Nano-boosted concrete: revolutionizing strength and durability for modern constructionABSTRACT
Nanotechnology is being investigated as a potential solution to enhance the strength and durability of high-strength concrete (HSC). The aim of this paper is to provide high-strength concrete, silica fume has used to provide matrix densification of concrete with high silica content and pozzolanic reactivity, and metakaolin which is well known for its high purity and fine particle sizes. Totally 13 mixes were prepared one with control mix and 12 mixes with the variation of metakaolin and silica upto 20% replacement individually with the increment of 5% from 5% to 20% in OPC. The results show that silica fume and metakaolin used in combination has been significantly effective not only in increasing the engineering properties of HSCs but also in significantly increasing the resistance to environmental damage. The findings suggest that using these nanomaterials includes in concrete mixes, which can lead to more flexible industries underlining the potential that could revolutionize concrete by providing a way to manufacturing better building materials that last longer and reduce maintenance costs.
Keywords:
Silica fume; Metakaolin; Strength; Durability; Numerical approach
1. INTRODUCTION
The building sector is continuously searching for innovative and novel materials to enhance concrete’s performance. High-strength concrete, or HSC, is a crucial component of a contemporary structure because of its exceptional mechanical qualities and lengthy life [1]. The application of nanotechnology, such as the use of silica fume (SF) and metakaolin (MK), had non-table effects on HSC viability and lifespan and showed encouraging outcomes in increasing these features [2]. The combination of SF and MK greatly enhances the mechanical qualities of concrete. For instance, it has been demonstrated that SF combined with MK binary and ternary with nano silica improves the elasticity, flexural toughness, and compressive rigidity of ultra-high performance concrete (UHPC) [3]. Use of graphene oxide have attempted for the assessment of the performance improvement on workability, strength and stability of concrete [4]. This synergy between SF, MK and nano silica concrete in the matrix provides a denser microstructure, which strengthens the environmental resistance and extends the life of the product, such advances in concrete technology are essential for improved durability for a long time to endure [5].
It has been found that the optimum replacement rate of metakaolin (MK), ranging from 10% to 15%, will give the best mechanical performance This rate increases the tensile strength of concrete, and leads to overall system integrity and high strength results in high strength concrete (HSC) mix [6]. Bentonite have replaced in cement as a binder in concrete for the performance evaluation characteristics [7]. In addition to the use of metakaolin was found to improve early compressive strength by up to 123%. This significant increase in early strength is attributed to the high reactivity and ability of metakaolin to repair the concrete microstructure, resulting in a strong and stable matrix which is particularly important for the long-term performance of concrete at chaotic environments [8]. Research has demonstrated that adding SF and metakaolin to concrete strengthens its defense against sulfate, acid, and chloride ion penetration [8]. This improved strength is important for buildings, as it extends the life of concrete and reduces maintenance costs. It can also achieve sustainable projects, which can meet the increasing demands for performance and sustainability function [9].
These particles provide a filler effect that strengthens concrete’s resistance to chemical attack and increases overall durability. These particles improve the waterproofing of concrete by reducing pore structure and filling microvoids, providing strength resistance to harsh environmental conditions is enhanced, prolonging its service life [10]. Concrete bond strength was found to enhance the corrosion resistance of environmental factors, metakaolin and nano silica fumes by (NSF) are used, increasing the longevity of the material. These nanoparticles improve ITZ between cement and paste aggregates [11].
The best HSC performance is also achieved by the ratio of the specific combination of SF and MK. For example, to get the greatest results in terms of strength and durability, three mixes of 10% SF and 10% MK are advised [12]. Through the synergistic actions of SF and MK, this compact combination improves the mechanical characteristics and durability of concrete [13]. Concrete technology has significantly advanced; for example, it is now more resistant to ion penetration by acids, sulfates, and chlorides, and its qualities have been enhanced by NMK [14].
The combination of 6% silica fume (SF) and 15% metakaolin (MK) has been found to be effective in improving the physical and durability properties of concrete. This particular mixture for pozzolanic reactions and filler effects good, resulting in increased tensile strength, as well as increased resistance to chemical attack and environmental degradation [15]. Silica fume (SF) and Metakaolin (MK); incorporation into HSC is a topic of extensive research due to its ability to enrich the durability and strength of concrete [16, 17, 18]. These filler cementitious materials (SCMs) are known for their pozzolanic properties helps for improving performance. Mixing concrete silica fume (SF) with metakaolin (MK) significantly increases compressive strength, flexural strength and elasticity parameters, ternary mixes show the best results. This synergistic combination for concrete microstructure is better, resulting in better mechanical properties compared to binary mixes or unmodified concrete and improvement [19].
The combination of silica fume (SF) and metakaolin (MK) improves water content, thereby reducing leakage and increasing resistance to acid and sulphate attack. Such pore cleaning this improvement reduces the permeability, thereby protecting concrete from the penetration of harmful chemicals and extending its durability and strength [20]. In order to obtain the best results in terms of how durable inside, a mixture containing 10% silica fume (SF) and 10% metakaolin (MK) is recommended. This optimal mixture maximizes the advantages of the cohesive nanomaterials, providing strain absorbing strength surface, improve pore structure and enhance resistance to chemical attack and environmental degradation [21]. The use of 15% metakaolin (MK) and 5–10% silica fume (SF) showed significant improvements in compressive strength and durability properties This grade provides flexibility to improve the workability of concrete, increasing its mechanical strength, resistant to both environmental and longevity [22].
Since metakaolin (MK) and silica fume (SF) re-lower the embodied carbon in the concrete mix, they can lessen the environmental effect of concrete. The quantity of carbon emissions related to the production of cement is decreased when these extra cementitious materials are utilized to partially substitute cement [23]. The long-term advantages in terms of durability and lower maintenance costs may outweigh the initial expenses related to adding metakaolin (MK) and silica fume (SF). These additives reduce the need for regular repairs and replacements by strengthening and prolonging the concrete’s lifespan [24, 25]. Blending silica fume and metakaolin in high-strength concrete is a proven method to enhance both durability and strength. The optimal blend typically includes around 10% of each material, and proper curing techniques are vital to achieving the best results [26].
The compressive strength of high-strength concrete is greatly increased by the combination of metakaolin and silica fume. Compressive strength was best in mixtures with 7.5% silica fume and 10% metakaolin and its predicted model using anova [27, 28]. For ultra-high performance concrete (UHPC), the greatest results were obtained when nano silica and metakaolin were combined in a binary fashion, and when nano silica, metakaolin, and silica fume were combined in a ternary fashion [29, 30]. The incorporation of mineral additives in concrete mix designs to achieve high concrete strength. Silica fume, metakaolin, and superplasticizer were blended with conventional cement-aggregate mixes in various proportions, targeting a compressive strength of approximately 60 N/mm2 [31]. Attempts also made for the computations of performance improvement in porous concrete [32]. Regression models were then employed to analyze the experimental data, generating predictive models for 7, 14, and 28 days. The key finding revealed that at a 10% replacement level, concrete with metakaolin attained a 28-day strength of 76.04 N/mm2, while silica fume yielded 73.76 N/mm2 at the same replacement level. The average error between experimental and predicted data stood at approximately 3.85% [33]. Based on the above insight this study focuses on the incorporation of nano silica and metakaolin, two prominent nanomaterials, into HSC to assess their impact on its mechanical and durability properties.
2. MATERIALS AND METHODS
Cement is a fundamental material in construction, and the physical and chemical composition of OPC 53 grade significantly influences its properties and performance. OPC 53 grade cement is grey in color and has a specific gravity of 3.14, indicating it is denser than water. The surface area of this cement is 2250 cm2/g, which affects its reactivity and strength development. In its physical state, it is a solid with particles averaging less than 90 microns in size. The volume expansion of OPC 53 grade cement is 3 mm, which is vital in determining the soundness and durability of concrete structures. Chemically, OPC 53 grade cement comprises several oxides, each contributing to its overall properties. The primary component is calcium oxide (CaO), making up 61.34% of the composition. The hydration events that produce calcium silicate hydrate, which gives cement its strength, depend on this high CaO concentration. Making about 20.15%, silicon dioxide (SiO2) aids in the development of calcium silicate phases. At 4.51% and 2.57%, respectively, aluminum oxide (Al2O3) and ferric oxide (Fe2O3) are present. They help generate different calcium aluminates and ferrites, which affect the cement’s setting time and strength growth. While the loss on ignition, a sign of the existence of volatile compounds, is 2.45%, magnesium oxide (MgO) is found at 1.05%.
The characteristics of river sand, a prevalent fine aggregate in concrete mixtures, significantly influence the final product. This type of sand exhibits a grainy texture, with a relative density of 2.55 and a bulk density of 2.55 g/cc. Its water absorption rate is 1.50%, impacting the w/c ratio and, thereby, the concrete’s workability and strength. The moisture content is recorded at 1.28%, reflecting the amount of inherent water before mixing. Classified as Zone II, this sand has a white hue and a fineness modulus of 2.72, which assesses the average particle size and gradation. The average grain size is 1.13 mm, ensuring effective packing and reducing voids in the concrete mix.
Coarse aggregate properties are equally important for the concrete mix. The aggregates are angular and have a size of 12.5 mm. With a relative density of 2.65, these aggregates are slightly denser than water. Their water absorption rate is 1.04%, which impacts the concrete’s workability and mix design. The crushing value is 17.54%, indicating the resistance of the aggregates to crushing under gradually applied compressive loads, while the impact strength is 14.69%, reflecting the aggregates’ resistance to impact loads. The bulk density of coarse aggregates is 1620 kg/m3, which influences the weight and density of the concrete. The moisture content is 0.8%, and the fineness modulus is 6.8%, which measures the coarseness of the aggregate particles. Chemically, coarse aggregates contain various oxides. Silicon dioxide (SiO2) is the most significant component at 55.7%, contributing to the aggregate’s hardness and strength. Calcium oxide (CaO) is 13.33%, which can react with atmospheric carbon dioxide over time, potentially influencing durability. Magnesium oxide (MgO) constitutes 9.58%, and aluminum oxide (Al2O3) is 0.77%, which may affect the thermal properties of the aggregates. Other minor components include sodium oxide (Na2O) at 0.14%, potassium oxide (K2O) at 0.09%, and ferric oxide (Fe2O3) at 0.37%. These minor constituents can influence the durability and chemical reactivity of the aggregates.
Nano silica is another material that can enhance concrete properties. It is white in color and has a specific surface area of 202 m2/g, significantly higher than traditional cement particles, which means it has a high reactivity due to the large surface area available for chemical reactions. The relative density of nano silica is 1.20, much lower than that of cement and aggregates, which indicates its lightweight nature. When added to concrete, nano silica can fill the micro-pores and refine the microstructure, leading to improved strength and durability.
Metakaolin is a pozzolanic material with a specific gravity of 3.05 and chemical composition primarily consisting of 53.0% silicon dioxide (SiO2) and 43.0% aluminum oxide (Al2O3), which contribute to its high reactivity and ability to enhance concrete strength and durability. It contains minor amounts of ferric oxide (Fe2O3) at 0.5%, calcium oxide (CaO) at 0.1%, sulfur trioxide (SO3) at 0.1%, sodium oxide (Na2O) at 0.1%, and potassium oxide (K2O) at 0.4%. The loss on ignition is 0.7%, indicating minimal volatile components. This composition makes metakaolin effective in improving the performance of cementitious materials.
3. METHODOLOGY
Nanotechnology is being researched as a potential means of enhancing the strength and durability of high-strength concrete (HSC). This paper aims to assess the impact of two widely used nanomaterials on the mechanical and durability properties of HSC: metakaolin and silica fume. Using metakaolin, which is renowned for its high purity and small particle size, and silica fume, which has a high silica content and pozzolanic reactivity, may greatly increase the densification of concrete. This study assesses the durability attributes, including resistance to sulfate attack and chloride penetration, as well as the compressive and tensile strengths, using a number of experimental and computational methods. The findings show that the combined application of metakaolin and silica fume improves HSC’s resilience to environmental degradation while also increasing the material’s compressive and tensile strengths.
The carefully chosen resources that form the basis of this research guarantee the dependability and the findings of the OPC, which is renowned for its excellent binding qualities and compatibility with a variety of additives and admixtures, is the principal binder utilized. Natural sand provides the required fine particles for the matrix, while crushed granite aggregates are selected for their exceptional mechanical qualities. Aggregates are an important aspect of mix design. Uncontaminated potable water guarantees steadiness in hydration and strength building. The two main nanomaterials used in this investigation are metakaolin and silica fume. Because of its tiny particle size and high purity, metakaolin—which is produced by calcining kaolinite clay—has the potential to enhance the microstructure of concrete and exhibit pozzolanic activity. The w/c ratio, which is maintained at 0.30 for all combinations, is not compromised by the use of superplasticizers of 1.25% and water-reducing agents of 0.5%, which guarantee that the HSC mix remains workable. Conventional mix proportions of 1:1.38:2.85.
The first step in the experimental phase is to produce several concrete mixes with variable proportions of metakaolin and silica fume. The percentages are modified by cement weight to 0%, 5%, 10%, 15%, and 20% in order to get the optimal dosage for the desired outcomes. Each mix is thoroughly mixed using a mechanical mixer to ensure that the nanoparticles are evenly dispersed throughout the concrete matrix. The mixes are then poured into molds to produce standard cylindrical specimens for compressive strength testing and prisms for tensile strength testing. Seven, fourteen, and twenty-eight-day curing durations are selected to evaluate the evolution of mechanical properties and durability over time. This methodical technique makes it possible to evaluate the HSC mixtures’ long-term and early-age performance.
Tests for both tensile and compressive strength are necessary to assess the mechanical characteristics of high-strength concrete (HSC) mixes. In accordance with ASTM C39/C39M standards, cylindrical specimens of 150 mm in diameter and 300 mm in height are used to calculate compressive strength. The accuracy and comparability of the outcomes are guaranteed by this procedure. The split-cylinder test, as outlined in ASTM C496/C496M, is used to determine tensile strength. For structural applications, this testing offers crucial information regarding the concrete’s resistance to different loads and stresses. Testing for durability is also necessary to evaluate how well HSCs operate in challenging environments. The ASTM C1202-recommended Rapid Chloride Permeability Test (RCPT) gauges a concrete specimen’s resistance to chloride ion penetration by passing an electrical charge through it. For buildings exposed to sea conditions or deicing salts, where corrosion of reinforcement caused by chloride is a serious problem, this test is essential. According to ASTM C1012, resistance to sulfate attack is evaluated by submerging specimens in a 5% sodium sulfate solution and tracking mass loss and visual degradation over time. This mimics groundwater and soil conditions that are high in sulfate, which can lead to expansion reactions and the deterioration of concrete. The mix proportions are displayed in Table 1.
4. RESULTS AND DISCUSSION
4.1. Slump cone test
The impact of incorporating silica fume and metakaolin on the workability of HSC, as measured by slump values. The slump test indicates concrete’s flowability, with higher values suggesting better workability. The control mix (M1), representing conventional concrete without nanomaterials, has a slump value of 85 mm, serving as a baseline. Adding 5% silica fume and 5% metakaolin (M2) increases the slump to 89 mm, indicating improved workability due to the fine particle size and pozzolanic activity of the nanomaterials, which enhance particle packing and reduce water demand. Increasing silica fume to 10% while maintaining 5% metakaolin (M3) yields the highest slump value of 90 mm, suggesting an optimal mix. However, further increasing silica fume to 15% (M4) slightly reduces the slump to 88 mm, and at 20% (M5), it returns to 85 mm, indicating diminishing returns at higher silica fume contents.
Similar trends are observed with 10% metakaolin mixes. With 5% silica fume (M6), the slump value remains at 89 mm, and at 10% silica fume (M7), it reaches 90 mm. However, increasing silica fume to 15% (M8) and 20% (M9) reduces the slump to 87 mm and 84 mm, respectively. These results suggest that combining 10% metakaolin with around 10% silica fume provides the best workability. For mixes with 15% metakaolin, the trend of decreasing workability is more pronounced. M10 (5% silica fume) has a slump of 86 mm, slightly lower than the control, while M11 (10% silica fume) drops to 84 mm. M12 (15% silica fume) and M13 (20% silica fume) further decrease to 80 mm and 75 mm, respectively. This significant reduction highlights the challenge of maintaining workability at high levels of both nanomaterials, as increased surface area and water demand make the mix more cohesive and less flowable.
It indicates that silica fume and metakaolin enhance HSC workability at moderate levels. The optimal workability is achieved with 5% to 10% of each additive. However, higher contents significantly reduce workability. Figure 1 shows the slump cone test results of various mix.
4.2. Compaction factor
The influence of silica fume and metakaolin on the compaction factor and overall workability of high-strength concrete (HSC). The compaction factor is a critical measure of concrete’s workability, indicating how easily the concrete can be compacted to ensure quality and durability. The control mix (M1), consisting of conventional concrete without nanomaterials, had a compaction factor of 0.825, serving as a baseline. In mixes M2 to M5, where 5% metakaolin was combined with increasing silica fume (5%, 10%, 15%, and 20%), notable improvements in workability were observed. Mix M2 (5% silica fume and 5% metakaolin) and M3 (10% silica fume and 5% metakaolin) exhibited compaction factors of 0.864 and 0.874, respectively, suggesting that the fine particle size and high pozzolanic activity of these nanomaterials enhance the concrete’s flow and compaction. However, increasing silica fume to 15% in M4 reduced the compaction factor to 0.854, and at 20% in M5, it dropped back to 0.825, indicating that excessive silica fume increases water demand, negating initial benefits.
Similar trends were observed with 10% metakaolin mixes. M6 (5% silica fume and 10% metakaolin) and M7 (10% silica fume and 10% metakaolin) had compaction factors of 0.864 and 0.874, respectively. However, increasing silica fume to 15% in M8 reduced the compaction factor to 0.845, and at 20% in M9, it further dropped to 0.816. This suggests that higher silica fume content, even with 10% metakaolin, negatively affects workability due to increased water demand. For mixes with 15% metakaolin, the decline in workability was more pronounced. M10 (5% silica fume) had a compaction factor of 0.835, while M11 (10% silica fume) dropped to 0.816. M12 (15% of both) further decreased to 0.777, and M13 (20% silica fume and 15% metakaolin) showed the lowest compaction factor of 0.728. These results indicate that higher levels of both nanomaterials significantly reduce workability, making the mix more cohesive and harder to compact due to increased surface area and water demand.
Silica fume and metakaolin can enhance the physical properties and durability of HSC, their proportions need to be optimized to maintain workability. Optimal workability is achieved with 5% to 10% of each nanomaterial, balancing enhanced performance and practical usability. Figure 2 shows the compaction factor test results of various mix.
4.3. Compressive strength
The compressive strength of concrete mixes incorporating varying proportions of silica fume and metakaolin at 7, 14, and 28 days. Conventional concrete (M1) served as the baseline with compressive strengths of 46.68 MPa, 58.36 MPa, and 66.57 MPa at the respective intervals. The addition of silica fume and metakaolin generally improved compressive strength. Mixes M2 through M5, containing 5% to 20% silica fume with 5% metakaolin, showed an increasing trend in strength, with M4 (15% silica fume + 5% metakaolin) achieving the highest strength of 72.74 MPa at 28 days. Mixes M6 through M9, combining 5% to 20% silica fume with 10% metakaolin, displayed higher early strengths but did not significantly surpass the 5% metakaolin mixes at 28 days. Notably, M7 (10% silica fume + 10% metakaolin) reached 71.67 MPa at 28 days. Mixes M10 through M13, with 15% metakaolin, showed comparable early strengths but slightly lower 28-day strengths, with M13 (20% silica fume + 15% metakaolin) having the lowest strength of 58.95 MPa at 28 days. The results indicate that moderate levels of silica fume (10-15%) combined with lower metakaolin content (5–10%) yield the best compressive strengths, as demonstrated by M4 and M7. Conversely, higher metakaolin content beyond 10% with increasing silica fume resulted in a reduction in strength, suggesting a threshold beyond which the benefits diminish. The inclusion of silica fume and metakaolin enhances concrete performance, with optimal results seen in balanced combinations. Figure 3 shows the compressive strength test.
4.4. Split tensile strength
The split tensile strength of concrete mixes with varying proportions of silica fume and metakaolin at 7, 14, and 28 days. Conventional concrete (M1) served as the baseline with strengths of 3.51 MPa, 3.91 MPa, and 4.19 MPa, respectively. The addition of silica fume and metakaolin generally improved tensile strength, with mixes M2 through M5 (5% to 20% silica fume and 5% metakaolin) showing an increasing trend. Notably, M4 (15% silica fume + 5% metakaolin) achieved the highest strength of 4.60 MPa at 28 days. Mixes M6 through M9 (5% to 20% silica fume and 10% metakaolin) displayed higher early strengths but did not significantly surpass the 5% metakaolin mixes at 28 days, with M7 (10% silica fume + 10% metakaolin) reaching 4.53 MPa. Mixes M10 through M13, containing 15% metakaolin, showed slightly lower 28-day strengths,with M13 (20% silica fume + 15% metakaolin) having the lowest strength of 3.73 MPa at 28 days. Optimal results were found in mixes with moderate levels of silica fume (10–15%) and lower metakaolin content (5–10%), specifically M4 and M7. It indicates that use of silica fume and metakaolin enhance split tensile strength, higher metakaolin content beyond 10% diminish these benefits. Figure 4 shows the split tensile strength test for various mixes.
4.5. Flexural strength
The flexural strength of concrete mixes incorporating varying proportions of silica fume and metakaolin at 7, 14, and 28 days. Conventional concrete (M1) served as the baseline with flexural strengths of 4.26 MPa, 5.79 MPa, and 6.61 MPa at the respective intervals. The addition of silica fume and metakaolin generally enhanced the flexural strength. Mixes M2 through M5, which included 5% to 20% silica fume with 5% metakaolin, exhibited an increasing trend in strength. Specifically, M4 (15% silica fume + 5% metakaolin) achieved the highest flexural strength across all ages, reaching 7.22 MPa at 28 days, a substantial improvement over the conventional mix. Mixes M6 through M9, combining 5% to 20% silica fume with 10% metakaolin, displayed higher early strength (7 days) compared to their 5% metakaolin counterparts but did not significantly surpass them at 28 days. Among these, M7 (10% silica fume + 10% metakaolin) stood out with a notable 7.12 MPa at 28 days. Mixes M10 through M13, incorporating 15% metakaolin with varying silica fume content, exhibited a different trend; while early strengths were comparable to other groups, the 28-day strengths were slightly lower. Notably, M12 and M13 with higher metakaolin content (15%) and varying silica fume showed a decline in flexural strength, with M13 (20% silica fume + 15% metakaolin) showing the lowest strength across all periods, reducing to 5.85 MPa at 28 days. From these results, it can be inferred that moderate levels of silica fume (10–15%) combined with lower metakaolin content (5–10%) yielded the best flexural strengths, with M4 and M7 being the top performers. The inclusion of silica fume and metakaolin enhances flexural strength, but higher metakaolin content beyond 10% with increasing silica fume can lead to a reduction in strength, indicating a threshold beyond which the benefits diminish. Figure 5 shows the flexural strength test.
4.6. Sulphate attack test
The percentage loss in compressive strength of several mixes of concrete exposed to sulphate attack. The control mix, ordinary concrete (M1), showed a decrease in compressive strength of 5.08%. The percentage of compressive strength loss in concrete was significantly decreased by the addition of metakaolin and silica fume, which improved the concrete’s resistance to sulphate assault. Mixes M2 through M5, which included 5% to 20% silica fume with 5% metakaolin, showed a consistent improvement in sulphate resistance. Notably, M4 (15% silica fume + 5% metakaolin) exhibited the lowest percentage loss at 4.65%, indicating superior performance in maintaining compressive strength under sulphate exposure. Mixes M6 through M9, combining 5% to 20% silica fume with 10% metakaolin, also demonstrated enhanced sulphate resistance compared to the control mix. Among these, M7 (10% silica fume + 10% metakaolin) achieved the lowest percentage loss at 4.60%, suggesting that an increase in metakaolin content from 5% to 10% can further improve sulphate resistance. Mixes M10 through M13 incorporated 15% metakaolin with varying silica fume content and presented a mixed trend. While some, like M10 and M11, showed reasonable performance with percentage losses around 4.65% and 4.78%, respectively, others, such as M12 and M13 with higher silica fume content (15–20%), exhibited increased losses of 5.22% and 5.61%. This suggests that extremely high levels of metakaolin combined with high silica fume content may not be as effective in enhancing sulphate resistance. The inclusion of silica fume and metakaolin generally improves concrete’s resistance to sulphate attack, with optimal performance observed in mixes containing moderate levels of both additives. Mixes M4 (15% silica fume + 5% metakaolin) and M7 (10% silica fume + 10% metakaolin) were particularly effective, showing the lowest percentage losses. However, excessively high metakaolin content combined with high silica fume may not provide additional benefits and could potentially reduce the concrete’s sulphate resistance. Figure 6 shows the sulphate attack test.
4.7. Rapid Chloride Penetration Test (RCPT)
RCPT revealed interesting insights into the chloride ion permeability of concrete mixes incorporating silica fume and metakaolin. Across all mixes, there was a general trend of decreased total charge passed during the 28-day testing period compared to conventional concrete (M1), indicating improved resistance to chloride penetration. Mixes containing higher percentages of silica fume and metakaolin generally exhibited lower total charges, suggesting enhanced durability against chloride ingress. Notably, mix M7 with 10% silica fume and 10% metakaolin showed the lowest total charge at 28 days, with a value of 1531 Coulombs. This indicates that the combination of silica fume and metakaolin at moderate proportions effectively reduces the permeability of concrete to chloride ions, potentially attributed to the pozzolanic reactions and densification of the microstructure. However, it’s interesting to note that mixes M9 and M13, which contained 20% silica fume and 10% metakaolin, and 20% silica fume and 15% metakaolin respectively, exhibited slightly higher total charges compared to some other mixes. This could suggest a threshold beyond which further increases in supplementary materials may not proportionally enhance chloride resistance, possibly due to changes in pore structure or other factors affecting concrete microstructure. Findings highlight the potential of silica fume and metakaolin as effective additives in mitigating chloride-induced corrosion in concrete structures. Figure 7 shows the rapid chloride penetration test results.
4.8. SEM analysis
The SEM analysis of conventional concrete (M1) and concrete with 10% silica fume and 10% metakaolin (M7) reveals notable differences in their elemental composition, reflecting the impact of the supplementary materials on the concrete’s microstructure. Oxygen (O) content increased from 51.20% to 53.50% (weight %) and from 60.40% to 62.10% (atomic %) in M7 compared to M1. This increase is consistent with the higher oxygen content in silica fume and metakaolin, suggesting an overall increase in the oxygen-rich phases in M7. Silicon (Si) levels rose from 13.75% to 15.20% (weight %) but decreased slightly from 10.12% to 11.05% (atomic %). This indicates that while the silica content contributed to a higher weight percentage, its atomic fraction decreased, likely due to the dilution effect of other elements in the mix. Calcium (Ca) content decreased from 13.85% to 12.50% (weight %) and from 8.25% to 7.75% (atomic %). The reduction in calcium aligns with the partial replacement of traditional cementitious materials by silica fume and metakaolin, which contain less calcium. Carbon (C) showed a slight decrease from 9.65% to 8.95% (weight %) and a minimal decrease from 18.51% to 17.85% (atomic %). This reduction may be attributed to the lower carbon content in the supplementary materials compared to conventional cement. Aluminum (Al) increased from 3.99% to 4.10% (weight %) and from 3.21% to 3.51% (atomic %). This rise is consistent with the higher aluminum content in metakaolin, contributing to the overall increase in aluminum in M7. Sodium (Na) and Iron (Fe) exhibited slight variations in both weight and atomic percentages, which are minimal and likely within the range of experimental error. Figures 8 and 9 shows the SEM image of M1 mix and M7 mix.
5. NUMERICAL APPROACH
5.1. Workability parameter – slump cone test
The findings of the slump cone test, which is frequently used to evaluate the consistency of newly mixed concrete, are shown in Table 2 as an ANOVA (Analysis of Variance). The source of variation, sum of squares (SS), degrees of freedom (df), mean squares (MS), F-value, P-value, and critical F-value (F crit) are among the important statistical metrics included in the table. The rows have a total sum of squares of 109.6154, 12 degrees of freedom, and an F-value of 1. The mean square for the rows is determined to be 9.134615. The P-value of 0.5 indicates that there may not be a significant difference between the rows, since it is greater than the vital value of 2.686637. Likewise, with one degree of freedom, the sum of squares for the columns is 1.884615, which yields a mean square of 1.884615 and an F-value of 0.206316. The P-value of 0.657777 suggests that there is no significant difference between the columns, since it is greater than the critical threshold of 4.747225. The erroneous sum of squares with 12 degrees of freedom is 109.6154, which adds up to a total sum of squares with 25 degrees of freedom of 221.1154. These findings imply that there is no statistically significant difference between rows and columns in the slump cone test.
5.2. Strength parameter – compressive strength
The findings of an ANOVA (Analysis of Variance) performed on compressive strength testing are shown in Table 3. The source of variation, sum of squares (SS), degrees of freedom (df), mean squares (MS), F-value, P-value, and the critical F-value (F crit) are among the significant statistical metrics included in the table. The rows have a mean square of 31.04472, an F-value of 71.37559, and a total sum of squares of 372.5366, with 12 degrees of freedom. The P-value for the rows is 9.16E-16, suggesting a very significant difference between them, and well below the crucial threshold of 2.18338. With two degrees of freedom and a total sum of squares of 2702.717 for the columns, the F-value is 3106.939 and the mean square is 1351.359. The P-value for the columns is 1.05E-29, which indicates a highly significant variation among the columns and is significantly lower than the critical value of 3.402826. The erroneous sum of squares with 24 degrees of freedom is 10.43877, which adds up to a total sum of squares of 3085.693 across 38 degrees of freedom. According to these findings, the compressive strength test’s row and column variances are both statistically significant.
5.3. Durability parameter
5.3.1. Sulphate attack test
Table 4 presents the ANOVA (Analysis of Variance) results for the sulfate attack test. The table includes key statistical metrics such as source of variation, sum of squares (SS), degrees of freedom (df), mean squares (MS), F-value, P-value, and the critical F-value (F crit). For rows, the analysis shows 12 degrees of freedom and a total sum of squares of 0.483872. This results in a mean square of 0.040323 and an F-value of 1. The P-value of 0.5, which exceeds the critical value of 2.686637, indicates no significant variation across rows. Conversely, the columns have a sum of squares of 154.9423 with one degree of freedom, leading to a mean square of 154.9423 and an F-value of 3842.562. The P-value for columns is 2.06E-16, substantially lower than the critical value of 4.747225, highlighting a highly significant difference across columns. With 12 degrees of freedom, the error sum of squares is 0.483872, contributing to a total sum of squares of 155.9101 across 25 degrees of freedom. These results suggest that variations in columns are highly significant, while variations in rows are not statistically significant in the sulfate attack test.
5.3.2. Rapid chloride penetration test
Table 5 provides the ANOVA (Analysis of Variance) results for the fast chloride penetration test. The table includes several key statistical metrics: source of variation, sum of squares (SS), degrees of freedom (df), mean squares (MS), F-value, P-value, and the critical F-value (F crit). For rows, with 12 degrees of freedom, the total sum of squares is 80,775.54, resulting in a mean square of 6,731.295 and an F-value of 1. The P-value of 0.5, which is higher than the critical value of 2.686637, indicates no significant variation among rows. Conversely, for columns, the sum of squares is 598,334.3 with one degree of freedom, leading to a mean square of 598,334.3 and an F-value of 88.88843. The P-value of 6.74E-07 for columns, which is much lower than the critical threshold of 4.747225, indicates a highly significant variance across columns. With 12 degrees of freedom, the error sum of squares is 80,775.54, contributing to a total sum of squares of 759,885.4 across 25 degrees of freedom. These results demonstrate that differences among columns are extremely significant, whereas variations among rows are not statistically significant in the fast chloride penetration test.
6. CONCLUSION
The incorporation of silica fume and metakaolin in concrete mixes significantly influences their properties. Mixes with higher percentages of silica fume and metakaolin generally demonstrate improved compressive, tensile, and flexural strengths. Specifically, Mixes M4 (15% Silica fume + 5% Metakaolin) and M7 (10% Silica fume + 10% Metakaolin) consistently outperform others in compressive strength and flexural strength tests, highlighting the benefits of these additives in enhancing concrete durability and performance. However, mixes with the highest metakaolin content (M12 and M13) exhibit lower workability, as indicated by reduced slump values and compaction factors. The optimal combination appears to be around 10–15% silica fume and 5–10% metakaolin, balancing strength and workability effectively.
The incorporation of silica fume and metakaolin in concrete significantly reduces the percentage loss of compressive strength after sulphate attack and the total charge passed in the RCPT. Mixes with 10–15% silica fume and 5–10% metakaolin, such as M4 and M7, demonstrate the best resistance to sulphate attack and chloride penetration, suggesting enhanced durability. However, higher metakaolin content (M12 and M13) slightly increases vulnerability, as indicated by higher loss percentages and charge values. The incorporation of silica fume and metakaolin in M7 leads to changes in the elemental composition, reflecting the specific contributions of these materials to the concrete’s microstructural characteristics. The increase in silicon and aluminum, along with the decrease in calcium, highlights the influence of these supplementary materials on the concrete’s.
The ANOVA results for the slump cone test, compressive strength test, sulphate attack test, and rapid chloride penetration test reveal significant insights. For both the slump cone and rapid chloride penetration tests, row variations are not statistically significant, indicating consistent performance across different samples. However, column variations in the compressive strength and sulphate attack tests are highly significant, suggesting substantial differences between the tested conditions. Notably, the rapid chloride penetration test shows significant column variation, underscoring its sensitivity to varying conditions.
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