Effect of aging on the mechanical properties of woven fabric-reinforced calcined diatomite substituted cement-based composites

The mechanical characteristics of polyester and flax woven fabric-reinforced, diatomite-substituted, cement-based composites have been examined at different ages within the scope of this study. The use of calcined diatomite in combination with a cement-based matrix aims to improve the mechanical performance within the composite as well as reduce carbon emissions. The consistency of cement-based and diatomite-substituted matrices with water-to-binder proportions of 0.28 and 0.45 was maintained at a fixed flow diameter of 235 mm with the adjusted use of a superplasticizer. The stress–strain graphs of the composites were obtained using an axial tensile testing machine and Linear Variable Differential Transformers (LVDT). The tensile strength, ductility, toughness development, and multi-crack performance of WFRC were obtained as a function of fabric type and aging. The effects of aging on tensile properties are discussed separately for each fabric type. Polyester woven fabric-reinforced composites were found to be superior to flax WFRC in terms of several mechanical properties at all ages. The substitution of diatomite further improved the tensile performance of the polyester woven fabric-reinforced composites. The fabric-matrix interface densification role of diatomite was determined by SEM/EDS line analysis. Evidence of a pozzolanic reaction between portlandite and diatomite was obtained through microstructure studies. Carbon emission analysis revealed that equivalent CO2 emissions could be reduced using diatomite in woven fabric reinforced composites. However, diatomite substitution caused a cost increasing effect.


Introduction
Woven fabric-reinforced composites have been increasingly preferred in engineering applications over the last decades.In recent years, structural applications have seen a significant increase in the use of composite materials formed by textile products such as fibers, yarns, or fabrics in conjunction with cement-based matrices [1].Furthermore, woven fabric-reinforced concrete stands out as an innovative structural material for addressing the corrosion issues associated with steel rebar in reinforced concrete structures [2].In brittle cement-based structural materials such as concrete, failure involves the initiation of a single crack within the material under tensile stresses, rapid propagation of the initiated crack, and catastrophic loss of load-carrying capability through fracture [3].In this respect, reinforcement is an indispensable principle for the design of concrete that exhibits ductile behavior.
Woven fabric-reinforced concrete is widely used not only for strengthening load-bearing systems but also for producing non-structural elements such as sandwich panels, exterior claddings, and walls [4].Additionally, the use of woven fabric-reinforced composites is prevalent in a wide range of applications such as ventilated facade systems, bridges, storage units, balcony floorings, structural elements exposed to high chloride levels, and shotcrete applications [5].In the production of woven fabric-reinforced composites, various combinations of existing natural and synthetic fibers with matrices can be used.These materials, designed for reinforcement purposes, can consist of combinations of S-glass, R-glass, carbon, polymeric fibers, aramid fibers, or natural fibers such as flax and jute.Woven fabric reinforced concrete materials have the potential to reduce manufacturing costs and increase productivity by enhancing mechanical characteristics [6].
The behavior of fabric-reinforced cementitious composites under tensile loads can be characterized by multiple cracking modes [7].Under axial tensile loading, after the first crack formation in the composite, transverse cracks gradually develop [8].Multiple cracking performance and the resulting stress-strain curve depend on the woven fabric features, cement matrix properties, and fiber-matrix interfacial bonding [9].Inadequate inter-adhesion within the fibers and matrix can result in inadequate bridging of the fibers, which plays a critical role in controlling crack development [10].In addition, the durability of the fiber, resistance to environmental effects, and especially the alkaline environment resistance in cement-based composites is crucial [11].The polymer structure of the fiber, the duration of exposure to an alkaline environment, and the alkaline concentration are important parameters that influence the alkaline resistance of the fiber [12].Additionally, heating-cooling and wetting-drying environmental cycles are among other significant factors affecting alkaline durability [13].
Aging may affect the performance of fibers embedded in cement-based matrices owing to the alkaline environment.Fibers may become brittle due to the alkaline pore solution.The loss of tensile strength of polymeric fibers can be ascribed to the breakdown of the polymer chain structure by alkaline attack.This negative effect can be mitigated by reducing the matrix's internal connected pore structure using appropriate pozzolanic additives with fine granulometry [14].Pozzolanic additive modification may also decrease pore solution alkalinity by binding portlandite phases.Therefore, mineral additives like high calcium fly ash and, silica fume, etc. have a wide range of applicability owing to their advantages in improving the mechanical and physical properties of concrete [15].In this context, diatomite as an alternative effective pozzolana, a naturally occurring mineral formed over time through the fossilization of single-celled algae, which is both organic and sedimentary in nature, has gained popularity in recent years.Diatomite is also recognized as the only natural mineral of biological origin, and its high amorphous silica content makes it a preferred alternative secondary binder choice [16].Diatomite's pozzolanic reactivity has been reported which resulted in the generation of more hydration products, including secondary C-S-H, at later ages [17].In addition, blended cement with enhanced mechanical properties can be developed by utilizing calcined diatomite, which combines high reactive silica content and high Blaine fineness [18].Furthermore, the high-water absorption capacity of diatomite may also lead to the entrapment of water within its structure, causing a loss of workability.Excessive water addition to combat the workability loss problem can lead to strength decrement.Increasing the dosage of superplasticizers may provide an uneconomical solution to workability loss.Thus, it is beneficial to limit the upper substitution ratio of calcined diatomite to 20% of cement by weight [19].In a study discussing the performance of diatomite with aging, samples with different content of ground diatomite at 7, 28, and 91 days were considered.The tensile and compressive strength values of specimens incorporating ground diatomite were higher in all series compared to the control sample.Results indicated that diatomite substitution up to a limit provides positive results in terms of alkali environment stability and composite durability in cement-based matrices over the long term [20].
Literature review suggest that the conscious selection of matrix and woven fabric components can significantly increase the tensile strength and deformation capacity of load-bearing elements, thereby enhancing structural ductility and toughness at the same time.The effective selection of alternative materials should not only aim for an increase in mechanical performance but also reduce carbon emissions.Considering that cement production is the most efficient activity in terms of energy and emissions, it would be beneficial to partially replace cement clinker with additional cement additives.In line with this goal, the substitution of diatomite in cement can effectively reduce carbon emissions [21].Moreover, the energy usage of the textile industry in the production of high-strength fibers developed as an alternative to steel reinforcement is 1/5 of that of the steel industry [22].
The current experimental study has been evaluated by considering both the goal of improving the tensile properties and environmental sensitivity of woven fabric reinforced cement-based composites.The role of diatomite incorporation on the tensile characteristics and equivalent CO 2 emissions of composites reinforced by woven fabrics produced with flax and polyester yarns has been studied in a systematic manner.For this purpose, the matrix phase binder was substituted with calcined diatomite at different water-to-binder ratios.The mechanical performance of the matrix phase and tensile characteristics of the fabric-reinforced composites were determined at 7, 28, and 90 days.The matrix phase of the composite was produced with a fixed slump flow by adjusting the superplasticizer dosage.The effects of fabric type and aging on the tensile stress-strain graph and multiple crack growth behavior was investigated.Based on the experimental results, the matrix-polyester fabric interface characteristics of the outperformed composites were studied by SEM-EDS linear analysis.Four different matrices were prepared with and without calcined diatomite substitution at two different water-to-binder ratios.The calcined diatomite substitution rate was fixed at 20% of the cement weight.The decision to use this ratio was based on literature recommendations and preliminary experiments [19].

Materials and method
The composite mix ratios are shown in table 3. The dosage of the superplasticizer was adapted to maintain a constant mortar consistency (flow diameter of 235 ± 10 mm).

Woven fabrics
Composites were prepared using 100% polyester and polyester-flax hybrid woven fabrics.The structural properties of fabrics are given in table 4. In textile terminology, warp yarns are positioned parallel to the machine in the fabric, whereas the weft yarns are positioned at perpendicular direction to the warp orientation.The density value mentioned in the table corresponds to the number of yarns per centimeter in the weft and warp directions of the fabric.The linear density (Nm) indicates how many meters of yarn weigh one gram.In the schematic representation of fabrics in table 4, polyester yarns are indicated by green lines, whereas flax yarns are indicated by red lines.The schematical views present the 2 × 2 cm unit cells of woven fabrics and they were created thanks to the WiseTex program [23].Woven fabrics, due to their sparse structure, were produced in a plain weave to maintain their form in composite production.Thus, the number of intersections between yarns within the fabric was maintain at the highest value.

Tensile test molding system
Bone-type steel molds were designed and manufactured to achieve a uniform distribution of the woven fabric in the matrix with consistent orientation.Matrix layers were formed with each having a 5-mm thickness, accommodating up to two fabric layers.The mold produced for this study is shown in figure 1.The dimensions were based on the specifications set by the Japan Society of Civ.Eng.(JSCE, 2008) [24] for high-performance fiber composites.Experiments were conducted in accordance with the TS EN 12350-5 standard, in which the composite matrix mixture was filled into truncated conical molds with a bottom diameter of 100 mm and an upper diameter of 70 mm.The cone was then lifted, and the average of two perpendicular final flow diameters was averaged.The flow diameter was maintained constant in the range of 235 ± 10 mm.Such a flowable matrix was found to be efficient in covering the surrounding textile reinforcement.

Direct tensile testing and crack counting methodology
A SHIMADZU tensile test machine with a 50 kN capacity was used to load samples at 0.5 mm min −1 displacement-controlled mode.The lower and upper supports of the tensile machine are fixed and hinged, respectively, to provide better alignment at the stage of axial loading.Four LVDTs were mounted on the specimen, two at the front and two at the back, to measure the elongation under tensile stress.The specimen elongation was determined by averaging the values from the four LVDTs.Stress-strain diagrams were obtained, and the tensile strain capacity (ductility), tensile strength, and toughness values up to the tensile strength (peak toughness) were determined for each specimen.Four samples were prepared for each mix composition.After the tensile test, to characterize the multiple cracking behavior in bone-shaped specimens, a line was drawn from the longitudinal symmetry axis, and the cracks intersecting the line were counted.This count represents the number of cracks for the entire specimen surface.In addition, cracks intersecting the line drawn from the longitudinal symmetry axis on a 100 mm-wide surface covered by the narrowed cross-section of the specimen were counted, and this count was referred to as the narrow region crack count.The purpose of determining these two different crack counts is to numerically assess the potential for cracking, even in larger areas rather than narrow lengths.Composite materials capable of cracking outside the narrowed section were evaluated to demonstrate higher performance in terms of multiple cracking potential.

Matrix compressive strength test
The matrix compressive strength values were determined on 50 × 50 × 50 mm cube specimens according to ASTM C 109.Tests for compressive strength were performed at 7, 28, and 90 days with a loading rate of 1.40 kN/ s.The average of three samples was determined.

Sample coding principle and experiment plan
The parameters expressed in first place, indicate the type of binder used.In matrices denoted by 'C,' only Portland cement is used as the binder, while matrices denoted by 'D' represent a matrix created by substituting 20% by weight of cement with calcined diatomite.The parameters expressed in the second place represent the

SEM and EDS analysis
Scanning Electron Microscopy and Energy Dispersive Spectroscopy (SEM-EDS) analyses were performed to obtain information about the fabric-matrix interface topography and elemental composition of the outperformed composites.The analyses were performed using a ZEISS Gemini SEM 560 within the range of 10-15 kV.The Gemini SEM 560, with its Schottky-type field emission and column internal beam deceleration design, enables high-resolution imaging and high-quality surface morphology observation even at low acceleration voltage values.In addition, an instrument with a high EDS signal collection efficiency was used with an analytical operation distance of 8.5 mm [25].10 × 10 × 5 mm-sized pieces containing one or two fabric fibers cut from the specimens with a precision saw.Pieces were vacuum coated with gold.Subsequently, EDS line analysis was conducted to examine the elemental distribution around the fabric-matrix interface.

Matrix compressive strength results
The compressive strength results for 7, 28, and 90 days of the four matrix types are shown in figure 3.In the C-H series, where the binder is only Portland cement, decrements of approximately 34%, 38%, and 45% in compressive strength results were determined for 7, 28, and 90 days compared with the C-L series, respectively.The loss in matrix compressive strength can be explained by the increase in the water-to-binder ratio from 0.28 to 0.45.This situation was parallel to the diatomite substituted D-L and D-H series where a 36% decrease in compressive strength was recorded.When the C-L and D-L series prepared at low water-to-binder ratio are compared at 90 days, although the diatomite substitution causes some reduction in compressive strength values in the early age, both series reached the same compressive strength values.When C-H and D-H series are compared, diatomite substitution provides an average increase in compressive strength of approximately 20% at all ages.Strength improvement can be attributed to the pozzolanic reaction developed between portlandite, generated by cement hydration reactions, and calcined diatomite, which contains high amorphous silica.This pozzolanic reaction is effective in homogenizing the paste, reducing the porous structure, and densifying the matrix microstructure [20].The slow early-age strength development followed by an increase at later ages can be associated with the high-water absorbing ability of diatomite.This characteristic may cause diatomite to retain the necessary water for the early hydration of C 3 S. Consequently, secondary C-S-H formation was limited at early ages because of the reduced portlandite content.Over time, the water released from diatomite can support the generation of second-stage hydration products, positively influencing long-term strength development [3].This phenomenon could be associated with the ability of porous diatomite particles to trap water and gradually release it into the matrix.The transportation of trapped water into the capillary pores over the long term contributes to strength development in similarity with the self-curing phenomena [26].

Tensile strength results of the fabric-reinforced composites
The 7, 28, and 90-day tensile strengths of the flax and polyester woven fabric reinforced cementitious and, diatomite substituted composites are shown in figure 4. The C-K-L series showed a slight decrease of approximately 3% in tensile strength with aging, while the C-K-H series showed a decrease of approximately 15% from 7 days to 90 days.The reduction in the tensile strength of the composites may be related to the weakening matrix interface resulting from the increase in the porous structure of the matrix parallel to the increase in the water/cement ratio.Moreover, the fabric degradation mechanisms that may arise from the chemical interaction between the basic matrix and the woven fabric should also be considered.The low alkali resistance of flax is responsible for the reduction in the tensile strength of flax-reinforced cement-based  composites [27].The main deterioration phenomena of cellulosic fibers in cement are the decomposition of hemicellulose and lignin in the porous structure of cementitious matrices.The alkaline hydrolysis of alkaline cellulose-based molecules is responsible for the deterioration of the molecular chains, which results with a decrement in both polymerization degree and tensile strength [28].Additionally, crystallization of lime in the middle lamellar layers and lumen, which are parts of the cellular structure of flax, may affect the cellulosic fibers by reducing their flexibility and strength [29].Furthermore, the hydrophilic behavior of flax fabrics and their porous structure facilitate the rapid and homogeneous spread of the alkali solution within the matrix.These factors contribute to acceleration of the degradation process [30].As deterioration progresses, compounds such as soluble portlandite and hydration products like C-S-H slowly enter the cell walls.This infiltration results in the mineralization and increased fragility of cellulose fibers [31].All of these mechanisms explain the aginginduced tensile strength decrement of flax woven fabric reinforced composites tested in our current study to the great extent.The C-P-L series exhibited a maximum tensile strength decrement of approximately 10% with aging, while in the C-P-H series, the reduction became more pronounced, reaching levels of up to 23% after 90 days.Similarly, in the flax-woven fabric reinforced composite, an increase in the water/cement ratio induced a more pronounced decrease in strength.The H series exhibited a 20% reduction in tensile strength compared with the L series after 90 days.The occurrence of this reduction may once again be attributed to a capillary porous matrix structure due to excessive water content.
Conflicting views emerge in the literature on the alkali resistance of polyester, with some studies highlighting its high alkali resistance [32] and others indicating the opposite [13].Considering both the variations in alkali solution concentration and the differences in the chemical structure of polyester, it can be anticipated that evaluating the alkali resistance of polyester on a case-by-case basis would provide a more accurate and insightful understanding, as these factors can influence degradation processes and mechanisms.Zeronian and Collins [32] explained the degradation mechanism of polyester in an alkaline environment as follows: Under alkaline conditions, polyesters undergo nucleophilic replacement and are hydrolyzed by hydrated sodium hydroxide.Hydroxyl ions lead to chain cleavage at that site and the formation of an amide by attacking electron-deficient carbonyl carbons in the polyester.If the polymer yields easily dispersible products (oligomers) as a result of the reaction, significant mass loss occurs.As a result, polyester threads, initially exhibiting hydrophobic behavior, transition to a hydrophilic tendency due to hydrolysis [33].The observed slight decrease in tensile strength due to aging in the current experiments can also be associated with the shrinkage effect on the diameter and dimensions of polyester threads resulting from the hydrolysis of polyester in an alkaline environment.The strength reduction, intensified by an increasing water-to-binder ratio, can be interpreted as a consequence of both the hydrolysis of polyester threads and the development of a defective fabric-matrix interface owing to the increasing porous structure of the matrix.Additionally, the formation of a porous structure and the hydrophilic behavior of polyester undergoing hydrolysis at alkaline conditions may contribute to the accelerated decomposition process by allowing easier diffusion of the solution of alkalis into the polyester structure.
When the cement-based diatomite substituted flax woven fabric reinforced D-K-L series was examined, a gradual tensile strength decrease was observed from 7 days to 28 and 90 days.In the D-K-H series, a gradual decrease similar to the previous series was observed, with tensile strength decrement of 8% and 16% at 28 and 90 days, respectively.
There are literature studies claiming that the substitution of diatomite increases the tensile strength of flaxderived fibrous structures by lowering the alkalinity level of the environment [34].In contrast to these views, the experimental results in this study indicated a reduction in the tensile strength of the flax-fabric reinforced composites with time.This trend became more evident in composites prepared with high water-to-binder ratios.Researchers reported that substituting diatomite into a cement-based matrix decreases the composite unit volume weight and increases the apparent porosity.Furthermore, ultrasonic tests confirmed an increase in matrix porosity with the increment in the diatomite substitution ratio [35,36].The interface strengthening role of diatomite by reacting with hydration products seems less efficient when hydrophilic flax fabric is employed as reinforcement.Flax fabrics may degrade even in a moist environment [30].The water molecules trapped in the porous structure may release at the interface of the fabric-matrix and degrade the flax fabric structure over time.
The D-P-L series achieved a tensile strength of 6.16 MPa on day 7 and maintained its strength.Similarly, D-P-H series had the highest tensile strength among all samples, reaching 6.27 MPa on the 28th day, showing a 6% increase compared with the 7th day.The tensile strength at 90 days was found to be similar to that at 7 days.In summary, no significant differences in tensile strength were observed in both the L and H series at all ages.The tensile strength values remained within a certain range and maintained their stability.Diatomite substitution further strengthened the matrix interface.While composites in the polyester woven fabric-reinforced C series exhibited a time-dependent decrement in strength, the diatomite-substituted D series exhibited no significant loss in strength values over time.When comparing the H series of polyester woven fabric-reinforced composites, diatomite substitution resulted in a nearly 50% improvement in tensile strength on the 90th day.In the L series, the improvement rate was approximately 13%.It is concluded that diatomite substitution enhanced the polyester fabric resistance against alkali degradation and improved the time-dependent tensile strength retention performance.The decrease in alkalinity due to the pozzolanic hydration reactions probably slows down the hydrolysis reactions of polyester.Thus, the structure of polyester fabrics remains stable compared with that of flax fabrics.

Ductility results of the fabric-reinforced composites
The ductility of cementitious composites reinforced by flax and polyester woven fabrics are shown in figure 5 The initial ductility of the C-K-L and C-K-H series at 7 days was 8.31% and 8.06%, respectively.The ductility of flax-reinforced composites decreased with aging.Fiber embrittlement due to increased density of the matrix around the flax fabrics and penetration of hydration products from the matrix into the cell walls of the fibers are considered as the main factors causing the initial crack strength to increase and the decrement in the ductility of flax yarn embedded composite [28].
In general, composites prepared with high water-to-binder ratios exhibited higher ductility compared to their low water-to-binder counterparts.The fabric-matrix interface with a low water-to-binder ratio has a much denser structure, which reduces the frictional slipping potential of single fiber from matrix.In this context, the difference in the density of hydration products accumulated at fabric-matrix interface may have played a frictional slip-enhancing role in the H series.Thus, these composite series exhibited a behavior that is more ductile with aging compared to the L series.
Polyester fabric reinforced C-P-L series presented a comparatively higher ductility value of 13.29% on the 7th day.However, the C-P-L series experienced a slight ductility decrement with aging.The C-P-H series, a ductility similar to the L series, was around 12.74%, with no significant change with aging.The structure of polyesters can degrade in a moist and alkaline environment, and the hydrolysis rate associated with degradation is characterized by the nature of the chain end tips.An increase in the carboxyl end group concentration accelerates the hydrolysis rate of the polymer [37].Therefore, the time-dependent ductility decrease in the C-P-L series can be associated with the alkaline environment, which causes a slight mass loss due to the degradation of the polyester yarn structure.Although there is a slight loss of ductility over time in the C-P-H series, it is not as significant as the loss observed in the L series.This difference may be associated with changes in hydration progression due to water absorption and changes in pore water alkalinity around fabric-matrix interface.
When the D-P-L series was analyzed, despite showing around a 10% loss in ductility on the 7th day compared with the C-P-L series, it was able to reach similar values on the 90th day with aging.The D-P-L series exhibited a 12% decrease in ductility on the 28th day compared with the 7th day but later reached a value close to the 7th-day ductility on the 90th day.The D-P-H series showed similar ductility to the C-P-H series on the 7th and 28th days.On the 90th day, the diatomite-substituted series achieved a 10% ductility improvement compared with the C series.In the D series, there has been an average improvement of approximately 17% in ductility values with increment in the water-to-binder ratios of matrices.This improvement can be associated with the ability of diatomite substitution to develop pozzolanic reactions, leading to a reduction in the alkalinity of the environment, which is beneficial for structural integrity of the polyester fabric.From this viewpoint, the enhanced compatibility of polyester woven fabrics with the diatomite-modified matrix can be pronounced with aging.In addition, the more ductile behavior of the D-H series compared to the D-L series can be associated with the dominant behavior of the fabric yarns in the composite by frictional slipping of the polyester woven fabric from the matrix-fabric tunnel.Considering all these results, it can be concluded that by adding diatomite into the matrix and increasing the water-to-binder ratio, the alkali source is diluted, and the polyester woven fabric embedded composites outperformed in terms of tensile strength, ductility, and durability performance.

Peak toughness results of fabric reinforced composites
The time-dependent peak toughness values of the cement-based composites reinforced with flax and polyester woven fabrics are illustrated in figure 6.The 7-day toughness values of the C-K-L and C-K-H series are 138 kJ m −3 and 139.1 kJ m −3 , respectively.Toughness values decreased by 17% and 27% with 90 days of aging.The decrease in toughness values due to aging can be associated with fiber weakening through alkali attack.This is associated with the channeling of products of hydration, particularly portlandite, into the voids and lumen of fibers, resulting in increased fiber mineralization.Consequently, a reduction in the tensile strength of fibers is observed [38].As the matrix water-to-binder ratio increased, the deterioration progressed further, and toughness loss became more pronounced.The increase in alkalinity, coupled with the proliferation of hydration reaction products, especially portlandite, is believed to be associated with the increased brittleness of the fiber, contributing to the manifestation of this condition.
The initial toughness values of the C-P-L and C-P-H series were 428.3 kJ m −3 and 404.3 kJ m −3 at 7 days, respectively.Aging up to 90 days resulted with a decrement of 15% and 28% in toughness values, respectively.Silva et al's research [39] on the decomposition of recovered PET fibers in Portland cement-based materials indicated that polyester fibers degrade in a cement-based mortar, a phenomenon confirmed through SEM analyses conducted over a period of 104 days.This degradation is associated with both topochemical and permanent deterioration processes.The topochemical deterioration process is defined as a reaction primarily limited to the fiber surface, leading to the cleavage of polymer chains.Permanent deterioration is defined as a reaction causing chain cleavage along the length of the fiber [40].After all these processes, the composite loses its toughness over time.The toughness reduction in the C-P-L series becomes even more pronounced in the C-P-H series with an increasing water-to-binder ratio.This situation results in an increase in the pore water alkalinity due to hydration reactions that progress with aging.
The D-K-L series had a toughness value of 67.5 kJ m −3 on the 7th day, experiencing nearly 50% toughness loss compared with the C-K-L series.However, with aging, these values exceeded those of the C series, showing an increase of over 100% compared with the 7th day.Toughness values of 136.1 kJ/m 3 were recorded after 90 days.The initial toughness value of the D-K-H series was 125.7 kJ m −3 at 7 days.Approximately 20% of the toughness decrement is recorded at 90 days.Although the alkalinity of the environment may lead to a decrease in composite tensile strength, the frictional slippage of flax fabric at comparatively lower stress levels from the fiber-matrix tunnel has contributed to an increase in composite ductility.In addition, the further increase in the area resulting from the fluctuations of stress (deviation from linearity in both horizontal and vertical directions) in the tensile stress-strain curve, displaying multiple crack behavior, can be considered as another reason for the toughness variations of the composite.In the D-K-H series, the toughness value decrement over time can be associated with the combined decrement in both tensile strength and ductility.
The initial toughness value of the D-P-H series was recorded as 434.8 kJ/m 3 at 7 days.A toughness enhancement reaching the value of 466.9 kJ m −3 was determined by 7% improvement compared to that of 7 days.Event better, the D-P-H series achieved a 60% improvement in toughness value after 90 days compared to that of C-P-H series.This outstanding enhancement can be attributed to the long-term pozzolanic reaction of diatomite, which result in a decrease in environmental alkalinity.Such conditions effectively maintained the stability of polyester fabrics, which enhanced composite performance in terms of both tensile strength and ductility.
The polyester fabric-reinforced composites exhibited a dominant superiority over the flax fabric-reinforced composites.This dominance is evident in both diatomite-substituted and cement series.The most significant toughness improvement was observed on the 7th day of the D-K-L and D-P-L series, with a 673% increase.In terms of long-term comparisons, polyester woven fabric-reinforced composites consistently demonstrate an average superiority of over 300% compared to that of flax woven fabric-reinforced composites.In this respect, it can be concluded that both polyester and flax fibers undergo degradation in an alkaline environment.Flax fibers are much more sensitive to such kind of deterioration.The effect of diatomite substitution on the long-term performance of polyester woven fabric-reinforced composites was found to be beneficial by maintaining stability in mechanical properties or exhibiting a certain degree of toughness improvement.

Multiple cracking capacity along with tensile stress-strain behavior
The development of multiple cracks under tensile stress can be explained by a four-stage mechanism for woven fabric-reinforced composites.The first stage involves elastic deformation up to the initial cracking strength.The second stage involves the formation of additional cracks and the stable development of existing cracks with increased tensile stress after the first cracking due to effective fiber bridging.The third stage is characterized by the transformation of one of the steady-state cracks (weakest crack in terms of bridging stress) into a Griffithtype crack, leading to an increase in its width.The fourth stage is defined by a significant decrease in the bridging stress capacity and composite failure [41].These multiple crack stages are illustrated in figure 7 for specimen D-P-L-28-1.The first step is characterized by linear or quasi-linear behavior and results in the beginning of the first crack.In the second stage, the composite shows both nonlinear response and drastic variation in stiffness.Multiple cracks may occur at this stage.The release of the elastically saved energy causes a sudden reduction in stress with the development of additional matrix cracks.In the post-cracking step (third stage), the interface friction bond formed between the woven fabrics and matrix determines the tensile behavior of the composite material, and the bulk matrix behavior can be neglected.This process continues until the peak load is reached, after which failure occurs [42].
The number of cracks formed at narrow region and overall length for flax and polyester woven fabric composites are shown in figure 8.The examinations were performed on dog-bone geometrically shaped tensile specimens categorized into two groups: narrow region length (where crack localization is expected in the case of conventional strain softening fiber-reinforced composites with single cracking) and overall specimen length along the axial tensile loading axis.
The C-K-L series exhibited average crack numbers of 6.25, 4, and 5.25 in the narrow region at 7, 28, and 90 days, respectively.These crack numbers slightly increased to 6.75, 5.63, and 7.50 in the case of overall specimen length.In the C-K-H series, there was no major change in the number of cracks in both the narrow region and overall length with aging.The average crack number in the narrow region was 3.7, and that in the overall length of the specimen was 5.7.In both the L and H series, no significant differences in crack numbers were detected with aging.In the C-H series, both the narrow region and overall length of the specimen exhibited fewer cracks than those in the C-L series (figure 8(a)).
Unlike the flax fabric reinforced series, there is a noticeable increment in the crack number at both the narrow region and overall specimen length for composites prepared with polyester fabrics (C-P-L and C-P-H series).The C-P-L series exhibited average crack numbers of 7.1, 7.4, and 8 in the narrow region at 7, 28, and 90 days, respectively.These crack numbers distinctly increased to 11.6, 13, and 11.5 in the case of overall specimen length (figure 8(a)).When the average crack counts of all 7, 28, and 90-day specimens are evaluated together in the whole and narrow regions of the specimen, it was determined that the number of cracks in the whole region is 60% more than the number of cracks in the narrow region.The formation of additional cracks even in the regions with widened cross-sections shows that the textile fabric plays an active role in force transmission even at widened sections, which enhances the multiple crack behavior of composite.The C-P-H series exhibited average crack numbers of 4.6, 5.9, and 5.6 in the narrow region at 7, 28, and 90 days, respectively.The average crack numbers of 10.3, 12.3, and 9 were counted in the overall specimen length.The crack numbers in the overall specimen were nearly twice as large as those in the narrow region.Similar to the results in the flax composite, the lower crack numbers in the H series compared to the L series are thought to be due to the differentiation of the matrix interface compatibility with polyester fabrics during the hydration process.The D-K-L and D-K-H series can be listed as the weakest series in terms of multiple cracking potential (figure 8(b)).They exhibited average crack numbers of only 2.9, 3.8, and 4.4 in the narrow region at 7, 28, and 90 days, respectively.No significant variation was observed with aging in the crack number counted overall length of the specimens.In the D-K-H series, there was a similar increase in crack numbers with aging, both in the narrow region and in the overall specimen.
The D-P-L series exhibited average crack numbers of 8.6, 9.8, and 11 in the narrow region and 13.6, 15.3, and 18.1 in the overall length at 7, 28, and 90 days, respectively (figure 8(b)).The diatomite substituted polyester series with a low water-to-binder ratio exhibited the highest crack number values among all the series, both in the region and on the overall specimen length.Aging further promoted the number of cracks.The crack number formed on the overall specimen length is 60% higher than that formed in the narrow region.The D-P-H series exhibited average crack numbers of 5.6, 7.4, and 6.8 in the narrow region and 10.6, 13.6, and 11.3 in the overall length at 7, 28, and 90 days, respectively (figure 8(b)).The D-P-H series appears to fall behind the D-P-L series in terms of crack number development.The average crack number across the surface in all examined specimens of D-P-H series is approximately 80% higher than those in the narrow region.In general, diatomite replacement contributed to the crack number development in both the L and H series.Multiple matrix cracking occurs due to the weakening of fiber-matrix interface bond and frictional debonding process.The post-cracking resistance is primarily provided by the performance of fiber pullout from crack surfaces [43].The intense and saturated development of multiple cracks was recorded in the D-P-L-90 series.The cracked photographs of composites prepared with cement-and diatomite-substituted matrices after tensile testing are presented in figure 9.These composites exhibited the worst and best performance in terms of multiple crack development.The tensile stress-strain relationships of these series are also shown in figure 10.When compared on a fiber basis, woven fabric-reinforced composites with polyester have demonstrated a clear superiority in terms of multiple crack development compared with those with flax fabric reinforcement.This superiority becomes even more pronounced in the diatomite-substituted series.Upon examining both the narrowed region and the entire specimen surface, the most intense crack development was observed in the polyester woven fabric-reinforced composite with a low water-to-binder ratio, particularly in the 90-day samples.

Microstructural investigations
The fabric-matrix interface morphological structure, microtopography and elemental composition were investigated using the SEM-EDS analysis.For this purpose, 10 × 10 × 5 mm specimens including embedded fibers were detached from cracked composites by cutting perpendicular to the fiber direction using a precision saw.Microphotographs were taken at the optical zoom ranges of 100X, 500X and 1000X.Linear EDS analyses were particularly performed to determine the elemental composition of the fabric-matrix interface, which covers the interface from the fiber to the matrix and is approximately 160 μm in length.
Figure 11 illustrates the linear EDS analyses of the C-P-L series composite, including the elements Ca, C, O, Si, Al, and Fe.It is assumed that local Ca(OH) 2 densities are associated with the Ca/Si ratio, particularly in areas abundant in calcium, where the probability of Ca(OH) 2 formation is relatively high [44].Linear EDS analysis in figure 11 revealed a significant accumulation of Ca(OH) 2 , particularly around the fibers, in the 20-70 μm range near the polyester fibers, indicating their tendency to retain Ca 2+ ions.This finding is consistent with those of previous studies [45].When examining the fiber-matrix interface, it was observed that there is a porous structure spanning approximately 50-60 μm in size.This indicates that under tensile loading, polyester fibers can easily slip away from the matrix interface, resulting in wide crack widths.Throughout the matrix, dominance of Ca, O, C, and Si elements was observed, with small amounts of Fe and Al density detected as well.
Linear EDS analyses of the D-P-L series composite are presented in figure 12.The fiber-matrix interface in the D-P-L matrix is quite compact, and the interface porosity content is quite low with a dense microstructure.EDS analysis revealed that there were small amounts of Ca/Si peaks around the polyester fiber and the nearby 0-40 μm range, and that a dense Ca(OH) 2 or CaCO 3 accumulation was localized on this region, possibly due to the surface cutting procedure.
Figure 12 clearly indicates that the fiber-matrix interface of D-P-L composite is compact and there is no visible porosity at the interface with diatomite substitution.In addition, linear EDS analyses revealed the presence of silicon elements in the structure of diatomite, which is different from C-P-L composite.Furthermore, the silicon-rich regions detected in the matrix phase of composite D-P-L at 1000X optical zoom range from point EDS spectra are evidence of the presence of diatomite (figure 13).The dense rims appear to be the product of a topochemical pozzolanic reaction between diatomite and Ca(OH) 2 known as secondary C-S-H.The formation of this secondary binder has also increased the adhesion of the fiber with matrix, allowing for better load transfer and playing a positive role in the development of multiple cracks.

Carbon emissions and cost analyses
The environmental impacts and economic efficiency of woven fabric reinforced composites are important parameters to be questioned in addition to their mechanical performance.Material Sustainability Indicators (MSIs) are used to analyze environmental impact of composites [46].These indicators provide information on waste, energy consumption, and pollutant emissions.In this study, the use of the equivalent carbon dioxide (eqCO 2 ) indicator is considered.Carbon dioxide gas emission and cost analysis of composite components; cement, diatomite, superplasticizer, water, flax, and polyester woven fabrics in different matrices were carried out.The unit values used in the calculations of CO 2 emission and cost analysis are collected in the direction of literature information and are presented in table 5.
Accordingly, the highest carbon emission value per unit weight belongs to the polyester woven fabric.The highest cost per unit kilogram belongs to flax woven fabric with $6.75.Although the literature research [55] for polyester woven fabric states that the average kilogram cost is 0.84$, the average woven polyester fabric cost is determined to be 1.91$ according to the cost amount declared by the manufacturer as 3$.The equivalent CO 2 emission values of the composite components are shown in figure 14.When all composite types were analyzed, C-P-H type had the highest carbon emission value while D-K-H had the lowest value.When the composite components are analyzed, it is seen that Portland cement is the most CO 2 emitting component in all series.Portland cement was followed by polyester woven fabric and hybrid flax woven fabrics.While the contribution of water and calcined diatomite to CO 2 emission is almost negligible, superplasticizers contribute to CO 2  emission at very small values.The addition of calcined diatomite to the same type of series provided approximately 14% CO 2 emission reduction in composites with the addition of flax woven fabric, while this value yields 7%-10% CO 2 emission reduction in polyester series.Flax woven fabric reinforced composites contributed to an average CO 2 emission reduction of around 19% compared with polyester woven fabrics.In general, CO 2 emission decreases inversely proportional to the increase in water-to-binder in all series.
The components cost analysis of the composites is shown in figure 15.When the composites were analyzed in terms of cost, it is seen that the D-K-L series had the highest cost, while the C-P-H series had the lowest cost.When the average costs of the materials across all series were analyzed, it was seen that the flax hybrid woven fabric had the highest cost.This was followed by polyester woven fabric and Portland cement.While the cost of water was almost negligible, the costs of calcined diatomite and superplasticizer were considerably lower than those of the composite components.

Conclusion
• The calcined diatomite improved the compressive strength development rate during initial periods of hydration due to the reduction of the porous structure through the pozzolanic reaction.Calcined diatomite provided compressive strength values comparable to those obtained from series containing only cementbased binder in the long term.• Woven fabric-reinforced composites with polyester have demonstrated significant tensile strength superiority over those with flax fabric reinforcement.The differences in the degradation mechanisms of composites during the aging process are associated with variations in the resistance of fabrics to the alkaline environment provided by Portland cement hydration products.
• The substitution of calcined diatomite into the Portland cement-based matrix either improved or maintained the toughness values in both the flax and polyester fabric-reinforced series compared to the plain Portland cement binder series.
• Polyester woven fabric reinforced composites performed much better than compared to flax fabric reinforced composites in terms of multiple cracking potential.The D-P-L-90 series, with an average of 18 overall crack formations, showed the best multiple cracking performance.
• In general, the polyester woven fabric reinforced series showed superiority compared to the flax woven fabric reinforced series.With the substitution of calcined diatomite in the composite, the performance difference of the fabrics became even more pronounced.
• The linear SEM-EDS analyses revealed the densified matrix-fabric interface and the presence of silicon-rich regions in the matrix phase of the composite D-P-L in the 1000X optical zoom range.The dense rims are associated with the product of the topochemical pozzolanic reaction between diatomite and Ca(OH) 2, known as secondary C-S-H.
• Carbon emission analysis revealed that the diatomite incorporation reduced the CO 2 emission of woven fabric reinforced composites.However, diatomite substitution caused a cost-increasing effect.

Figure 1 .
Figure 1.Mold system developed within the scope of the study (a) design and dimensions, (b) mold produced, (c) sample samples produced with the mold.

Figure 2 .
Figure 2. Sample preparation and experimental plan.

Figure 3 .
Figure 3. 7, 28, and 90 days of compressive strength results of the matrix series (bars denotes standard deviation of each test group).

Figure 8 .
Figure 8. Crack counts of composites at 7, 28, and 90 days.(a) cement-based composites and-) diatomite substituted composites.* NRC: crack number counted at narrow region length (100 mm).* ORC: crack number counted at the overall length of the specimen along the axial tensile loading.

Figure 9 .
Figure 9. Different crack development of composites of C and D series.

Figure 10 .
Figure 10.Tensile strength and strain relationships of composites of different C and D series.

Figure 11 .
Figure 11.(a) 100X optical SEM photograph and (b) 500X optical SEM photograph of type C-P-L composite.(c) Normalized EDS line profile and d) EDS analysis along the line profile, indicating the proportional changes of Ca/Si -Ca(OH) 2 .

Figure 12 .
Figure 12.(a) 100X optical SEM photograph and (b) 500X optical SEM photograph of composite type D-P-L.It also includes (c) Normalized EDS line profile and d) EDS analysis along the line profile and the proportional changes of Ca/Si -Ca(OH) 2 .

Figure 13 .
Figure 13.EDS spectrum analyses of the D-P-L matrix.

Figure 14 .
Figure 14.Equivalent CO 2 emission values of the composite components.

Figure 15 .
Figure 15.Cost analysis of composite components.

Table 1 .
Mechanical, chemical, physical, and chemical properties of Portland cement.

Table 2 .
Mechanical, chemical, physical, and chemical properties of the calcined diatomite.

Table 4 .
Structural properties of the woven fabrics used in composite production.

Table 5 .
Unit material quantities used in the cost analysis and calculation of carbon emissions from composites.