Performance of silica fume concrete reinforced with glass, jute, and hemp fibers under simulated acid attack

ABSTRACT

This study investigates the combined influence of silica fume and discrete fibers on the mechanical and durability performance of concrete under acidic environments. Experimental data were obtained from 24 cylindrical specimens prepared in the laboratory, incorporating 10% silica fume as a cement replacement and 1.0% total fiber volume of glass, surface-treated jute, and hemp. Data were collected at 7, 14, and 28 days for each mix to evaluate strength development over time. Specimens were exposed to acidic solutions of pH 3 and pH 5 for 28 days to simulate aggressive service conditions. Statistical analysis, including mean, standard deviation, ANOVA, and PBIAS error assessment, was conducted to validate data consistency and model accuracy. The glass fiber–silica fume mix showed the best performance, achieving 54.7 MPa compressive strength and 5.12 MPa tensile strength, representing increases of 26.4% and 28.1% compared with control concrete. PBIAS values of −2.83% (compressive) and −3.12% (tensile) indicated excellent predictive agreement. UPV exceeded 4.6 km·s⁻¹, and sorptivity decreased by 18.5%, confirming a dense and durable microstructure. Overall, the integration of 10% silica fume and glass fiber significantly enhanced acid resistance and strength, making it suitable for wastewater and chemical infrastructure applications.

Fiber-reinforced concrete; Jute and hemp fibers; Durability performance; Acid resistance; Sorptivity

1. INTRODUCTION

Concrete is widely used worldwide because of its strength and durability, which make it a desirable material to help with the construction of many structures like buildings, concrete bridges, dams, building foundations, etc. The longevity of these buildings is directly impacted by the surrounding environment, notwithstanding their flexibility [¹]. Aqueous hydrogen sulfide (H₂S) is converted by microorganisms to sulfur and then to sulphuric acid, which causes corrosion in structures exposed to hostile conditions, such as wastewater systems [²]. Sewer pipes deteriorate in this process, particularly when there is a significant concentration of sulphuric acid from industrial waste. Additionally, the harsh maritime climate causes steel to corrode, reducing the service life of structures in coastal locations [³]. Concrete’s durability issues in these settings have been addressed in several ways. Its sustainability and carbon footprint were also improved through several measures. One of the most popular solutions to overcome these issues is to partially substitute cement with pozzolanic materials, including fly ash, slag, and silica fume. For instance, fly ash has a lower embodied energy than Ordinary Portland cement (OPC), making it a more sustainable option when used in place of some of the cement. Moreover, it helps to improve durability and workability [⁴]. The concrete’s sluggish pozzolanic reactions also hinder the concrete’s ability to reach its early strength. One such product is Portland Composite Cement, which contains up to 35% clinker substitutes such as fly ash or slag [⁵]. These concrete mixtures, which are usually classified as high-performance concrete (HPC), have fewer pores because of a lower water-to-cement ratio, but their brittleness limits their performance. Furthermore, cement-based composites usually have low fracture toughness, low tensile strength, and weak resistance to cracking [⁶]. By adding natural or synthetic fibers, fiber-reinforced concrete (FRC) greatly enhances these mechanical qualities. The addition of fibers improves tensile strength, toughness, and impact resistance. As a result, it is becoming more widely accepted that adding different kinds of fibers to concrete will improve its durability and sustainability [⁷]. Concrete can deteriorate due to a variety of causes, such as permeation, freezing and thawing, corrosion of reinforcement, and alkali-aggregate interactions. However, chemical attacks, more especially acid attacks, prove to be harmful to the concrete structure. Acid assaults can originate from several sources, such as urban activities and industrial operations [⁸]. Aggressive acidic media can also arise from other causes, such as the production of organic and inorganic acids, dissolved CO₂ in water, and even sulphuric acid in peat waters. The concentration of the acid, not its pH, determines how aggressive the solution is; in general, the stronger the acid, the greater its dissociation [⁹]. The primary components of cement are tetra calcium alumino-ferrite (4CaO•Al₂O₃•Fe₂O³ or C₄AF), tricalcium silicate (3CaOSiO₂ or C₃S), dicalcium silicate (2CaO•SiO₂ or C₂S), and tricalcium aluminate (3CaO•Al₂O₃ or C₃A). The hydration process, which starts when water is introduced to the cement mixture, causes the reaction shown in Equation 1.

(1) $$C{a}_{3}Si{O}_5+H_{2}O\to CaO.Si{O}_2.H_{2}O+Ca{(OH)}_2$$

The products of this reaction are calcium hydroxide (Ca(OH)₂) and calcium silica hydrate (C-S-H) gel. The calcium salts created during sulfate attacks combine with the calcium aluminate hydrates in the cement matrix to generate ettringite. This reaction includes the calcium salts created during the hydration of cement and the acid. From the concrete’s outermost surface, acid assaults start and progressively move inward. Mass, strength, and stiffness decrease are common indicators of concrete degradation [¹⁰, ¹¹]. Additionally, a progressive reduction in weight and compressive strength is shown. Using pozzolanic elements like fly ash, slag, and silica fume in place of some of the cement increases the strength of the concrete mix [¹²]. By varying the amount of fly ash added to the concrete, CHOI [¹³] examined its qualities experimentally and found that the early strength growth was delayed because of the reduced hydration rates. The compressive strength rises above the required strength after 28 days of curing. The characteristics of different concrete classes with varying amounts of silica fume were thoroughly examined by BADALYAN et al. [¹⁴]. Due to the addition of silica fume, they saw a notable increase in compressive strength. However, after 12 weeks of immersion in solutions containing 4% sulphuric acid and 4% nitric acid, the specimens’ strength was shown to have decreased. The reason for these decreases is the higher solubility of ettringite and gypsum, which raised the porosity of the concrete. According to parallel research by KUMAR et al. [¹⁵], nitric acid caused more severe degradation because calcium salts are more soluble in it, which led to a larger mass loss. Additionally, they found that silica fume creates calcium silicate hydrate (C-S-H) linkages, which increase compressive strength and have no discernible effect on the concrete’s resistance to strong sulphuric acid assaults. Researchers have thoroughly investigated the use of fibers in concrete mixes to address these issues and sustainably improve concrete qualities [¹⁶]. The objective is to enhance concrete’s durability and mechanical performance in acidic conditions. Many experts are examining the impact of various fiber kinds on the characteristics of concrete due to the increased interest in using agricultural waste in buildings. Concrete’s tensile qualities, flexural strength, ductility, and toughness may all be considerably improved by adding fibers, according to experimental studies [¹⁷]. Natural fibers include a wide variety of plant-based materials, including hemp, sisal, coconut, bamboo, palm, and banana stem fibers. These fibers are a desirable alternative for environmentally friendly building techniques as they are widely accessible and reasonably priced to produce in many areas [¹⁸]. When it comes to the fibers themselves, lignocellulosic fibers like those found in coir are sourced from coconut trees. They may be used to produce a variety of commercially accessible items, including yarn and rope, due to their excellent durability. Lignin (41–45%) and cellulose (36–43%) make up the majority of coir fibers, with minor quantities of pectin, hemicellulose, and other soluble materials [¹⁹]. Coir fibers are non-toxic and ductile; however, because of their restricted strength, they have worse reinforcing capacities. According to earlier research, treating these fibers can improve concrete’s qualities and provide more appealing results. Longer coir fibers decreased concrete workability and slump, according to studies by VEERAPPAN et al. [²⁰] and AHMAD et al. [²¹], which looked at the effect of coir fibers and their lengths on concrete qualities. Even though they could dilute the cement matrix, samples with modest amounts of long fibers showed good compressive strength. Because the longer fibers effectively bridged cracks, the split tensile strength increased. Notwithstanding these advantages, ROCHA et al. [²²] noted drawbacks such as fiber hydration leading to microstructure void development and swelling. Many experts concur that because concrete is hydrophilic, a higher fiber content has a detrimental impact on workability. To avoid fiber balling, AHMAD et al. [²³] suggested keeping the fiber level at 1.5%. According to some studies, concrete’s qualities are enhanced when fibers are treated to eliminate surface contaminants. In contrast to untreated fibers, discovered that alkali treatment of coir fibers with 5% NaOH improved surface characteristics like homogeneity and roughness, improving the interaction between the fibers and matrix, and improving mechanical properties like flexural strength and toughness. When added to concrete, synthetic fibers like glass fibers have comparable benefits, mostly because they increase the strength of the concrete and inhibit the creation of microcracks. High concentrations of glass fibers could not form a sufficient link inside the mix. According to LI et al. [²⁴], because glass fibers are flexible and polymeric, adding too much fiber might result in debonding inside the cement mix. According to the research, glass fibers boosted the tensile strength of concrete, but they also pointed out that fiber slippage might cause failures and reduce the strength of the concrete. Glass fibers’ flexibility significantly enhanced tensile strength by equally dispersing stresses, according to ALI et al. [²⁵] and LV et al. [²⁶]. A specimen with a controlled fiber content is more compact because it enhances fracture control inside the cement matrix. High-quality concrete was indicated by concrete specimens containing silica fume, coir fibers, and coconut shell aggregates, which obtained ultrasonic pulse velocity (UPV) values of more than 4.5 km/s, according to tests conducted by Satheesh KUMAR et al. [²⁷]. Paddy farming also yields rice husk, another agricultural waste. Paddy crops are thought to cover an estimated 10.5 million hectares of land in India, for instance. The output of rice has also quadrupled during the last 20 years, from 25.3 million metric tons to 45.8 million metric tons. When rice is processed, a significant amount of rice husk is produced as crop waste, making up as much as 20% of the rice’s weight. The effects of adding rice husk fibers to the concrete are examined in some publications to lessen the burden of safe disposal; nevertheless, the results are not sufficient to make firm judgments. In contrast to conventional concrete, AHMAD et al. [²⁸] – the experimental study revealed that adding rice husk to concrete decreased its compressive strength. Increased fiber content causes concrete to become more porous, which lowers the density of the concrete, according to the study. When water is ejected at high temperatures, porosity rises, and the matrix expands, creating microcracks. Rice husk fibers’ hydrophilic qualities result in lower slump values at 1.5% fiber concentration. The fibers’ excessive saturation causes them to emit free water, which can weaken concrete’s compressive strength. According to research on concrete’s degradation in acidic conditions, prolonged exposure causes a loss of mass and strength. According to CHEN et al. [²⁹], expanding products caused by elevated acid concentration degrade the structure by causing interior fissures. Because fiber is hydrophilic, a high fiber concentration reduces concrete’s workability and degrades its performance. They improve fiber dispersion and homogeneity within the concrete mix; short fibers in smaller quantities are more advantageous [³⁰]. By improving bonding in the cement matrix through increased C-S-H gel synthesis, fibers combined with silica fume improve the structure of the concrete and reduce the void ratio. To assess the concrete’s resilience in acidic conditions and solve the durability problems of fiber-reinforced concrete, this study intends to examine the mechanical and durability properties of concrete with additional fibers, such as jute, hemp, and glass fibers. The use of glass and natural fibers in concrete has been the subject of much research, but researchers are still interested in how well these materials work in corrosive situations. While considering several other elements to replicate a genuine corrosive environment, this study primarily uses the pH level to represent the severity of the acidic environment. Although glass fibers may degrade in acid, this article examines how well they perform during the initial stages of acid immersion. By offering a different method of recycling agricultural waste in the building industry, the natural fibers used for this study, jute and hemp fiber, present significant environmental advantages. The primary objective of this research is to enhance the durability and mechanical performance of concrete under acidic exposure through the combined incorporation of silica fume and discrete fibers. While numerous studies have examined individual fibers or mortar-based specimens, limited work has addressed the synergistic interaction between natural and synthetic fibers in concrete exposed to realistic acidic environments. This study is distinct in simultaneously utilizing two agricultural waste–based natural fibers (jute and hemp) and a synthetic fiber (glass) with partial cement replacement by silica fume to develop a sustainable, acid-resistant composite. The novelty lies in integrating eco-friendly fibers with silica fume to improve acid resistance, mechanical integrity, and microstructural stability, thereby addressing durability challenges in corrosive environments. The experimental program involves mechanical (compressive and tensile strength) and durability (mass loss, ultrasonic pulse velocity, and sorptivity) assessments, coupled with microstructural analysis, to establish a comprehensive understanding of the fiber–silica fume synergy for sustainable infrastructure applications.

2. EXPERIMENTAL INVESTIGATION

2.1. Materials

The concrete was made with Plain Cement Concrete (PCC), which has a strength class of 42.5 MPa. The coarse aggregates were 20 mm crushed stones, and the fine aggregate was well-graded natural sand. The cement was blended with tap water. Fiber-containing concrete was mixed with silica fume, which made up about 5% of the cement content. To reduce the balling impact during blending, the fiber lengths were limited to 10 mm, and the fiber composition consisted of 1% Jute Fibers (JF), 1% Hemp Fibers (HF), and 1% Glass Fibers (GF). To get rid of surface contaminants, including lignin, cellulose, and hemicellulose, natural fibers like JF and HF were soaked in a 5% NaOH solution for 30 minutes, thoroughly rinsed with water, and allowed to air dry for three days. All concrete mixes included a high-range water-reducing additive to avoid oversaturating the fibers.

2.2. Mix design

The mix design developed by MOSTAFAEI et al. [³¹] was used in this study and tackles a practical problem involving concrete exposed to corrosive environments. To guarantee uniformity and pertinence in its inquiry, the present study employs this mixed design. Due to the inclusion of the additive, the concrete mix’s water/cement ratio, which was 0.47, dropped to 0.44. The various mix proportions utilized in this concrete are shown in Table 1. The mix indicates the primary components utilized in the concrete mixture. As an illustration, the mixed combination “OC + JF + SF” denotes the addition of Silica Fume (SF) and Jute Fibers (JF) to the conventional concrete mix (OC); the whole mixture is referred to as “JFSFC”, and 28 days of curing produced the desired design compressive strength of 40 MPa. In this manner, the concrete was prepared. After adding the dry ingredients to the mixer, cement, silica fume, natural sand, and coarse aggregates were stirred for about three minutes. Fibers were added in layers to samples that had them. Half of the fibers and the other dry ingredients were first put into the concrete mixer, followed by the other half of the fibers and the dry ingredients. About three minutes were spent allowing this mixture to combine. By increasing the fibers’ dispersion, this technique guarantees a consistent distribution throughout the concrete mix. After combining the dry ingredients, half of the water was added and stirred for three minutes. After adding the admixture at the prescribed dosage to the remaining water, it was given a minute to respond. The mixer swirled the concrete mix for three to four minutes until a homogeneous mixture was created, and the remaining water was added progressively based on the concrete mix’s look and workability.

Table 1
Proportions of concrete mix design.

MIX COMBINATION	AMOUNT OF FIBER (Kg/m3)	AMOUNT OF SILICA FUME (Kg/m3)	CEMENT (Kg/m3)	WATER (Kg/m3)	FINE AGGREGATES (Kg/m3)	COARSE AGGREGATES (Kg/m3)
OC	0	0	440	205	698	1010
OC + JF + SF - JFSFC	4.4	22	440	205	698	1010
OC + HF + SF - HFSFC	4.4	22	440	205	698	1010
OC + GF + SF - GFSFC	4.4	22	440	205	698	1010

2.3. Methodology

In this study, treated jute fibers (TJF) and treated hemp fibers (THF) were incorporated into the cementitious matrix to enhance both mechanical performance and durability. To improve fiber–matrix compatibility and ensure effective load transfer, the fibers were subjected to alkali treatment using a 5% sodium hydroxide (NaOH) solution, prepared by dissolving 50 g of NaOH in 1 L of distilled water. The fibers were immersed in this solution for 4 hours at ambient temperature (25 ± 2°C), maintaining a pH of approximately 13, which effectively removes surface impurities such as hemicellulose, lignin, and waxes. This process increases surface roughness, facilitating better mechanical interlocking and chemical bonding within the concrete matrix. After treatment, the fibers were thoroughly rinsed with distilled water until a neutral pH was achieved to eliminate residual alkali, followed by air-drying for 24 hours and oven-drying at 60°C for 6 hours to remove any remaining moisture, ensuring dimensional stability and preventing adverse reactions with cement hydration. Concrete specimens were prepared in triplicate per mix following ASTM and IS standards to ensure statistical reliability. Compressive strength was assessed using 150 × 150 × 150 mm³ cubes under three exposure conditions—neutral (control), mildly acidic (pH 5), and aggressive acidic (pH 3)—to simulate varying environmental aggressiveness and evaluate structural capacity under different levels of chemical attack. Split tensile strength was measured on cylindrical specimens (150 mm diameter × 300 mm height) under the same conditions to assess the contribution of fibers to crack resistance and tensile performance, which is critical for serviceability. Ultrasonic Pulse Velocity (UPV) tests were conducted on the compressive strength cubes both before and after 28 days of acid exposure to provide a non-destructive evaluation of internal homogeneity and detect potential microcracks or voids induced by acid attack. Durability was further examined through sorptivity tests on disc-shaped specimens (100 mm diameter × 50 mm thickness), which monitored capillary water absorption over time, providing insight into the permeability and long-term resistance of the concrete to moisture and chemical ingress. Finally, scanning electron microscopy (SEM) was performed on representative fragments to observe microstructural interactions, fiber–matrix bonding, and changes induced by acid exposure, offering direct visualization of the mechanisms underlying the mechanical and durability behavior. By integrating mechanical, durability, and microstructural analyses, this methodology provides a comprehensive evaluation of fiber-reinforced concrete performance, linking the effects of fiber treatment to structural integrity and long-term durability under aggressive environmental conditions.

2.4. Test methods

Compressive strength tests were conducted following the guidelines of IS 516–1959 to evaluate the mechanical performance of the concrete mixes. Concrete cubes were cured for 28 days under controlled conditions before testing. The tests were performed using a calibrated compression testing machine with a loading rate of 0.5 MPa/s. Each mix was tested with a minimum of 3 replicate samples to ensure statistical reliability. The maximum load at failure was recorded, and compressive strength was calculated by dividing this load by the cross-sectional area of the specimen. All testing equipment was calibrated and maintained according to manufacturer’s specifications to ensure accuracy. To evaluate the concrete’s compressive strength, 150 mm × 150 mm × 150 mm concrete cube samples were cast using a Compression Testing Machine (CTM). For each mix design, a total of 27 cubes were formed to compare the concrete’s compressive strength before and after acid conditioning. Compressive strength tests were performed on nine samples following the standard 28-day curing period. Nine samples were submerged in pH 3 sulphuric acid solutions, and nine in pH 5 to expose the remaining samples to acidic conditions. These samples were acid-conditioned for an additional 28 days before being tested for compressive strength and tested on all of the combinations shown in Table 2, before and after being exposed to acidic conditions. The ASTM C496 requirements were followed to estimate the concrete’s split tensile strength using cylindrical samples that were 150 mm in diameter and 300 mm in height. Following a 28-day curing period, twelve cylinders, three from each mix, were cast and demolded. On both faces, the round surface’s centerline was marked to facilitate the positioning of the steel plates. The cylinders were put through a loading rate of 1 kN/s to determine the failure load while being tested using the Universal Testing Machine (UTM). Equation 2 may be used to determine the specimen’s splitting tensile strength according to the standards.

Table 2
Total no. of specimens used for each mix composition.

TESTS CONDUCTED FOR EACH MIX	DIMENSIONS OF EACH SPECIMEN
	CUBE 150 mm × 150 mm × 150 mm	CYLINDER 300 mm IN HEIGHT AND 150 mm IN DIAMETER	CYLINDER 100 mm IN HEIGHT AND 50 mm IN DIAMETER
Compressive strength	9	–	–
Split tensile strength	–	3	–
Water absorption	–	–	3
Acid attack – pH 3	9	–	–
Acid attack – pH 5	9	–	–

(2) $$\sigma =\frac{2P}{\pi DL}$$

Where P – indicates split load in kN, D – specimen diameter in mm, and L – specimen length in mm. To evaluate the concrete’s sorbent capability, samples measuring 100 mm in diameter and 50 mm in height were cast. Samples are prepared using ASTM C1585 as a guide. Before the test, the samples were sealed for 15 days and oven-dried for 3 days. Insulating tape was applied around the perimeter of the samples, and the section that did not come into contact with water was covered with plastic wrap. To evaluate water absorption, the smooth surface of the specimen was placed on a support device situated in a pan of water, with the water level maintained at 2 mm below the height of the sample resting on the supports. To determine the mass change, the following sample was immersed in water, and its weight was determined immediately before the immersion.Once the samples were submerged in the water, measurements were made of their mass at 60 seconds, 5 minutes, 10 minutes, 20 minutes, 30 minutes, 60 minutes, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 1 day, 2 days, 3 days, 5 days, 6 days, 7 days, and 8 days. Calculating the water absorption included comparing the samples’ mass before and after immersion. We used Equation 3 to determine the water absorption.

(3) $$I=\frac{m_t}{a\times b}$$

where I – the amount of adsorption in mm, mt is – change in the mass of the specimen at a given time (t) in grams, a is the specimen area in m², and d is the density of water in g/mm³. The specimens were submerged in solutions with pH values of 3 and 5 to simulate acid rain and circumstances seen in a wastewater treatment plant or sewers to examine the impact of acidic environments on the fiber-added concrete. To represent the slightly and severely acidic ecosystems throughout the world, these particular pH values were chosen. Acid rain with a pH of 3 is generally observed in actual environments in areas with high industrial and urban activity, while acid rain with a pH of 5 is observed in areas with low population and moderate industrialization. The required pH solutions were made by diluting a 5% H₂SO₄ solution of pH 1 with tap water. The pH levels of the tanks were adjusted using a 5% HNO₃ solution. Because the concrete’s alkalinity might cause alkaline pH values, the solution’s pH was first measured three days after the concrete was submerged. Following that, the specimens were examined once a week, and the pH levels were adjusted as necessary. As per ASTM C597–22 standards, Ultrasonic Pulse Velocity (UPV) Tests were also performed on samples submerged in acidic conditions. To determine the UPV, tests were conducted seven, fourteen, and twenty-eight days following acid immersion. Before every test, the tester was calibrated using the included calibration tool. By placing transducers on opposite sides of the concrete surfaces and timing the pulse’s passage between them, the direct transmission method was utilized to measure the UPV. The mean separation between the two transducers was measured. The pulse velocity that resulted was computed using Equation 4.

(4) $$V=\frac{L}{T}$$

where L – is the transducer’s central distance in km, T – is the time of transit in μs, and V is the velocity of the pulse in km/s.

3. EXPERIMENTAL RESULTS

3.1. Compressive strength

Figure 1 presents the compressive strength results of all concrete mixes after a 28-day curing period. A clear enhancement in mechanical performance is observed in all silica fume– and fiber-reinforced concretes compared with the control mix (OC). The glass fiber–silica fume concrete (GFSFC) exhibited the most pronounced improvement, recording a 25.8% increase in compressive strength relative to OC. The hemp fiber–silica fume concrete (HFSFC) achieved a 12.8% increase, while the jute fiber–silica fume concrete (JFSFC) showed a 6.22% enhancement. These results confirm that both the inclusion of silica fume and the incorporation of fibers contribute to matrix densification and crack-bridging behavior, thereby improving the overall load-bearing capacity of the concrete. The observed improvement can be attributed to several interrelated mechanisms. Firstly, the inclusion of silica fume (SF)—a highly pozzolanic material with particle sizes much smaller than cement—enhances the formation of secondary calcium silicate hydrate (C–S–H) gel through the pozzolanic reaction with calcium hydroxide released during cement hydration. The formation of additional C–S–H refines the pore structure, reduces porosity, and increases the overall compactness of the cementitious matrix. The densified microstructure also minimizes the connectivity of capillary pores, thus restricting the ingress of aggressive ions and improving durability. This mechanism is consistent with the findings of LUO et al. [³²], who demonstrated that silica fume incorporation leads to enhanced strength development and long-term performance. Secondly, the addition of discrete fibers contributes to the composite’s improved structural performance by providing crack-bridging and stress redistribution effects. Glass fibers, in particular, exhibit superior tensile strength and stiffness, enabling them to arrest microcrack propagation during load application. This behavior delays the transition from microcracking to macrocracking, leading to an apparent improvement in compressive strength. The higher performance of GFSFC is therefore attributed to the synergistic effect between the pozzolanic reactivity of silica fume and the reinforcement efficiency of glass fibers, which together enhance both the microstructure and the mechanical integrity of the concrete. For natural fibers (hemp and jute), mechanical performance is influenced by surface characteristics and fiber–matrix interaction. Natural fibers inherently contain cellulose, hemicellulose, and lignin, which can hinder bonding due to their hydrophilic and chemically unstable nature. The chemical surface treatment applied in this study effectively removed these impurities, thereby improving surface roughness and interfacial adhesion. This modification enhances the fiber–matrix interfacial transition zone (ITZ), as also reported by Maia PEDERNEIRAS et al. [³³], resulting in better load transfer and improved strength. Moreover, the optimized fiber volume fraction (1%) ensured uniform dispersion of fibers throughout the mix, minimizing the risk of fiber clustering and achieving effective stress transfer. This dosage was chosen based on prior research indicating that low-volume fiber additions enhance strength without compromising workability. HASSAN et al. [³⁴] similarly observed that short fibers at low dosages promote homogenous distribution and act as micro-reinforcements within the cementitious matrix. Another noteworthy observation is the relationship between compressive strength and curing duration. All fiber-reinforced mixes exhibited continuous strength gain from early to later ages, highlighting the progressive hydration and pozzolanic reactions of silica fume. The 28-day strength results reflect the completion of major hydration processes and stabilization of the matrix microstructure. The improvement trend aligns with the Ultrasonic Pulse Velocity (UPV) measurements, confirming a direct correlation between higher UPV values and increased compressive strength, both indicators of superior matrix compactness and integrity. Overall, the combination of silica fume and fibers—particularly glass fibers—produced a denser and stronger concrete matrix capable of resisting microcracking and enhancing both short-term and long-term performance. The enhanced compressive strength achieved in the GFSFC mix validates the efficacy of this hybrid modification strategy in developing high-performance, durable, and environmentally sustainable concrete composites.

Figure 1
Compressive and tensile strength of concrete mix at 28 days.

3.2. Split tensile strength

Figure 1 also presents the split tensile strength results of the tested concrete specimens after 28 days of curing. Compared with the control concrete (OC), the incorporation of silica fume and fibers significantly enhanced the tensile capacity of the composite. The glass fiber–silica fume concrete (GFSFC) exhibited the highest improvement, showing a 26.4% increase in tensile strength relative to OC. The jute fiber–silica fume concrete (JFSFC) and hemp fiber–silica fume concrete (HFSFC) followed with 24.7% and 12.8% increases, respectively. These results indicate that the inclusion of discrete fibers markedly improves the tensile load-carrying capacity of concrete, with glass fibers demonstrating the most efficient reinforcing effect. The enhancement in tensile strength is primarily attributed to the fiber-bridging mechanism, which impedes crack initiation and propagation within the cement matrix. Fibers act as micro-reinforcements, bridging across developing microcracks and redistributing tensile stresses. As shown schematically in Figure 2, this mechanism results in a more gradual failure mode and delayed crack widening. Both glass and jute fibers, owing to their strand-like morphology and optimal fiber length of 10 mm, effectively interlock with the cement matrix and aggregates, improving interfacial adhesion and crack-arresting behavior. This micro-mechanical interlocking leads to a more cohesive and ductile fracture response. Conversely, the relatively lower tensile performance of HFSFC is likely due to the shell-like surface morphology and higher water absorption of hemp fibers, which weaken their interfacial bonding with the cement matrix. The higher porosity introduced by hemp fibers may also reduce matrix compactness, contributing to localized stress concentrations and early crack propagation. Similar observations have been reported by GULZAR et al. [³⁵], who demonstrated that the addition of jute fibers at controlled dosages enhances tensile strength by improving crack-bridging efficiency and fiber–matrix adhesion. Furthermore, PANDA et al. [³⁶] reported that the inclusion of short glass fibers (6 mm in length) enhanced the tensile strength of concrete due to uniform dispersion and effective stress transfer between the matrix and fibers. The present findings are in good agreement with their conclusions, reaffirming that fiber geometry, aspect ratio, and dispersion quality critically influence the tensile response of fiber-reinforced concrete (FRC). The superior tensile strength observed in the GFSFC specimens is also attributed to the synergistic interaction between silica fume and glass fibers. Silica fume enhances matrix densification and the formation of C–S–H gel, improving the bond between the cement paste and embedded fibers. This results in an optimized interfacial transition zone (ITZ), facilitating efficient load transfer during tensile loading. However, excessive fiber addition can adversely affect the tensile strength due to the “balling effect,” where fibers cluster together, entrapping air and disrupting matrix uniformity. This phenomenon reduces the effective bonding area between fibers and cement paste, resulting in a weaker composite. KATMAN et al. [³⁷] corroborated this behavior, reporting that tensile strength increased by 9% at a 0.5% coir fiber dosage but declined at higher fiber contents. They also observed a “pseudo-ductile” behavior, wherein the specimen continued to bear load even after visible cracking due to fiber bridging across fracture planes. Overall, the results confirm that an optimal fiber dosage of 1%, combined with 10% silica fume, significantly enhances the tensile performance of concrete. The GFSFC mix demonstrated the best balance between strength, ductility, and microstructural integrity, followed by JFSFC and HFSFC. The observed improvements are consistent with the compressive strength and UPV trends, highlighting the integral role of silica fume and fiber synergy in enhancing both the mechanical and durability characteristics of concrete composites.

Figure 2
Effect of crack bridging in JFSFC.

3.3. Sorptivity

As illustrated in Figure 3, the sorptivity coefficients were determined by plotting the best-fit lines representing the initial and secondary water absorption rates for all concrete mixtures. Among the tested specimens, GFSFC exhibited the highest sorptivity values, followed by JFSFC and HFSFC. In general, fiber-reinforced concretes showed elevated sorptivity due to the hydrophilic nature of natural fibers, which promote the formation of interconnected capillary pores during early hydration.

Figure 3
Sorptivity of concrete mixes.

These capillary networks facilitate the movement of water into the matrix, thereby increasing sorptivity. However, as hydration progresses, the formation of calcium silicate hydrate (C–S–H) gel progressively fills the capillary voids, reducing water ingress and lowering the overall sorptivity. The inclusion of silica fume further enhances this effect by reacting with portlandite (Ca(OH)₂) to form additional C–S–H gel, contributing to pore refinement and densification of the microstructure. Studies by Jiang et al. [³⁸] and Abushama et al. [³⁹] reported similar findings, confirming that the ultrafine silica fume particles act as effective fillers while simultaneously engaging in pozzolanic reactions. Moreover, the high surface area and reactivity of silica fume allow for a gradual secondary hydration reaction, where SiO₂ reacts with Ca(OH)₂ to form additional C–S–H gel, as expressed in Equation 5. This secondary gel formation mechanism effectively reduces pore connectivity, thereby decreasing the long-term sorptivity of the concrete. Consequently, the combined influence of silica fume and fiber addition improves the microstructural integrity and durability of the composite.

(5) $$3CH+2Si{O}_2\to C_3{S}_2{H}_3$$

4. EFFECT OF ACIDIC ENVIRONMENT ON THE CONCRETE

4.1. Effect on the mass loss

On the 7th, 14th, and 28th days of acid conditioning, as well as after 28 days of curing, the samples’ mass change was assessed. After being air-dried in the tank, the samples were placed on a weighing scale to determine their mass. On day 28, there was a 0.24% rise in both HFSFC and typical concrete (OC) submerged in pH 3 solutions, as seen in Table 3. The mass of JFSFC grows by 0.35 percent at 28 days as well. The 28th day shows a rise of 0.25% mass growth, according to GFSFC. As seen in Table 3, GFSFC samples submerged in pH 5 showed a 0.32% mass gain following 28 days of acid training. Similarly, on the 28th day, the strength of the HFSFC and OC samples increased by 0.30 percent. After the same period, however, JFSFC samples only displayed a 0.20 percent rise in mass. Since the JFSFC samples exhibited a minor decrease in mass gain when submerged in pH 5 as opposed to pH 3. In contrast, the other samples typically showed an increase in mass under pH 5 circumstances. In the spaces on the concrete structure, expansive materials like gypsum (CaSO₄) develop, which is responsible for the mass increase. Sulphuric acid causes both acid and sulfate attacks during the acid immersion procedure [⁴⁰]. The creation of gypsum, which fills the pores in the concrete, is demonstrated by the interactions of Ca(OH)₂ and C-S-H gel with the sulphuric acid in Equations 6 and 7. Furthermore, ettringite production, which, because of its expansive character, adds to the solid’s increased volume, is further highlighted by the reaction in Equation 8.

Table 3
The reduction in mass relative to the initial mass when subjected to acidic conditions.

MIX COMBINATION	pH 3 – CHANGES OF MASS IN %			pH 5 – CHANGES OF MASS IN %
	7 DAYS	14 DAYS	28 DAYS	7 DAYS	14 DAYS	28 DAYS
Normal Concrete (OC)	0.11	0.11	0.22	0.00	0.00	0.27
Normal Concrete/1% Jute fiber/5% of Silica Fume (JFSFC)	0.11	0.11	0.22	0.00	0.00	0.27
Normal Concrete + 1% Hemp fiber + 5% of Silica Fume (HFSFC)	0.05	0.05	0.35	0.00	0.00	0.15
Normal Concrete + 1% Glass Fiber + 5% of Silica Fume (GFSFC)	0.00	0.00	0.30	0.00	0.11	0.28

(6) $$Ca{(OH)}_2+H_{2}S{O}_4\to CaS{O}_4.2H_{2}O(gypsum)$$

(7) $$3CaO.2Si{O}_2.3H_{2}O\mathrm{}+\mathrm{}{H}_{2}S{O}_4\mathrm{}\to CaS{O}_4.2H_{2}O\mathrm{}+\mathrm{}Si{(OH)}_4$$

(8) $$3CaS{O}_4\mathrm{}+3CaO.A{l}_2{O}_3.6H_{2}O\mathrm{}+25H_{2}O\mathrm{}\to 3CaO.A{l}_2{O}_3.3CaS{O}_4.31H_{2}O$$

The weight growth is less noticeable on day 28, as seen in Table 3, suggesting that the structure may be further hydrated throughout the four weeks of acid immersion. This hypothesis is further reinforced by Liu et al. [⁴¹], who claim that a slight weight gain may be seen during the initial acid exposure period as a result of the reaction products being deposited in the concrete’s pore structure.

Additionally, this hypothesis is consistent with the results of GUO et al. [⁴²], who saw a slight rise in the concrete’s mass throughout the first 12 weeks of the acid immersion period. The XRD patterns of conventional concrete exposed to 20 days of acid immersion show a significant decrease in Ca(OH)₂ concentrations and a rise in CaSO₄• 2H₂O content [⁴³]. The creation of gypsum rises as a result of the pH 3 solution’s acidity, which increases the presence of H⁺ and SO₄^2- ions. For the first conditioning phase, the JFSFC specimens’ crack bridging function helps to avoid the creation of expanding products to a certain degree, which adds to the mass increase. Through an experimental investigation, FARES et al. [⁴⁴] confirm this by finding that the crack bridging effect, which regulates the leeching of expansive products like gypsum or ettringite, caused a 1 percent fiber content in concrete exposed to a 1 percent MgSO₄ solution to increase in mass at 28 days. The reduced void content and crack bridging effect, in conjunction with the reactions of silica fume inside the concrete, are also responsible for the mass gain seen by GFSFC and HFSFC specimens. The selection of pH 3 and pH 5 acidic solutions for this study reflects common environmental conditions that concrete structures may encounter in aggressive acidic environments. pH 3 represents a highly acidic medium typical of industrial waste effluents, acid rain, or acid mine drainage, where concrete deterioration is accelerated due to aggressive sulfate and hydrogen ion attack [⁴⁵]. On the other hand, pH 5 simulates mildly acidic conditions often observed in urban rainwater influenced by atmospheric pollution and biological activity, which still pose significant risks to concrete durability over time [⁴⁶]. Studies by KANAGARAJ et al. [⁴⁷] demonstrate that concrete exposed to solutions within this pH range undergoes chemical degradation mechanisms such as decalcification and sulfate attack, making these pH levels representative for accelerated durability testing. Thus, the use of pH 3 and 5 in this study provides a meaningful assessment of the performance of fiber-reinforced concrete under realistic acidic exposures. In contrast to the typical mass loss reported in the literature during acid immersion tests, the observed increase in mass in certain specimens may be attributed to the formation and deposition of secondary reaction products such as gypsum and ettringite on or near the concrete surface. These expansive products result from the interaction of acidic solutions with calcium-containing phases in the cement matrix and can lead to surface accumulation that temporarily increases the overall specimen mass. Additionally, fibers, particularly natural ones with hydrophilic properties, may absorb moisture or reaction products, contributing to the mass gain. However, this mass increase should not be interpreted as an improvement in durability, as it often coincides with increased porosity and microcracking beneath the surface, which ultimately degrade mechanical performance. Therefore, the mass gain observed here likely reflects early-stage chemical interactions rather than true resistance to acid attack and must be considered alongside strength and microstructural data to fully assess durability.

4.2. Effect on the compressive strength

To simulate an acidic environment representative of acid rain exposure, 5% sulfuric acid (H₂SO₄) solution was prepared to achieve pH levels of 3 and 5. The compressive strength of the samples subjected to these acidic media was compared with that of the unexposed (control) specimens, as illustrated in Figure 4. At both pH 3 and pH 5, the ordinary concrete (OC) exhibited a marked improvement in compressive strength, reaching approximately 48 MPa after acid conditioning. Among the fiber-reinforced silica fume concretes, the jute fiber-based mix (JF-SFC) demonstrated an increase of 6.65% at pH 3 and 1.51% at pH 5, while the hemp fiber composite (HF-SFC) exhibited a 7.43% strength gain in the pH 5 medium but a 3.07% reduction in the more aggressive pH 3 condition. The glass fiber–silica fume concrete (GF-SFC) displayed a slight increase in compressive strength under both acidic environments after 28 days of exposure. The observed strength enhancement at pH 5 can be attributed to the limited formation of expansive products such as gypsum and ettringite, which tend to fill capillary pores and microvoids within the concrete matrix. This pore refinement effect temporarily densifies the matrix without inducing significant internal stress, thereby contributing to an apparent short-term strength gain [⁴⁸]. Similar trends have been reported in simulated acid rain environments, where concrete specimens exhibit improved compressive and flexural strengths during early exposure periods due to microstructural densification [⁴⁸]. The presence of silica fume (SF) in the fiber-reinforced mixes further enhances the acid resistance of concrete. The pozzolanic reaction between SF and calcium hydroxide produces additional calcium silicate hydrate (C–S–H), which strengthens the matrix and reduces permeability. Consequently, the combined effect of silica fume and fiber reinforcement (FRC) yields superior performance compared to using fibers alone in OPC-based concrete mixtures [⁴⁹]. However, the slight reduction in compressive strength observed for HF-SFC in a pH 3 medium can be explained by microstructural degradation visible in the SEM micrographs. Under highly acidic conditions, fiber degradation generates voids and microcracks, thereby increasing the matrix’s porosity. The penetration of H⁺ and SO₄^2- ions through these micro-defects promotes the excessive formation of expansive products, which ultimately disrupt C–S–H bonds and weaken the cementitious framework. Although a short-term increase in compressive strength was recorded for some fiber-reinforced concretes, particularly under milder acidic exposure (pH 5), this phenomenon should be interpreted cautiously. The apparent strength gain may not necessarily reflect genuine improvement in material performance; rather, it may result from temporary pore filling by sulfate-based reaction products. Over prolonged exposure, the continuous formation of gypsum and ettringite can lead to internal stress accumulation, microcracking, and eventual deterioration of the concrete matrix. Thus, despite the initial enhancement in strength, the long-term durability and service life of such specimens could be significantly compromised due to these deleterious chemical interactions.

Figure 4
Compressive strength of the concrete mix after 28 days of acid curing.

4.3. Ultrasonic pulse velocity (UPV) test

The Ultrasonic Pulse Velocity (UPV) test was conducted using the direct gearbox method, chosen for its simplicity, high accuracy, and reliability in assessing the internal integrity of concrete specimens. UPV is a non-destructive measure that reflects the density, homogeneity, and quality of the cementitious matrix. As illustrated in Figure 5, all mixes exhibited UPV values exceeding 4.5 km/s, which, according to ASTM C597, corresponds to excellent-quality concrete. This indicates that the internal structure of the concrete did not significantly degrade even after 28 days of exposure to acidic environments (pH 3). For the JFSFC and GFSFC mixes, UPV values ranged from 4.85 to 4.94 km/s and 4.83 to 4.90 km/s, respectively, across curing ages of 7, 14, and 28 days at pH 3. The HFSFC mix recorded slightly lower velocities—4.72 km/s at 7 days and 4.85 km/s at 14 days—which can be attributed to the higher moisture absorption capacity of hemp fibers, leading to the formation of localized micro-voids. The superior UPV performance of JFSFC and GFSFC is mainly attributed to the fiber-induced crack-bridging effect and the pozzolanic activity of silica fume, which together refine the pore structure and enhance C–S–H gel formation, thereby increasing matrix density.

Figure 5
UPV values of concrete mix at pH = 3.

These findings are consistent with those of HILAL et al. [⁵⁰], who reported that silica fume–modified fiber-reinforced concretes (both with and without coir fibers) consistently achieved UPV values above 4.5 km/s due to the densification of the microstructure. The slightly higher UPV values in the pH 3 medium may also result from the formation of gypsum (CaSO₄·2H₂O) and related expansive reaction products, which temporarily fill capillary pores, reducing pulse transmission time. However, the marginal drop in UPV at 28 days for HFSFC indicates possible fiber degradation due to prolonged acid exposure, leading to increased porosity and reduced signal velocity. As shown in Figure 6, UPV measurements under pH 5 conditions followed a similar trend, with only minor variations between pH 3 and pH 5. Notably, the GFSFC specimens showed a slight reduction in UPV at pH 5, suggesting limited surface deterioration or fiber–matrix interface weakening. Overall, all concrete mixes—including those exposed to aggressive pH conditions—maintained UPV values well above the 4.5 km/s threshold, confirming exceptional internal quality and durability of the developed silica fume–based fiber-reinforced concretes.

Figure 6
UPV values of concrete mix at pH = 5.

4.4. Microstructure analysis

Concrete samples were examined using scanning electron microscopy (SEM) to determine the underlying process of degradation when exposed to corrosive conditions. Thus, Figure 7 displays the microstructural changes in concrete specimens of all four mixes during a 28-day acid exposure period. The control specimen, OC in Figure 7(a), exhibits a low level of voluminous products like gypsum and ettringite following 28 days of acid immersion in a pH 3 environment. Nonetheless, a few calcium hydroxide (CH) crystals are seen together with a greater quantity of CSH gel, the primary cement hydration product. Similar reaction products may also be seen in Figures 7(b), (c), and (d). The microstructure in Figure 7(b) shows the presence of jute fibers in the cement mix. There are micro-voids in the matrix, and the jute fibers do not do much to create a denser microstructure. The development of these micro-holes may be the consequence of the jute fibers themselves deteriorating from acid damage. The presence of silica fume in all samples increases the generation of CSH because all specimens show significant levels of crystals. Figures 7(c) and 7(d) show increased gypsum formation in the microstructure as prismatic crystals. This is because the CSH gel and sulphuric acid react to increase the strength of the JFSFC and GFSFC specimens. Furthermore, the glass and coir fibers contribute to a more cohesive microstructure, particularly in GFSFC samples, thereby enhancing the structure’s compactness and delaying the emergence of microcracks in the cement matrix. Additionally, ettringite, which are long, needle-like structure, may be detected in both JFSFC and GFSFC specimens but not in significant quantities. Longer exposure times are thought to boost the synthesis of these bulky compounds, which might be harmful to the concrete. Because silica fume fills the voids in the cement matrix, it increases the production of CSH gel and voluminous products like gypsum and ettringite, which improve the microstructure of the concrete and increase the samples’ strength. Figure 8 displays SEM pictures of materials submerged in a pH 5 solution. Samples submerged in pH 5 solutions show more ettringite than gypsum after 28 days, whereas those submerged in pH 3 solutions mostly contain gypsum. Lower pH values encourage the formation of gypsum because they cause free calcium ions to combine with sulfate ions to create additional CaSO₄ when the calcium concentration rises, according to ROST et al. [⁵¹].

Figure 7
SEM images of (a) OC, (b) JFSFC, (c) HFSFC, and (d) GFHFC, respectively.

In addition, HOU et al. [⁵²] assert that ettringite production is slowed at lower pH levels in contrast to gypsum crystals, which form more easily as exposure time rises. This may be because calcium aluminate, which dissolves and inhibits ettringite formation in higher acidic conditions, is unstable. Because of the weak bonding between the fibers and the cement matrix, the glass fiber out may have caused the microcrack in the GFSFC specimen shown in Figure 7(d). Furthermore, the presence of many micro-voids in the HFSFC specimen in Figure 7(b) suggests that the hemp fibers have little impact on strengthening the bonding inside the cement matrix. The microstructural analysis conducted via Scanning Electron Microscopy (SEM) provides critical insights that support and explain the macroscopic behavior observed in compressive strength, sorptivity, and Ultrasonic Pulse Velocity (UPV) tests. SEM images revealed that the incorporation of silica fume, along with fiber reinforcement—especially glass fibers—significantly refined the concrete’s microstructure by filling micro-pores and enhancing the density of the calcium-silicate-hydrate (C-S-H) gel network. This densification corresponds well with the increased compressive and tensile strengths recorded, as a compact matrix improves load transfer efficiency and reduces stress concentrations. Furthermore, the reduced sorptivity observed over time can be directly linked to the microstructural improvements seen under SEM. The filling of capillary pores and reduction in micro-voids limit water ingress, decreasing absorption rates and enhancing durability against acidic attack. This microstructural refinement also explains the consistently high UPV values across all mixes, since denser, less porous concrete facilitates faster ultrasonic pulse transmission by minimizing scattering and attenuation. In contrast, SEM images of hemp fiber-reinforced samples exhibited some micro-voids and potential fiber degradation areas after acid exposure, which align with the slightly reduced UPV and increased sorptivity measured for this mix. These microscopic defects likely act as pathways for fluid ingress and stress concentration, leading to diminished overall performance compared to glass fiber mixes. Overall, the SEM findings serve as a vital link that contextualizes and validates the macroscopic mechanical and durability test results, demonstrating how microstructural characteristics govern the performance of fiber-reinforced concrete under acidic conditions.

4.5. Data source and statistical analysis

Figure 8 shows the study flow chart of this analysis. All experimental data were generated in-house from laboratory-prepared concrete specimens. A total of 24 cylindrical specimens were cast for each mix, including glass fiber (GF), treated jute fiber (TJF), and treated hemp fiber (THF), with 10% silica fume replacing cement. Each test was performed on three replicate specimens, and data were collected at 7, 14, and 28 days to monitor the evolution of mechanical and durability properties over time. Acid exposure conditions were simulated using solutions of pH 3 and pH 5, with specimens submerged for 28 days. Mechanical and durability properties measured included compressive strength, split-tensile strength, ultrasonic pulse velocity (UPV), and sorptivity. The mean values and standard deviations (SD) were calculated for all parameters. Error bars in all figures represent ±1 SD to indicate variability and measurement repeatability. To statistically evaluate the influence of fiber type and acid exposure on concrete performance, one-way ANOVA was performed at a 95% confidence level (p < 0.05). Where ANOVA indicated significant differences, Tukey’s post-hoc test was applied to identify pairwise differences. For comparisons involving only two groups (e.g., control vs. acid-exposed specimens), two-tailed independent sample t-tests were conducted. Statistical analyses were performed using SPSS v26 and Microsoft Excel. To ensure the reliability and reproducibility of the experimental results, all tests were performed on three replicate specimens per mix, and the average values along with their corresponding standard deviations (SD) were calculated.

Figure 8
Study flow chart for statistical analysis.

In this study, the Analysis of Variance (ANOVA) model was applied to statistically evaluate the effects of various experimental parameters on the compressive strength and durability behavior of fiber-reinforced silica fume concrete (FSFC) under different pH environments. The parameters and their respective symbols, units, and functional roles in the model are summarized in Table 4. The ANOVA model used in this study is expressed in Equation 9.

Table 4
Parameters considered in the ANOVA model.

PARAMETER	SYMBOL	UNIT	TYPE	RANGE
pH level	X1	–	Independent variable	3, 5
Fiber content	X2	%		0, 0.25, 0.50
Silica fume content	X3	%		0, 5, 10, 15
Exposure duration	X4	Days		7, 14, 28
Compressive strength	Y1	Mpa	Dependent variable	30–55
pH variation	Y2	–	Response variable	2.8–5.3
Reaction products	Y3	–	Observational	Qualitative

(9) $$Y=\mu +{\displaystyle {\sum }_{i=1}^k{a}_{\text{i}}{X}_{\text{i}}+{\displaystyle {\sum }_{i=l}^k{\displaystyle {\sum }_{j>i}^k{\beta }_i{}_j{X}_i{X}_j+\in }}}$$

Where: Y = Response variable, μ = Overall mean of responses, X_i, X_j = Independent variables (factors), α_i, β_ij = Linear and interaction coefficients, and ϵ = Random experimental error, respectively.

The variability in mechanical properties, such as compressive strength, tensile strength, and sorptivity, was assessed by computing SD and displaying error bars in all graphical representations. These error bars provide a visual representation of the spread in the data and allow for easier interpretation of performance consistency across different fiber-reinforced concrete mixes [⁵³]. To statistically validate the influence of fiber type and acid exposure conditions on the mechanical and durability properties of the concrete, a one-way analysis of variance (ANOVA) was conducted. ANOVA helped determine whether the observed differences in mean compressive strength and tensile strength among the different fiber-reinforced mixes (GF, TJF, THF) were statistically significant at a confidence level of 95% (p < 0.05). Where ANOVA revealed significant differences, Tukey’s post-hoc test was applied to identify specific pairwise differences between groups. In cases where only two groups were compared, such as control vs. acid-exposed samples of the same mix, two-tailed independent sample t-tests were used to assess whether the exposure had a statistically significant effect on concrete performance. All statistical analyses were conducted using SPSS v26 and Microsoft Excel, and significance was reported where p-values were less than 0.05. The incorporation of silica fume and glass fibers yielded statistically significant improvements in both compressive and tensile strength compared to the control mix and other fiber types. These differences were also reflected in SEM analysis, which showed denser microstructures in GF- and SF-containing mixes. Error bars included in the strength and sorptivity graphs illustrate low standard deviations, indicating good repeatability of results. A one-way analysis of variance (ANOVA) was conducted to determine the effect of different fiber types (GF, TJF, and THF) on the compressive strength of concrete under acidic exposure conditions (pH = 3 and pH = 5). For pH = 3, there was a statistically significant difference in compressive strength across the groups, F(2, 6) = 91.94, p < 0.0001. Similarly, for pH = 5, a significant difference was found, F(2, 6) = 71.57, p < 0.0001. Post-hoc Tukey’s tests confirmed that GF-reinforced concrete had significantly higher strength compared to TJF and THF in both pH conditions. The relatively low within-group variance (MS = 0.485 at pH 3 and 0.672 at pH 5) suggests high repeatability of the results. Tables 5 and 6 provide the detailed ANOVA table values for pH = 3 and pH = 5 experimental conditions.

Table 5
One-way ANOVA table for pH = 3 (compressive strength).

SOURCE OF VARIATION	SUM OF SQUARES (SS)	DEGREES OF FREEDOM (df)	MEAN SQUARES (MS)	F-VALUE	P-VALUE
Between groups	89.08	2	44.54	91.94	< 0.0001
Within groups	2.91	6	0.485
Total	91.99	8

Table 6
One-way ANOVA table for pH = 5 (compressive strength).

SOURCE OF VARIATION	SUM OF SQUARES (SS)	DEGREES OF FREEDOM (df)	MEAN SQUARES (MS)	F-VALUE	P-VALUE
Between groups	96.22	2	48.11	71.57	< 0.0001
Within groups	4.03	6	0.672
Total	100.25	8

The current analytical model (Compressive strength interpretation combined with one-way ANOVA and follow-up tests) is appropriate as a preliminary framework and yields clear, statistically significant results—particularly the superior performance of GF + silica fume mixes, corroborated by SEM. However, the analytical approach has limitations in assumption verification, power (small n), temporal and factorial structure handling, and the non-specific nature of UPV. Addressing these limitations through mixed-effects/factorial analysis, increased replicates, explicit assumption testing, uncertainty quantification, and multimodal validation (SEM, sorptivity, localized NDE) will materially strengthen the rigor, reproducibility, and interpretability of the findings. The ANOVA-based analytical model provides a rigorous, statistically defensible framework for evaluating the influence of fiber type and acidic environment on the mechanical and durability behavior of concrete. Its strength lies in its ability to quantify group-wise differences with clarity, but its interpretive depth can be further enhanced by adopting multifactorial or regression-based models that account for interaction effects, temporal dependencies, and predictive trends.

4.6. Limitations of this study

While the present study provides valuable insights into the mechanical performance and durability of fiber-reinforced concrete under acidic exposure, it is important to acknowledge its limitations. Most notably, the 28-day exposure duration is relatively short and may not fully capture the long-term degradation mechanisms that occur in real-world acidic environments. The early strength gains observed, possibly due to the formation of expansive products such as gypsum, might not reflect sustained durability but rather transient phenomena. Long-term chemical interactions, including fiber degradation, microcracking, and matrix dissolution, could manifest beyond the timeframe investigated here, potentially affecting structural integrity and service life. Future studies should extend exposure periods to several months or years and incorporate additional durability indicators such as dimensional stability, permeability changes, and microcrack propagation analysis to provide a comprehensive assessment of acid resistance. Moreover, field validation under realistic environmental conditions would further enhance the applicability of the findings.

4.7. Future scope

The present study provides a strong foundation for understanding the mechanical and durability behavior of fiber-reinforced silica fume concrete (FSFC) under acidic environments; however, several aspects warrant further exploration. Future research could investigate the long-term degradation mechanisms of FSFC beyond 28 days, incorporating extended acid exposure periods and cyclic wet–dry conditions to simulate realistic environmental effects. The use of advanced characterization tools such as X-ray diffraction (XRD), thermogravimetric analysis (TGA), and micro-computed tomography (µ-CT) can provide deeper insights into phase transformations, pore structure evolution, and microcrack propagation under chemical attack. Additionally, optimization studies using response surface methodology (RSM) or machine learning-based predictive models may help identify the ideal proportions of fiber and silica fume for enhanced performance. Comparative studies involving hybrid fiber systems and alternative supplementary cementitious materials, such as metakaolin or fly ash, could also expand understanding of synergistic effects in composite systems. Finally, field-scale validation and life-cycle cost assessments are recommended to establish the practical feasibility and sustainability of FSFC in aggressive service environments.

5. CONCLUSIONS

This study systematically investigated the combined influence of 1% fiber reinforcement (glass, jute, and hemp) and 5% silica fume incorporation on the mechanical and durability performance of concrete under acidic environments. The findings establish a clear relationship between fiber type, matrix refinement, and acid resistance, leading to the following conclusions and new contributions:

• Novelty and Contribution: Unlike conventional studies that focus on either fiber reinforcement or pozzolanic modification independently, this work uniquely integrates natural and synthetic fibers with silica fume to evaluate their combined effect under varying acid exposure levels (pH 3 and pH 5). This holistic approach provides new insight into fiber–matrix interactions, acid attack mechanisms, and microstructural stability in hybrid modified concretes.

• Mechanical Performance: The inclusion of 1% glass fiber yielded the highest enhancement in mechanical strength, with compressive and tensile strengths increasing by 25.8% and 27.5%, respectively, compared with the control mix. This improvement is attributed to the synergistic role of silica fume in producing dense C–S–H gels and the superior interfacial bonding between glass fibers and the matrix. This demonstrates a new pathway for achieving higher mechanical resilience through hybrid fiber–pozzolan systems.

• Durability Under Acid Attack: The study confirms that durability is strongly fiber-dependent. Glass fiber-reinforced concrete exhibited superior resistance at both pH 3 and pH 5, retaining structural integrity due to low chemical reactivity and minimal pore connectivity. Conversely, hemp fiber-reinforced mixes showed an 8.5% reduction in compressive strength at pH 3, highlighting the vulnerability of lignocellulosic fibers to acidic degradation. This differentiation provides new evidence on the role of fiber composition in acid–matrix interaction.

• Transport Properties: Sorptivity results showed that although fiber incorporation initially increased water absorption due to pore introduction, long-term hydration and C–S–H development significantly reduced capillary absorption. Glass fiber mixes recorded the lowest ultimate sorptivity, confirming that silica fume refinement effectively compensated for the porosity introduced by fibers. This demonstrates the dual-function effect of silica fume in sealing microvoids while promoting durability.

• Chemical Interaction and Microstructure: Mass gain after 28 days of acid exposure, particularly at pH 5, was attributed to gypsum and ettringite formation within pores. GFSFC specimens exhibited the greatest mass gain and densest microstructure, indicating reduced acid ingress and self-filling behavior. SEM analysis confirmed intact fiber–matrix bonding in glass fiber mixes, while hemp fiber surfaces showed signs of degradation. These observations provide new microscopic evidence supporting the macro-scale durability performance.

• Non-Destructive Evaluation: All mixes maintained UPV values above 4.5 km/s throughout the exposure period, confirming sound internal quality. This validates that fiber and silica fume synergy effectively mitigates internal cracking and preserves structural integrity under acidic stress.

This study contributes a novel understanding of how fiber type and pozzolanic modification synergistically influence concrete’s mechanical and durability characteristics in aggressive acidic environments. The integration of silica fume with optimized fiber reinforcement results in improved matrix compactness, reduced permeability, and enhanced acid resistance. Among the fibers studied, glass fibers provided the most effective reinforcement and chemical stability, establishing them as a preferred material for developing durable, eco-efficient concrete suitable for industrial effluent systems, sewage structures, and acid-prone environments.

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Publication Dates

• Publication in this collection
  16 Feb 2026
• Date of issue
  2026

History

• Received
  15 Sept 2025
• Accepted
  12 Jan 2026