Wastepaper fiber-reinforced concrete containing metakaolin: Effect on fracture behavior

Plasterboard, a commonly utilized construction material, comprises a gypsum core nestled between two paper layers. Gypsum after demolition of plasterboards is a recyclable waste that has been reused into new plasterboard or other purposes such as agricultural products. However, there is a lack of understanding on the potential for recycling the paper layers. This study investigates the use of wastepaper fibers, obtained from the paper layers, as a reinforcing material and metakaolin as a partial cement replacement material in concrete. This study demonstrates the ability of the paper from waste plasterboard for reinforcing concrete. Wastepaper fibers were used at different concentrations ranging from 0 to 2.5% by weight of binder. Bending test was conducted for assessing fracture behavior of concretes, including load bearing capacity, modulus of rupture, crack mouth opening displacement (CMOD) at the load bearing capacity, fracture toughness, and fracture energy. Slump, axial compression, and scanning electron microscopy (SEM) were also conducted on the concretes. It is found that incorporating wastepaper fiber by up to an optimum content of 1.5% results in an increase in the compressive strength (57%), flexural load bearing capacity and modulus of rupture (31%), CMOD displacement at load capacity (14%), fracture toughness (37%), and fracture energy (73%) of the metakaolin-based concrete. However,

in an increase in the compressive strength (57%), flexural load bearing capacity and modulus of rupture (31%), CMOD displacement at load capacity (14%), fracture toughness (37%), and fracture energy (73%) of the metakaolin-based concrete.However, further increase in the wastepaper fiber content results in decreased mechanical properties of the concrete, which is due to the fiber agglomeration and non-uniform distribution within the concrete matrix.Based on the results, the concrete with 20% metakaolin and 1.5% wastepaper fiber experiences similar mechanical properties to the conventional concrete.The results of this study underscore the substantial potential of leveraging waste materials such as wastepaper and by-products like metakaolin to diminish reliance on conventional cement.This approach not only enhances the mechanical properties of concrete but also fosters sustainable building practices by promoting the recycling of waste materials, reducing carbon footprint, and mitigating the environmental impacts inherent in traditional concrete production.

| INTRODUCTION
Concrete is the second most consumed material worldwide after water. 1 The manufacturing process of ordinary Portland cement (OPC), utilized as the binding agent in concrete, accounts for approximately 8% of global carbon dioxide emissions. 2The growth in concrete production due to the increased urbanization and industrialization has led to high demand for the OPC, which has put pressure on the availability of raw materials required for cement manufacturing. 3Meanwhile, construction and demolition (C&D) operations generate a substantial volume of waste, exerting a notable environmental impact through soil erosion, sediment deposition, and climate alteration. 4Recycling the C&D waste has been a promising strategy for sustained development of infrastructure. 5lasterboard is a widely used building material in interior construction. 6It consists of gypsum as a core sandwiched between two layers of paper. 7After buildings are demolished, the plasterboard becomes a recyclable C&D waste that can be repurposed into new plasterboard or transformed into agricultural products. 8In Europe, it is estimated that 2.35 million tons of the waste plasterboard are generated annually. 9In Australia's recycling endeavors, more than 200,000 tons of the plasterboard are diverted from landfills, finding diverse applications across various industries. 10Each year, Australia discards approximately 1 million tons of the plasterboard, yet a significant portion of it can be recycled, potentially saving around 13 trees for every ton recycled. 10However, the paper used to cover the plasterboard, considered as wastepaper, is presently neglected and discarded in landfills, where it is ultimately incinerated.One potential method of recycling the wastepaper derived from discarded plasterboard is to utilize it as a reinforcing fiber material within concrete.
The application of wastepaper fiber in cementitious materials, while promising, remains an underexplored area in literature.These fibers, derived from various sources like discarded newspapers, office papers, and packaging materials, are primarily composed of cellulosic fiber.Their strength, flexibility, and ability to form cohesive networks with cementitious materials make them a valuable addition. 11However, their high water absorption necessitates a careful balancing of content in cementitious mixes to avoid compromising the material properties. 12,13Jiang et al. 14 demonstrated that wastepaper fibers sourced from newspapers, when added to concrete in concentrations between 0.1% and 0.4%, not only mitigated self-shrinkage cracking but also offered internal curing benefits in concrete.This finding is significant as it highlights the dual functional benefits of wastepaper fibers in concrete.Praburanganathan et al. 13 further contributed to this understanding by showing that incorporating up to 20% wastepaper sludge in concrete increased the water absorption by 17% and enhanced the compressive strength by 30%.This illustrates the delicate balance between improving mechanical properties and managing water absorption in concrete mixes.Zaki et al. 15 expanded this knowledge by revealing that the inclusion of wastepaper from newspapers up to a 5% concentration by weight of cement increased compressive, splitting tensile, and flexural strengths of concrete by approximately 10%.This improvement in various strength parameters underscores the multifaceted benefits of wastepaper in concrete.Conversely, Cardinale et al. 16 observed that increased wastepaper content from office papers led to reduced strengths and thermal conductivity, alongside heightened capillary absorption in cement mortars.This underscores the importance of optimizing wastepaper content to maintain a balance between mechanical properties and durability of cementitious materials.Lastly, de Azevedo et al. 17 conducted wetting and drying cycle tests on cement mortars with wastepaper sludge, and reported significant degradation in mechanical properties of the mortars.Their findings highlight the potential challenges in durability that need to be addressed in future research.
The durability of cellulosic fibers, such as those from wastepaper, in cementitious composites presents a complex challenge, particularly under humid conditions.The aging phenomenon in these fibers, often observed in such environments, contributes to a decline in strength and toughness of the composite post-cracking. 18This issue is a multifaceted one, stemming from a series of interrelated factors.Alkali attack on the fibers leads to their weakening over time, while the migration of hydration products can result in the mineralization of the fibers.Additionally, the inherent high water absorption capacity of cellulosic fibers causes size fluctuations, further impacting the long-term durability of the composite. 11Understanding and addressing these durability challenges is crucial for the effective use of wastepaper fibers in concrete, particularly in environments with variable or high humidity levels.
To tackle the degradation issue of cellulosic fibers, Toledo Filho et al. 19 studied mechanical performance of mortars reinforced with sisal fiber under 100 wetting and drying cycles.As one of the most efficient methods, they suggested that the replacement of 50% of the OPC with metakaolin reduces the water absorption of the mortar and the degradation issue of the fiber by decreasing the available calcium hydroxide (Ca(OH) 2 ) amount of the mortar.Furthermore, it was indicated that the inclusion of the fiber into calcium hydroxide-free mortar improves durability throughout the process of maturation. 19Metakaolin is a highly reactive pozzolanic material that is produced by the calcination or thermal activation of kaolin clay.Metakaolin has an extensive application as an additive cementitious material in the manufacturing of concrete. 20When added to concrete mixes, metakaolin undergoes a reaction with the released calcium hydroxide during the hydration process of cement, resulting in the formation of supplementary calcium silicate hydrate gel. 21By incorporating metakaolin into the composite, the overall water absorption of the material can be reduced. 22This limits the volume variations and dimensional instability typically associated with cellulosic fibers, mitigating the degradation caused by excessive moisture.It can also help in reducing the carbon footprint of concrete production by partially replacing the OPC, thereby reducing the overall greenhouse gas emissions associated with cement manufacturing. 23ased on the above literature review, in this study metakaolin was used as a partial replacing material for the OPC to develop concrete reinforced with wastepaper fiber produced from waste plasterboard.The objective of this study was to determine the optimum content of the wastepaper fiber to develop concrete with similar flexural behavior to conventional concrete.Different tests including slump, axial compression, and three-point bending were conducted.In the bending test, different properties including load bearing capacity, modulus of rupture, crack mouth opening displacement (CMOD) at the load capacity, and fracture toughness and energy of the wastepaper fiberreinforced concretes were investigated.Scanning electron microscopy (SEM) test was also undertaken for assessing the microstructure of the concretes at 28 days of curing.A summary of the experimental plan was presented initially, which was followed by the results and discussion.

| RESEARCH SIGNIFICANCE AND NOVELTY
The urgency of mitigating environmental crises and the continued need for infrastructural development compel researchers to explore sustainable alternatives in all sectors, including the construction industry.This research delves into this urgent need, focusing on the potential of repurposing the C&D waste materials, particularly waste plasterboard, in the production of concrete.The global output of the waste plasterboard, as one of the most abundant C&D wastes, is about 15 million tons annually, with most of it ending up in landfill. 24The novel approach of this study lies in the exploration of using wastepaper fibers derived from the waste plasterboard as a reinforcing material in concrete, thereby addressing the dual issues of waste management and sustainable development.Prior studies have indicated that wastepaper fibers exhibit degradation issues because of their high water absorption, leading to decreased mechanical properties of cementitious materials.This study introduces a novel approach to mitigate the degradation issue of the fibers by using metakaolin as a cement replacement material in wastepaper fiber-reinforced concrete.This article presents the first study in the literature on using the wastepaper fiber produced from the waste plasterboard and determining their optimal ratios to develop concrete with similar fracture properties to conventional concrete.

| TESTING PROGRAM
Prior to establishing the experimental matrix, a comprehensive literature review was conducted to identify the desired properties of materials suitable for the concrete mixes in this study.The selection of materials aimed to optimize the performance characteristics of the resulting concrete mixes.

| Materials
The OPC and metakaolin were sourced from Adbri and Boral in Australia, respectively.They were dark gray and plain white in color, respectively.The metakaolin had a percentage passing of 99%, 98%, and 97% for particle sizes of 300, 150, and 75 μm, respectively.Its bulk density was 2670 kg/m 3 .Table 1 shows the chemical composition of the OPC and metakaolin.The river sand was procured from Marion Sand and Metal Paving Centre in Australia and was creamy-yellow in color.Its maximum particle size, bulk density, and water absorption were 4.75 mm, 2610 kg/m 3 , and 2.24%, respectively.The coarse aggregate, supplied by Marion Sand and Metal Paving Centre, had a size range of 10-14 mm and was light gray in color.Its bulk density and water absorption were 2720 kg/m 3 and 0.35%, respectively.MasterGlenium SKY 8700 was used as superplasticizer.The wastepaper fiber was obtained from discarded plasterboard sourced from Sunshine Groupe in Australia.The wastepaper fiber had a density of 50 kg/m 3 and a water absorption of 824%.

| Concrete mix design and preparation
A total of seven concrete mixes were prepared, including one conventional concrete mix and six concrete mixes with 20% OPC replaced with metakaolin.In the six metakaolin mixes, wastepaper fibers were added at 0%, 0.5%, 1%, 1.5%, 2%, and 2.5% by binder weight.The conventional concrete mix was designed to have the target 28-day compressive strength of 35 MPa, aligning with common structural-grade concrete specifications and providing a baseline for comparison with the modified mixes.This target strength is typically required for structural components subjected to moderate loading conditions and is a standard benchmark within the industry.All mixes had a water-to-binder (w/b) ratio of 0.50.Table 2 outlines various concrete mix proportions.To identify the mix ID in the table, the letters CC and MC stand for conventional concrete and metakaolin concrete mixes, respectively.The numbers following the MC indicate the percentage of the wastepaper fiber in the mixes.For example, MC1.5 is a concrete mix with 20% metakaolin and 1.5% wastepaper fiber.
Based on trial mixes, it was found that at the maximum superplasticizer dosage the replacement of the OPC with more than 20% metakaolin led to zero slump mixes, necessitating additional water which caused a significant reduction in the compressive strength of the mixes at 28 days.In addition, Nežerka et al. 25 reported that using metakaolin by more than 20% led to a significant decrease in the strength of concrete mixes.Therefore, it was decided to utilize 20% metakaolin in the mixes.In addition, it was decided to utilize the sand in saturated surface dry state to prevent its water absorption in the concrete mixes.To prepare the mixes, binder, sand, and aggregate were thoroughly blended for a duration of 2 min.Subsequently, the wastepaper fiber was gradually introduced and mixed for an additional 2 min to ensure their uniform distribution.Finally, water and superplasticizer were slowly added at 50%, 75%, and 100% and mixed for 3 min.The materials were then poured into the relevant molds, and the prepared specimens were demolded after 24 h.The dimension of the concrete specimens for the axial compression test was 100 Â 200 mm (cylindershaped) and for the flexure test was 100 Â 100 Â 400 mm (prism-shaped).Three identical specimens were used for the compression test and two identical specimens were used for the flexure test for each mix.The specimens were submerged in a water tank with a constant temperature of 23 C for 28 days.
The method used for the preparation of the wastepaper fiber from the waste plasterboard was detailed to ensure a thorough understanding of the separation process. 12Initially, the waste plasterboard was manually disassembled to separate the gypsum core from the paper layers.The paper layers were then soaked in a bucket containing an appropriate amount of water to soften the papers and facilitate the separation of any residual gypsum.After soaking for a specified period, the saturated plasterboard paper was processed into smaller fragments using a blender for a duration of 15 seconds.This action helped to disaggregate the paper fibers effectively.Excess water was then removed from the disintegrated materials, ensuring that the paper fibers were sufficiently moist but not overly saturated.The resulting moist paper fibers underwent 24 h of drying in an oven set at a controlled temperature, ensuring the removal of moisture without degrading the fiber quality, resulting in a cohesive chunk of dry fibers.Finally, the dried paper fiber was reintroduced into the blender for an additional 30 s to further refine its size and achieve the desired fiber consistency for reinforcement purposes.The resultant wastepaper fiber was characterized by a length ranging from 1 to 4 mm, an average diameter of 70 μm, and an aspect ratio ranging from 5 to 14. Figure 1 shows the production process and Figure 2 shows the produced wastepaper fibers.

| Testing
Slump test on fresh concrete mixes was done in accordance with ASTM C143/C143M. 26Axial compression test according to ASTM C39 27 and three-point bending test according to ASTM C293 28 were performed on the hardened concrete mixes.The compression test was done with a loading rate of 20 MPa/min and the bending test was done with a displacement rate of 0.5 mm/min.The bending test was done to investigate the load bearing capacity, modulus of rupture, the CMOD at the load capacity, and fracture toughness and energy of the concrete mixes.Figure 3a,b shows a beam notch wedge and two steel plates on the bottom of the beam surface in the bending test, respectively.The beam specimens had a notch with a depth of 25% of the beam depth (25 mm) and a 4 mm thickness, as illustrated in Figure 3a.A clip gage was utilized for measuring the CMOD, while an inbuilt extensometer was employed for measuring vertical deflection at the mid-span of the beams.To obtain the CMOD values, two steel plates, each with a 3 mm thickness, were securely attached to the concrete surface, as illustrated in Figure 3b, thereby the gage was attached to the steel plates by positioning the notched area facing a fan for 10 min for ensuring a dry surface.A superglue adhesive was then applied to affix the plates to the surface of concrete on both sides of the notch near the edge.
The adhesive was allowed to be set for a duration of 3 h, ensuring a strong connection between the surface of concrete and steel plates.
Figure 4a shows the arrangement of the beams in flexure test.A testing machine with a maximum capacity of 300 kN was used for the flexure test.The beam specimens were positioned on the support rollers, and they had a span of 300 mm.The specimens were subjected to continuous loading until failure was reached.Figure 4b illustrates typical failure mode of the specimens under the bending test.
The procedure for assessing fracture toughness adhered to ASTM E1820. 29For determining the fracture parameters an established literature methodology 30 was followed.The primary parameter, the effective critical crack length (a c ) in m, was initially determined.This parameter was essential for computing both fracture toughness and energy of the beams, as derived from Equation (1).
where H is the beam height (in m), E is the beam elastic modulus (in MPa), B is the beam width (in m), P Vmax is the peak vertical load (in kN), and CMOD c is the critical crack mouth opening displacement (in m).Following the determination of the effective crack length, the a c value was employed to calculate the function associated with a c /H as shown in Equation (2).
The fracture toughness was then determined using Equation (3).
F I G U R E 1 Production of wastepaper fiber (WPF) from waste plasterboard.
where K IC is the fracture toughness (in kN/m 1.5 ) and S is the beam span length (in m).
Flexural modulus of rupture represents the maximum bending stress that a beam can withstand before it fails in tension.The modulus of rupture (f ) was determined using Equation (4). 31 where a 0 is the initial notch depth (in m).
Fracture energy as a critical property of a material fracture signifies the energy necessary for increasing the fracture surface area per unit till the point of failure is reached. 32The fracture energy was calculated following an established literature method. 31This energy was derived using Equation ( 5) from the flexural load-CMOD curves.
where G F denotes the fracture energy (in N/m), δ 0 is maximum CMOD displacement (in m), W 0 is area under load-CMOD curve (in N m), P W is equivalent self-weight force determined by (S/2L)mg, in which S is beam span (in m), L is beam total length (in m), m is beam mass between supports (in kg), and g is gravitational acceleration (in m/s 2 ).
The SEM was performed in the Flinders Microscopy and Microanalysis Laboratory using an FEI Inspect F50 SEM with an EDS detector 33 for microstructural evaluation of MC1.5 and MC2.5 mixes.The SEM samples were obtained from the specimens used in the compressive strength test.

| Slump
The slump test, a measure of concrete's workability, is significantly impacted by the water absorption characteristics of the mix constituents.Figure 5 depicts the slump values of various mixes.As can be observed, the conventional concrete mix exhibited the highest slump value.When 20% metakaolin was incorporated into the mix, the slump decreased by $6%.This observation agrees with previous studies. 34,35The decrease in slump upon the addition of metakaolin can be ascribed to its specific particle shape (plate-shaped) and size, which possesses a higher surface area to volume ratio than cement, leading to an increased water demand and, consequently, a modest reduction in workability. 36,37s shown in Figure 5, with the introduction of the wastepaper fiber, there was a decrease in the slump.MC0.5, MC1.0, MC1.5, MC2.0, and MC2.5 mixes exhibited 15%, 38%, 59%, 62%, and 74% lower slump than MC0 mix, respectively.The slump reduction associated with increased wastepaper fiber content is due to the fiber's high water absorption capacity.As the fibers absorb water, less free water remains available, thereby reducing the fluidity of the mix and impairing its workability.Furthermore, the addition of the wastepaper fiber increases the volume of non-cementitious material in mixes.This, along with the irregular geometry and rough texture of the fibers, can hinder the movement of concrete constituents during mixing, leading to a further decrease in workability. 16t is important to note that while the slump of concrete mixes decreases with incorporating wastepaper fiber, the inclusion of the fiber may enhance certain characteristics of the concrete, including strength, toughness (crack resistance), and energy absorption. 38Therefore, achieving a balance between workability and other properties of concrete is essential.The slump test results of this study suggest that the inclusion of metakaolin and wastepaper fiber in the concrete mixes has clear implications for workability.The effect of the metakaolin was relatively minor compared to the significant reduction in workability caused by the wastepaper fiber, owing to the water absorption capacity and physical characteristics of the wastepaper fiber.

| Compressive strength
Figure 6 shows the average compressive strength of mixes at 28 days of curing.Incorporating 20% metakaolin led to 39% decrease in the compressive strength of the mix.This observation is consistent with previous studies 34,39,40 and can be attributed to the reduced cement hydration as binding material, retarding calcium silicate hydrate nucleation in the mix 41 and the dilution effect of the metakaolin as it is a pozzolanic material and introduces additional inert particles into the mix. 42igure 6 shows that incorporating wastepaper fiber increased the compressive strength of the metakaolin mix.With an increase of the wastepaper fiber by up to 1.5% the compressive strength increased by 57%.However, increasing the wastepaper fiber by more than 1.5% declined the strength.As can be seen, the MC1.5 mix, which contained 1.5% wastepaper fiber, developed nearly similar compressive strength to the CC mix. Figure 7a,b shows the SEM images of MC1.5 and MC2.5 mixes, respectively.According to the figures, the wastepaper fibers were uniformly and randomly distributed in the microstructure of the MC1.5 mix.However, they were clogged and agglomerated in the microstructure of the MC2.5 mix.An increased compressive strength with an increased cellulosic fiber content is attributed to the uniform distribution of the fibers in the concrete matrix when they are utilized by up to an optimum content.This uniform fiber distribution leads to a reduced size and propagation of microcracks and uniform distribution of stress within the concrete matrix. 43The decrease in the strength with an increased fiber content after the optimum content is due to the fiber agglomeration and non-uniform distribution within the concrete matrix.This leads to the development of voids and cracks in the concrete mix. 44

| Flexural load bearing capacity and modulus of rupture
Load-vertical deflection curves of the concrete beams at mid-span at 28 days subjected to three-point bending are shown in Figure 8. Figure 8a,b shows the curves for the first and second repetitions for each mix, respectively.Based on the figures, the curves gradually reached the load bearing capacity and then gradually declined.Figure 9a,b shows the average load bearing capacity and modulus of rupture of different mixes, respectively.It is shown that the load bearing capacity and modulus of rupture decreased by 24% when 20% of the OPC was replaced with metakaolin.This observation agrees with a previous study. 34Although metakaolin is known for its pozzolanic activity which can contribute to the strength of concrete, it requires a certain period for the pozzolanic reaction to occur, hence, the contribution may not be as immediate as the OPC. 45The lower rate of reaction in the mix with metakaolin can be because of the lower concentration of Ca(OH) 2 available for the pozzolan to react with, compared to the conventional concrete. 45Additionally, the particle size and shape of the metakaolin might also influence the packing density and bonding within the concrete matrix, thereby affecting the load bearing capacity and modulus of rupture. 46It was also reported previously that an increase of the metakaolin above 15% can generate more pores in the concrete mix. 47ccording to Figure 9, the incorporation of 0.5% wastepaper fiber led to a 7% increase in the load bearing capacity and modulus of rupture of the metakaolin mix.An increase in the wastepaper fiber by up to 1.5% increased the load bearing capacity and modulus of rupture.MC1.0 and MC1.5 mixes exhibited 15% and 31% higher load bearing capacity and modulus of rupture compared to MC0 mix, respectively.The ability of cellulosic microfibers to bridge microcracks and hinder their growth within the concrete matrix is not merely a function of their tensile strength, but is largely attributed to their distribution and the tortuosity they introduce to crack paths, which requires more energy for crack propagation, thus enhancing load bearing capacity up to an optimal fiber content. 48,49ased on Figure 9, further increase in the wastepaper fiber content after 1.5% led to a decreased load bearing capacity and modulus of rupture.Incorporating 2% and 2.5% wastepaper fiber caused 13% and 18% reduction of the load bearing capacity and modulus of rupture when Compressive strength of mixes with various fiber percentages.compared to 1.5% wastepaper fiber, respectively.These observations suggest that while the initial addition of the wastepaper fiber improves the load bearing capacity, possibly due to the fibers' ability to transfer stress across cracks, which contributes to an increase in post-cracking load bearing capacity and crack resistance, there is an optimal content beyond which further wastepaper fiber addition can be detrimental.This strength reduction can be attributed to the fiber agglomeration, as corroborated by the SEM analysis (Figure 7), which may create inhomogeneities within the matrix, effectively acting as stress concentrators rather than bridges, thus reducing the overall load bearing capacity. 50ased on the obtained results, all concrete mixes developed a lower load bearing capacity and modulus of rupture than the conventional concrete mix, except for the concrete mix with 20% metakaolin and 1.5% wastepaper fiber which exhibited a similar load bearing capacity and modulus of rupture to the conventional concrete mix.This suggests that while wastepaper fibers are less robust than conventional reinforcement materials, when used within an optimal range, they can still fulfill a role akin to crack-bridging, contributing to enhanced mechanical performance of concrete.
Alomayri and Ali 51 examined the flexural performance of conventional concrete with three types of cellulosic fibers, including jute, bamboo, and coconut.They reported that the load bearing capacity of the concrete incorporating 0.3% jute, bamboo, and coconut was 23%, 25%, and 15% higher than that of the conventional concrete, respectively.These improvements were believed to be due to the fibers' crackbridging capabilities rather than their intrinsic strength.the figure, the curves exhibited a gradual progression towards reaching the load bearing capacity at the peak CMOD and then slightly declined.The CMOD at load bearing capacity (as peak CMOD) is crucial for assessing the crack control and serviceability aspects of concrete beams.By measuring the peak CMOD, engineers can observe and analyze the extent of crack opening of the beam under load, which is important for understanding the structural performance and integrity of beams. 52igure 11 shows the average CMOD values at load bearing capacity of various mixes.The peak CMOD decreased (18%) with incorporating metakaolin in the mix.This behavior can be because of the decrease in the strength and overall stiffness of the concrete mix due to the replacement of cement, limiting crack opening at the peak load. 53t is also observed from the figure that increasing the wastepaper fiber by up to 1.5% caused an increase in the peak CMOD when compared to the unreinforced metakaolin mix.This observation agrees with previous studies on steel 54,55 and basalt fiber-reinforced concrete. 56The addition of uniformly distributed cellulosic fibers by up to an optimum dosage can bridge cracks and provide additional reinforcement.Thereby, the fibers can allow for increased crack opening and deformation before failure occurs. 38This increased capacity for crack opening leads to an increased CMOD at peak flexural load.However, based on the figure, an increase in the wastepaper fiber above 1.5% caused a decreased peak CMOD.This can be due to the fibers clumping and agglomerating within the concrete matrix as shown in Figure 7.This creates stress concentration points, which leads to premature cracking and reduced crack opening. 57Based on the results, all concrete mixes developed a lower CMOD at load bearing capacity than the conventional concrete mix, except for the concrete mix with 20% metakaolin and 1.5% wastepaper fiber which exhibited nearly similar CMOD to the conventional concrete mix.

| Flexural load-CMOD curves and fracture toughness
Fracture toughness (K IC ) characterizes the ability of a brittle material to withstand crack propagation under the influence of an applied stress, ultimately leading to fracture. 58Figure 12 represents the average fracture toughness of different concrete mixes at 28 days of curing.Based on the figure, 20% replacement of the OPC with metakaolin led to 27% decrease in the fracture toughness of the concrete mix.This observation agrees with previous studies. 34,40etakaolin particles are finer than cement particles, and their addition can affect the packing density of the concrete mix.Increased porosity of the mix leads to the creation of voids and microcracks, which can act as stress concentration points and promote crack initiation and propagation. 53The presence of these defects weakens the concrete and lowers its fracture toughness.
It can be observed in Figure 12 that, with the incorporation of 0.5% wastepaper fiber in the metakaolin mix, the fracture toughness increased by 13% compared to the MC0 mix.Increasing the fiber content to 1.5% led to 37% increase in the fracture toughness when compared to the unreinforced metakaolin mix.In agreement with previous studies in the literature on the use of cellulosic fibers, 57,59 the fracture toughness decreased with a further increase in the fiber content.Based on the results, although all mixes experienced a lower fracture toughness in comparison to the conventional concrete, the MC1.5 mix developed similar fracture toughness to the conventional concrete.Nelson et al. 60 examined the fracture toughness of conventional concrete with 5% kraft pulp fiber and reported that the reinforced concrete exhibited 40% higher fracture toughness than unreinforced concrete.Benaimeche et al. 57 found that using 2% date palm mesh fiber in cement mortar developed a similar fracture toughness to mortars reinforced with synthetic fibers.Cellulosic fibers act as reinforcement within the concrete matrix.When cracks initiate in the concrete, the fibers at an optimum content can bridge across the cracks and distribute the applied load, effectively impeding the crack propagation.This crack bridging mechanism helps in absorbing energy and increasing the fracture toughness of the concrete. 60However, when cellulosic fibers are added in excessive amounts, they tend to agglomerate or clump together within the concrete matrix.Fiber agglomerations create stress concentration points, making the concrete more susceptible to crack initiation and propagation.This clustering of fibers can hinder their effective dispersion and bridging capability, leading to decreased fracture toughness of the concrete. 31

| Fracture energy
The energy that a material absorbs during the formation of a crack is referred to as fracture energy. 61This energy is important for understanding a material's resistance to fracture, particularly in the context of dynamic loads or environments prone to cracking.Figure 13 shows the average fracture energy of different concrete mixes at 28 days of curing.It is notable that the fracture energy was calculated at 0.5 mm CMOD displacement, where most of the mixes started to develop a constant CMOD rate.Like the trend observed for the compressive strength, load bearing capacity, and fracture toughness, the MC0 mix experienced a lower fracture energy (41%) compared to the CC mix.This observation agrees with previous studies. 34,35As already discussed, this behavior can be due to the increased porosity and enhanced crack initiation and propagation in the mixes with metakaolin. 53s can be observed in Figure 13, incorporating 0.5% wastepaper fiber in the metakaolin mix led to 15% increase in the fracture energy of the mix.In addition, an increase in the fiber content from 0.5% to 1.5% led to 50% increase in the fracture energy of the mix, with 73% higher fracture energy than the unreinforced metakaolin mix.Kesikidou and Stefanidou 62 studied the fracture energy of cement mortars with 1.5% jute, coconut, and kelp fibers and reported that coconut fiber was more efficient than jute and kelp fibers in enhancing the fracture energy of the mortar owing to the higher ductility of the Fracture energy of different concrete mixes at 0.5 mm CMOD displacement.
coconut fiber.Ruano et al. 63 found that bagasse fiber improved the flexural strength while hemp fibers the fracture toughness of cement mortars.An increase in the fracture energy of the metakaolin mixes with an increased fiber content by up to an optimum content in the present study is due to the enhancement of the load bearing capacity and ductility of the concrete by incorporating uniformly distributed fibers, allowing for greater plastic deformation and more shallow postcracking behavior, which results in improved fracture energy before failure. 30According to the results, all the metakaolin concrete mixes experienced a lower fracture energy compared to the conventional concrete mix, except for the MC1.5 mix which developed similar fracture energy to the conventional concrete mix.Based on the obtained results in this study, it is crucial to maintain the optimum content of the wastepaper fibers in the metakaolin concrete to ensure the desired enhancement of the fracture properties.Excessive fiber content can disrupt the fiber-matrix interaction, dilute the matrix, and lead to fiber agglomeration, ultimately reducing the fracture performance of the concrete.

| CONCLUSIONS
This study has evaluated the fracture performance of wastepaper fiber-reinforced concrete incorporating metakaolin as binder.The following conclusions can be derived: • Adding wastepaper fiber results in a decrease (74% at 2.5% fiber content) in the workability of metakaolinbased concrete, which is attributed to the water absorption and rough texture of the fiber.• 20% replacement of cement with metakaolin leads to 39%, 24%, 18%, 27%, and 41% decrease in the compressive strength, flexural load bearing capacity, CMOD displacement at the load bearing capacity, fracture toughness, and fracture energy of the concrete, respectively.This behavior can be attributed to the increased porosity and enhanced crack initiation and propagation in the concrete with metakaolin.• The introduction of the wastepaper fiber to the metakaolin concrete by up to an optimum content of 1.5% results in an increase in the compressive strength (57%), flexural load bearing capacity and modulus of rupture (31%), CMOD displacement at the load bearing capacity (14%), fracture toughness (37%), and fracture energy (73%) of the concrete.Based on the SEM images, this behavior can be attributed to the uniform distribution of the fibers within the concrete matrix when they are used at up to an optimum content, thereby efficiently bridging the microcracks and reducing their propagation.
• An increase in the wastepaper fiber content from 1.5% to 2.5% causes a decrease in the compressive strength (33%), flexural load bearing capacity (18%), CMOD displacement at the load bearing capacity (50%), fracture toughness (18%), and fracture energy (29%) of the metakaolin concrete.According to the SEM images, this observation can be due to the fiber agglomeration and non-uniform distribution within the concrete matrix when they are used in excessive contents.
This study acknowledges the high water absorption capacity of the wastepaper fibers, which poses a challenge to the workability of concrete mixes.This effect becomes particularly pronounced with a higher fiber content, necessitating adjustments in the concrete mix design or the use of water-reducing admixtures to achieve desired workability levels.The potential for moisture release from the wastepaper fibers over time warrants further investigation to understand its implications for the long-term performance of the concrete.The findings of this study are significant as they indicate that the integration of the metakaolin and recycled wastepaper fibers into concrete mixes does not detract from the structural integrity of the material.Moreover, the utilization of these materials aligns with the broader goals of sustainable construction by reducing reliance on conventional cement, which in turn contributes to the conservation of natural resources and reduction of environmental footprint of concrete production.Furthermore, the repurposing of the wastepaper, particularly from waste plasterboard, presents a viable strategy for waste management, potentially diminishing the volume of the C&D waste directed to landfills.

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I G U R E 2 (a) Prepared wastepaper fiber (left) versus original waste plasterboard (right) and (b) SEM image of the prepared fibers.F I G U R E 3 (a) Concrete beam notch wedge and (b) two steel plates on the concrete surface in bending test.

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I G U R E 4 (a) Flexure testing and (b) typical failure mode.F I G U R E 5 Slump test results.

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I G U R E 7 SEM image of (a) MC1.5 and (b) MC2.5 mixes.

Figure
Figure 10a,b shows flexural load-CMOD relationships of different concrete mixes at 28 days of curing.Based on

F I G U R E 1 0
Flexure load-CMOD curve of different mixes: (a) first and (b) second series.F I G U R E 1 1 CMOD values at load bearing capacity of different concrete mixes.
Concrete mix proportions.
T A B L E 2