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Effects of carbon fiber on mechanical and electrical properties of fly ash geopolymer composite
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 14017–14025
www.materialstoday.com/proceedings
SACT 2016
Effects of carbon fiber on mechanical and electrical properties of fly ash geopolymer composite Panjasil Payakanitia,*, Supree Pinitsoonthornb,c, Prasit Thongbaib,c, Vittaya Amornkitbamrungb,c, Prinya Chindaprasirtd a
Materials Science and Nanotechnology Program, Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand Integrated Nanotechnology Research Center, Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand c Nanotec-KKU Center of Excellence on Advanced Nanomaterials for Energy Production and Storage, Khon Kaen University, Khon Kaen 40002, Thailand d Sustainable Infrastructure Research and Development Center, Department of Civil Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen 40002, Thailand b
Abstract High calcium fly ash geopolymer composite was prepared by incorporating short carbon fiber in the geopolymer matrix. The carbon fiber (CF) content was varied from 0 to 1%, the liquid to ash ratio was controlled at 0.5. The workability, mechanical and electrical properties of the geopolymer composites were examined. The result showed that the incorporation of 0.5%CF and above resulted in the ductile failure of specimen. The increasing CF content also resulted in decreasing workability and mechanical strength. The electrical resistivity was also reduced and was more stable to the curing age and temperature. The cyclic voltammetry (CV) curve of 60°C curing provided less peaks than that of 25°C. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of SACT 2016. Keywords: Geopolymer; Alkaline activated binder; Mechanical Properties; Electrical properties
1. Introduction Geopolymer is an alumina-silica based binder material, invented to supplement the ordinary Portland cement (OPC) usage. The major issue of OPC is that OPC production does not only consume huge amount of energy and
* Corresponding author. E-mail address: [email protected], [email protected] (S. Pinitsoontorn) 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of SACT 2016.
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raw materials. It is also release significantly high amount of green house gases to the atmosphere, especially carbon dioxide (CO2) [1]. On the other hand, the starting materials for geopolymer preparation are waste, recycled and byproduct materials. As a consequence, much less energy is required and the gas releasing from the production processes is lower [2]. Some examples of the starting materials are metakaolin, rice husk ash, fly ash, glass waste, phosphorus slag and blast furnace slag [3]. Among these materials, fly ash (FA) is one of the most favorable raw materials due to its high chemical reactivity for geopolymerization to form the three dimensional structure of alumina and silica. Also, FA is widely available since it is a by-product from coal burning in power plant. Besides the raw materials, alkaline activators such as alkaline hydroxide and alkaline silicate, are needed for generating geopolymerization reaction. Overall, geopolymer provides excellent mechanical and thermal properties, high durability and low shrinkage. However, it still has a drawback about brittleness similar to OPC. Therefore, reinforcing materials are usually added in the geopolymer matrix to enhance toughness and other specific properties. The reinforcing materials can be either particulate, short fiber or continuous fiber. Considering cost efficiency, ease of handling and manufacturing, short fibers are most favorable for civil engineering applications [4,5]. There are many experiments about geopolymer composite reinforced with carbon-based fiber such as cotton fiber, polyvinyl alcohol (PVA), polyvinyl chloride (PVC), polypropylene (PP), carbon nanotube (CNT) and carbon fiber (CF) [6-8]. With an appropriate concentration, the fibers improve the mechanical properties of geopolymer composites, viz. compressive strength, flexural strength and toughness. In the case of CNT and CF, their valuable properties for improving electrical properties of geopolymer are also remarkable. Conductive binding materials recently gain interest because of their unique properties. They can be use in wide applications, lightning protection and building grounding, electromagnetic shielding for electronics in power plant and telecommunications and deicing construction such as airport runway, bridge, road and footpath [9].Generally, CF and CNT would be selected over the other carbon-based materials for the electrical improvement due to their low electrical resistivity. Saafi et al. [10] added multi-walled carbon nanotube (MWCNT) in geopolymer matrix. It was claimed that the conductivity of MWCNT/geopolymer composite was increased and reached the maximum value at 1 wt% MWCNT. Also, Kusak et al. [11] found that electrical capacity of MWCNT/geopolymer composite increased after high MWCNT content was added. Above all, it was mentioned that the conductivity of geopolymer is influenced by factors such as moisture content, pore volume, size and connectivity, ions concentration and degree of geopolymerization[10,11]. In this work, short CF was used for adding in geopolymer matrix CF concentration, curing temperature and aging time were varied and compressive strength was measured using a universal testing machine (UTM). For electrical properties determination, I-V measurement and cyclic voltammetry (CV) were carried out. Microstructure of CF/geopolymer composite was investigated under a scanning electron microscope (SEM). 2. Experimental procedure 2.1 Material A high calcium fly ash (FA) (ASTM C618) was obtained from lignite Mae Moh power plant in Lampang, Thailand. Commercial chopped CF was purchased from New bright carbon fiber products Co., Ltd, Zhejiang, China. The properties of CF are shown in Table 1. The alkaline activating solutions were 10M sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). NaOH was prepared by dissolving 98% caustic soda flake (AGC Chemicals Co., Ltd, Thailand) with deionized (DI) water, while Na2SiO3 composed of 12.53 wt% Na2O, 30.24 wt% SiO2 and 57.23 wt% H2O. Table 1. Properties of the purchased short CF. Properties Fiber diameter (μm) Carbon content (%) Density (g/cm3) Tensile strength (GPa) Tensile modulus (GPa) Elongation (%) Volume resistivity (Ω cm)
Value 7 ≥95 1.6-1.76 2.6-3.8 220-240 1.5 1.5 x 103
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2.2 Sample preparation CF/geopolymer composite was prepared with fixed liquid to solid ratio (L/A) at 0.5. The composite was firstly prepared by dispersing CF in sodium hydroxide (NaOH) for 5 minutes manually to avoid agglomeration. FA was added in the mixing bowl and mixed at high speed (1400 rpm) for 5 minutes. Then, sodium silicate (Na2SiO3) was added and mixed at the same speed for another 5 minutes. The ratio of Na2SiO3 to NaOH was kept at 1 and the CF content was varied from 0 to 1 wt%. Next, the mixed slurry was poured into acrylic molds and vibrated for ten seconds to eliminate pores. The specimen size for mechanical testing was 2.5 x 2.5 x 2.5 cm3. For the electrical testing, the specimens were prepared followed Kim et al. [12]’s methods. They were casted in 2.5 x 2.5 x 10 cm3 acrylic mold. Then, left in ambient for 5 minutes 4 well-polished brass electrodes were inserted at the middle of the specimens. The sizes of the electrodes were 1 x 10 cm2 and the space between each probe was 1.2 cm. At 60 minutes after the mixing process started, the specimens were wrapped with cling film to protect them from moisture loss. The specimens were cured at 25°C and 50% humidity in a control room or 60°C in an electric oven. After 24 hours of curing, the specimens were taken out from the oven, left in ambient until cooled down. Then, they were demolded and wrapped again with cling film, and kept in the control room for 7, 14 and 28 days for testing. 2.3 Characterization techniques The chemical composition of FA was determined by X-ray fluorescence spectroscopy (XRF, S8 Tiger, Bruker) and X-ray diffraction (XRD, PANalytical, Empyrean, USA). Microstructure images of FA was investigated using SEM (Zeiss LEO 1450VP, UK) and the particle size distribution was tested using (Mastersizer 3000, Malvern, UK) The workability of geopolymer composite was determined to find an optimized condition for mixing, by miniature slump (mini-slump) cone test [13,14]. The testing was designed according to ASTM 143. The freshly mix geopolymer was pour in a truncated conical mold, vertically lifted and left to spread. Then, three diameters of the spreaded geopolymer paste were measured at 30 s, 1, 2, 3, 4 and 5 min. The diameters were averaged and the workability was calculated using this following equation %
=
× 100
Equation 1
where %W is workability, d is the averaged diameter and d0 is the bottom diameter of the conical molds. The mechanical testing was done at the age of 7 days. A universal testing machine (UTM, CY 6040A1, Chun Yen Testing Machines Co., Ltd., Taiwan) was used to apply 50 N/min compression load to the specimens (ASTM C109/C109M-13). Post-cracking specimens were collected and used for further chemical and microstructural examinations. An alternative current (AC) with the maximum current of 100 mA was applied for I-V measurement (VersaLabTM, Quantum Design, Inc, USA) and cyclic voltammetry (CV, Gamry 3000 auxiliary, USA). For I-V measurement, the applied frequency was in the range of 0.3-100 Hz. Resistivity was calculated from the slope of the I-V curve using equation 2. For CV, the scan rate was varied between 5 and 100 mV/s. =
Equation 2
3. Results and discussion 3.1 Raw materials From XRF result in Table 2, calcium oxide (CaO) content was 24.2795%, so it was confirmed that the FA is a Class C, according to ASTM C618. Particle size distribution (Fig. 1) showed a wide distribution from a few micron to thousands micron. The mean size of FA particles was 21.6 μm with the standard deviation of 0.639. The XRD pattern (Fig. 2) showed the broad peak indicating that the FA was amorphous and the peak was generally broad. Some crystalline phases were detected. Anhydrite (A) was the tallest peak and the others were quartz (Q), hematite (F), calcium oxide (C) and mullite (M).
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P. Payakaniti et al./ Materials Today: Proceedings 5 (2018) 14017–14025 Table 2. Chemical composition of FA. Oxide SiO2 CaO Al2O3 Fe2O3 SO3 K2O MgO Na2O
Mass (%) 34.0610 24.2795 16.5085 13.9696 4.3136 2.4679 1.9776 1.2102
Fig. 1. Particle size distribution of FA.
Fig. 2. XRD pattern of Mae Moh FA.
3.2 Rheology of CF/geopolymer composite Fig. 3 shows that the workability of 0%CF was highest at 244%. With the increase in CF content in the geopolymer matrix, the workability continuously reduced. At 0.5%CF, the workability was 225% and the lowest value was 176% at 1%CF. It was found that the suitable value for workability of CF/geopolymer was between 150 and 250% [15]. Thereby, the added CF content in the present work is appropriate for the flow of the paste. However, if the added CF content was increased, the manufacturing cost will be higher and might not practical for industrial scale manufacturing.
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Fig. 3. Workability of L/A 0.5 geopolymer composite.
3.3 Mechanical properties of CF/geopolymer composite The mechanical strength of 7 days geopolymer composites, cured at 60°C is shown in Fig. 4. The compressive strength was slightly reduced with higher concentration of CF. The strength of 0%, 0.5% and 1%CF were 81.0, 67.3 and 59.4 MPa, respectively. The decreasing in compressive strength probably affected by clustering of the CF during the fabrication processes [16,17]. However, at 0.5%CF and above, the post-cracking specimens still stuck together. This was resulted from bridging effect of CF and bonding between the fiber and the matrix. It could be referred that the behavior of geopolymer was less brittle and provided more plastic deformation.
Fig. 4. Compressive strength of L/Aa0.5 geopolymer composite.
3.4 Electrical properties of CF/geopolymer composite I-V measurement of CF/geopolymer composite is shown in Fig. 5 and Fig. 6. It is clear that the resistivity was reduced by adding CF and the value was lower when higher CF content was introduced to the geopolymer matrix. The resistivity of 28-days age geopolymer is reported in Fig. 5. It can be seen that the curing temperature influenced the resistivity of the composite at low CF concentration. However, the effect of curing temperature on the resistivity diminished after the CF content was 0.4% or higher. For 25°C curing, the resistivity of 0% was 320.42 Ω cm, 0.3% was 208.33 Ω cm, 0.6% was 31.04 Ω cm and 0.9% was 34.79 Ω cm. In the case of 60°C, the resistivity was 4466.04, 328.96, 78.75 and 22.92 Ω cm for 0, 0.3, 0.6 and 0.9%CF, respectively. The 60°C cured specimens obviously had much higher resistivity than 25°C cured specimen due to their higher degree of geopolymerisation. At 0.4%CF concentration, the resistivity of composite was significantly lower for both curing temperature.
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Fig. 5. Effect of CF content to resistivity of geopolymer composite, measured at 28 days.
When comparing the effect of curing day on resistivity, it is obviously that the resistivity of 25°C curing increased from 7 days to 14 days as shown in Fig. 6(a). For example, the resistivity of 0%CF at 7 days was 92.29 Ω cm and increased more than two times to 200.63 Ω cm at 14 days. However, there were differences of resistivity increasing for the specimens curing from 14 days to 28 days age. Considering in increment of the resistivity, they can be divided into 3 groups. The first group was 0-0.3%CF, the resistivity was still clearly increased and the highest resistivity belonged to 0%CF at 28 days with the resistivity of 320.42 Ω cm. The second group was 0.40.5%CF with moderate increase in the 28 days resistivity. The last group was 0.6-1%CF with only slight increase in resistivity. The lowest resistivity was 1%CF with the 28 days resistivity of 30.42 Ω cm. In the case of 60°C curing (Fig. 6(b)), only 0 and 0.1%CF presented the changes from 7 to 28 days curing age. At 7 days, the resistivity of 0%CF was 2556.25 Ω cm and that of 0.1%CF was 1736.25 Ω cm. By 28 days their resistivities were 4466.04 and 269.76 Ω cm, respectively. Therefore, it in confirmed that the degree of geopolymerization was better at 60°C curing temperature, since the resistivity was nearly unchanged compared with that of 25°C curing.
Fig. 6. Effect of curing age to electrical resistivity of geopolymer composite curing at (a) 25°C and (b) 60°C.
The CV measurement result was illustrated in Fig. 7 (25°C) and Fig. 8 (60°C). From the figure, it is obviously that the CV curve of 25°C contains a lot more peaks than 60°C in every CF concentration. According to Haynes [18,19], the peaks corresponded to the chemical reactions in geopolymer. There are two possible reactions that occur inside the geopolymer which are a redox reaction of 2H2O + 2e- ↔ H2(g) + 2OH- or a redox reaction of Na+ ions,
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trapped inside the geopolymer structure (Na+ + e- ↔ Na(s)). Besides the number of peaks, the breadths of peaks also changes with different curing temperatures and CF contents. The amplitudes of 25°C curves were larger than those of 60°C at the same concentration and curing temperature. The curves were narrower at higher CF concentration. It could be explained that at 60°C curing, geopolymerization of the geopolymer matrix was better formed. Moreover, the curves at higher CF were smaller probably because the conductivity of CF was predominant than the conductivity of geopolymer matrix [15]. Furthermore, the scanning rate also influences the curves. The curves with slower scan rate were smaller than the faster one. Also, the peaks and troughs shifted to the higher voltage when the higher scanning rate was applied.
Fig. 7. CV curve of 25°C curing CF/geopolymer with (a) 0%CF, (b) 0.2%CF, (c) 0.4%CF and (d) 0.9%CF.
Fig. 8. CV curve of 60°C curing CF/geopolymer with (a) 0%CF, (b) 0.2%CF, (c) 0.4%CF and (d) 0.9%CF.
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3.5 Microstructure of CF/geopolymer composite The microstructure of the geopolymer composite was investigated under SEM in Fig. 9. Fig. 9(a) shows that there were unreacted spherical particles of FA all over the fracture surface of the matrix. For the CF added geopolymer, it can be seen that the CFs were aligned in random direction without agglomeration. Fig. 9(b)-(f) show that although the CF content was higher, the fiber still did not agglomerate. These can prove that the mixing processes did solve the CF agglomeration problem. Also, when CFs content was higher electron paths shorten as distance between adjacent CF was narrower and resulted in the decreasing in electrical resistivity.
Fig. 9. SEM images of L/A 0.5 CF/geopolymer containing (a) 0%CF, (b) 0.2%CF, (c) 0.4%CF, (d) 0.6%CF, (e) 0.8%CF and (f) 1%CF.
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4. Conclusion Geopolymer was composited with 0-1%CF at L/A ratio of 0.5. The workability of CF/geopolymer composite was decreased with increasing CF concentration. The mechanical and electrical properties measurements were carried out. The compressive strength test showed that the geopolymer without CF provided the highest strength and the strength slightly decreased with increasing CF concentration. However, at the CF content of 0.5% and over, the post-cracking specimen pieces did not separate from each other and its behavior was more ductile. The electrical determination showed that both curing temperature and CF contents affected the geopolymer composites’ resistivity. The electrical resistivity of 60°C was higher and more stable with time than that with 25°C. Also, less peaks and troughs of CV curves was found for 60°C specimens. These were affected from the higher degree of geopolymerization of 60°C geopolymer composite. Acknowledgements This work was supported by the Thailand Research Fund (TRF) and Khon Kaen University under the TRF Senior Research Scholar, Grant No. RTA5780004, the Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program of Center of Excellence Network. References 1. Kupaei RH et al. Mix design for fly ash based oil palm shell geopolymer lightweight concrete. Constr Build Mater 2013;43: 490-496. 2. Shaikh FUA. Review of mechanical properties of short fibrereinforced geopolymer composites. Constr Build Mater 2013;43: 37–49. 3. Author. Handbook of Alkali-activated Cements, Mortars and Concretes. In: Pacheco-Torgal F et al., editors. Handbook of Alkali-activated Cements, Mortars and Concretes, Cambridge: Woodhead Publishing; 2015. 4. Masi G et al. The effect of organic and inorganic fibres on the mechanical and thermal properties of aluminate activated geopolymers. . Compos Part B-ENG 2015;76: 218-228. 5. Matthews FL,Rawlings RD. Composite Materials: Engineering and Science. London: Chanpman & Hall; 1999. 6. Alomayri T et al. Characterisation of cotton fibre-reinforced geopolymer composites. Compos Part B-ENG 2013;50: 1-6. 7. Bernal S et al. Performance of an alkali-activated slag concrete reinforced with steel fibers. Constr Build Mater 2010;24: 208-214. 8. Ranjbar N et al. A Comprehensive Study of the Polypropylene Fiber Reinforced Fly Ash Based Geopolymer. Plos One 2016;1-20. 9. Chung DDL. Multifunctional Cement-Based Materials. New York: CRC Press; 2003. 10. Saafi M et al. Multifunctional properties of carbon nanotube/fly ash geopolymeric nanocomposites. Constr Build Mater 2013;49: 46-55. 11. Kusak I et al. Electric conductivity changes in geopolymer samples with added carbon nanotubes. Procedia Engineering 2016;151: 157-161. 12. Kim HK et al. Enhanced effect of carbon nanotube on mechanical and electrical properties of cement composites by incorporation of silica fume. Composite Structures 2014;107: 60-69. 13. Kantro DL. Influence of Water-Reducing Admixtures on Properties of Cement Paste - A Miniature Slump Test. Cement Concrete And Aggregates 1980;2: 95-102. 14. Mebrouki A et al. A Self-Compacting Cement Paste Formulation using Mixture Design. Journal of Applied Sciences 2009;9: 4127-4136. 15. Payakaniti P et al. Electrical conductivity and compessive strength of carbon fiber reinforced fly ash geopolymeric composites. CONSTR BUILD MATER 2017;135: 164-176. 16. Vilaplana JL et al. Mechanical properties of alkali activated blast furnace slag pastes reinforced with carbon fibers. construction and Building Materials 2016;116: 63-71. 17. Zhang H et al. Fiber Reinforced Geopolymers for Fire Resistance Applications. Procedia Engineering 2014;71: 153 – 158. 18. Haynes WM. CRC Handbook of Chemistry and Physics. 93rd ed. FL: CRC Press; 2012. 19. Haynes WM et al. CRC Handbook of Chemistry and Physics: A ready Reference Book of Chemical and Physical Data 92nd ed. Boca Raton : CRC, cop.; 2011.
ScienceDirect Materials Today: Proceedings 5 (2018) 14017–14025
www.materialstoday.com/proceedings
SACT 2016
Effects of carbon fiber on mechanical and electrical properties of fly ash geopolymer composite Panjasil Payakanitia,*, Supree Pinitsoonthornb,c, Prasit Thongbaib,c, Vittaya Amornkitbamrungb,c, Prinya Chindaprasirtd a
Materials Science and Nanotechnology Program, Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand Integrated Nanotechnology Research Center, Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand c Nanotec-KKU Center of Excellence on Advanced Nanomaterials for Energy Production and Storage, Khon Kaen University, Khon Kaen 40002, Thailand d Sustainable Infrastructure Research and Development Center, Department of Civil Engineering, Faculty of Engineering, Khon Kaen University, Khon Kaen 40002, Thailand b
Abstract High calcium fly ash geopolymer composite was prepared by incorporating short carbon fiber in the geopolymer matrix. The carbon fiber (CF) content was varied from 0 to 1%, the liquid to ash ratio was controlled at 0.5. The workability, mechanical and electrical properties of the geopolymer composites were examined. The result showed that the incorporation of 0.5%CF and above resulted in the ductile failure of specimen. The increasing CF content also resulted in decreasing workability and mechanical strength. The electrical resistivity was also reduced and was more stable to the curing age and temperature. The cyclic voltammetry (CV) curve of 60°C curing provided less peaks than that of 25°C. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of SACT 2016. Keywords: Geopolymer; Alkaline activated binder; Mechanical Properties; Electrical properties
1. Introduction Geopolymer is an alumina-silica based binder material, invented to supplement the ordinary Portland cement (OPC) usage. The major issue of OPC is that OPC production does not only consume huge amount of energy and
* Corresponding author. E-mail address: [email protected], [email protected] (S. Pinitsoontorn) 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of SACT 2016.
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raw materials. It is also release significantly high amount of green house gases to the atmosphere, especially carbon dioxide (CO2) [1]. On the other hand, the starting materials for geopolymer preparation are waste, recycled and byproduct materials. As a consequence, much less energy is required and the gas releasing from the production processes is lower [2]. Some examples of the starting materials are metakaolin, rice husk ash, fly ash, glass waste, phosphorus slag and blast furnace slag [3]. Among these materials, fly ash (FA) is one of the most favorable raw materials due to its high chemical reactivity for geopolymerization to form the three dimensional structure of alumina and silica. Also, FA is widely available since it is a by-product from coal burning in power plant. Besides the raw materials, alkaline activators such as alkaline hydroxide and alkaline silicate, are needed for generating geopolymerization reaction. Overall, geopolymer provides excellent mechanical and thermal properties, high durability and low shrinkage. However, it still has a drawback about brittleness similar to OPC. Therefore, reinforcing materials are usually added in the geopolymer matrix to enhance toughness and other specific properties. The reinforcing materials can be either particulate, short fiber or continuous fiber. Considering cost efficiency, ease of handling and manufacturing, short fibers are most favorable for civil engineering applications [4,5]. There are many experiments about geopolymer composite reinforced with carbon-based fiber such as cotton fiber, polyvinyl alcohol (PVA), polyvinyl chloride (PVC), polypropylene (PP), carbon nanotube (CNT) and carbon fiber (CF) [6-8]. With an appropriate concentration, the fibers improve the mechanical properties of geopolymer composites, viz. compressive strength, flexural strength and toughness. In the case of CNT and CF, their valuable properties for improving electrical properties of geopolymer are also remarkable. Conductive binding materials recently gain interest because of their unique properties. They can be use in wide applications, lightning protection and building grounding, electromagnetic shielding for electronics in power plant and telecommunications and deicing construction such as airport runway, bridge, road and footpath [9].Generally, CF and CNT would be selected over the other carbon-based materials for the electrical improvement due to their low electrical resistivity. Saafi et al. [10] added multi-walled carbon nanotube (MWCNT) in geopolymer matrix. It was claimed that the conductivity of MWCNT/geopolymer composite was increased and reached the maximum value at 1 wt% MWCNT. Also, Kusak et al. [11] found that electrical capacity of MWCNT/geopolymer composite increased after high MWCNT content was added. Above all, it was mentioned that the conductivity of geopolymer is influenced by factors such as moisture content, pore volume, size and connectivity, ions concentration and degree of geopolymerization[10,11]. In this work, short CF was used for adding in geopolymer matrix CF concentration, curing temperature and aging time were varied and compressive strength was measured using a universal testing machine (UTM). For electrical properties determination, I-V measurement and cyclic voltammetry (CV) were carried out. Microstructure of CF/geopolymer composite was investigated under a scanning electron microscope (SEM). 2. Experimental procedure 2.1 Material A high calcium fly ash (FA) (ASTM C618) was obtained from lignite Mae Moh power plant in Lampang, Thailand. Commercial chopped CF was purchased from New bright carbon fiber products Co., Ltd, Zhejiang, China. The properties of CF are shown in Table 1. The alkaline activating solutions were 10M sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). NaOH was prepared by dissolving 98% caustic soda flake (AGC Chemicals Co., Ltd, Thailand) with deionized (DI) water, while Na2SiO3 composed of 12.53 wt% Na2O, 30.24 wt% SiO2 and 57.23 wt% H2O. Table 1. Properties of the purchased short CF. Properties Fiber diameter (μm) Carbon content (%) Density (g/cm3) Tensile strength (GPa) Tensile modulus (GPa) Elongation (%) Volume resistivity (Ω cm)
Value 7 ≥95 1.6-1.76 2.6-3.8 220-240 1.5 1.5 x 103
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2.2 Sample preparation CF/geopolymer composite was prepared with fixed liquid to solid ratio (L/A) at 0.5. The composite was firstly prepared by dispersing CF in sodium hydroxide (NaOH) for 5 minutes manually to avoid agglomeration. FA was added in the mixing bowl and mixed at high speed (1400 rpm) for 5 minutes. Then, sodium silicate (Na2SiO3) was added and mixed at the same speed for another 5 minutes. The ratio of Na2SiO3 to NaOH was kept at 1 and the CF content was varied from 0 to 1 wt%. Next, the mixed slurry was poured into acrylic molds and vibrated for ten seconds to eliminate pores. The specimen size for mechanical testing was 2.5 x 2.5 x 2.5 cm3. For the electrical testing, the specimens were prepared followed Kim et al. [12]’s methods. They were casted in 2.5 x 2.5 x 10 cm3 acrylic mold. Then, left in ambient for 5 minutes 4 well-polished brass electrodes were inserted at the middle of the specimens. The sizes of the electrodes were 1 x 10 cm2 and the space between each probe was 1.2 cm. At 60 minutes after the mixing process started, the specimens were wrapped with cling film to protect them from moisture loss. The specimens were cured at 25°C and 50% humidity in a control room or 60°C in an electric oven. After 24 hours of curing, the specimens were taken out from the oven, left in ambient until cooled down. Then, they were demolded and wrapped again with cling film, and kept in the control room for 7, 14 and 28 days for testing. 2.3 Characterization techniques The chemical composition of FA was determined by X-ray fluorescence spectroscopy (XRF, S8 Tiger, Bruker) and X-ray diffraction (XRD, PANalytical, Empyrean, USA). Microstructure images of FA was investigated using SEM (Zeiss LEO 1450VP, UK) and the particle size distribution was tested using (Mastersizer 3000, Malvern, UK) The workability of geopolymer composite was determined to find an optimized condition for mixing, by miniature slump (mini-slump) cone test [13,14]. The testing was designed according to ASTM 143. The freshly mix geopolymer was pour in a truncated conical mold, vertically lifted and left to spread. Then, three diameters of the spreaded geopolymer paste were measured at 30 s, 1, 2, 3, 4 and 5 min. The diameters were averaged and the workability was calculated using this following equation %
=
× 100
Equation 1
where %W is workability, d is the averaged diameter and d0 is the bottom diameter of the conical molds. The mechanical testing was done at the age of 7 days. A universal testing machine (UTM, CY 6040A1, Chun Yen Testing Machines Co., Ltd., Taiwan) was used to apply 50 N/min compression load to the specimens (ASTM C109/C109M-13). Post-cracking specimens were collected and used for further chemical and microstructural examinations. An alternative current (AC) with the maximum current of 100 mA was applied for I-V measurement (VersaLabTM, Quantum Design, Inc, USA) and cyclic voltammetry (CV, Gamry 3000 auxiliary, USA). For I-V measurement, the applied frequency was in the range of 0.3-100 Hz. Resistivity was calculated from the slope of the I-V curve using equation 2. For CV, the scan rate was varied between 5 and 100 mV/s. =
Equation 2
3. Results and discussion 3.1 Raw materials From XRF result in Table 2, calcium oxide (CaO) content was 24.2795%, so it was confirmed that the FA is a Class C, according to ASTM C618. Particle size distribution (Fig. 1) showed a wide distribution from a few micron to thousands micron. The mean size of FA particles was 21.6 μm with the standard deviation of 0.639. The XRD pattern (Fig. 2) showed the broad peak indicating that the FA was amorphous and the peak was generally broad. Some crystalline phases were detected. Anhydrite (A) was the tallest peak and the others were quartz (Q), hematite (F), calcium oxide (C) and mullite (M).
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P. Payakaniti et al./ Materials Today: Proceedings 5 (2018) 14017–14025 Table 2. Chemical composition of FA. Oxide SiO2 CaO Al2O3 Fe2O3 SO3 K2O MgO Na2O
Mass (%) 34.0610 24.2795 16.5085 13.9696 4.3136 2.4679 1.9776 1.2102
Fig. 1. Particle size distribution of FA.
Fig. 2. XRD pattern of Mae Moh FA.
3.2 Rheology of CF/geopolymer composite Fig. 3 shows that the workability of 0%CF was highest at 244%. With the increase in CF content in the geopolymer matrix, the workability continuously reduced. At 0.5%CF, the workability was 225% and the lowest value was 176% at 1%CF. It was found that the suitable value for workability of CF/geopolymer was between 150 and 250% [15]. Thereby, the added CF content in the present work is appropriate for the flow of the paste. However, if the added CF content was increased, the manufacturing cost will be higher and might not practical for industrial scale manufacturing.
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Fig. 3. Workability of L/A 0.5 geopolymer composite.
3.3 Mechanical properties of CF/geopolymer composite The mechanical strength of 7 days geopolymer composites, cured at 60°C is shown in Fig. 4. The compressive strength was slightly reduced with higher concentration of CF. The strength of 0%, 0.5% and 1%CF were 81.0, 67.3 and 59.4 MPa, respectively. The decreasing in compressive strength probably affected by clustering of the CF during the fabrication processes [16,17]. However, at 0.5%CF and above, the post-cracking specimens still stuck together. This was resulted from bridging effect of CF and bonding between the fiber and the matrix. It could be referred that the behavior of geopolymer was less brittle and provided more plastic deformation.
Fig. 4. Compressive strength of L/Aa0.5 geopolymer composite.
3.4 Electrical properties of CF/geopolymer composite I-V measurement of CF/geopolymer composite is shown in Fig. 5 and Fig. 6. It is clear that the resistivity was reduced by adding CF and the value was lower when higher CF content was introduced to the geopolymer matrix. The resistivity of 28-days age geopolymer is reported in Fig. 5. It can be seen that the curing temperature influenced the resistivity of the composite at low CF concentration. However, the effect of curing temperature on the resistivity diminished after the CF content was 0.4% or higher. For 25°C curing, the resistivity of 0% was 320.42 Ω cm, 0.3% was 208.33 Ω cm, 0.6% was 31.04 Ω cm and 0.9% was 34.79 Ω cm. In the case of 60°C, the resistivity was 4466.04, 328.96, 78.75 and 22.92 Ω cm for 0, 0.3, 0.6 and 0.9%CF, respectively. The 60°C cured specimens obviously had much higher resistivity than 25°C cured specimen due to their higher degree of geopolymerisation. At 0.4%CF concentration, the resistivity of composite was significantly lower for both curing temperature.
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Fig. 5. Effect of CF content to resistivity of geopolymer composite, measured at 28 days.
When comparing the effect of curing day on resistivity, it is obviously that the resistivity of 25°C curing increased from 7 days to 14 days as shown in Fig. 6(a). For example, the resistivity of 0%CF at 7 days was 92.29 Ω cm and increased more than two times to 200.63 Ω cm at 14 days. However, there were differences of resistivity increasing for the specimens curing from 14 days to 28 days age. Considering in increment of the resistivity, they can be divided into 3 groups. The first group was 0-0.3%CF, the resistivity was still clearly increased and the highest resistivity belonged to 0%CF at 28 days with the resistivity of 320.42 Ω cm. The second group was 0.40.5%CF with moderate increase in the 28 days resistivity. The last group was 0.6-1%CF with only slight increase in resistivity. The lowest resistivity was 1%CF with the 28 days resistivity of 30.42 Ω cm. In the case of 60°C curing (Fig. 6(b)), only 0 and 0.1%CF presented the changes from 7 to 28 days curing age. At 7 days, the resistivity of 0%CF was 2556.25 Ω cm and that of 0.1%CF was 1736.25 Ω cm. By 28 days their resistivities were 4466.04 and 269.76 Ω cm, respectively. Therefore, it in confirmed that the degree of geopolymerization was better at 60°C curing temperature, since the resistivity was nearly unchanged compared with that of 25°C curing.
Fig. 6. Effect of curing age to electrical resistivity of geopolymer composite curing at (a) 25°C and (b) 60°C.
The CV measurement result was illustrated in Fig. 7 (25°C) and Fig. 8 (60°C). From the figure, it is obviously that the CV curve of 25°C contains a lot more peaks than 60°C in every CF concentration. According to Haynes [18,19], the peaks corresponded to the chemical reactions in geopolymer. There are two possible reactions that occur inside the geopolymer which are a redox reaction of 2H2O + 2e- ↔ H2(g) + 2OH- or a redox reaction of Na+ ions,
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trapped inside the geopolymer structure (Na+ + e- ↔ Na(s)). Besides the number of peaks, the breadths of peaks also changes with different curing temperatures and CF contents. The amplitudes of 25°C curves were larger than those of 60°C at the same concentration and curing temperature. The curves were narrower at higher CF concentration. It could be explained that at 60°C curing, geopolymerization of the geopolymer matrix was better formed. Moreover, the curves at higher CF were smaller probably because the conductivity of CF was predominant than the conductivity of geopolymer matrix [15]. Furthermore, the scanning rate also influences the curves. The curves with slower scan rate were smaller than the faster one. Also, the peaks and troughs shifted to the higher voltage when the higher scanning rate was applied.
Fig. 7. CV curve of 25°C curing CF/geopolymer with (a) 0%CF, (b) 0.2%CF, (c) 0.4%CF and (d) 0.9%CF.
Fig. 8. CV curve of 60°C curing CF/geopolymer with (a) 0%CF, (b) 0.2%CF, (c) 0.4%CF and (d) 0.9%CF.
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3.5 Microstructure of CF/geopolymer composite The microstructure of the geopolymer composite was investigated under SEM in Fig. 9. Fig. 9(a) shows that there were unreacted spherical particles of FA all over the fracture surface of the matrix. For the CF added geopolymer, it can be seen that the CFs were aligned in random direction without agglomeration. Fig. 9(b)-(f) show that although the CF content was higher, the fiber still did not agglomerate. These can prove that the mixing processes did solve the CF agglomeration problem. Also, when CFs content was higher electron paths shorten as distance between adjacent CF was narrower and resulted in the decreasing in electrical resistivity.
Fig. 9. SEM images of L/A 0.5 CF/geopolymer containing (a) 0%CF, (b) 0.2%CF, (c) 0.4%CF, (d) 0.6%CF, (e) 0.8%CF and (f) 1%CF.
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4. Conclusion Geopolymer was composited with 0-1%CF at L/A ratio of 0.5. The workability of CF/geopolymer composite was decreased with increasing CF concentration. The mechanical and electrical properties measurements were carried out. The compressive strength test showed that the geopolymer without CF provided the highest strength and the strength slightly decreased with increasing CF concentration. However, at the CF content of 0.5% and over, the post-cracking specimen pieces did not separate from each other and its behavior was more ductile. The electrical determination showed that both curing temperature and CF contents affected the geopolymer composites’ resistivity. The electrical resistivity of 60°C was higher and more stable with time than that with 25°C. Also, less peaks and troughs of CV curves was found for 60°C specimens. These were affected from the higher degree of geopolymerization of 60°C geopolymer composite. Acknowledgements This work was supported by the Thailand Research Fund (TRF) and Khon Kaen University under the TRF Senior Research Scholar, Grant No. RTA5780004, the Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program of Center of Excellence Network. References 1. Kupaei RH et al. Mix design for fly ash based oil palm shell geopolymer lightweight concrete. Constr Build Mater 2013;43: 490-496. 2. Shaikh FUA. Review of mechanical properties of short fibrereinforced geopolymer composites. Constr Build Mater 2013;43: 37–49. 3. Author. Handbook of Alkali-activated Cements, Mortars and Concretes. In: Pacheco-Torgal F et al., editors. Handbook of Alkali-activated Cements, Mortars and Concretes, Cambridge: Woodhead Publishing; 2015. 4. Masi G et al. The effect of organic and inorganic fibres on the mechanical and thermal properties of aluminate activated geopolymers. . Compos Part B-ENG 2015;76: 218-228. 5. Matthews FL,Rawlings RD. Composite Materials: Engineering and Science. London: Chanpman & Hall; 1999. 6. Alomayri T et al. Characterisation of cotton fibre-reinforced geopolymer composites. Compos Part B-ENG 2013;50: 1-6. 7. Bernal S et al. Performance of an alkali-activated slag concrete reinforced with steel fibers. Constr Build Mater 2010;24: 208-214. 8. Ranjbar N et al. A Comprehensive Study of the Polypropylene Fiber Reinforced Fly Ash Based Geopolymer. Plos One 2016;1-20. 9. Chung DDL. Multifunctional Cement-Based Materials. New York: CRC Press; 2003. 10. Saafi M et al. Multifunctional properties of carbon nanotube/fly ash geopolymeric nanocomposites. Constr Build Mater 2013;49: 46-55. 11. Kusak I et al. Electric conductivity changes in geopolymer samples with added carbon nanotubes. Procedia Engineering 2016;151: 157-161. 12. Kim HK et al. Enhanced effect of carbon nanotube on mechanical and electrical properties of cement composites by incorporation of silica fume. Composite Structures 2014;107: 60-69. 13. Kantro DL. Influence of Water-Reducing Admixtures on Properties of Cement Paste - A Miniature Slump Test. Cement Concrete And Aggregates 1980;2: 95-102. 14. Mebrouki A et al. A Self-Compacting Cement Paste Formulation using Mixture Design. Journal of Applied Sciences 2009;9: 4127-4136. 15. Payakaniti P et al. Electrical conductivity and compessive strength of carbon fiber reinforced fly ash geopolymeric composites. CONSTR BUILD MATER 2017;135: 164-176. 16. Vilaplana JL et al. Mechanical properties of alkali activated blast furnace slag pastes reinforced with carbon fibers. construction and Building Materials 2016;116: 63-71. 17. Zhang H et al. Fiber Reinforced Geopolymers for Fire Resistance Applications. Procedia Engineering 2014;71: 153 – 158. 18. Haynes WM. CRC Handbook of Chemistry and Physics. 93rd ed. FL: CRC Press; 2012. 19. Haynes WM et al. CRC Handbook of Chemistry and Physics: A ready Reference Book of Chemical and Physical Data 92nd ed. Boca Raton : CRC, cop.; 2011.