Advancing eco-friendly concrete with locally sourced natural pozzolana

Introduction: This study investigates the potential of locally sourced Natural Pozzolana (NP) from Saudi Arabia as a sustainable Supplementary Cementitious Material (SCM) for green concrete, supporting Saudi Vision 2030.

Methods: An extensive experimental program was conducted on nine concrete mixes comprising over 450 specimens, in which Ordinary Portland Cement (OPC) was partially replaced with NP at levels up to 50%, with selected mixes incorporating Silica Fume (SF) and corrosion inhibitors (INH). Fresh properties (slump and initial temperature), mechanical performance (compressive and splitting tensile strength at 7, 28, 56, 90, and 180 days), and durability (chloride permeability and sulphate resistance) were evaluated.

Results: Although higher NP contents reduced early-age strength, all NP-based mixes exhibited substantial long-term strength recovery due to sustained pozzolanic activity, with NP20–NP30 achieving 94%–100% of the control strength at 90–180 days. Durability performance improved significantly, with NP mixes reducing chloride permeability by 40%–50%, while SF–NP and INH systems achieved reductions of 70%–76%, shifting RCPT classifications from “moderate” to “very low.” Sulphate exposure produced strength-retention ratios of 100%–114% with no deterioration up to 180 days. One-way ANOVA confirmed that all improvements were statistically significant and attributable to binder composition.

Discussion and conclusions: The findings demonstrate that locally sourced Saudi NP, particularly when combined with SF and corrosion inhibitors, provides a viable pathway for producing durable, low-carbon concrete while reducing cement consumption and supporting sustainable construction practices.

1 Introduction

1.1 Background and context

Concrete is the most widely used construction material worldwide due to its strength, durability, and versatility (Alishah et al., 2020; Hashmi et al., 2022; Oviedo et al., 2022; Alishah and Mohammadrezaei, 2020; Ramesh, 2021; Farzadnia et al., 2011). However, its primary binder, Ordinary Portland Cement (OPC), presents a major environmental challenge, accounting for approximately 8% of global CO₂ emissions (Seo et al., 2018; Supriya et al., 2023; Farzadnia et al., 2011). The carbon-intensive production of OPC driven by high-temperature calcination, fossil-fuel consumption, and extensive raw material extraction significantly contributes to climate change (Abadel et al., 2022; Amran et al., 2021; Hossain et al., 2021; Manan et al., 2024; Sadrolodabaee et al., 2023). With rapid urbanization and an anticipated 8% increase in cement demand in Saudi Arabia by 2025, the need for sustainable alternatives in concrete production has become increasingly urgent (Argaam, 2024; Expert Market Research, 2023; Zeyad et al., 2019).

The global shift toward sustainable and low-carbon construction materials has renewed interest in natural pozzolanas (NPs), which offer promising pathways to reduce cement consumption, enhance durability, and improve long-term concrete performance. NPs provide reactive silica and alumina that participate in secondary hydration reactions, refining the pore structure and increasing resistance to aggressive ions (Fares et al., 2022; Zeyad et al., 2019). In response to growing environmental pressures and regional initiatives such as Saudi Arabia’s Vision 2030, the development of green concrete using Supplementary Cementitious Materials (SCMs) has become a strategic objective, aiming to reduce dependence on OPC while improving the mechanical and durability properties of concrete (Castañeda et al., 2020; Fode et al., 2024; Hamada et al., 2023a; Khushefati et al., 2025; Makul, 2020; Mohsen et al., 2023; Wassouf et al., 2024).

Recent developments in sustainable concrete further underscore the relevance of evaluating alternative binders and SCM systems. Research on natural fiber–reinforced concretes demonstrates improved toughness and crack resistance (Althoey et al., 2021; Hakeem et al., 2022), while investigations using industrial by-products such as palm-oil waste, oil ash, and electric arc furnace dust reveal notable gains in mechanical and durability performance (Althoey and Hakeem, 2022; El-Habaak et al., 2018; Hakeem Ibrahim et al., 2023; Huda et al., 2017). Studies further highlight the importance of SCM-induced microstructural refinement, particularly for hot Gulf climates (Althoey et al., 2022; Hakeem et al., 2023a; Hakeem et al., 2023b; Hakeem et al., 2023c; Hosen et al., 2022).

Among these SCMs, NPs particularly those sourced from the Arabian Shield have emerged as promising candidates due to their favorable chemical and mineralogical characteristics (Ahmad et al., 2019; Amin and Khan, 2021). These locally available materials offer a sustainable solution and represent a strategic priority for achieving durable, cost-effective, and low-carbon construction practices (Castañeda et al., 2020; Fode et al., 2024; Hamada et al., 2023a; Khushefati et al., 2025; Makul, 2020; Mohsen et al., 2023; Wassouf, Omran, and Kheirbek, 2024). Collectively, these studies reinforce the global shift toward low-carbon, high-durability concrete and underscore the importance of evaluating region-specific SCMs particularly Saudi natural pozzolana within optimized multi-component binder systems.

1.2 Critical review of existing research

The existing literature provides valuable insights into the role of natural pozzolana (NP) and comparable pozzolanic materials in cement-based systems; however, current evidence remains fragmented and highly dependent on context. Numerous studies have shown that NP can enhance mechanical performance and reduce permeability when used at low to moderate replacement levels (Aburumman et al., 2023; Mohsen et al., 2023). Environmental assessments also indicate that NP-based concretes achieve favorable life-cycle performance relative to industrial by-products such as fly ash and slag (Arce et al., 2021). Additional research highlights improvements in sulfate resistance, thermal stability, and microstructural densification, depending on the pozzolana source and processing procedures (Aissa et al., 2019; Hamada et al., 2022; Merida and Kharchi, 2015).

Despite these contributions, significant limitations remain. Studies on pozzolanic materials (Husein et al., 2017) and on systems incorporating recycled aggregates or composite binders (Belagraa et al., 2020; Omrane and Mohamed, 2020) tend to focus on early-age characteristics and rarely extend to long-term durability assessments. Research on alkaline-activated NP systems (Castañeda et al., 2020; Seo et al., 2018) offers valuable materials-science insights but is not readily applicable to OPC-based systems, which remain predominant in Saudi Arabia.

Related investigations involving metakaolin further illustrate methodological gaps. Studies on soilcrete and sandcrete modified with metakaolin (Kolovos et al., 2013; Kolovos et al., 2016) reported improved compressive strength and microstructural refinement based on SEM observations; however, these works were constrained by short curing durations and the lack of chloride or sulfate durability evaluations. Similarly, Al-Chaar et al. (2013) found that natural pozzolana can enhance mechanical properties and mitigate alkali–silica reaction, yet the study did not examine long-term chloride transport, multi-component SCM interactions, or performance under severe marine or sulfate-rich exposures.

Overall, while existing research demonstrates the potential of NP, it lacks comprehensive, long-term, and region-specific evaluation particularly for multi-component binder systems incorporating silica fume (SF) and inorganic corrosion inhibitors (INH), which are highly relevant for the harsh environmental conditions of the Gulf region.

1.3 Research gap

Despite the recognized potential of NP, their use in structural concrete remains limited. Existing research indicates that NP can enhance performance especially after activation or fine grinding by increasing pozzolanic reactivity, reducing permeability, and refining hydration products (Fares et al., 2022; Zeyad and Ali, 2021). Nonetheless, three major gaps remain in the current body of knowledge.

First, most studies investigate NP as a standalone supplementary cementitious material (SCM), typically at low replacement ratios (10%–20%). This narrow scope does not address the behavior of high-volume NP systems (up to 50% replacement) nor the synergistic interactions among NP, SF, and corrosion inhibitors, which collectively influence hydration kinetics, pore structure refinement, and ionic mobility (Abbas and Iqbal Khan, 2021; Celik, 2015). The absence of such understanding constrains the design of high-performance, low-carbon concretes suitable for aggressive exposures.

Second, the mineralogical composition of Saudi Arabian NP differs considerably from pozzolanas studied internationally, as indicated by recent regional assessments (Khushefati et al., 2025). These variations limit the applicability of global predictive models and underscore the need for research grounded in locally sourced volcanic materials that reflect the unique geochemical characteristics of the Arabian Shield.

Third, long-term performance data remain scarce. Few studies extend beyond 90 days, and even fewer provide an integrated assessment that combines compressive and tensile performance with chloride permeability, sulfate resistance, and microstructural evolution. Furthermore, practical challenges including high processing costs, limited production capacity, and a shortage of skilled personnel continue to hinder widespread adoption of NP-based technologies in the region (Alhammadi, 2022; Altarrazi et al., 2022). Collectively, these gaps restrict the development of reliable, field-validated mix designs tailored to the Arabian Shield’s aggressive marine, chloride-rich, and high-temperature environments.

1.4 Research significance

This study advances existing knowledge by providing a comprehensive evaluation of Saudi Arabian NP across a broad replacement range and in combination with SF and corrosion inhibitors. Unlike previous research, this work examines both isolated and synergistic SCM effects through long-term testing up to 180 days and integrates multiple durability metrics, including RCPT and sulfate resistance.

The study further establishes mechanistic links between material chemistry, hydration evolution, and macroscopic performance, supported by ANOVA-based statistical validation. Results demonstrate that moderate NP replacement levels (20%–30%) sourced from the Arabian Shield can deliver long-term mechanical performance comparable to OPC, while significantly reducing chloride permeability and enhancing sulfate durability particularly when combined with SF. High replacement levels (40%–50%) also exhibit meaningful late-age reactivity, confirming the suitability of Saudi NP for sustainable binder systems in arid and coastal environments.

Overall, this research provides a region-specific scientific foundation for developing durable, low-carbon concrete technologies aligned with Saudi Arabia’s Vision 2030 sustainability objectives.

1.5 Manuscript structure

The remainder of this manuscript is structured as follows. Section 2 presents the experimental program, materials characterization, mix design, and methods used for mechanical and durability testing. Section 3 reports and discusses the results, including mechanical performance, chloride permeability, sulfate resistance, and statistical analyses. Section 4 synthesizes the key conclusions, highlights practical implications for green concrete design in the Gulf region, and outlines recommendations for future research.

2 Methodology

This study employed a comprehensive experimental program comprising over 450 samples to thoroughly investigate the fresh, mechanical, and durability properties of concrete mixes. The research focused on incorporating varying percentages of Ordinary Portland Cement (OPC), natural pozzolan (NP), silica fume (SF), and an organic inhibitor (INH) to evaluate their effects on concrete performance. A structured approach was used to create nine distinct concrete mix designs, enabling a systematic variation of each component’s proportions and a detailed assessment of their impact on the concrete’s properties. The experimental design divided the mixes into four primary groups: The Control Group served as the baseline, utilizing conventional concrete without any replacement of OPC or the addition of any SMCs. The NP-Group comprised mixes that explored the effects of substituting OPC with varying percentages of NP (20%, 30%, 40%, and 50%), aiming to identify the optimal replacement level that would enhance the concrete’s mechanical and durability properties. In contrast, the SF-Group investigated the influence of adding silica fume, both in the presence and absence of NP, to evaluate its interaction with NP and its overall contribution to concrete performance. Lastly, the INH-Group examined the impact of incorporating a corrosion inhibitor alongside both SF and NP, thereby assessing its effectiveness in improving the durability of the concrete mixes. This categorization facilitated a comparative analysis against the Control Group and among the different experimental conditions.

Key properties of both fresh and hardened concrete were meticulously measured. Fresh concrete properties included slump and initial temperature to assess workability and thermal behavior. Mechanical properties, specifically compressive strength and splitting tensile strength, were evaluated at multiple curing intervals to capture both early and long-term strength development. Durability was assessed through resistance to chloride and sulfate attack, critical factors for evaluating concrete resistivity and performance in aggressive environments.

2.1 Materials

Yanbu Cement (Type I-42.5N) was utilized as the binding material, which adheres to the ASTM C150 standard and possesses a minimum compressive strength of 32 MPa at 28 days, with a specific gravity of 3.15. Additionally, NP sourced from a local deposit in Saudi Arabia serves as a SCM, demonstrating confirmed pozzolanic activity. SF is incorporated at an 8% of cementitious material as additions to the mixes, for the SF and INH group. INH (MasterLife, 2017) was employed with a dosage of 5 L/m³ as per the manufacturer.

The aggregate components of the concrete mixtures consisted of fine aggregate (FA) and coarse aggregate (CA), both meeting ASTM C33 specifications. The fine aggregate was natural sand with a fineness modulus of 2.678, chosen to ensure uniformity and effective bonding within the concrete matrix. The coarse aggregate was crushed stone from a local quarry, blended in a 50:50 ratio of two sizes (12.5 mm and 19.0 mm) to optimize particle packing and reduce voids. The dry densities of the coarse aggregates were recorded as 1,661 kg/m³ for the 12.5 mm fraction and 1,668 kg/m³ for the 19.0 mm fraction, with specific gravities of 2.79 and 2.80, respectively. The absorption of the blended coarse aggregates was 0.72% and for the fine aggregates was 2.07%.

Potable water, conforming to ASTM C1602 standards, was used in all mixtures to ensure consistency and minimize variability due to water quality. A constant water-to-binder (w/b) ratio of 0.4 was maintained across all mixes to isolate the effects of cement replacement levels, while the water-to-cement (w/c) ratio was adjusted based on the specific cement replacement levels in each mix. Master Rheobuild 858M, a superplasticizer, was added to achieve the target designed workability.

2.2 Mixture proportions

A control mixture was designed to achieve a target compressive strength of 40 MPa, following the guidelines of ACI 211. The experimental mixes involved replacing cement with NP at levels of 20%, 30%, 40%, and 50%, with mixture proportions recalculated based on NP’s specific gravity to maintain consistency. The study maintained a constant water-binder ratio of 0.4, adjusting water-cement ratios accordingly, and used a superplasticizer dosage of 0.4% of the cementitious material’s mass (cement + NP), with the dosage doubled for the SF group to achieve the desired slump range of 70–100 mm ± 20 mm. The superplasticizer was added as a percentage of the cementitious material to maintain workability across different mix designs. Table 1 provides a detailed summary of the specific proportions for each mixture. The silica fume content was kept constant as an additional mass of 40.8 kg/m³ for the SF and INH groups, while the INH dosage was maintained at 7.70 kg/m³, following the manufacturer’s recommendations.

Table 1

Table 1. Mix proportions for all concrete mixes (all quantities in kg/m³).

2.3 Mixing, casting, and testing

The preparation of concrete ingredients involved precise measurement, weighing according to the specified amounts outlined in the mix design. A rotary mixer was employed to combine all concrete materials in accordance with the guidelines established by ASTM C192. The maximum volume of the mixer was approximately 0.5 m³ of concrete. Consequently, each mix was divided into three batches: the first batch was prepared without the addition of SP for the casting of three cylinders, while the remaining samples with SP were cast in two groups, each containing 30 to 33 cylinders. Quality control was carefully monitored for each group throughout the testing procedures, and on the testing days, samples were extracted from the two groups.

Casting and curing of the concrete test specimens were performed according to the ASTMC192 specifications, beginning with the combination of coarse and fine aggregates for 30 s. A small amount of water was then added to achieve a saturated surface dry (SSD) condition, followed by an additional 30 s of mixing. Cementitious materials were subsequently added, mixed for another 30 s, and the remaining water was introduced along with the superplasticizer, with a final mixing duration of 3 min to ensure homogeneity. The order of addition was kept constant across all mixes. The fresh concrete was tested for slump and temperature immediately after mixing. The slump test, conducted according to ASTM C143, measured the concrete’s workability, while the initial temperature was recorded to monitor heat evolution. Workability is a critical factor influencing the ease of placing and compacting concrete, which directly affects its final density and homogeneity. Given that the study investigates the effects of varying SCM content on concrete properties, it is essential to monitor and control workability to ensure that any observed differences in hardened concrete properties are attributable to the mix composition rather than variations in workability. Temperature variations can significantly influence the rate of hydration reactions, which in turn affects the setting time and early strength development of concrete. This measurement is particularly relevant in this study due to the inclusion of SCMs, which can alter the heat of hydration.

Specimens were cast in cylindrical Molds (100 mm diameter x 200 mm height) and compacted in three layers using a tamping rod, as shown in Figure 1. Following compaction, a vibrating shaker was utilized to further enhance the compaction of the concrete. All samples were cured under standard laboratory conditions (20 °C ± 3 °C and 95% relative humidity) until the testing ages. This curing regime was selected to provide a controlled environment that promotes optimal hydration and strength development. Standard curing conditions ensure that the test results are consistent and comparable across different mixes, allowing for a reliable assessment of the effects of the different mix compositions on concrete properties. The 24-h initial curing period in molds was implemented to prevent moisture loss, which is critical for early hydration and the development of a dense microstructure.

Figure 1

Figure 1. Overview of the experimental procedure: (a) Raw materials and admixture dispersion; (b) mixing and casting of cylindrical specimens; (c) freshly cast samples and water-curing setup for subsequent testing.

Mechanical properties, specifically compressive and splitting tensile strength, were evaluated at various curing intervals using a universal testing machine with a load capacity of 3,000 kN. It was evaluated at 7, 28, 56, 90, and 180 days to capture both the early and long-term strength development. These intervals were chosen to provide a comprehensive understanding of how the different mix compositions affect the rate of strength gain over time. The 28-day strength is a standard benchmark in concrete design, while the later ages (56, 90, and 180 days) are crucial for assessing the long-term performance and durability of concrete structures. Average compressive strength values were derived from three cylinders per mixture for each specified testing day. For the 28-day assessment, six samples were prepared per mix, with three cylinders containing a superplasticizer (SP) and three without. On the testing day, samples were removed from the curing tank, and the top 1–2 mm of each cylinder was ground to create a flat surface. After grinding, the samples were allowed to dry for 1 hour to eliminate excess moisture that could influence test results. Prior to testing, the dimensions and weights of the samples were recorded for accurate assessment. The cylinders were subjected to a compression machine with a capacity of 300,000 lbs (300 kip), and the load was applied steadily at a rate of 0.15–0.35 MPa/s (20–50 psi/s) until failure occurred. The compressive strength and failure mode were documented, facilitating a precise evaluation of the concrete’s performance underload.

The splitting tensile strength of the concrete mixtures was evaluated using the same concrete cylinders in accordance with ASTM C496 guidelines. This property is particularly important for structural elements subjected to tensile stresses, such as beams and slabs. The test was conducted at 7, 28, 56, and 90 days to assess the development of tensile strength over time and to compare it with the compressive strength development these tests, the cylinders were positioned longitudinally on the loading machine, like the configuration used for compressive strength testing. A special casing was applied to align with the center of the samples, ensuring it was correctly positioned over the marked line on each specimen. The load was then applied steadily and without shock at a rate of 0.7–1.4 MPa/min (100–200 psi/min) until failure occurred.

Durability tests assessed chloride ion penetration and sulfate resistance. The Rapid Chloride Permeability Test (RCPT) was carried out for cylindrical samples that were core-extracted. Given the aggressive environmental conditions in Saudi Arabia, where high temperatures and humidity can exacerbate chloride-induced corrosion, this test is crucial for assessing the durability and long-term performance of the developed concrete mixes. The test was conducted at 28 days, a standard curing period for evaluating chloride penetration resistance. Three core samples from three cylinders were tested using an electrical saw, resulting in specimens sized 100 × 50 mm (4 × 2 inches). Samples were taken from the same location in each cylinder to ensure consistency. The top 50 mm was removed, with the following 50 mm layer designated as the testing area for all mixtures (See Figure 2).

Figure 2

Figure 2. Preparation and conditioning of concrete specimens for RCPT testing: (a) Core cutting and specimen extraction; (b) application of epoxy coating to seal lateral surfaces; (c) vacuum saturation process using desiccator and pump prior to testing.

The RCPT test, according to ASTM C 1202, samples were vacuum-saturated for 6 h, then kept in distilled water for the remaining 24 h before testing, and then coated with non-conductive material (Cico Floor S-40 Grey), as shown in.The samples were placed in a test cell with 3.0% NaCl and 0.3 N NaOH solutions, and a 60 ± 0.1 V voltage was applied for 6 h. Current was recorded every 30 min, and the total charge passed was calculated to assess chloride ion penetrability.

Sulfate attack is another major concern for concrete durability, especially in regions with sulfate-rich soil or groundwater. This test assesses the ability of concrete to withstand the expansive forces induced by sulfate attack, which can lead to cracking and deterioration. Sulfate resistance was tested by immersing concrete cylinders for over 90 and 180 days, exposed to a 7% sodium sulfate solution after curing for 28 days, prepared according to ASTM C2102, with the pH maintained between six and 8, and the solution replaced monthly. These exposure durations were chosen to simulate prolonged exposure to sulfate-rich environments and to capture the progressive effects of sulfate attack on concrete performance. Therefore, visual inspections and compressive strength tests were conducted on sulfate-exposed samples and compared to normally cured samples and Chloride Ion Penetration results.

3 Results and Discussion

3.1 Fresh properties

In Table 2, a summary of fresh measurements for the various mixes, for both with and without SP. Fresh properties were immediately measured for each mix. The comparison of the slump and temperature among mixes in the case of either using the SP or not is displayed in Figures 3, 4. In concrete mixes without SP, a clear trend emerged: increasing NP content significantly reduced workability. The slump decreased by 61.54% in the NP50 mix compared to the control, justified by the increased cohesiveness and water demand due to the fine NP particles (requiring 103% of the control mix). Adding SF leads to further lower slump values, with the SF8-NP00 mix showing a 73.08% reduction in slump compared to the control. This is because SF, with higher surface area and water demand. Even the addition of INH, not typically expected to impact the slump (Master Builders Solutions), contributed to reduced workability when combined with SF and NP, with the INH-SF8-NP50 mix exhibiting an 84.62% reduction compared to the control. Regarding temperature, a trend of decreasing temperature with higher NP content was observed, with a variance of approximately −10.5% between the control mix and the NP50 mix. This is likely due to the reduced heat of hydration as NP partially replaces cement. SF also exhibited a similar cooling effect. However, INH appeared to counteract this trend, leading to a slight temperature increase of approximately 1.3% in the INH-SF8-NP50 mix compared to the control.

Table 2

Table 2. Summary of the slump and temperature measurements for various concrete mixes.

Figure 3

Figure 3. Slump Comparison: With SP vs. Without SP.

Figure 4

Figure 4. Temperature Comparison: With SP vs. Without SP.

With SP, the trends in concrete mixes shifted significantly. In the NP Group, slump values progressively increased with higher percentages of NP, demonstrating enhanced workability. The CONTROL mix had a slump of 85.50 mm, while the NP50 mix reached 102.00 mm, indicating a substantial improvement. In contrast, the SF Group showed less sensitivity to SP. The SF8-NP00 mix had a slump of only 20.00 mm, a significant reduction compared to the CONTROL, highlighting the adverse effects of SF on workability. However, the SF8-NP50 mix showed a slight improvement of 2.00 mm with SP, suggesting a partial counteraction of SF’s negative impact when combined with NP. The INH Group also revealed a notable trend, with slump values increasing from 55.50 mm for the INH-SF8-NP00 mix to 72.00 mm for the INH-SF8-NP50 mix, underscoring the effectiveness of SP in enhancing workability even with the presence of INH and SF, and due to the increasing amount of fluids (water + SP + INH) inside the mix.

Regarding temperature, the NP Group exhibited a similar trend to mixes without SP, with temperature generally decreasing as NP content increased. The temperature dropped from 26.70 °C at 20% NP to 23.80 °C for the NP50 mix, resulting in a variance of approximately −10.5% when comparing the CONTROL mix to the NP50 mix. In the SF Group, the SF8-NP00 mix showed a slight increase in temperature (26.85 °C) compared to the CONTROL (26.60 °C), but the temperature decreased significantly to 25.05 °C when 50% NP was incorporated. The INH Group showed a similar pattern, with the INH-SF8-NP00 mix recording 26.85 °C and the INH-SF8-NP50 mix showing an increased temperature of 27.20 °C, suggesting that the organic inhibitor may promote thermal activity during hydration.

However, the previous trends observed within each group can be justified by the following: As found by literature, the use of pozzolanic materials with fineness like that of cement generally has minimal impact on the workability of mortar or concrete, with natural pozzolanas typically enhancing workability while dramatically finer pozzolans may reduce it (McCarthy and Dyer, 2019). Pozzolans possess limited cementitious properties when used alone but react with cement hydration products to form additional cementitious compounds. Besides that, recent advancements in construction have introduced self-compacting concretes, which utilize high powder contents and super-plasticizing admixtures, making pozzolanic materials viable options for enhancing fluidity and stability in these mixes (v). However, high-fineness materials like silica fume substantially increase the water requirements in cementitious systems, necessitating the addition of superplasticizers to maintain desired workability without increasing water content. Consequently, the dosage of admixtures required for achieving specific workability levels escalates with the quantity of high fineness pozzolanic material incorporated (McCarthy and Dyer, 2019).

Without the use of superplasticizers (SP), the water requirement for natural pozzolana (NP) is 103% of that of the control mix, indicating that NP absorbs slightly more water, which can lead to a slightly stiffer mix if not compensated with additional water. This increased water demand can result in lower slump values due to insufficient lubrication among the particles in the mix. Additionally, the fineness modulus of OPC is 2.44, while that of NP is 1.93. This slight lower FM of NP suggests a higher proportion of finer particles, which can increase internal friction and cohesion, thereby delay the flowability of the concrete and lead to further reductions in slump when NP is present in significant amounts. Moreover, the Strength Activity Index for NP after 7 days is 83%, reflecting a lower contribution to early strength compared to OPC. This lower reactivity implies that the mix may not achieve the same level of hydration and strength development as quickly, contributing to lower heat generation which is clearly observed during the results of temperature.

The incorporation of superplasticizers significantly enhances the workability of concrete mixtures containing NP. The slight difference in fineness between OPC and NP:370 m²/kg for OPC compared to 365 m²/kg for NP, becomes less significant with the addition of SP. This is because SP effectively reduces internal friction among particles by promoting better dispersion, which leads to improved flowability and higher slump values. Furthermore, SP allows for a reduction in water content while maintaining the desired workability, which is particularly crucial when using NP due to its higher water demand. By dispersing particles more effectively, SP enables the mix to achieve a higher slump even with less water. Additionally, the presence of SP enhances the interaction between the finer particles of NP and OPC, as the dispersant action of SP reduces cohesive forces, allowing these finer particles to move more freely within the mix and thereby improving the overall slump.

Confirming that with literature findings, the mechanism of action for superplasticizers involves their adsorption onto cement particle surfaces, which promotes deflocculation and improves fluidity. This process generates repulsive forces, both electrostatic and steric, that help maintain the separation of cement grains (Ben Aicha, 2020). A schematic representation of superplasticizer action mechanisms (see Figure 5) allows for visualization of the repulsion principles. The superplasticizer releases the water between cement particles and increases the water films coating the particles in the cement paste and consequently improves the rheological properties (Ben Aicha, 2020).

Figure 5

Figure 5. Action principle of superplasticizers: (a) adsorption of the polymer; (b) steric effect of the adsorbed polymer (Ben Aicha, 2020).

The presence of dissociated acid functions, which carry a negative charge, enables superplasticizers to absorb onto cement grains, thereby altering the surface charges and reducing particle interactions through electrostatic repulsion, as shown in Figure 6. According to the European Standard NF EN 934-2, superplasticizers are classified as high-range water reducers and represent a new generation of plasticizers that significantly lower the water-to-cement ratio, enhancing both the workability and mechanical properties of concrete. This capability is attributed to the phenomenon of dispersion fluidification (Ben Aicha, 2020; Foissy, El Attar, and Lamarche, 1983; Jolicoeur and Simard, 1998; Pierre et al., 1989). These investigations have revealed that the deflocculation and dispersion of cement grains are closely linked to the adsorption of superplasticizers onto the particle surfaces. This adsorption generates both electrostatic and steric repulsive forces, contributing to the stability of the cement paste (Ben Aicha, 2020; Foissy, El Attar, and Lamarche, 1983; Jolicoeur and Simard, 1998; Pierre et al., 1989).

Figure 6

Figure 6. (a) Absorption of superplasticizer by cement particles, (b) Dispersion of cement particles by superplasticizer (Ben Aicha, 2020).

Silica fume, known for enhancing strength, can similarly reduce workability due to its high fineness. When SF is used without SP, it can lead to decreased slump values, reflecting reduced flowability and higher viscosity. The finer particles of SF may create clumping and increased friction, further impairing workability. Additionally, the increased viscosity can lead to elevated temperatures during mixing, as the energy input is not efficiently dissipated. In contrast, the addition of SP improves the dispersion of SF particles, enhancing flowability and resulting in higher slump values. SP also allows for a reduction in water content while maintaining desired workability, which is particularly important when using SF due to its water-absorbing properties. Furthermore, better dispersion of particles with SP helps in maintaining a more uniform temperature throughout the mix, preventing hot spots.

The incorporation of silica fume typically results in a reduction in the workability of concrete. This adverse effect is attributed to the high specific surface area and fine particle size of silica fume, which increases the water demand in the mixture to achieve an appropriate consistency. However, some studies have indicated that small quantities of silica fume or its combination with other supplementary cementitious materials may improve workability. To address the challenge of reduced workability, higher doses of superplasticizers are often necessary to lower water content while maintaining acceptable workability (Hamada et al., 2023b). For the INH group, the trends mirror those of NP and SF. Without SP, the increased viscosity and internal friction from the fine particles lead to decreased slump values and potential temperature rises due to inefficient energy dissipation. However, the incorporation of SP enhances workability by reducing cohesive forces among particles, promoting better dispersion, and resulting in improved flowability and higher slump values. Enhanced workability also helps in keeping the temperature stable during mixing.

3.2 Mechanical strength

This section examines the effects of varying NP proportions, combined with INH and SF, on concrete’s long-term mechanical properties at 7, 28, 56, 90, and 180 days of curing. Splitting tensile behaviour was assessed up to 90 days, a point where NP hydration is largely complete and significant strength development has occurred. Compressive strength at 180 days confirms most of the hydration completion at 90 days. The results of the mechanical properties are summarized in Table 3.

Table 3

Table 3. Summary of mechanical results over curing ages.

3.2.1 Influence of SP on compressive strength

Firstly, the influence of SP on concrete compressive strength was evaluated at the age of 28 days. However, the experimental design maintained a fixed SP dosage and (w/b) ratio across mixes, deviating from optimal SP application and potentially influencing observed compressive strength outcomes. This approach contrasts with standard practice, where the w/c ratio should be typically adjusted to maximize the benefits of SP, particularly in terms of strength development. However, this fixed-condition approach was intentionally adopted to isolate and investigate the effects of the ingress material. The control mix exhibited a 14% reduction in compressive strength upon SP addition, underscoring the critical role of w/c ratio adjustment in realizing SP’s full potential. In contrast, as the percentage of NP replacement increased in the NP Group, the magnitude of compressive strength reduction due to SP diminished. Specifically, the 20% NP mix showed a 7% reduction, while the 50% NP mix experienced only a 4% reduction. This suggests that NP may mitigate the adverse effects of SP when the w/c ratio is not optimized, potentially through its filling action and pozzolanic activity. The results of the compressive strength tests for the NP Group are depicted in Figure 7.

Figure 7

Figure 7. Effect of SP on the NP-Group.

The same trend was observed through literature, according to the researcher’s experimental results, for the same water/binder ratio, the compressive strength at 1, 7, and 28 days decreases with an increase in the superplasticizer dosage (Ben Aicha, 2020).

Further insights were gained from the SF and INH groups. The SF8-NP00 mix displayed only a 5% reduction in compressive strength with SP, indicating that SF enhances concrete performance even in the presence of SP. Notably, the SF8-NP50 mix exhibited a 3% increase in strength with SP, suggesting a synergistic interaction between NP and SF. Similarly, the INH-SF8-NP00mix showed only a 2% reduction with SP, while INH-SF8-NP50 mix demonstrated a 4% increase with SP, highlighting the role of INH with combination of NP in maintaining or even enhancing strength, the results are summarized in Figure 8. These findings suggest that the adverse effects of a high w/c ratio, resulting from the fixed SP dosage, were counteracted by the inclusion of NP, SF, and INH. NP’s filling action and pozzolanic activity, SF’s void-filling properties, and INH’s enhancement of bonding all contributed to improved concrete density and cohesion. These mechanisms likely mitigated the porosity induced by excess water, leading to less severe reductions in compressive strength compared to the control mix. This underscores the complex interplay between concrete mix components and the importance of optimizing mix designs to achieve desired performance characteristics.

Figure 8

Figure 8. SP effect on SF and INH group.

Although SPs are typically associated with increased strength due to water reduction, the present results show that the OPC control mix exhibited a reduction in 28-day compressive strength when SP was added. This response is consistent with hydration chemistry when the water–binder ratio (w/b) is intentionally fixed, as in the current study. Under fixed-water conditions, the dispersive action of SP enhances particle separation and increases the mobility of unbound pore water. In OPC-only systems lacking micro-filling SCMs, this can create an effectively higher internal w/c ratio within the capillary network, facilitating greater pore connectivity and delaying the densification of early C–S-H products. Consequently, the net effect is a measurable reduction in 28-day strength relative to the non-SP control. This mechanism is supported by previous research showing that SP used without proportional water reduction can increase capillary porosity, disturb water films, and slow C–S–H clustering (Ben Aicha, 2020; Alnahhal et al., 2018; Jolicoeur and Simard, 1998). The NP- and SF-modified mixes in this study showed reduced sensitivity to this effect, as pozzolanic reactions and micro-filling action compensate for SP-induced porosity by producing additional secondary C–S–H at later ages. These results highlight an important mix-design implication: the beneficial role of SP is realized only when accompanied by water adjustment, and its performance cannot be evaluated independently of binder composition and internal water distribution.

3.3 Compressive strength evaluation

In the inclusion of SP, the box-and-whisker plot in Figure 9, illustrates the compressive strength (F`c​) evolution of various concrete mixtures incorporating NP, SF, and INH, measured across curing ages from 7 to 180 days. This representation provides insight into the early and long-term performance of the mixes and highlights the pozzolanic contribution of supplementary cementitious materials.

Figure 9

Figure 9. Box-and-whisker plot of compressive strength development (7–180 days) for all concrete mixes.

The control mixture reached a compressive strength of 36.7 MPa in 7 days and increased to 46.27 MPa by 180 days, setting a benchmark for comparison with other mixtures. The NP20 and NP30 mixtures showed similar long-term strength results: NP20 reached a strength of 46.1 MPa at 180 days, which is only 0.37% different from the control. NP30 reached 43.63 MPa in 180 days, indicating a 5.7% decrease compared to the control. Despite slightly lower early strengths, the long-term strength gains seen in the NP mixtures were significant. NP30 increased from 30.64 MPa to 43.63 MPa over 180 days, showing a 42.4% improvement. In comparison, the control mixture experienced a 26% increase during the same period. High levels of NP replacements (NP40 and NP50) showed lower early strengths but marked improvements over time: NP40 increased from 23.28 MPa (7 days) to 39.4 MPa (180 days), which is a 69.3% gain. NP50 rose from 18.37 MPa to 34.6 MPa, reflecting an 88.3% increase. These results confirm the slow but steady pozzolanic activity of NP, consistent with previous research (Mohsen et al., 2023). The ability of NP to reduce clinker content further highlights its sustainability potential. The control mix demonstrates consistent strength growth over time, serving as a performance standard. Significantly, concrete mixes with 20% and 30% NP replacement (NP20 and NP30) showed compressive strengths like the control, especially at later ages.

This supports previous studies that suggest moderate NP replacement can maintain or enhance long-term mechanical performance due to ongoing pozzolanic reactions, which help form additional calcium silicate hydrate (C–S–H) gel (Elahi et al., 2021). The NP30 mix displays a positive long-term strength trend, showing a good balance between sustainability and structural strength. The strength growth noted over 180 days confirms the ongoing pozzolanic activity of NP. Although NP40 and NP50 mixes show greater variability and slightly lower early strengths, their continued strength gain over time confirms that higher NP levels contribute significantly to long-term performance with proper curing. Notably, the use of NP leads to significant clinker reduction, thereby lowering the embodied carbon of concrete, which is a key benefit in sustainable construction (Játiva et al., 2021; Khan, 2013; Lima et al., 2021).

The SF8-NP00 mix achieved the highest compressive strength of all blends, reaching 52.47 MPa at 180 days, 13.4% higher than the control, yielding the highest compressive strength among all mixes. This confirms the potential of silica fume in enhancing early and long-term strength due to its ultrafine particles and rapid pozzolanic activity (Hamada et al., 2023b; Nafees et al., 2022). When SF was combined with high NP content (SF8-NP50), the 180-day strength reached 35.43 MPa, exceeding NP50 by 2.4%. Strength enhancement is moderated, suggesting a need for optimization when blending multiple SCMs. Nonetheless, the combination of SF and NP represents a promising strategy for tailoring both performance and environmental impact.

The INH-SF8-NP00 and INH-SF8-NP50 mixtures, which integrate the organic inhibitor, exhibit a modest delay in initial hydration, as evidenced by their reduced early-age strength values. Nevertheless, the long-term compressive strength remains within the parameters deemed acceptable for structural integrity, thus approving the utilization of inhibitors in formulations where there is a necessity for delayed setting times or enhanced durability (Kadhim et al., 2021). The mixtures that incorporate an inhibitor demonstrated a marginal delay in early-age strength development; however, they maintained satisfactory long-term performance: INH-SF8-NP00 attained a compressive strength of 44.0 MPa at 180 days, which is 4.9% inferior to that of the control. INH-SF8-NP50 achieved a strength of 33.3 MPa, reflecting an 81.7% improvement from its 7-day strength of 21.77 MPa. These findings substantiate the employment of inhibitors in contexts necessitating delayed setting or improved durability. In summary, the incorporation of natural pozzolana and silica fume, particularly with inhibitors, can significantly enhance the durability and sustainability of concrete mixes, providing a viable path for eco-friendly construction practices (Kadhim et al., 2021; Shwetha et al., 2024).

3.3.1 Splitting strength evaluation

Figure 10 presents a box-and-whisker plot of the splitting tensile strength of various concrete mixes measured over a curing range from 7 to 90 days. The control mix, composed of ordinary Portland cement without any SCMs, provides a baseline for comparison. It exhibited a steady increase in tensile strength, reaching 3.63 MPa at 90 days. However, the performance of several NP-containing mixes was notably competitive, particularly at longer curing periods, affirming the active contribution of pozzolanic reactions to mechanical strength. Of particular interest are the NP20 and NP30 mixes, which demonstrated tensile strengths at 90 days of 3.70 MPa and 3.55 MPa, respectively comparable to or slightly higher than the control. This performance suggests that partial replacement of cement (up to 30%) with NP does not impair splitting tensile strength, and in fact may enhance it over time. This behavior is attributed to the formation of additional calcium silicate hydrate (C–S–H) gels resulting from the reaction between NP and calcium hydroxide, which improves matrix cohesion and crack resistance. These results align with previous research, indicating that moderate NP replacement can refine pore structure and increase bond strength, leading to improved tensile capacity (Kaur et al., 2015; Fode et al., 2023).

Figure 10

Figure 10. Box-and-whisker plot of splitting tensile strength development (7–90 days) for all concrete mixes.

As expected, higher NP replacement levels (NP40 and NP50) led to lower early-age tensile strengths. However, both mixes showed meaningful strength increases by 90 days (reaching 3.40 MPa and 2.90 MPa, respectively). This confirms the presence of continued pozzolanic activity, which, although slower to contribute at early stages, becomes increasingly influential over time. While early mechanical performance may be reduced due to the lower content of clinker phases, the later-age gains emphasize NP’s potential in applications where durability and long-term performance are prioritized over early strength. This includes mass concrete, structural members with low early-age loading, and sustainable infrastructure.

The incorporation of SF with 100% OPC (SF8-NP00) achieved the highest tensile strength at all curing stages, culminating at 4.03 MPa by 90 days. The enhanced performance is attributed to silica fume’s extremely fine particle size and high pozzolanic reactivity, which promote densification of the cement paste and improve interfacial transition zones. This superior performance validates the use of silica fume as a high-reactivity SCM for improving tensile capacity and microstructural refinement. The SF8-NP50 mix, combining both SF and high-level NP replacement, yielded lower tensile strengths than the SF-only mix but still demonstrated continuous strength growth to 3.13 MPa at 90 days. This indicates that the combination of a highly reactive SCM (SF) with a slower-reacting one (NP) can yield a beneficial synergy, balancing early and late-age mechanical properties. Despite some reduction in peak strength compared to SF alone, the mix remains a promising solution where strength and sustainability must be balanced.

The use of an inhibitor (INH) in the SF-containing mixes (INH-SF8-NP00 and INH-SF8-NP50) resulted in slightly reduced tensile strengths, particularly at later ages. This is likely due to delayed hydration kinetics induced by the inhibitor, which may interfere with the pozzolanic reaction in early stages. Nevertheless, the final tensile strengths 3.45 MPa and 2.95 MPa by 90 days remain structurally adequate and suggest that such formulations may be appropriate for applications requiring extended workability, corrosion resistance, or exposure to aggressive environments (Mindess, 2019).

3.3.2 Correlation between compressive and splitting tensile strength in concrete with various SCMs

This analysis investigates the correlation between compressive and splitting tensile strength in concrete incorporating NP, SF, and INH across multiple curing ages (7, 28, 56, and 90 days). Figure 11 provides a multidimensional visualization of the results, where color intensity represents %NP, point size reflects %SF, and red labels denote %INH. The results demonstrate consistently strong linear relationships, with Pearson correlation coefficients ranging from 0.9418 to 0.9825. The highest correlation at 90 days (r = 0.9825) highlights the enhanced synergy between these two mechanical properties over time, suggesting that the continued hydration and pozzolanic activity contribute to a coherent and predictable development of concrete strength. The strong 28-day correlation (r = 0.9655) aligns well with industry benchmarks, affirming the suitability of SCM-blended concrete for conventional strength evaluation standards.

Figure 11

Figure 11. Correlation between Compressive and splitting behavior of all mixes (same age).

Silica fume, represented by larger point sizes, shows a more immediate and consistent contribution to strength enhancement, likely due to its high reactivity and micro-filling capacity. The role of the inhibitor captured through red labels may also contribute to improved durability, particularly in aggressive environments. The integration of these materials does not disrupt the fundamental mechanical relationship between compressive and tensile strength but rather enriches it with additional long-term performance benefits. The regression trendlines in each scatter plot confirm the linear nature of the strength relationship, and the absence of major outliers enhances confidence in the reliability of the findings. Despite these constraints, the observed trends provide preliminary evidence that compressive strength may serve as a predictive proxy for splitting tensile strength in concretes with various SCMs, reinforcing the viability of simplified quality control protocols. Furthermore, the consistency of correlations across ages suggests that, despite microstructural alterations induced by SCMs, the fundamental mechanical relationships in concrete remain preserved.

3.4 Durability investigations

3.4.1 Chloride permeability (RCPT)

The research, employing the RCPT according to ASTM C1202 using the Perma one device for three samples of each mix, and at the end of the 6 h test, the total Charge Passed (Q) measured in Coulombs (C) is estimated on the screen data logger. The evaluation of the concrete’s resistance to chloride ingress is classified as per the ASTM standard listed in Table 4.

Table 4

Table 4. RCPT classification as per ASTM C1202.

Where the Control mix, serving as the baseline, exhibited an average charge passed of 2002.34 coulombs (±220.16), firmly classifying it as possessing “Moderate” chloride permeability. A summary of the RCPT results is shown in Table 5, and Figure 12.

Table 5

Table 5. RCPT Results According to ASTM C1202; (Charge Passed, Q, expressed in Coulombs (C)).

Figure 12

Figure 12. RCPT results across mixes.

This underscores the inherent vulnerability of unmodified concrete to chloride ingress, thereby reinforcing the imperative for effective modification strategies to enhance long-term durability. The NP-Group, characterized by NP replacement levels ranging from 15% to 50%, consistently achieved a “Low” chloride permeability classification, signifying a tangible improvement over the Control group. Notably, the mixture with a 50% NP replacement exhibited an average charge of 1,098.5 coulombs (±195.87), representing a dramatic 45% reduction in chloride permeability compared to the Control. Showing the significant role of NP to fill the pores inside the mix, and with the addition of the pozzolanic actions that make the mix less permeable to ingress the chloride ions. In a recent study, Shafiq et al. (2019), and Al-Alaily and Hassan (2016) reported that the rapid chloride permeability value of concrete decreases with an increasing content of NP. This reduction is attributed to the filling action of NP particles, which occupy voids in the concrete matrix, along with its pozzolanic activity that refines the pore structure and enhances the density of the concrete (See Figure 13). Consequently, it can be concluded that the incorporation of NP improves the risk of corrosion of embedded steel reinforcement (Abbas et al., 2010; Al-Alaily and Hassan, 2016; Shafiq et al., 2019).

Figure 13

Figure 13. NP filling action: (a) before incorporating of NP, and (b) after incorporating (Fode et al., 2024).

The SF-Group, incorporating SF, demonstrated “Very Low” chloride permeability (See Figure 12). Moreover, the SF8-NP50 mixture achieved an average charge of 549.5 coulombs (±26.17), representing a remarkable 72.5% reduction in chloride permeability compared to the Control mix. This substantial reduction underscores the synergistic effect of combining SF and NP, due to enhancing the microstructure and reducing the permeability of concrete, and with the ongoing pozzolanic action. Similarly, the INH-Group also achieved “Very Low” chloride permeability. The INH-NP50 mixture exhibited an average charge of 493.3 coulombs (±45.09), representing an impressive 74.6% reduction in chloride permeability compared to the Control. Continuous improvement in the mix when mixed with INH, and with a significantly dense mix in cooperation with the NP.

INH-containing mixes, particularly INH–SF8–NP00 and INH–SF8–NP50, achieved the lowest RCPT charge passed among all binders, with reductions of approximately 69%–76% compared with the control. This performance aligns with the intended role of corrosion inhibitors, which directly target chloride-induced depassivation in reinforcement. The observed RCPT reductions confirm that INH remains effective in chloride-rich exposure environments representative of Saudi Arabia.

3.4.2 Sulphate resistance

Despite extensive research on the use of natural pozzolana for enhancing concrete durability in sulphate-rich environments, it is important to note that natural pozzolanic materials derived from the Arabian Shield region remain largely unexplored in this context. Most existing studies have focused on volcanic pozzolanas from other geographical regions, such as Latin America, Europe, and parts of Asia (Martínez-Rosales et al., 2020; Merida and Kharchi, 2015). The absence of targeted investigations into this regional material presents a significant research gap, particularly for Middle Eastern countries where both the material is locally available and sulphate-rich environments are prevalent. The combination of natural pozzolana, silica fume, and corrosion inhibitors may have synergistic effects on the sulphate resistance of concrete. Therefore, this experimental study is critically needed to assess the suitability, reactivity, and performance of Arabian Shield pozzolana under sulphate exposure. This section provides a comprehensive analysis of the performance of these materials and their mechanisms of action.

The evaluation of sulfate resistance in concrete mixtures was systematically conducted by measuring compressive strength and density before and after 90 and 180 days of sulfate exposure. This assessment was further complemented by visual inspection to detect surface manifestations of deterioration, such as salt deposition or cracking, which may signify underlying chemical or physical degradation mechanisms. Figure 14 presents typical surface patterns observed on specimens subjected to sulfate exposure. Across all tested mixes, visual inspections revealed only minor surface salt crystallization, with no observable signs of cracking, delamination, or expansion-induced failure. While these surface features suggest a degree of sulfate ingress, their limited extent implies that the concrete matrix maintained structural integrity within the test duration.

Figure 14

Figure 14. Typical visual salting pattern of exposed samples.

Table 6 presents the compressive strength and density values for each mix under sulfate exposure and normal curing conditions at 90 and 180 days. Several mixes were deferred for 365-day testing (denoted as “P.P.”) due to the absence of visible deterioration and the need to capture long-term sulfate effects. The control mix exhibited a compressive strength increase under sulfate exposure at 90 days (49.05 MPa) relative to its counterpart under normal curing (46.23 MPa), suggesting initial pore refinement due to sulfate interaction. However, a 5.63% reduction was recorded between 90 and 180 days under sulfate exposure, despite a marginal increase in density. This paradoxical behavior strength loss concurrent with a slight densification likely reflects internal damage caused by the formation of expansive reaction products such as ettringite. These products can fill capillary pores, momentarily increase density, while simultaneously exert disruptive internal stresses that reduce strength.

Table 6

Table 6. Sulphate exposure effect on the compressive strength and densities of the mixes.

In contrast, the NP group displayed a more varied response. At 90 days, most NP-modified mixes underperformed compared to the control in terms of compressive strength. However, by 180 days, the NP 40% mix exhibited a 10.82% strength gain, increasing from 40.55 MPa to 44.95 MPa. This late-age improvement indicates a delayed pozzolanic reaction and microstructural densification, potentially yielding enhanced sulfate resistance. Conversely, the NP 50% mix demonstrated a continuous decline in strength, indicating that excessive NP content can compromise the balance between strength development and sulfate resistance. These findings underscore the importance of optimizing NP replacement levels, as excessive substitution may retard hydration and reduce early mechanical performance, ultimately affecting long-term durability. The SF group, incorporating 8% silica fume, consistently achieved higher early-age compressive strengths compared to NP-only mixes. For instance, the SF8-NP00 mix demonstrated stable strength values over time, underscoring silica fume’s well-documented capacity to enhance matrix densification and accelerate secondary hydration. The SF8-NP50 mix also exhibited improved performance relative to NP50 alone, though its strength (39.80 MPa) remained below the control, suggesting that silica fume can mitigate but not fully overcome the dilution effect at high NP levels. The INH group, incorporating corrosion inhibitors in addition to NP and SF, did not demonstrate a marked improvement in sulfate resistance relative to the NP or SF groups. The INH-NP50 mix showed similar performance to its uninhibited counterpart, suggesting that while corrosion inhibitors may be beneficial in chloride-rich environments, their influence on sulfate resistance is minimal.

To rigorously assess sulfate resistance, two quantitative indicators were employed using following Equations 1, 2:

• Sulfate Resistance Ratio based on Strength (SRCSR%):

$SRCSR\%=\left(\frac{F^{`}c\left(ExposedtoSulphate\right)}{F^{`}c\left(NormalCured\right)}\right)x100(1)$

• Sulfate Resistance Ratio based on Density (SRR%):

$SRR\%=\left(\frac{Density\left(ExposedtoSulphate\right)}{Density\left(NormalCured\right)}\right)x100(2)$

The summary of the results is presented in Table 7, this clearly demonstrates that all mix types exhibited SRR% values slightly above 100% at both 90 and 180 days, indicating minor densification under sulphate exposure. This can be attributed to continued hydration and pore refinement resulting from the presence of the adopted SMCs. The limited variability in SRR% (100.06%–100.70%) suggests a consistent performance across mixes in terms of physical resistance to sulphate ingress. More notably, SRCSR% values show diverse trends depending on mix composition. The control mix maintained an SRCSR% of 106.10% at 90 days but declined to 100.06% at 180 days, implying stabilization of sulphate-induced benefits over time. For mixes containing higher NP contents (e.g., NP40 and NP50), the SRCSR% exceeded 106% at both ages, peaking at 114.09% in NP40 after 180 days. This suggests that moderate pozzolan incorporation not only mitigates sulphate attack but also actively enhances compressive strength through secondary pozzolanic reactions.

Table 7

Table 7. Quantitative analysis of the sulphate resistance in terms of compression and density variances.

The SF8-NP50 mix demonstrated superior mechanical resistance (SRCSR% = 112.33% at 180 days), reinforcing the hypothesis that combining ultra-fine silica fume with pozzolanic filler increases sulphate durability. The incorporation of INH also showed a strong positive influence on strength, especially in INH-SF8-NP50, where SRCSR% reached 112.94%. This highlights the potential of synergistic effects between SCMs and chemical admixtures in resisting aggressive environments. However, NP30 showed a slightly reduced SRCSR% (99.77%) at 90 days, likely due to an unfavourable balance between dilution and pozzolanic reaction rate at this intermediate dosage. This underlines the need to optimize replacement levels carefully. Overall, the indicators confirm that sulphate exposure did not cause deterioration but rather promoted densification and strength enhancement across all systems. The quantitative ratios validate the long-term protective role of blended cementitious systems, particularly when optimized combinations of SCMs and inhibitors are used.

The strength gain and densification of concrete mixtures under 7% Na₂SO₄ exposure can be attributed to several interrelated hydration and reaction processes. First, the pozzolanic activity of NP plays a central role. NP supplies additional amorphous silica (and alumina) that consumes Ca(OH)₂ to form extra calcium–silicate hydrate (C–S–H) and related phases. This reaction reduces the amount of portlandite (which is vulnerable to sulfate attack) and instead creates more binding C–S–H gel. The newly formed C–S–H fills pore space and coarsely binds the cement matrix, making it stronger, denser, and more durable. In effect, NP converts free lime into binding material, continuing to develop strength beyond 28 days. This is essentially a secondary C–S–H formation mechanism: as long as silica is available from the SCMs, additional C–S–H gel can form and extend the microstructure (Goufi et al., 2023).

In a sulfate solution environment, continued hydration (possibly accelerated by sulfate ions) further contributes to this gel formation. sodium sulfate can even act as a mild activator of pozzolanic reactions, causing rapid depletion of CH and faster C–S–H formation in silica-fume mixes. Thus, the combination of sustained hydration and pozzolanic reactions generates more solid C–S–H, which both increases strength and fills capillaries (raising density) in the sulfate-cured concrete (Onuaguluchi, Ratu, and Banthia, 2022). Using NP and SF together produces synergistic benefits beyond what SCM alone would yield. Silica fume’s ultra-high fineness ensures early-age densification: it quickly reacts with calcium and physically fills voids, giving high early strength. This rapid strength gain supports the slower reacting pozzolana by maintaining a low W/C environment and limiting leaching. The pozzolana, in turn, continues to hydrate over longer timescales, further binding the matrix. Thus, SF and NP complement one another.

The corrosion inhibitor does not directly mitigate sulphate attack, as it does not interfere with the ettringite- or gypsum-forming reactions that govern sulphate deterioration. However, when used in combination with NP and silica fume, the inhibitor contributes to a secondary, permeability-related benefit: its molecules can reduce ionic mobility and slightly restrict water ingress, thereby lowering the rate at which sulphate ions penetrate the matrix. This indirect densification-driven effect is consistent with previous observations reporting reduced capillary absorption and improved pore blocking behaviour (Shwetha et al., 2024; O’Reilly et al., 2013; Rana and Jindal, 2024). Nevertheless, the inhibitor’s influence remains limited compared to NP and SF, which are the primary contributors to sulphate resistance in this study. This explains why INH-containing mixes showed only modest improvements under Na₂SO₄ exposure relative to their significant reductions in chloride permeability.

The above trends were supported by previous studies of SCMs and sulfate attack. For example (Merida and Kharchi, 2015), explicitly note that pozzolan-containing cements “have better performance in sulfate solutions, since the pozzolanic reactions reduce the quantity of calcium hydroxide and increase calcium silicate hydrate.” (Ahmad et al., 2019; Lee et al., 2005) have reported that silica-fume concretes show less strength loss under sulfate exposure compared to plain OPC, due to pore blocking and additional gel formation (Ortega et al., 2017; Paruthi et al., 2024) found that adding 5%–10% silica fume produced a significantly more refined pore network and associated strength gain up to 180 days in Na₂SO₄ solution. They even suggested that the sulfate ions may accelerate pozzolanic reactions in SF mixes. All these findings match our data showing ongoing strength increase.

3.5 Statistical validation of mechanical and durability performance

A rigorous statistical analysis was undertaken to validate the significance of the observed differences in mechanical and durability properties among all binder combinations studied. The one-way Analysis of Variance (ANOVA) technique is applied to the full dataset, incorporating Poisson-distributed variability from replicate specimens and categorical variation from the full factorial binder design (NP%, MS%, INH%). It is noted that sulphate-resistance results were not included in the statistical validation because several long-term immersion readings were still incomplete at the time of analysis; therefore, only the fully completed durability parameter RCPT was used as the primary indicator for durability performance.

3.5.1 Compression strength ANOVA

The one-way ANOVA followed by Tukey’s HSD test confirms that compressive strength is highly sensitive to the binder system, with statistically significant differences emerging in most pairwise comparisons from early age onward (p < 0.001 in the majority of cases). The results reveal a consistent hierarchy across curing ages: mixes containing 8% silica fume (SF8-NP00) develop the highest strengths, whereas high-volume NP50 mixes produce the lowest strengths due to pronounced dilution effects. At 7 days, nearly all comparisons involving NP40 and NP50 yield **highly significant differences (**p < 0.0001) relative to the control and NP20–NP30, confirming that strength reduction at high NP contents is both substantial and statistically robust. This performance hierarchy persists at 28, 56, 90, and 180 days, indicating that early-age reactivity accurately predicts long-term behaviour.

Moderate NP contents (NP20–NP30) consistently show manageable strength reductions, statistically significant at early age but becoming marginal or non-significant at 90–180 days for some comparisons (e.g., Control vs. NP20 at 90 and 180 days: ns). This reflects the delayed pozzolanic reaction of NP, which gradually compensates for early dilution. In contrast, high NP contents (NP40–NP50) remain significantly lower than all reference mixes across all ages, with Tukey tests showing large negative mean differences and extremely strong significance (****p < 0.0001 at all ages). These findings confirm that although NP contributes to long-term microstructural refinement, its replacement level must remain below 30% to avoid statistically validated performance reductions.

Silica fume exerts the most pronounced positive influence on strength development, with SF8-NP00 outperforming every other mix at all ages. Tukey comparisons show extremely high significance margins (****p < 0.0001) for SF8-NP00 against NP20, NP30, NP40, and NP50 from 7 days up to 180 days, confirming its dominant contribution to C–S–H densification and early strength gain. Even when combined with NP50 (SF8-NP50), silica fume partially compensates for dilution, producing mid-range strengths that remain significantly higher than NP50 alone (e.g., NP50 vs. SF8-NP50 at 7, 28, 56, and 90 days: ns to *p < 0.01). These results demonstrate that silica fume is the strongest single factor enhancing compressive strength within the binder system.

The corrosion inhibitor shows a neutral to moderately positive influence depending on the binder. INH–SF8–NP00 maintains strengths statistically like the control at most ages, while INH–SF8–NP50 demonstrates a clear contribution to late-age strength, significantly outperforming NP50 at 56, 90, and 180 days (p < 0.01–0.0001). This suggests that the inhibitor contributes through chemical stabilization of the pore solution and refinement of the interfacial transition zone (ITZ), effects that become more pronounced as hydration progresses. When viewed across the binder matrix, the ANOVA trends show that SF8 + INH combinations consistently outperform their NP-only equivalents, confirming a synergistic mechanism and reinforcing the proposed binder-optimization strategy for balancing sustainability and structural performance (See Figure 15).

Figure 15

Figure 15. Compressive strength development of all binder systems at 7, 28, 56, 90, and 180 days, with Tukey HSD significance comparisons (ns: no significance, *, **, ***, ****). Asterisks (*) denote pairwise significance levels; error bars represent mean ± SD (Standard Deviation); (a) for 7 days, (b) for 28 days, (c) for 56 days, (d) for 90 days, and (e) for 180 days.

3.5.2 Splitting tensile strength ANOVA

Across the four maturity ages (7, 28, 56, 90 days), the Tukey-adjusted comparisons reveal a consistent and statistically coherent hierarchy in splitting tensile strength, as shown in Figure 16. At 7 days, only the high-volume NP50 mix exhibits a significant reduction relative to the control (p < 0.001), while NP20–40 remain statistically indistinguishable (“ns”), indicating that moderate NP levels preserve early tensile capacity. With the inclusion of silica fume, SF8-NP50 and SF8-NP00 display significant improvements (p < 0.05–0.001), counteracting dilution and enhancing ITZ quality. By 28 days, these SF-bearing mixes again outperform NP40 and NP50, with strong significance (e.g., NP40 vs. SF8-NP00, p = 0.0012), confirming accelerated microstructural development. The inhibitor has minimal influence at early age but begins to produce measurable effects in synergy with SF, especially in mixes containing both SF and NP.

Figure 16

Figure 16. Splitting tensile strength of all binder combinations at 7, 28, 56, and 90 days with Tukey-adjusted pairwise significance; (a) for 7 days, (b) for 28 days, (c) for 56 days, and (d) for 90 days.

By 56 and 90 days, the significance patterns stabilise into a coherent performance hierarchy. High NP replacement (50%) remains consistently the weakest group (p < 0.01–0.001 vs. most mixes), whereas SF-bearing mixes (both with and without inhibitor) increasingly dominate the upper range of tensile strength improvements (p < 0.05–0.001). The inhibitor (INH) produces delayed but statistically relevant effects at later ages, especially when combined with SF, showing significant improvements over NP50 and some NP30–NP40 mixes at 90 days (p < 0.01–0.001). The overarching trend across all ages indicates that (i) moderate NP levels (20%–40%) retain tensile strength, (ii) silica fume is the most influential modifier, consistently elevating tensile resistance across ages, and (iii) high NP (50%) requires SF and/or inhibitor to recover structural performance. These statistically validated patterns align with the mechanistic expectations of pozzolanic reaction kinetics, ITZ refinement, and long-term microstructural development.

3.5.3 RCPT statistical interpretation

The 28-day RCPT results (Figure 17) demonstrate a statistically robust hierarchy of chloride-ion penetrability across the binder systems. All experimental binders produced significantly lower charge passed than the control mixture (p < 0.01 [marked **]), establishing that the incorporation of natural pozzolana, silica fume, and inhibitor fundamentally alters the transport characteristics of the composite matrix. The NP-only mixes exhibited moderate but statistically significant permeability reductions (mean reductions ranging from 196 to 903 Coulombs), reflecting the partial refinement of the capillary network through pozzolanic reactions. Importantly, no significant differences were observed among NP20, NP30, NP40, and NP50, indicating that once the primary pore refinement associated with NP takes place, additional NP content does not proportionally enhance transport resistance. This plateau effect is consistent with established percolation-threshold behaviour in SCM-modified concretes.

Figure 17

Figure 17. One-way ANOVA with Tukey’s HSD demonstrating significant differences in RCPT (Q) among binder types at 28 days. Asterisks denote pairwise significance levels; error bars represent mean ± SD.

In contrast, mixes containing silica fume produced the largest and most statistically distinguishable reductions in RCPT, with mean decreases exceeding 1,300 Coulombs relative to the control (p < 0.0001). The inclusion of silica fume in NP50 mixes further deepened the decline in ionic conductivity, confirming the dominant role of ultrafine silica in generating a discontinuous pore structure and highly polymerised C–S–H phases. Notably, the presence of the corrosion inhibitor did not significantly alter RCPT when used in isolation but yielded an additional reduction when co-incorporated with silica fume, as evidenced by the marked separation of the INH-SF systems from their SF-only counterparts. This behaviour suggests a synergistic mechanism, wherein the inhibitor modifies pore-solution chemistry and the interfacial transition zone to enhance the already refined silica-fume microstructure, thus lowering ionic mobility. Collectively, the RCPT statistical outputs confirm a clear mechanistic progression of moderate transport refinement via NP substitution, pronounced refinement via silica fume, and a synergistic peak in performance when silica fume and inhibitor are jointly introduced, providing rigorous quantitative substantiation for the proposed binder-optimization framework.

3.5.4 Cross-property interpretation of mechanical and durability performance

A holistic assessment of the dataset demonstrates that the mechanical and durability domains diverge systematically, especially in NP-rich binders. While high NP contents lower early and long-term strengths (e.g., NP50 remaining ∼12–18 MPa below control at 7 days and ∼11–17 MPa below at 28–180 days), they concurrently yield statistically superior durability, with RCPT reductions exceeding 60%–75% relative to control and achieving p < 10⁻⁶ significance against most mechanical high-strength mixes. This contrast underscores that strength significance and durability significance follow different trajectories, confirming that structural design and exposure performance cannot be inferred from compressive data alone.

Silica fume establishes the strongest multi-property significance, as SF8–NP00 consistently ranks among top performers in both domains: its compressive strength surpasses NP-only mixes by +10 to +23 MPa across all ages (p < 0.0001), while simultaneously producing some of the lowest RCPT charges (<600 C). This dual-domain significance, driven by accelerated C–S–H formation and advanced pore refinement, positions SF-modified binders as the only group where mechanical and transport significance converge, marking them as the most statistically balanced performers.

Inhibitor-containing systems exhibit a selective shift in significance: although INH–SF8 mixes consistently improve strength by +4 to +10 MPa over NP-only systems (p < 0.01), their RCPT improvements plateau relative to SF-only mixes, remaining within ±100 °C of SF8–NP00 and not reaching the same ultra-low transport category. The statistical pattern, therefore, supports a mechanistic interpretation where INH modifies corrosion kinetics rather than pore structure, strengthening concrete but not fundamentally altering ion transport pathways.

Collectively, cross-property significance mapping reveals that no single binder dominates all domains; instead, the optimal balance emerges from NP–SF synergy. These mixes deliver robust strength development and substantial reductions in transport coefficients, achieving significant effects across properties (p < 0.001) while maintaining the sustainability benefits of high-volume NP replacement. This integrated interpretation establishes a statistically defensible framework for selecting green binders in exposure-driven design, where durability significance can outweigh strength significance depending on project priorities.

Although the present study did not include SEM, XRD, or MIP analyses, the mechanistic interpretations are supported by previously published microstructural evidence on natural pozzolana and silica fume systems. Prior studies (Hamada et al., 2023a; Kupwade-Patil et al., 2018; Merida and Kharchi, 2017; Merida and Kharchi, 2015) have demonstrated that NP and SF promote pore refinement, increase C–S–H gel formation, and reduce capillary porosity mechanisms consistent with the improved mechanical and durability performance observed herein. Future work will incorporate SEM, XRD, and MIP testing to directly visualize hydration products and microstructural evolution in NP–SF–INH blended concretes.

4 Conclusion

This research presents a comprehensive experimental investigation into the properties of concrete mixtures incorporating locally sourced NP as a SCM, alongside SF, INH, and SP. The study systematically evaluated the fresh, mechanical, and durability characteristics of these modified concretes through a series of standardized tests, including slump, initial temperature, compressive strength, splitting tensile strength, rapid chloride permeability, and sulphate resistance. The findings provide valuable insights into the complex interactions between these constituents and their collective impact on concrete behavior, where several key conclusions can be drawn:

4.1 Fresh properties

• NP and SF significantly reduced slump and mix temperature due to increased water demand and lower heat of hydration.

• SP markedly improved workability in NP-rich mixes but only partially compensated for the severe slump reduction in SF-containing binders.

• The enhanced rheology with SP is attributed to improved dispersion and reduced interparticle friction, consistent with established fluidification mechanisms.

4.2 Mechanical properties

• NP exhibited clear long-term pozzolanic activity; NP20–NP30 achieved 180-day strengths comparable to the control, while higher NP contents (40%–50%) showed strong late-age recovery but remained below the control in absolute values.

• SF8-NP00 delivered the highest compressive and tensile strengths, demonstrating the dominant role of silica fume in pore refinement and C–S–H densification.

• Combined NP + SF mixes achieved balanced long-term performance, though optimization is required at high NP replacement levels.

• Splitting tensile strength strongly correlated with compressive strength (r = 0.94–0.98), confirming consistent mechanical behaviour across binder systems.

• INH induced slight early-age delays but maintained adequate long-term mechanical performance.

4.3 Durability performance

• NP reduced chloride ion penetrability by 40%–45%, achieving “Low” permeability per ASTM C1202.

• SF and INH further reduced charge passed by up to 75%, placing these mixes in the “Very Low” permeability category.

• All mixes remained structurally intact under sulphate exposure, showing only superficial salting.

• Moderate NP and SF combinations (e.g., NP40, SF8-NP50) exhibited strength enhancement under sulphate attack, with SRCSR values exceeding 110%.

• NP + SF + INH systems achieved the highest densification (SRR >100%), confirming synergistic improvements in long-term chemical and physical durability.

4.4 Statistical significance and cross-property Insight

• ANOVA revealed that binder composition had a highly significant effect on strength and transport properties (p < 0.001).

• High-volume NP mixes demonstrated statistically significant reductions in strength yet produced the most substantial enhancements in chloride resistance indicating the need for dual performance criteria in binder selection.

• SF emerged as the most influential modifier, consistently forming the top-performing cluster for both mechanical and durability outcomes.

• INH improved mechanical responses relative to NP-only binders but offered limited additional reduction in chloride transport.

• Cross-property analyses confirmed that mechanical strength and durability do not follow identical significance patterns, emphasizing the importance of performance-based mix optimization in exposure-driven environments.

4.5 Recommendations for further studies

• Investigate variable SP dosages in relation to water/binder ratios to maximize strength gain, especially in high-SF and high-NP mixes.

• Extend sulfate exposure testing to 365 days and beyond to observe late-age degradation or stabilization, particularly in high NP and INH combinations.

• The microstructural mechanisms inferred in this work need to be validated in future research through direct SEM/MIP/XRD characterization to further substantiate the observed performance trends.

• Explore the use of multiple supplementary cementitious materials (SCMs) in varied ratios to identify the best performing combinations for strength and durability.

• Conduct electrochemical corrosion tests to validate reinforcement protection in chloride-rich environments.

• Investigate drying shrinkage and creep behavior of pozzolan-modified concretes to ensure structural stability over time.

• Integrate multi-objective optimization and AI-driven significance modelling to predict binder compositions that simultaneously maximize strength, durability, and sustainability metrics based on statistically validated datasets.

• Expand statistical analysis to microstructural predictors using mixed-effects and regression-hierarchical modelling. This will enable quantifying why specific binders show significance patterns, bridging microstructure–property statistical relations.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Ethics statement

Written informed consent was obtained from the individual(s)/minor(s)’ legal guardian/next of kin, for the publication of any potentially identifiable images or data included in this article.

Author contributions

WK: Conceptualization, Supervision, Validation, Writing – review and editing. EA: Methodology, Investigation, Formal analysis, Writing – original draft. AS: Supervision, Validation, Writing – review and editing. RD: Supervision, Writing – review and editing.

Funding

The author(s) declared that financial support was not received for this work and/or its publication.

Acknowledgements

The authors gratefully acknowledge the invaluable support provided by King Abdulaziz University, including access to experimental laboratory facilities. Furthermore, the authors also wish to express their gratitude to Yanbu Cement Factory for their kind provision of the cement required for the experimental investigations. Also, the authors acknowledge Teba factory for their kind provision of the natural pozzolana required for the experimental investigations.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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