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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 12115–12129
Correlation of microstructure and mechanical properties of various fabric reinforced geo-polymer composites after exposure to elevated temperature Sneha Samala,n, Nhan Phan Thanha,c, Iva Petríkováa, Bohdana Marvalováa, Katleen A.M. Vallonsb, Stepan V. Lomovb a
Department of Applied Mechanics, Technical University of Liberec, Studentská 2, 46117 Liberec, Czech Republic b Department of Materials Engineering, KU Leuven, Kasteelpark Arengberg 44, 3001 Heverlee, Belgium c Department of Mechanics & Materials, Nha Trang University, 02 Nguyen Dinh Chieu, Nha Trang, Khanh Hoa, Vietnam Received 7 March 2015; received in revised form 8 May 2015; accepted 5 June 2015 Available online 17 June 2015
Abstract This article presents the correlation between microstructure and mechanical properties such as flexural strength, flexural modulus of fabric reinforced geo-polymer composite after their exposure to elevated temperature (up to 1000 1C). Geo-polymer matrices with Si:Al ratio of 15.6 were synthesized and the samples of geo-polymer composite were fabricated with three different types of fabric reinforcements such as carbon, Eglass and basalt fibers. The residual strength of carbon based geo-polymer composite increased with increase of the processing temperature after 600 1C. The basalt reinforced geo-polymer composite strength decreased at higher processing temperature, which may be due to the strong interaction between matrix and fiber and transformation of the composite into a ceramic-like structure. The non-adhesion of E-glass fiber to geopolymer matrix and pyrolysis of the fiber at high temperature has been observed in the composite. Carbon reinforced geo-polymer composite can be possible candidate for the high temperature applications in thermal barrier coatings and panels. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: C. Mechanical properties; Carbon fiber; Geocomposite; E-glass fiber; Basalt fiber
1. Introduction Geo-polymers are inorganic polymeric materials with amorphous nature, possessing ceramic-like structures and properties [1– 3]. These are the materials that are synthesized from two main constituents such as silicon and aluminum entities dissolved in the alkaline medium. The process named as geo-polymerisation is the combination of alkaline medium solutions with silicates. The chemical unit sialate, which basically consists of silicon and aluminum by bridging oxygen, is the basic building block of the geo-polymers [4]. Geo-ploymers are based on polysialates such as sialate (–Si–O–Al–O–), sialate-siloxo (–Si–O–Al–O–Si–O–) and sialate disiloxo (–Si–O–Al–O–Si–O–Si–O–). They contain the n
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[email protected] (S. Samal).
http://dx.doi.org/10.1016/j.ceramint.2015.06.029 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
silicon and aluminum tetrahedral units with charge balance by Na þ or K þ ions [5]. The applications of geo-polymers range from conventional building materials to numerous other materials for many industrial sectors [6]. The most promising use of the geopolymers is for high temperature insulators [7], which should withstand elevated temperature for a prolonged period of time. Geo-polymers are the most promising eco-friendly materials and an alternative to Portland cement and other cementitious materials due to their proven durability, mechanical strength and thermal properties [8,9]. However, despite these features, poor tensile and bending strength has been exhibited by the materials due to their brittle nature [10,11]. The ceramic-like nature can lead to catastrophic and unpredictable failure that is the main obstacle at high temperature application in several areas [12] such as civil engineering and in industry.
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Table 1 (a) Physical and mechanical properties of geo-polymer. Physical properties
Matrix (geo-polymer)
Type Density (g cm 3) Composition of matrix Span of testing (80 mm) Flexural Strength Rmf (Mpa) Flexural Modulus E (Gpa) Failure strain ε (%) Name of the supplier
Inorganic 2.070.01 Si:Al¼ 16:1 3 Point bending 18.57 0.1 27.87 0.1 0.447 0.1 Research institute
Compression 11.770.1 88.970.2 3.2370.1 of inorganic chemistry Inc., Czech Republic
(b) Physical and mechanical properties of fabrics Physical properties Carbon fabric Type Biaxial-plain Density (g cm 3) of fiber 1.657 0.02 Composition of fiber Carbon 4 99.9 Wt%
E-glass fabric Biaxial-plain 2.5770.02 SiO2:54 Wt%, Al2O3:14 Wt%, CaOþMgO :22 Wt%, B2O3:10 Wt% Thickness (mm) 0.24 0.38 Ends/picks count (/10 mm) 4.5/4.5 4.5/3.5 Yarn linear density (g/km) 1600 1064 Areal density of fabrics (g/m2) 200 486 Warp direction Weft direction Warp direction Weft direction Tensile module E (GPa) 25.17 0.1 25.370.1 19.670.2 16.47 0.1 Tensile strength σ (Mpa)a 282.27 0.1 288.670.1 129.870.2 104.47 0.1 Failure strain ε (%) 1.327 0.2 1.1870.1 1.5170.1 1.437 0.2 Name of the supplier Havel Composite, Czech Havel composite, Czech Republic Republic a
Basalt fabric Biaxial-plain 2.7 70.02 SiO2 E50 Wt%, CaO, Na2O, FeO, Аl2O3 0.16 6.5/6.5 1200 200 Warp direction Weft direction 24.270.2 23.270.1 219.470.1 224.670.1 1.2670.1 1.2170.2 Havel Composite, Czech Republic
Tensile properties for fabrics measured according to standard EN ISO 13934. Stresses are calculated based on the fabric thickness given in the table.
Table 2 Volume fraction of constituents. Sample
Matrix (polymer) content (vol%)
Fiber content (vol%)
Void content vol%)
Geo-polymer Carbon reinforced polymer E-glass reinforced polymer Basalt reinforced polymer
99.87 0.02 40.57 0.8
0 39.470.5
0.27 0.2 217 0.2
37.37 0.2
41. 170.3
227 0.2
45.37 0.6
40.370.7
157 0.1
Curing temperature fixed at 70 1C for 24 h.7 designated standard deviation in five tests.
Fiber or fabric reinforced geo-polymers have been shown to have improved mechanical strength and thermal behavior [13,14]. The addition of fiber has also been reported to improve properties such as strength to weight ratio, corrosion resistance and ease of application in compare to pure geo-polymer matrix and geopolymer based composite such as particulate, continuous and short fiber [15]. Different types of fiber reinforcements improve the flexural strength, impact behavior and failure mode in composites by generating a bridging effect in geo-polymer matrix. It has also been reported that reinforcement by fibers helps to control micro and macro-cracks diffusion through the material by generating a fiber bridging effect as well as to change the post-cracking behavior of the material from a brittle fracture mode to a ductile one, thanks to its enhanced strain energy dissipation ability [16]. Carbon and Eglass fabrics are the most commonly used materials for FRCs
composites that are able to encounter harsh operating conditions such as high stress, speed, temperature [17,18]. Woven fabrics, mats and unidirectional fibers such as carbon, E-glass and basalt have been so far the most widely used to cast continuous fiberreinforced composites in civil engineering applications [13]. More recently, structural composite materials obtained from inorganic matrix have been designed to deal with the major drawbacks for the use of organic polymer resins, as significantly low resistance to UV radiation and high-temperature [19]. This study investigates the effect of various fabrics reinforcements on the behavior of geo-polymer matrix composites after exposure to elevated temperature. Carbon, E-glass, basalt fabrics were chosen and their geo-polymer composites were developed to examine the adhesion of each type of fiber to the matrix, microstructure development and their ability to improve the mechanical and thermal properties of the geocomposite at elevated temperature. Correlations between fabric types, the microstructure evolution and the performance of overall properties in terms of physical and mechanical properties were investigated. 2. Materials and experimental methods 2.1. Materials Geo-polymer matrix was prepared with metakaolin (6.88%), alumino-silicate powder (49%) and alkali activator with the NaOH/KOH (44.12%) content ratio of 15.6 in free dissolution, supplied by the Research institute of inorganic chemistry
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Fig. 1. (a) Schematic diagram of the fabrication stage of the geo-polymer composite. (b) Heat treatment stages of the geo-polymer composite: (a) initial stage, (b) necking stage (melting stage of the matrix), (c) coagulation stage for formation of gaps, (d) partial dissolution of fabric for initiation of oxidized layer, and (e) final stage of the fabric reinforced geo-polymer composite.
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Inc., Czech Republic. The elemental chemical composition of geo-polymer in percentage is shown as Al (2.04), Si (31.80), P (0.08), K (15.5), Zr (1.76), Na (0.63), Ca(0.24) and O (48.32). Woven fabrics (carbon, E-glass and basalt) are identified as representative fabrics for the composite development. Physical and mechanical properties of the fabrics and geo-polymer are listed in Table 1. 2.2. Geo-polymer and fabric reinforced geo-polymer composite synthesis The content of fibers and matrix in geo-polymer composites is shown in Table 2. The geo-polymer (matrix) was initially prepared by mixing Al:Si powder, metakaloin and alkali activator solution for 10 min to obtain the homogenous mixture. The fabrics were manually filled by geo-polymer mix, stacked together in the identical direction, compressed by a roller till the desired thickness of about 3 mm was achieved. The assembled fabric reinforced geo-polymer composites were placed in a vacuum bag and cured at 0.003 MPa at ambient temperature for 2 h. The bag was then placed in a curing oven at 70 1C for a period of 2 h, and finally the samples were cured in air environment for 20 h at ambient temperature. Fig. 1 displays the schematic presentation for the fabrication of fabric reinforced geo-polymer at room temperature. The second stage represents the evolution of cracks and fragmentation of fabrics at various elevated temperature.
to stack different number of fabric layers, such as 10 layers of carbon (designated below as 10C), 7 layers of E-glass (7E) and 15 layers of basalt (15B) fabrics for around 3.187 0.27 mm, 3.047 0.35 mm, 3.50 7 0.12 mm thickness of composites. The geo-polymer resins are poured in to the molds and cured in oven at 70 1C. The samples were tested under threepoint bending and compression in accordance with ASTM D790 (3). Geo-polymer composites (3 15 220 mm) were tested with universal tester TIRA test 2810 (Germany) and INSTRON (U.K.) model 4202, in bending and tension in both warp and weft directions to determine the directional Young's moduli and strength using the standard ASTM C 1275-00 (standard test method for tensile properties of continuous fiber reinforced advanced ceramic composite). For the bending tests the stress, strain and modulus of elasticity were derived from the maximum failure load, deflection and other material parameters. σ¼
3PS 2bt 2
ð4Þ
ε¼
6Dt S2
ð5Þ
E¼
Δσ S3 ΔP ¼ U Δε 4bt 3 ΔD
ð6Þ
where Vf, Vm, Vv are the volume fractions of fabric, matrix and void in the geo-polymer composites, n is the number of fabric layers in the composite samples, t, L, b are the thickness, length and width of the composite [m], respectively, ρw is the density of fiber layer [g/m2], ρf is the density of a single fiber [g/m3], ρm is the density of matrix [g/m3].
where σ is the maximum stress in the outer fiber, S is the support span, b is the width and t is the thickness of the sample. D is the deflection of the beam center, ε is the maximum strain in the outer fiber, E is the modulus of elasticity in bending, ΔP/ΔD is the ratio of force difference and deflection difference at beam center, measured at the linear segment of the graph. Series of the V-shaped notched samples fabricated are used to measure in-plane shear properties of geo-polymer composite. Specimens were griped by four halves of Arcan fixture and mounted on testing machine [20]. One DIC camera of Dantec Dynamics Company was used to analyze strain field of specimens (Fig. 4a). All the tests were performed at the same loading speed of 1 mm/min and at room temperature (20– 25 1C), approximately 70% relative humidity. Due to high structural heterogeneity of material, it leads to the state of stress at a center of a specimen is not completely a pure shear states. However, plane shear stress can be measured thanks to loads recorded from a load cell of a testing machine and plane shear strain of the central part of a specimen can be determined optically by the Q-400 camera system. The optical device was set up to take one picture per one second. Both testing machine and the optical device were synchronized together. Hence the magnitude of shear stress was calculated as follows:
2.4. Mechanical properties of geo-polymer composite
τxy ¼
The geo-polymer composites were fabricated with three types of fabrics, each of different thickness. To acquire the desired uniform thickness of composite samples, it is necessary
where F is the load and b, t are the dimensions of middle cross section of the samples. b ¼ 40 mm for all samples, t¼ 4.47 mm for carbon geo-polymer composite, t¼ 3.73 for
2.3. Density measurements The density of the samples such as geo-composites without heat treatment and with heating test (1000 1C) was determined. Samples having dimension 3 15 220 mm were used for density measurements. All reported density results are the average of measurements from 5 samples. The volume fraction of fabric, matrix and porosity is estimated in fabric reinforced geo-polymer composites using the following equation: nρf Vf ¼ U100% ð1Þ tρf Vm ¼
1 mc nρw U100% tρm Lb
V v ¼ 100 ðV f þ V m Þ
ð2Þ ð3Þ
F bdt
ð7Þ
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E-glass geo-polymer composite, t ¼ 4.63 for basalt geopolymer composite.
2.5. Properties of geo-polymer composite after exposure to elevated temperature The fabric reinforced geo-polymer composites (3 14 90 mm3) were exposed to heat in an electric furnace with the radiant heat source 25 kW/m2. After the heating to a chosen temperature of the range from 200 to 1000 1C and a holding time of 30 min, the samples were cooled down in air to room temperature. The various factors in relation to heating conditions, such as density, flexural strength and modulus (tested in the three point bending), mass loss and thickness of the heat treated composites were investigated.
Fig. 2. Density for geo-polymer composite with incorporation of carbon, E-glass and basalt fabrics.
3. Results 2.6. Surface morphology observation The surface morphology of the samples was examined by a scanning electron microscope (SEM, ZEISS) with field emission source and using a higher resolution optical microscope (OM, NIKON EPIPHOT 200). The elemental analysis was carried out by the Energy Dispersive X-ray Analysis (EDX). Sample fragments were mounted onto aluminum stubs and out-gassed in a desiccator over 48 h period before being coated with 4 nm layer of platinum prior to imaging in the SEM using 30 kV acceleration voltages.
2.7. Out-of-plane thermal conductivity measurements The out-of-plane thermal diffusivity of the rectangular composite samples 12.6 12.6 3 mm3 was determined by a NETZSCH model LFA 447 NanoFlashs using a laser flash technique. The specific heat is determined simultaneously by employing the comparative method. The calibration standard for the specific heat determination was Pyroceram 9606. The out-of-plane thermal conductivity is calculated from these two measured values and from the known bulk density of the sample by means of the formula: κ ¼ ρC p α
ð8Þ
where κ is the thermal conductivity (W m 1 K 1), ρ is the density of the sample (g cm 3), Cp is the specific heat capacity of the samples (J g 1 K 1) and α is the thermal diffusivity (m2 s 1). Thermal conductivity of geo-polymer composites with carbon, E-glass and basalt fabric were determined in temperature range from 25 1C to 250 1C. The samples 12.6 12.6 3 mm3 were coated with graphite in order to increase the absorption of flash light on the front surface and to increase the emissivity on the back surface. Average five samples were measured at each temperature 25, 100, 150, 200 and 250 1C.
3.1. Physical properties of the geo-polymer and fabric reinforced geo-polymer composites Table 2 represents volume fraction of constituents such as matrix, fabric and void content in only geo-polymer and fabric reinforced geo-polymer composite. The sizes of the pores vary from several microns up to maximal width 2.070.2 mm. The porosity in the geo-polymer may be developed due to the curing and drying of the inorganic matrix. The density of the geo-polymer is 2.070.05 g/cm3 with volume shrinkage around 15.570.8%. The volume fractions of voids in carbon and E-glass geopolymer (approx. 56 V%) composites are approximately equal; however there is a remarkable difference observed in basalt geo-polymer (58 V%) composites. Fig. 2 shows the density of fabric reinforced geo-polymer composites without a temperature treatment and after the heating. The density of fabric reinforced geo-polymer composites is lower than that of the matrix (2.0 7 0.05 g/cm3), and for the materials without temperature treatment corresponds to the density calculated using the linear mixture rule and volume fraction values shown in Table 2: ρcomp ¼ V g Uρg þ V gp U ρgp þ V void U0:
ð9Þ
The density of the geo-polymer composites after heating to 1000 1C is lower in comparison to the non-heated material. At higher temperature matrix (geo-polymer) may degrade, so that the volume fraction of the geo-polymer decreases. Pyrolysis of the fiber is prominent in this temperature for the E-glass and basalt fabric. So the density of the composites in case of E-glass and basalt decreases drastically as a result of combined effects from geo-polymer degradation (hence creation of voids) and fabric burn out. 3.2. Mechanical properties of fabrics reinforced composite at room temperature Fig. 3(a–c) represents the tensile diagrams of fabric in warp and weft direction at room temperature. The diagram of carbon
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Fig. 3. (a) Tension diagram of carbon fabric, (b) tension diagram of E-glass fabric, (c) tension diagram of Basalt fabric, (d) Tensile failure of the geo-composite: (a) basalt, (b) carbon, (c) E-glass fabric. Table 3 Tensile properties of geo-polymer composites. Geo-polymer composites with fabrics
Carbon E-glass Basalt
Mechanical properties in a warp direction
Mechanical properties in a weft direction
Rmt [MPa]
E εmt [GPa] [%]
Rmt [MPa]
E εmt [GPa] [%]
191.4 76.3 35.8
12.6 9.6 10.2
190.4 65.6 35.6
12.5 8.5 11.3
2.40 3.19 1.75
2.43 2.72 1.72
fabric composite (Fig. 3a) shows linear dependency up to the strength of carbon with the average modulus of 193 GPa, whereas E-glass fabric displays elongation before stiffening (Fig. 3b). The value 11 GPa corresponds to Fig. 3b for warp, and the strength is 78 MPa different in warp and 68 Mpa weft direction. However basalt reinforced geo-polymer composite shows very brittle behavior, with disturbances/noise or errors during the analysis of the stiffness. Note that the strain was calculated based on the grips displacement of the machine; hence the values of moduli could be underestimated. Carbon reinforced geo-polymer composite shows better mechanical strength compared to E-glass and basalt geo-polymer composites. The breakdown of the basalt reinforced geo-polymer composite that was observed in early stage of applied stress, which may be attributed to the catastrophic failure (or may be due to poor adhesion) of the basalt fabric within matrix. The linear behavior demonstrates the brittle nature of material; low strength could be a result of damage of the specimen surfaces at clamping position/
supports. Fig. 3d shows tensile failure mode of geo-polymer composite with basalt, carbon and E-glass fabric. Basalt fabric reinforced geo-polymer composite exhibits sharp, distinct cutting edges; however carbon and E-glass shows delamination and fiber rupture. Table 3 shows the tensile strength measured for various fabric reinforced geo-polymer composite. The strength and young's modulus values of carbon geo-composite are higher than those of E-glass and basalt geo-composites. The values of elastic modulus and strength of geocomposites reinforced with carbon and basalt fabrics are relatively similar in warp and weft direction. However E-glass composite shows variation in tensile properties rather than in warp direction are better than in weft direction. The strength of basalt geocomposite is too low. 3.3. In plane shear properties of geo-polymer composites Fig. 4(a–c) presents the in-plane shear stress–strain diagram for carbon and E-glass composites with the exception of basalt that failed at very early stage of the experiment. Fig. 4b presents the distribution of principal strains in the middle zone of the carbon fabric reinforced geo-polymer composite. The direction of strains on the optical image indicates non uniform nature of the shear deformation. Failure mode of geo-polymer composites samples for carbon and E-glass are presented in Fig. 5. The images show the delamination with narrow band of swelling, that is due to the both compression and tension as depicted in the scheme shown in Fig. 5. The specimen is buckled and delaminated in the compression (Fig. 6). The shear resistance is still present after this failure (see Fig. 4b) because the fibers in the specimen still carry load or may be due to friction.
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Fig. 4. (a) Geo-polymer specimen, holding apparatus and experiment technique, (b) in-plane shear-strain diagrams of the geo-composite, and (c) the first principal strain field of geo-polymer composite from DIC camera.
3.4. Thermal properties The essential characteristic of geo-polymer materials is their resistance to high temperature, so the materials are ideal for high temperature applications. Fig. 7 displays the results of the differential thermal analysis (DSC) for the geo-polymer and carbon, E-glass and basalt fabrics. Matrix geo-polymer
exhibits two exothermic peaks around the temperature 50– 100 1C and at 250–300 1C. The first peak may be explained by the dehydration and degassing of binder as chemicals. The second exothermic peak shows deformation or degradation of geo-polymer. This observation implicates that fabric are stable at high temperature (500 1C), whereas the matrix geo-polymer is very unstable and swells with cracks.
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Fig. 5. Faliure mode of geo-composite under shear load.
Fig. 6. Faliure mode of geo-composite (a) with carbon, (b) E-glass, and (c) basalt fabric in inter-laminar shear strength measurement.
exhibit increase in modulus at higher temperature. This may be possibly attributed to the strong adhesion exhibited between carbon fabric and geo-polymer matrix. Fig. 9a shows the thickness expansion of various fabric reinforced geo-polymer composites after heat treatment and subsequent cooling. Carbon reinforced composite showed less expansion in comparison to E-glass and basalt reinforced geopolymer composite, for which the thickness of the samples increases drastically. Fig. 9b represents mass retention of fabric reinforced composites in function of temperature. Mass loss is observed in all three types of composites, carbon geocomposite remains stable after 600 1C; however, it is more stable and less loss for E-glass and basalt reinforced composite. 3.6. Out-of-plane thermal conductivity Fig. 7. Thermal properties (differential scanning calorimetry) of geo-polymer and carbon, E-glass and basalt fabrics as function of temperature.
3.5. Mechanical testing of fabric reinforced geo-polymer composites after exposure to elevated temperature Samples were heat treated in the range from 200 to 1000 1C for the duration of 30 min. The mechanical properties of the heat-treated samples are shown in Fig. 8(a–b). Fig. 8a shows the flexural strength of geo-polymer composites after exposure to high temperature. The flexural strength for all the geo-polymer composite decreases up to 600 1C, then the increase in strength observed in carbon reinforced composite. Fig. 8b represents the flexural modulus of various composites with exposure to different temperatures. Fabric reinforced composite shows decrease in flexural modulus up to 600 1C. After 600 1C, flexural modulus increases at higher temperature in carbon and E-glass fabric reinforced geopolymer composite. Basalt and E-glass geo-polymer composite shows fall in flexural modulus at 1000 1C; however carbon
Thermal insulation properties of the geo-polymer composites were assessed by measuring the thermal conductivity at elevated temperature. Fig. 10(a–c) shows specific heat, thermal diffusivity and thermal conductivity and of the fabric reinforced geo-polymer composites as the function of temperature of exposure. The relative error standard bar (coefficient of variation) of the data points is 1% from the standard deviation of the data determined for average five samples. The specific heat capacity increases with temperature and is shown in Fig. 10a. But specific heat gradient of carbon geocomposite to temperature achieves maximum value while those of two the other seemed much lower. Specific heat capacity rises almost linearly from 950 to 1650 J/(kg K) over the temperature range. Similarly thermal diffusivity of the geo-composite decreases with respect to temperature. Fig. 10b shows the thermal diffusivity values decreases with temperature and remain constant at 250 1C. Thermal conductivity of the geo-polymer composites is almost constant for carbon and glass reinforced geo-polymers and decreases with the increase of the heat treatment temperature for basalt. The values obtained match
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Fig. 8. (a) Flexural strength and (b) thickness as a function of firing temperature for fired fabric reinforced geo-composites.
Fig. 9. (a) Elastic modulus strength and (b) mass loss as a function of firing temperature for fabric reinforced geo-composites.
well with thermal insulation materials such as brick (1.31 W/ m k), soil/clay (1.1 W/m k) and concrete (0.4–0.7 W/m k) [21].
3.7. Micro-structural evolution Samples of geo-polymer and fabric reinforced geocomposite and geo-composites treated at elevated temperature were cut cross section wise. SEM test were carried out on the samples collected from the center of cross section that are considered for testing of mechanical strength. Fig. 11(a–b) shows the changes in the microstructure of the geo-polymer before and after heat treatment. On increasing the temperature from 600 to 1000 1C, the cracks paths and surface deformation of the matrix is observed (Fig. 11c–e).
Geo-polymer fabric composites before heat treatment show impregnation of the matrix within the fabric layers and within the yarn. (Figs. 12a, 13a and 14a). Geo-polymer heated at 600 1C (Figs. 12d, 13d and 14d) show slight deformation/ degradation and cracks on the surface morphology. The deformation/degradation is more prominent at 800 1C with larger cracks. However at 1000 1C, the geo-polymer shows melting stage of the matrix. Fig. 12(a–f) shows the microstructure of the carbon fabric reinforced geo-polymer composite at various temperature range firing test from 200 to 1000 1C. Carbon reinforced fabric geo-polymer composite at room temperature shows the good packing of the matrix within fabric layers. At 200 1C, there is no remarkable change in the composite. Melting and initial dissolution of the geo-polymer begins; as a result necking of the geo-polymer matrix happens
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Fig. 10. (a) Specific heat capacity as the function of temperature for fabric reinforced geo-composites, (b) thermal diffusivity as the function of temperature for fabric reinforced geo-composites, and (c) thermal conductivity as the function of temperature for fabric reinforced geo-composites.
within the fabric layers at 400 1C. On increasing towards higher temperature (600 1C), due to melting and shrinkage the porosity and voids are created between fabric and geo-polymer matrix. Debonding between fiber and matrix increases during heating may be due to the shrinkage and dehydration of the matrix. At 800 1C, the pyrolysis of the fabric begins. The volatilization of fiber and matrix deformation results in the voids (Fig. 15b and c) in the geo-polymer composite. At the highest temperature (1000 1C) the carbon geo-composite surface looks more homogeneous and an oxidized layer is more prominent in the outer surface. Carbon reinforced fibers before and after heat treatment show good adhesion although minor cracks are observed in the composite. Cracks may have been created during dehydration at higher temperature forming a pathway for moisture. The partial carbon fiber oxidizing of the outer layers at temperature higher 400 1C (outer surface of carbon fabric reinforced geo-polymer composite forms a white layer, which indicates oxidizing process, as the heat treatment were carried out in air). It was estimated that approximately
14 wt% (Fig. 9b) of carbon fibers is volatized (the percentage of fabric loss deduced from mass retention of geo-polymer composite as the function of temperature). Fig. 13 shows the E-glass fabric reinforced geo-polymer composite at room temperature and after exposure to heat in the temperature range from 200 to 1000 1C. At 200 1C, there is no change in the geo-polymer composite. Similarly to carbon and basalt geo-polymer composites, initiation of dissolution of matrix begins within fabric at 400 1C. At 600 1C, the matrix geo-polymer debonding and partial dissolution of the E-glass fabric is observed that may be arises from the lack of impregnation of matrix geo-polymer in the fabrics. E-glass fiber detachment from the matrix of the geo-polymer is observed, which may be due to the wide difference thermal expansion between matrix (12.3 10 6) and E-glass (9.18 10 6) at higher temperature [22,23] (Fig. 15b). Fig. 14 shows the basalt fabric reinforced geo-polymer composite at room temperature and after exposure to heat in the temperature range from 200 to 1000 1C. Basalt reinforced
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Fig. 11. SEM images of (a) geo-polymer at RT, (b) geo-polymer heat treated at 200 1C, (c) geo-polymer heat treated at 600 1C, (d) geo-polymer heat treated at 800 1C, and (e) geo-polymer heat treated at 1000 1C.
Fig. 12. SEM images of (a) carbon fabric reinforced geo-polymer composites (RT), (b) carbon-reinforced geo-polymer composite (200 1C), (c) carbon reinforced geo-composite (400 1C), (d) carbon reinforced geo-polymer composite at 600 1C, (e) carbon reinforced geo-polymer composite (600 1C), and (f) Carbon reinforced geo-polymer composite at 1000 1C.
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Fig. 13. (a) E-glass fabric reinforced geo-polymer composite (RT), (b) E-glass reinforced geo-polymer composite (200 1C), (c) E-glass reinforced geo-composite (400 1C), (d) E-glass reinforced geo-composite at 600 1C, (e) E-glass reinforced geo-composite (600 1C), and (f) E-glass reinforced geo-composite at 1000 1C.
fabric in geo-polymer composite is very well packed at room and 200 1C. Similar to other geo-polymer composites discussed in Fig. 13 phenomena like dissolution and voids creation are observed such as 400 and 600 1C, respectively. However, at 800 1C, deformation/degradation of the geopolymer matrix is observed in the composite which induces creation of more porosity. The composite becomes concrete-like structure at higher temperature. Basalt reinforced geo-polymer composite showed good adhesion and interaction between basalt fiber and geo-polymer. The interaction during the heat treatment may be attributed towards the mineral composition of basalt and geo-polymer composition. The ionic exchange between the basalt fiber and matrix has been observed in developing agglomeration and coarse concrete like structure. In case of E-glass and basalt fabric, pyrolysis (Fig. 15b–c) of the part of the fabric in the geo-polymer composite observed at elevated temperature at 1000 1C. The pyrolysis of fabric is revealed in the microstructural evolution of the basalt and E-glass composite. Heat-treated E-glass fabric reinforced composite has shown no fiber bridging and cracking. Elongated pores in the E-glass composite arise due to less
oxygen or adhesion between matrix and fiber that induces cracking behavior. However carbon reinforced composite showed the homogeneity, good adhesion behavior with good mechanical strength towards elevated temperature (Fig. 15a– c). Fig. 16 shows the effect of temperature as the function of fiber diameter. E-glass reinforced fiber geo-polymer composite shows maximum increase in thickness, that may arise from the contribution of both matrix and E-glass fiber swelling. 4. Discussion: carbon fabric reinforced geo-polymers Carbon fabric reinforced geo-polymer composites performed the best among the composites tested, and were found to be a suitable geo-polymer composite for application at elevated temperatures, with good maintaining of physical and mechanical properties after exposure to temperatures over 600 1C. Overall the properties of the composites change drastically after exposing up to higher than 600 1C, with the flexural strength remaining only 37% and elasticity modulus approximately 20% compared to the original ones. It can be seen from Fig. 13 that the non-adhesion behavior of the fiber
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Fig. 14. (a) Basalt fabric reinforced geo-polymer composite (RT), (b) Basalt reinforced geo-composite (200 1C), (c) Basalt reinforced geo-composite (400 1C), (d) Basalt reinforced geo-composite at 600 1C, (e) Basalt reinforced geo-composite (600 1C), and (f) Basalt reinforced geo-composite at 1000 1C.
and geo-polymer matrix is observed, resulting in pores and cracks in the matrix. The reason for the maximum reduction at 600 1C is unclear although there is a suspicion that the difference of thickness expansion of fabric and matrix could be increased with respect to temperature (Fig. 9a), which should be further investigated. However, for carbon fabric geopolymer composites, when the temperature of heating was raised further (4 600 1C), the mechanical properties of the composites recovered (Fig. 8a) because may be the adhesion was improved and initial reaction layer might be created, so the flexural strength gained 54% and remained around 50% after heating to 800 1C and 1000 1C, respectively, meanwhile the flexural modulus was 67% of those of composites at room temperature (Figs. 5a and 6a). Carbon fiber exhibits good adhesion between fiber and matrix in compare with E-glass fiber. The physical and chemical properties of the carbon fabric reinforced geo-polymer composite remained stable at high temperature. This may be beneficial for improving the strength towards higher temperature. The lower density samples contain less water as they had a lower portion of the hydrated geopolymer. Water evaporates at high temperature. This absorbs energy from the furnace that would have otherwise been
imparted to the sample. This is beneficial for materials as it reduces the temperature of the sample. Thermal conductivity is an intrinsic property of a material and remains relatively constant for the duration of heat treatment. Carbon fibers showed best adhesion to the geopolymer matrix, whereas the E-fiber showed the pull out and volatilization within the matrix at high temperature. Basalt fiber may induce the chemical reaction within matrix, judging by agglomeration and sintering observed in the geo-polymer composite. The final product with basalt reinforced composites became hard and porous in nature. The strong bonding and effect of carbon fiber bridging with the matrix hinders the pull out and increases the strength of the material. Amongst these fabrics, carbon fabric not only offers maximum strength but also boost the thermal conductivity at elevated temperature that is the crucial point from tribology point of view. The rapid dissipation of frictional heat produced at the contacts protects the matrix from degradation. Moreover, in general carbon fibers help in imparting additional lubricity because of layerlattice structure of lubricity [24]. The carbon reinforced composite retains more than 85% of the average mass after the heating.
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Fig. 15. (a) Carbon fabric reinforced geo-polymer composite (1000 1C) shows homogenous layer, (b) E-glass fabric reinforced geo-polymer composite (1000 1C shows de-bonding and elongated pores and (c) Basalt fabric shows pyrolysis of fiber (voids) in geo-composite.
of mass loss is observed may be due to volatilization of fibers and losses of water, organic additives and hydroxyl group losses. In addition, experimental findings show that composites based on geo-polymers are very good at thermal dimensional stability. Judging by SEM images, carbon reinforced geopolymer composite seems to have good adhesion between the matrix with the fabric layers, being able to control microcracks propagation along the matrix and creating a fiber bridging effect. At elevated temperature carbon reinforced geo-polymer composite exhibited higher strength, better homogeneity, and can be proven as one of the candidate of geopolymer composite for high temperature application. This composite can be used as thermal insulation and fire resistance properties at elevated temperature with wide potential applications. Fig. 16. Fiber diameter as the function of heating temperature.
5. Conclusion Fabric reinforced composite materials based on sustainable geo-polymer matrix has been investigated on the various aspects at elevated temperature. The combined performance in terms of physical, mechanical properties of the fabrics (multi-layer) reinforced geo-polymer at elevated temperature has been investigated. The geo-polymer resins can protect carbon fibers from oxidation; however, approximately 14 wt%
Acknoweldgements This work was supported by ESF operational programme “Education for Competitiveness” in the Czech Republic in the framework of project “Support of engineering of excellent research and development teams at the Technical University of Liberec” No. CZ.1.07/2.3.00/30.0065. References [1] J. Davidovits, Geopolymers:inorganic polymeric new materials, J. Ther. Anal. 37 (1991) 1633–1656.
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