a load was applied at the midpoint until failure occurred. The maximum load at failure was recorded, which was then used to calculate the flexural strength of the concrete samples. This test provided insights into the tensile behavior of the HPC mixes and their potential performance in real-world applications, where flexural forces are prevalent.

Brazilian tensile strength test

The Brazilian tensile strength test was employed to evaluate the indirect tensile strength of the concrete samples, providing an additional perspective on the material’s performance under tensile loading conditions. For this purpose, the cylindrical samples of 200 mm height and 100 mm diameter, identical to those used in the compressive strength test, were utilized (Fig. 2c). During testing, these cylindrical samples were placed horizontally in a universal testing machine.

A compressive load was applied along the diameter of the sample until failure occurred. The test was conducted in accordance with ASTM C496/C496M standards³⁶, which outline the procedures for determining the indirect tensile strength of concrete. The maximum load at failure was recorded, allowing for the calculation of the Brazilian tensile strength using the formula:

1

Where σ_t is the tensile strength, P is the maximum load at failure, L is the length of the specimen, and t is the thickness of the specimen. This testing method is particularly relevant for assessing the performance of concrete subjected to tensile stresses, thereby providing a comprehensive understanding of the mechanical properties of the developed HPC mixes.

Conclusion of testing section

The results obtained from these tests will contribute to a detailed evaluation of the mechanical performance of the HPC mixes. By analyzing compressive strength, flexural strength, and Brazilian tensile strength, this research aims to assess the viability of incorporating recycled rubber powder and slag in concrete formulations, thereby advancing sustainable practices in the construction industry while ensuring that performance standards are met.

Life cycle assessment (LCA)

Life Cycle Assessment (LCA) is an established and systematic approach used to quantify the environmental impacts associated with concrete systems throughout their entire lifecycle. In the context of HPC, LCA enables a comprehensive evaluation of embodied carbon by accounting for greenhouse gas emissions arising from each stage of production^37,38. This includes raw material extraction, processing, transportation, mixing, placement, and end-of-life scenarios such as recycling, reuse, or disposal^39–42. Conducting LCA for HPC is essential since cement manufacturing, particularly clinker production, remains one of the most energy-intensive industrial processes and is responsible for a major share of global CO₂ emissions due to both thermal energy consumption and calcination of limestone^43–46.

Incorporating low-carbon materials has become a key strategy for reducing the environmental footprint of HPC⁴⁷. Supplementary cementitious materials (SCMs) such as fly ash, slag, silica fume, and metakaolin partially replace Portland cement and contribute to significant emission reductions while enhancing durability and mechanical performance^48–51. Likewise, utilizing waste-derived constituents such as recycled rubber powder from discarded tires not only promotes circular material use but also reduces landfill accumulation and improves the energy absorption capacity of concrete^44,52,53. Through material substitution and mix optimization based on LCA outcomes, HPC can achieve a lower carbon footprint without compromising structural performance.

To ensure accuracy of results, carbon footprint calculations in this study were performed using SimaPro supported by environmental databases and impact modelling tools. Impact evaluation was performed using IMPACT 2002+, allowing comparison of environmental indicators beyond CO₂ emissions, including human health, ecosystem quality, climate change, and resource consumption. The system boundaries considered in this research start from raw material procurement to concrete production, as illustrated in Fig. 3, enabling assessment of embodied emissions associated with each constituent material and mix configuration.

Fig. 3.

The boundary condition for LCA.

Results and discussion

Slump

The slump test results for the various HPC mix designs are presented in Fig. 4. Across all mixes, the slump values were maintained within a relatively narrow range of 202–216 mm, ensuring consistency in the targeted workability. A clear trend can be observed with respect to the type and proportion of replacement materials. As the percentage of slag substituted for cement increased, the workability of the concrete improved. Specifically, a 50% slag replacement yielded the highest slump value of 216 mm, indicating enhanced flowability and reduced internal friction within the fresh mix. This improvement can be attributed to the finer particle size and latent hydraulic properties of slag, which contribute to better particle packing and lubrication effects in the cementitious system.

Fig. 4.

Slump results of HPC samples.

Conversely, increasing the proportion of rubber powder had the opposite effect on workability. When the rubber content was raised from 10 to 30%, the slump value decreased from 212 to 202 mm. This reduction reflects the hydrophobic nature and irregular surface texture of rubber particles, which hinder effective dispersion and water availability in the mix. As a result, the fresh concrete becomes less workable, with reduced ease of placement and compaction. These findings highlight the contrasting influence of slag and rubber powder on the rheological behavior of HPC: while slag enhances fluidity through improved matrix packing, rubber powder introduces resistance to flow due to poor compatibility with the cement paste.

Specific weight

Figure 5 reveal a clear trend regarding the impact of slag and waste rubber powder on the specific weight of concrete samples. As illustrated, the specific weight of the concrete decreases systematically with an increase in the percentage of slag used as a partial replacement for cement. The reference sample, which contains 100% cement without any slag, has the highest specific weight recorded at 2448 kg/m³. This high value is expected as traditional cement is denser compared to the alternatives being tested. As the percentage of slag in the concrete mix increases from 10 to 50%, there is a corresponding decrease in specific weight, illustrating a shift in material specific weight. This decrease in specific weight can be attributed to the lower specific weight of the slag compared to that of traditional cement, as well as the possible changes in the microstructure of the concrete, which can lead to a more porous composition. Specifically, when 10% slag is introduced, the specific weight drops to approximately 2428 kg/m³, and at 50% slag replacement, it reaches around 2342 kg/m³. This trend underscores the effectiveness of slag not only as a binder but also as a lightweight replacement material, enhancing the sustainability of concrete by reducing the overall weight.

Fig. 5.

Specific weight results of HPC samples.

Furthermore, when waste rubber powder is incorporated into the concrete mix as a partial cement replacement, a similar pattern of decreasing specific weight is observed. The experiments varied the amount of waste rubber powder from 10 to 30%, and the specific weight of the concrete declines from 2286 to 2157 kg/m³. This reduction is significant and suggests that waste rubber powder, which has a notably lower specific weight than cement, contributes to a lighter concrete mix. This finding is particularly relevant for applications where reducing the weight of concrete can be beneficial, such as in precast concrete elements, transportation, and structural applications.

Compressive strength

Figure 6 presents images of the samples following the compressive strength test. The compressive strength results of concrete samples with varying percentages of slag as a replacement for cement at 28 and 90 days are presented in Fig. 7.

Fig. 6.

Test specimens; (a) before and (b) after compressive strength test.

Fig. 7.

Compressive strength results of HPC samples.

The results indicate that substituting cement with slag initially leads to a moderate reduction in compressive strength. Specifically, up to a 30% replacement of slag results in a decrease of less than 5% in compressive strength, indicating that this level of slag incorporation does not significantly compromise the structural integrity of the concrete. However, as the slag replacement increases beyond 40%, there is a marked decline in compressive strength, suggesting that excessive substitution impacts the performance of the concrete negatively. This observation emphasizes the critical balance needed when formulating concrete mixes with slag. Based on the results, it can be concluded that an optimal replacement percentage of up to 30% slag is recommended, as it allows for the benefits of enhanced sustainability without severely compromising the compressive strength. Moreover, a significant enhancement is observed at 90 days, where long-term hydration of slag improves matrix densification. At 90 days, mixes with 10–30% slag achieved 109–103 MPa, which is comparable to or slightly higher than the control mix (113 MPa).

In addition to the effects of slag, the figure also illustrates the impact of incorporating rubber powder. At 28 days, compressive strength decreased from 89 MPa (10%R) to 73 MPa (30%R), indicating a progressive decline with increasing rubber content. However, results at 90 days show that rubber mixes still gain additional strength over time, rising to 97 MPa at 10%R, 90 MPa at 20%R, and 80 MPa at 30%R. This decline in strength suggests that while rubber powder can enhance certain properties of concrete, such as flexibility and impact resistance, it also introduces a trade-off with compressive strength. Thus, the findings indicate that the optimal percentage for replacing cement with rubber powder is 10%, as this level provides a favorable balance between performance and functional benefits. Combining these insights, a proposed mixing plan has emerged, suggesting a formulation that includes 30% slag and 10% rubber powder as optimal substitutions for cement. This mix not only minimizes the compressive strength loss but also capitalizes on the environmental advantages of recycling industrial materials and waste products. The selected mixing proportions provide a pragmatic approach, addressing both the strength and sustainability goals inherent in contemporary concrete design practices. Ultimately, this mixing design demonstrates an effective strategy for achieving a lower environmental impact while maintaining adequate structural performance, making it a viable option for future construction applications.

Slag contributes to secondary hydration reactions by consuming portlandite and generating additional calcium silicate hydrate (C–S–H) gel, which densifies the matrix and enhances overall microstructural integrity. In contrast, recycled rubber powder is chemically inert and hydrophobic, meaning it does not participate in hydration reactions; instead, it disrupts the continuity of the matrix and weakens the interfacial transition zone (ITZ). When slag and rubber are incorporated together, slag can partially offset the negative effects of rubber by refining the pore structure and reducing portlandite content, thereby improving matrix compactness. Nevertheless, the hydrophobic nature and poor wettability of rubber particles continue to hinder local hydration near their surfaces, often resulting in microvoids and weaker bonding within the ITZ.

To enhance the reliability and reproducibility of the experimental results, a statistical evaluation was carried out using the coefficient of variation (CV). This parameter indicates the relative degree of variability observed in each measured property across the different mix designs. The CV values for five principal characteristics—slump flow, density, compressive strength, flexural strength, and tensile strength—are summarized in Table 3.

Table 3.

Coefficient of variation (CV) of specimens/tests.

Mix design	Compressive strength	Flexural strength	Tensile strength
HPC	0.013	0.019	0.019
HPC (10%S)	0.021	0.014	0.018
HPC (20%S)	0.014	0.022	0.015
HPC (30%S)	0.016	0.017	0.014
HPC (40%S)	0.017	0.016	0.012
HPC (50%S)	0.014	0.016	0.016
HPC (10%R)	0.015	0.014	0.017
HPC (20%R)	0.014	0.016	0.014
HPC (30%R)	0.019	0.021	0.014

Tensile strength

The tensile strength results for concrete samples containing varying percentages of slag as a cement replacement are meticulously illustrated in Fig. 8. A thorough examination of the data reveals significant trends and patterns that highlight the complex interactions between these materials. The findings indicate that replacing cement with slag leads to a measurable yet modest reduction in tensile strength. Specifically, when up to 30% of cement is substituted with slag, the decrease in tensile strength remains below 10%. This observation suggests that low to moderate levels of slag can be effectively incorporated into concrete without substantially compromising its tensile performance.

Fig. 8.

Tensile strength results of HPC samples.

The minimal decline in tensile strength within this range implies that slag serves as a viable alternative for partial cement replacement in concrete mixes. The pozzolanic properties of slag contribute to this effect, as they react with calcium hydroxide (CH) to form additional calcium silicate hydrate (C–S–H), which enhances concrete durability. This reaction improves the microstructure of concrete, potentially leading to better long-term performance by increasing its resistance to environmental factors.

However, when slag replacement exceeds 40%, a significant decline in tensile strength is observed. This pronounced drop highlights the potential challenges and limitations associated with excessive slag incorporation in concrete production. One primary mechanism behind this strength reduction is the interference with C–S–H formation, as excessive slag content may disrupt the development of this crucial component, leading to a weaker cement matrix. Additionally, an overreliance on slag increases concrete porosity, which negatively impacts tensile strength. Increased porosity creates stress concentrators that facilitate crack propagation under tensile stress, further weakening the material.

Based on these findings, an optimal replacement percentage of up to 30% slag is recommended. This proportion strikes a balance between sustainability—by recycling industrial byproducts—and maintaining adequate tensile strength necessary for structural integrity. Utilizing slag within this range supports environmentally friendly concrete production while ensuring the material’s long-term performance remains uncompromised.

The substitution of cement with rubber powder in concrete mixes represents another strategy aimed at enhancing sustainability while introducing unique material properties. The analysis of tensile strength in relation to rubber powder replacement percentage provides valuable insights into its optimal utilization in concrete production. Results indicate that replacing cement with rubber powder leads to a notable decrease in tensile strength, with values dropping from 5.1 to 4.2 MPa as the replacement percentage increases from 10 to 30%. This reduction suggests that while rubber powder offers certain benefits, its use in higher concentrations may compromise the concrete’s overall tensile performance.

The percentage loss in tensile strength, calculated at approximately 17.6%, underscores the limitations of rubber powder when used in higher amounts. This finding highlights the necessity of carefully determining the replacement percentage to maintain a balance between performance and sustainability. Despite the observed reduction in tensile strength, rubber powder may provide additional advantages, including improved ductility, energy absorption, and impact resistance. These properties can enhance the functional performance of concrete, particularly in applications where flexibility and resilience are beneficial.

Considering both the benefits and drawbacks, an optimal replacement of 10% rubber powder is deemed the most effective. At this level, sufficient tensile strength is maintained while still capitalizing on the unique properties of rubber. This approach supports the development of concrete with enhanced functionality and sustainability, making it a viable alternative for eco-friendly construction materials.

Flexural strength

Figure 9 shows the samples before and after the flexural test. The flexural strength results for concrete samples incorporating varying percentages of slag as a cement replacement are illustrated in Fig. 10. The results demonstrate that slag replacement influences flexural strength in distinct phases, with different thresholds affecting performance.

Fig. 9.

Test specimens; (a) before and (b) after flexural strength test.

Fig. 10.

Flexural strength results of HPC samples.

For slag replacement up to 30%, the reduction in flexural strength remains modest, with a decrease of less than 10%. This minor decline indicates that concrete can still maintain adequate strength for practical applications while benefiting from the environmental advantages of reduced cement usage. The results suggest that this level of replacement provides an effective compromise between performance and sustainability. However, when slag replacement exceeds 40%, a significant drop in flexural strength is observed. This pronounced reduction highlights the potential risks of excessive slag incorporation, likely caused by weakened interfacial bonding and aggregate matrix interactions. These findings emphasize that while slag can serve as a beneficial alternative to cement, its usage must be carefully controlled to prevent structural deficiencies. Based on this evaluation, a 30% slag replacement is identified as the optimal proportion, striking a balance between sustainability and mechanical integrity.

In parallel, the study also examined the effect of rubber powder as a cement replacement, revealing a substantial reduction in flexural strength as its proportion increased. With replacement levels ranging from 10 to 30%, the flexural strength decreased from 6.6 to 5.2 MPa, representing an approximate decline of 21%. This drop underscores the limitations of rubber powder in maintaining structural performance when used in high concentrations. While incorporating rubber waste into concrete enhances sustainability by promoting recycling, its negative impact on mechanical properties cannot be overlooked. The findings suggest that limiting rubber powder replacement to 10% provides the best balance between sustainability and strength retention, ensuring that the concrete remains suitable for structural applications.

Considering both slag and rubber powder replacement findings, an optimized mixing design has been proposed. A composition incorporating 30% slag and 10% rubber powder is recommended as an effective formulation. This combination minimizes strength loss while maintaining mechanical properties within acceptable limits for conventional structural applications. Additionally, the integration of these materials contributes to environmental sustainability by reducing cement consumption and utilizing industrial byproducts. The adoption of this optimized mix design not only enhances concrete’s environmental footprint but also aligns with sustainable construction practices by addressing waste management challenges and reducing carbon emissions.

The stress-displacement curves of concrete mixtures incorporating different percentages of rubber powder (10%, 20%, and 30%) as a cement replacement are illustrated in Fig. 11. This analysis provides critical insights into the mechanical behavior and energy absorption capacity of rubber-modified concrete, highlighting the trade-offs between strength, ductility, and toughness.

Fig. 11.

Stress-displacement curves of HPC samples.

The findings indicate a reduction in maximum stress when rubber powder is introduced into the mixture, suggesting a decline in load-bearing capacity compared to the reference sample without rubber. This decrease can be attributed to the intrinsic properties of rubber, which, while enhancing flexibility and energy absorption, do not provide the same structural rigidity as traditional cement. This trade-off is particularly relevant in applications where maintaining high load-bearing capacity is essential. While rubber modification improves other mechanical aspects, its effect on maximum stress must be carefully considered when determining the feasibility of its use in structural applications.

A significant increase in displacement at the rupture moment was observed in rubber-modified samples, with values rising by approximately 100%. This substantial increase in deformation before failure indicates improved ductility, making rubber-modified concrete more resilient under strain. Enhanced displacement is particularly beneficial for seismic-resistant designs, where structures must accommodate dynamic loading conditions. The ability of rubber-modified concrete to absorb and dissipate energy through increased deformation can contribute to structural stability under extreme stress, making it a promising material for specific engineering applications.

Further analysis of rupture energy, as illustrated in Fig. 12, reveals that the highest energy absorption was recorded in samples containing 10% rubber powder, reaching 636.6 J/m². This metric is crucial in assessing the material’s toughness and ability to withstand impact loads. The superior energy absorption at this optimal percentage suggests that a well-balanced interaction between rubber particles and the cement matrix enhances the composite’s ability to resist failure under impact. However, as the rubber content increases beyond this threshold, the effectiveness diminishes, indicating that excessive rubber incorporation may disrupt the structural cohesion of the concrete.

Fig. 12.

Fracture energy absorption of HPC samples.

These findings suggest that while rubber powder reduces maximum stress, it significantly enhances ductility and impact resistance. An optimal replacement level of 10% appears to provide the best balance between maintaining structural integrity and leveraging the benefits of rubber modification. This level ensures that concrete retains sufficient mechanical strength while offering improved energy absorption, making it suitable for applications requiring enhanced toughness and flexibility.

XRD analysis of HPC mixing designs

The XRD analysis was conducted on three different HPC mixtures to examine the impact of slag and rubber powder as partial cement replacements. The evaluated samples included an HPC mix with 30% slag, another with 30% slag and 10% rubber powder, and a control sample without replacements. The results, illustrated in Fig. 13, provide valuable insights into the mineralogical composition and crystallographic changes induced by these variations in mix design.

Fig. 13.

XRD results, (a) HPC, (b) HPC (30%S) and (c) HPC (10%R).

The phase identification results reveal the presence of key hydration compounds, namely dicalcium silicate (C₂S) and CH, across all samples. These phases are crucial indicators of cement hydration and contribute to the strength and durability of concrete. Among the three mixes, the highest amount of C₂S was detected in the control sample, indicating that conventional cement hydration remains most effective in forming silicate compounds. In contrast, the sample incorporating 30% slag exhibited a lower C₂S content, suggesting that while slag enhances sustainability and influences mechanical properties, it may alter the hydration kinetics of silicate phases. The lowest amount of C₂S was observed in the sample containing both 30% slag and 10% rubber powder, implying that the addition of rubber powder adversely affects the reactive potential of the cement matrix. This reduction in C₂S formation could hinder the overall strength development of the concrete.

The implications of these phase changes are significant, particularly regarding the long-term mechanical performance of the concrete. C₂S plays a fundamental role in the strength evolution of cementitious materials, contributing to improved durability over time. A decrease in C₂S content in rubber-modified mixes may lead to less efficient hydration, potentially compromising structural integrity under sustained loading conditions. Since the mechanical properties of concrete, such as compressive and flexural strength, are closely linked to its hydration products, variations in mineralogical composition provide essential predictions for its performance across different applications.

The role of CH in these mixtures further adds to the discussion of durability and environmental resistance. While CH is a natural byproduct of cement hydration, its presence is often linked to alkali-silica reactions, which can impact long-term stability. However, in slag-modified concrete, increased CH content can enhance resistance to sulfate attack and corrosion, particularly in aggressive environments. This suggests that while slag replacement alters hydration reactions, it may offer additional benefits in terms of durability under harsh exposure conditions. The higher CH values observed in slag-rich mixes can be explained by two factors: (i) the slower reactivity of slag compared to Portland cement, which delays pozzolanic consumption of CH at early ages, and (ii) the limitations of XRD in detecting amorphous phases, meaning that part of the C–S–H formed from slag hydration is not fully captured in the crystalline phase analysis. As a result, the apparent increase in CH in slag-rich mixes at the tested age does not contradict the long-term pozzolanic role of slag; rather, it reflects the balance between cement hydration (which generates CH) and the slower slag reaction kinetics (which consume CH over extended curing periods).

Overall, the XRD analysis highlights the complex interplay between cement replacement materials and hydration reactions in HPC. While slag replacement promotes sustainability and durability improvements, the incorporation of rubber powder can alter hydration kinetics, potentially affecting mechanical performance. These findings underscore the need for an optimized mix design that balances sustainability with structural reliability.

Using Rietveld refinement analysis, the relative proportions of crystalline and amorphous phases in the different mix designs were quantified, and the results are summarized in Table 4. The data reveal that the dominant phase formed within the matrix is calcium silicate hydrate (C–S–H), which constitutes both the principal amorphous and crystalline component of the system. Notably, the base sample exhibited the highest concentration of C–S–H, indicating a more extensive hydration process and a denser binding matrix compared to the modified mixes. This observation underscores the critical role of C–S–H in governing the mechanical strength and durability of the composite.

Table 4.

Mineralogical composition of binder for HPC (%).

HPC (10%R)	HPC (30%S)	HPC	Phases
37.2	52.2	65.8	C–S–H
31.8	24.8	13	GGBS
0.7	0.6	0.4	Calcite
9.6	8.6	7.8	Portlandite
20.7	13.8	13	Other

In contrast, the amount of portlandite (Ca(OH)₂) was lowest in the base sample. This reduction is significant because portlandite is generally considered less desirable in cementitious systems due to its limited contribution to strength and its susceptibility to leaching and chemical attack. The lower portlandite content in the base mix suggests a more efficient consumption of calcium hydroxide during pozzolanic reactions, leading to enhanced stability of the matrix. Together, these findings highlight the superior phase composition of the base sample, characterized by a higher proportion of strength-contributing C–S–H and a reduced presence of portlandite, which collectively improve the performance of the system.

It should be noted that rubber particles, being inert and hydrophobic, and sometimes releasing trace retarders such as zinc, impede slag dissolution and hinder the nucleation of C–S–H. Consequently, hydration kinetics are delayed and the overall formation of C–S–H is reduced. Isothermal calorimetry confirms this effect, showing an extended induction period and lower heat release peaks, indicative of slower reaction rates and diminished hydration progress.

Thermogravimetric analysis further supports these findings, revealing decreased chemically bound water and reduced portlandite consumption, both consistent with limited slag activation and lower C S H yield. Taken together, the calorimetry and TGA results reconcile the observed strength losses: fewer hydrates are produced, porosity increases, and the interfacial transition zone (ITZ) around rubber particles weakens, ultimately leading to reductions in compressive and flexural strength^54,55.

FTIR test

The FTIR analysis results for two different HPC mixing designs, one of which includes 10% recycled content (designated as HPC (10% R)), are illustrated in Fig. 14. This figure provides a comparative overview of the spectral characteristics associated with each mixing design. Previous research has identified that the band peak observed within the range of 960–1120 cm⁻¹ corresponds to the Si–O–Si bonding structure within Calcium-Silicate-Hydrate (C–S–H), which is a crucial component in determining the structural properties of concrete.

Fig. 14.

FTIR analysis results for two different HPC mixing designs.

Upon examining the data presented in the figure, it is evident that the desired Si–O–Si peak appears more prominently in the HPC mixing design compared to the HPC (10% R) formulation. This pronounced peak suggests a greater level of polymerization or order within the C–S–H gel of the HPC design, which is often linked to enhanced mechanical strength and durability. Consequently, the superior intensity of this peak could be a significant factor that contributes to the optimal mechanical properties observed in the HPC mixing design, highlighting its advantages over the HPC (10% R) formulation.

SEM

The SEM micrographs of the base HPC sample and the HPC sample incorporating 10% rubber powder (denoted as HPC–10%R) are presented in Fig. 15. A comparative examination of these images reveals distinct differences in the microstructural characteristics of the two systems. In the base HPC sample, the cementitious matrix demonstrates a relatively dense and homogeneous structure, with a well-defined interfacial transition zone (ITZ) that ensures adequate bonding between the matrix and the aggregates. In contrast, the HPC–10%R sample exhibits noticeable microstructural disruptions. The incorporation of rubber powder, in combination with slag as a partial replacement for cement, has adversely influenced the overall integrity of the composite.

Fig. 15.

SEM results, (a) HPC, and (b) HPC (10%R).

Specifically, the ITZ in the modified sample appears less cohesive, with visible microvoids and weak contact regions between the cement paste and the aggregates. This weakened bond suggests a reduction in the efficiency of stress transfer across the matrix–aggregate interface, which is critical for maintaining mechanical strength. The diminished bonding can be attributed to two primary factors: (i) the poor chemical compatibility and hydrophobic nature of rubber particles, which hinder their integration into the cement hydration products, and (ii) the altered hydration process induced by slag substitution, which modifies the formation and distribution of calcium silicate hydrate (C–S–H) gel. Together, these effects contribute to a more porous, discontinuous, and heterogeneous microstructure.

At ~ 10% rubber, the rubber–paste ITZ shows roughened interfaces and partial mechanical interlock that enhance crack-bridging and energy dissipation, explaining the peak fracture energy. However, SEM/EDS studies consistently reveal weak bonding, higher porosity, and microcrack initiation at the ITZ, while micro-CT imaging demonstrates that at higher rubber contents (> 15–20%), voids and defects become interconnected, indicating a percolation threshold that leads to sharp reductions in fracture energy and strength⁵⁶.

Life cycle assessment

The LCA outcomes presented in Table 5 reveal distinct environmental trends between slag-based and rubber-based HPC mixtures. A progressive and consistent improvement is observed when slag is used as a cement replacement. Increasing slag content results in a uniform reduction across nearly all environmental impact categories, including carcinogens, non-carcinogens, respiratory inorganics, ionizing radiation, ecotoxicity, acidification potential, mineral extraction, and non-renewable energy demand. For example, carcinogenic impact decreased from 6.331 kg C₂H₃Cl eq for the control mix to 5.002 kg C₂H₃Cl eq at 50% slag, while global warming potential reduced from 1075 to 617 kg CO₂-eq over the same range. Similarly, non-renewable energy consumption dropped from 5542 to 3996 MJ, reflecting the lower embodied energy associated with slag production. These results confirm that slag replacement beyond 30% substantially mitigates environmental burden, with 40–50% slag achieving the most pronounced sustainability gains.

Table 5.

Life cycle assessment results for all HPC mix designs incorporating slag and rubber as partial cement replacements, evaluated using the IMPACT 2002 + method across multiple mid-point environmental impact categories.

Impact category	Unit	HPC	HPC (10%S)	HPC (20%S)	HPC (30%S)	HPC (40%S)	HPC (50%S)	HPC (10%R)	HPC (20%R)	HPC (30%R)
Carcinogens	kg C2H3Cl eq	6.331	6.134	5.822	5.511	5.199	5.002	6.719	7.927	9.136
Non-carcinogens	kg C2H3Cl eq	13.257	12.253	11.209	10.165	9.121	8.117	9.764	9.362	8.961
Respiratory inorganics	kg PM2.5 eq	0.583	0.551	0.516	0.481	0.446	0.414	0.483	0.485	0.487
Ionizing radiation	Bq C-14 eq	2053	2011	1953	1895	1836	1794	2204	2513	2823
Ozone layer depletion	kg CFC-11 eq	3.59E-05	3.38E-05	3.13E-05	2.89E-05	2.64E-05	2.43E-05	3.29E-05	3.70E-05	4.11E-05
Respiratory organics	kg C2H4 eq	0.126	0.118	0.109	0.100	0.091	0.083	0.132	0.164	0.195
Aquatic ecotoxicity	kg TEG water	60,166	56,383	52,318	48,253	44,187	40,405	48,271	48,290	48,308
Terrestrial ecotoxicity	kg TEG soil	16,153	15,148	14,058	12,969	11,880	10,875	13,000	13,031	13,062
Terrestrial acid/nutri	kg SO2 eq	12.370	11.468	10.539	9.610	8.681	7.780	9.359	9.109	8.858
Land occupation	m2org.arable	5.901	5.627	5.332	5.037	4.741	4.467	5.203	5.370	5.536
Aquatic acidification	kg SO2 eq	2.656	2.556	2.448	2.340	2.232	2.132	2.332	2.324	2.316
Aquatic eutrophication	kg PO4 P-lim	0.063	0.061	0.057	0.054	0.051	0.048	0.058	0.062	0.067
Global warming	kg CO2 eq	1075	984	892	800	707	617	759	719	678
Non-renewable energy	MJ primary	5542	5262	4933	4605	4276	3996	5236	5868	6500
Mineral extraction	MJ surplus	9.888	9.556	9.071	8.587	8.102	7.770	9.954	11.321	12.688

Rubber-based mixtures demonstrate different environmental behavior. While rubber incorporation contributes considerably to reducing carbon footprint, falling to 678 kg CO₂-eq at 30% rubber replacement, the improvement is not uniform across all indicators. In contrast to slag, several categories such as carcinogens, ionizing radiation, and respiratory organics increase at higher rubber contents, indicating potential toxicity-linked trade-offs. Carcinogenic impact rises to 9.136 kg C₂H₃Cl eq at 30% rubber, surpassing the control mix. Likewise, non-renewable energy consumption increases steadily with higher rubber dosage, reaching 6500 MJ for the 30% replacement mix. This suggests that although rubber effectively lowers CO₂ emissions and supports waste valorization, excessive replacement may introduce additional impacts related to polymer processing and chemical additives present in recycled tire rubber.

Overall, the comparative trends indicate that slag delivers the most robust environmental benefits across nearly all categories, particularly at 40–50% replacement levels, with consistent reductions in emissions, energy use, and resource extraction impacts. Rubber offers environmental advantages in terms of global warming potential and circular material use; however, its optimal use lies in moderate replacement levels (10–20%), where environmental gains are present without significant increases in toxicity-related categories. Therefore, from a sustainability perspective, slag emerges as the most effective SCM for holistic environmental improvement, while rubber can be considered a complementary eco-material when applied with controlled dosage to balance emissions reduction with potential environmental trade-offs.

Figure 16a, b illustrate the contribution of individual constituent materials to the overall environmental impact of HPC, allowing a direct comparison between the control mix and the rubber-modified mix. In the reference HPC, cement dominates the environmental profile across all categories, frequently accounting for 50–75% of total impact, depending on the indicator. This confirms that cement is the principal driver of emissions in high-performance concrete due to its energy-intensive clinker production and calcination process. Other materials such as silica sand, slag, water, and superplasticizer contribute relatively small shares, generally below 15%, indicating that their environmental relevance is minor compared to cement.

Fig. 16.

Material contribution shares across key impact categories calculated using the IMPACT 2002 + method: (a) base HPC; (b) HPC (10%R).

When 10% of cement is replaced with rubber (Fig. 16b), notable variations in material contribution trends emerge. The share of cement decreases visibly, reducing its impact dominance across categories, particularly in climate change, acidification, non-renewable energy, and mineral extraction indicators. Rubber powder begins to contribute a measurable environmental load, especially in categories linked to carcinogens, ionizing radiation, and respiratory organics, where its relative share increases. This is associated with the chemical composition, processing energy of tire recycling, and additives in rubber granules. Nevertheless, because rubber has a significantly lower embodied CO₂ than cement, its introduction reduces overall global warming potential compared to conventional HPC.

A comparison between figures indicates a shifting distribution of impacts rather than a uniform reduction. Slag and rubber both reduce the environmental burden tied to cement; however, rubber introduces trade-offs in toxicity-related indicators where its share becomes more pronounced. This means that adding rubber does not eliminate cement’s influence but redistributes part of the environmental load across different impact categories. The presence of rubber reduces climate-linked emissions yet increases contribution to categories relating to long-term ecosystem and human health exposure.

Table 6 summarizes the single-score damage indicators for all mixes based on Human Health, Ecosystem Quality, Climate Change, and Resource Depletion. A clear downward trend is observed with increasing slag content, indicating substantial environmental improvement relative to the control HPC. Human Health reduces from 4.64E-04 DALY in HPC to 3.27E-04 DALY at 50% slag, corresponding to a 29% reduction in damage potential. Similarly, Ecosystem Quality decreases progressively from 150.09 PDF·m²·yr to 101.01 PDF·m²·yr, marking a 33% improvement when half of the cement is replaced. Resource consumption shows the largest reduction, dropping from 5552 to 4004 MJ, equal to a 28% reduction, while Climate Change impact decreases from 1075 to 617 kg CO₂-eq, representing a 43% improvement. These results confirm that higher slag substitution substantially reduces long-term environmental damage across all endpoint categories.

Table 6.

Damage assessment results of HPC mixes from LCA based on the IMPACT 2002 + endpoint method.

Damage category	Unit	HPC	HPC (10%S)	HPC (20%S)	HPC (30%S)	HPC (40%S)	HPC (50%S)	HPC (10%R)	HPC (20%R)	HPC (30%R)
Human health	DALY	4.64E-04	4.38E-04	4.09E-04	3.81E-04	3.53E-04	3.27E-04	3.85E-04	3.89E-04	3.93E-04
Ecosystem quality	PDF*m2*yr	150.09	140.71	130.60	120.49	110.39	101.01	120.66	120.83	120.99
Climate change	kg CO2 eq	1075	984	892	800	707	617	759	719	678
Resources	MJ primary	5552	5272	4943	4613	4284	4004	5246	5880	6513

Rubber-modified mixes exhibit a different pattern. Although rubber improves Climate Change performance, with values dropping from 759 kg CO₂-eq (10%R) to 678 kg CO₂-eq (30%R), representing a 11% reduction across the rubber series compared to the control, the gains are less pronounced than those achieved with higher slag levels. For Human Health, rubber mixes remain lower than the control HPC at moderate replacement (10%R = 3.85E-04 DALY), but the impact gradually rises with increasing rubber content, reaching 3.93E-04 DALY at 30% replacement. This results in only a 15–17% improvement compared to HPC, noticeably lower than the 29% achieved at 50% slag. A similar trend appears in Resources, where 10% rubber begins with 5246 MJ (6% improvement), but impacts increase considerably at higher contents, reaching 6513 MJ at 30%R, which exceeds even the baseline HPC. Ecosystem Quality remains almost unchanged around ~ 120 PDF·m²·yr for all rubber mixes, suggesting rubber does not meaningfully alleviate ecological impacts in this dimension.

When comparing the two material groups directly, slag mixtures provide broad and consistent improvements in all environmental damage metrics, especially beyond 30% replacement, whereas rubber primarily reduces climate-related impacts but introduces trade-offs in human health and resource categories at higher dosages. The best-performing mix overall is 50% slag, demonstrating the lowest scores in every damage category. Rubber is environmentally advantageous only up to ~ 10–20% replacement, where CO₂ emissions are reduced without excessive increase in resource or toxicity burdens.

Figure 17 compares the contribution percentage of raw materials to total environmental damage for four endpoint categories—Human Health, Ecosystem Quality, Climate Change, and Resource depletion—in both the reference HPC and the 10% rubber-modified mix. In all categories, cement remains the dominant contributor, consistently accounting for the largest environmental share. In the reference HPC, cement contributes approximately 65–80% of total damage depending on category, confirming its influence as the primary environmental hotspot due to clinker production and fuel consumption in cement manufacturing.

Fig. 17.

Contribution share of constituent materials to endpoint damage categories for HPC and HPC with 10% rubber replacement.

When 10% cement is replaced with rubber powder, the overall share of cement decreases across all categories, most visibly in Human Health and Resource damage. However, a new contribution component emerges from rubber, which begins to occupy 6–14% of total share depending on category. While rubber reduces cement-related emissions, its introduction shifts part of the environmental burden toward toxicity-related categories due to rubber processing and additives in recycled tire waste.

For Human Health, cement dominates in both mixes, yet its contribution reduces after rubber incorporation. Rubber introduces a recognizable share, while slag and silica sand maintain smaller contributions. Ecosystem Quality follows a similar trend, with slight reduction in cement share and increased representation from rubber and SP. In Climate Change, the cement share decreases further in HPC(10%R), reflecting the effectiveness of rubber in lowering carbon emissions compared to the original mix. However, rubber remains a noticeable contributor, indicating that CO₂ reduction is achieved through trade-offs in other categories. Resource depletion shows one of the strongest redistributions, with cement’s share dropping while rubber and SP contributions grow, illustrating energy input associated with rubber processing.

Figure 18 presents the carbon footprint of various HPC mix designs, incorporating slag and rubber as partial cement replacements. The data highlights the significant impact of these alternative materials in reducing the environmental burden associated with cement production.

Fig. 18.

Carbon footprint of HPC samples.

The control mix (HPC) exhibits the highest emission value (1,075 kg CO₂-eq), reflecting the dominant role of cement clinker in embodied carbon. Progressive substitution with slag leads to a clear reduction trend: 10% slag reduces emissions to 984 kg CO₂-eq, 20% slag to 892 kg CO₂-eq, and 30% slag further down to 800 kg CO₂-eq. With higher incorporation, emissions drop substantially, reaching 707 kg CO₂-eq at 40% slag and the lowest footprint of 617 kg CO₂-eq at 50% slag, corresponding to a reduction of approximately 43% relative to the reference. This reduction highlights the advantage of using slag—an industrial by-product with significantly lower embodied energy—over ordinary Portland cement.

Rubber-modified mixes also demonstrate considerable emission savings. Carbon footprint values decline to 759 kg CO₂-eq for 10% rubber, 719 kg CO₂-eq at 20% rubber, and 678 kg CO₂-eq at 30% rubber, confirming rubber’s effectiveness in lowering environmental impact. Although reductions are slightly less pronounced than high-percentage slag mixes, all rubber replacements still achieve significant carbon savings compared to the control HPC. This is attributed to the low embodied carbon of recycled rubber sourced from waste tires, which not only diverts waste from landfills but also offsets cement consumption.

Overall, both slag and rubber substantially reduce the embodied carbon of HPC, with slag showing maximum impact at high replacement levels (40–50%) and rubber offering meaningful reductions at moderate dosages (10–30%). These findings reinforce the role of industrial waste utilization as a viable pathway toward low-carbon concrete production.

The reported CO₂ emission reductions of 31% (slag) and up to 56% (rubber) compare favorably with international benchmarks. EC3 typically identifies ~ 30% reductions as achievable through low-carbon procurement, while LEED v5 awards maximum credits for embodied carbon reductions in the 20–40% range. Our results not only meet but exceed these thresholds. Furthermore, the EU taxonomy requires life-cycle GHG reductions consistent with net-zero trajectories; the reductions achieved in this study align with these technical screening criteria, underscoring the potential of the proposed HPC mix to contribute to sustainable construction under recognized international frameworks.

Figure 19 illustrates the strength per impact (MPa/kg CO₂-eq) for different HPC mix designs incorporating slag and rubber as partial cement replacements. The results highlight how these alternative materials enhance eco-efficiency, defined as the ratio of mechanical performance to environmental impact.

Fig. 19.

Strength per impact for different HPC mix designs.

The reference HPC mix exhibits the lowest value (0.094 MPa/kg CO₂-eq), indicating higher cost associated with achieving its compressive strength. Introducing slag progressively enhances cost efficiency, where HPC with 10% slag increases to 0.103 MPa/kg CO₂-eq, followed by further improvement at 20% slag (0.110 MPa/kg CO₂-eq). The highest value within slag series is observed at 30% slag (0.119 MPa/kg CO₂-eq), demonstrating the most economically advantageous replacement level. At higher dosages, 40% and 50% slag mixes show slight decreases (0.120 → 0.118 MPa/kg CO₂-eq), yet still remain superior to the control mix. This trend suggests an optimum range around 20–40% slag, where the balance between mechanical strength and material cost is most favorable.

Rubber-modified mixes also show notable improvement compared to the control specimen. HPC containing 10% rubber reaches 0.117 MPa/kg CO₂-eq, exceeding all slag mixes except 30–50% replacements. The 20% rubber mixture maintains competitive cost efficiency at 0.113 MPa/kg CO₂-eq, while 30% rubber achieves 0.108 MPa/kg CO₂ eq, slightly lower than other modified mixes but still significantly higher than the unmodified HPC baseline. These results indicate that moderate rubber contents can enhance economic performance, although excessive replacement may begin to reduce overall strength efficiency.

Overall, both slag and rubber incorporation increase cost-effectiveness relative to conventional HPC. Slag mixtures show a clear optimum around 30–40%, whereas rubber achieves maximum benefit at low to moderate levels (10–20%). These findings highlight the potential of industrial by-products to reduce construction costs while maintaining mechanical performance, supporting their integration into sustainable concrete technologies.

Lyfe cycle costing

The cost analysis of different HPC mix designs, as illustrated in Fig. 20. The cost analysis of different HPC mixes reveals a significant trend of cost reduction as cement is partially replaced with slag and rubber. The reference HPC mix, which consists entirely of cement as the primary binder, has the highest cost at $543 per cubic meter. This is expected since traditional cement is energy-intensive to produce and comes with a higher market price. However, as slag content increases, the cost declines. The cost reduction reaches its peak with the HPC (50%S) mix, which achieves a cost of $374, marking a 31.1% decrease from the original HPC mix. This demonstrates that slag is an effective cost-reducing material, making it a viable alternative to traditional cement, particularly in large-scale construction projects where cost savings are crucial.

Fig. 20.

Cost analysis of different HPC mix designs.

When rubber powder is added alongside 30% slag, the cost remains significantly lower than the reference mix, but slightly higher than the HPC (50%S) mix. The HPC (10%R) mix, which includes 30% slag and 10% rubber powder, results in a cost of $432, reducing costs by 20.4% compared to the reference mix. As the rubber powder percentage increases to 20% and 30%, the costs continue to decline to $423 and $414, respectively. This trend suggests that while rubber powder contributes to cost reduction, the effect is slightly less pronounced than with slag alone. The mix with 30% slag and 30% rubber powder (HPC 30%R) demonstrates one of the lowest costs at $414, making it a highly cost-effective alternative.

The strength-to-cost of different HPC mix designs is presented in Fig. 21. It provides insights into how effectively the mechanical strength of a concrete mix is achieved relative to its cost. The reference HPC mix, which consists entirely of cement, serves as the baseline for comparison, with a strength-to-cost ratio of 0.19 MPa/$. As slag is introduced as a partial replacement for cement, the efficiency of strength utilization improves, reflecting the cost benefits of using slag. Among the slag-modified mixes, HPC (30%S) exhibits the highest strength-to-cost ratio at 0.22 MPa/$, making it the most cost-effective option. This suggests that 30% slag replacement provides an optimal balance between strength retention and cost savings. Beyond this level, the efficiency starts to plateau or slightly decline, as seen in HPC (50%S), which achieves 0.21 MPa/$. This trend implies that while slag remains beneficial at higher percentages, the additional cost savings do not significantly enhance strength efficiency beyond a 30% replacement level.

Fig. 21.

Strength-to-cost of different HPC mix designs.

The incorporation of rubber alongside 30% slag leads to a gradual decline in the strength-to-cost ratio. The HPC (10%R) mix maintains a 0.19 MPa/$ ratio, comparable to the reference mix. However, as the rubber powder content increases, the efficiency decreases further, with HPC (20%R) dropping to 0.19 MPa/$ and HPC (30%R) recording the lowest value at 0.18 MPa/$. This decline suggests that rubber powder, while beneficial for sustainability and flexibility, does not contribute to strength in the same way as slag. The cost reduction achieved by using rubber powder is not sufficient to compensate for the mechanical strength loss, making it less suitable for applications where high compressive strength is a priority.

From an economic efficiency standpoint, the HPC (30%S) mix emerges as the most cost-effective formulation, offering the highest strength per cost ratio. This mix leverages the cost-saving benefits of slag while maintaining sufficient mechanical performance for structural applications.

Design code limitations, practical challenges, and implementation considerations for rubber and slag in concrete

An important consideration in the practical application of the proposed HPC mix designs is compliance with existing design codes and standards. Although the present study has primarily focused on mechanical and microstructural performance, the incorporation of slag and recycled rubber powder in concrete is governed by specific limitations and requirements established by international and national regulations. Ground granulated blast furnace slag (GGBFS) is widely recognized as a SCM and is explicitly permitted in major standards such as ASTM C989, EN 15167, and ACI 233. These codes define requirements related to chemical composition, fineness, and activity index to ensure consistent performance. Depending on the application and exposure conditions, slag replacement levels typically range between 20 and 50% of the cement mass, with higher replacement ratios requiring verification of strength development and long-term durability. Design provisions further emphasize the assessment of durability-related properties, including sulfate resistance, chloride penetration, and mitigation of alkali–silica reaction when slag is incorporated.

In contrast, the use of recycled rubber powder or crumb rubber in structural concrete is not yet fully codified. Existing standards, such as ASTM C33 and EN 12620, do not explicitly recognize rubber as an approved aggregate or cement replacement material. As a result, rubberized concrete is generally regarded as an experimental or non-structural material, with applications commonly limited to pavements, lightweight blocks, vibration-damping elements, and shock-absorbing components. Design guidelines consistently highlight concerns regarding reduced workability, lower compressive strength, and weakened interfacial transition zones (ITZ), which must be carefully addressed before rubber-modified concrete can be approved for structural use. In practical applications, rubber additions are often restricted to relatively low contents, typically less than 20% by volume of fine aggregate, unless extensive mechanical and durability validation is provided.

The findings of the present study demonstrate the mechanical and microstructural behavior of HPC mixes incorporating slag and recycled rubber powder; however, field implementation requires careful alignment with design code requirements. In particular, practical adoption necessitates verification that slag replacement levels satisfy the chemical and performance criteria outlined in ASTM, EN, and ACI standards, recognition that recycled rubber powder currently falls outside the scope of conventional structural concrete codes, and compliance with code-mandated testing protocols, including compressive strength benchmarks, shrinkage and creep limits, and durability performance under aggressive environmental exposures. Consequently, further code-based validation under standardized testing frameworks is required to support the transition from laboratory-scale investigations to real-world applications.

Despite their clear sustainability benefits and promising laboratory performance, several challenges may hinder the widespread adoption of slag- and rubber-modified HPC in construction practice. Variability in the chemical composition and fineness of GGBFS, depending on its source and production process, can influence reactivity and long-term strength development. Similarly, recycled rubber powder often exhibits inconsistent particle size distribution, morphology, and surface characteristics, leading to unpredictable effects on workability, hydration, and bonding within the cementitious matrix. From a construction perspective, rubber incorporation tends to reduce slump and fresh-state flowability due to its hydrophobic nature and irregular surface texture, complicating placement and compaction in field conditions. Although slag can enhance workability at moderate replacement levels, it may also delay early-age strength development, potentially affecting construction schedules.

Mechanical and durability trade-offs also remain critical concerns. Rubber particles weaken the ITZ, resulting in reductions in compressive strength and stiffness, while high slag replacement levels (typically above 40–50%) may compromise early-age strength and require extended curing periods. Moreover, the long-term durability of rubber-modified concrete under aggressive environmental conditions—such as chloride ingress, freeze–thaw cycles, and sulfate attack—has not yet been fully established, raising uncertainties regarding service life and structural reliability. Additional constraints include regulatory limitations due to the absence of formal code provisions for recycled rubber, economic and logistical challenges related to the availability and consistency of raw materials, additional processing costs associated with rubber pretreatment to improve bonding, and potential resistance from industry stakeholders toward materials perceived as experimental.

Finally, the long-term behavior of slag- and rubber-modified HPC under real service conditions remains insufficiently understood. Potential risks associated with shrinkage, creep, and microcracking may compromise dimensional stability and durability in large-scale applications. Addressing these uncertainties through extended durability testing and code-compliant validation is essential to bridge the gap between experimental innovation and practical acceptance, thereby enabling the safe and effective implementation of sustainable HPC in real-world construction projects.

Potential challenges in adopting slag- and rubber-modified mixes in real construction projects

Although slag and recycled rubber powder offer clear sustainability benefits and have demonstrated promising mechanical and microstructural performance in laboratory studies, their adoption in real construction projects faces several challenges that must be carefully addressed.

Variability in material properties

Slag The chemical composition and fineness of ground granulated blast furnace slag (GGBFS) can vary depending on the source and production process, which may affect reactivity and long-term strength development.

Rubber powder Recycled rubber particles often exhibit inconsistent size distribution, morphology, and surface chemistry, leading to unpredictable effects on workability and bonding within the cementitious matrix.

Workability and placement issues

Rubber incorporation tends to reduce slump and fresh mix flowability, complicating placement and compaction in field conditions. Slag, while improving workability at moderate replacement levels, can slow early strength gain, which may delay construction schedules.

Mechanical and durability trade-offs

• Rubber particles weaken the interfacial transition zone (ITZ), reducing compressive strength and stiffness.

• High slag contents (> 40–50%) may compromise early-age strength, requiring extended curing times.

• Long-term durability under aggressive exposures (chloride ingress, freeze–thaw cycles, sulfate attack) remains insufficiently validated for rubberized mixes, raising concerns about service life.

Code and standard limitations

Current design codes explicitly recognize slag as a supplementary cementitious material but do not formally permit rubber powder as an replacement in structural concrete. This regulatory gap restricts large-scale adoption and necessitates extensive performance validation before approval.

Economic and logistical considerations

• Supply chain Reliable sourcing of high-quality slag and consistent rubber powder is essential but may be limited regionally.

• Processing costs Rubber requires pre-treatment (e.g., surface modification) to improve bonding, which adds cost and complexity.

• Market acceptance Contractors and stakeholders may be hesitant to adopt mixes perceived as experimental without clear cost–benefit evidence.

Long-term performance uncertainty

The long-term behavior of slag- and rubber-modified HPC under real service conditions is not yet fully understood. Potential risks include shrinkage, creep, and microcracking, which could compromise dimensional stability and durability in large-scale applications.

Conclusions

This research explored the development of high-performance concrete (HPC) incorporating significant percentages of slag and rubber powder as partial replacements for cement. A comprehensive evaluation was conducted, including compressive strength, flexural strength, Brazilian tensile strength, XRD analysis, SEM analysis, and FTIR spectroscopy, to assess both the mechanical properties and microstructural composition of the modified mixtures. Additionally, life cycle analysis and life cycle costing evaluations were performed to quantify the environmental and economic benefits of cement replacement. The key findings can be summarized as follows:

• Optimal slag replacement Increasing the replacement percentage of cement with slag up to 30% resulted in a decrease in compressive, flexural, and tensile strengths of less than 10%. However, further increases in slag replacement from 30 to 50% led to a substantial decline in mechanical properties. Thus, the optimal slag replacement percentage is determined to be 30%, balancing sustainability with adequate strength.

• Rubber powder replacement The study found that increasing the replacement percentage of cement with rubber powder from 10 to 30% resulted in reductions in compressive strength (18%), tensile strength (17%), and flexural strength (21%). Consequently, a 10% replacement of rubber powder is recommended to achieve an optimal blend of flexibility and structural integrity.

• XRD findings XRD analysis revealed that the highest concentration of C2S was present in the sample with no cement replacement. This indicates that while incorporating slag and rubber powder can enhance certain properties of HPC, it may also hinder the formation of crucial hydration products necessary for long-term strength development.

• Carbon footprint findings Incorporating both slag and rubber in HPC significantly reduces its carbon footprint. The standard HPC mix has the highest emissions (1,075 kg CO₂-eq), but with increasing GBFS replacement levels, emissions steadily decline, reaching 617 kg CO₂-eq at 50% slag replacement. This trend highlights the effectiveness of GBFS in reducing cement-related emissions. Similarly, replacing cement with rubber powder also leads to substantial reductions, with 10% rubber reducing emissions to 759 kg CO₂-eq, 20% rubber to 719 kg CO₂-eq, and 30% rubber achieving the lowest footprint of 678 kg CO₂-eq. The 30% slag mix (HPC 30%S) exhibits the highest eco-efficiency (0.12 MPa/kg CO₂-eq), demonstrating that incorporating slag and rubber is effective in balancing mechanical performance and environmental impact in sustainable concrete.

• Life cycle costing A key finding from the cost and strength-to-cost analysis is that HPC (30%S) emerges as the most cost-effective mix, offering the highest strength-to-cost ratio of 0.22 MPa/$. While the incorporation of slag significantly reduces costs, with HPC (50%S) achieving the lowest cost at $374 per cubic meter, the optimal balance between strength retention and cost savings is observed at 30% slag replacement. In contrast, adding rubber powder alongside 30% slag further reduces costs, but its strength-to-cost efficiency declines, making it less suitable for applications where high compressive strength is critical. These findings highlight that 30% slag replacement is the ideal solution for achieving cost savings while maintaining structural integrity, making it a practical choice for sustainable and economically efficient concrete production.

In conclusion, this research highlights the potential of using slag and rubber powder in HPC formulations, finding important trade-offs between mechanical performance and sustainability. Further studies could focus on optimizing other parameters or evaluating the long-term durability of these mixtures in various environmental conditions.

Although the present study demonstrates that slag and rubber can significantly reduce the carbon footprint of HPC, their combined influence on transport properties and durability-based service life has not yet been quantified. Moderate slag contents are expected to refine the pore system and enhance resistance to ingress, whereas rubber may locally increase heterogeneity and potential transport pathways. Future work will therefore include systematic measurements of chloride diffusion, water sorptivity, and gas permeability, followed by performance-based service-life modelling. This will allow reassessment of whether the CO₂ reductions identified here remain favorable once potential changes in cover thickness, repair needs, and service life are incorporated into an integrated durability–sustainability framework. Future research will systematically assess durability under aggressive environmental conditions, including sulfate attack, chloride ingress, carbonation, and freeze–thaw cycling, to better quantify long-term service performance. Dimensional stability will also be evaluated through measurements of autogenous shrinkage, drying shrinkage, and creep, which are critical for HPC with low water-to-binder ratios.

In addition, advanced mixture-design and response-surface optimization techniques will be employed to explore multi-objective trade-offs among workability, strength, durability, cost, and environmental impact. Rather than identifying a single optimal mixture, this approach will enable visualization of performance synergies and constraints, providing a more holistic framework for sustainable HPC design. Collectively, these future investigations will bridge laboratory findings with practical implementation, supporting the safe and effective adoption of environmentally responsible HPC technologies.

Author contributions

Hasan Mostafaei: Conceptualization, data curation, formal analysis, investigation, writing—original draft. Moataz Badawi: Methodology, Validation, formal analysis, writing—review & editing. Hadi Bahmani: Conceptualization, data curation, formal analysis, investigation, writing—original draft. Moataz Badawi: Methodology, Validation, formal analysis, writing—review & editing. All authors reviewed the manuscript.

Funding

No funding has been received for conducting this research.

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.