Optimizing concrete sustainability with bagasse ash and stone dust and its impact on mechanical properties and durability

Abstract

Addressing environmental challenges such as pollution and resource depletion requires innovative industrial and municipal waste management approaches. Cement production, a significant contributor to greenhouse gas emissions, highlights the need for eco-friendly building materials to combat global warming and promote sustainability. This study evaluates the simultaneous use of Sugarcane Bagasse Ash (SCBA) and Stone Dust (SD) as partial replacements by volume for cement and sand, respectively, at varying ratios in eco-strength concrete mixes designed for 28 MPa (ES-28) and 34 MPa (ES-34), emphasizing their economic and environmental benefits. The influence of SCBA and SD on workability, mechanical properties, and durability were experimentally investigated. Results reveal that for ES-28, with 9% SCBA and 50% SD, compressive and tensile strengths were nearly equal to the control mix, while flexural strength improved by 6.86%. For ES-34, with 9% SCBA and 50% SD, compressive strength was enhanced by 10.16%, tensile strength by 11.68%, and flexural strength by 5.22%, compared to the control mix. This improvement is attributed to pozzolanic reactions, enhanced particle packing, and optimal curing conditions. However, water absorption increased significantly, with ES-28 showing a 31.61% rise and ES-34 a 22.32% rise when SCBA was 9% and SD was 50%. These results highlight the trade-offs between mechanical performance and durability. The optimized mix, derived from response surface analysis, demonstrates significant potential as a sustainable alternative to conventional concrete, aligning with environmental and structural performance objectives.

Introduction

Concrete, the second most consumed material globally after water, plays a crucial role in the construction sector due to its versatility, durability, and cost-effectiveness^1–4. However, rapid urbanization, technological advancements, and economic growth have significantly increased the demand for natural resources, leading to their depletion^5–8. Among these, sand and cement are essential raw materials, with sand serving as a fine aggregate and constituting over one-third of the aggregate volume in concrete, while cement functions as the primary binder^9,10. The annual consumption of sand for applications such as concrete, mortar, glass, ceramics, and road construction is estimated to range between 3.2 billion and 5.0 billion tons^10,11. Similarly, global cement consumption exceeds 4 billion tons annually, contributing significantly to environmental concerns due to its energy-intensive production process¹². The production of concrete accounts for approximately 28% of global CO₂ emissions, with the cement industry alone responsible for 5–8% of these emissions^13–17. Significant efforts have been made to integrate industrial and agricultural byproducts into construction materials to overcome these environmental challenges generated by concrete production. Such approaches are particularly critical in low-income countries, where over 90% of waste is improperly disposed of, often through unregulated or open incineration, exacerbating environmental issues¹⁸.

Sugarcane Bagasse Ash (SCBA), a byproduct of the sugar industry, has garnered significant attention due to its high amorphous silica content and excellent pozzolanic properties, making it an effective Supplementary Cementitious Material (SCM)^19–21. Annually, over 1,500 million tons of sugarcane are harvested globally, yielding significant quantities of bagasse fibers, with 1 ton producing 25–40 kg of SCBA²². Major sugarcane-producing countries include Brazil (46%), India (26%), and Pakistan (6%), among others. SCBA’s high silica content meets the requirements of ASTM C618, qualifying it as a pozzolanic material²³. During cement hydration, SCBA reacts with calcium hydroxide (Ca(OH)₂) to form additional calcium-silicate-hydrate (C–S–H) gel, enhancing the strength, durability, and resistance of concrete to sulfate attack. Research indicates that incorporating SCBA at 10–15% replacement levels can increase compressive strength by up to 20% while reducing permeability due to its fine particle size and reactive properties^24–30. Beyond improving concrete’s mechanical and durability properties, SCBA offers an environmentally friendly solution for managing this agricultural byproduct, particularly in sugarcane-producing regions such as Brazil, India, and Pakistan. Its successful application in rural infrastructure projects highlights its potential to improve construction sustainability and cost-effectiveness^31,32. Further research is needed to understand its synergistic effects when combined with other alternative materials.

Similarly, Stone Dust (SD), a byproduct of the quarrying industry, has proven to be a viable alternative to natural sand in concrete production due to its chemical composition, which closely resembles that of natural sand^33,34. Using SD as a replacement for sand in proportions of 30–50% enhances mechanical properties, such as compressive and flexural strength, while reducing costs and mitigating the depletion of global sand resources^33–38. SD’s fine particle size improves particle packing and matrix density, reducing voids and improving the overall performance of concrete^33,39. However, it’s angular particles and rough surface textures can reduce slump and workability but enhance adherence to the cement paste^33,40,41. The global quarrying industry generates significant SD waste, much of which is landfilled, contributing to environmental challenges^42–44. Utilizing SD as a replacement for fine aggregate in concrete helps address the challenges associated with sand scarcity and excessive extraction. By reducing the reliance on natural sand, SD mitigates the environmental impact of over-extraction, which depletes natural resources and causes significant ecological degradation^45,46. SD has been employed in road construction and low-rise building projects in areas with abundant quarrying activities, effectively reducing the reliance on natural sand and minimizing environmental degradation^47–50.

The use of SCBA and SD offers a sustainable solution to pressing environmental challenges. SD addresses the depletion of natural sand reserves, while SCBA mitigates the environmental impact of cement production. Together, they present a promising approach to promoting environmentally friendly construction practices. However, despite the individual benefits of these materials, research on their combined use in concrete remains limited. Most studies focus on fixed substitution levels or individual performance metrics, leaving a critical gap in understanding their synergistic effects across different concrete grades and varying mix proportions. This gap is particularly significant as it limits the ability to optimize replacement levels for specific structural and workability requirements. Recognizing this, the present study investigates the concurrent use of SD and SCBA as replacements for sand and cement, aiming to enhance performance and sustainability while addressing this research void.

The objectives of this study are to evaluate the mechanical properties (compressive, tensile, and flexural strengths) and durability aspects (including permeability) of concrete incorporating SCBA and SD, systematically identify optimal replacement ratios, and assess their environmental and economic impacts, as well as quantify their potential to reduce carbon footprints, conserve natural resources, and promote sustainable construction practices. Moreover, Scanning Electron Microscopy (SEM) analysis is conducted on the controlled and optimized sample to examine microstructural properties and validate the observed performance enhancements. The methodology involves designing tailored eco-strength (ES) concrete mixes for target strengths of 28 MPa and 34 MPa, supported by an extensive experimental program encompassing 390 recipes. Similarly, response surface methodology is utilized to evaluate the interaction between replacement ratios of SCBA and SD and their corresponding performance outcomes, enabling precise optimization of concrete properties. This study bridges the gap between laboratory findings and practical applications by simulating field conditions and addressing challenges in large-scale implementation, providing a robust framework for sustainable construction practices.

Experimental program

This study tested the workability of fresh concrete and the mechanical properties of hardened concrete, including compressive strength, split tensile strength, flexural strength, and water absorption, following ASTM standards. Three hundred ninety recipes were prepared, considering various replacement ratios of cement by SCBA and sand by SD.

Characterization of materials

This study utilized locally sourced materials, including Ordinary Portland Cement (OPC), fine aggregates, coarse aggregates, SCBA, and SD, to produce sustainable and eco-friendly concrete mixtures. These materials were carefully selected to meet ASTM standards, ensuring their suitability for use in high-performance concrete. OPC conforming to ASTM C150/C150M-22 Type I standards was used⁵¹, and its essential properties are summarized in Table 1. Sand used as fine aggregate was graded between 4.75 mm (No. 4) and 150 μm (No. 100) sieves. Its particle size distribution was analyzed following ASTM C136/C136M-19 standards and compared with ASTM C33/C33M-23 specifications^52,53. The sand complied with these standards, with its particle size distribution shown in Fig. 1, and its fineness modulus and specific gravity detailed in Table 2. Similarly, coarse aggregates with a maximum size of 19 mm (3/4 inch) the crushed stone of Margalla hill rock, were tested following ASTM C136/C136M-19 and compared to ASTM C33/C33M-23 specifications^52,53. The particle size distribution, illustrated in Fig. 2, also met ASTM standards. Additional properties, including specific gravity, absorption, impact value, and crushing value, were determined per relevant ASTM and BS standards and are summarized in Table 2. This comprehensive evaluation ensured compliance with required material standards for concrete production.

Table 1.

Physical properties of OPC.

S. No.	Characteristics	Standard Values	Obtained Value	Standard Followed
1	Normal Consistency	25 − 30% by weight of cement	28%	[54]
2	Specific Gravity	3.10–3.25	3.20	[55]
3	Fineness	%age of cement-passing on a 90 μm sieve < 10%	4.5%	[56]

Fig. 1.

Sieve analysis of fine aggregate and SD.

Table 2.

Physical properties of aggregates.

S. No.	Property	Fine Aggregate (Sand)	Coarse Aggregate	Standard Followed
1	Fineness Modulus	2.46	–	[53]
2	Specific Gravity	2.69	2.76	[57] & [58]
3	Absorption Percentage	1.7%	0.43%	[58]
4	Impact Value Test	–	16.15%	[59]
5	Crushing Value Test	–	28.65%	[60]

Fig. 2.

Sieve Analysis of coarse aggregate.

SCBA used in this study was obtained from the Mardan Sugar Mill, Khyber Pakhtunkhwa (KPK), Pakistan, where sugarcane bagasse is combusted to generate power for the mill’s boilers. Optimizing SCBA’s pozzolanic effectiveness involves focusing on finer particles, typically below 45 microns, as they enhance dispersion and interaction with cementitious materials, leading to improved microstructure and densification^61,62. The obtained SCBA was in wet form as it is residual waste therefore it was left to air-dry for a short duration, usually spanning a few hours or overnight. To achieve a finer particle size, the dried ash was sieved through a No. 200 sieve to eliminate unburned and fibrous particles, as their presence can diminish pozzolanic activity⁶³. This process resulted in a fineness value of 35.2 microns, meeting the desired particle size range^61,62. The density and surface area of the SCBA were measured as 2.54 g/cm³ and 2250 cm²/g, respectively. Additionally, the absorption percentage was 16.57%, reflecting its ability to retain water, which can be attributed to its porous structure. Its oxide composition and crystallographic phases were analyzed using X-ray Fluorescence (XRF) and X-ray Diffraction (XRD) to assess its potential as a pozzolanic material. XRD analysis was conducted on a JDX-3532 JEOL diffractometer (Japan) with CuKα radiation, operating at 40 kV and 25 mA. The diffraction patterns were interpreted using the International Crystallographic Diffraction Database (ICDD) and processed with MATCH Phase Identification Software (version 3.1) within a 2θ range of 20°–80° to identify the chemical phases. The oxide composition was determined through XRF analysis. XRD analysis revealed the crystallographic characteristics of SCBA, confirming its predominantly amorphous nature due to the absence of sharp peaks. However, diffraction peaks corresponding to Quartz were observed at a phase diffraction angle of 2θ = 27.24° (3.27 Å), as illustrated in Fig. 3. Chemical properties of SCBA are summarized in Table 3.

Fig. 3.

XRD analysis of SCBA.

Table 3.

Chemical oxide composition of SCBA.

S. No.	Oxides	(%)	SCBA after passing from 200 sieve
1	SiO2	63.78
2	Al2O3	8.32
3	Fe2O3	1.68
4	CaO	10.54
5	MgO	4.76
6	MnO	0.09
7	Na2O	1.32
8	K2O	2.60
9	TiO2	0.28
10	P2O5	1.42
11	LOI	4.21

SD was collected from a crushing plant in KPK, Pakistan, and evaluated for its suitability as a fine aggregate in concrete. The material was graded to pass through a 4.75 mm (No. 4) sieve and retained on a 150 μm (No. 100) sieve. Sieve analysis revealed that SD particles passed through a 4.75 mm (No. 4) sieve and were retained on a 150 μm (No. 100) sieve, aligning with standard grading requirements. The fineness modulus of SD was determined to be 3.28, indicating a coarser texture compared to typical fine aggregates. This coarser nature can enhance particle packing and reduce voids in concrete mixtures, potentially improving strength and durability^64,65. However, it is important to note that a fineness modulus exceeding the typical range for fine aggregates (2.6 to 3.1) suggests that SD is coarser than standard sand, which may affect workability and require adjustments in mix design. However, the particle size distribution, shown in Fig. 1, is almost within acceptable limits, and a specific gravity of 2.92 and a density range of 1480 kg/m³ to 1824 kg/m³ are also comparable to sand^66,67. The chemical composition of SD was analyzed using the XRF technique, and the elemental percentages obtained are summarized in Table 4. The chemical properties of SD, particularly its high SiO₂ and CaO content, along with its low alkali and sulfate levels, make it an excellent candidate for replacing sand in concrete. These properties ensure improved strength, durability, and resistance to chemical attacks while being environmentally and economically advantageous^67,68.

Table 4.

Chemical oxide composition of SD.

S. No.	Oxides	(%)	SD Sample
1	SiO2	51.82
2	CaO	13.20
3	Al2O3	9.24
4	Fe2O3	9.10
5	MgO	12.52
6	SO3	0.12
7	Na2O	0.29
8	K2O	0.93

Benchmark mixes and design strategies

This study developed and evaluated concrete formulations incorporating SCBA as a partial replacement for OPC and SD as a partial replacement for sand. Two benchmark formulations were designed based on compressive strength targets: Eco-strength 28 MPa (ES-28) and Eco-strength 34 MPa (ES-34). The ES-28 mixture followed a mix ratio of 1:1.5:3 (binder: sand: coarse aggregate) by volume (v/v), at a water-to-binder (w/b) ratio of 0.57. Meanwhile, the ES-34 mixture used a mix ratio of 1:1:2 by volume (v/v), at a w/b ratio of 0.45. These w/b ratios were selected to achieve the desired workability and strength, aligning with recommendations from ACI 211.1 and BS 8500, which advocate for higher w/b ratios for medium-strength concrete and lower w/b ratios for enhanced durability and strength^69,70. The 0.57 w/b ratio for ES-28 was optimized for medium-strength applications, balancing workability and strength, while the 0.45 w/b ratio for ES-34 minimized porosity and enhanced durability for higher strength requirements. These mix designs were refined through an iterative process based on relevant standards and supported by findings from the literature to achieve optimal mechanical performance and workability. Aligned with Pakistan’s conventional construction practices, the formulations reflect local material availability, environmental resilience, and cost-effectiveness, making them suitable for rapid urban development^71,72.

To explore the potential for enhancing the sustainability and mechanical performance of these formulations, SCBA replaced OPC at levels of 3%, 6%, and 9%, and SD replaced sand at 20%, 30%, 40%, and 50% by volume, as detailed in Table 5. For clarity, sample codes were used to represent each recipe, combining numerical values and specific codes corresponding to the proportions and constituents in the concrete mixtures. For example, “3BA”, “6BA”, and “9BA” represent mixtures containing 3%, 6%, and 9% SCBA by volume, respectively, while “20SD,” “30SD,” “40SD,” and “50SD” indicate mixtures with 20%, 30%, 40%, and 50% SD by volume.

Table 5.

Sample designations.

Mixtures	Designation	Binder (%)		Fine Aggregate (%)
		OPC	SCBA	River Sand	SD
ES-28	0BA:0SD	100	0	100	0
ES-34
ES-28	3BA:20SD	97	3	80	20
ES-34
ES-28	6BA:20SD	94	6	80	20
ES-34
ES-28	9BA:20SD	91	9	80	20
ES-34
ES-28	3BA:30SD	97	3	70	30
ES-34
ES-28	6BA:30SD	94	6	70	30
ES-34
ES-28	9BA:30SD	91	9	70	30
ES-34
ES-28	3BA:40SD	97	3	60	40
ES-34
ES-28	6BA:40SD	94	6	60	40
ES-34
ES-28	9BA:40SD	91	9	60	40
ES-34
ES-28	3BA:50SD	97	3	50	50
ES-34
ES-28	6BA:50SD	94	6	50	50
ES-34
ES-28	9BA:50SD	91	9	50	50
ES-34

The selected replacement levels were guided by findings from recent studies, which demonstrate the effectiveness of these percentages in enhancing concrete properties. Research shows that incorporating uncontrolled SCBA up to 10% enhances compressive strength and durability. Similarly, replacing natural sand with SD up to 50% improves mechanical properties, with 40% SD frequently achieving the best results in terms of compressive strength and durability^{31–33,38,73}. These findings underscore the benefits of SCBA and SD in promoting sustainability through the use of industrial by-products while improving the performance of concrete.

The specific mix proportions by volume, presented in Table 6, balance the binder, fine aggregate, and coarse aggregate components to achieve the desired mechanical performance and workability. This integrated design approach ensures compliance with industry standards, advances sustainable construction practices, and provides an eco-friendly and cost-effective solution for modern concrete production.

Table 6.

Design mix proportions of different recipes.

Mixtures	Designation	W/B Ratio	Binder (m3/m3)		Fine Aggregate (m3/m3)		Coarse Aggregate (m3/m3)	Water (m3/m3)
			OPC	SCBA	River Sand	SD
ES-28	0BA:0SD	0.57	0.165	0.000	0.247	0.000	0.494	0.094
ES-34		0.45	0.225	0.000	0.225	0.000	0.449	0.101
ES-28	3BA:20SD	0.57	0.160	0.005	0.198	0.049	0.494	0.094
ES-34		0.45	0.218	0.007	0.180	0.045	0.449	0.101
ES-28	6BA:20SD	0.57	0.155	0.010	0.198	0.049	0.494	0.094
ES-34		0.45	0.211	0.013	0.180	0.045	0.449	0.101
ES-28	9BA:20SD	0.57	0.150	0.015	0.198	0.049	0.494	0.094
ES-34		0.45	0.204	0.020	0.180	0.045	0.449	0.101
ES-28	3BA:30SD	0.57	0.160	0.005	0.173	0.074	0.494	0.094
ES-34		0.45	0.218	0.007	0.157	0.067	0.449	0.101
ES-28	6BA:30SD	0.57	0.155	0.010	0.173	0.074	0.494	0.094
ES-34		0.45	0.211	0.013	0.157	0.067	0.449	0.101
ES-28	9BA:30SD	0.57	0.150	0.015	0.173	0.074	0.494	0.094
ES-34		0.45	0.204	0.020	0.157	0.067	0.449	0.101
ES-28	3BA:40SD	0.57	0.160	0.005	0.148	0.099	0.494	0.094
ES-34		0.45	0.218	0.007	0.135	0.090	0.449	0.101
ES-28	6BA:40SD	0.57	0.155	0.010	0.148	0.099	0.494	0.094
ES-34		0.45	0.211	0.013	0.135	0.090	0.449	0.101
ES-28	9BA:40SD	0.57	0.150	0.015	0.148	0.099	0.494	0.094
ES-34		0.45	0.204	0.020	0.135	0.090	0.449	0.101
ES-28	3BA:50SD	0.57	0.160	0.005	0.124	0.124	0.494	0.094
ES-34		0.45	0.218	0.007	0.112	0.112	0.449	0.101
ES-28	6BA:50SD	0.57	0.155	0.010	0.124	0.124	0.494	0.094
ES-34		0.45	0.211	0.013	0.112	0.112	0.449	0.101
ES-28	9BA:50SD	0.57	0.150	0.015	0.124	0.124	0.494	0.094
ES-34		0.45	0.204	0.020	0.112	0.112	0.449	0.101

Tests on fresh concrete

Slump tests and compaction factor tests were conducted to investigate the workability of fresh concrete. The slump test followed ASTM C143/C143M-20 guidelines⁷⁴. The targeted slump (50–100 mm) was successfully attained by minor modifications to the concrete mixture. Similarly, as per IS 1199–1959⁷⁵, the compaction factor test was conducted on a fresh concrete sample to ensure that the concrete mix had the desired compaction factor (0.95 to 0.97) and to identify any potential issues with the mix’s consistency and compatibility.

Tests on hardened concrete

Compressive strength, split tensile strength, flexural test, and water absorption test were conducted to investigate hardened concrete’s mechanical properties and durability.

Manufacture of specimens.

Five groups of three samples were prepared to determine the effect of using SCBA and SD as replacement materials in conventional concrete. These groups were designated for specific tests: two groups for compressive strength (at 7 and 28 days), one group for split tensile strength (at 28 days), one group for flexural strength (at 28 days), and one group for water absorption (at 28 days). In total, 156 samples were created for compressive strength tests, and 78 samples were designated for split tensile strength, flexural strength, and water absorption tests as shown in Table 7. During the casting of various samples, strict adherence to ASTM standards was maintained. Proper curing procedures were diligently carried out to accurately assess the concrete’s performance with partial replacement of essential concrete ingredients. Standard specimens were prepared, which included cylinders measuring 6 inches (0.1524 m) in diameter and 12 inches (0.3048 m) in height, cubes with sides of 6 inches (0.1524 m) in length, and prisms featuring dimensions of 4 inches (0.1016 m) in width, 4 inches (0.1016 m) in height, and 12 inches (0.3048 m) in length. These specimens underwent a 28-day curing period, during which the concrete was carefully cured in a controlled environment at approximately 20 °C with a humidity level of 90% within a curing tank.

Table 7.

Sample distribution across various tests.

Mixtures	Designation	Compressive Strength @ 7 & 28 day	Tensile Strength @ 28 day	Flexural Strength @ 28 day	Water Absorption @ 28 day	Total Sample
ES-28	0BA:0SD	3 + 3	3	3	3	30
ES-34		3 + 3	3	3	3
ES-28	3BA:20SD	3 + 3	3	3	3	30
ES-34		3 + 3	3	3	3
ES-28	6BA:20SD	3 + 3	3	3	3	30
ES-34		3 + 3	3	3	3
ES-28	9BA:20SD	3 + 3	3	3	3	30
ES-34		3 + 3	3	3	3
ES-28	3BA:30SD	3 + 3	3	3	3	30
ES-34		3 + 3	3	3	3
ES-28	6BA:30SD	3 + 3	3	3	3	30
ES-34		3 + 3	3	3	3
ES-28	9BA:30SD	3 + 3	3	3	3	30
ES-34		3 + 3	3	3	3
ES-28	3BA:40SD	3 + 3	3	3	3	30
ES-34		3 + 3	3	3	3
ES-28	6BA:40SD	3 + 3	3	3	3	30
ES-34		3 + 3	3	3	3
ES-28	9BA:40SD	3 + 3	3	3	3	30
ES-34		3 + 3	3	3	3
ES-28	3BA:50SD	3 + 3	3	3	3	30
ES-34		3 + 3	3	3	3
ES-28	6BA:50SD	3 + 3	3	3	3	30
ES-34		3 + 3	3	3	3
ES-28	9BA:50SD	3 + 3	3	3	3	30
ES-34		3 + 3	3	3	3
Total samples for each test		156	78	78	78	390

Test setup.

According to ASTM: C39/C39M-21, a concrete cylinder was tested for concrete compressive strength at a loading rate of 0.2 MPa/s at the age of 7 and 28 days⁷⁶. To uncover valuable data that will contribute to a comprehensive understanding of the material’s properties and its behavior under various conditions, ultimately enhancing the quality and durability of concrete structures, Scanning Electron Microscope (SEM) analysis was performed on both the standard mix and the samples that showed the highest increase in compressive strength. Meanwhile, concrete samples were moist cured for 28 days for split tensile strength and tested in a saturated surface dry condition following the standard ASTM C496/C496M-17⁷⁷.

Following the ASTM standards C78/C78M-22, flexural tests were conducted on a prism sample subjected to third-point loading to induce flexural stresses in the 4” deep section of the beam⁷⁸. The test results were based on the maximum load recorded on the dial. All the samples exhibited collapse in the middle third portion of the span, eliminating the need for any modification factor in the test results. Also, a water absorption test was conducted using ASTM specification C642–13⁷⁹. Standard cubes were cast for each mix. After 28 days of moist curing, test specimens were retrieved from the curing tank. Subsequently, the cured samples were subjected to a water absorption test.

Experimental results

Workability of fresh concrete

Slump test

Figure 4 comprehensively analyzes the slump values observed in various concrete mixtures. Mixtures with lower w/b ratios exhibit reduced slumps, indicating greater stiffness and lower workability, while mixtures with higher w/b ratios demonstrate higher slump values, signifying increased fluidity and improved workability. The data suggests that including SCBA and SD can impact the workability of concrete mixtures for 28 and 34 MPa strength. As the proportion of SCBA increases while keeping the SD content constant, the slump values subtly decline. Similarly, the slump values also gradually reduce when the SD content increases while maintaining consistent SCBA content. As the variation in slump values is very low in millimeters, it might have minimal impact on the performance of concrete, especially in situations where slight differences in workability are acceptable or can be compensated during construction.

Fig. 4.

Slump values for various concrete mixtures.

Compaction factor test

Figure 5 provides a detailed analysis of the compaction factor values observed in multiple concrete mixtures. The graph illustrates the compaction factor, where a higher value indicates more workable concrete and a lower value suggests stiffer mixtures. The results show that the proportions of SCBA and SD have minimal effects on the compaction factor for both ES-28 and ES-34 mixtures, with values ranging between 0.92 and 0.96. An increase in SCBA proportion while keeping SD constant results in subtle variations in the compaction factor, while increasing SD content generally leads to slightly reduced compaction factor values. These reductions are minor and unlikely to cause significant issues, demonstrating the feasibility of incorporating SCBA and SD without severely compromising workability.

Fig. 5.

Compaction factor values for various concrete mixtures.

Mechanical properties and durability of hardened concrete

Concrete mechanical properties were analyzed based on the data obtained through the concrete compressive strength test, split tensile strength test, and flexural strength test of different samples.

Concrete compressive strength

Figure 6 presents the compressive strength values for concrete mixtures designed for 28 MPa strength at both 7-day and 28-day curing periods. Varying proportions of SCBA and SD influence these values. At seven days, the compressive strength values vary across the different mixtures. Generally, the mixtures containing higher proportions of SCBA tend to exhibit slightly lower compressive strength values at this early age than the control mix (0BA:0SD). At 28 days, the compressive strength values for most mixtures have increased significantly compared to the 7-day values.

Fig. 6.

Average compressive strength at 7 & 28 days of ES-28 mixtures.

Figure 7 presents compressive strength values for concrete mixtures designed for 34 MPa strength at both 7- and 28-day curing periods. At seven days, the compressive strength values vary across the different mixtures. Generally, the mixtures containing higher proportions of SCBA tend to exhibit slightly lower compressive strength values at this early age than the control mix (0BA:0SD). The 28-day compressive strength data reveals an interesting trend when considering different replacement proportions of SCBA and SD in comparison to the control mix. Across the board, the mixtures with various SCBA and SD replacement proportions tend to show an overall increase in compressive strength at 28 days compared to the control mix.

Fig. 7.

Average compressive strength at 7 and 28 days of ES-34 mixtures.

Concrete split tensile strength

Figure 8 illustrates the split tensile strength values for concrete mixtures with varying proportions of both SCBA and SD. These mixtures were formulated to achieve target strengths of 28 MPa and 34 MPa and were subjected to a curing period of 28 days. Comparing the 20% SD mixes with different percentages of BA in the 28 MPa mix, it’s evident that the presence of SCBA, regardless of the percentage, tends to reduce the split tensile strength compared to the control mix. The effect of SD is more variable, with potential improvements in split tensile strength observed at higher percentages (30% and 40%). In the case of the 34 MPa mix, it seems that the presence of SD, especially at higher percentages, can mitigate the reduction in split tensile strength caused by higher percentages of SCBA.

Fig. 8.

Average split tensile strength at 28 days of ES-28 & ES-34 mixtures.

Concrete flexural strength

Figure 9 depicts the flexural strength values for concrete mixtures with varying proportions of both SCBA and SD, formulated for strengths of 28 MPa and 34 MPa and subjected to a curing period of 28 days. SD, especially at higher percentages, appears to mitigate the potential reduction in flexural strength caused by SCBA concerning the 28 MPa control mix. The data suggests that mixes with 6% or 9% SCBA and 30% or higher SD (e.g., 6BA:30SD, 9BA:30SD, 6BA:40SD, 9BA:40SD, 6BA:50SD, 9BA:50SD) tend to have higher flexural strengths compared to the control mix and in 34 MPa mix the data suggests that mixes with 9% SCBA and 30% or higher SD (e.g., 9BA:30SD, 9BA:40SD, 9BA:50SD) tend to have significantly higher flexural strengths compared to the control mix.

Fig. 9.

Average flexural strength at 28 days of ES-28 & ES-34 mixtures.

Concrete water absorption for durability

Figure 10 provides a visual representation of water absorption values, which indicate the extent to which each concrete mixture is susceptible to absorbing water. The presence of both SCBA and SD in Mix-28 and Mix-34 seems to contribute to increased water absorption in the concrete, with the effect being more noticeable when both materials are present in the mix. As the amount of SCBA and SD is increased in the sample mixes, the water absorption value rises accordingly.

Fig. 10.

Average WA at 28 days of ES-28 & ES-34 mixtures.

Discussion

The findings of this study demonstrate the feasibility of using SCBA and SD as partial replacements for OPC and sand, respectively, in concrete formulations. The mechanical performance, as evaluated through compressive strength, tensile strength, flexural strength, and water absorption, provides insight into the interplay between material proportions and concrete behavior.

Influence of SCBA and SD on properties of concrete

Influence on compressive strength

The compressive strength of ES-28 mixes with varying proportions of SD and SCBA content at 28 days is presented in Fig. 11 (a). The results demonstrate that as SCBA replacement levels increase to 3%, 6%, and 9%, and SD content varies from 20 to 40%, the compressive strength shows a steady and consistent increase. This ultimately reaches 27.54 MPa, 27.77 MPa, and 27.93 MPa, respectively, which closely align with the compressive strength of the control mix (27.58 MPa). This enhancement can be attributed to the pozzolanic activity of SCBA, which reacts with calcium hydroxide during hydration, leading to the formation of additional calcium silicate hydrate (C–S–H) gel. Such reactions enhance the matrix density and strength, particularly at later curing stages. Similar findings reported in recent studies highlight SCBA’s role in improving concrete’s compressive strength up to an optimal replacement level^{3,4,6,27,31,80–83}.

Fig. 11.

Avg compressive strength on the 28th day: (a) ES-28 mixes with different SD content, (b) ES-34 mixes with different SD content.

However, with higher SD content (50%), a slight decrease in compressive strength is observed. This reduction can be linked to the increased porosity and altered particle packing caused by the angular shape of SD, which affects the density and bonding within the concrete matrix. Researchers have observed that while fine SD particles improve packing density, excessive replacement levels may compromise compressive strength due to increased microvoids^33,35,38,84. Notably, the combination of 9% SCBA and 40% SD resulted in the highest compressive strength improvement for ES-28, with a 1.26% increase compared to the control mix. This improvement highlights the synergistic effects of SCBA and SD, as SCBA’s pozzolanic activity complements SD’s filler properties, particularly at 28 days.

The compressive strength trends for ES-34 mixes with varying levels of SD and SCBA content at 28 days are presented in Fig. 11 (b). The results reveal a consistent upward trajectory in strength as SCBA levels increase to 3%, 6%, and 9%, and SD content varies from 20 to 50%. The corresponding compressive strength values reach 35.77 MPa, 37.34 MPa, and 38.11 MPa, signifying improvements of 4.29%, 8.31%, and 10.16%, respectively, compared to the control mix (34.24 MPa). These enhancements are primarily due to the high silica content in SCBA, which chemically reacts with calcium hydroxide and water, forming additional C–S–H gel that strengthens the matrix. Studies have similarly emphasized that the delayed pozzolanic activity of SCBA contributes to strength gains over time^{3,4,6,27,31,33,38,80–83}.

Interestingly, while strength improvement is evident at 28 days, prior studies have shown that compressive strength may initially decrease at early curing stages due to delayed hydration and changes in water demand caused by SCBA’s finer particle size^21,73. The increase in strength at later stages underscores the importance of sufficient curing periods for mixes containing SCBA. Furthermore, SD’s angular geometry enhances mechanical interlocking and contributes to compressive strength improvements at optimal replacement levels⁸⁵. However, as SD content increases beyond 40% in ES-28, the additional porosity outweighs these benefits, leading to slight strength reductions.

These findings emphasize the need for a balanced mix design when incorporating SCBA and SD. By optimizing replacement levels, it is possible to achieve compressive strength comparable to or better than conventional concrete mixes while leveraging the sustainability benefits of SCBA and SD. These results align with recent research advocating for the integration of supplementary materials in concrete to reduce environmental impacts without compromising structural performance.

Influence on split tensile strength

The split tensile strength of ES-28 mixes with varying levels of SD and SCBA content at 28 days is presented in Fig. 12 (a). The results demonstrate that the split tensile strength consistently increases with 3%, 6%, and 9% SCBA replacements as the SD content varies from 20 to 40%. This ultimately leads to values of 2.74 MPa, 2.77 MPa, and 2.79 MPa, respectively, which closely match the split tensile strength of the control mix at 2.74 MPa. However, as the SD content is further increased to 50%, there is a subsequent reduction in split tensile strength. Nonetheless, these values still align with those of the control mix. Mixes with 6% or 9% SCBA and 30% or higher SD could potentially result in split tensile strengths equal to or higher than the control mix. This behavior aligns with findings from recent studies, where researchers have reported that SCBA contributes to tensile strength improvements through pozzolanic reactions, which enhance the bonding between particles in the concrete matrix^{3,4,6,27,31,80–83}. Higher percentages of SD in some instances could improve tensile strength by filling voids or enhancing the interlocking effect^33,35,38,84.

Fig. 12.

Avg split tensile strength on the 28th day: (a) ES-28 mixes with different SD content, (b) ES-34 mixes with different SD content.

Similarly, the split tensile strength of ES-34 mixes with varying levels of SD and SCBA content at 28 days is presented in Fig. 12 (b). The findings indicate that, with 3%, 6%, and 9% BA replacements, as the SD content fluctuates from 20 to 50%, there is a consistent upward trajectory in split tensile strength. Ultimately, this leads to values of 3.57 MPa, 3.74 MPa, and 3.84 MPa, respectively. These figures signify increases of 5.07%, 9.43%, and 11.63% when compared to the split tensile strength of the control mix, which stands at 3.39 MPa. The observed increases in split tensile strength could be attributed to a synergy between the pozzolanic effect of SCBA and the improved particle packing resulting from SD^{3,4,6,27,31,33,38,80–83}. When combined, these factors lead to an overall enhancement in the concrete’s performance. The pozzolanic reactions initiated by SCBA contribute to the development of additional cement hydration products within the concrete matrix, effectively binding the particles together. Simultaneously, the improved packing and densification of the mixture, facilitated by SD, further enhance the concrete’s structural integrity. Achieving the observed increases in split tensile strength likely requires careful mix design, where the proportions of SCBA and SD are balanced to harness their beneficial effects. The optimal mix design may vary depending on factors such as the specific properties of the SCBA and SD, the cementitious materials used, and the project’s requirements.

Influence on flexural strength

Figure 13 (a) illustrates the 28-day flexure strength of ES-28 mixes, showcasing the impact of varying SD and SCBA content. The results demonstrate that the flexure strength consistently increases with 3%, 6%, and 9% SCBA replacements as the SD content varies from 20 to 40%. This ultimately results in values of 5.76 MPa, 6.80 MPa, and 7.05 MPa, respectively. These figures indicate an increase in flexural strength up to 0.26%, 15.46%, and 18.39%, respectively, compared to the flexural strength of the control mix at 5.75 MPa, aligning with latest findings which attributed this to the enhanced bonding and load transfer mechanisms facilitated by pozzolanic activity and angular aggregate geometry^86,87. However, with a further increase in SD content to 50%, there is a subsequent reduction in flexural strength. The same phenomenon was observed in ES-34 mixes, as Fig. 13 (b) depicts. It showcased an increase in flexural strength of up to 0.17%, 6.29%, and 8.92% for 3%, 6%, and 9% SCBA replacements, respectively, compared to the flexural strength of the control mix at 7.65 MPa. The test data suggests that SD, particularly at higher percentages, appears to counteract the reduction in flexural strength caused by the inclusion of SCBA⁸⁸. This indicates that combining these two materials can produce concrete mixes at least as strong as the control mix, which is a promising outcome for sustainable construction practices.

Fig. 13.

Average flexural strength on the 28th day: (a) ES-28 mixes with different SD content (b) ES-34 mixes with different SD content.

The w/b ratios have a significant influence on the mechanical properties of concrete. A well-chosen w/b ratio can optimize the reaction between cementitious materials (cement and SCBA) and water, forming a stronger matrix. It also affects the workability of the mix, which is crucial for proper compaction and curing. If the mix is too dry (too low w/b ratio), it may not be workable enough, leading to poor compaction and, thus, lower strength. Conversely, if the mix is too wet (too high w/b ratio), it may lead to segregation and excessive shrinkage upon drying, also reducing strength⁸⁹.

A lower w/b ratio in ES-34 generally makes concrete stronger because it leads to a denser matrix and fewer pores within the hardened cement paste, which means there are fewer weak points in the structure. The finer particles of SD fill the voids more effectively, leading to a denser and more cohesive matrix that can withstand compressive and tensile stresses better. SCBA, containing silica, can react with calcium hydroxide released during cement hydration to form an additional calcium silicate hydrate (C-S-H), the primary compound responsible for strength in concrete. As the percentage of SCBA increases, the trend of increasing compressive and split tensile strength with SD content is maintained, suggesting that SCBA, up to a point, is also contributing positively to the compressive strength, potentially due to its pozzolanic properties, which can enhance the microstructure of the hardened concrete. Unlike compressive and tensile strengths, flexural strength decreased with a 50% replacement of sand with SD, as flexural strength measures the material’s ability to resist deformation under load. The decrease in flexural strength suggests that while the internal cohesion of the material is improved (as indicated by increased compressive and tensile strengths), the material’s ability to distribute stress across the composite is compromised. This might occur because the flexural strength is more sensitive to the bond between the aggregates and the cement paste. Despite its positive effects on void filling and possibly initial inter-particle strength, SD might not bond as effectively with the cement paste in bending. The different trends in these strength measures can also be due to the angularity and surface texture of SD particles, which may lead to an improved mechanical interlock for compression and tension, but create stress concentrations under bending loads that promote crack initiation and propagation. Moreover, at 50% replacement, there might be a threshold beyond which the benefits of finer SD particles and pozzolanic reaction of SCBA are overshadowed by the adverse effects on the concrete’s flexural performance, like altered aggregate-paste bond strength and the effectiveness of the SD particles in the matrix.

Similarly, In ES-28 a higher w/b ratio is used, which would generally reduce strength. However, up to the optimal point of SD content, the mix may still be workable and cohesive enough to achieve good compaction and strength. After passing the optimal point, the excess SD might make the mix too stiff or cause segregation, resulting in weaker concrete. The peak and subsequent decrease in compressive, split tensile, and flexural strength with higher SD content could also be due to the dilution of the cement paste, as more SD means less cementitious binder relative to aggregates, leading to a less effective paste that binds aggregates together.

Influence on water absorption

Figure 14 illustrates the 28-day water absorption of ES-28 and ES-34 mixes, respectively, showcasing the impact of varying SD and SCBA content. The results indicate that incorporating SCBA and SD in Mix-28 and Mix-34 increases water absorption within the concrete. This is due to the effect of SCBA in the mixes when used in higher percentages, may introduce porosity and affect the binding properties of the concrete, leading to increased water absorption and enhanced durability. Similarly, SD especially at higher percentages, can also increase the porosity of the concrete, which in turn leads to higher water absorption. These findings emphasize the importance of selecting the right mix proportions based on project requirements, mainly when aiming to achieve superior water resistance in concrete structures.

Fig. 14.

Average water absorption on the 28th day: (a) ES-28 mixes with different SD content, (b) ES-34 mixes with different SD content.

Higher water absorption suggests increased porosity within the concrete. Higher water absorption diminished protection from sulfate attack of concrete, and the connection between surface water absorption and protection from sulfate attack was roughly straight⁹⁰. The SD particles may not pack as densely as the sand particles they are replacing, potentially due to their size, shape, or surface texture. With a higher w/b ratio, the increased water in the mix also contributes to a higher porosity, as excess water leaves behind more voids when it evaporates from the concrete⁹¹. The compressive and split tensile strengths improved by adding SD and SCBA. This does not necessarily correlate with a decrease in porosity. The finer particles of SD may fill the spaces between the coarser aggregates, leading to denser packing and a higher compressive strength. However, the increased porosity indicated by higher water absorption rates suggests that these finer particles may not be bonding as effectively with the cement paste, which can negatively impact the durability and flexural strength of the concrete. The SCBA, despite providing a pozzolanic reaction, does not seem to reduce the porosity sufficiently to counteract the effects of SD on water absorption.

Factors influencing Mechanical properties

Several factors influenced the observed mechanical properties, including the w/b ratio, curing regime, aggregate gradation, and replacement of SCBA and SD. The w/b ratio was a key determinant of strength, as the lower w/b ratio of 0.45 in ES-34 reduced porosity and improved strength, while the higher ratio of 0.57 in ES-28 ensured better workability at the expense of strength and durability. Additionally, the pozzolanic activity of SCBA and the particle geometry of SD were critical in determining the microstructural and mechanical outcomes.

Microstructure analysis

To study the microstructure of control concrete and concrete mixture which gives higher compressive strength value, i.e., 9BA:50SD having w/b ratio of 0.45, the SEM analysis was carried out to provide a close-up view of the microstructure of concrete mixes, which can give us insights into the differences in mechanical strength between the control mix and the mix with cement replaced by 9% SCBA and sand replaced by 50% SD. The variation in the properties of concrete studied so far was validated with the micrographs of SEM. Figure 15 collectively provides insight into the structural composition of concrete and its implications for strength. Figure 15 (a) illustrates a large aggregate with a clearly defined contact zone, emphasizing the significance of a well-bonded interface between aggregate and cement paste for enhanced strength. In contrast, Fig. 15 (b) exposes pores within this contact zone, highlighting their potential to weaken the concrete by serving as initiation points for cracks. Figure 15 (c) captures hydrated particles in the cement paste, underscoring the importance of hydration level for strength, with complete hydration correlating to a stronger matrix. Finally, Fig. 15 (d) reveals the presence of calcium hydroxide (Ca(OH)₂) and calcium-silicate-hydrate (C-S-H) gel, the latter being the primary contributor to concrete’s strength. At the same time, the former is a by-product of the hydration process. Together, these figures depict the complex interplay of components and processes that determine the strength of concrete.

Fig. 15.

(a)–(d) Micrograph of sample 1 - control mix of 34 MPa strength concrete under different magnifications.

It was observed that the micrographs of 9BA:50SD showed better properties as far as the dense matrix is concerned. Figure 16 (a) presents the aggregate and its contact zone, suggesting a possibly more defined or densely packed area with hydration products than the control mix, potentially contributing to increased strength. Figure 16 (b), showing pores within the contact zone, reveals fewer and smaller pores relative to the control mix, indicative of a denser matrix that could account for heightened strength. In Fig. 16 (c), the presence of hydrated particles alongside pores is observed, with the hydration appearing more complete and the distribution of particles more uniform, which are advantageous for strength. Lastly, Fig. 16 (d) displays the calcium-silicate-hydrate (C-S-H) gel and calcium hydroxide (Ca(OH)₂), similar to the control mix, but with a possibly more significant presence of C-S-H gel due to the pozzolanic reaction of SCBA, enhancing the concrete’s strength.

Fig. 16.

(a)–(d) Micrograph of Sample 2 (34 MPa) – 9% SCBA and 50% SD under different magnifications.

The modified mix displays a denser microstructure with more C-S-H gel, which is the strength-giving phase in concrete. As a pozzolan, SCBA reacts with the Ca(OH)₂ to form additional C-S-H gel, which fills pores and makes the cement matrix more homogenous and less porous, increasing the compressive strength. Furthermore, the SD particles could be filling the voids between the larger aggregate particles more effectively than the sand, leading to a denser packing and better particle interlock. The reduction in porosity, as evidenced by the lower prevalence of pores and the denser packing of hydrated particles, is likely a significant contributor to the increased compressive strength observed in the modified mix. In addition, the pozzolanic reaction of SCBA consumes the Ca(OH)₂, which is relatively weak and can be leached out, transforming it into additional C-S-H gel, thereby enhancing the durability and mechanical properties of the concrete.

Response surface analysis

In this research Design Expert software was used for Response Surface Analysis in which Compressive strength, Tensile strength, Flexural strength, and Water absorption tests (for both mixes) were the factors for which response surface methodology was used for the optimization of any of these parameters keeping other factors constant. For illustration Compressive strength, Tensile strength, Flexural strength, and Water absorption have been taken as response and w/b ratio, % replacement ratio of cement with SCBA, and sand with SD as factors. The test results were subjected to independent response surface methodology to obtain the best ratios for each property, as shown in Figs. 17, 18 and 19, and Fig. 20. The optimization process aimed to determine the best ratios of SCBA and SD replacement that would yield the most favorable outcomes in terms of concrete’s mechanical properties and durability as indicated by water absorption.

Fig. 17.

Surface Plot of Compressive Strength (a) ES-28 (b) ES-34.

Fig. 18.

Surface Plot of Split Tensile Strength (a) ES-28 (b) ES-34.

Fig. 19.

Surface Plot of Flexural Strength (a) ES-28 (b) ES-34.

Fig. 20.

Surface Plot of Water Absorption (a) ES-28 (b) ES-34.

The analysis of the optimized mix compared to the control mix across various parameters—compressive strength, split tensile strength, flexural strength, and water absorption—shows varying degrees of difference. The optimized mix demonstrates 34.28 MPa, 3.43 MPa, 7.97 MPa, and 5.45% respectively. The values obtained using response surface methodology are indicated in Table 8 for 34 MPa, which shows the best ratio of cement replacements with SCBA and sand with SD for the targeted results of each property. These figures represent differences of 0.12%, 1.18%, 4.18%, and a significant 27.34% in each parameter. Such variations indicate that the optimized mix broadly matches the control mix in mechanical strengths, underscoring its suitability for structural use. However, the noTable 27.34% increase in water absorption in the optimized mix points to increased porosity or reduced density, which might adversely affect its long-term durability. This increased water absorption, while a drawback, is a consequence of optimizing the mix for mechanical strengths, highlighting a potential compromise in the material’s overall performance and durability in structural applications.

Table 8.

Optimization outcomes for concrete Mix properties utilizing response surface analysis.

Property	W/B Ratio	Replacement ratio of SCBA (%)	Replacement Ratio of SD (%)	Value (With Response Surface) (A)	Value (Control Mix) (B)	Percent difference b/w A & B
Compressive Strength (MPa)	0.50	9	50	34.28	34.24	0.12%
Split tensile Strength (MPa)				3.43	3.39	1.18%
Flexural Strength (MPa)				7.97	7.65	4.18%
Water absorption (%)				5.45	4.28	27.34%

In short, optimizing one property does not necessarily lead to improvements across all properties; trade-offs must be considered. While the mechanical properties show favorable increases, the durability indicated by water absorption is compromised, which could lead to longer-term issues such as increased permeability and potential for damage from freeze-thaw cycles or chemical attacks.

Carbon Footprint reduction through optimal sustainable concrete mix design

Cement production is a significant contributor to global CO₂ emissions, releasing approximately 0.885 metric tons of CO₂ for every metric ton of cement produced^92–94. Pakistan’s cement industry, with its 26 operational plants and an annual capacity of 83.179 million metric tons, plays a vital role in the nation’s infrastructure development but also adds to the environmental burden through carbon emissions⁹⁵. Similarly, the production of sand or gravel through open-pit mining emits approximately 0.008 tons of CO₂ per ton⁹⁶.

This study focuses on optimizing concrete mixes by incorporating SCBA as a partial replacement for OPC and SD as a natural sand substitute. Based on w/b ratios of 0.45 and 0.57, the optimal mix design includes 9% SCBA and 50% SD replacement. These proportions were selected to balance structural performance and environmental sustainability while achieving the design objectives. The following analysis quantifies the reduction in CO₂ emissions facilitated by these substitutions.

Quantification of CO₂ emission reductions

Cement Replacement with SCBA (9%):

• For a w/b ratio of 0.45, replacing 48.42 kg of cement with SCBA in a mix with a total cement content of 537.96 kg/m³ reduced CO₂ emissions by approximately 42.85 kg per cubic meter of concrete.

• For a w/b ratio of 0.57, replacing 35.61 kg of cement with SCBA in a mix with a total cement content of 395.71 kg/m³ resulted in a CO₂ emission reduction of approximately 31.51 kg per cubic meter of concrete.

Sand Replacement with SD (50%):

• For a w/b ratio of 0.45, substituting 268.79 kg of sand with SD in a mix with a total sand content of 537.96 kg/m³ reduced CO₂ emissions by approximately 2.15 kg per cubic meter of concrete.

• For a w/b ratio of 0.57, substituting 296.79 kg of sand with SD in a mix with a total sand content of 593.57 kg/m³ reduced CO₂ emissions by approximately 2.37 kg per cubic meter of concrete.

The results confirm that the selected replacement proportions of SCBA and SD effectively reduce CO₂ emissions for both w/b ratios^92,94,96. The higher cement content in the 0.45 w/b mix results in greater CO₂ emissions compared to the 0.57 w/b mix; however, the integration of SCBA mitigates these emissions significantly. Similarly, the 50% SD substitution in sand further contributes to minor but meaningful reductions in CO₂ emissions.

Sustainability and practical implications

The optimized mix designs featuring 9% SCBA and 50% SD replacement highlight a practical and eco-friendly strategy for reducing the environmental impact of concrete production. By incorporating these sustainable materials, the reliance on conventional cement and natural sand is reduced, promoting resource conservation and energy efficiency. These formulations align with global efforts to mitigate greenhouse gas emissions while ensuring the durability and strength of concrete structures.

This approach not only adheres to sustainable construction practices but also offers a regionally appropriate solution for Pakistan’s construction industry, which is rapidly evolving to meet increasing urbanization demands. By achieving the optimal mix designs for 0.45 and 0.57 w/b ratios, this study underscores the viability of integrating industrial byproducts like SCBA and SD into concrete production for enhanced environmental and structural benefits.

Conclusions and recommendations

This research explored the simultaneous incorporation of SCBA and SD as partial replacements for cement and sand at various ratios in eco-strength concrete mixes targeting 28 MPa (ES-28) and 34 MPa (ES-34). The experimental investigation evaluated the influence of SCBA and SD on workability, mechanical properties, and durability, with the following conclusions:

Mechanical strength

For ES-28, with 9% SCBA and 50% SD, compressive and tensile strengths matched the control mix, while flexural strength improved by 6.86%. For ES-34, the same proportions enhanced compressive, tensile, and flexural strengths by 10.16%, 11.68%, and 5.22%, respectively. These improvements were attributed to pozzolanic reactions, enhanced particle packing, optimal curing, and the synergistic effects of SCBA and SD. SEM analysis confirms that the combined use of SCBA and SD creates a denser microstructure with higher C-S-H gel concentration and reduced porosity, further enhancing mechanical performance. To further validate the findings, it is recommended that strength testing be extended to include both short-term (1 and 3 days) and long-term (56 and 91 days) curing periods. This would provide a more comprehensive understanding of the early-age strength development and long-term performance of the optimized mixes, ensuring that the potential benefits of SCBA and SD in concrete applications are fully realized over time.

Durability

Water absorption increased significantly with higher SCBA and SD levels. For ES-28, a rise of 31.61% was observed, while ES-34 showed a 22.32% increase at 9% SCBA and 50% SD, indicating increased porosity and permeability. While the optimized particle distribution and pozzolanic reactions enhance mechanical properties, the higher water absorption highlights a trade-off between strength and durability. This increased porosity underscores the need for proper mix design adjustments, such as the inclusion of supplementary admixtures or further optimization of curing conditions, to mitigate this effect and achieve a balance between mechanical performance and durability.

Workability

Increased SCBA and SD proportions reduced slump and compaction values, reflecting a decline in workability. This reduction is likely due to the finer particle size and angular shape of SCBA and SD, which increase water demand and friction within the mix, leading to stiffer and less flowable concrete.

Sustainability and environmental impact

By combining SCBA and SD as partial replacements in conventional concrete, sustainable construction practices can effectively address resource scarcity, waste management, environmental concerns, and cost-effectiveness. The optimized mix designs featuring 9% SCBA and 50% SD replacement significantly reduce CO₂ emissions, with reductions of approximately 42.85 kg and 31.51 kg per cubic meter of concrete for w/b ratios of 0.45 and 0.57, respectively, due to SCBA substitution. Additionally, sand replacement with SD contributes further reductions of 2.15 kg and 2.37 kg per cubic meter for the respective w/b ratios. These reductions highlight the potential of these sustainable materials to mitigate greenhouse gas emissions while conserving resources.

Author contributions

Muhammad Adeel Khan: conceptualization, methodology, data curation, software, validation, formal analysis, investigation, writing—original draft. Boshan Zhang: conceptualization, methodology, validation, writing—review and editing, supervision. Mahmood Ahmad: methodology, writing—review and editing, resources, investigation, project administration. Mariusz Niekurzak: methodology, resources, investigation, project administration. Muhammad Salman Khan: methodology, data curation, software, validation, formal analysis, writing—review and editing. Mohanad Muayad Sabri Sabri: conceptualization, methodology, writing—review and editing, resources, project administration, funding acquisition. Weizhen Chen: conceptualization, methodology, writing—review and editing, visualization, resources, investigation, supervision.

Funding

This research was supported by a grant from the Russian Science Foundation No. 22-79-10021, https://rscf.ru/project/22-79-10021/.

Data availability

The data presented in this study are available on request from the corresponding author.

Declarations

Competing interests

The authors declare no competing interests.