Mechanical, durability, and microstructure analysis of concrete made with metakaolin and copper slag for sustainable construction
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Amruta Yadav
, Ajay Gajbhiye , Dhiraj Agrawal, Khalid Ansari
, Abdullah H. Alsabhan
, Krishna Prakash Arunachalam , Siva Avudaiappan, Nelson Maureira-Carsalade
and Osamah J. Alsareji
Abstract
The upsurge in urbanization steered a substantial proliferation in the demand for concrete in the construction industry. As a result, there is a higher demand for cement and fine aggregate, as constituents of concrete. The detrimental effect of cement generation on the environment has been witnessed over the last 4–5 decades. Similarly, the increased demand for fine aggregate has led to extensive extraction of river sand, which results in negative impacts on the environment. Utilizing industrial waste materials like copper slag (CS) and mineral admixtures such as metakaolin (MK) in eco-friendly concrete production can help reduce this environmental impact. The primary objective of this research is to examine the impact on the mechanical and durability properties of concrete when using CS as a partial replacement for sand, combined with MK as a partial replacement for cement. To evaluate the mechanical characteristics of concrete, tests were conducted to measure split tensile strength, compressive strength, and flexural strength, and its durability was assessed via a rapid chloride ion penetration test. The microstructural analysis of concrete was also performed using scanning electron microscopy and energy dispersive spectroscopy. The optimum percentages of the MK and CS were as fractional switches of cement and sand, respectively, were assessed using the response surface methodology (RSM). The optimal strengths were found at 15% MK and 31.533% CS, while experimentally, the best performance of modified concrete was obtained at 30% CS. The experimental findings and predicted values from the RSM model showed a strong correlation with R-squared (R 2) values obtained as 0.9880 and 0.9552 for compressive and flexural strengths, respectively. The findings have significant implications, as they provide a sustainable alternative to sand while enhancing the performance and longevity of concrete structures.
1 Introduction
The construction industry requires three essential ingredients: aggregate, sand, and cement. Cement and sand are two crucial elements in making concrete, which significantly impact mixed design, and strength is primarily determined by cement paste quality [1,2]. The cement manufacturing process includes calcination, a chemical reaction that releases substantial amounts of carbon dioxide into the air by adding to greenhouse gas emissions and harming the environment [3]. Conventional concrete (CC) typically has low strength, durability, and workability, along with high fragility, which can negatively impact the performance and weight of structural elements [4,5].
These environmental issues highlight the need for sustainable substitutes for these materials in the construction industry. Industrialization is essential for the nation’s development. Annually, industrial sectors generate substantial quantities of waste that have resulted in a concomitant increase in both the volume and diversity of waste produced, and in this context, slag-based geopolymer concrete emerges as a promising green alternative to conventional concrete, offering improved performance in aggressive sulfate environments, as demonstrated by a comprehensive comparative study of mechanical and durability properties [6]. In developing economies, the management of power plant byproducts such as fly ash, bottom ash, and biomass ash plays a pivotal role in advancing circular economy principles within sustainable construction practices, and aligning with this approach, another study carried out by Ashish investigates the effective utilization of marble powder combined with supplementary cementitious materials like metakaolin (MK) and silica fume, demonstrating enhanced strength and durability properties of concrete while maintaining favorable setting and soundness characteristics in blended cement pastes [7]. This approach not only mitigates environmental risks but also enhances resource efficiency and economic sustainability [8]. Another study conducted by Gill et al. presents a unified empirical equation to non-destructively predict geopolymer concrete strength using UPV, AAL/B ratio, and density with high accuracy [9]. Copper slag (CS) is a mining waste product that is extracted while producing copper. It is created in gigantic amounts when copper is extracted [10]. Approximately 2.23 million tonnes of CS are produced for every million tonnes of extracted copper ore [11–13]. A major environmental concern is the disposal of such a massive amount of waste produced by the industries. In the near future, an additional challenge triggered by the weakening of natural aggregates will need a viable solution; wastes from the mining industry can be employed as a non-natural aggregate [14,15]. CS can be utilized as an aggregate or as a fractional substitute for cement in concrete due to its distinctive mechanical and chemical qualities. CS has several beneficial mechanical properties when used as aggregate, such as good stability, abrasion resistance, and soundness characteristics. Additionally, due to the presence of other oxides such as SiO2, Fe2O3, and Al2O3, as well as a low CaO level, CS displays pozzolanic abilities [16]. It is granular and shiny, and when added to concrete, it improves the material’s strength, durability, workability, and other properties [10,17]. The addition of CS from 0 to 100% improved the workability of the concrete and nearly raised its density by 5%. However, in high-performance concrete, segregations and bleeding at larger substitution levels of 80 and 100% CS were observed [11,18].
Nevertheless, the cement hydration and manufacturing processes generate a substantial amount of heat and release carbon dioxide, a greenhouse gas that exacerbates global warming and climate change [19]. Also, Karatas et al. highlighted the environmental benefits of alkali-activated metakaolin cement as a sustainable alternative to Portland cement, which is responsible for approximately 7% of the global CO2 emissions [20]. Kaolinite, a clay mineral, is transformed into MK through dihydroxylation [21]. In addition to being frequently utilized in the manufacturing of ceramics, MK can also be substituted for cement in concrete. Because of its excellent pozzolanic properties, the MK material can serve as a cement substitute to create high-performance concrete [22]. MK is a synthetic pozzolana that is produced by heating kaolinite clays to between 700 and 850°C. The incorporation of MK enhances the mechanical and durability characteristics of concrete because of its high pozzolanic activity [18,23]. Arslan et al. found that the SCGC mixtures exhibited significant strength gains, particularly in the first 24 h of curing. Ambient-cured GBFS-based mixtures showed strength increases ranging from 10.39 to 26.49% after 24 h, while 28-day strength gains were lower, between 3.04 and 9.19% [24]. Major constituents of MK include amorphous silica and amorphous alumina, which react with calcium hydroxide (CH) and water to yield primarily calcium aluminate hydrates and aluminum silicate hydrates [23]. At all ages up to 180 days, the concrete containing 10% MK was stronger than the control concrete [22]. Note that impurities in MK can limit its reactivity, leading to lower-than-expected strength outcomes [25,26]. Concrete can be made in large amounts without the use of cement and sand through the utilization of industrial wastes such as GGBS, MK, and CS [27–30]. By incorporating MK, concrete achieves lower permeability and greater strength, minimizes chloride ion penetration and water requirements, and improves workability [31]. The addition of 10% MK reduces the workability of concrete containing CS from the River Beas as fine aggregates, although workability increases with higher CS content [32]. Also, the studies carried out show that the inclusion of waste tire aggregates (WTA) and MK significantly improved the mechanical properties of slag/MK-based rubberized semi-lightweight geopolymer composites. With a mixture of 60% WTA and 10% MK, a compressive strength of 25.10 MPa was achieved at a curing temperature of 100°C [33].
It is evident from the research background that several studies have been carried out on CS as a substitute for natural sand (NS) and MK as a partial cement replacement. Concrete with 30% CS replacement exhibited a more compact microstructure than conventional concrete, confirming that the AAM concrete with a higher Si/Al ratio had a more densely packed structure [34]. The potential for using CS, particularly at a 30% level, in micro surfacing treatments is substantial, as it enhances asphalt performance with respect to the curing time, abrasion resistance, and both vertical and lateral displacements due to its angularity and elevated Fe2O3 and SiO2 percentages [35]. Zhang et al. showed that using CS as an supplementary cementitious material (SCM) poses no significant environmental risks, as the leaching concentrations of heavy metals in CS comply with regulatory limits. This finding supports the safe use of CS in the construction materials [25].
However, several research studies provided insights into how these materials behave under elevated temperatures, including calcined kaolin and MK, which improve the compressive strength of self-compacting concrete (SCMs) when exposed to high temperatures [36]. For instance, mortars with MK showed increased compressive strength at moderate temperatures (up to 200°C) due to enhanced pozzolanic activity, which contributes to a denser microstructure [37,38]. However, at very high temperatures (e.g., 1,000°C), MK-based SCMs may experience significant strength loss due to microstructural degradation. This transformation results in an amorphous phase that contributes to improved pozzolanic reactivity and mechanical performance at elevated temperatures [39]. MK-based geopolymers also exhibit superior thermal resistance, maintaining structural integrity and reducing cracking tendencies at temperatures above 200°C [40,41]. The relationship of various investigations related to mechanical, durability, and analysis of concrete made with MK and CS for Sustainable Construction is shown in Figure 1 as a cluster diagram from recent years [1–32] with the help of VOS viewers software (v.19).

Cluster diagram showing the relationship of mechanical properties.
To bridge the demand–supply gap of raw materials in the construction industry and address the disposal issues of industrial wastes like CS, incorporating such by-products into concrete offers a sustainable solution, and in this direction, fly ash-based geopolymer concrete with alccofine emerges as an eco-friendly alternative, demonstrating enhanced workability and compressive strength even at ambient conditions, making it suitable for both general and precast concrete applications [42,43]. Utilizing waste materials reduces the cost of concrete, and recycling waste has been considered to be the most environmentally friendly approach to solve the waste disposal challenge [44–48]. Because of this, the current study attempts to mitigate environmental impacts by partially replacing cement with MK and sand with CS in sustainable concrete production, and in line with this approach, Ashish and Verma explored the potential of industrial symbiosis by utilizing waste foundry sand as a viable resource in self-compacting concrete, revealing improved strength and microstructural properties over time while maintaining environmental safety through leaching analysis [49].
Previous research has highlighted the potential of using industrial byproducts like CS and MK in concrete to enhance its mechanical properties and sustainability. However, these studies often faced limitations in terms of achieving strength using CS or MK individually, rather than in combination. Also, optimal replacement percentages for both CS and MK in combination were not well established. Therefore, the primary aim of this study is to explore the use of CS as a partial replacement for sand and MK as a partial replacement for cement in concrete [50]. This study aims to overcome these limitations by conducting a comprehensive analysis of the mechanical and durability properties of concrete and optimizing response surface methodology (RSM) to determine the optimal proportions of CS and MK for enhanced performance with CS and MK [51]. This research advances knowledge by providing a detailed understanding of how CS and MK can be effectively used in concrete to achieve sustainability goals. It fills a previously unconsidered gap by offering a systematic approach to optimizing material proportions and assessing their environmental impact, thus paving the way for more eco-friendly construction practices worldwide [52].
2 Experimental methods
2.1 Test materials
The binding material used is the Ultratech 53 Grade of OPC, and the corresponding supplementary cementitious material utilized in this investigation is MK. As reported by IS 269 [53] guidelines, the material complies with multiple standard tests. Table 1 shows the chemical composition of cement and MK, and Figure 2 illustrates the X-ray diffraction of MK. Cement has a specific gravity of 3.15, compared to 2.6 for MK. The combined quantities of SiO2, Al2O3, and Fe2O3 were greater than the required minimum of 70. Designed for applications that demand significant water reduction and extended workability, Viscoflux 5507 is a high-performance superplasticizer. In this case, 1.0% of the cement’s weight was used as a plasticizer in the mixture.
Chemical properties of cement and MK
| Component | Cement | MK |
|---|---|---|
| SiO2 | 18.89 | 54.899 |
| K2O | 1.14 | 0.162 |
| CaO | 62.37 | 0.279 |
| MgO | 0.99 | 0.914 |
| P2O5 | 0.12 | 0.03 |
| Fe2O3 | 3.83 | 0.799 |
| Al2O3 | 4.24 | 38.145 |
| Na2O | 0.12 | 0.01 |
| TiO2 | 0.30 | 2.061 |

X-ray diffraction of MK.
Locally sourced coarse aggregates with nominal sizes of 10 and 20 mm were used in equal proportions of 50% each. The specific gravity, water absorption, and fineness modulus are conducted as per IS 2386 Part III [54].
NS that was readily available locally was utilized in concrete mixtures that conformed to IS 383 [55]. It is classified into four zones by the IS 383 [55] based on the particle size determined by sieve analysis; Zone II was found to be the case for this particular sand. The physical and chemical properties of NS are shown in Table 2. The X-ray diffraction of CS is shown in Figure 3.
Physical properties of coarse and fine aggregate
| Component | NS | CS | Coarse aggregate |
|---|---|---|---|
| Specific gravity (g/cc) | 2.64 | 3.31 | 2.89 |
| Water absorption (%) | 1.06 | 0.87 | 0.89 |
| Fineness modulus | 2.84 | 2.86 | 6.93 |
| Bulk of sand (%) | 28 | — | — |
| Chemical properties of fine aggregates | |||
| SiO2 | 73.79 | 20.328 | — |
| K2O | 3.46 | 0.723 | — |
| CaO | 1.79 | 2.721 | — |
| MgO | 0.47 | 0.859 | — |
| P2O5 | 0.13 | 0.1 | — |
| Fe2O3 | 2.69 | 64.733 | — |
| Al2O3 | 15.39 | 3.076 | — |
| Na2O | 0.59 | 0.425 | — |
| TiO2 | 0.34 | 0.391 | — |

X-ray diffraction of CS.
CS from Birla Copper in Gujarat is utilized in this study. The material is heavier due to its high specific gravity. Table 2 shows the physical and chemical properties of NS compared to CS. Figure 4 shows the particle size distribution for NS and CS.

Particle size distribution of NS and CS.
2.2 Test procedure
A total of eight concrete mixes were used for this experimental work and are shown in Table 3. The study involved preparing, casting, and testing different mixes to determine the mechanical properties using different proportions of CS and MK. The curing of specimens was carried out for 7, 14, 28, and 56 days to determine the mechanical properties of the concrete. The experimental work was divided into two phases: the first phase tested CS as a partial substitute for fine aggregate in amounts from 10 to 40%, increasing by 10% increments.
Percentage of constituents of concrete for different mixes
| S.N | Mix | Proportion | ||||
|---|---|---|---|---|---|---|
| Cement | Fine aggregate | CA | ||||
| OPC | MK | NS | CS | |||
| 1 | CC | 100 | 0 | 100 | — | 100 |
| 2 | CS10 | 100 | 0 | 90 | 10 | 100 |
| 3 | CS20 | 100 | 0 | 80 | 20 | 100 |
| 4 | CS30 | 100 | 0 | 70 | 30 | 100 |
| 5 | CS40 | 100 | 0 | 60 | 40 | 100 |
| 6 | MK5CS30 | 95 | 5 | 70 | 30 | 100 |
| 7 | MK10CS30 | 90 | 10 | 70 | 30 | 100 |
| 8 | MK15CS30 | 85 | 15 | 70 | 30 | 100 |
OPC: Ordinary Portland cement; NS: natural sand; CA: coarse aggregate.
The second phase involved adding MK as a partial replacement for cement at 5, 10, and 15%, along with 30% CS replacing sand. The optimum percentage obtained for CS as a part switch of sand is obtained in phase I. The concrete mixes blended with MK and CS are prepared, cast, and tested for strength. The quantity of all the materials is shown in Table 4 for all the mixes. The methodology adopted in this research work is also depicted in Figure 5.
Quantity of constituents of concrete (in kg·m−3)
| S.N | Mix | Cement (kg·m−3) | Fine aggregate (kg·m−3) | CA | Super plasticizer | W/C ratio | ||
|---|---|---|---|---|---|---|---|---|
| OPC | MK | NS | CS | |||||
| 1 | CC | 350 | — | 701.18 | — | 1368.17 | 3.50 | 0.45 |
| 2 | CS10 | 350 | — | 631.06 | 88.08 | 1368.17 | 3.50 | 0.45 |
| 3 | CS20 | 350 | — | 560.94 | 176.16 | 1368.17 | 3.50 | 0.45 |
| 4 | CS30 | 350 | — | 490.83 | 264.25 | 1368.17 | 3.50 | 0.45 |
| 5 | CS40 | 350 | — | 420.71 | 352.33 | 1368.17 | 3.50 | 0.45 |
| 6 | MK5CS30 | 332.5 | 17.5 | 490.05 | 263.83 | 1366.02 | 3.50 | 0.45 |
| 7 | MK10CS30 | 315.0 | 35.0 | 489.28 | 263.41 | 1363.86 | 3.50 | 0.45 |
| 8 | MK15CS30 | 297.5 | 52.5 | 488.51 | 263.00 | 1361.70 | 3.50 | 0.45 |

Methodology for the work performed.
In the initial state, the workability test was carried out as per IS 1199 [56]. Workability of the fresh concrete was measured using the slump cone test.
To evaluate the mechanical properties of the hardened concrete, strength tests were conducted on both the control and developed concrete, including tests for compressive strength, split tensile strength, and flexural strength [57].
Following IS Code 516 [58], compressive strength testing was carried out on cubes of size 150 mm × 150 mm × 150 mm at 7, 14, 28, and 56 days of moist curing. The tests were performed using a 2,000 kN capacity compression testing machine, as depicted in Figure 6a. The average of strengths for three specimens is considered the strength of the concrete mix.

Testing of specimens. (a) Cube specimen, (b) beam specimen, (c) cylindrical specimen, (d) RCPT testing, and (e) RCPT specimen.
The flexural strength of concrete was evaluated as per IS516 [58] on the beam specimen of size 100 mm × 100 mm × 500 mm. The specimens were extracted from the curing tank at 28 and 56 days of moist curing and tested, as illustrated in Figure 6b.
The split tensile strength of concrete was conducted as per IS 5816 [59] on the cylindrical specimens, measuring 150 mm in diameter and 300 mm in height, at 28 and 56 days of moist curing. A 2,000 kN capacity compression testing machine was used for the tests, as demonstrated in Figure 6c.
For durability assessment, the rapid chloride permeability test was conducted as per ASTM C1202 [60], as shown in Figure 6d and 6e. The test specimen of cubes was tested for resistance against chloride ion penetration after 28 and 56 days of moist curing in water.
3 Results and discussion
3.1 Workability
The results indicate that the workability of the mix CS40 is a maximum of 120 mm as compared to the CC mix, i.e., 90 mm, as presented in Table 5. The increase can be explained by the excess free water remaining post-absorption and hydration, as CS’s glassy or smooth surface results in a lower water absorption rate compared to NS [61,62]. The mix MK15CS30 has a minimum workability of 91 mm, as shown in Figure 7; however, the workability progressively declines due to the addition of MK, as presented in Table 5.
Test results for 7, 14, 28, and 56 days curing
| S.N | Mix | Slump (mm) | Compressive strength (N·mm−2) | Flexural strength (N·mm−2) | Split tensile strength (N·mm−2) | Density (kg·m−3) | |||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 7 | 14 | 28 | 56 | 28 | 56 | 28 | 56 | ||||
| 1 | CC | 90 | 36.69 | 37.78 | 39.53 | 41.22 | 7.85 | 8.11 | 2.86 | 2.91 | 2630.37 |
| 2 | CS10 | 100 | 39.80 | 42.67 | 46.23 | 48.76 | 8.90 | 9.28 | 2.96 | 3.03 | 2634.07 |
| 3 | CS20 | 105 | 41.78 | 44.44 | 48.55 | 50.92 | 9.24 | 9.88 | 3.15 | 3.24 | 2654.81 |
| 4 | CS30 | 110 | 43.56 | 45.78 | 50.33 | 53.16 | 9.52 | 10.1 | 3.46 | 3.55 | 2723.70 |
| 5 | CS40 | 120 | 37.78 | 40.00 | 48.66 | 51.98 | 8.40 | 8.87 | 3.25 | 3.43 | 2752.59 |
| 6 | MK5CS30 | 100 | 40.22 | 48.41 | 53.33 | 56.43 | 9.60 | 10.15 | 3.54 | 3.66 | 2666.67 |
| 7 | MK10CS30 | 94 | 41.56 | 49.44 | 57.78 | 61.43 | 10.2 | 10.65 | 3.70 | 3.81 | 2652.00 |
| 8 | MK15CS30 | 91 | 40.11 | 48.54 | 56.89 | 60.11 | 9.80 | 10.45 | 3.64 | 3.75 | 2622.22 |

Density and workability for different concrete mixes.
3.2 Density
The increase in the density of the concrete mix with increasing CS content is illustrated in Figure 7. This is because CS has a specific gravity of 3.31, which is greater than that of NS(2.64). Incorporating MK into the concrete mix as a cement reduces the density of concrete, as shown in Table 5. The study found that the workability of concrete increases with higher CS content, while the addition of MK as a cement replacement decreases workability. This is attributed to the smoother texture of CS, which enhances workability, but excessive CS can lead to bleeding and segregation if used beyond 50% replacement. The density of concrete increases with more CS but decreases with MK, indicating a trade-off between workability and density when using these materials.
3.3 Compressive strength
Figure 8 illustrates the compressive strength of concrete made with CS and MK. The results from the compressive strength test show that the CS30 mix in the first phase outperforms all other mixes. The highest compressive strength values for CS30 are 50.33 N·mm−2 at 28 days and 53.16 N·mm−2 at 56 days, showing an increase of 27.32 and 28.97%, respectively. This improvement can be attributed to the higher iron oxide (Fe2O3) content and the inherent toughness of CS.

Variation in the compressive strength for concrete mixes.
All of the mixes containing MK show higher compressive strength as compared to the concrete made only with CS. The MK10CS30 mix outperformed all the other mixes, as it exhibited higher compressive strengths of 57.78 and 61.43 N·mm−2 with an increase of 46.16 and 49.03% at 28 and 56 days, respectively. Also, MK15CS30 shows higher strengths of 43.91 and 45.83% at 28 and 56 days, as compared to the CC mix.
Mixes CS10, CS20, CS30, CS40, MK5CS30, MK10CS30, and MK15CS30 show an increase in compressive strength by 16.95, 22.82, 27.32, 23.10, 34.92, 46.16, and 43.91% at 28 days and 18.29, 23.53, 28.97, 26.10, 36.90, 49.03, and 45.83% at 56 days, respectively, with respect to CC. This suggests that the combination of MK and CS optimizes the compressive strength of concrete.
3.4 Flexural strength
The flexural tensile strength of CS10, CS20, CS30, and CS40 increases by 13.38, 17.71, 21.27, and 7.01% at 28 days and 14.43, 21.82, 24.54, and 9.37%, respectively, with respect to the CC mix in the first phase of research. Mix CS30 exhibited higher flexural strengths of 9.52 and 10.1 N·mm−2, as shown in Figure 9, with an increase of 21.27 and 24.54% at 28 and 56 days. The highest flexural strengths of 10.2 and 10.65 N·mm−2 were achieved for MK10CS30 with an increase of 29.94 and 31.32%, as compared to the CC mix.

Variation in the flexural strength for concrete mixes.
The other MK blended concrete samples, i.e., MK5CS30, MK10CS30, and MK15CS30, show the maximum flexural strengths of 22.29, 29.94, and 24.84% at 28 days, and 25.15, 31.32, 28.85% at 56 days, respectively, as associated with CC. This indicates that MK and CS not only improve the compressive strength but also enhance the tensile and flexural properties of the concrete.
3.5 Split tensile strength
The splitting tensile strength test results in the first phase indicated that the strengths of concrete mix CS30 are 3.46 and 3.55 N·mm−2, which are higher by 20.98 and 21.99% at 28 and 56 days, respectively, as compared to CC, as shown in Figure 10.

Variation in the splitting tensile strength for concrete mixes.
In the second phase of research, the flexural strengths for MK5CS30, MK10CS30, and MK15CS30 increase by 23.73, 29.37, and 27.27% at 28 days and 25.77, 30.93%, and 28.87 at 56 days, respectively. The concrete mix MK10CS30 outperformed among all the other mixes as it exhibited maximum strengths of 3.7 and 3.81 N·mm−2 with an increase of 29.37 and 30.93% at 28 and 56 days, respectively. The incorporation of MK as a fractional substitution of cement induced a remarkable strength to the concrete. Hence, the splitting tensile strength of the modified concrete mixes is enhanced compared to the control concrete.
3.6 Rapid chloride ion penetration test
The durability of concrete is assessed by using the rapid chloride ion penetration test on the concrete cube of 100 mm × 50 mm, as shown in Figure 6e. The Coulomb's charge for the CC mix was found to be 2133.45 and 1880.87 on the 28th and 56th day, showing moderate penetration as per ASTM C1202. The charge passed for CS30 showed higher values than the control concrete, i.e., 3512.07 and 3220.34 at 28 and 56 days, respectively, as shown in Figure 11, but within the limits as per ASTM C1202. Due to the addition of MK in the second phase, i.e., for MK5CS30, MK10CS30, and MK15CS30, the Coulomb charge reduces as compared to the CC and CS30 mixes. The resistance against chloride ion penetration for the MK10CS30 and MK15CS30 mixes was found to be at a lower level as per ASTM C1202. The Coulomb charges for this mix are 1520.14 and 1390.36 at 56 days, respectively, as shown in Figure 11. This shows remarkably lower permeability than the CC and CS30 mixes. MK reduced the chloride ion permeability, indicating lower permeability of the MK- CS concrete. The addition of MK reduces the pores of concrete and makes the concrete impermeable.

Variation in charge passes for different concrete mixes.
3.7 Microstructural analysis
After observing promising results in the strength and durability of MK10CS30, an SEM investigation was conducted. The SEM analysis confirmed a denser microstructure, clearly illustrating the benefit of CS inclusion. Reduced porosity, as evidenced by the SEM microstructure in Figure 12(a), minimizes pathways for harmful substances, thereby enhancing durability by lowering the potential for corrosion, cracking, and deterioration. Additionally, Figure 12(b) shows aluminosilicate bonding at the interface. In this study, supplementary cementitious materials like MK, which are rich in aluminosilicate, were added. These materials undergo pozzolanic reactions with CH, leading to the formation of the additional C–S–H gel. This gel contributes to the overall strength and durability of concrete. In addition, the CS added to the concrete enriches the bonding by facilitating the formation of calcium aluminosilicate bonding, which can be ascertained through the energy dispersive spectrograph shown in Figure 12(c) and (d), which indicates the presence of Ca and Al at the interface. Thus, the mix MK10CS30 has proved beneficial, contributing to the strength of the material.

Microstructural analysis of the concrete with 10% MK and 30% CS. (a) No porosity observed, (b) aluminosilicate bonding between interface, (c) presence of Ca and Al at the interface, and (d) energy dispersive spectrograph of concrete.
3.8 Optimization of MK and CS using RSM
Considering the results obtained for the mechanical properties of concrete using MK and CS, the optimization of these elements is conducted using the Central Composite Design process in RSM [63]. A quadratic model is fitted for predicting the optimization. A statistical model of analysis is conducted to optimize the quantities of CS and MK in concrete by obtaining the maximum compressive and flexural strengths for the tailored concrete. Table 6 presents the responses recorded for the experiment. The regression equations obtained from the analysis of variance are shown in Table 7.
Responses provided for the MK and CS factors
| S.N | Factor 1, MK (%) | Factor 2, CS (%) | Response 1, compressive strength (MPa) | Response 2, flexural strength (MPa) |
|---|---|---|---|---|
| 1 | 0 | 0 | 39.53 | 7.85 |
| 2 | 0 | 10 | 46.23 | 8.90 |
| 3 | 0 | 20 | 48.55 | 9.24 |
| 4 | 0 | 30 | 50.33 | 9.52 |
| 5 | 0 | 40 | 48.66 | 8.40 |
| 6 | 5 | 30 | 53.33 | 9.60 |
| 7 | 10 | 30 | 57.77 | 10.20 |
| 8 | 15 | 30 | 56.88 | 9.80 |
Regression equations for the various parameter levels
| Responses | Regression model | R 2 | Eq. |
|---|---|---|---|
| Compressive strength (MPa) | =54.81 + 3.61A + 4.40B + 0.0000AB − 2.18A² − 4.84B² | 0.9880 | (1) |
| Flexural strength (MPa) | =10.09 + 0.3108A + 0.3309B + 0.0000AB − 0.3884A² − 1.24B² | 0.9552 | (2) |
The equation, expressed in terms of coded factors, can be used to predict the response for specific levels of each factor. Here, A represents the percentage of MK, while B indicates the percentage of CS. The predicted versus actual compressive and flexural strengths are shown in Figure 13. An F-value analysis was performed to determine the optimal outcome [15,64]. The F-values obtained for this analysis were 61.57 and 15.98 for compressive and flexural strengths, respectively. The P-values were less than 0.05, which indicates that the models were significant. The experimental findings and predicted values from the RSM model showed a strong correlation as R squared (R 2) values obtained were 0.9880 and 0.9552 for compressive and flexural strengths, respectively.

Predicted and actual (a) compressive strength and (b) flexural strength.
As depicted in Figure 14, a three-dimensional surface plot was employed to describe the optimization and verification of the models. The study extensively explored the complex relationship between MK and CS levels and the concrete’s mechanical properties, with a focus on compressive and flexural strengths. The desirability function was used in the optimization process to provide a quantitative method for determining the upper and lower limits of these strengths, enabling the determination of the desirability level. The study reached a combined desirability score of 0.8903, as shown in Figure 15. The optimal strengths were found at 15% MK and 31.533% CS. The contour plots used for optimization are shown in Figure 16.

Three-dimensional surface representation for (a) compressive strength and (b) flexural strength.

Desirability chart for factors and responses considered in the study.

Contour surface plots for (a) compressive strength, (b) flexural strength, and (c) desirability of the model.
4 Conclusion
Concrete’s workability increases with higher CS content, but adding MK as a replacement for cement decreases workability.
With a compressive strength of roughly 50.33 N·mm−2, the CS30 mix outperformed the control mixture, which had a value of 39.53 N·mm−2. This indicates an almost 27.32% increase in strength compared to the control mix. However, the lowest compressive strength was found in CS80 and CS100 with greater CS concentrations.
The highest compressive strengths achieved for MK10CS30 were 57.78 and 63.13 N·mm−2 at 28 and 56 days, respectively, enhancing the strength by 46.16 and 49.02% as compared to the CC mix.
The maximum split tensile strength was found for MK10CS30 at an increment of 29.37 and 30.93% at 28 and 56 days, respectively.
For MK10CS30, the highest flexural strengths of 10.2 and 10.65 N·mm−2 with an increase of 29.94 and 31.32% at 28 and 56 days, respectively.
All the mixes containing MK showed higher strength as compared to other mixes.
Concrete becomes denser with an increased amount of CS, while incorporating MK leads to a reduction in density.
According to the RCPT results, concrete blended with slag and MK greatly densified the matrix, which reduces permeability and enhances the strength as seen in compression and flexural strengths. It was found that MK concrete exhibits a significantly higher resistance to chloride ion penetration.
Moreover, the SEM micrograph of MK10CS30 demonstrated a homogeneous and dense structure of the C–S–H gel.
The RSM used in the study revealed that MK can be used as a partial replacement of cement up to 15% and the fine aggregates can be exchanged up to 31.533% using CS.
Finally, this study shows that the use of CS as a partial replacement of NS has some limitations, as the texture of CS is smoother, and hence it makes concrete more workable, and there might be the chances of bleeding and segregation if the partial replacement is beyond 50%. However, the utilization of MK is itself challenging, as the high-performance MK is not available in all parts of the globe; in addition, water demand and early age shrinkage can be the concerning factors for MK as a fractional switch of cement. Further research for life cycle assessment and carbon footprint emission can be carried out for utilizing the MK and CS as partial replacements of cement and NS to make construction activity sustainable.
Overall, this research provides significant insights into the use of alternative materials in concrete, offering a pathway to more sustainable and efficient construction practices, offering a practical solution to environmental challenges while potentially improving concrete performance and economics in the construction industry.
Acknowledgments
The authors would like to acknowledge the Ongoing Research Funding program (ORF-2025-473), King Saud University, Riyadh, Saudi Arabia, and Dirección de Investigación de la Universidad Católica de la Santísima Concepción, Concepción, Chile.
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Funding information: The authors would like to acknowledge the support provided by the Ongoing Research Funding program (ORF-2025-473), King Saud University, Riyadh, Saudi Arabia, Dirección de Investigación de la Universidad Católica de la Santísima Concepción, Concepción, Chile, and Departamento de Ciencias de la Construcción, Facultad de Ciencias de la Construcción Ordenamiento Territorial, Universidad Tecnológica Metropolitana, Santiago.
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Author contributions: Amruta Yadav – conceptualization, methodology, software, validation, formal analysis, investigation, data curation, writing – original draft, writing – review and editing, and resources. Ajay Gajbhiye – investigation, data curation, writing – original draft, writing – review and editing, resources, and visualization. Dhiraj Agrawal – conceptualization, methodology, software, validation, formal analysis, investigation, data curation, writing – original draft, writing – review and editing, resources, visualization, supervision, and project administration. Khalid Ansari – conceptualization, methodology, software, validation, formal analysis, investigation, data curation, writing – original draft, writing – review and editing, resources, visualization, supervision, and project administration. Abdullah H. Alsabhan – conceptualization, methodology, software, validation, formal analysis, investigation, data curation, writing – original draft, writing – review and editing, resources, and funding acquisition. Krishna Prakash Arunachalam – software, validation, formal analysis, investigation, data curation, writing – original draft, and writing – review and editing. Siva Avudaiappan – investigation, data curation, writing – review and editing, resources, visualization, and funding acquisition. Nelson Maureira-Carsalade – investigation, data curation, writing – review and editing, resources, visualization, and funding acquisition. Osamah J. Alsareji – software, validation, formal analysis, investigation, data curation, writing – original draft, writing – review and editing, and resources. All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
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Conflict of interest: The authors state no conflict of interest.
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Data availability statement: The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.
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- Eco-friendly waste plastic-based mortar incorporating industrial waste powders: Interpretable models for flexural strength
- Bioactive potential of marine Aspergillus niger AMG31: Metabolite profiling and green synthesis of copper/zinc oxide nanocomposites – An insight into biomedical applications
- Preparation of geopolymer cementitious materials by combining industrial waste and municipal dewatering sludge: Stabilization, microscopic analysis and water seepage
- Seismic behavior and shear capacity calculation of a new type of self-centering steel-concrete composite joint
- Sustainable utilization of aluminum waste in geopolymer concrete: Influence of alkaline activation on microstructure and mechanical properties
- Optimization of oil palm boiler ash waste and zinc oxide as antibacterial fabric coating
- Tailoring ZX30 alloy’s microstructural evolution, electrochemical and mechanical behavior via ECAP processing parameters
- Comparative study on the effect of natural and synthetic fibers on the production of sustainable concrete
- Microemulsion synthesis of zinc-containing mesoporous bioactive silicate glass nanoparticles: In vitro bioactivity and drug release studies
- On the interaction of shear bands with nanoparticles in ZrCu-based metallic glass: In situ TEM investigation
- Developing low carbon molybdenum tailing self-consolidating concrete: Workability, shrinkage, strength, and pore structure
- Experimental and computational analyses of eco-friendly concrete using recycled crushed brick
- High-performance WC–Co coatings via HVOF: Mechanical properties of steel surfaces
- Mechanical properties and fatigue analysis of rubber concrete under uniaxial compression modified by a combination of mineral admixture
- Experimental study of flexural performance of solid wood beams strengthened with CFRP fibers
- Eco-friendly green synthesis of silver nanoparticles with Syzygium aromaticum extract: characterization and evaluation against Schistosoma haematobium
- Predictive modeling assessment of advanced concrete materials incorporating plastic waste as sand replacement
- Self-compacting mortar overlays using expanded polystyrene beads for thermal performance and energy efficiency in buildings
- Enhancing frost resistance of alkali-activated slag concrete using surfactants: sodium dodecyl sulfate, sodium abietate, and triterpenoid saponins
- Equation-driven strength prediction of GGBS concrete: a symbolic machine learning approach for sustainable development
- Empowering 3D printed concrete: discovering the impact of steel fiber reinforcement on mechanical performance
- Advanced hybrid machine learning models for estimating chloride penetration resistance of concrete structures for durability assessment: optimization and hyperparameter tuning
- Influence of diamine structure on the properties of colorless and transparent polyimides
- Post-heating strength prediction in concrete with Wadi Gyada Alkharj fine aggregate using thermal conductivity and ultrasonic pulse velocity
- Experimental and RSM-based optimization of sustainable concrete properties using glass powder and rubber fine aggregates as partial replacements
- Special Issue on Recent Advancement in Low-carbon Cement-based Materials - Part II
- Investigating the effect of locally available volcanic ash on mechanical and microstructure properties of concrete
- Flexural performance evaluation using computational tools for plastic-derived mortar modified with blends of industrial waste powders
- Foamed geopolymers as low carbon materials for fire-resistant and lightweight applications in construction: A review
- Autogenous shrinkage of cementitious composites incorporating red mud
- Mechanical, durability, and microstructure analysis of concrete made with metakaolin and copper slag for sustainable construction
- Special Issue on AI-Driven Advances for Nano-Enhanced Sustainable Construction Materials
- Advanced explainable models for strength evaluation of self-compacting concrete modified with supplementary glass and marble powders
- Analyzing the viability of agro-waste for sustainable concrete: Expression-based formulation and validation of predictive models for strength performance
- Special Issue on Advanced Materials for Energy Storage and Conversion
- Innovative optimization of seashell ash-based lightweight foamed concrete: Enhancing physicomechanical properties through ANN-GA hybrid approach
- Production of novel reinforcing rods of waste polyester, polypropylene, and cotton as alternatives to reinforcement steel rods
Articles in the same Issue
- Review Articles
- Utilization of steel slag in concrete: A review on durability and microstructure analysis
- Technical development of modified emulsion asphalt: A review on the preparation, performance, and applications
- Recent developments in ultrasonic welding of similar and dissimilar joints of carbon fiber reinforcement thermoplastics with and without interlayer: A state-of-the-art review
- Unveiling the crucial factors and coating mitigation of solid particle erosion in steam turbine blade failures: A review
- From magnesium oxide, magnesium oxide concrete to magnesium oxide concrete dams
- Properties and potential applications of polymer composites containing secondary fillers
- A scientometric review on the utilization of copper slag as a substitute constituent of ordinary Portland cement concrete
- Advancement of additive manufacturing technology in the development of personalized in vivo and in vitro prosthetic implants
- Recent advance of MOFs in Fenton-like reaction
- A review of defect formation, detection, and effect on mechanical properties of three-dimensional braided composites
- Non-conventional approaches to producing biochars for environmental and energy applications
- Review of the development and application of aluminum alloys in the nuclear industry
- Advances in the development and characterization of combustible cartridge cases and propellants: Preparation, performance, and future prospects
- Recent trends in rubberized and non-rubberized ultra-high performance geopolymer concrete for sustainable construction: A review
- Cement-based materials for radiative cooling: Potential, material and structural design, and future prospects
- A comprehensive review: The impact of recycling polypropylene fiber on lightweight concrete performance
- A comprehensive review of preheating temperature effects on reclaimed asphalt pavement in the hot center plant recycling
- Exploring the potential applications of semi-flexible pavement: A comprehensive review
- A critical review of alkali-activated metakaolin/blast furnace slag-based cementitious materials: Reaction evolution and mechanism
- Dispersibility of graphene-family materials and their impact on the properties of cement-based materials: Application challenges and prospects
- Research progress on rubidium and cesium separation and extraction
- A step towards sustainable concrete with the utilization of M-sand in concrete production: A review
- Studying the effect of nanofillers in civil applications: A review
- Studies on the anticorrosive effect of phytochemicals on mild steel, carbon steel, and stainless-steel surfaces in acid and alkali medium: A review
- Nanotechnology for calcium aluminate cement: thematic analysis
- Towards sustainable concrete pavements: a critical review on fly ash as a supplementary cementitious material
- Optimizing rice husk ash for ultra-high-performance concrete: a comprehensive review of mechanical properties, durability, and environmental benefits
- Research Articles
- Investigation of the corrosion performance of HVOF-sprayed WC-CoCr coatings applied on offshore hydraulic equipment
- A systematic review of metakaolin-based alkali-activated and geopolymer concrete: A step toward green concrete
- Evaluation of color matching of three single-shade composites employing simulated 3D printed cavities with different thicknesses using CIELAB and CIEDE2000 color difference formulae
- Novel approaches in prediction of tensile strain capacity of engineered cementitious composites using interpretable approaches
- Effect of TiB2 particles on the compressive, hardness, and water absorption responses of Kulkual fiber-reinforced epoxy composites
- Analyzing the compressive strength, eco-strength, and cost–strength ratio of agro-waste-derived concrete using advanced machine learning methods
- Tensile behavior evaluation of two-stage concrete using an innovative model optimization approach
- Tailoring the mechanical and degradation properties of 3DP PLA/PCL scaffolds for biomedical applications
- Optimizing compressive strength prediction in glass powder-modified concrete: A comprehensive study on silicon dioxide and calcium oxide influence across varied sample dimensions and strength ranges
- Experimental study on solid particle erosion of protective aircraft coatings at different impact angles
- Compatibility between polyurea resin modifier and asphalt binder based on segregation and rheological parameters
- Fe-containing nominal wollastonite (CaSiO3)–Na2O glass-ceramic: Characterization and biocompatibility
- Relevance of pore network connectivity in tannin-derived carbons for rapid detection of BTEX traces in indoor air
- A life cycle and environmental impact analysis of sustainable concrete incorporating date palm ash and eggshell powder as supplementary cementitious materials
- Eco-friendly utilisation of agricultural waste: Assessing mixture performance and physical properties of asphalt modified with peanut husk ash using response surface methodology
- Benefits and limitations of N2 addition with Ar as shielding gas on microstructure change and their effect on hardness and corrosion resistance of duplex stainless steel weldments
- Effect of selective laser sintering processing parameters on the mechanical properties of peanut shell powder/polyether sulfone composite
- Impact and mechanism of improving the UV aging resistance performance of modified asphalt binder
- AI-based prediction for the strength, cost, and sustainability of eggshell and date palm ash-blended concrete
- Investigating the sulfonated ZnO–PVA membrane for improved MFC performance
- Strontium coupling with sulphur in mouse bone apatites
- Transforming waste into value: Advancing sustainable construction materials with treated plastic waste and foundry sand in lightweight foamed concrete for a greener future
- Evaluating the use of recycled sawdust in porous foam mortar for improved performance
- Improvement and predictive modeling of the mechanical performance of waste fire clay blended concrete
- Polyvinyl alcohol/alginate/gelatin hydrogel-based CaSiO3 designed for accelerating wound healing
- Research on assembly stress and deformation of thin-walled composite material power cabin fairings
- Effect of volcanic pumice powder on the properties of fiber-reinforced cement mortars in aggressive environments
- Analyzing the compressive performance of lightweight foamcrete and parameter interdependencies using machine intelligence strategies
- Selected materials techniques for evaluation of attributes of sourdough bread with Kombucha SCOBY
- Establishing strength prediction models for low-carbon rubberized cementitious mortar using advanced AI tools
- Investigating the strength performance of 3D printed fiber-reinforced concrete using applicable predictive models
- An eco-friendly synthesis of ZnO nanoparticles with jamun seed extract and their multi-applications
- The application of convolutional neural networks, LF-NMR, and texture for microparticle analysis in assessing the quality of fruit powders: Case study – blackcurrant powders
- Study of feasibility of using copper mining tailings in mortar production
- Shear and flexural performance of reinforced concrete beams with recycled concrete aggregates
- Advancing GGBS geopolymer concrete with nano-alumina: A study on strength and durability in aggressive environments
- Leveraging waste-based additives and machine learning for sustainable mortar development in construction
- Study on the modification effects and mechanisms of organic–inorganic composite anti-aging agents on asphalt across multiple scales
- Morphological and microstructural analysis of sustainable concrete with crumb rubber and SCMs
- Structural, physical, and luminescence properties of sodium–aluminum–zinc borophosphate glass embedded with Nd3+ ions for optical applications
- Eco-friendly waste plastic-based mortar incorporating industrial waste powders: Interpretable models for flexural strength
- Bioactive potential of marine Aspergillus niger AMG31: Metabolite profiling and green synthesis of copper/zinc oxide nanocomposites – An insight into biomedical applications
- Preparation of geopolymer cementitious materials by combining industrial waste and municipal dewatering sludge: Stabilization, microscopic analysis and water seepage
- Seismic behavior and shear capacity calculation of a new type of self-centering steel-concrete composite joint
- Sustainable utilization of aluminum waste in geopolymer concrete: Influence of alkaline activation on microstructure and mechanical properties
- Optimization of oil palm boiler ash waste and zinc oxide as antibacterial fabric coating
- Tailoring ZX30 alloy’s microstructural evolution, electrochemical and mechanical behavior via ECAP processing parameters
- Comparative study on the effect of natural and synthetic fibers on the production of sustainable concrete
- Microemulsion synthesis of zinc-containing mesoporous bioactive silicate glass nanoparticles: In vitro bioactivity and drug release studies
- On the interaction of shear bands with nanoparticles in ZrCu-based metallic glass: In situ TEM investigation
- Developing low carbon molybdenum tailing self-consolidating concrete: Workability, shrinkage, strength, and pore structure
- Experimental and computational analyses of eco-friendly concrete using recycled crushed brick
- High-performance WC–Co coatings via HVOF: Mechanical properties of steel surfaces
- Mechanical properties and fatigue analysis of rubber concrete under uniaxial compression modified by a combination of mineral admixture
- Experimental study of flexural performance of solid wood beams strengthened with CFRP fibers
- Eco-friendly green synthesis of silver nanoparticles with Syzygium aromaticum extract: characterization and evaluation against Schistosoma haematobium
- Predictive modeling assessment of advanced concrete materials incorporating plastic waste as sand replacement
- Self-compacting mortar overlays using expanded polystyrene beads for thermal performance and energy efficiency in buildings
- Enhancing frost resistance of alkali-activated slag concrete using surfactants: sodium dodecyl sulfate, sodium abietate, and triterpenoid saponins
- Equation-driven strength prediction of GGBS concrete: a symbolic machine learning approach for sustainable development
- Empowering 3D printed concrete: discovering the impact of steel fiber reinforcement on mechanical performance
- Advanced hybrid machine learning models for estimating chloride penetration resistance of concrete structures for durability assessment: optimization and hyperparameter tuning
- Influence of diamine structure on the properties of colorless and transparent polyimides
- Post-heating strength prediction in concrete with Wadi Gyada Alkharj fine aggregate using thermal conductivity and ultrasonic pulse velocity
- Experimental and RSM-based optimization of sustainable concrete properties using glass powder and rubber fine aggregates as partial replacements
- Special Issue on Recent Advancement in Low-carbon Cement-based Materials - Part II
- Investigating the effect of locally available volcanic ash on mechanical and microstructure properties of concrete
- Flexural performance evaluation using computational tools for plastic-derived mortar modified with blends of industrial waste powders
- Foamed geopolymers as low carbon materials for fire-resistant and lightweight applications in construction: A review
- Autogenous shrinkage of cementitious composites incorporating red mud
- Mechanical, durability, and microstructure analysis of concrete made with metakaolin and copper slag for sustainable construction
- Special Issue on AI-Driven Advances for Nano-Enhanced Sustainable Construction Materials
- Advanced explainable models for strength evaluation of self-compacting concrete modified with supplementary glass and marble powders
- Analyzing the viability of agro-waste for sustainable concrete: Expression-based formulation and validation of predictive models for strength performance
- Special Issue on Advanced Materials for Energy Storage and Conversion
- Innovative optimization of seashell ash-based lightweight foamed concrete: Enhancing physicomechanical properties through ANN-GA hybrid approach
- Production of novel reinforcing rods of waste polyester, polypropylene, and cotton as alternatives to reinforcement steel rods