Lais Alves
email: lais.alves@cefet-rj.br@b

Nordine Leklou
email: nordine.leklou@univ-nantes.fr

Pascal Casari
email: pascal.casari@univ-nantes.frd

Silvio De Barros
email: silvio.debarros@gmail.com

Fiber-matrix bond strength by pull-out tests on slag-based geopolymer with embedded glass and carbon fibers

Keywords: blast furnace slag, cement Portland, sodium silicate activating solution, pull-out test, glass fiber, carbon fiber

The reinforcement efficiency on a composite depends on the effective transfer of the stress between matrix and fiber. This work presents an experimental and comparative study of fiber-matrix bond strength for fiber-matrix interface between glass fibers and carbon fibers added to the slag-based geopolymer matrix. This analysis was performed by pull-out test. A total of 18 tests have been conducted, three for each type of fiber at each embedded length of 10 mm, 20 mm and 30 mm. The critical embedded length and the maximum interfacial shear (bond) strength were analyzed, and SEM observations were carried out for the cross-section of each fiber to measure diameter and observe the interface. It was found that the greatest efficiency was obtained by reinforcing with the glass fibers, incorporated at 20 mm in the slag-based matrix.

Introduction

The geopolymer began to be engendered in the 1970s by Davidovits, emerging as a new class of material derived from rocks and not from oil, as organic polymers [START_REF] Davidovits | From ancient concrete to geopolymers[END_REF]. It consists of inorganic polymers obtained by the alkaline activation of materials rich in silica (SiO2) and alumina (Al2O3) [START_REF] Alves | A Bibliographic Historical Analysis on Geopolymer as a Substitute for Portland Cement[END_REF] and have similar physical and mechanical characteristics as Cement Portland (CP) [START_REF] Buchwald | Condensation of Aluminosilicate Gels: model system for geopolymer binders[END_REF]. CP is the most used binder to produce cement and mortar, and the second most used material in the world, behind only water [START_REF] Nogueira | Fiber matrix adhesion on industrial geopolymer[END_REF]. But the production process classifies it as the third-largest source of anthropogenic emissions of carbon dioxide (CO2) [START_REF] Andrew | Global CO2 emissions from cement production[END_REF]. In 2018, global process emissions reached 1.50 ± 0.12 Gt CO2, which corresponds to about 4% of emissions from fossil fuels and cumulative emissions from 1928 to 2018 result in 38.3 ± 2.4 Gt CO2, from which 71% occurred after 1990 [START_REF] Andrew | Global CO2 emissions from cement production[END_REF]. Thus, the need to find an alternative material for construction processes arises.

Compared to CP, geopolymer emissions of CO2 are approximately 43% less. Previous studies on these materials show that values for compressive strength are comparable [6 -9], with values depending on the precursor material and activator solution. Correia et al. [START_REF] Correia | Mechanical Performance of Natural Fibers Reinforced Geopolymer Composites[END_REF] found that geopolymers maintain considerable mechanical properties at temperatures up to 1000° C, whereas the application of most polymer resins is often limited to temperatures below 400° C. The material also presents high durability due to low apparent porosity, that results in low water permeability [START_REF] Alves | A Quick Overview on Geopolymer Chemistry and General Properties[END_REF]. The main precursor materials are clays such as metakaolin [12 -14] which is an artificially calcined kaolinite clay, and industrial waste such as blast furnace slag [START_REF] Mauri | Geopolymer mortar: degradation study by sodium sulfate and sulfuric acid (in Portuguese)[END_REF]15] and fly ash [16 -20].

Geopolymers have low tensile strength and low deformation capacity, which require the use of reinforcements [21]. Geopolymer matrix composites reinforced with particulates and fibers can be considered a solution to improve flexural strength and compressive strength [22 -28]. The fibers increase the ductility of the material, preventing abrupt rupture [START_REF] Silva | Cracking mechanisms in durable sisal fiber reinforced cement composites[END_REF]. Natural fibers, such as jute, and synthetic fibers, including glass and carbon fibers, can be used to reinforcement the geopolymer.

There is a direct proportional relationship between the fiber-matrix interaction force, the strength, and the adhesion between the surface of the geopolymer matrix and the surface of the fiber [30 -32].

The mechanical properties of fiber-reinforced composite materials highly depend by how efficient the load transfer through the interface between the fiber and the matrix [31, 33 -36]. When a composite has a high adhesive force at the fiber-matrix interface, it exhibits high strength. This is due to a greater efficiency of the tension undergone by the matrix to the fibers. On the other hand, a high value of causes the system to have a low tenacity, since the energy spent during the crack propagation is low, thus the failure of the matrix will propagate through the fiber-matrix transfer of the tension to the fibers, and a high tenacity, since a high energy value would be expended not only by the cracks, but by the decoupling of the fiber [START_REF] Yue | Assessment of fibre-matrix adhesion and interfacial properties using the pull-out test[END_REF].

A factor that influences the bond strength between the composite and the fiber is the critical embedded length (Lc) [START_REF] Nogueira | Fiber matrix adhesion on industrial geopolymer[END_REF]. Fibers with shorter lengths results in deficiencies of the transmission of the external loads, besides causing debonding or decoupling. It can also be led to failures in regions that present lower values of resistance, either in the matrix or in the interface [START_REF] Stamboulis | Interfacial characterisation of flax fibrethermoplastic polymer composites by the pull-out test[END_REF]. The critical length can be analyzed through the pull-out test and represents the optimum length capable of promoting the greater adhesion and better mechanical performance of the composite.

The analysis of the fiber-matrix adhesion can be performed through the study of the materials involved, their geometries, the loads, and the relative displacements, which provide the adhesion stress values [START_REF] Silva | Effect of fiber shape and morphology on interfacial bond and cracking behaviors of sisal fiber cement-based composites[END_REF], fundamental for the knowledge of the shear stress transfer between the fiber and the matrix. Different types of tests to study this interface have been developed, namely pull-out tests [START_REF] Quentin | Interfacial Characterization by Pull-Out Test of Bamboo Fibers Embedded in Poly (Lactic Acid)[END_REF][START_REF] Zhandarov | Investigation of load transfer between the fiber and the matrix in pull-out tests with fibers having different diameters[END_REF], push-out tests [START_REF] Parthasarathy | Extraction of Interface Properties from a Fiber Push-Out Test[END_REF][START_REF] Zhou | Interfacial properties of polymer composites measured by push-out and fragmentation tests[END_REF], micro-bond tests [START_REF] Sockalingam | Fiber-Matrix Interface Characterization through the Microbond Test: A Review[END_REF][START_REF] Zhandarov | Indirect estimation of fiber/polymer bond strength and interfacial friction from maximum load values recorded in the microbond and pull-out tests. Part I: local bond strength[END_REF] and fiber fragmentation tests [START_REF] Zhou | Interfacial properties of polymer composites measured by push-out and fragmentation tests[END_REF][START_REF] Huber | Fibre matrix adhesion of natural fibres cotton, flax and hemp in polymeric matrices analyzed with the single fibre fragmentation test[END_REF][START_REF] Kim | Observations of Fiber Fracture and Interfacial Debonding Phenomena Using the Fragmentation Test in Single Fiber Composites[END_REF].

The main difference between these tests is the test geometry [START_REF] Zhandarov | Peak force as function of the embedded length in pull-out and microbond tests: effect of specimen geometry[END_REF]. However, the results analyzed by Pitkethly et. al [START_REF] Pitkethly | A Round-Robin Programme on Interfacial Test Methods[END_REF] from different tests or from the same test applied by different researchers showed that the scatter was high.

To evaluate the bond quality at the fiber-matrix interface and the ability of stress transfer between the fiber and the matrix, the fiber pull-out test is one of the most important test methods developed [START_REF] Fu | Comparison of the stress transfer in single-and multi-fiber composite pull-out tests[END_REF]. The pull-out test is done by embedding the fiber up to a certain length into a specimen of the matrix material. Then, the two ends are attached, the test body and the tip of the fiber, and a tensile force (F) is applied on the fiber. If the length of the fiber is greater than Lc, the fiber will break.

Otherwise, the fiber slips from within the matrix without breaking. By pull-out test it is also possible to study the influence of fiberby the pressure on the fiber (P0) on the mechanical properties of the composite [START_REF] Fu | Comparison of the stress transfer in single-and multi-fiber composite pull-out tests[END_REF][START_REF] Yue | Assessment of fibre-matrix adhesion and interfacial properties using the pull-out test[END_REF].

Therefore, research work on fiber pull-out from a matrix is essential to understand the stress transfer on different types of composites. [START_REF] Fu | Comparison of the stress transfer in single-and multi-fiber composite pull-out tests[END_REF]. This work seeks to verify the interaction between a geopolymer slag-based matrix and synthetic glass and carbon fibers, comparing the fiber-matrix bond strength between the fiber and the matrix obtained by pull-out test. It aims to examine the cracking pattern of the fibers embedded in the specimens to discover the critical embedded length (Lc) and analyze relative displacements occurring at the fiber-matrix interface.

Materials and methods

This research used 100% ground blast furnace slag (GBFS), provided by ECOCEM from France, as the basic precursor material for the fabrication of the geopolymer paste specimens. The chemical composition for GBFS can be found in Table 1. Blast furnace slag is produced by drying and grinding granulated blast furnace slag. The degree of depolymerisation (DP) for the GBFS was found 1.44, which is an indicator of slag activity, that is considered good within a range from 1.3 to 1.5 [START_REF] Duxson | Designing Precursors for Geopolymer Cements[END_REF]. D50

represents the average particle size in the production and application of powder materials, which can affect the durability of the geopolymer, since a large surface area leads to a higher polymerization rate and a difference in the number of voids [START_REF] Lateef | Investigation of Early Compressive Strength of Fly Ash-Based Geopolymer Concrete[END_REF]. For the GBFS used, D50 .

Table 1 -Chemical composition (%) of precursors

The alkaline activator solution employed in the mixtures, in its proper proportions, were prepared by mixing the sodium hydroxide (NaOH) solution with the sodium silicate (Na2SiO3) solution.

Sodium silicate (Na-Si) activator was prepared by mixing 10M sodium hydroxide (NaOH) with sodium silicate (Na2SiO3), maintaining a Na2SiO3/NaOH mass ratio of 2.0. The produced Na-silicate activator contained 66.7% water with 0.7 Na2O/SiO2 molar ratio. NaOH was purchased from ALFA-AESAR in the form of pellets, white colored with 98% purity. Sodium silicate (Na2SiO3) was purchased from VWR in soluble form, of pH between 11-11. A total of 18 rectangular specimens were tested to find the critical embedded length and the maximum interfacial shear (bond) strength. Three essays were carried out for each type of fiber, carbon (CF) and glass (GF) at each embedded length of 10 mm, 20 mm and 30 mm [START_REF] Zulfiati | Mechanical properties of fly-ash based geopolymer with natural fiber[END_REF]. The specimens were named according to the type of fiber used and the embedded length, e.g., for carbon fiber embedded 10 mm, specimen was named CF_10mm. The geopolymer paste specimens all had five cm in height and cross section of approximately one cm 2 [START_REF] Trindade | Development and mechanical behavior of textile geopolymer composites reinforced with jute fiber[END_REF]. Each fiber strand, composed of multiple fibers, had approximately one millimeter in diameter, measured by SEM observations [START_REF] Nguyen | A Pull-out Fiber/Matrix Interface Characterization of Vegetal Fibers Reinforced Thermoplastic Polymer Composites: The Influence of the Processing Temperature[END_REF].

The specimens were prepared at 20 and 50% humidity, demolded after 24 hours and kept in a plastic film until testing date. All the specimens were tested at seven days [START_REF] Trindade | Development and mechanical behavior of textile geopolymer composites reinforced with jute fiber[END_REF]. As observed in Alves

[6], for this mixture, the GBFS had already been activated at this curing time.

A displacement of 0.5 mm/min [START_REF] Liu | Development and characterization of alkali treated abaca fiber reinforced friction composites[END_REF][START_REF] Lima | Structural monitoring of helicopter rotor blades using intelligent materials[END_REF] was applied to the fiber on a Swift/Roell Z050 machine (Figure 2a), adapted with a Restrained Top Constraint (RTC) grapple, with a load cell of 50kN, following the schematics presented on Figure 2b (adapted from Yue [START_REF] Yue | Assessment of fibre-matrix adhesion and interfacial properties using the pull-out test[END_REF]). While the force was applied, the behavior at the fiber-matrix interface was verified through the graph generated. The test was finished once the fiber is totally pulled-out of the matrix. The data obtained from the fiber pullout test determines the applied loads from the shear stress and the relative displacements occurring at the fiber-matrix interface. With these results, it is possible to compute the relative adhesion stress and the strength effectiveness conferred by the fibrous reinforcement [START_REF] Sorensen | Fiber pull-out test and single fiber fragmentation test analysis and modelling[END_REF].

Results and discussion

Through the data analysis of the applied loads (F) and relative displacements that occur between the fiber and the matrix, it is possible to define the values of adhesion stress 39].

Compiled results, according to the embedded length of the reinforcing fiber, are shown in Table 2 (standard deviation values are in parentheses) and Figure 3.

Table 2 Results for the different specimens of the pull-out test but higher force for CF_30mm. Specimen CF_20mm presented the best adherence due to the higher value of applied load, without fiber fracture. Figure 4 shows SEM observations of the cross section and interface for one of the carbon fiber specimens with embedded length of 20 mm. Glass fiber results showed same trend as that of carbon fiber. For specimens GF_10mm and GF_20mm the fiber was pulled out, with adhesion stress values of 0.7242 MPa and 1.2163 MPa, respectively. For 30 mm embedded length, a fracture on the fiber can be observed, at due to the low value of 0.2811 MPa for adhesion stress. Thus, it is possible to affirm that the embedded length of 20 mm has the greatest capacity for transferring stresses between glass fiber and the matrix.

Figure 5 shows SEM observations of the cross section and interface for one of the glass fiber specimens with embedded length of 20 mm. The critical length (Lc) is a possible explanation for the fact that the embedded lengths of 20 mm had higher stress transfer capacity between the fiber and the matrix [START_REF] Chandran | A comparative investigation of interfacial adhesion behaviour of polyamide based selfreinforced polymer composites by single fibre and multiple fibre pull-out tests[END_REF]. Lengths below Lc present a deficiency in transfer of loads and the fiber slides through the matrix. For lengths above Lc, a greater interaction between the fibers can be noted, causing failure, entanglement and decrease of the effective length. According to Feih et. al [START_REF] Feih | The influence of fibre sizing on the strength and fracture toughness of glass fibre composites[END_REF], sizing controls the interface for glass fiber composites, because it is responsible for the physical-chemical link between the fiber surface and the matrix system.

The fiber material influences the efficiency of the reinforcement and of the displacement related to the applied load, due to chemical interactions [START_REF] Feih | The influence of fibre sizing on the strength and fracture toughness of glass fibre composites[END_REF] between the type of fiber and the matrix material.

From the results, the fiberfiber system. This implies the glass fiber system has a matrix-to-fiber stress transfer more effective, which suggests higher tensile strength for the composite. Applied loads (F) and relative displacements

The applied load (F) vs displaceme for the specimen CF_20mm, which presented bond strength and interaction in the fiber-matrix interface superior to the other specimens analyzed, is plotted in Figure 6. When studying the results for the specimen that presented the best adherence (Figure 6), it is important to notice that the pull-out test can be divided into three phases [START_REF] Yue | Assessment of fibre-matrix adhesion and interfacial properties using the pull-out test[END_REF][START_REF] Sorensen | Fiber pull-out test and single fiber fragmentation test analysis and modelling[END_REF]. The initial debonding and sliding phase, found in region I of the graph, is characterized by being an elastic-linear section, where the load is constantly increased until reaching a nonlinear stretch. Region II is qualified as maximum fiber stress, where the extraction force reaches the maximum value (Fmax), and decohesion becomes partial. After this phase, there is a constant drop in the load, corresponding to region III. This region is controlled by the friction resistance of the interface and consists of sliding and pull-out until the fiber is extracted and completely withdrawn [START_REF] Yue | Assessment of fibre-matrix adhesion and interfacial properties using the pull-out test[END_REF].

Conclusions

Understanding the interface properties helps in enhancing the structural properties of composite materials by improving the interfacial bonding. This work described a procedure to measure apparent interfacial shear strength from pull-out tests on fibrous reinforcements. The pull-out test provides results of fiber-matrix adhesion and interfacial properties that can characterize and assess fiberreinforced composites. The interfacial parameters of applied loads (F), and can be determined from pull-out data.

Two types of fiber were studied, glass fiber and carbon fiber, in three different embedded lengths of 10 mm, 20 mm and 30 mm. This study is fundamental to determine which material can be considered more efficient and makes the matrix more resistant to tensile and deformation strength. In addition to the different behaviors noticed due to the type fibers used, the influence of the embedded length of the fiber was another factor analyzed. The embedded length affects the load transfers and, therefore, the reinforcement efficiency. It is possible, through these data, to infer which composite has a more effective matrix-to-fiber stress transfer and determine the fiber critical embedded length.

It was concluded that the fiber that presented the best adherence for GBFS-based matrix is glass fibers, with 20 mm embedded length. The value obtained for adhesion stress of glass fiber at 20 mm embedded length is 68% higher than for glass fiber at 10 mm embedded length. When comparing with the carbon fiber system that has the higher value of applied load without fiber fracture, the glass fiber system value corresponding to fiber-matrix interface is 83% higher. Critical length was considered 20 mm. With embedded lengths superior to the critical length, the fiber failure can occur before interface debonding. Lengths bellow critical length also causes deficiency in transfer of loads because there is a minimum critical length of the fiber required for a valid shear pull-out.

  5 and density 1.35 g/cm at 20ºC.The precursor material (1,458.3 kg/m 3 ) was mixed for three minutes to have a more homogeneous mixture. Following, the activating solution (335.3 kg/m 3 ) mixed with the water (208.3 kg/m 3 ) was added to the dry mixture and blended for three minutes more. The percentage of water in the mixture was 12.4% and solid-to-liquid ratio 2.0.The glass fiber used was commercial S2-glass which have high tensile strength of 3700 to 4300 MPa [52], without alkaline oxides, containing 65% SiO2, 10% MgO and 25% Al2O3. The diameter for a single fiber was measured, by Scanning Electronic Microscopy (SEM), electron microscope equipped with a secondary electron sensor and a backscattered electrons sensor.After the pull-out test, prisms samples cores were sliced and polished into square shapes of 2cm×2cm by 0.5 cm thickness. Single glass fibers used measured around 17-20 µm and single carbon fibers had a diameter of 5-8 µm, as show in Figure1. The fibers were not preconditioned as the aim was to study the interaction of the raw materials.

Figure 1 -

 1 Figure 1 -Diameter of glass fiber (a) and carbon fiber (b) measured by SEM observations

Figure 2

 2 Figure 2 Experimental setup of pull-out test (a) and pull-out test schematics (b)

Figure 3 -

 3 Figure 3 -Compiled values from the pull-out test

Figure 4 -

 4 Figure 4 -SEM observations for cross section of carbon fiber (a) and interface after pull-out (b)

Figure 5 -

 5 Figure 5 -SEM observations for cross section of glass fiber (a) and interface after pull-out (b)

Figure 6

 6 Figure 6 Pull-out test results for specimen GF_20mm

Acknowledgement The authors acknowledge the Brazilian institutions CAPES (Coordenação de Aperfeiçoamento de pessoal de nível superior -Coordination of Improvement of senior staff), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico -National Council for Scientific and Technological Development) and Institut de Recherche en Génie Civil et Mécanique Research Institute on Civil and Mechanical Engineering for the financial support and their support in conducting experiments.