Sulfate activation of wheat straw ash to enhance the properties of high-performance concrete with recycled aggregates and waste tire steel fibers

Abstract

A sustainable alternative to conventional concrete involves using recycled aggregates (RA) instead of natural aggregates (NA) and incorporating wheat straw ash (WSA) as a partial replacement for Portland cement. The demand for high-performance concrete (HPC) is rising due to the need for architecturally complex structures and long-span bridges, but HPC’s low ductility necessitates reinforcement. Waste tire steel fibers (WTSFs) are gaining popularity for their tensile strength. However, WSA-RA concrete’s low early strength is a challenge. Chemical activators like sodium sulfate can enhance early-age strength. This study evaluated the durability and strength of fiber-reinforced concrete with both inactivated and activated WSA. Tests included compressive strength, indirect tensile strength, modulus of rupture (MOR), acid attack resistance, chloride penetration, sorptivity, and water absorption. Activated WSA-RA concrete showed significantly improved early strength. The mixture with 30% RA, 40% WSA, WTSFs, and activator exhibited the highest strength at 90 days. At 60% RA content, activated concrete with 40% WSA and 2.5% WTSFs outperformed the control. Durability was enhanced with a 14–17% reduction in water absorption and sorptivity and a 25.2% decrease in chloride penetration. Acid resistance improved by 26%. X-ray diffraction (XRD) confirmed these findings with elevated hydration product peaks. This study demonstrates that chemical activation of WSA optimizes the engineering properties of WSA-modified HPC with WTSFs and RA, providing a sustainable solution to their challenges.

1. Introduction

The consequences of producing and utilizing cement concrete can be challenging to comprehend, as they can be positive and negative, depending on the situation. A key component in creating concrete is Ordinary Portland Cement (OPC), which Abadel et al. [1, 2] noted that it has various economic, social, and environmental implications that apply to concrete. Althoey et al. [3, 4] revealed that the emission of greenhouse gases (GHGs) from the production of OPC accounts for approximately 7% of the total global greenhouse gas emissions. Rapid urban growth and the need for modern structures have increased the demand for OPC, leading to adverse effects on the eco-friendliness and social aspects of OPC production. Past literature revealed [5–7] that China is the largest cement producer, with an annual output of nearly 22000 million tons. At the same time, the USA, Japan, and India collectively contribute to over 80% of the world’s OPC production. Zaid et al. [8] revealed that developing 1.0 kg of OPC generates approximately 0.55 to 1.0 kilograms of global GHGs, depending on the type of energy source and production method used. Although other concrete components, such as fine and coarse aggregates, have a smaller Carbon dioxide (CO₂) footprint, their cost and carbon dioxide emissions are heavily influenced by the transportation distances between the concrete batching plant and the aggregate stone quarry. As a result, almost 85% of OPC-based concrete emissions depend on the binder component of concrete. From studying the literature [9–12], it was noted that minimizing cement consumption is the most effective approach to making concrete eco-friendly. Using agricultural waste or discarded materials containing binding and pozzolanic properties can effectively obtain alternative OPC replacements. Incorporating these discarded materials in place of OPC can significantly reduce the CO₂ footprint of concrete. It is essential to evaluate the performance of potential OPC substitute products by examining the improvements in the engineering properties achieved through these agricultural waste or discarded materials [13].

Presently, requests for natural aggregates (NA) have surpassed 5 billion cubic tons, which strains raw materials and poses a considerable challenge to making a sustainable society. Also, demolition and construction waste (DCW) formation is increasing alarmingly. In 2016, China generated 1.5 billion metric tons of DCW, while European countries produced almost 1 billion metric tons of DCW in 2019. Zaid et al. [14] observed that conventional disposal of DCW in landfills requires space, contributes to soil toxicity, and causes sustainability and land pollution issues. Many DCW can be repurposed, primarily consisting of aggregates, concrete, and bricks. Using recycled aggregates (RA) in concrete assists in addressing eco-friendliness concerns. This approach addresses environmental issues, as revealed in the relevant past research [15–18]. RA is a suitable option for recycled construction material, albeit with slightly reduced strength compared to the original material.

Wheat straw ash (WSA) is a byproduct of the combustion of wheat straw, an agricultural waste material. Given the large-scale production of wheat in countries like India (170 million tons in 2022), China (135 million tons), Russia (86 million tons), and the USA (51 million tons), WSA offers significant potential as a sustainable alternative to traditional pozzolans. WSA is enriched with minerals, predominantly silica (SiO₂), along with potassium oxide (K₂O) and calcium oxide (CaO), which contribute to its pozzolanic properties. Compared to established pozzolans such as rice husk ash (RHA), fly ash, and silica fume, WSA’s pozzolanic activity is competitive, primarily due to its high silica content. This silica reacts with calcium hydroxide [Ca(OH)₂] during the cement hydration process, forming additional cementitious compounds, which enhance the strength and durability of concrete [15]. However, the utilization of WSA is not without challenges. Variability in chemical composition due to differences in combustion conditions and straw source can affect pozzolanic performance. Additionally, unlike more established pozzolans, WSA’s availability and consistent quality control are less predictable, which could impact its widespread adoption. Nevertheless, using WSA in concrete aligns with sustainable practices, reducing the environmental impact associated with waste disposal and leveraging a renewable resource. Therefore, further research and standardization efforts are necessary to optimize its use in concrete mixtures.

Various relevant past research works [16–20] have studied the joint employment of RA and WSA in concrete making, considering factors such as engineering performance, economical parameters, and environmental constraints. The adverse effects of incorporating high amounts of RA include reduced strength and durability performance compared to NA-based concrete. It has been found that combining RA and WSA results in fewer adverse effects on rheology, durability, cost, and mechanical behavior than when each is used individually. This can be ascribed to the pozzolanic reaction between WSA components and the calcium hydroxide present in RA. Using a higher volume of WSA in RA mixtures is reasonable, as WSA particles act as binders rather than fillers. However, it is widely recognized that incorporating large amounts of WSA and RA can negatively affect concrete properties to a significant extent. Adding a high volume of WSA can harm the early-age strength performance of concrete. This is because when WSA replaces a considerable portion of OPC [21], many particles in the binding matrix remain unreacted due to the reduction in calcium hydroxide. Consequently, the incorporation of high volumes of RA and WSA in concrete is limited, and alternative solutions are needed to increase their usage in concrete production. Traditionally, WSA is employed at inclusion rates ranging from 10% to 25% in various cementitious composites. Some researchers have explored adding more than 25% WSA to reduce OPC content in concrete mixtures. Past studies [22, 23] have confirmed that adding agricultural husks can lead to low early strength, reduced carbonation and chloride attack resistance, and undesired setting times. To address these challenges, researchers have proposed the use of intermediate binders. These binders involve mixtures containing a combination of WSA and OPC, such as 50% WSA and 50% OPC or 40% WSA and 60% OPC, along with suitable chemical activators. Adopting these intermediate binder formulations makes mitigating the adverse effects on concrete properties associated with high WSA content possible.

The practical application of high-performance concrete (HPC) has grown significantly due to advancements in material availability, design methods, and construction techniques. The increasing trend toward large-span bridges, buildings, and lightweight materials has driven demand for HPC and UHPC [24]. However, designers may hesitate to use HPC because of its low ductility. Researchers [25, 26] have explored adding various types of fibers to HPC to address these issues. These fibers act as reinforcements within the concrete matrix, enhancing its mechanical properties. Common types of fibers used in past studies in HPC include steel [27], carbon [28], coconut [29], and synthetic [30] fibers in the mix. Adding fibers to HPC can improve its ductility by enhancing its strain capacity and crack resistance. Fibers act as microscopic reinforcements within the concrete, bridging cracks and distributing stress more evenly. Steel fibers provide high tensile strength and improve the post-cracking behavior of HPC. Carbon fibers offer excellent mechanical properties and can significantly increase the ductility of HPC. Also, past relevant literature studies [31, 32] confirmed that synthetic fibers improve the tensile strain capacity and control crack propagation in HPC. However, the addition of fibers to HPC comes with specific challenges. One of the main challenges is the increased cost associated with producing and incorporating fibers [33]. Steel and carbon fibers, in particular, can be expensive, making the final product cost-prohibitive in some cases.

Researchers have shown increasing interest in using waste tire steel fibers (WTSFs) as a substitute for conventional steel fibers in HPC [34–37]. WTSFs are derived from discarded tires, making them an attractive eco-friendly alternative. WTSFs offer several advantages, including lower cost and a potential solution to waste tire management, as observed by Althoey et al. [38]. When incorporated into HPC, WSTFs have demonstrated the ability to enhance the material’s ductility. Alsaif et al. [39] revealed that WTSFs possess high tensile strength, deformability, and good bond strength with the concrete matrix. These properties allow them to effectively bridge cracks, increase energy dissipation capacity, and improve crack control in HPC. Furthermore, the use of WTSFs contributes to the reduction of waste tires, addressing an environmental concern. Ongoing research and development efforts focus on optimizing the dosage and dispersion techniques of WTSFs in HPC to maximize their effectiveness.

The increased incorporation of Wheat Straw Ash-Recycled Aggregates (WSA-RA) in concrete presents a compelling opportunity for its eco-friendly and sustainable utilization as a construction material. However, as identified by the authors, an important issue pertains to the low mechanical and durability properties, particularly in the initial stages, which hinder the commercial viability of WSA-RA concrete. Prior research [40–42] has highlighted the effectiveness of integrating significant quantities of WSA and RA into concrete, aided by an appropriate chemical activator. Nevertheless, there is a lack of information regarding the effects of sulfate activation on high-performance fiber-reinforced WSA-RA concrete, and incorporating a substantial volume of WSA and RA in high-performance waste steel fiber-reinforced concrete remains unexplored. This study aims to evaluate the impact of high WSA-RA content on the strength and durability of high-performance fiber-reinforced concrete. It uses sodium sulfate as a chemical activator to enhance pore structure in the early stages. The concrete mix included a significant proportion of WSA (40% WSA, 60% Ordinary Portland Cement), combined with three different levels of RA (30%, 60%, 100%) and an addition of 2% waste tire steel fibers to improve ductility. The research examined various engineering properties of the concrete, such as strength, compression, splitting tensile strength, and modulus of rupture, as well as durability aspects like chloride penetration, water absorption, sorptivity, and acid resistance. X-ray diffraction analysis was conducted to analyze sample morphology. The findings contribute to understanding sulfate activation in high-performance fiber-reinforced WSA-RA concrete and its potential in sustainable construction.

2. Materials and methods

2.1 Materials

2.1.1 Cement and wheat straw ash.

Per ASTM C150 [43], 53-grade type-I cement was used as a primary binder in the current research. OPC’s surface area and specific gravity were 341 m²/kg and 3.18. The wheat straw was collected from local sellers and harvested for making WSA as agricultural waste. The straw was then transported to a combustion facility in Mardan City, where controlled burning occurred. During combustion, the wheat straw was subjected to temperatures around 500–600 degrees Celsius. This process resulted in the transformation of the straw into ash, which was collected and further processed. The collected WSA underwent a series of treatments to enhance its suitability as a cement replacement material. The ash was subjected to grinding and sieving processes to achieve a fine and uniform particle size distribution. This step ensured better reactivity and dispersion of the ash in the concrete mixture.

Additionally, any impurities or unwanted materials were removed through sieving. After the processing stage, the WSA, as presented in Fig 1(A), was ready for use as a partial replacement for OPC in concrete. X-ray diffraction analysis of WSA was performed, as presented in Fig 1(B). XRD analysis was performed to identify and characterize the crystalline phases present in the ash. This technique provides valuable information about the mineral composition and structure of WSA. The mean particle size, specific area, fineness, and specific gravity of WSA was 17 microns, 3230 cm²/kg, 95.5%, and 1.97, respectively. The chemical composition of OPC and WSA is provided in Table 1.

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(a). Wheat Straw Ash. (b). XRD Spectra of Wheat Straw Ash.

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2.1.2 Aggregates.

The fine aggregate (FA) used in the present research was sourced from Lawrencepur, Attock City, Pakistan, with a maximum particle size of 3.85 mm. As for the natural coarse aggregates, crushed granite stone from the Jehangira quarry in Nowshera City, Pakistan, was employed. Additionally, three groups of RAs were developed, namely RA-30, RA-60, and RA-100. These RA were obtained from discarded concrete samples available in the laboratory, which exhibited a compressive strength ranging from 50 to 65 MPa. To ensure the durability and strength of the concrete, the maximum particle size for both natural coarse aggregates and recycled coarse aggregates was limited to 10 mm. This decision was made to maintain uniformity in the concrete mixture and ensure consistent performance and properties. By setting a maximum particle size, potential variations in aggregate sizes are minimized, which can significantly affect the concrete’s workability, strength, and durability. Restricting the maximum size to 10 mm allows for better control over the concrete’s characteristics and facilitates accurate evaluation of the impact of other variables on the concrete’s performance. This decision was based on relevant past research [68] indicating that using larger coarse aggregates, e.g., 18 mm and 22 mm, can have more detrimental effects on concrete durability and strength. Fig 2(A) presents the sieve analysis for recycled and coarse aggregates, and Fig 2(B) presents the particle sieve analysis for fine aggregates per ASTM C33, respectively. Table 2 presents the physical characteristics of the FA as well as the coarse and recycled aggregates used in the study.

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Particle Size Distribution: (a) Recycled and Coarse Aggregates, (b) Fine Aggregates.

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2.1.3 Sulfate chemical.

To chemically activate the hybrid binder composed of WSA-OPC, a high-purity chemical activator, sodium sulfate (Na₂SO₄), was employed. The sodium sulfate used in this study had a purity of 99%, consistent with laboratory-grade standards, and a specific gravity of approximately 2.68 g/cm³. This chemical was sourced from Merck (Pvt.) Ltd., Pakistan, a reputable supplier known for its high-quality laboratory chemicals. To ensure effective activation, sodium sulfate was added to the mixes at a dosage of 3% relative to the weight of the binder.

2.1.4 Admixture.

This study employed 1.1% (by binder’s wt.) polycarboxylate ether (PCE) based superplasticizers. PCEs are synthetic polymers that possess unique molecular structures with pendant carboxylate functional groups. This structure gives them a high affinity for cement particles and water molecules. When added to concrete mixtures, PCE-based superplasticizers disperse the cement particles more effectively, reducing the interparticle friction and enabling the concrete to exhibit higher fluidity and improved rheological properties in fiber-based concrete.

2.1.5 Waste tire steel fibers.

Developing WTSFs suitable for fiber reinforcement in high-performance concrete involves several vital steps. Firstly, discarded truck waste tires were collected from a local seller in Mardan City, Pakistan, and a comprehensive recycling process was undertaken. The tires were shredded into smaller pieces to separate the embedded steel wires. These steel wires were then extracted using magnetic separation techniques, yielding a valuable waste tire steel wire product. Once the waste tire steel wires were obtained, they were subjected to further processing to transform them into steel fibers suitable for reinforcement in high-performance concrete. The physical properties of WTSFs are presented in Table 3. The wires were cut and straightened to the desired length and diameter. A mechanical cutting method was employed to achieve the required fiber dimensions.

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2.2 Mix design

In this study, 12 concrete mixtures were formulated to investigate the effects of sulfate activation of WSA on the properties of high-performance fiber-reinforced recycled aggregate concrete incorporating various dosages of RA. The composition and details of each mixture can be found in Table 4. A water-to-binder ratio of 0.38 was maintained to achieve workable and flowable concrete, and an admixture was incorporated at a dosage of 1.1% (by binder’s wt.) for all mixtures. The superplasticizer was necessary due to the presence of WTSF, which tends to make the freshly mixed concrete less workable. Consequently, the slump values of the fibers-containing mixes ranged from 130 to 185 mm, while the mixes without fibers exhibited slump values ranging from 195 to 210 mm. The NA was partially replaced with RA at proportions of 30%, 60%, and 100% to form 3 different groupings of mixtures labeled as RA-30, RA-60, and RA-100, respectively. In each group, WSA was used to replace OPC (by wt.) at 0% and 40% levels. The WSA mixes (RA-0, RA-30, RA-60, and RA-100) were activated using sodium sulfate at a dosage of 3% (by binder’s wt.) (WSA + OPC). Additionally, 2.5% of WTSFs were included in all mixes except those without WSA and RA. The complete mix details of all specimens are provided in Table 4. The terminology for the mixtures follows a specific format: the number after "WSA" indicates the percentage of WSA used as an OPC substitute, the number following "RA" represents the proportion of RA concerning NA, "OPC" suggests the absence of WSA with only ordinary Portland cement used as the binder, "WTSFs" signifies the presence of waste tire steel fibers, and "CA" denotes the use of a chemical activator in the respective mixture.

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The selection of WSA, RA, and steel fiber percentages in our concrete mix design was a strategic decision to balance sustainability and performance. For WSA, a 40% replacement level of Ordinary Portland Cement was chosen to significantly leverage its pozzolanic properties while ensuring the structural integrity of the concrete. This proportion was critical to evaluate the effectiveness of WSA as a sustainable binder component and its impact on concrete performance under sulfate activation. Regarding RA, substitution levels of 30%, 60%, and 100% were selected to explore the behavior of high-performance concrete with varying degrees of natural aggregate replacement. These proportions allowed us to assess the feasibility and implications of using RA in different scenarios, contributing to waste reduction and resource conservation in construction practices. The addition of 2.5% waste tire steel fibers was carefully calculated to enhance the ductility of the concrete. This percentage was optimized to improve the mechanical properties of the concrete without compromising its workability or other key attributes. Including these steel fibers was crucial in mixes with high WSA and RA content, where alterations in the concrete matrix demanded compensatory measures to maintain performance.

The final mix proportions were determined through a combination of theoretical principles and practical trial and error. This approach allowed us to fine-tune the mix to achieve the desired balance between workability, strength, and durability. The proportions of OPC, WSA, RA, and WTSFs were selected based on their potential to contribute to the concrete’s mechanical and durability properties. For instance, the mix identified as RA30-WSA40-WTSFs-CA emerged as the most optimal, demonstrating enhanced performance in both mechanical and durability tests. The careful selection and optimization of these proportions were essential to ensure that the resulting concrete mixes met the performance requirements and aligned with sustainability goals by maximizing the use of recycled and waste materials.

Overall, the chosen percentages resulted from careful planning to create a series of concrete mixes that could provide valuable insights into the potential of using sustainable materials in high-performance concrete applications.

2.3 Mixing technique

The mixing process began by combining the dry ingredients, including OPC, WSA, and the appropriate amount of RA based on the desired proportion (30%, 60%, or 100%). These dry materials were thoroughly mixed in an electrically powered mixer for approximately 2 minutes to ensure proper blending. Next, the calculated amount of water was added gradually while mixing continued. The mixture was stirred for about 3 minutes to distribute uniform water throughout the dry ingredients. Once the desired consistency was reached, the superplasticizer was added gradually, and mixing was continued for 2 minutes. This extended mixing time ensures proper dispersion and activation of the superplasticizer, facilitating improved workability and flowability. After adding the superplasticizer, the WTSFs were introduced into the mixture, and mixing was continued for 4 minutes. This duration allows for thorough dispersion of the fibers, ensuring their even distribution throughout the concrete matrix. After the fibers were uniformly incorporated, the mixture was checked for slump values. The mixes containing fibers were expected to exhibit slump values ranging from 130 to 185 mm, while those without fibers were expected to have slump values ranging from 195 to 210 mm. Adjustments were made by adding small amounts of water or dry materials to achieve the desired slump. Once the desired slump was completed, the concrete was mixed for 2 minutes to ensure overall homogeneity. It is important to note that during the mixing process, care should be taken to prevent the segregation of the aggregates and maintain consistent mixing speeds. The resulting concrete mixtures were then ready for casting into molds and then placed for curing at a room temperature of 22°C.

2.4 Description of test methods

The compressive strength was determined at 28, 56, and 90 days per ASTM C39 [44] by subjecting cylindrical specimens (300 mm x 150 mm) to axial loading until failure occurs. The maximum load at failure was recorded, and the compressive strength was calculated based on the cross-sectional area of the specimen. The splitting tensile strength was evaluated at 28, 56, and 90 days per ASTM C496 [45] by applying a diametrical load to cylindrical specimens (300 mm x 150 mm) until failure. The maximum load at failure was recorded, and the splitting tensile strength was calculated using the appropriate formula. To assess the modulus of rupture, prismatic beams of 750 mm x 200 mm x 200 mm were subjected per ASTM C78 [46] to a three-point bending test until failure. The maximum load at failure was recorded, and the modulus of rupture was calculated based on the dimensions of the specimen.

Water absorption was determined per ASTM C642 [47] by immersing dried specimens (100 mm diameter discs) in water for 56 and 90 days, after which the specimens were weighed to measure the increase in weight due to water absorption. Water absorption was calculated as a percentage of the weight gain relative to the initial dry weight. To evaluate sorptivity ASTM C1585 [48], the specimens (100 mm diameter discs) were partially submerged in water, and the water absorption rate was measured over 56 and 90 days. The test setup of sorptivity is presented in Fig 3. The sorptivity coefficient was determined based on the water absorption rate and the specimens’ dimensions.

[Figure omitted. See PDF.]

The specimens (150 mm x 100 mm) were exposed to a 3% acidic solution (hydrochloric acid) for acid attack resistance. After curing in water for 28 days, the samples were carefully dried in an oven at 40°C for 3 days. After drying, the samples were left to cool in ambient air for 24 hours. Subsequently, the samples were immersed in hydrochloric acid (HCl). The weight loss of the concrete samples was then measured after 28, 56, and 90 days of immersion in the HCl solution. This allowed for the assessment of the mass loss experienced by the samples over time as a result of exposure to the acidic environment.

The rapid chloride penetration test (RCPT) was conducted using an immersion method on 100 mm height and diameter cylinders. These samples were placed in water for 28 days and dried in an oven at 40 degrees Celsius for 3 days. After drying, the specimens were air-cooled for 1-day and immersed in a salt solution. At intervals of 28, 56, and 90 days, the specimens were split and sprayed with a 0.1M AgNO₃ chemical on the cut place to see and note the chloride penetration depth. Based on a previous study [6], this method involves recording chloride penetration at six different spots around the sample (refer to Fig 4A), and an average value is considered the final result. It is important to note that this method of assessing concrete’s endurance to chloride penetration differs from the conventional methods such as chloride ions penetration (ASTM C 1202) [49], chloride ion diffusion test (ASTM C 1556) [50], and NT build 492 chloride ion migration [51]. The immersion method allows chloride ions to enter the concrete from all sides, enabling the assessment of chloride penetration depth from multiple directions. In comparison, the standard methods typically allow chloride ions to enter from one side with a lower concentration (as shown in Fig 4B). The test setup used in this study provides a simple alternative when standard apparatus is unavailable for evaluating chloride penetration in specimens.

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Chloride ion Penetration in HPC; (a) Immersion process; (b) NT Build 492 test for migration.

X-ray Diffraction (XRD) analysis is a crucial tool for ascertaining the mineralogical composition of concrete specimens. This process began with carefully preparing the samples, where concrete specimens were ground into a fine powder to ensure uniformity and accuracy in the analysis. The powdered samples were then exposed to X-ray diffraction, which involves directing X-rays at the material and analyzing the resulting diffraction pattern. This pattern indicates the crystalline phases in the concrete, as each mineral has a unique diffraction signature. The diffracted patterns obtained from the samples were methodically analyzed to identify these crystalline phases, enabling us to quantify the mineralogical composition of the concrete. This analysis is critical as it provides insights into the fundamental properties of the concrete, including aspects related to its strength, durability, and resistance to various environmental factors. By enhancing the description of the XRD methodology in our paper, we aim to offer readers a clearer understanding of the process and its significance in evaluating the quality and characteristics of the concrete specimens.

3. Results and discussions

3.1 Strength properties

3.1.1 Compressive strength.

Fig 5 illustrates the compressive strength results for each mixture. As anticipated, the concrete’s compressive strength decreased as the RA content increased. Compared to a control mixture, the concrete with RA experienced a reduction in compressive strength of 4.57%, 11.78%, and 23.34% at 90 days for 30%, 60%, and 100% RA, respectively. This reduction in the strength can be credited to deficiencies observed in relevant past studies [51–53] present in the RA, specifically the interfacial transition zone (ITZ) between the old aggregate and old paste of the RA. The additional ITZ facilitates rapid cracking propagation, thereby diminishing the load-bearing capacity of the mixtures. Moreover, the paste within the RA is less densely packed than the NA, reducing the concrete strength with RA. The presence of WSA had a negative impact on the 28-day compressive strength. Samples containing WSA (with no activator) exhibited up to 15.86% lower strength at 30% to 100% of RA than those without WSA. Furthermore, the compressive strength of the sample with 100% RA and WSA was 20.77% lower than that of the sample without WSA at 56 days.

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The compressive strength of the sample with WSA exhibited a notable increase in compressive strength at 90 days, despite WSA concrete not reaching the same level of compressive strength as the sample without WSA. This behavior of WSA can usually be attributed to the gradual chemical reaction between alumino-silicate particles and the available portlandite (Ca (OH)₂), resulting in the formation of binding components such as aluminum silicate hydrate (A-S-H) or calcium silicate hydrate (C-S-H) as revealed by Ramasamy et al. [16]. However, when WSA-RA concrete was not activated with a chemical activator, it underperformed in compressive strength compared to the control sample at 56 and 90 days despite exhibiting higher pozzolanic activity. Surprisingly, when WTSFs, WSA, and RA were combined, their impact on concrete behavior exceeded expectations. Previous research studies [20, 54] have revealed that mineral additives like WSA have minimal adverse effects on the strength properties of RA concrete, unlike their impact on NA concrete. This is because of calcium hydroxide in the attached mortar of RA aggregate, which chemically reacts with WSA at the ITZ between the WSA+OPC matrix and mortar in RA concrete. This mitigates the adverse effects of incorporating WSA in high volumes on the compressive strength of RA concrete. The progression of compressive strength in WSA concrete between 56 and 90 days accelerates with increasing dosage of RA, demonstrating the pozzolanic potential of concrete, which improves with higher amounts of RA. Zaid et al. [55] discovered that utilizing WSA and steel fibers significantly enhances the compressive strength of high-strength concrete compared to its counterparts. The incorporation of WTSFs and the chemically activating WSA demonstrated a positive influence on compressive strength at every curing duration. Specifically, at 56 and 90 days, chemically activated samples of WSA and WTSFs exhibited significantly higher compressive strength than concrete without WSA, regardless of the dosage of RA. The chemically activated WSA mixes showed a 5–8% increase in compressive strength compared to mixes without WSA at 56 and 90 days. Additionally, the chemical activation of WSA aided in mitigating the strength loss caused by the presence of RA. For example, concrete containing 70% RA exhibited higher compressive strength than the control mixture at 56 and 90 days. Similarly, concrete with 100% RA achieved an identical compressive strength as the control mixture at 90 days. This improvement can be credited to the increased reactivity of WSA with calcium hydroxide in sodium sulfate. It has been documented in past research [56] that the sulfate activator directly reacts with portlandite, contributing to the formation of ettringites.

The chemical reaction resulting from the presence of sodium sulfate leads to the formation of potent alkalis like NaOH. This increase in alkalinity around the components of WSA aids in dissolving the silica content of WSA. Previous studies [40, 57, 58] have also observed similar accelerating effects of sodium sulfate on concrete containing a high volume of mineral additives, showing promising results in concrete with RA. This can be attributed to the reinforcement of the ITZ between the new and attached mortar in RA, as the sulfates from the chemical activator react chemically with calcium hydroxide.

Moreover, concrete with RA exhibits an excess amount of calcium hydroxide compared to the control mixture, which increases the likelihood of chemical reactions between calcium hydroxide and the activator in the RA concrete. For instance, in the case of the mix RA0-WSA40-WTSFs-CA, there was an approximately 29.27% increase in compressive strength between 56 and 90 days. Similarly, at 90 days, the mixtures RA30-WSA40-WTSFs-CA, RA60-WSA40-WTSFs-CA, and RA100-WSA40-WTSFs-CA exhibited strength gains of 21.80%, 26.66%, and 34.93%, respectively, compared to their counterparts in the control sample. These results indicate that the chemical activator present in WSA significantly contributes to the properties of HPC with RA, surpassing those of the control sample.

3.1.2 Splitting tensile strength.

The results of the tests conducted on each specimen are depicted in Fig 6. When the test result was analyzed, the splitting tensile strength exhibited a consistent decline as the proportion of RA increased from 30% to 100%. However, the inclusion of WTSFs mitigated the reduction in strength significantly compared to the reduction observed in compressive strength due to the rising dosage of RA. Concrete incorporating RA demonstrated diminished strength compared to concrete with NA. This reduction can be attributed to extra ITZ in RA concrete, facilitating easier separation of failed surfaces under the applied loading. While the decline in compressive strength is more pronounced with an enhancing dosage of RA, this is primarily because of the distinctive behavior of the applied loading. Concrete containing RA exhibits high porosity, which leads to increased lateral expansion, adversely affecting the axial load capacity of the specimen. Other similar research studies [59, 60] have also documented that the compressive strength of concrete is more significantly affected by the inclusion of RA than the indirect tensile strength. When comparing the performance of concrete incorporating WSA with other pozzolans such as RHA, FA, and SF, distinct trends emerge. RHA has been extensively studied for its high silica content, which contributes to pozzolanic activity and enhances the strength and durability of concrete. For instance, studies have shown that incorporating RHA can significantly improve both compressive and tensile strengths, often surpassing those of OPC mixes due to the refined pore structure and increased density of the matrix [61]. Similarly, FA and SF are well-documented for their ability to enhance the mechanical properties of HPC, with SF being particularly effective due to its ultra-fine particles that enhance the microstructure by filling voids and reducing permeability [62].

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In this study, the addition of WSA to concrete is observed to reduce the indirect tensile strength at all stages, particularly noticeable in the early phases. At 28 days, the tensile strength of samples containing WSA alone (e.g., RA0-WSA0) was significantly lower than those without WSA, indicating an initial reduction in mechanical performance due to the slow-reacting nature of WSA and a decrease in the binder’s Ca(OH)₂ content. This trend aligns with initial reductions observed in RHA and FA mixes, where early strength gain is often slower due to the delayed pozzolanic reaction. However, by the 90-day mark, the tensile strength recovery in WSA-containing samples was notable, achieving around 94–96% of the control sample’s strength. This recovery can be attributed to the prolonged hydration and pozzolanic reactions facilitated by WSA, which enhance the concrete matrix over time.

SF typically shows less initial strength reduction and faster recovery due to its higher reactivity and contribution to early-age strength development [63]. However, WSA, particularly when used with WTSFs and chemical activators, shows comparable or even superior long-term performance, which can be attributed to the combined effects of crack arresting by WTSFs and the densification of the microstructure by the activators. To mitigate the initial strength reduction caused by WSA, the inclusion of WTSFs and chemical activators proved beneficial across all levels of RA. The combination of WTSFs and CA significantly improved the splitting tensile strength, often surpassing the control samples. For instance, the RA30-WSA40-WTSFs-CA mixture showed substantial strength gains, with a 33–37% increase at 28 days compared to the control, demonstrating the effectiveness of this combination in enhancing early-stage tensile strength. This improvement persisted at 56 and 90 days, with increases of 19–25% and 12–17%, respectively, highlighting the long-term benefits of incorporating WTSFs and chemical activators. The influence of the activator diminished over time, while WTSFs acted as crack arrestors, further enhancing the strength. All samples incorporating WTSFs and activators exhibited higher indirect tensile strength than the control sample. Furthermore, chemical activators demonstrated compatibility with concrete containing RA. Concrete with RA showed greater strength enhancements when WSA and WTSFs were included than normal concrete with WSA. The sulfate ions in the activator reacted readily with the C₃A present in the OPC, forming ettringite, which contributed to a denser microstructure. Nawaz et al. [64] revealed that with a higher presence of calcium hydroxide in concrete with RA, ettringites were formed at the ITZ between the RA and the OPC matrix, further reinforcing the microstructure. These findings highlight the positive effects of using WSA, WTSFs, and chemical activators in improving the strength and performance of concrete, particularly when incorporating RA, thereby offering potential applications in sustainable and durable construction practices.

3.1.3 Modulus of rupture.

The test results of the MOR at 56 and 90 days are presented in Fig 7(A). Incorporating high-volume WSA as a partial replacement of cement in HPC and its interaction with various proportions of RA and WTSFs has significant consequences for the MOR of the concrete at 56 and 90 days. When WSA is added to the concrete mixtures in all variants, there is a noticeable reduction in MOR, which can be attributed to several factors. Firstly, adding WSA introduces additional porous components into the concrete matrix. WSA particles, less dense than traditional cementitious materials, create voids and interconnected pore spaces within the hardened concrete. As Kou et al. [65] disclosed, these voids act as weak points, reducing the concrete’s overall structural integrity and load-bearing capacity. Consequently, the MOR of the HPC mixtures incorporating WSA is lower than the reference mixtures without WSA. When comparing these findings with other studies involving pozzolans like RHA, FA, and SF, it becomes evident that each pozzolan influences MOR differently. For example, due to its high silica content and finer particle size, RHA tends to improve the MOR by refining the microstructure and enhancing the bond between the cement paste and aggregates [66]. On the other hand, FA, which is often used for its pozzolanic and filler effects, can either maintain or slightly improve MOR depending on its fineness and replacement level [67]. SF is known for significantly enhancing MOR due to its ultra-fine particles, which fill voids and contribute to a denser and more homogeneous matrix [63]. In contrast, WSA, while beneficial in some aspects, tends to introduce porosity that negatively affects the MOR, especially in the early stages. Analyzing Fig 7 and the provided discussion text, it is apparent that the recycled aggregates have a notable impact on the MOR of HPC at both 56 and 90 days. As the proportion of RA increases from 30% to 100% in the concrete mixtures, there is a discernible trend of decreasing MOR. This trend suggests that replacing natural aggregates with recycled aggregates may introduce inconsistencies and potential weaknesses in the concrete matrix. Recycled aggregates often come with attached mortar and varying degrees of porosity, affecting the ITZ between the aggregates and the cement paste. This results in reduced tensile strength, as reflected in MOR values. For instance, the RA100-WSA0 mixture shows a lower MOR than the RA30-WSA0 at both time intervals, underscoring the influence of RA content on concrete’s flexural performance.

[Figure omitted. See PDF.]

(a). Modulus of Rupture of HPC at 56 and 90 days. (b). Statistical analysis of Compressive Strength between Splitting Tensile Strength and Modulus of Rupture.

However, the presence of WSA and its chemical activation with sodium sulfate slightly mitigates this reduction. This suggests that while the inclusion of RA may initially decrease the MOR due to factors like increased porosity and altered aggregate-paste bonding, the activation process and the presence of steel fibers can counteract these effects by enhancing the concrete matrix’s cohesion and strength. This is particularly evident in mixtures like RA100-WSA40-WTSFs-CA, which show improved MOR values compared to their counterparts without WTSFs or chemical activation. El-Sayed et al. [68] observed that the slow-reacting nature of WSA constituents, such as silica and alumina, decreases the proportion of calcium oxide within the binder. Calcium oxide is crucial in forming C-S-H gel, contributing to the cementitious matrix’s strength and cohesion. The reduction in CaO content negatively affects the formation and development of C-S-H, resulting in a weaker and less interconnected network of hydration products. Consequently, the MOR of the concrete is compromised. Evaluating the MOR values at 90 days, it becomes evident that the incorporation of WSA and WTSFs impacts the MOR of HPC. When comparing the mixtures to the reference mixture (RA0-WSA0), it is clear that the MOR is reduced when WSA is added without WTSFs (e.g., RA0-WSA40), as indicated by the lower MOR value of 42.1 MPa. This decrease signifies a decline in the concrete’s ability to withstand bending forces. However, the chemical activation of WSA with sodium sulfate, as seen in the RA0-WSA40-WTSFs-CA mix, significantly improves the MOR to 51.3 MPa, indicating that this process can offset the MOR reduction typically caused by WSA addition. This technique bolsters the concrete’s mechanical properties, compensating for the lower CaO content by promoting ettringite formation, contributing to a denser and stronger microstructure. This reaction not only enhances the reactivity of WSA particles but also fills the voids they create, resulting in denser, more cohesive concrete that offers greater durability and bending resistance. These findings align with previous research [69–71], which highlights similar benefits when using chemical activators with other pozzolans like FA and SF.

Fig 7(B) presented demonstrate clear empirical relationships between the compressive strength, splitting tensile strength, and modulus of rupture for HPC incorporating WSA, RA, and WTSFs. The first figure illustrates the relationship between splitting tensile strength (ft) and compressive strength (fc), revealing that ft increases with fc, following the power-law relationship ft = 6.48×(fc)^0.27. This indicates that as the compressive strength of the concrete increases, there is a corresponding but more gradual increase in tensile strength. This trend is consistent with the known behavior of concrete, where tensile strength typically constitutes a smaller proportion of compressive strength, reflecting the material’s inherent brittleness and lower tensile capacity. The second figure depicts the MOR and compressive strength (fc) relationship, described by the equation MOR = 0.34×(fc)^1.09. This relationship suggests a slightly stronger correlation between compressive strength and flexural strength (as measured by the modulus of rupture) compared to the tensile strength relationship. The exponent close to 1.09 indicates that the modulus of rupture increases at a rate slightly higher than the compressive strength, which is indicative of the improved crack resistance and flexural behavior in concrete mixes with higher compressive strengths, particularly in those modified with WSA, RA, and WTSFs. These statistical relationships highlight the effectiveness of WSA, RA, and WTSFs in enhancing the mechanical properties of HPC. Using these empirical equations, the ability to predict tensile and flexural strengths from compressive strength is crucial for designing and optimizing concrete mixes for specific structural applications. Furthermore, the close fit of the experimental data to the modeled equations highlights the reliability of these relationships for the materials used in this study. Incorporating sustainable materials such as WSA and RA not only maintains but, in some cases, improves the performance characteristics of concrete, as reflected in these relationships, making these mixes suitable for high-performance structural applications where both strength and sustainability are paramount.

3.2 Durability properties

3.2.1 Acid attack test.

The acid attack test plays a crucial role in evaluating the durability of specimens, particularly in situations involving industrial chemical diluted water. The primary vulnerable component of the binder is Ca (OH)₂, which reacts with external elements, leading to the development of salts. These salts cause expansion within the concrete, resulting in the deterioration of OPC-based concrete. In this research, the acid attack was conducted to assess the loss in mass of the specimens caused by acid erosion after 28, 56, and 90 days. Fig 8 presents the test results. As the percentage of RA increases, the concrete’s endurance to acid diminishes rapidly. This decline can be mainly attributed to the higher Ca (OH)₂ concentration in RA, which is highly vulnerable to acid. Additionally, the high permeability of RA allows harmful elements to infiltrate deeply into the concrete matrix. However, including WSA improves the concrete’s endurance against acid attacks at all levels of RA, enhancing its overall performance. Adding WSA, a material rich in amorphous silica, reduces calcium hydroxide concentrations within the concrete matrix [21]. This is pivotal because Ca (OH)₂ is known to react with acids to form soluble salts, which not only get leached out, leading to mass loss but also cause internal expansion and subsequent deterioration. By decreasing the availability of Ca (OH)₂ through the substitution with WSA, the binder becomes less susceptible to these harmful reactions. Moreover, WSA’s pozzolanic reaction produces additional calcium silicate hydrate, which densifies the concrete matrix, significantly lowering its permeability and sorptivity. This densification effectively hinders the ingress of acids, thereby enhancing the durability of the concrete against acid attack. These improvements are consistent across all levels of RA, as demonstrated by the superior performance of mixtures with WSA under acidic conditions relative to those without WSA. This synergy between WSA and RA contributes to the heightened endurance of the concrete, reinforcing the material’s overall structural integrity in aggressive chemical environments.

[Figure omitted. See PDF.]

Masood et al. [56] also revealed in their research study that the improvements observed can be mainly credited to reduced Ca (OH)₂ development when high-SiO₂-rich and low-Ca mineral additives are used as substitutes for OPC. Additionally, the inclusion of WSA decreases the permeability and sorptivity of the concrete, thereby slowing down the entry of acidic chemicals. Using sodium sulfate as an activator in concrete mixtures has been instrumental in reducing mass loss from acid attacks at all evaluated ages. Notably, the mix with 100% recycled aggregates and 40% WSA, activated with sodium sulfate (RA-100-WSA40-WTSFs-CA), displayed mass loss comparable to the control sample at 56 and 90 days. This is significant because data on WSA’s role in enhancing acid resistance is scarce. Studies [72, 73] on metakaolin-activated geopolymer concrete confirm that activators can bolster acid resistance by converting amorphous binder components into more crystalline structures. Moreover, Palomo et al. [73] have shown that mineral additives activated with a substance like sodium sulfate lead to concrete with improved acid resistance relative to conventional concrete. The activation process, involving sodium hydroxide release after sulfate ion depletion, may promote the development of durable binder components akin to those in alkali-activated binders, further enhancing concrete’s resistance to acid environments.

3.2.2 Penetration of chloride.

The deterioration of reinforced concrete structures in marine environments is primarily caused by the corrosion of steel reinforcement due to the infiltration of chlorides. This research assessed the chloride penetration by exposing concrete to a NaCl solution. No technique was employed to accelerate the process of chloride penetration, and the mixes were evaluated after 28, 56, and 90 days of immersion. The test results depicting chloride penetration can be found in Fig 9. The results indicate that the use of WTSFs and RA tends to decrease the resistance of concrete against chloride ion penetration. The medium’s permeability, particularly its absorption capacity, directly influences chloride penetration. Consequently, increasing the amount of RA worsens the concrete’s susceptibility to chloride infiltration. However, incorporating WSA into the concrete mix improved particle size distribution within the matrix, reducing porosity. The test results revealed that including a chemical activator significantly enhanced the concrete samples’ resistance to chloride penetration. Compared to the control sample, activated WSA concrete exhibited a 15–25% decrease in chloride permeability at all testing ages. These enhancements could be attributed to the densification of the concrete matrix, which occurs due to the early consumption of calcium hydroxide. This process reduces pore connectivity and size, ultimately improving the resistance of the concrete to chloride penetration.

[Figure omitted. See PDF.]

Several observations can be made when comparing WSA’s performance to other well-known pozzolans like RHA, FA, and SF. Silica fume, known for its ultra-fine particles and high pozzolanic reactivity, typically provides superior resistance to chloride penetration due to its ability to significantly reduce porosity and refine concrete microstructure [74]. Similarly, RHA has been shown to improve the durability of concrete against chloride ingress by enhancing the microstructure and reducing the pore size distribution. However, its effect might be less pronounced than SF [75]. Depending on its composition and fineness, fly ash contributes to improved resistance to chloride penetration, primarily through the pozzolanic reaction that densifies the matrix and reduces permeability [76]. WSA demonstrates comparable or slightly lower performance in reducing chloride penetration compared to these pozzolans. The effectiveness of WSA can be attributed to its high silica content, which, like RHA and SF, participates in pozzolanic reactions that enhance the concrete’s microstructure. However, the overall performance of WSA may be influenced by the specific characteristics of the ash, such as particle size and the presence of impurities, which can vary depending on the source material and processing conditions. Using a chemical activator with WSA further enhances the resistance of both NA and RA concrete to chloride penetration. While some studies [77, 78] have suggested that the rapid chloride migration (RCM) test may overestimate the resistance of activated materials due to their high alkalinity [79], the present research allowed chloride ingress to occur naturally, providing a more accurate assessment of the material’s performance. The findings of this study revealed the positive influence of activators on the concrete’s resistance to chloride penetration, highlighting that the benefits of chemical activation apply to both NA and RA concrete when using WSA.

3.2.3 Sorptivity test.

This test evaluated Darcian flow over the HPC under capillary force to assess its sorptivity, a measure of the material’s ability to absorb and transmit water through capillary action. The materials’ permeability and pore structure arrangement heavily influence sorptivity. Fig 10 displays the sorptivity results for each mixture. The results indicate that the presence of mortar with low density significantly increases the sorptivity of the concrete as the amount of RA used increases. At 56 days, the sorptivity increased by 35.2% when NA) was wholly substituted with RA. This increase is primarily due to the higher porosity and weaker ITZ in RA concrete, which facilitate capillary action and moisture ingress.

[Figure omitted. See PDF.]

The introduction of a chemical activator led to a reduction in the sorptivity coefficient. This reduction can be attributed to the enhanced pozzolanic reactions facilitated by the activator, which improves the microstructure by reducing porosity and refining the pore size distribution. However, the adverse effects of RA on sorptivity were significantly mitigated by adding WSA. WSA, due to its high silica content and pozzolanic activity, helps to densify the concrete matrix, thus reducing sorptivity. Additionally, using sodium sulfate as an activator further reduced the sorptivity coefficient, likely by promoting the formation of additional binding phases that fill voids within the concrete matrix. The mix RA60-WSA40-WTSFs-CA exhibited sorptivity identical to the control mixture at 56 and 90 days, highlighting the effectiveness of combining WSA with a chemical activator and WTSFs in maintaining low permeability even with a significant proportion of RA. However, at 90 days, the mix RA100-WSA40-WTSFs-CA had a slightly higher sorptivity (0.24 mm/min^0.5) than the control sample (0.17 mm/min^0.5). This indicates that while the combination of WSA and chemical activators significantly improves the performance of RA concrete, the extent of improvement may be limited at very high levels of RA replacement. Some distinctions can be made when comparing WSA to other pozzolans like RHA, FA, and SF. Silica fume is known for its ability to significantly reduce sorptivity due to its ultra-fine particle size, which leads to a highly dense and impermeable matrix [74]. Fly ash also effectively reduces sorptivity, particularly in the long term, by filling voids and refining the pore structure through ongoing pozzolanic reactions [75]. Rice husk ash, similar to WSA, reduces sorptivity by improving particle packing and enhancing the pozzolanic activity. However, its performance can vary depending on the quality and processing of the ash [76].

In comparison, WSA provides a sustainable alternative that effectively enhances the concrete’s resistance to moisture ingress, though its performance might not surpass that of highly reactive pozzolans like SF. The effectiveness of WSA in reducing sorptivity is particularly notable when combined with chemical activators, which further improve the microstructure by producing a densely packed alumino-silicate gel, as observed in alkali-activated materials [79]. This gel is more compact and impermeable than the C-S-H gel typically formed in conventional concrete, as noted by Alanazi et al. [80]. The present sorptivity results emphasize the importance of chemical activators in decreasing the porosity of RA concrete. By leveraging the pozzolanic potential of WSA with chemical activators, it is possible to achieve concrete mixes with lower sorptivity, enhancing their durability in moisture-prone environments.

The durability of high-performance concrete is significantly influenced by its resistance to acid attack, chloride penetration, and sorptivity, which are critical for ensuring long-term structural integrity, particularly in aggressive environments. The test results indicate that the mix RA0-WSA40-WTSFs-CA consistently exhibited the lowest mass loss during the acid attack test, the slightest chloride penetration, and the lowest sorptivity at 28, 56, and 90 days, underscoring its superior durability. These improvements can be attributed to the inclusion of wheat straw ash and chemical activators, which reduce the calcium hydroxide content susceptible to acid erosion and densify the concrete matrix, thereby limiting the ingress of harmful ions and moisture. When correlated with the strength data, the RA0-WSA40-WTSFs-CA mix also demonstrates high compressive strength, splitting tensile strength, and modulus of rupture, making it the most well-rounded performer among the tested mixes. This mix provides enhanced durability in terms of resistance to acid attack, chloride ingress, and moisture absorption and maintains superior mechanical properties, making it particularly suitable for high-performance applications in harsh environmental conditions. The integration of WSA, RA, and WTSFs, especially with chemical activation, thus optimizes both the mechanical and durability properties of HPC, establishing the RA0-WSA40-WTSFs-CA mix as the best option for ensuring long-term durability and structural integrity.

3.2.4 Water absorption.

The water absorption test results for all samples are presented in Fig 11, providing insights into the concrete’s durability and ability to absorb water containing potentially harmful substances. This test also indicates the connectivity of pores within the concrete’s microstructure. As expected, the water absorption increased with higher RA proportions in the mix. At 56 days, including WTSFs and RA led to a corresponding increase in water absorption of approximately 2.4%, 8.95%, and 14.68% at 30%, 60%, and 100% of RA, respectively. Thomas et al. [81] revealed that this can be attributed to the higher porosity of RA, which facilitates the easy flow of fluids into the concrete due to their low-density mortar content. The presence of WSA did not have an adverse influence on the water absorption of the concrete. The mixtures with WSA displayed lower water absorption than the control sample, with reductions of approximately 7–13% observed at 56 and 90 days. This can be attributed to the densifying effect of WSA on the microstructure of the concrete. Firstly, during the pozzolanic reaction, WSA consumes excess calcium hydroxide, forming additional calcium silicate hydrate gel, contributing to a denser microstructure. Secondly, as WSA acts as a filler material, it effectively fills the gaps between the OPC particles, further enhancing the compactness of the concrete.

[Figure omitted. See PDF.]

Although slow, the pozzolanic reaction contributes to the concrete’s strength. WSA also plays a role in reducing the porosity of RA by effectively filling the pores. Similar findings by Dimitriou et al. [82] regarding mineral additives’ impact on RA’s porosity have been documented in previous research. Additionally, incorporating a chemical activator further reduces the water absorption of concrete with every level of RA. The activation of WSA results in a significant reduction in water absorption, with cuts of up to 14.75% observed. Gao et al. [83] noted that this activation process also improves the micro-hardness of the ITZ in concrete with NA, contributing to the enhanced performance of RA concrete containing WTSFs and WSA. The higher permeability of RA concrete allows for better filling of the concrete matrix by the (WSA + OPC) paste, increasing the potential of effective binding between the paste and the RA, thus reducing water absorption. Furthermore, the test results indicate that chemically activated WSA concrete exhibits better suitability for RA concrete than NA concrete, as incorporating a chemical activator leads to a more significant reduction in water absorption in RA concrete compared to NA concrete.

The water absorption results highlight the influence of recycled aggregates on the porosity of the concrete, with higher proportions leading to increased water absorption. However, including WSA mitigates this effect by densifying the microstructure through pozzolanic reactions and efficient filling of voids. These findings contribute to understanding concrete durability and the potential benefits of incorporating WSA in concrete mixtures to improve water resistance.

4. XRD spectra of HPC

Fig 12 shows the XRD test result of the most optimal mixture (RA30-WSA40-WTSFs-CA) observed during the mechanical and durability tests. The optimal concrete mixture exhibited enhanced performance during the XRD test, prompting the conduction of X-ray diffraction analysis to assess the variations in the progression of calcium hydroxide and other hydration products. The XRD analysis aimed to examine the peak intensity of calcium hydroxide and calcium silicate hydrate, both key hydration products in concrete. The peak intensities of calcium silicate hydrate and portlandite were observed at 2-theta values of 38°, 40°, 25°, and 32°, respectively. The XRD spectra showed that the peak intensity reduced as WSA was added, indicating a lower presence of calcium hydroxide due to its consumption in pozzolanic reactions. Furthermore, the X-ray diffraction spectra indicated that the highest peak intensity was observed at 90 days, suggesting the continued progression of strength at later ages in the mixtures, likely due to the pozzolanic activity of WSA and the effect of sodium sulfate as a chemical activator. A similar trend was reported by Chindaprasirt et al. [84], where the use of fly ash as a fractional replacement for OPC resulted in the appearance of Ca(OH)₂ peaks at later ages. This comparison highlights that, like fly ash, WSA effectively contributes to the formation of secondary hydration products, enhancing concrete’s long-term strength and durability. The pozzolanic behavior of WSA is influenced by factors such as the proportion of amorphous SiO₂ and the fineness of the WSA. Appropriately treated WSA can be an effective replacement material for OPC, enhancing the chemical reaction and providing increased strength during hydration, particularly in the later stages.

[Figure omitted. See PDF.]

When comparing WSA with other pozzolans such as RHA, FA, and SF, it becomes evident that WSA’s effectiveness in altering the hydration process and forming hydration products is comparable to these well-established materials. Silica fume, for instance, is known for its ability to significantly reduce the amount of portlandite due to its high pozzolanic activity, which results in a dense microstructure and enhanced durability [74]. Rice husk ash, similar to WSA, also contributes to the pozzolanic reaction by consuming calcium hydroxide and forming additional calcium silicate hydrate, which refines the concrete’s microstructure [75]. Fly ash is well-documented for its delayed pozzolanic reaction, which continues to enhance the microstructure over time, as evidenced by the XRD peaks related to calcium silicate hydrate at later ages [76]. The observed shift in the XRD peaks after 28 days, as depicted in Fig 12, can be attributed to several factors related to the physicochemical changes occurring within the concrete matrix over time. Typically, peak shifts in XRD patterns can result from changes in the crystal structure, lattice parameters, or the formation of new phases. The peak shifts in the concrete mixture containing WSA may indicate a transformation in the hydration products. Over the initial 28 days, the ongoing hydration of cement and the pozzolanic reactions involving WSA led to the formation and growth of calcium silicate hydrate and other secondary products. This process can alter the microstructure and cause changes in the interplanar spacing of the crystalline phases, which is detected as a shift in the 2-theta angle. Additionally, incorporating sodium sulfate may also influence the formation of new phases like ettringite or mono-sulfate, which can further contribute to peak shifts. Studies on FA and SF have similarly reported the formation of secondary phases like ettringite, which contribute to microstructural refinement and durability [85]. The combination of these evolving reactions and phase developments is likely responsible for the alterations in the XRD peaks observed after 28 days. This indicates the concrete mixture’s dynamic and continuing strength gain and microstructural refinement process.

5. Conclusions

This research evaluated the properties of high-performance recycled aggregate concrete modified with wheat straw ash and WTSFs; the following conclusions are drawn from this research:

* The addition of sodium sulfate effectively addresses early-age compressive strength issues in WSA concrete, showing significant improvement regardless of the RA percentage. Exceptional performance was observed in samples with 30% RA, 40% WSA, and 2.5% WTSFs, surpassing the control sample at 56 and 90 days.

* At 90 days, the activated WSA concrete containing 60% RA, 40% WSA, and 2.5% WTSFs exhibited enhanced mechanical performance compared to the control sample.

* Chemical activation with WSA enhances both early and later-age mechanical performance in HPC, contributing significantly to RA concrete’s strength formation compared to Normal Aggregate concrete.

* Similar trends are observed in the results of the indirect tensile strength and modulus of rupture tests, aligning with the observations made for the compressive strength test.

* Including activators leads to a notable decrease in water absorption and sorptivity, reducing them by approximately 14–17%. This decrease can be attributed to the enhanced density of the (WSA + OPC) matrix. Concrete samples containing 30% and 60% RA and activated WSA exhibit significantly lower water porosity than the control sample.

* Using activated WSA significantly improves the resistance of RA concrete against chloride ion penetration. Both activated and inactivated WSA contribute to enhancing the behavior of RA concrete. Notably, the mixtures with activated WSA exhibit a 25.2% reduction in chloride penetration compared to the control sample, regardless of the content of RA.

* Adding an activator in RA concrete significantly reduces its vulnerability to acid attacks. Compared to a sample containing only OPC, activated WSA concrete exhibits a 26% higher resistance to acid attack. Additionally, 100% RA and 40% WSA specimens exhibit comparable acid resistance to the control sample during the acid attack test.

* The X-ray diffraction spectra provide evidence of the concrete’s increased strength through elevated peaks corresponding to hydration products after 90 days.

Based on the observations above, the present study reaffirmed that sulfate activation played a crucial role in maximizing the utilization of RA, WTSFs, and WSA to achieve high-performance concrete. Sodium sulfate in WSA concrete improves early-age strength, especially in 30% RA, 40% WSA, and 2.5% WTSFs samples. It enhances both early and later-age mechanical performance in HPC. Activators reduce water absorption by 14–17%, improving density. They also increase resistance to chloride penetration by 25.2% and acid attacks by 26%. X-ray diffraction shows increased strength through hydration product peaks after 90 days.

As part of future research, attention should be given to optimizing the mix design of HPC incorporating WSA, RA, and WTSFs to achieve an ideal balance between mechanical performance and sustainability. Additionally, studies on long-term durability, environmental impact assessments, economic feasibility, and microstructural analysis could further enhance understanding and application of these materials in sustainable construction.

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Citation: Althoey F, Zaid O, Elhadi KM (2024) Sulfate activation of wheat straw ash to enhance the properties of high-performance concrete with recycled aggregates and waste tire steel fibers. PLoS ONE 19(10): e0311838. https://doi.org/10.1371/journal.pone.0311838

About the Authors:

Fadi Althoey

Roles: Project administration, Supervision, Visualization

Affiliation: Civil Engineering Department, College of Engineering, Najran University, Najran, Saudi Arabia

Osama Zaid

Roles: Conceptualization, Visualization, Writing – original draft, Writing – review & editing

E-mail: [email protected]

Affiliation: Department of Civil Engineering, Swedish College of Engineering and Technology, Wah Cantt, Pakistan

ORICD: https://orcid.org/0000-0002-8071-1341

Khaled Mohamed Elhadi

Roles: Funding acquisition, Resources

Affiliations: Department of Civil Engineering, College of Engineering, King Khalid University, Abha, Kingdom of Saudi Arabia, Center for Engineering and Technology Innovations, King Khalid University, Abha, Saudi Arabia

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