Fly Ash and Natural Pozzolana Impacts on Sustainable Concrete Permeability and Mechanical Properties

: This paper studied the e ﬀ ect of ﬂ y ash (FA) and natural pozzolana (NP) as partial cement substitutes on sustainable concrete permeability and mechanical and microstructural properties. Batches with 10, 20, 30, 40, and 50% FA and NP replacements for cement were prepared and tested for compressive strength after 7, 28


Introduction
Pozzolanic materials are environmentally friendly supplemental cementitious materials (SCMs) widely used in concrete as a partial cement substitute. SCMs have recently become popular due to the growing need for durable and sustainable concrete with strong resistance to harmful long-term chemical reactions [1,2]. Furthermore, ecofriendly cement alternatives help reduce carbon dioxide emissions in the atmosphere [3]. Also, using recycled and waste materials in concrete is considered the most practical alternative due to its sustainability and environmental advantages [4]. The most widely used SCM in cement-based products is currently fly ash (FA), which is a by-product of pulverized coal combustion in electric power plants [5]. FA reacts rapidly with calcium hydroxide Ca(OH)2 to produce cementitious compounds promoting concrete hydration [6,7]. It has become an industry standard for generating high-quality, durable concrete due to its low cost and extensive availability in many locations. Numerous novel research results have been achieved on the mechanical properties and microstructure analysis of FA cementitious composites. De Maeijer et al. [8] studied the effect of FA on cementitious composites. The findings showed that an increased fineness of FA contributes to better mixture workability. Also, replacing cement with FA enhanced the strength, resistivity, chloride migration coefficient, and alkali-silica reaction for concrete samples. Teixeira et al. [9] reported that FA improves the environmental performance of concrete when used as a partial cement replacement. Moreover, De Matos et al. [10] reported that FA replacement of cement would enhance fresh and hardened concrete properties, such as compressive and flexural strengths and modulus of elasticity, particularly at replacement ratios of 50 and 60%. Similarly, Huang et al. [11] reported that concrete mixes containing FA demonstrated significant improvement in their mechanical properties after 365 curing days. On the other hand, refs. [12,13] reported the possibility of using up to 80% Class F fly ash as a cement replacement in concrete. Moreover, several experimental studies reported improvements in fracture toughness [14][15][16], corrosion resistance, pore structure [17], ion penetration resistance [18], and high-temperature resistance [19]. Thus, FA produces an environmentally safe green concrete material and improves structural, economic, and environmental performances. However, the availability of FA is currently threatened by several environmental restrictions [20]. As a result, viable natural alternatives for FA with similar pozzolanic reactivity that might lead to enhanced concrete mechanical and rheological qualities became necessary. Recently, Liu et al. [21] have investigated the potential of producing alkali-activated cementitious material with municipal solid waste incineration fly ash (MSWIFA) as an economical and green material to replace traditional cement. Although the utilization of MSWIFA to produce alkaliactivated cementitious materials was found beneficial, the authors have reported that MSWIFA was encapsulated in such alkali-activated materials, which promotes the risk of dioxin leakage. Hence, they have demonstrated the urgent necessity of finding low-cost dioxin leaching agents to be incorporated into the production process. Li et al. [22] utilized three types of cementitious materials, namely, ground granulated blast furnace slag (GGBFS), fly ash (FA), and ordinary Portland cement (OPC), to produce different types of artificial lightweight coarse aggregates (ALCAs) using cold bonding with the municipal solid waste incineration bottom ash (MSWIBA). The produced ALCAs were used to replace 0%, 30%, 60%, and 100% of natural aggregates used in the production of lightweight concrete. The results showed that the ALCAs produced with GGBFS and FA adhesives (with no OPC) are most efficient in improving the durability of the concert and reducing CO2 emissions as well.
On the other hand, natural pozzolana (NP), which is a natural material resulting from volcanic tuff, with variable chemical, mineralogical, and reactivity properties, has recently experienced high use in concrete [23][24][25][26]. NP, which is found abundantly in many geographical locations, is a cost-effective material to be used in concrete [27]. Many researchers investigated the benefits of using NP in concrete. Cheng Liu et al. [28] found that NP may effectively replace OPC by 30-50% in volume to produce durable and costcompetitive concrete while meeting a range of strength criteria for varied applications. Ababneh and Matalkah [29] reported that concrete batches with high SiO2 content produced high early-age compressive strengths, while those with high CaO concentration showed better long-term strength development. Moreover, the experimental results showed a reduction in the susceptibility to deleterious alkali-silica reactions. Hijazin et al. [30] reported that replacing 39% of ordinary Portland cement (OPC) with NP decreased concrete compressive strength by 15% but improved its durability by reducing its permeability to chloride ions by 66% and its self-shrinkage by 40%. Hossain [31] studied the rheological and mechanical properties of concrete containing NP. The findings demonstrated that mixtures containing NP have better durability properties. Also, the 28day compressive strength of concrete containing high content of VA by 40-50% by weight was accepted, which can hardly satisfy the requirement for structural concrete applications. Other studies also concluded that 50% NP replacement of cement improved the concrete resistance to alkali-silica reactions and sulfate attacks [32].
This study is novel and different from other previous similar studies as it does not only investigate the effect of utilizing the FA and NP on the mechanical properties of ordinary concrete but also compares the effect of different dosages of both additives on the same concrete mixes and investigates the microstructure, permeability, and density of such mixes. That is because several impending environmental and safety regulations currently threaten the availability and quality of FA, and it has become imperative to find viable natural alternatives to FA with similar pozzolanic reactivity that can lead to improved concrete mechanical and rheological properties. Accordingly, the results of this study shed light on the overall behavior of concrete mixes containing FA and NP, demonstrate the potential of utilizing NP instead of FA and recommend the optimum dosage to be utilized in concrete mixes. For this purpose, 10, 20, 30, 40, and 50% FA and NP replacements of cement were performed. Compressive strengths were determined after 7, 28, and 56 days of curing, while flexural strengths and permeability were obtained after 28 curing days. Density values were also obtained at various ages. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) were used to analyze the microstructure of the fractured samples. Therefore, the results of this study will allow future research to focus on improving and modifying the physical properties and compositions of these pozzolanic materials to achieve better replacement ratios for cement in concrete mixtures.

Materials
In this study, Portland cement (CEM II/B-P 42.5 N), complying with EN 197-1:2011, was provided by the Manaseer Industrial Complex in Jordan. The chemical composition and physical properties of the cement used are provided in Tables 1 and 2, respectively. The locally available coarse and fine aggregates were compliant with ASTM C-33 and consisted of dolomite limestone and natural sand, respectively. A polycarboxylate-ether superplasticizer with the commercial name, Master Glenium 51, was made available by BASF Construction Chemicals, UAE LLC. The properties of Master Glenium 51 are summarized in Table 3. The Class F fly ash, which was used in this study, was in compliance with ASTM C-618. The fly ash was provided by Al-Faiha for Engineering Products, Inc. Class N natural pozzolana, compliant with ASTM C-618, was also provided by the Manaseer Industrial Complex. The chemical compositions of FA and NP, which are summarized in Table 4, were determined using an ARL 9900 IntelliPower X-ray spectrophotometer. Moreover, FA and NP TEM pictures, which are shown in Figure 1, were taken using a 500 µm scale and are shown the distribution, shape, and size of the particles of the pozzolanic materials.

Samples' Preparation
Sample preparation was carried out in two stages. Firstly, dry materials, including cement and fine and coarse aggregates, and cement substitutes, whether FA or NP, were dry mixed for 15 min using the proportions summarized in Table 5. The concrete mix proportions were determined in accordance with ACI 211.4R-93 [33]. A 28-day compressive strength of 50 MPa was targeted in our mix design. After that, a watercontaining superplasticizer was mixed with the dry materials in a rotary mixer for 30 min. After the concrete mixing process was completed, the material was poured into steel molds. After 24 h, the concrete samples were removed from the molds and stored in a curing tank at a constant temperature of 25 °C and relative humidity of 90%. Figure 2 shows concrete samples after demolding.  A Malvern particle size analyzer was used to determine the particle size distribution (PSD) of FA and NP. The angular scattered light intensity variation is measured after a laser beam passes through a dispersed small sample (0.25 g). Then, Mie's theory was used to determine the PSD in the samples.

Slump
To determine the workability of the concrete batches, a slump test was performed according to ASTM C 143 [34]. Fresh concrete is poured into the slump cone in three layers (1/3 height of mold), and each layer is compacted 25 times. After that, the cone is lifted, and the height of the fresh concrete is measured. The workability of fresh concrete for each batch was determined before pouring the concrete into the molds.

Mechanical Properties
The compressive strength test was performed in accordance with ASTM C39 [35]. For each mix, nine 100 mm × 100 mm × 100 mm cubes were prepared and tested after 7, 28, and 56 curing days using the strength testing machine. Also, three beams of 550 mm in length and 150 mm × 150 mm in cross-section were tested to determine the flexural strength in accordance with ASTM C78 [36].

Permeability
The resistance of concrete to water penetration under exerting pressure was determined according to DIN-1048 (Part 5) (DIN 1991) after 28 curing days [37]. Three cubic (150 mm × 150 mm × 150 mm) concrete samples from each batch were exposed from below the surface to a constant water pressure of 0.5 N/mm 2 for three days. Immediately after the pressure release, the sample was removed and split down along the center. After 10 min, the drying signs appeared on the divided face. After that, the maximum penetration depth was measured to determine the water permeability.

Microstructural Analysis
The microstructure analysis of the fractured samples was performed to understand the impacts of using FA and NP on concrete properties. High-resolution images were taken using an FEI-Nova Nano SEM Model microscope. The samples were prepared by coating their surfaces with gold-palladium to remove excess surface charges after drying them in a vacuum chamber. Then, the scanning images were taken. In addition, an EDX analysis was performed by analyzing the sample surfaces to verify their elemental composition.

Density Evolution
The density evolution of all concrete batches was evaluated by the difference between the sample average density before and after curing. The samples were weighed immediately after removal from the molds and before curing. Then, the samples were weighed after curing for 7, 28, and 56 days. The density percentage difference for each batch relative to the reference was calculated to know the effect of FA and NP on the density.

Cost Analysis
The concrete batch costs included those of the materials required to prepare each batch according to the commercial market price. Then, an evaluation of the batches was carried out depending on their cost and their role in the concrete strength using the economy index, which represents (compressive strength/cost).

Particle Size Distribution (PSD)
The fineness of FA and NP is one of the important properties that have a role in the performance of concrete and has an impact on the rheological and mechanical properties, which cannot be excluded. FA and NP samples particle size distribution curves are shown in Figure 3. The FA and NP percentages passing the 25-micrometer sieve were 69 and 75%, respectively. The NP material was generally finer than Class F fly ash. However, FA particle size distribution was larger than that of NP. PSD is among the most important physical properties affecting the reactivity of pozzolanic materials in concrete mixes. The materials with larger particle size distributions and finer particles generally have an increased reactivity in the mix, which optimizes the concrete pore structure. So, the filling effect of pozzolana particles has reduced the void between the particles of cement and aggregates, thus resulting in improved density and strength generally in the concrete mix, as reported by Mohapatra et al. [8,38].

Workability
Slump tests were performed to investigate the impacts of FA and NP on concrete workability. Figure 4 shows the obtained slump results. Compared with the control (0% fly ash), the slump values of the concrete batches prepared using 10, 20, 30, and 40% FA decreased by 16.7, 33.3, 66.7, and 38.9%, respectively. By contrast, the slump of the 10% NP concrete batch increased by 11.1%. However, it decreased for 20% and 30% NP concrete batches to 11.1 and 55%, respectively. After that, the slump increased again by 55 and 66% for 40 and 50% NP concrete batches, respectively. Generally, FA content increase contributes to a water demand reduction for fresh concrete. Class F fly ash is generally characterized by particle smooth surface and spherical shape. Furthermore, the fly ash fine particle's suitable size distribution improves the workability properties of concrete [39]. Despite the porous structure, the increase in the slump for concrete batches is attributed to small particle size and lower specific gravity rendering it to fill a higher volume of pores in a concrete matrix, thus providing a lubricating surface to other cement particles as well as aggregate [3]. The same conclusions can be drawn for NP, where the water demand reduction becomes more prominent as the cement replacement content increases to a limit of 30%. Beyond this percentage, more NP use will not add any additional benefits in terms of water demand. Concrete mixes with high slump values will generally yield lower strength values. Thus, the quantities of FA and NP need to be optimized to achieve a desirable strength for concrete. Also, the water-reducing agent was used in this study at a constant rate among all mixtures, which is 200 g, i.e., a low percentage compared to the percentage of water in the mixture, which is 10 kg. However, with the addition of water-reducing agents, the rheological parameters of the cementitious composites were significantly improved. As the workability of the concrete mixture improves, but to a specific limit when the work to maintain stability, the amount of waterreducing agent is saturated content.

Mechanical Properties
The mechanical properties results for the concrete batches are summarized in Table  6, including the percent change in the compressive and flexural strengths when compared with those of the plain concrete samples (0% FA or NA). The relationships between the compressive strength results for the FA and NP concrete batches at 7, 28, and 56 curing days are shown in Figures 5 and 6, respectively. Moreover, Figure 7 shows the flexural strength test results after 28 curing days. Standard deviations were also determined for each batch.      Figure 5 shows the concrete compressive strength development results for FA batches. The results show a significant improvement for concrete batches with low FA content. The compressive strength growth rate reached its maximum of 3.24% with an FA content of 10% at the 7 days. However, the compressive strength of batches with 20%, 30%, 40%, and 50% FA content decreased by 16.45%, 16.19%, 32.98%, and 39.72%, respectively. FA batches need a moist treatment for more than 7 days because the pozzolanic reaction of FA is very slow at early ages, so it behaves as a fine aggregate that does not change the initial pore size distribution in the concrete matrix. But, at later ages, fly ash starts to have greater chemical effects and improve concrete properties. The strength of the 10% FA batch increased by 8.92% after 28 days. This is due to the fact that with increasing curing time, FA enhances cement hydration when it reacts with other compounds, such as silicates and aluminates, by contributing to the formation of dense hydration products such as calcium-silicates-hydrates (C-S-H), which changes the bigger pores to smaller pores because a large amount of hydration products fills the initial pore system and improves the concrete matrix structure, and this is responsible for the strength performance. However, the compressive strength decreased by 12.97% and 21.6% when the FA replacement content increased to 40% and 50%, respectively. The substitution of FA came at the expense of using less cement. The decrease in compressive strength was more pronounced when higher FA percentages were used as cement replacements. After 56 moist curing days, the compressive strength has increased by 11.63%, 1%, and 6.06% for batches with 10%, 20%, and 30% FA content, respectively. This increase may be attributed to a large amount of Ca (OH)2 that was generated from cement and FA particle hydration due to extended curing days. But, the compressive strength of FA batches also decreased when the FA content reached 40% and 50% by 13.5% and 24.3%, respectively, compared to the control batch. The compressive strength of concrete samples decreased with increasing FA content. This is an indication that FA is not efficiently used and acts as a fine aggregate in the mix instead of a supplementary cementitious material. Because it does not react as required, FA exhibits negative impacts on compressive strength. Moreover, the hydration product decreases as the cement content in the concrete mix drops. As a result, the amount of Ca(OH)2, which reacts with pozzolanic materials, decreases. Therefore, the produced amounts of C-S-H and C-H decrease. Thus, FA is used less efficiently.

Compressive Strength Development
It is worth noting that the free CaO content is higher in cement (greater than 56%) than in Class F FA (<2%) and NP (<10%). The component CaO is the main responsible for concrete strength development. At higher FA contents, the concrete mix design needs to be revised by adjusting the water-to-cementitious ratio or adding silica fume, etc. The Control 10% 20% 30% 40% 50%

Replacement of Cement Ratio%
FA NP lower compressive strength could be acceptable in certain concrete applications such as pipelines, sidewalks, curbs, culverts, etc. Similar outcomes were reported by Wang et al. [40]. The compressive strength results for NP-modified batches are presented in Figure 6. The compressive strengths of 10% NP concrete samples increased by 7.95, 5.55, and 8.75% after 7, 28, and 56 curing days, respectively, when compared to those of the controls. A strength decrease was observed when NP content was increased beyond 10% in the remaining batches Figure 6. The use of high NP contents lowered the compressive strength even at later curing ages (up to 56 days). These findings can be attributed to the relatively low rate of pozzolanic reaction activities when compared with that of the control and also to the presence of NP-less reactive constituents. Earlier discussions on fly ash performance in concrete are valid when discussing natural pozzolanic materials. These findings are in agreement with those obtained by Ababneh and Matalkah [29], who attributed the strength reduction to the NPʹs relatively low SiO2 content, 47.85 wt.%. As shown in Table 4, the SiO2 content has a significant influence because of its major role in pozzolanic reactions [41]. Nazari et al. [42] have reported that the NP components Fe2O₃ and Al2O3 contribute to the enhancement of the concrete mix's compressive strength. However, the impact of the two NP components was not prevalent in our results.

Flexural Strength
As shown in Figure 7, the flexural strengths of all FA concrete batches increased compared to those of the controls. The flexural strength for 10, 20, 30, 40, and 50% FA concrete batches increased by 30.1, 26.8, 29.7, 25.1%, and 15.6%, respectively, compared to those of the controls. The flexural strengths for 10 and 20% NP concrete mixes increased to 6.17 and 5.40 MPa, respectively. It is worth noting that the 28-day flexural strength for the control mix was only 4.55 MPa. However, the flexural strengths for 30, 40, and 50% NP concrete mixes decreased by 2.4, 2.8, and 23.7%, respectively, when compared to those of the controls. The flexural strength improvements for concrete containing low quantities of pozzolanic materials can be attributed to the continuity of the pozzolanic reactions, which strengthens the bond between the aggregates and the paste. The presence of large quantities of unreacted particles acts more as fine aggregates that fill the pores in the concrete matrix and decrease the interface porosity [43]. Figure 8 shows the impacts of FA and NP on concrete water penetration depths. It can be observed that FA concrete batches had lower water permeability values than those of NP concrete mixes. The water penetration depths for 10, 20, 30, 40, and 50% FA batches were equal to 34.8, 34.8, 30.3, 60.9, and 78.3%, respectively. Moreover, the water penetration depths for 10, 20, 30, 40, and 50% NP concrete batches were equal to 26.1, 43.5, 21.7, 43.5, and 56.5%, respectively. The pozzolana particles, which did not react and remained in the micropores in the mix, improved the concrete pore microstructure and lowered its permeability. Increasing FA and NP contents generally leads to increased contents of the compounds such as SiO2, Al2O3, and Fe2O3. Consequently, such compounds react with Ca(OH)2 to produce more calcium-silicate-hydrate (C-S-H) and calcium hydrate (C-H), which significantly improves the density of the concrete microstructure by reducing the micropore structure. Additionally, the hydration products have a positive impact on the transition zone properties by making concrete denser. Because of their high pozzolan contents, FA and NP improve the microstructure by adding more C-S-H gel in place of CH crystals. As a result, the transition zone gets thinner, and the probability of micro-cracking decreases. This results in the reduction in permeability in concrete [37]. Figure 9 shows a concrete sample after splitting it along the center. The maximum penetration depth was measured when the drying signs appeared on the divided face.

Microstructural Properties
The microstructure and morphology of randomly selected samples were analyzed after 28 days. Figure 10 shows the cement hydration and pozzolanic reaction hydration products. It is worth noting that FA particles in the concrete mix slowly reacted with CH crystals to form more compact and cohesive C-S-H gel flakes, which contributed to further mix densification through pore filling and hydration product capillary blockage. The increase in the percentage of pozzolanic material contents led to additional production and growth of hydrates, which contributed to mechanical strength and permeability improvements compared to those of the control batch. On the other hand, large amounts of FA remained unreacted or partially reactive, particularly the large particles, when the percent cement replacement reached 40 and 50%, which reduced concrete property improvement. Meanwhile, additional NP content of up to 10% enhanced the matrix microstructure due to the increased quantity of C-S-H gel compared to that of the control batch, as shown in Figure 11. However, increasing NP content led to a concrete mix with poor porous and voided microstructure [44]. The weak and porous microstructure resulted in low compressive strength and high porosity. This finding is supported by the EDX analysis results, which are summarized in Table 7. The results show the elemental composition of randomly selected areas from the tested samples. It is known that the Ca/Si atom ratio indicates the amount of C-S-H, which is responsible for concrete strength. The Ca/Si ratios were 1.41, 1.75, and 1.68 for 10, 20, and 30% FA batches, respectively, while the control Ca/Si ratio was 2.83. On the other hand, the Ca/Si ratios in 40 and 50% FA batches increased to 3.40 and 3.58, respectively. These findings agree with 10% 20% 30% 40% 50%

Replacement of Cement Ratio%
FA NP the results reported by Ghanim et al. [4], who indicated that a Ca/Si ratio lower than 2.0 results in higher pozzolanic activity, improved mechanical properties, and denser microstructure. It is worth noting that the Ca/Si ratio of the 10% NP batch decreased to reach the value of 2.51. However, the ratio has increased for the remaining NP percent contents, as shown in Table 7. Table 4 shows that FA contains a lower amount of Ca compared to that NP and a high amount of Si, which has decreased its Ca/Si ratio. In other words, an increase in the NP contents led to an increase in the Ca/Si ratio, which contributed to the reduction in NP batch compressive strengths.    Table 8 shows the density for all concrete batches compared to those of the control mix (i.e., a difference in density before curing and after 7, 28, and 56 curing days). Most batches had higher densities than that of plain concrete. Generally, the density increased with curing age, as shown in Figure 12.

Density Evolution
The 10, 20, and 30% FA batches resulted in the best improvement in terms of density. There was a positive trend between increased density with strength. Also, there was an increase in density with curing times for all NP batches. The density improvement with curing time can be attributed to the pozzolanic material presence in the batches, as cement alone cannot fill the voids in the concrete matrix. Therefore, the FA and NP materials, which are characterized by their small size, large surface areas, and smooth texture, fill any voids present in the concrete matrix. They also contribute to the pozzolanic reactions creating a dense microstructure [45], which leads to increased strength and reduced permeability, as discussed previously.   Table 9 summarizes the cost analysis results for all batches. The economy index (compressive strength/cost) is used herein to evaluate the cost of the pozzolanic materials and their contribution to the concrete strength. Table 9 shows that the 10% FA batch had the highest economy index value. Consequently, the 10% FA batch is the ideal one, followed by the 10% NP mix.

Cost Analysis
Summarizing all of the results presented herein and placing emphasis on the importance of this study as a trial of figuring out the effect of different FA and NP replacement percentages of cement on the concrete's properties, including not only strength but also workability, durability, microstructure, density, and cost, it could be demonstrated that both FA and NP can be utilized as partial replacement to cement in concrete mixes and the optimum value of such replacement could be concluded. The slump values were reduced for all FA mixes while it increased for 10% NP and decreased for higher NP dosages up to 30%. For the compressive and flexural strengths, the results  showed enhancement in the 28-day compressive strength of FA and NP mixes up to 10% replacement only. On the contrary, the 28-day flexural strength was increased for all FA replacement percentages and for NP replacement by up to 20%. Moreover, the permeability of all concrete mixes was reduced by up to 20% with the replacement of both FA and NP. Studying the samples' microstructures, the calculated Ca/Si ratio decreased for FA mixes by up to 30% and for NP mixes by up to 10%, suggesting enhancement of the concrete's mechanical properties of such mixes. The density of the prepared mixes was found optimal for 20% and 30% replacement in the FA mixes, and 10% for the NP mixes. Finally, according to the calculated economy indexes, a 10% replacement of both FA and NP was found to be the most convenient. Considering the abovementioned observations, one may conclude that both FA and NP could be utilized as partial replacements of cement in concrete mixes, and 10% replacement may be considered the optimum replacement percentage regardless of the type of cementitious material to achieve the most favorable fresh and hardened concrete properties as well as economic and environmental benefits.

Conclusions
The paper investigated the fly ash and natural pozzolana impacts on fresh and hardened concrete properties. The main findings are presented below: 1. Replacing the cement with 30% FA and 30% NP improved the workability of fresh concrete by up to 66.6% and 55%, respectively, when compared with that of the control mix (0% FA or NP). This shows the importance of these pozzolanic materials in improving fresh concrete properties.
2. The 56-day compressive strengths for the 10%, 20%, and 30% FA concrete batches increased by 11.63%, 1%, and 6.06%, respectively, compared to those of the control mixes. Meanwhile, the 56-day compressive strength for the 10% NP batch increased by 8.75% over that of the control mix; 3. The 28-day flexural strengths for the 10, 20, 30, 40, and 50% FA concrete batches increased by 30.1, 26.8, 29.7, 25.1, and 15.6%, respectively, compared to those of the control mixes. The 28-day flexural strengths for the 10 and 20% NP concrete batches increased by 35.60 and 18.68%, respectively, compared to those of the control mixes; 4. Class F fly ash and natural pozzolana improved the permeability of concrete mixes.
The permeability for 50% FA and NP concrete batches decreased by 78.3% and 56.5%, respectively, compared to those of the control mixes; 5. A microstructural analysis showed an improvement in the densification of the concrete matrix by adding FA and NP due to the formation of C-S-H crystals, which contributed to improved concrete performance; 6. FA and NP concrete had an improved density after longer curing periods when compared with those of control mixes. The products of the pozzolanic reactions generally fill the pores and create a dense microstructure leading to improvements in the concrete properties; 7. Based on the economic index results, the 10% FA concrete batch is the ideal one, followed by the 10% NP mix. Considering the environmental restrictions on FA, NP can be utilized instead with an optimum value of 10% replacement of cement content; 8. The results obtained in this are consistent with the recommendations for the use of FA and NP in concrete admixtures, according to ASTM C618-17, which stipulates that Class F pozzolan can be used with a replacement ratio of 12-20% by the cement content. Funding: This research received no external funding.

Data Availability Statement:
The data used to support the findings of this study are available, upon request, from the corresponding author.