The standard consistency of cement paste was determined via Vicat apparatus in line with ASTM C187 (Astm, 2004) for different levels of GGBS replacement (0%–90%).

Initial and final setting times were determined following the procedures outlined in ASTM C191 (International, 2008). The initial setting time was assessed using a 1 mm square needle, with measurements taken until the penetration depth reached 5 ± 0.5 mm from the base of the mold. The final setting time was assessed with an annular attachment, taking measurements every 30 min until the paste surface showed no visible indentation.

Concrete workability was evaluated through the slump test, conducted immediately after mixing and in accordance with ASTM C143 (ASTM, 2014). The slump was maintained within the specified range to ensure pumpability.

The rheological behavior of each concrete mix was assessed using a state-of-the-art rheometer equipped with a four-blade vane, measuring 50.1 mm in length and 15.3 mm in diameter. The sample volume was selected to approximate an infinite medium, ensuring reliable measurement conditions (Nguyen and V Boger, 1992). To maintain consistency, the concrete paste was manually remixed before testing to prevent particle settlement and ensure homogeneity. A 30-second resting period was followed to allow the dissipation of residual stresses induced during mixing.

All fresh concrete and mortar samples were tested at approximately 15 min after the mixing water was introduced. To minimize any thixotropic effects or early structural buildup, we began each test with a 30-second high-speed shearing phase (Roussel, 2006). Next, the rotational speed of the rheometer was decreased stepwise over 0.6 rev/s to 0.1 rev/s range, corresponding to a maximum shear rate of about 10 s^-1. Each speed step included a 2-second transition followed by a 3-second measurement period to allow the system to reach near steady-state conditions. Following these measurements, the Bingham model was employed to analyze the data and extract values for yield stress and plastic viscosity (Feys et al., 2007).

To validate the applicability of the Bingham model for analyzing the rheological behavior of the concrete mixes, experimental shear stress versus shear rate data were plotted alongside model predictions. As shown in Figure 1, the data for mixtures A6, B6, and C6 closely follow linear trends, indicating a strong fit with the Bingham model. This supports the model’s assumption of a linear relationship between shear stress and shear rate for cementitious systems with varying GGBS contents. The consistency of the linear fits across mix types with different aggregate proportions (1:1:2, 1:2:4, and 1:3:6) further reinforces the appropriateness of using the Bingham model in this study (Park et al., 2005).

Compressive strength was evaluated by calculating the average crushing strength of three cylindrical specimens for each mix, tested at 7, 14, 21, 28, and 56 days. The splitting tensile and flexural strengths were determined using the mean values of three cylinders and prisms, tested at 28 and 56 days.

A summary of the different tests conducted on paste and fresh concrete is presented in Table 4, while the tests for hardened concrete specimens are detailed in Table 5.

The normal consistency of cement paste was measured for different levels of GGBS replacement, ranging from 0% to 90%, with the results shown in Figure 2. The values of water required for normal consistency exhibited an incremental trend as the percentage of GGBS replacement increased. For pure cement paste (0% GGBS), the consistency was measured as 23%, which increased gradually to 30.1% at 90% GGBS replacement. GGBS contributes to higher consistency values primarily due to its physical and chemical properties. The smoother and glassy texture and reduced friction of GGBS particles enhance the paste’s cohesiveness, which requires more water to achieve the standard flow (Siddique and Bennacer, 2012). Furthermore, the higher fineness of GGBS contributes to an increased specific surface area, leading to greater water retention in the paste. The consistency values of 28% at 50% GGBS replacement and 31.1% at 90% GGBS replacement suggest that GGBS enhances the paste’s plasticity while maintaining its stability. A linear regression of the data points yielded the best‐fit line y = 0.080x+23.38, as shown in Figure 2, with a high coefficient of determination (R² = 0.98). This strong correlation confirms a near‐linear relationship between the GGBS replacement level and the paste consistency.

The setting times of cement paste serve as key indicators of the hydration process and are notably affected by the partial substitution of cement with GGBS. The initial setting time indicates when the paste begins to lose its plasticity, whereas the final setting time signifies when it has hardened enough to withstand defined pressure levels. These parameters are particularly important in evaluating the suitability of GGBS-blended cement paste for practical construction applications, especially for pumpable concrete.

The results, presented in Figure 3, show a progressive increase in both the initial and final setting times as the percentage of GGBS replacement increased (Mark et al., 2019). For 100% OPC (0% GGBS), the initial setting time was recorded at 82 min, and the final setting time at 182 min. As the GGBS replacement level increased to 90%, the initial and final setting times extended significantly to 192 min and 298 min, respectively.

This pattern aligns with the observations of Yu et al., who attributed the extended setting times to the slower hydration kinetics associated with GGBS (Yu et al., 2021). GGBS, being a latent hydraulic material, relies on the activation provided by OPC and water to contribute to hydration. As the OPC content decreases with higher GGBS replacement, the availability of active hydration compounds, such as tri-calcium silicate (C₃S), diminishes, leading to slower setting (Liew et al., 2024).

Additionally, the fineness of GGBS, combined with its smoother surface texture, reduces its immediate reactivity compared to OPC. This delayed reaction is particularly evident in the slower initial hydration phase, which is critical for the development of early strength. However, the extended setting times also offer some practical advantages. The decrease in heat of hydration observed with increased GGBS content helps reduce the risk of thermal cracking in large-scale concrete elements, thereby making GGBS-based binders well-suited for mass concrete applications (Lawrence et al., 2012).

The findings also indicate a consistent rise in final setting time as the GGBS content increases. Specifically, the final setting time extended by around 63%, increasing from 182 min at 0% GGBS to 298 min at 90% replacement. However, at high GGBS levels, delayed setting may become a challenge for rapid construction schedules, particularly in cold climates where hydration is naturally slowed. Moreover, linear regressions of the data indicate a near‐linear trend: for the initial setting time, y = 1.192x+75.18 with R² = 0.9544, and for the final setting time, y = 1.234x+170.67 with R² = 0.9245. These high coefficients of determination confirm a strong correlation between GGBS replacement level and the corresponding increases in setting times.

While a linear regression was applied here to describe the trend between GGBS replacement level and setting time, it is recognized that this approach is empirical in nature. For a more mechanistic understanding of the setting process, models such as the Avrami hydration kinetics equation could be applied in future studies to better capture the nucleation and growth behavior of hydration products in cement-GGBS systems (Sun et al., 2022).

Figures 4a–c illustrate the variation in yield stress for pumpable concrete across different GGBS replacement levels in all mix series. All mixes followed a general trend of increasing yield stress with time, an expected phenomenon reflecting ongoing hydration and structural build-up in the fresh cementitious system (Arularasi et al., 2021). During the early stages (first 30 min), the yield stress was comparatively low, promoting ease of flow and pumpability. As hydration products accumulated, intra-particle bonding became more pronounced, which raised the yield stress noticeably beyond the 30-minute mark.

In Mix Series A, which contains a higher cement-to-aggregate ratio, the larger paste volume contributed to a milder early increase in yield stress, indicating the mix retained sufficient fluidity to support pumping operations during the initial period. In contrast, the leaner mixes (Series B and Series C) had less paste available to lubricate the aggregates, thus displaying steeper increases in yield stress after the 30-min mark. These variations highlight how both paste content and mix proportions can influence the kinetics of structural build-up, highlighting the need to balance early workability with the desired rate of stiffening.

Figure 4d further illustrates the effect of increasing levels of GGBS on the yield stress for all mixes. In general, partial replacements of cement with GGBS (e.g., 20%–40%) consistently lowered the initial yield stress, fostering enhanced workability and more extended pumpability windows. This improvement is primarily linked to GGBS particles refining the paste matrix, reducing internal friction, and prolonging the induction period of hydration, particularly in the early minutes after mixing.

These observations align well with the findings of Park et al. (2005), who studied the rheological behavior of cementitious systems incorporating GGBS using a cylindrical spindle rheometer. In their study, cement was partially replaced with GGBS at various levels. The results revealed that replacing cement with GGBS in the range of 15%–45% led to a noticeable reduction in yield stress, indicating improved flowability and workability of the concrete mixtures.

However, as the GGBS content increases beyond 50%, a more pronounced rise in yield stress is observed after the initial low values. This behavior results from the diminished OPC content, which reduces the availability of tricalcium silicate (C₃S), the main compound responsible for initiating early hydration. As a consequence, the dormant phase concludes more abruptly, and the activation of GGBS becomes the dominant source of hydration. Although this delayed activation initially promotes fluidity, it also leads to a steeper post-induction structuration due to the build-up of hydration products. The sudden stiffening effect, coupled with reduced OPC, can present challenges in pumping applications where extended flowability is needed. Therefore, while moderate GGBS levels optimize initial yield stress and enhance early workability, excessive replacement requires careful mix design control to avoid premature loss of flow during placement.

Past literature reports a similar trend in yield stress with increasing replacement of cement by GGBS. Several studies indicate that replacing cement with GGBS in the range of 15%–30% results in the lowest (i.e., optimum) yield stress, enhancing workability (Adjoudj et al., 2014; Amran et al., 2021). However, beyond this level, further increases in GGBS content tend to increase the yield stress, likely due to changes in particle packing and reduced early reactivity at high replacement levels (Montes et al., 2012).

Furthermore, at higher GGBS levels, the reduced calcium silicate content shifts the hydration mechanism toward aluminate-dominant reactions, particularly involving phases such as C₄AF and the alumina-rich components of GGBS. These aluminates can rapidly react with available sulfates, leading to early ettringite formation and localized flocculation, which contributes to the sudden yield stress increase observed after the dormant phase (Nedunuri and Muhammad, 2024). Additionally, chemical shrinkage from hydration reactions may enhance particle crowding in the paste, particularly under reduced OPC content, triggering early-age structuration and stiffening. These microstructural phenomena likely play a complementary role in the sharp rise in yield stress seen at GGBS levels exceeding 50%, and reinforce the importance of mix control when targeting extended pumpability.

The plastic viscosity of all mix series was monitored over time to assess its evolution during the fresh state. Figures 5a–c illustrate the trends observed for each mix series. The results indicate that plastic viscosity generally increased with time, which can be attributed to the ongoing hydration of cementitious materials and the flocculation of particles in the fresh concrete. This phenomenon aligns with previous studies, which report that the time-dependent increase in plastic viscosity is primarily due to the progressive formation of hydration products and the reduction in free water available for lubrication (Ferraris, 2001; Roussel et al., 2010).

For Mix Series A, the plastic viscosity exhibited a steady increase over time, reflecting the gradual structuration of the cement paste. Similarly, Mix Series B showed a comparable trend, albeit with slightly higher initial plastic viscosity due to differences in mix design parameters. Interestingly, Mix Series C demonstrated a more pronounced increase in plastic viscosity over time, suggesting that specific mix proportions or admixture effects could be influencing the structuration rate.

The observed increase in plastic viscosity over time is a critical consideration in pumpability, as excessive stiffening of the mix during pumping could lead to blockages in pipelines. This highlights the importance of optimizing mix proportions and incorporating appropriate admixtures to regulate viscosity development.

The impact of GGBS replacement on plastic viscosity was analyzed for all mix series, as illustrated in Figure 5d. The values plotted in Figure 5d correspond to measurements taken at 0 min, i.e., immediately after mixing. The results indicate that increasing GGBS content generally led to a reduction in plastic viscosity. This decrease is primarily due to the finer particles and smooth surface texture of GGBS, which promote better particle packing and improve the dispersion of cementitious components. Consequently, the mix exhibits lower internal friction, resulting in reduced plastic viscosity.

Notably, the effect of GGBS replacement varied among the different mix series. In Mix Series A, incorporating GGBS resulted in a noticeable decline in plastic viscosity, particularly at higher replacement levels. The dilution effect and latent hydraulic activity of GGBS contribute to a less viscous paste. In Mix Series B, a similar trend was observed, albeit with a slightly less pronounced reduction in viscosity. This could be attributed to variations in mix proportions and water-to-binder ratios, which influence the overall rheological response.

Mix Series C exhibited a consistent reduction in plastic viscosity across all levels of GGBS replacement, reinforcing the idea that GGBS contributes positively to reducing the internal resistance to flow in fresh concrete. These findings align with previous studies, which highlight that GGBS reduces the overall viscosity of cement paste due to its lower water demand and enhanced dispersion characteristics (Vikan and Justnes, 2007).

While GGBS generally reduces plastic viscosity due to its spherical shape, high fineness, and lower water demand, this trend begins to plateau beyond 50% replacement. At such high levels, the dilution of OPC reduces the C-S-H framework formation in early hydration but increases alumina-based reactions, which can promote particle agglomeration and reduced fluid dispersion. Additionally, as superplasticizers are less effective in highly aluminate-rich environments, the dispersion efficiency may drop, leading to slight increases or stagnation in viscosity reduction. These effects, combined with increased paste volume and particle packing density, help explain the more complex plastic viscosity behavior observed at high GGBS levels.

The observed reduction in plastic viscosity with increasing GGBS content is consistent with the past studies (Grzeszczyk and Janowska-Renkas, 2012). For example, Park et al. (2005) explored the rheological behavior of cementitious systems containing mineral admixtures, including GGBS. They reported that partial replacement of cement with GGBS led to a notable decrease in plastic viscosity. This effect was attributed to the smoother particle morphology and lower early-age reactivity of GGBS, which helped reduce interparticle friction and enhance the dispersion of solid particles within the mix.

Compressive strength is a key parameter for assessing the structural performance and load-bearing capacity of concrete. It directly governs the load-bearing capacity, durability, and serviceability of concrete in construction applications. In pumpable concrete, compressive strength is significantly influenced by concrete composition, water-to-cement ratio, admixture content, and the fractional substitution of cement with GGBS.

The incorporation of GGBS as an SCM enhances long-term strength development, but at the cost of lower early-age strength due to its slower pozzolanic reaction (Monteiro, 2006). This research investigated the compressive strength performance of 30 pumpable concrete mixes, comprising three series (A, B, and C), each with GGBS content varying from 0% to 90%. The compressive strength was measured at 7, 14, 21, 28, and 56 days, ensuring a comprehensive understanding of strength development trends. The experimental values are presented in Figure 6, illustrating the result of curing (age) and GGBS content on compressive-strength performance.

At early ages (7–21 days), the results indicate a decrease in compressive strength as the GGBS content increases. This is primarily due to the slower initial hydration reaction of GGBS compared to OPC, leading to delayed strength gain (Monteiro, 2006). The trends show that low GGBS content (0%–20%) exhibits higher early-age strength, as OPC dominates the hydration process. With moderate GGBS content (30%–50%), early-age strength is slightly reduced but remains sufficient for practical applications. However, a high GGBS content (exceeding 50%) leads to noticeably reduced early-age strength, primarily due to the slower pozzolanic activity of GGBS. These findings align with studies by Oh et al. (2024) and Li et al. (2014), which reported delayed early-age strength development in GGBS-based concrete.

Notably, Series A (1:1:2), with richer cement content, shows comparatively higher early-age strengths across all GGBS replacement levels, while Series C (1:3:6), being the leanest mix, consistently yields lower strengths at these ages. Series B (1:2:4) lies between these two extremes. Despite these differences, the general trend of reduced early strength with higher GGBS replacement remains consistent across all three series.

Regarding later-age strength gain (28 and 56 days), the pozzolanic activity of GGBS accelerates, leading to continued strength development beyond 28 days. Most mixes containing GGBS catch up or surpass the control mix by 56 days. The enhancement in strength is primarily due to the increased formation of calcium silicate hydrate (C-S-H) gel, which strengthens the concrete’s microstructure. Furthermore, improved matrix densification and lower porosity contribute to higher compressive strength at later curing stages, as supported by microstructural evidence in previous studies (He and Lu, 2024a; He et al., 2024). It is important to note that this explanation is based on indirect mechanical evidence and literature precedent. No direct microstructural validation (e.g., SEM, MIP) was performed in this study. Therefore, the interpretation of C-S-H densification remains speculative and should be confirmed through future microstructural analyses.

The results show that, at 28 days, the maximum compressive strength is typically achieved with a 40% GGBS replacement level across all three mix series (A, B, and C). By 56 days, mixes within the 30%-50% GGBS range show the highest strengths, often exceeding the strength of the control mix (0% GGBS). For instance, in Series A, the peak 56-day strength (49.5 MPa) is achieved at 40% GGBS content. Beyond 50% GGBS, strength continues to increase with curing time but remains lower compared to the 30%–50% range. These findings align with previous research (Chen et al., 2024), where GGBS improved long-term strength but required extended curing durations to match or exceed OPC concrete strength.

An interesting observation is that even with high GGBS content (e.g., 90%), later-age strengths can still reach structurally viable strength under carefully managed laboratory conditions. For Series A, 90% GGBS and 10% OPC can yield a 56-day compressive strength of about 37.5 MPa, provided that the mix design includes a suitably low water-to-binder ratio, adequate superplasticizer dosage, and proper curing. Although early-age strength (7–21 days) is significantly reduced at these high replacement levels, the slower pozzolanic reaction of GGBS can still lead to substantial long-term strength when given sufficient curing time and favorable conditions (Monteiro, 2006). Nevertheless, for practical purposes, such high‐volume GGBS mixes may not be ideal where early strength is critical.

The results confirm that low GGBS content (0%–20%) maintains high early-age strength but does not offer the most significant long-term benefits. Conversely, moderate GGBS content (30%–50%) achieves the best balance between early-age and later-age strength, making it ideal for structural applications requiring good workability, pumpability, and enhanced durability. High GGBS content (>50%) excessively delays strength gain and reduces the 28-day strength, although continued curing can still improve the final strength somewhat. As illustrated in Figure 6d, the maximum compressive strength at 28 and 56 days was specifically observed at 40% GGBS replacement. Although mixes with 30% and 50% GGBS also showed considerable long-term gains, exceeding 50% GGBS generally resulted in lower 28-day strength due to the dilution of OPC and slower hydration reactions. These findings are consistent with previous studies suggesting that an optimal GGBS replacement range for concrete lies between 30% and 50% (Ganesh and Murthy, 2019).

While the observed strength enhancement at moderate GGBS levels is likely associated with increased C-S-H formation and matrix densification, this explanation remains interpretative. Since the present study does not include microstructural validation techniques (e.g., SEM or MIP), the proposed mechanisms are based on indirect evidence from mechanical trends. Future studies incorporating such analyses could more definitively verify these observations.

Cracking in hardened concrete initiates when the locally induced tensile stress exceeds the concrete’s intrinsic tensile capacity. The Splitting Tensile Strength, obtained in compliance with ASTM C496 (ASTM: C496/C496M − 17, 2011), is therefore a direct indicator of the stress level at which first micro-cracks form under restraint loading. Likewise, the Flexural Strength or Modulus of Rupture (ASTM C78 (ASTM International, 2009)) governs crack initiation and propagation under bending; it is the parameter used in serviceability-limit calculations for pavements, slabs and beams. Mixtures that exhibit higher Splitting Tensile and Flexural Strength delay crack initiation, reduce crack spacing, and limit crack widths, thereby restricting the ingress of aggressive agents such as chlorides, sulfates and CO₂. Numerous durability models (e.g., fib Model Code 2010 (Taerwe and Matthys, 2013), ACI 546) embed these parameters when predicting service-life or allowable crack width.

Since concrete is inherently poor in tension, its splitting tensile strength is significantly influenced by the mix proportions, aggregate interlocking, paste cohesion, and hydration reactions (Neville, 2011). The effect of GGBS replacement, w/c ratio, and superplasticizer dosage on splitting tensile strength was investigated at 28 and 56 days, with results illustrated in Figure 7.

The results confirm that splitting tensile strength increases with curing age, aligning with the findings of Güneyisi et al. (2013), who observed a similar gain in tensile resistance due to continued hydration and pozzolanic reactions. At 28 days, mixes with low GGBS content (0%–30%) exhibit higher splitting tensile strength because of the early strength contribution of OPC hydration (Ramzi and Hajiloo, 2023).

The highest splitting tensile strength at 28 days was observed at 40% GGBS replacement, with Mix A5 achieving 4.6 MPa, Mix B5 reaching 3.7 MPa, and Mix C5 attaining 3.1 MPa. At 56 days, an increase in tensile strength is observed across all mixes, with the maximum tensile strength still recorded at 40% GGBS replacement. Mix A5 achieves 5.1 MPa, Mix B5 reaches 4.1 MPa, and Mix C5 records 3.5 MPa, confirming that 40% GGBS replacement provides the optimal balance for splitting tensile strength improvement. These results align with the pozzolanic reaction that enhances the binding properties of the paste over time, as also observed by Venkitasamy et al. (Venkitasamy et al., 2024).

Beyond 40% GGBS replacement, a decreasing trend is observed, suggesting that excessive GGBS incorporation leads to higher paste viscosity, poor aggregate bonding, and delayed hydration effects, leading to tensile strength reduction (Siddique, 2014). Mixes with high GGBS content exhibited the lowest tensile strengths at both 28 and 56 days, with Mix A10, B10, and C10 showing 2.9 MPa, 2.1 MPa, and 1.6 MPa at 28 days, respectively, and further reductions at 56 days.

A direct correlation between compressive and splitting tensile strength is observed, consistent with past research. The optimal replacement range (30%–50% GGBS) ensures both compressive and tensile strength enhancement, making it ideal for structural applications requiring high durability. Excessive GGBS replacement (>50%) adversely affects early-age compressive and tensile strength, suggesting that beyond this limit, GGBS inclusion leads to excessive cohesiveness and lower aggregate-paste bonding (Sankar and Ramadoss, 2022).

As illustrated in Figure 7, horizontal reference lines indicate the control mix’s 28-day and 56-day splitting tensile strengths, allowing a clear comparison of each GGBS-based mixture against the baseline. With respect to tensile strength, the results show that mixes incorporating 30%–50% GGBS either match or outperform the control mix, while higher replacement levels generally lead to a decline in performance, reinforcing 40% GGBS as the optimal replacement level.

Splitting tensile strength, unlike compressive strength, is more directly influenced by the formation and propagation of internal microcracks and the quality of the paste–aggregate bond. At moderate GGBS levels (30%–40%), enhanced matrix packing and reduced porosity support better crack resistance. However, at higher GGBS levels, the slower pozzolanic activation can weaken early-age ITZ development, potentially leading to microstructural discontinuities that impair tensile resistance. These observations align with studies where tensile performance has been linked to early microcracking behavior and delayed matrix cohesion in GGBS-blended systems (He et al., 2024).

Flexural strength is an essential mechanical property that evaluates the ability of concrete to resist bending stresses, particularly in beam and slab structures. The flexural performance of pumpable concrete is significantly influenced by the mix composition, w/c ratio, GGBS replacement level, and curing age. Partially replacing cement with GGBS modifies the hydration process, which in turn influences the flexural strength of concrete over time. The flexural strength results for all 30 mix designs were evaluated at 28 and 56 days. The age-dependent flexural strength development is presented in Figures 8a–c, which demonstrate the flexural strength variations for Mix A, Mix B, and Mix C, respectively. At 28 days, the results indicate a steady increase in flexural strength with increasing GGBS replacement up to 40%, beyond which a decline is observed. The maximum flexural strength at 28 days was recorded at 40% GGBS replacement, with Mix A5 achieving 5.8 MPa, Mix B5 reaching 5.0 MPa, and Mix C5 attaining 4.0 MPa. This trend aligns with previous studies (Dom et al., 2022) attributing the improvement at moderate GGBS levels to the pozzolanic activity of GGBS, which leads to the formation of secondary C-S-H gel, enhancing the matrix’s load-bearing capacity.

At 56 days, most of the mixes demonstrated continued strength gain, indicating the prolonged hydration process facilitated by GGBS reactivity at later ages. Similar to compressive and splitting tensile strength trends, Mixes with 40% GGBS consistently achieved the highest flexural strength at 56 days. The extended curing period allowed for increased pozzolanic reactions, forming additional C-S-H gel, which improved the tensile load distribution capacity (Ayub et al., 2014). However, for mixes exceeding 50% GGBS replacement, the continued strength gain was relatively lower due to the lack of sufficient OPC content, which is essential for initiating early hydration and strength development.

The relationship between flexural strength and GGBS replacement at 28 and 56 days is illustrated in Figure 8d. The results indicate that low GGBS content (0%–20%) results in lower flexural strength due to the lack of supplementary pozzolanic reactions, which are essential for improving the concrete matrix’s tensile resistance. In contrast, moderate GGBS content (30%–50%) provides optimum flexural performance, enhancing load transfer capability and minimizing microcracking under bending stresses. This is attributed to the increased formation of C-S-H gel, which strengthens the bond between the aggregates and the cementitious matrix. However, at high GGBS content (>50%), a noticeable decline in flexural strength occurs, likely owing to the excessive dilution of OPC and increased porosity, which weakens the mix’s ability to resist tensile stresses (Samad and Shah, 2017).

These findings align with past studies (Karri et al., 2015; Ahmad et al., 2022), which emphasize that an optimum range of 30%–50% GGBS replacement maximizes mechanical properties without compromising early-age strength. The results further confirm that while GGBS enhances long-term mechanical properties, excessive replacement reduces early-age strength and negatively impacts flexural behavior. Therefore, careful optimization of GGBS content is essential to achieve a balance between workability, strength, and long-term performance, ensuring structural integrity in high-performance applications.

A comparative analysis of Mix A, B, and C shows that Mix A exhibited the highest flexural strength, followed by Mix B and then Mix C. This can be attributed to the lower w/c ratio in Mix A, leading to a denser and more compact concrete matrix. Mix C, having the highest w/c ratio, exhibited the lowest flexural strength due to increased porosity and reduced cohesion between cementitious particles. These patterns align with the results observed in compressive and splitting tensile strength, highlighting the interconnected nature of concrete strength properties.

Flexural strength is particularly governed by how well tensile stresses are redistributed across the cross-section and how effectively cracks are bridged at the bottom fiber. In this context, the bond between coarse aggregates and paste plays a dominant role. Moderate GGBS content improves ITZ characteristics, resulting in higher energy absorption and delayed crack widening under flexural loading. However, at GGBS levels above 50%, reduced early hydration limits the formation of a dense bond network, which can initiate premature cracking and more brittle behavior—especially in leaner mixes like Series C. Prior microstructural research has linked such flexural failure modes to insufficient ITZ formation during early curing (He and Lu, 2024b).

In this study the mixes containing 30%–50% GGBS achieved the highest Splitting Tensile and Flexural Strength, which implies a lower risk of early micro-cracking and, consequently, enhanced long-term durability through improved resistance to deleterious ion penetration and freeze–thaw damage. These observations are consistent with the crack-resistance mechanisms reported by Neville (2011) and Venkitasamy et al. (2024) for tensile behaviour, and by Dom et al. (2022) for flexural performance, reinforcing the durability benefits of moderate GGBS replacement.