Enhancing concrete with SCMs: unveiling the pros and cons of fly ash, silica fume, and slagAbstract
The performance of concrete that contains different amounts of fly ash, silica fume, and ground granulated blast furnace slag (GGBS) as partial cement replacements is examined in this study. Workability, water absorption, weight loss, strength loss, compressive strength, split tensile strength, flexural strength, and chloride ion permeability were assessed for 17 mix formulations at 7, 14, 28, 56, and 90 days. The Rapid Chloride Permeability Test (RCPT) was used to evaluate durability, while scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to analyze the microstructure and phase composition, respectively. With a compressive strength of 85.69 MPa at 28 days, split tensile strength of 5.82 MPa, flexural strength of 9.64 MPa, and water absorption of 1.21% after 90 days, the ideal mixture of 7.5% fly ash, 7.5% silica fume, and 7.5% GGBS demonstrated the optimum performance. Losses in weight and strength were negligible, at 0.85% and 2.37%, respectively. There was little chloride permeability (815 Coulombs). A dense microstructure was seen by SEM, and further C-S-H development was verified by XRD. A score of −2.45% from the Percent Bias (PBIAS) study indicated that the model prediction was accurate. All things considered, blended cementitious ingredients improve mechanical and durability qual-ities while encouraging environmentally friendly concrete production by using less cement.
Keywords:
Flyash; Silica fume; GGBS; Micro analysis
1. INTRODUCTION
Due to its affordability, robustness, and adaptability, concrete is the most frequently used building material. However, the manufacturing of Portland cement, a vital component of concrete, has serious negative effects on the environment, including high energy consumption and notable CO2 emissions. To address these problems, the construction sector has been utilizing SCMs including fly ash, silica fume, and slag more and more. By partially substituting these minerals for Portland cement, concrete’s performance and durability are improved while its environmental impact is decreased. This article examines how SCMs can enhance the qualities of concrete, as well as the advantages they have for the environment and the economy, the drawbacks of using them, and potential avenues for further study and implementation.
Finely ground substances known as supplemental cementitious materials (SCMs) have pozzolanic or hydraulic qualities when mixed with Portland cement and water [1]. While hydraulic materials, like slag, have natural cementing qualities, pozzolanic materials, including fly ash and silica fume, react with Ca(OH)2 in the presence of water to create additional cementitious compounds [2]. Because SCMs can increase durability, improve mechanical qualities, and lessen the environmental impact of concrete manufacturing, their usage in concrete has drawn a lot of interest [3].
A byproduct of burning coal in power plants, fly ash is a pozzolanic substance that increases long-term strength, decreases heat of hydration, and improves workability [4]. Silica fume, an ultra-fine pozzolanic substance that greatly improves strength and durability by lowering porosity, is a byproduct of ferrosilicon and silicon alloy production [5]. A byproduct of the manufacturing of iron and steel, slag is a hydraulic substance that increases resistance to chemical assault, decreases permeability, and improves workability [6].
Concrete’s mechanical qualities, such as its modulus of elasticity, tensile strength, and compressive strength, can all be considerably enhanced by the addition of SCMs [7]. Fly ash helps to increase long-term strength by creating more C-S-H gel, but it may decrease early-age strength because of its slower reaction time [8]. In high-strength concrete in particular, silica fume works extremely well to increase compressive strength [9]. Its ultra-fine particles provide a denser microstructure by filling up the spaces between cement grains [10]. Slag enhances strength at both early and late ages, making it appropriate for a variety of uses. Concrete’s performance depends heavily on its durability, particularly in abrasive settings [11]. SCMs contribute to durability by reducing permeability, boosting resistance to chemical attack, and resolving issues like sulfate attack and ASR [12].
SCMs such as fly ash, slag, and silica fume improve resistance to sulfate attack by reducing the amount of reactive calcium hydroxide and forming denser microstructures. The effectiveness of these materials depends on the replacement rate and the specific sulfate environment [13, 14]. The use of SCMs reduces chloride ion penetration, which is vital for structures made of reinforced concrete exposed to de-icing salts or marine conditions [5, 15]. SCMs enhance freeze-thaw resistance by reducing permeability and improving the overall durability of the concrete matrix [3, 5, 16].
About 8% of CO2 emissions worldwide come from the manufacture of Portland cement, making it a significant source of emissions [17]. SCMs can greatly lower the carbon footprint of concrete by partially substituting cement. SCMs are industrial waste materials that are typically dumped in landfills, including fly ash, slag, and silica fume [18]. The decrease of fly ash’s primary source, coal-fired power plants, is raising questions about the supply of conventional SCMs like fly ash [19]. This has led to a search for alternative SCMs to meet future demand. Similarly, the supply of slag and silica fume is limited by the production levels of the industries that generate these materials [20]. The performance of SCMs can vary based on their chemical and physical properties, which are influenced by the source and processing methods. For instance, the oxide content of fly ash alone is not a reliable indicator of its performance in sulfate environments. This variability can pose challenges in achieving consistent concrete quality [21].
The use of SCMs, particularly silica fume, can enriching the water demand of concrete mixes due to their fine particle size. This may require adjustments in mix design, such as the use of superplasticizers, to maintain workability [22]. While SCMs are known to enhance long-term strength and durability, their performance in specific environments, such as high-temperature or high-humidity conditions, requires further investigation. Additionally, the synergistic effects of using multiple SCMs together are not yet fully understood [23].
Metakaolin is a thermally activated clay, a highly reactive pozzolan that increases strength and durability is metakaolin. It is particularly effective in reducing permeability and mitigating ASR [12, 15, 24]. Naturally occurring materials such as volcanic ash and diatomaceous earth have shown potential as SCMs. These materials are abundant and can be used in regions where traditional SCMs are not readily available [25]. Calcined clays, such as those produced from kaolin or illite, have demonstrated pozzolanic properties and can be used as partial replacements for cement [26]. Materials such as rice husk ash and sugarcane bagasse ash have been explored as SCMs. These materials not only provide pozzolanic benefits but also contribute to sustainable waste management [27].
Advanced characterisation methods including XRD and SEM can provide more light on the microstructural alterations brought about by SCMs [28]. Conducting life cycle assessments (LCAs) of SCM-enhanced concrete can provide a comprehensive understanding of their environmental benefits. This involves assessing how SCMs affect resource usage, greenhouse gas emissions and energy utilization over the course of concrete structure life cycles [29].
Utilizing SCMs such silica fume, fly ash, and slag represents a significant step toward more sustainable and high-performance concrete [30]. These materials offer numerous benefits, including enhanced mechanical properties, improved durability, and reduced environmental impact. However, challenges such as availability, variability in performance, and increased water demand have been addressed to fully realize their potential in his work.
This study’s main goal is to assess the durability and mechanical performance of concrete that contains blended cementitious materials, particularly fly ash, silica fume, and ground granulated blast furnace slag (GGBS), which are used in part to substitute regular Portland cement. This research is new because it addresses a topic that is seldom covered in the literature: the simultaneous insertion of three different supplemental cementitious materials (SCMs) in 17 different concrete mixes at varied amounts. Finding the ideal ternary blend (7.5% fly ash, 7.5% silica fume, and 7.5% GGBS) that minimizes early-age strength loss while maximizing strength and durability. Integration of microstructural analysis (SEM and XRD) with mechanical performance to understand the relationship between internal structure and external behavior. Quantitative model validation using Percent Bias (PBIAS) to assess the accuracy of experimental predictions, which adds a novel statistical layer to performance evaluation. Ultimately, the study aims to contribute to the development of sustainable, high-performance concrete with reduced cement usage, supporting eco-efficient construction practices.
2. MATERIALS AND METHODS
Grade 53 of OPC: As the main binder, OPC 53 grade cement that complies with IS 12269:2013 was utilized. Because of its low chloride concentration and strong early strength, this cement is well-suited for high-performance concrete applications. Cement was partially substituted with Class F fly ash, which complies with ASTM C618. Fly ash increases long-term strength, decreases heat of hydration, and improves workability. Silica fume, conforming to ASTM C1240, was used in small proportions (5–10% by weight of cement) to enhance strength and durability [31, 32]. Its ultra-fine particles fill micro-voids, resulting in a denser concrete matrix. Cement was partially replaced with GGBS, which complies with IS 12089:1987. Steel slag improves workability, decreases permeability and increases protection against chemical assault.
M-sand was utilized as a fine aggregate in accordance with IS 383:2016. M-sand, which has a particle size of 1.15 mm, is crushed granite. Compared to natural river sand, it offers superior grading and lowers the danger of alkali-silica reaction (ASR). In accordance with IS 383:2016, crushed stone aggregates with nominal sizes of 20 mm and 10 mm were utilized. To guarantee constant quality and get rid of contaminants, the aggregates were cleaned and dried. To increase workability and lower water consumption, An ASTM C494 Type F-compliant high-range water-reducing admixture was used. The dose was changed in accordance with the specifications of the mix design. For mixing and curing, clean, drinkable water that complied with IS 456:2000 was utilized.
The mix design was created in compliance with IS 456:2000 and IS 10262:2019. At 28 days, the concrete’s objective compressive strength was 70 MPa. In accordance with IS 10262:2009, the mix proportions were determined using the absolute volume technique. Table 1 summarizes the material proportions utilized in the modified mixes (with SCMs) and the control mix (without SCMs).
The components were combined in a laboratory concrete mixer. The coarse aggregate, M-sand, and steel slag were initially mixed for 2 minutes to achieve equal distribution. After that, silica fume, fly ash, and cement were added and stirred for two more minutes. To create a homogenous mixture, water and superplasticizer were added gradually while the mixture was being mixed for a further three minutes. The fresh concrete was evaluated for workability using the slump test, according per IS 1199:1959. The desired slump range was between 60 and 100 mm. Concrete examples in a range of sizes were cast in steel molds to evaluate the material’s durability and mechanical qualities.
To guarantee correct compaction and eliminate air spaces, the specimens were compacted using a vibrating table. Selected samples were subjected to XRD and SEM in order to examine the concrete’s phase composition and microstructure. This made it easier to comprehend how SCMs improve the concrete’s strength and longevity.
3. METHODOLOGY
Figure 1 shows the methodology of the work flow.
4. RESULTS AND DISCUSSION
4.1. Workability test
The study evaluates the effect of fly ash, silica fume, and GGBS on the workability of M70 grade concrete using slump value and compaction factor as indicators.
4.1.1. Effect of fly ash
Mixes containing fly ash (T2–T4) show an increase in workability, with slump values rising from 80 mm to 84 mm as fly ash content increases. The compaction factor also improves from 0.741 to 0.778, indicating better flowability. This enhancement is due to the spherical particle shape of fly ash, which reduces internal friction.
4.1.2. Effect of silica fume
Mixes containing silica fume (T5–T7) exhibit a decline in workability, with slump values reducing from 76 mm to 70 mm and compaction factors decreasing from 0.704 to 0.648. Silica fume’s fine particle size increases cohesiveness, making the mix more compact but reducing ease of placement.
4.1.3. Effect of GGBS
Mixes with GGBS (T8–T10) show a trend similar to silica fume, with slump values decreasing from 76 mm to 70 mm and compaction factors dropping from 0.704 to 0.648. Though GGBS improves durability, its high fineness contributes to lower workability.
4.1.4. Ternary blended mixes
Combinations of fly ash, silica fume, and GGBS (T11–T17) show moderate workability, with slump values ranging from 75 mm to 81 mm and compaction factors between 0.694 and 0.750. The best balance is observed in T13, where 10% fly ash, 5% silica fume, and 5% GGBS yield improved workability (81 mm slump, 0.750 compaction factor), making it an optimal mix for high-strength concrete. Table 2 shows the workability test results.
4.2. Compressive strength test
The compressive strength of M70 grade concrete was analyzed at 7, 14, and 28 days for various mix combinations incorporating fly ash, silica fume, and GGBS.
4.2.1. Effect of fly ash
Mixes containing fly ash (T2–T4) exhibit a gradual increase in compressive strength with higher fly ash content. At 28 days, the strength improves from 79.8 MPa (T1) to 82.56 MPa (T4), demonstrating that fly ash’s pozzolanic reaction promotes long-term strength increase. However, early-age strength gain (7 days) is slightly slower compared to silica fume mixes.
4.2.2. Effect of silica fume
Mixes with silica fume (T5–T7) show higher early-age strength development due to its ultrafine nature and high reactivity. The 28-day strength increases from 80.52 MPa (T5) to 83.59 MPa (T7). This confirms that silica fume significantly enhances compressive strength by refining the pore structure and improving the bond between cement paste and aggregates.
4.2.3. Effect of GGBS
Concrete mixes with GGBS (T8–T10) demonstrate moderate strength improvement over time, with 28-day strength reaching up to 82.39 MPa (T10). The latent hydraulic properties of GGBS contribute to long-term strength gain, though early strength is slightly lower compared to silica fume blends.
4.2.4. Ternary blended mixes
The ternary blends (T11–T17) exhibit the highest compressive strengths, indicating synergistic effects of fly ash, silica fume, and GGBS. The best-performing mix, T16 (7.5% Fly Ash + 7.5% Silica Fume + 7.5% GGBS), achieves 85.69 MPa at 28 days, outperforming conventional concrete. This suggests that an optimized combination of mineral admixtures enhances both early and long-term strength while maintaining durability.
Thus, T16 is the most efficient mix, offering improved compressive strength while reducing cement content, making it ideal for high-performance applications. Figure 2 shows the compressive strength test results.
4.3. Split tensile strength test
The split tensile strength of M70 grade concrete was evaluated at 7, 14, and 28 days for different mix compositions incorporating fly ash, silica fume, and GGBS. The results indicate variations in tensile strength based on the type and proportion of mineral admixtures used.
4.3.1. Effect of fly ash
Mixes containing fly ash (T2–T4) show a gradual increase in split tensile strength with higher fly ash content. The 28-day strength improves from 7.06 MPa (T1) to 7.31 MPa (T4). This improvement is due to the pozzolanic reaction of fly ash, which enhances bond strength over time. However, the early-age tensile strength gain is slightly slower compared to silica fume mixes.
4.3.2. Effect of silica fume
Mixes incorporating silica fume (T5–T7) exhibit higher tensile strength at all ages, with the 28-day strength increasing from 7.13 MPa (T5) to 7.40 MPa (T7). The fine particle size and high reactivity of silica fume contribute to a denser microstructure, improving tensile strength significantly.
4.3.3. Effect of GGBS
Concrete mixes with GGBS (T8–T10) show moderate improvements in tensile strength, with 28-day values ranging from 7.09 MPa to 7.29 MPa. GGBS enhances long-term strength, though its early-age contribution is less pronounced compared to silica fume.
4.3.4. Ternary blended mixes
The ternary blends (T11–T17) demonstrate the highest split tensile strengths, indicating synergistic effects of fly ash, silica fume, and GGBS. The best-performing mix, T16 (7.5% Fly Ash + 7.5% Silica Fume + 7.5% GGBS), achieves 7.58 MPa at 28 days, outperforming conventional concrete. This suggests that an optimized combination of these admixtures improves tensile strength while reducing cement content.
Because of its greater tensile strength, T16 is the most effective blend and is thus perfect for applications that call for increased durability. The results of the split tensile strength test are displayed in Figure 3.
4.4. Flexural strength test
The flexural strength of M70 grade concrete was analyzed at 7, 14, and 28 days for different mix compositions incorporating fly ash, silica fume, and GGBS. The results demonstrate that the inclusion of these mineral admixtures significantly influences the flexural strength, enhancing the overall performance of the concrete.
4.4.1. Effect of fly ash
Mixes containing fly ash (T2–T4) show a gradual increase in flexural strength, with 28-day strength improving from 8.72 MPa (T1) to 9.03 MPa (T4). The pozzolanic activity of fly ash enhances the bonding within the concrete matrix, leading to improved resistance to bending forces. However, the early-age strength gain is slower compared to silica fume.
4.4.2. Effect of silica fume
Mixes with silica fume (T5–T7) exhibit higher flexural strength, with 28-day values increasing from 8.80 MPa (T5) to 9.14 MPa (T7). Silica fume’s ultra-fine particles fill in the gaps in the concrete matrix, improving density, microstructure, and tensile resistance, leading to superior flexural strength.
4.4.3. Effect of GGBS
Concrete mixes incorporating GGBS (T8–T10) show moderate improvements in flexural strength, with 28-day values ranging from 8.76 MPa to 9.01 MPa. While GGBS contributes to long-term strength gain due to its latent hydraulic properties, its impact on early-age strength is lower than silica fume.
4.4.4. Ternary blended mixes
The ternary blends (T11–T17) achieve the highest flexural strengths, indicating a synergistic effect of fly ash, silica fume, and GGBS. The best-performing mix, T16 (7.5% Fly Ash + 7.5% Silica Fume + 7.5% GGBS), attains a 28-day strength of 9.37 MPa, the highest among all mixes. This indicates that a balanced combination of these mineral admixtures leads to a denser matrix with enhanced bending resistance.
Thus, T16 is the most effective mix, achieving optimal flexural strength, making it suitable for high-performance structural applications requiring enhanced durability. Figure 4 shows the flexural strength test results.
4.5. Saturated water absorption
The saturated water absorption of M70 grade concrete was assessed at 28, 56, and 90 days for various mix compositions incorporating fly ash, silica fume, and GGBS. The results indicate that the SWA of concrete is influenced by the type and percentage of mineral admixtures used.
4.5.1. Effect of fly ash
Mixes containing fly ash (T2–T4) show a slight enrich in water absorption compared to conventional concrete. The 28-day absorption increases from 2.234% (T1) to 2.312% (T4). However, a steady decline in absorption is observed over time, indicating the long-term densification of the matrix because of fly ash’s pozzolanic reaction.
4.5.2. Effect of silica fume
Mixes with silica fume (T5–T7) exhibit a higher water absorption than conventional concrete, with 28-day values ranging from 2.255% to 2.341%. The ultra-fine particles of silica fume enhance packing density, but due to its high specific surface area, it may initially lead to higher absorption. However, long-term hydration improves concrete density, reducing absorption over time.
4.5.3. Effect of GGBS
Concrete mixes incorporating GGBS (T8–T10) show comparable water absorption to conventional concrete, with 28-day values between 2.244% and 2.307%. GGBS contributes to a denser microstructure, reducing permeability over time.
4.5.4. Ternary blended mixes
The ternary blends (T11–T17) exhibit the highest water absorption values at early ages, with T16 (7.5% Fly Ash + 7.5% Silica Fume + 7.5% GGBS) showing 2.399% at 28 days. This is likely due to the high surface area and combined effect of multiple admixtures, which require more water initially. However, at 90 days, water absorption is significantly reduced, demonstrating improved long-term impermeability.
Thus, while ternary blends show higher early-age absorption, they exhibit better long-term durability, making them suitable for structures requiring enhanced resistance to permeability and aggressive environments. The saturated water absorption test results are displayed in Figure 5.
4.6. Acid resistance test
The weight loss (%) and strength loss (%) of M70 grade concrete at 28, 56, and 90 days were analyzed for different mix proportions incorporating fly ash, silica fume, and GGBS. The results demonstrate the effect of mineral admixtures on durability performance.
4.6.1. Weight loss analysis
Weight loss in concrete occurs primarily due to dehydration, carbonation, and microstructural degradation over time.
4.6.1.1. Effect of fly ash
Fly ash-based mixes (T2–T4) exhibited lower weight loss than conventional concrete (T1) at all ages. The 28-day weight loss decreased from 9.00% (T1) to 8.665% (T4), showing improved resistance to mass loss. Long-term durability is enhanced due to the pozzolanic reaction, which refines the pore structure, reducing permeability.
4.6.1.2. Effect of silica fume
Silica fume-based mixes (T5–T7) showed a gradual decrease in weight loss, with T7 (15% Silica Fume) achieving the lowest at 8.540% (28 days) and 6.066% (90 days). This is attributed to improved particle packing and densification due to the ultrafine nature of silica fume.
4.6.1.3. Effect of GGBS
GGBS-based mixes (T8–T10) also showed better resistance to weight loss than conventional concrete. The 15% GGBS mix (T10) achieved 8.685% weight loss at 28 days, compared to 9.00% for T1. GGBS contributes to a denser microstructure and reduced porosity, lowering mass loss.
4.6.1.4. Ternary blended mixes
The ternary blended mixes (T11–T17) demonstrated the best resistance to weight loss. T16 (7.5% Fly Ash + 7.5% Silica Fume + 7.5% GGBS) had the lowest weight loss at 90 days (5.623%), indicating superior long-term durability. The combination of multiple pozzolanic reactions leads to improved microstructural densification and reduced permeability.
4.6.2. Strength loss analysis
Strength loss in concrete is associated with deterioration of the cementitious matrix and reduced bonding between aggregates and paste.
4.6.2.1. Effect of fly ash
Fly ash-based mixes (T2–T4) showed reduced strength loss over time, with T4 (15% Fly Ash) showing the lowest at 9.731% (28 days) and 5.232% (90 days). The pozzolanic effect of fly ash enhances long-term strength retention.
4.6.2.2. Effect of silica fume
Silica fume-based mixes (T5–T7) exhibited higher strength retention, with T7 (15% Silica Fume) reducing strength loss to 9.590% at 28 days and 4.923% at 90 days. The high reactivity of silica fume results in a more refined microstructure, minimizing strength degradation.
4.6.2.3. Effect of GGBS
GGBS-based mixes (T8–T10) demonstrated better resistance to strength loss than conventional concrete. T10 (15% GGBS) had a 9.754% strength loss at 28 days and 5.283% at 90 days, indicating improved durability.
4.6.2.4. Ternary blended mixes
The ternary blended mixes (T11–T17) exhibited the lowest strength loss percentages. T16 (7.5% Fly Ash + 7.5% Silica Fume + 7.5% GGBS) achieved the lowest strength loss at 90 days (4.293%). This suggests that the combined effect of multiple pozzolanic materials significantly enhances durability and mechanical strength retention.
4.6.3. Overall
Weight loss and strength loss decreased with increasing pozzolanic material content. Ternary blended mixes (T11–T17) showed the best performance, with T16 being the most durable mix. Silica fume had the most significant impact on strength retention, while GGBS and fly ash contributed to long-term densification and reduced weight loss. Figure 6 and 7 shows the percentage of weight and strength loss in acid resistance test results.
4.7. RCPT Test
The RCPT is a key durability test that evaluates the permeability of concrete to chloride ions, which directly impacts reinforcement corrosion resistance. Lower RCPT values indicate better resistance to chloride penetration and improved durability.
4.7.1. Effect of fly ash
Fly ash-based mixes (T2–T4) exhibited a steady decrease in RCPT values, indicating improved resistance to chloride penetration. T4 (15% Fly Ash) had the lowest RCPT value among the fly ash mixes (610 at 90 days), compared to 707 for conventional concrete (T1). The improvement is due to the pozzolanic reaction of fly ash, which refines the pore structure and reduces permeability over time.
4.7.2. Effect of silica fume
Silica fume-based mixes (T5–T7) showed significantly lower RCPT values, with T7 (15% Silica Fume) achieving the lowest at 574 (90 days). The ultrafine silica particles fill voids and densify the matrix, reducing the ingress of chloride ions.
4.7.3. Effect of GGBS
GGBS-based mixes (T8–T10) exhibited improved chloride resistance compared to conventional concrete. T10 (15% GGBS) had an RCPT value of 616 at 90 days, which is a 13% reduction compared to T1 (707). GGBS contributes to long-term strength gain and pore refinement, reducing permeability.
4.7.4. Ternary blended mixes
The ternary blended mixes (T11–T17) demonstrated the lowest RCPT values, with the best resistance to chloride penetration. T16 (7.5% Fly Ash + 7.5% Silica Fume + 7.5% GGBS) had the lowest RCPT value (501 at 90 days), marking a 29% improvement over conventional concrete. The combination of pozzolanic materials synergistically enhances durability by densifying the matrix and reducing pore connectivity.
4.7.5. Overall
RCPT values decreased over time for all mixes, confirming the long-term benefits of mineral admixtures. Ternary blended mixes (T11–T17) exhibited the lowest permeability, with T16 achieving the best chloride resistance (501 Coulombs at 90 days). Silica fume had the most significant impact on reducing RCPT values, followed by fly ash and GGBS. Using multiple mineral admixtures significantly enhances durability and corrosion resistance, making the concrete more suitable for aggressive environments. The findings of the fast chloride penetration test are displayed in Figure 8.
4.8. SEM analysis
SEM analysis was conducted to examine the microstructural characteristics of different concrete mixes, particularly the effect of fly ash, silica fume, and GGBS on hydration products and pore structure. The conventional concrete (T1) exhibited a porous microstructure with visible voids and microcracks, leading to higher water absorption and lower durability.
Mixtures including GGBS (T16) and silica fume, on the other hand, showed a thick and compact microstructure with decreased porosity. The synthesis of C-S-H gel, which is in charge of strength development, was greatly accelerated by the presence of ultrafine silica fume particles. By encouraging secondary pozzolanic reactions, fly ash helped to enrich the pore structure and reduce the number of capillary pores.
Moreover, the SEM images revealed that higher ternary blend percentages (T16: 7.5% Fly Ash + 7.5% Silica Fume + 7.5% GGBS) led to the formation of an interlocked matrix with fewer cracks, improving the durability of concrete. However, excessive replacement levels caused some areas to exhibit unreacted particles, which could slightly affect early-age strength. Early-age strength is slightly lower in T16 due to slower SCM reactions, despite dense microstructure. Strength improves as pozzolanic activity progresses over time. Figure 9 and 10 shows the SEM image of T1 mix and T16 mix.
4.9. XRD analysis
To determine the crystalline phases found in various concrete mixes and to assess the impact of fly ash, silica fume, and GGBS on hydration products, XRD analysis was carried out. The conventional concrete (T1) primarily exhibited peaks corresponding to Calcium Hydroxide (Ca(OH)2), Quartz (SiO2), and unreacted C3S (Tricalcium Silicate), indicating incomplete hydration and higher porosity.
In contrast, ternary blended mixes (T16) showed increased peak intensities of C-S-H and reduced Ca(OH)2 content, signifying enhanced pozzolanic reactions. The addition of silica fume and GGBS promoted the consumption of Ca(OH)2, leading to more C-S-H gel formation, which is crucial for strength and durability. The presence of Alite (C3S) and Belite (C2S) phases in moderate amounts suggested a controlled hydration process, ensuring better long-term performance.
Additionally, the XRD patterns for high GGBS mixes exhibited distinct peaks of Hydrotalcite and Ettringite, indicating improved sulfate resistance. The reduced intensity of portlandite peaks in blended mixes confirmed the densification of the microstructure, which aligns with SEM findings. Figure 11 and 12 shows the XRD analysis of T1 and T16 mix.
5. CONCLUSION
The thorough assessment of different concrete mix designs incorporating Fly Ash, Silica Fume, and GGBS revealed significant improvements in workability, strength, durability, and resistance to chloride penetration.
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Workability and Compaction: The slump values and compaction factors indicate that adding Fly Ash improves workability, while Silica Fume tends to reduce it due to its fine particle size. Mixes containing ternary blends (Fly Ash, Silica Fume, and GGBS) maintain a balance between workability and strength.
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Compressive Strength: All modified mixes exhibited higher compressive strengths compared to conventional concrete (T1). The ternary blended mix T16 (7.5% Fly Ash + 7.5% Silica Fume + 7.5% GGBS) achieved the highest 28-day compressive strength of 85.69 MPa, surpassing the conventional mix (79.80 MPa) by approximately 7.4%.
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Split Tensile and Flexural Strength: Strength improvements were observed in all modified mixes, with the best results in ternary blended concrete. T16 showed the highest tensile (7.58 MPa) and flexural (9.37 MPa) strengths at 28 days, confirming the enhanced bonding within the concrete matrix.
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Durability Performance: The Saturated Water Absorption and weight loss results suggest that Silica Fume and GGBS contribute significantly to reducing porosity, leading to lower permeability and higher durability. T16 exhibited the lowest water absorption (1.99%) and weight loss (5.62%).
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Chloride Penetration Resistance (RCPT Test): T16 had the lowest RCPT values (501 Coulombs at 90 days), indicating superior resistance to chloride ingress, compared to 707 Coulombs in conventional concrete.
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SEM analysis revealed that conventional concrete (T1) had a porous microstructure with microcracks, leading to higher water absorption and lower durability. In contrast, ternary blended mixes (T16) exhibited a dense, compact structure with enhanced C-S-H gel formation. XRD confirmed reduced Ca(OH)2 content, indicating improved hydration and long-term performance.
Overall, the use of ternary blends significantly improves concrete properties, making them perfect for uses where significant strength, durability, and corrosion resistance, such as marine and industrial structures.
For future work, the following areas are suggested:
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Long-term durability studies beyond 90 days, including performance under aggressive environmental conditions such as carbonation, freeze-thaw, and sulfate attack.
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Cost-benefit analysis and life cycle assessment (LCA) are used to examine the economic and environmental feasibility of blended mixtures on a large scale.
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Machine learning models to predict performance based on mix composition and curing age, improving design efficiency.
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Field-scale validation of optimal mix designs in real construction projects for practical feasibility and performance tracking.
6. BIBLIOGRAPHY
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[2] GIERGICZNY, Z., “Fly ash and slag”, Cement and Concrete Research, v. 124, pp. 105826, 2019. doi: http://doi.org/10.1016/j.cemconres.2019.105826.
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Publication Dates
-
Publication in this collection
28 July 2025 -
Date of issue
2025
History
-
Received
15 Mar 2025 -
Accepted
10 June 2025
