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Thermoelectric behaviors of fly ash and metakaolin based geopolymer
Construction and Building Materials 237 (2020) 117757
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Thermoelectric behaviors of fly ash and metakaolin based geopolymer Jingming Cai a,b, Jiawei Tan a,b,⇑, Xiaopeng Li c,⇑ a
Key Laboratory of Concrete and Prestressed Concrete Structures of Ministry of Education, Southeast University, Nanjing, China Department of Civil Engineering, KU Leuven, Bruges, Belgium c Department of Civil and Environmental Engineering, University of California, Irvine, USA b
h i g h l i g h t s The thermoelectric behaviours of fly ash and metakaolin based geopolymer paste were investigated. The effects of alkali activator concentration, curing temperature and slag substitution ratio were discussed. Geopolymer, especially metakaolin based geopolymer, was found to be a more promising thermoelectrical material.
a r t i c l e
i n f o
Article history: Received 29 August 2019 Received in revised form 19 November 2019 Accepted 28 November 2019
Keywords: Seebeck coefficient Thermoelectric behavior Geopolymer Curing temperature Slag
a b s t r a c t In this paper, the thermoelectric behaviors of fly ash and metakaolin based geopolymer were investigated. It was found that the seebeck coefficient of geopolymer paste is much higher than that of cement paste, indicating geopolymer may be a promising thermoelectrical material. The influences of different parameters were discussed. The seebeck coefficient for both fly ash and metakaolin based geopolymers were found to increase with the increase of alkali concentration. The curing temperature has a more significant influence on the thermoelectric behavior of fly ash based geopolymers. Meanwhile, the increase of slag replacement ratio would result in a decreased seebeck coefficient for fly ash based geopolymer. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Geopolymer is known as a good alternative for ordinary Portland cement because of its advantages of low energy consumption [1,2], reduction of carbon dioxide emission [3,4] and effective utilization of industrial by-products such as fly ash and slag [5,6]. Many researches have been conducted to study the mechanical behaviors of geopolymer based structural components, such as beams [7], columns [8] and beam column joints [9]. In recent years, geopolymer has been successfully used for constructions such as pavements, retaining walls, water tanks and precast bridge decks [10]. With the increase of applications in structural engineering, the electrical and thermal prosperities of geopolymers have also been attracting attentions in recent years. Saiprasad and Erez [11] monitored the electrical resistance of carbon fiber reinforced geopolymer under flexural and compressive load, the experimental results indicated that conductive geopolymer could serve as a
⇑ Corresponding authors at: Key Laboratory of Concrete and Prestressed Concrete Structures of Ministry of Education, Southeast University, Nanjing, China (J. Tan). E-mail addresses: [email protected] (J. Tan), [email protected] (X. Li). https://doi.org/10.1016/j.conbuildmat.2019.117757 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
smart material in health monitoring applications. Bi et al. [12] found that geopolymer could exhibit ultrahigh self-sensing performance with the addition of carbon nanotubes. Zhang et al. [13] investigated the microwave absorption properties of geopolymeric based composites with the addition of graphite, the dielectric constants of the geopolymeric composite was found to increase gradually with the increase of graphite content. Duan et al. [14] reported a novel thermal insulating and lightweight metakaolin based geopolymer with the addition of polystyrene particles, it proved the feasibility of making thermal insulation material with geopolymer, and the strength of geopolymeric composites even increased after 400 °C exposure. According to former researches, geopolymers possess the potential to be an important functional constructional material, which makes it possible of being promisingly applied in structural health monitoring, electromagnetic shielding and thermal insulation. However, limited attention has been paid on the thermoelectric behaviors of geopolymers. The thermoelectric behaviors of constructional material are worth being studied due to its ability of harvesting thermal energy from solar radiation and converting into electric energy directly, which would be helpful in terms of
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relieving the urban heat island effect and reducing greenhouse gas emission. For cementitious materials, after Sun et al. [15] first reported the seebeck effects of cement paste in 1998, the thermoelectric effects of cementitious materials have been widely investigated and different nanomaterials have been applied to increase the thermoelectric properties of cementitious materials [16–18]. By contrast, the basic thermoelectric properties of geopolymeric materials are still unknown even though much attention has been paid to their mechanical behaviors. Set against this background, the seebeck effect of fly ash and metakaolin based geopolymers were investigated and compared with that of normal Portland cement in this paper. The effects of curing temperature, alkali activator concentration and slag substitution ratio were discussed. It was found that the Seebeck coefficient for geopolymer paste is much higher than that for conventional cement paste, indicating geopolymeric materials could be promising thermoelectrical material. 2. Experiment 2.1. Materials Geopolymer paste is a kind of aluminosilicate binder, which is synthesized through reactions between solid precursors and alkali activator. Both fly ash and metakaolin based geopolymers are investigated in this study. The solid precursors (low calcium fly ash, metakaolin and slag) were purchased from Gongyi Chenyi Abrasion-proof Material Co., Ltd (China). The macro and micro morphology of raw materials are shown in Figs. 1 and 2. All these materials are powdery particles under macro scale, the particle size for metakaolin is much smaller than that for fly ash and slag, as shown in Fig. 2. The particle size distribution curves for raw materials are shown in Fig. 3, it can be concluded that the fineness of metakaolin is much higher than that of fly ash and slag, while slag contains the largest particle size even higher than 100 lm. The main chemical compositions for each material are shown in Table 1, it can be seen that slag has a much higher content of CaO than fly ash and metakaolin. A combination of Na2SiO3 (39.8 wt% Na2SiO3, relative density 1.42 g/cm3) and KOH (98% purity, density 2.1 g/cm3) purchased from Hengxi chemical Co., Ltd (China) was applied as the alkaline activator. The cement speci-
Fig. 1. Macro morphology of raw materials.
mens (Hailuo brand, Grade 42.5) with a constant water cement ratio of 0.4 were also tested and compared with geopolymer specimens. The main chemical compositions for cement are shown in Table 1. 2.2. Sample preparation The alkali activator was first prepared with KOH solution and Na2SiO3 solution. KOH solution with different concentrations (4 mol/L, 8 mol/L, 12 mol/L and 16 mol/L) were prepared by dissolving KOH flakes in water. A constant Na2SiO3 solution to KOH solution ratio of 1.5 by weight was used in this paper. Both Na2SiO3 and KOH activator solutions were mixed together and cooled for 24 h before being mixed with solid precursors [19]. A total of 11 geopolymer samples with different parameters were casted and the details for all samples are shown in Table 2. The solid-toliquid ratio for fly ash and metakaolin based geopolymers were set as 3 and 1, respectively. The geopolymer sample ‘FA-4-0’ is used to explain the nomenclature: the first two letters ‘FA’ denote the type of solid precursor, while the following number depicts the KOH concentration, the last number represents the slag replacement ratio, which is defined as the proportion of slag in the solid precursors by weight. To prepare geopolymer paste, the solid precursors were first mixed with an electric blender at a ratio of 140 rpm for about 5 min. After that, alkaline activator was carefully added in the blender and mixed for another 10 min until the mixture became homogeneous and uniform. Finally, the fresh paste was casted in the cylinder molds and was covered with plastic film. The cylinder specimens were first kept under room temperature for 24 h before demolding. Then, all the specimens were cured in an automatic oven (Hebei Rongyao Corporation, China) with the specified temperature (25 °C, 50 °C and 70 °C) for 28 days.
Fig. 3. Particle size distribution curves for raw materials.
Fig. 2. Micro morphology of raw materials.
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J. Cai et al. / Construction and Building Materials 237 (2020) 117757 Table 1 Chemical constituents of solid precursors (wt%). Compositions
CaO
SiO2
Al2O3
Fe2O3
MgO
K2O
Na2O
P2O5
MnO
Metakaolin Fly ash Cement Slag
0.13 3.71 61.13 34.93
61.45 50.86 21.45 40.28
32.45 28.16 5.42 10.11
0.89 6.24 2.89 0.07
2.08 1.28 2.37 8.14
0.81 0.67 2.08 1.12
0.77 1.27 0.81 1.12
0.07 0.12 2.50 0.08
– 0.07 0.77 1.14
Table 2 Details of all samples. Sample ID
Solid precursors
KOH concentrations (mol/L)
Slag replacement ratio %
Solid-to-liquid ratio
FA-4-0 FA-8-0 FA-12-0 FA-16-0 FA-8-30 FA-8-0 FA-8-90 MK-4-0 MK-8-0 MK-12-0 MK-16-0
Fly ash Fly ash Fly ash Fly ash Fly ash with slag Fly ash with slag Fly ash with slag Metakaolin Metakaolin Metakaolin Metakaolin
4 8 12 16 8 8 8 4 8 12 16
0 0 0 0 30 60 90 0 0 0 0
3 3 3 3 3 3 3 1 1 1 1
The flow chart of casting geopolymer specimen is shown in Fig. 4. The humidity was maintained as ambient humidity (about 40%) by water nebulizer. The dimensions of typical geopolymer specimen used for electrical resistance measurement are shown in Fig. 5. 2.3. Test set-up Seebeck effect is a phenomenon in which a temperature difference between two surfaces of semiconductors produces a voltage difference. The voltage difference of each geopolymer sample
was measured with two-probe measurement method. The schematic diagram of test set-up is shown in Fig. 5. The copper sheets with the width of 10 mm were attached on both ends of the specimen. Silver paint was applied between the copper sheets and the sample surface to further enhance the electrical contact. Two copper wires were wrapped around each copper sheet and connected to anodic and cathodic terminal of a digital multimeter (Fluke Technologies, 8808A), respectively. The resistance heater (Bailer Technologies, H32B) was applied at the bottom end of the specimen while the upper end was not heated. The temperature of
Fig. 4. The flow chart of casting geopolymer specimen.
Fig. 5. A schematic diagram of the test set-up.
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The temperature difference versus recorded seebeck voltage of typical cement, fly ash and metakaolin based geopolymer specimens are shown in Fig. 6. It can be seen that the seebeck voltage for both fly ash and metakaolin based geopolymers are all negative, indicating geopolymers can be classified as n-type semiconductor. When both ends of geopolymer specimen were under different temperature, the inner electrons would flow from the heating end to the cooling end, thus producing a seebeck voltage difference inside the geopolymer specimen. With the increase of temperature difference, as shown in Fig. 6, the seebeck voltage increased linearly as well, indicating more electrons were released and flowed from the heating end to the cooling end. The seebeck coefficient could be represented by the slope of each curve shown in Fig. 6 [17]. As can be seen in Table 3, the seebeck coefficient for pure cement paste is only 0.27 lV/°C, which is quite low and far from meeting the requirement of practical application. This may be attributed to the reason that cement is a kind of multiphase and porous material, the hydration products of cement are disordered and irregular. Even though the inner electrons could be activated under temperature difference, the directed transfer of electrons may still be disturbed or blocked by the needle-like, bar-like and sheet-like crystals inside the cement paste. By contrast, the seebeck coefficient for fly ash and metakaolin based geopolymers are as high as 3.36 lV/°C and 12.07 lV/°C,
respectively. It can be concluded that the seebeck coefficient for geopolymers could be an order of magnitude higher than cement paste. There are two possible reasons accounting for this phenomenon. First, geopolymer is characterized as three-dimensional aluminosilicate containing AlO-4 and SiO4 as tetrahedral subunits, as shown in Fig. 7 [20]. Thus, geopolymers may have more regular and homogeneous microstructure compared with conventional cement paste, this enables the activated electrons to be more easily transferred from the heating end to the cooling end with less interference and impediment, resulting a higher voltage difference between the two ends. It should be noted that for homogeneous elemental materials such like metallic materials and monocrystalline silicon, the existing thermoelectric theory can provide a clear theoretical illustration for their seebeck effect [21]. Therefore, it is feasible to increase the Seebeck coefficient by adjusting the morphology of homogeneous elemental materials [22]. However, for geopolymer, classified as typical heterogeneous material, the situation is much more complicated and the existing theoretical methods may not be directly applied. In this case, the microstructure may significantly influence the thermoelectric behavior of heterogeneous material. It may be also this reason that why the metakaolin based geopolymer showed a much higher seebeck coefficient than fly ash based geopolymer. As shown in Table 3, the seebeck coefficient for metakaolin based geopolymer is about 3.6 times higher than that for fly ash based geopolymer. It has been reported that the active amorphous phase material account for more than 80% of its total mass in metakaolin. By contrast, fly ash contains more than 60% chemically inactive glassy phases materials [23]. As can be seen in Table 1 and Fig. 2, metakaolin possess a much higher fineness as well as more aluminosilicate content than that of fly ash. Thus, with the same curing condition and alkali concentration, metakaolin based geopolymer can have a denser and more homogeneous structure than fly ash based geopolymer.
Fig. 6. Seebeck voltage versus temperature difference for geopolymers and cement.
Fig. 7. Typical geopolymer types with aluminosilicate structure.
the two ends were detected by two T-type thermocouples (CESMOOY brand, TT-T-24-SLE) attached on each end. The voltage and temperature difference of each specimen were recorded simultaneously by the multimeter. Then, the voltage difference divided by the temperature difference yielded the Seebeck coefficient. It should be noted the copper wires at the two ends were also at different temperature, thus the calculated Seebeck coefficient should plus the absolute thermoelectric power of copper (2.34 lV/°C) [20]. 3. Experimental results and discussions
Table 3 Linear fitting results. Specimen
Cement MK-12-0-50 °C FA-12-0-50 °C
Intercept
Slope (Seebeck coefficient)
Value
Standard Error
Value
Standard Error
0.28 4.68 8.95
0.14 2.25 2.85
0.27 3.36 13.28
0.01 0.08 0.11
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The second reason may be that geopolymers contain more lowdimensional nanomaterial than conventional cement paste. The typical SEM micrographs of metakaolin based geopolymer, fly ash based geopolymers and cement paste are shown in Fig. 8. Geopolymer possesses a compact and continues gel-like structure, attached with numerous zero-dimensional particulate matters, while many one-dimensional bar-like and two-dimensional sheet-like hydration products can be observed in cement paste. It has been reported that decreasing the micro size of semiconductors may cause a quantum confinement effect [24,25], which could dramatically increase the density of states (DOS). The DOSs for one-, two-, and three-dimensional materials are listed as Eqs. (1) and (3) [26], respectively.
g 1D;nm ðEÞ ¼ g 2D;n ðEÞ ¼
g 3D ðEÞ ¼
1=2 2md
1
aph2 md
aph2
2
h
; E En ;
ðE Enm Þ1=2 ; E Enm ; md ¼
md ¼ mx
p ffiffiffiffiffiffiffiffiffiffiffiffiffi 2 mx my
3=2 1 2md E1=2 ; E 0; 2p 2 h 2
md ¼
ð1Þ ð2Þ
p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 mx my mz
ð3Þ
where g 1D;nm ðEÞ, g 2D;n ðEÞ and g 3D ðEÞ are the DOS for one-, two-, and three-dimensional materials; h is Planck constant; mx;y;z is the effective mass in x, y and z directions; a is the width or thickness of the quantum wire; Enm and En are the confinement energies of quantum wire and quantum well, respectively. The total DOS is the sum of each individual bans, shown as follows:
g ðEÞ ¼
þ1 X
g n ðEÞ
ð4Þ
n¼1
The total DOS for semiconductors with different dimensions are shown in Fig. 9. It can be concluded that materials with lower dimensional structures exhibit sharper DOSs when compared with materials with higher dimensional structures. The direct relation between EDS and seebeck coefficient is shown as follow [27]:
S¼
1 eT
R þ1
gðEÞsðEÞEt2 ðEÞ dfde0 dE E EF R 0þ1 gðEÞsðEÞt2 ðEÞ dfde0 dE E0
! ð5Þ
where S is the seebeck coefficient; e is the electric quantity for a single electron; T is the absolute temperature; sðEÞ is the relaxation time of electrons; gðEÞ is the total DOS; f 0 ðEÞ is the distribution of electrons and EF is the energy of Fermi band. The formulas indicate that the increase of DOS can lead to an increased seebeck coefficient, and the zero-dimensional particles attached on the surface of geopolymer can dramatically increase the DOS of geopolymer specimens, which can consequently increase the seebeck coefficient of geopolymer paste. As shown in Fig. 8, some unreacted fly ash particles can be observed inside the fly ash based geopolymer, indicating that fly ash based geopolymer may contain less low dimensional aluminosilicate while being compared with metakaolin based geopolymer, which can be also the reason that the seebeck coefficient for fly ash based geopolymer is much lower than that for metakaolin based geopolymer. 3.1. Influence of alkali concentration The seebeck voltage versus temperature difference for fly ash and metakaolin based geopolymer paste with different alkali concentration are shown in Fig. 10. The alkali concentration varied from 4 mol/L to 12 mol/L. The seebeck coefficient can be represented by the slope of each curve, as shown in Table 4. For metakaolin based geopolymer, the seebeck coefficient increased with the increase of alkali concentration. This is quite reasonable since a higher alkali concentration means more OH– and K+ ions were introduced in the system. The higher alkali ion concentration would seriously influence the Young’s modulus, micro morphology as well as micro structure of metakaolin based geopolymer [28]. A higher alkali concentration would result in a denser microstructure, which would be beneficial for the transportation of electrons and subsequently introduce more low-dimensional aluminosilicate particles. The same phenomenon can be observed for fly ash based geopolymer: the seebeck coefficient for specimen FA-12-0-50 °C is about two times higher than that for
Fig. 8. SEM micrograph of metakaolin based geopolymers and cement.
Fig. 9. Schematic diagram for EDS of (a) three-; (b) two-; and (c) one-dimensional material.
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J. Cai et al. / Construction and Building Materials 237 (2020) 117757
Fig. 10. Seebeck voltage versus temperature difference for geopolymers with different alkali concentration: (a) metakaolin based geopolymers; (b) fly ash based geopolymers.
Table 4 Linear fitting results. Specimen
MK-4-0-50 °C
MK-8-0-50 °C
MK-12-0-50 °C
FA-4-0-50 °C
FA-8-0-50 °C
FA-12-0-50 °C
S
10.73
12.01
13.28
1.63
2.52
3.36
Note: S is seebeck coefficient (lV/°C).
FA-4-0-50 °C. The reason behind this phenomenon may be that the alkaline activator with higher alkali concentration provides more OH–, thus more Al and Si atoms will be dissolved from fly ash, followed by a higher concentration of Al and Si atoms in the alkaline solutions. These atoms would be converted into geopolymer gel with the geopolymerization process going on, thus the microstructure is likely to be denser with lower dimension gel-like particles. 3.2. Influence of curing temperature It has been reported that the curing temperature has a significant influence on the strength, pore distribution and microstruc-
ture of both fly ash and metakaolin based geopolymers, the chemical composition and physical structure of geopolymer could even be quite different with different curing conditions [29,30]. In this paper, the curing temperatures were set as 25 °C, 50 °C and 70 °C. For metakaolin based geopolymer, as can be seen in Fig. 11a and Table 5, the seebeck coefficient for all metakaolin based geopolymer samples are similar, indicating the curing temperature has negligible influence on the thermoelectrical properties for metakaolin based geopolymer at 28 days. Previous researches have also reported that the temperature curing conditions show limited influences on the quality and property of geopolymerization products for metakaolin based geopolymer, the 28-day compressive strength for metakaolin based geopolymer
Fig. 11. Seebeck voltage versus temperature difference for geopolymers with different curing temperatures: (a) metakaolin based geopolymers; (b) fly ash based geopolymers.
Table 5 Linear fitting results. Specimen
MK-8-0-25 °C
MK-8-0-50 °C
MK-8-0-70 °C
FA-8-0-25 °C
FA-8-0-50 °C
FA-8-0-70 °C
S
10.13
12.01
15.12
1.26
2.52
6.31
Note: S is seebeck coefficient (lV/°C).
J. Cai et al. / Construction and Building Materials 237 (2020) 117757
with high temperature curing conditions was even lower than that for geopolymer under ambient curing temperature [29]. For fly ash based geopolymer, as can be seen in Fig. 11(b), the influence of curing temperature was much more significant. When the curing temperature was 25 °C, the seebeck coefficient was only 1.26 lV/°C at 28 days, indicating the geopolymerization process was still incomplete. Previous researchers have also reported that the compressive strength of fly ash based geopolymer cured at ambient temperature was lower than 10 MPa at 28 days [31]. The seebeck coefficient for specimen FA-8-0-70 °C was about five time higher than that for specimen FA-8-0-25 °C, indicating much denser microstructure as well as more low-dimensional aluminosilicate particles were formed in the geopolymer paste with higher curing temperature. This is reasonable since fly ash generally manifests lower chemical activity and higher activation energy barrier, thus elevating the curing temperature would effectively facilitate the process of dissolving Al and Si atoms from inactive fly ash particles [32].
3.3. Influence of slag replacement ratio As mentioned above, the geopolymerization process for fly ash based geopolymer is very sensitive to curing temperature. The addition of slag in fly ash based geopolymer can dramatically increase the mechanical strength with ambient temperature curing conditions [33]. The effects of slag replacement ratio on the thermoelectrical properties of fly ash based geopolymers are shown in Fig. 12 and Table 6. It can be found that the seebeck coefficient decreased with the increase of slag replacement ratio, indicating that more high-dimensional aluminosilicate structures were formed with the addition of slag. It may be attributed to the reason that the addition of slag consumes more OH– and SiO2 3 while produces more C-A-S-H gel in the paste due to the pozzolanic reactions between slag and alkali activator. This reaction process was much more violent than geopolymerization process and more needle-like, bar-like, and sheet-like crystals were formed in the
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paste, which would decrease the DOS and seebeck coefficient of the specimen in the meantime. 4. Discussions In this paper, the highest seebeck coefficient for geopolymer paste, although much higher than conventional cement paste, was only 15.12 lV/°C, and this value could be further improved with the addition of various thermoelectric admixture (such as manganese dioxide nanorods and silicon nanowires) and conducting materials (such as carbon black and carbon fiber). The improvement of seebeck coefficient of geopolymer paste would also be helpful for the self-sensing behavior of geopolymer concrete. For example, it is feasible to monitor the temperature of mass geopolymer concrete by recording its voltage change, since the temperature difference in core area and surface area of geopolymer concrete will generate voltage due to seebeck effect. For the propose of energy-harvesting, the thermoelectric conversion efficiency depends on the thermoelectric figure of merit (ZT), which can be expressed as follows:
ZT ¼ S2 T=qk
ð6Þ
where S,T, q and k are the Seebeck coefficient, absolute temperature, electrical resistivity and thermal conductivity, respectively. It would be commercially-viable if the ZT value could be as high as 1. However this hard to achieve since all the parameters are interdependent [34]. The thermal conductivity as well as electrical resistivity of geopolymer would be further investigated in the next step. 5. Conclusions In this work, the thermoelectrical behaviors of fly ash and metakaolin based geopolymers were investigated. The main conclusions are shown as follows: 1. The seebeck coefficient for geopolymer is much higher than that for normal cement paste, since geopolymer is denser and it contains more low dimensional aluminosilicate particles. 2. The seebeck coefficient for both fly ash and metakaolin based geopolymers increased with the increase of alkali concentration. A higher alkali concentration would result in a denser micro-structure for geopolymers, which would be beneficial for the transportation of electrons. 3. The curing temperature has negligible influence on the seebeck effect of metakaolin based geopolymer. By contrast, curing temperature has a significant influence on the seebeck effect of fly ash based geopolymer. 4. For fly ash based geopolymer, the seebeck coefficient decreased with the increase of slag replacement ratio, since the addition of slag may introduce more high dimensional calcium silicoaluminate particles.
Declaration of Competing Interest Fig. 12. Seebeck voltage versus temperature difference for fly ash based geopolymers with different slag replacement ratio.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments
Table 6 Linear fitting results. Specimen
FA-8-0-50 °C
FA-8-30-50 °C
FA-8-0-650 °C
FA-8-90-50 °C
S
2.52
1.91
1.39
0.88
Note: S is seebeck coefficient (lV/°C).
This work was financially supported by Open Foundation of Key Laboratory of Concrete and Prestressed Concrete Structure of Ministry of Education CPCSME2018-06, National Natural Science Foundation of China under 51908117.
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Thermoelectric behaviors of fly ash and metakaolin based geopolymer Jingming Cai a,b, Jiawei Tan a,b,⇑, Xiaopeng Li c,⇑ a
Key Laboratory of Concrete and Prestressed Concrete Structures of Ministry of Education, Southeast University, Nanjing, China Department of Civil Engineering, KU Leuven, Bruges, Belgium c Department of Civil and Environmental Engineering, University of California, Irvine, USA b
h i g h l i g h t s The thermoelectric behaviours of fly ash and metakaolin based geopolymer paste were investigated. The effects of alkali activator concentration, curing temperature and slag substitution ratio were discussed. Geopolymer, especially metakaolin based geopolymer, was found to be a more promising thermoelectrical material.
a r t i c l e
i n f o
Article history: Received 29 August 2019 Received in revised form 19 November 2019 Accepted 28 November 2019
Keywords: Seebeck coefficient Thermoelectric behavior Geopolymer Curing temperature Slag
a b s t r a c t In this paper, the thermoelectric behaviors of fly ash and metakaolin based geopolymer were investigated. It was found that the seebeck coefficient of geopolymer paste is much higher than that of cement paste, indicating geopolymer may be a promising thermoelectrical material. The influences of different parameters were discussed. The seebeck coefficient for both fly ash and metakaolin based geopolymers were found to increase with the increase of alkali concentration. The curing temperature has a more significant influence on the thermoelectric behavior of fly ash based geopolymers. Meanwhile, the increase of slag replacement ratio would result in a decreased seebeck coefficient for fly ash based geopolymer. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Geopolymer is known as a good alternative for ordinary Portland cement because of its advantages of low energy consumption [1,2], reduction of carbon dioxide emission [3,4] and effective utilization of industrial by-products such as fly ash and slag [5,6]. Many researches have been conducted to study the mechanical behaviors of geopolymer based structural components, such as beams [7], columns [8] and beam column joints [9]. In recent years, geopolymer has been successfully used for constructions such as pavements, retaining walls, water tanks and precast bridge decks [10]. With the increase of applications in structural engineering, the electrical and thermal prosperities of geopolymers have also been attracting attentions in recent years. Saiprasad and Erez [11] monitored the electrical resistance of carbon fiber reinforced geopolymer under flexural and compressive load, the experimental results indicated that conductive geopolymer could serve as a
⇑ Corresponding authors at: Key Laboratory of Concrete and Prestressed Concrete Structures of Ministry of Education, Southeast University, Nanjing, China (J. Tan). E-mail addresses: [email protected] (J. Tan), [email protected] (X. Li). https://doi.org/10.1016/j.conbuildmat.2019.117757 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
smart material in health monitoring applications. Bi et al. [12] found that geopolymer could exhibit ultrahigh self-sensing performance with the addition of carbon nanotubes. Zhang et al. [13] investigated the microwave absorption properties of geopolymeric based composites with the addition of graphite, the dielectric constants of the geopolymeric composite was found to increase gradually with the increase of graphite content. Duan et al. [14] reported a novel thermal insulating and lightweight metakaolin based geopolymer with the addition of polystyrene particles, it proved the feasibility of making thermal insulation material with geopolymer, and the strength of geopolymeric composites even increased after 400 °C exposure. According to former researches, geopolymers possess the potential to be an important functional constructional material, which makes it possible of being promisingly applied in structural health monitoring, electromagnetic shielding and thermal insulation. However, limited attention has been paid on the thermoelectric behaviors of geopolymers. The thermoelectric behaviors of constructional material are worth being studied due to its ability of harvesting thermal energy from solar radiation and converting into electric energy directly, which would be helpful in terms of
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relieving the urban heat island effect and reducing greenhouse gas emission. For cementitious materials, after Sun et al. [15] first reported the seebeck effects of cement paste in 1998, the thermoelectric effects of cementitious materials have been widely investigated and different nanomaterials have been applied to increase the thermoelectric properties of cementitious materials [16–18]. By contrast, the basic thermoelectric properties of geopolymeric materials are still unknown even though much attention has been paid to their mechanical behaviors. Set against this background, the seebeck effect of fly ash and metakaolin based geopolymers were investigated and compared with that of normal Portland cement in this paper. The effects of curing temperature, alkali activator concentration and slag substitution ratio were discussed. It was found that the Seebeck coefficient for geopolymer paste is much higher than that for conventional cement paste, indicating geopolymeric materials could be promising thermoelectrical material. 2. Experiment 2.1. Materials Geopolymer paste is a kind of aluminosilicate binder, which is synthesized through reactions between solid precursors and alkali activator. Both fly ash and metakaolin based geopolymers are investigated in this study. The solid precursors (low calcium fly ash, metakaolin and slag) were purchased from Gongyi Chenyi Abrasion-proof Material Co., Ltd (China). The macro and micro morphology of raw materials are shown in Figs. 1 and 2. All these materials are powdery particles under macro scale, the particle size for metakaolin is much smaller than that for fly ash and slag, as shown in Fig. 2. The particle size distribution curves for raw materials are shown in Fig. 3, it can be concluded that the fineness of metakaolin is much higher than that of fly ash and slag, while slag contains the largest particle size even higher than 100 lm. The main chemical compositions for each material are shown in Table 1, it can be seen that slag has a much higher content of CaO than fly ash and metakaolin. A combination of Na2SiO3 (39.8 wt% Na2SiO3, relative density 1.42 g/cm3) and KOH (98% purity, density 2.1 g/cm3) purchased from Hengxi chemical Co., Ltd (China) was applied as the alkaline activator. The cement speci-
Fig. 1. Macro morphology of raw materials.
mens (Hailuo brand, Grade 42.5) with a constant water cement ratio of 0.4 were also tested and compared with geopolymer specimens. The main chemical compositions for cement are shown in Table 1. 2.2. Sample preparation The alkali activator was first prepared with KOH solution and Na2SiO3 solution. KOH solution with different concentrations (4 mol/L, 8 mol/L, 12 mol/L and 16 mol/L) were prepared by dissolving KOH flakes in water. A constant Na2SiO3 solution to KOH solution ratio of 1.5 by weight was used in this paper. Both Na2SiO3 and KOH activator solutions were mixed together and cooled for 24 h before being mixed with solid precursors [19]. A total of 11 geopolymer samples with different parameters were casted and the details for all samples are shown in Table 2. The solid-toliquid ratio for fly ash and metakaolin based geopolymers were set as 3 and 1, respectively. The geopolymer sample ‘FA-4-0’ is used to explain the nomenclature: the first two letters ‘FA’ denote the type of solid precursor, while the following number depicts the KOH concentration, the last number represents the slag replacement ratio, which is defined as the proportion of slag in the solid precursors by weight. To prepare geopolymer paste, the solid precursors were first mixed with an electric blender at a ratio of 140 rpm for about 5 min. After that, alkaline activator was carefully added in the blender and mixed for another 10 min until the mixture became homogeneous and uniform. Finally, the fresh paste was casted in the cylinder molds and was covered with plastic film. The cylinder specimens were first kept under room temperature for 24 h before demolding. Then, all the specimens were cured in an automatic oven (Hebei Rongyao Corporation, China) with the specified temperature (25 °C, 50 °C and 70 °C) for 28 days.
Fig. 3. Particle size distribution curves for raw materials.
Fig. 2. Micro morphology of raw materials.
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J. Cai et al. / Construction and Building Materials 237 (2020) 117757 Table 1 Chemical constituents of solid precursors (wt%). Compositions
CaO
SiO2
Al2O3
Fe2O3
MgO
K2O
Na2O
P2O5
MnO
Metakaolin Fly ash Cement Slag
0.13 3.71 61.13 34.93
61.45 50.86 21.45 40.28
32.45 28.16 5.42 10.11
0.89 6.24 2.89 0.07
2.08 1.28 2.37 8.14
0.81 0.67 2.08 1.12
0.77 1.27 0.81 1.12
0.07 0.12 2.50 0.08
– 0.07 0.77 1.14
Table 2 Details of all samples. Sample ID
Solid precursors
KOH concentrations (mol/L)
Slag replacement ratio %
Solid-to-liquid ratio
FA-4-0 FA-8-0 FA-12-0 FA-16-0 FA-8-30 FA-8-0 FA-8-90 MK-4-0 MK-8-0 MK-12-0 MK-16-0
Fly ash Fly ash Fly ash Fly ash Fly ash with slag Fly ash with slag Fly ash with slag Metakaolin Metakaolin Metakaolin Metakaolin
4 8 12 16 8 8 8 4 8 12 16
0 0 0 0 30 60 90 0 0 0 0
3 3 3 3 3 3 3 1 1 1 1
The flow chart of casting geopolymer specimen is shown in Fig. 4. The humidity was maintained as ambient humidity (about 40%) by water nebulizer. The dimensions of typical geopolymer specimen used for electrical resistance measurement are shown in Fig. 5. 2.3. Test set-up Seebeck effect is a phenomenon in which a temperature difference between two surfaces of semiconductors produces a voltage difference. The voltage difference of each geopolymer sample
was measured with two-probe measurement method. The schematic diagram of test set-up is shown in Fig. 5. The copper sheets with the width of 10 mm were attached on both ends of the specimen. Silver paint was applied between the copper sheets and the sample surface to further enhance the electrical contact. Two copper wires were wrapped around each copper sheet and connected to anodic and cathodic terminal of a digital multimeter (Fluke Technologies, 8808A), respectively. The resistance heater (Bailer Technologies, H32B) was applied at the bottom end of the specimen while the upper end was not heated. The temperature of
Fig. 4. The flow chart of casting geopolymer specimen.
Fig. 5. A schematic diagram of the test set-up.
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The temperature difference versus recorded seebeck voltage of typical cement, fly ash and metakaolin based geopolymer specimens are shown in Fig. 6. It can be seen that the seebeck voltage for both fly ash and metakaolin based geopolymers are all negative, indicating geopolymers can be classified as n-type semiconductor. When both ends of geopolymer specimen were under different temperature, the inner electrons would flow from the heating end to the cooling end, thus producing a seebeck voltage difference inside the geopolymer specimen. With the increase of temperature difference, as shown in Fig. 6, the seebeck voltage increased linearly as well, indicating more electrons were released and flowed from the heating end to the cooling end. The seebeck coefficient could be represented by the slope of each curve shown in Fig. 6 [17]. As can be seen in Table 3, the seebeck coefficient for pure cement paste is only 0.27 lV/°C, which is quite low and far from meeting the requirement of practical application. This may be attributed to the reason that cement is a kind of multiphase and porous material, the hydration products of cement are disordered and irregular. Even though the inner electrons could be activated under temperature difference, the directed transfer of electrons may still be disturbed or blocked by the needle-like, bar-like and sheet-like crystals inside the cement paste. By contrast, the seebeck coefficient for fly ash and metakaolin based geopolymers are as high as 3.36 lV/°C and 12.07 lV/°C,
respectively. It can be concluded that the seebeck coefficient for geopolymers could be an order of magnitude higher than cement paste. There are two possible reasons accounting for this phenomenon. First, geopolymer is characterized as three-dimensional aluminosilicate containing AlO-4 and SiO4 as tetrahedral subunits, as shown in Fig. 7 [20]. Thus, geopolymers may have more regular and homogeneous microstructure compared with conventional cement paste, this enables the activated electrons to be more easily transferred from the heating end to the cooling end with less interference and impediment, resulting a higher voltage difference between the two ends. It should be noted that for homogeneous elemental materials such like metallic materials and monocrystalline silicon, the existing thermoelectric theory can provide a clear theoretical illustration for their seebeck effect [21]. Therefore, it is feasible to increase the Seebeck coefficient by adjusting the morphology of homogeneous elemental materials [22]. However, for geopolymer, classified as typical heterogeneous material, the situation is much more complicated and the existing theoretical methods may not be directly applied. In this case, the microstructure may significantly influence the thermoelectric behavior of heterogeneous material. It may be also this reason that why the metakaolin based geopolymer showed a much higher seebeck coefficient than fly ash based geopolymer. As shown in Table 3, the seebeck coefficient for metakaolin based geopolymer is about 3.6 times higher than that for fly ash based geopolymer. It has been reported that the active amorphous phase material account for more than 80% of its total mass in metakaolin. By contrast, fly ash contains more than 60% chemically inactive glassy phases materials [23]. As can be seen in Table 1 and Fig. 2, metakaolin possess a much higher fineness as well as more aluminosilicate content than that of fly ash. Thus, with the same curing condition and alkali concentration, metakaolin based geopolymer can have a denser and more homogeneous structure than fly ash based geopolymer.
Fig. 6. Seebeck voltage versus temperature difference for geopolymers and cement.
Fig. 7. Typical geopolymer types with aluminosilicate structure.
the two ends were detected by two T-type thermocouples (CESMOOY brand, TT-T-24-SLE) attached on each end. The voltage and temperature difference of each specimen were recorded simultaneously by the multimeter. Then, the voltage difference divided by the temperature difference yielded the Seebeck coefficient. It should be noted the copper wires at the two ends were also at different temperature, thus the calculated Seebeck coefficient should plus the absolute thermoelectric power of copper (2.34 lV/°C) [20]. 3. Experimental results and discussions
Table 3 Linear fitting results. Specimen
Cement MK-12-0-50 °C FA-12-0-50 °C
Intercept
Slope (Seebeck coefficient)
Value
Standard Error
Value
Standard Error
0.28 4.68 8.95
0.14 2.25 2.85
0.27 3.36 13.28
0.01 0.08 0.11
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The second reason may be that geopolymers contain more lowdimensional nanomaterial than conventional cement paste. The typical SEM micrographs of metakaolin based geopolymer, fly ash based geopolymers and cement paste are shown in Fig. 8. Geopolymer possesses a compact and continues gel-like structure, attached with numerous zero-dimensional particulate matters, while many one-dimensional bar-like and two-dimensional sheet-like hydration products can be observed in cement paste. It has been reported that decreasing the micro size of semiconductors may cause a quantum confinement effect [24,25], which could dramatically increase the density of states (DOS). The DOSs for one-, two-, and three-dimensional materials are listed as Eqs. (1) and (3) [26], respectively.
g 1D;nm ðEÞ ¼ g 2D;n ðEÞ ¼
g 3D ðEÞ ¼
1=2 2md
1
aph2 md
aph2
2
h
; E En ;
ðE Enm Þ1=2 ; E Enm ; md ¼
md ¼ mx
p ffiffiffiffiffiffiffiffiffiffiffiffiffi 2 mx my
3=2 1 2md E1=2 ; E 0; 2p 2 h 2
md ¼
ð1Þ ð2Þ
p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 mx my mz
ð3Þ
where g 1D;nm ðEÞ, g 2D;n ðEÞ and g 3D ðEÞ are the DOS for one-, two-, and three-dimensional materials; h is Planck constant; mx;y;z is the effective mass in x, y and z directions; a is the width or thickness of the quantum wire; Enm and En are the confinement energies of quantum wire and quantum well, respectively. The total DOS is the sum of each individual bans, shown as follows:
g ðEÞ ¼
þ1 X
g n ðEÞ
ð4Þ
n¼1
The total DOS for semiconductors with different dimensions are shown in Fig. 9. It can be concluded that materials with lower dimensional structures exhibit sharper DOSs when compared with materials with higher dimensional structures. The direct relation between EDS and seebeck coefficient is shown as follow [27]:
S¼
1 eT
R þ1
gðEÞsðEÞEt2 ðEÞ dfde0 dE E EF R 0þ1 gðEÞsðEÞt2 ðEÞ dfde0 dE E0
! ð5Þ
where S is the seebeck coefficient; e is the electric quantity for a single electron; T is the absolute temperature; sðEÞ is the relaxation time of electrons; gðEÞ is the total DOS; f 0 ðEÞ is the distribution of electrons and EF is the energy of Fermi band. The formulas indicate that the increase of DOS can lead to an increased seebeck coefficient, and the zero-dimensional particles attached on the surface of geopolymer can dramatically increase the DOS of geopolymer specimens, which can consequently increase the seebeck coefficient of geopolymer paste. As shown in Fig. 8, some unreacted fly ash particles can be observed inside the fly ash based geopolymer, indicating that fly ash based geopolymer may contain less low dimensional aluminosilicate while being compared with metakaolin based geopolymer, which can be also the reason that the seebeck coefficient for fly ash based geopolymer is much lower than that for metakaolin based geopolymer. 3.1. Influence of alkali concentration The seebeck voltage versus temperature difference for fly ash and metakaolin based geopolymer paste with different alkali concentration are shown in Fig. 10. The alkali concentration varied from 4 mol/L to 12 mol/L. The seebeck coefficient can be represented by the slope of each curve, as shown in Table 4. For metakaolin based geopolymer, the seebeck coefficient increased with the increase of alkali concentration. This is quite reasonable since a higher alkali concentration means more OH– and K+ ions were introduced in the system. The higher alkali ion concentration would seriously influence the Young’s modulus, micro morphology as well as micro structure of metakaolin based geopolymer [28]. A higher alkali concentration would result in a denser microstructure, which would be beneficial for the transportation of electrons and subsequently introduce more low-dimensional aluminosilicate particles. The same phenomenon can be observed for fly ash based geopolymer: the seebeck coefficient for specimen FA-12-0-50 °C is about two times higher than that for
Fig. 8. SEM micrograph of metakaolin based geopolymers and cement.
Fig. 9. Schematic diagram for EDS of (a) three-; (b) two-; and (c) one-dimensional material.
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J. Cai et al. / Construction and Building Materials 237 (2020) 117757
Fig. 10. Seebeck voltage versus temperature difference for geopolymers with different alkali concentration: (a) metakaolin based geopolymers; (b) fly ash based geopolymers.
Table 4 Linear fitting results. Specimen
MK-4-0-50 °C
MK-8-0-50 °C
MK-12-0-50 °C
FA-4-0-50 °C
FA-8-0-50 °C
FA-12-0-50 °C
S
10.73
12.01
13.28
1.63
2.52
3.36
Note: S is seebeck coefficient (lV/°C).
FA-4-0-50 °C. The reason behind this phenomenon may be that the alkaline activator with higher alkali concentration provides more OH–, thus more Al and Si atoms will be dissolved from fly ash, followed by a higher concentration of Al and Si atoms in the alkaline solutions. These atoms would be converted into geopolymer gel with the geopolymerization process going on, thus the microstructure is likely to be denser with lower dimension gel-like particles. 3.2. Influence of curing temperature It has been reported that the curing temperature has a significant influence on the strength, pore distribution and microstruc-
ture of both fly ash and metakaolin based geopolymers, the chemical composition and physical structure of geopolymer could even be quite different with different curing conditions [29,30]. In this paper, the curing temperatures were set as 25 °C, 50 °C and 70 °C. For metakaolin based geopolymer, as can be seen in Fig. 11a and Table 5, the seebeck coefficient for all metakaolin based geopolymer samples are similar, indicating the curing temperature has negligible influence on the thermoelectrical properties for metakaolin based geopolymer at 28 days. Previous researches have also reported that the temperature curing conditions show limited influences on the quality and property of geopolymerization products for metakaolin based geopolymer, the 28-day compressive strength for metakaolin based geopolymer
Fig. 11. Seebeck voltage versus temperature difference for geopolymers with different curing temperatures: (a) metakaolin based geopolymers; (b) fly ash based geopolymers.
Table 5 Linear fitting results. Specimen
MK-8-0-25 °C
MK-8-0-50 °C
MK-8-0-70 °C
FA-8-0-25 °C
FA-8-0-50 °C
FA-8-0-70 °C
S
10.13
12.01
15.12
1.26
2.52
6.31
Note: S is seebeck coefficient (lV/°C).
J. Cai et al. / Construction and Building Materials 237 (2020) 117757
with high temperature curing conditions was even lower than that for geopolymer under ambient curing temperature [29]. For fly ash based geopolymer, as can be seen in Fig. 11(b), the influence of curing temperature was much more significant. When the curing temperature was 25 °C, the seebeck coefficient was only 1.26 lV/°C at 28 days, indicating the geopolymerization process was still incomplete. Previous researchers have also reported that the compressive strength of fly ash based geopolymer cured at ambient temperature was lower than 10 MPa at 28 days [31]. The seebeck coefficient for specimen FA-8-0-70 °C was about five time higher than that for specimen FA-8-0-25 °C, indicating much denser microstructure as well as more low-dimensional aluminosilicate particles were formed in the geopolymer paste with higher curing temperature. This is reasonable since fly ash generally manifests lower chemical activity and higher activation energy barrier, thus elevating the curing temperature would effectively facilitate the process of dissolving Al and Si atoms from inactive fly ash particles [32].
3.3. Influence of slag replacement ratio As mentioned above, the geopolymerization process for fly ash based geopolymer is very sensitive to curing temperature. The addition of slag in fly ash based geopolymer can dramatically increase the mechanical strength with ambient temperature curing conditions [33]. The effects of slag replacement ratio on the thermoelectrical properties of fly ash based geopolymers are shown in Fig. 12 and Table 6. It can be found that the seebeck coefficient decreased with the increase of slag replacement ratio, indicating that more high-dimensional aluminosilicate structures were formed with the addition of slag. It may be attributed to the reason that the addition of slag consumes more OH– and SiO2 3 while produces more C-A-S-H gel in the paste due to the pozzolanic reactions between slag and alkali activator. This reaction process was much more violent than geopolymerization process and more needle-like, bar-like, and sheet-like crystals were formed in the
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paste, which would decrease the DOS and seebeck coefficient of the specimen in the meantime. 4. Discussions In this paper, the highest seebeck coefficient for geopolymer paste, although much higher than conventional cement paste, was only 15.12 lV/°C, and this value could be further improved with the addition of various thermoelectric admixture (such as manganese dioxide nanorods and silicon nanowires) and conducting materials (such as carbon black and carbon fiber). The improvement of seebeck coefficient of geopolymer paste would also be helpful for the self-sensing behavior of geopolymer concrete. For example, it is feasible to monitor the temperature of mass geopolymer concrete by recording its voltage change, since the temperature difference in core area and surface area of geopolymer concrete will generate voltage due to seebeck effect. For the propose of energy-harvesting, the thermoelectric conversion efficiency depends on the thermoelectric figure of merit (ZT), which can be expressed as follows:
ZT ¼ S2 T=qk
ð6Þ
where S,T, q and k are the Seebeck coefficient, absolute temperature, electrical resistivity and thermal conductivity, respectively. It would be commercially-viable if the ZT value could be as high as 1. However this hard to achieve since all the parameters are interdependent [34]. The thermal conductivity as well as electrical resistivity of geopolymer would be further investigated in the next step. 5. Conclusions In this work, the thermoelectrical behaviors of fly ash and metakaolin based geopolymers were investigated. The main conclusions are shown as follows: 1. The seebeck coefficient for geopolymer is much higher than that for normal cement paste, since geopolymer is denser and it contains more low dimensional aluminosilicate particles. 2. The seebeck coefficient for both fly ash and metakaolin based geopolymers increased with the increase of alkali concentration. A higher alkali concentration would result in a denser micro-structure for geopolymers, which would be beneficial for the transportation of electrons. 3. The curing temperature has negligible influence on the seebeck effect of metakaolin based geopolymer. By contrast, curing temperature has a significant influence on the seebeck effect of fly ash based geopolymer. 4. For fly ash based geopolymer, the seebeck coefficient decreased with the increase of slag replacement ratio, since the addition of slag may introduce more high dimensional calcium silicoaluminate particles.
Declaration of Competing Interest Fig. 12. Seebeck voltage versus temperature difference for fly ash based geopolymers with different slag replacement ratio.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments
Table 6 Linear fitting results. Specimen
FA-8-0-50 °C
FA-8-30-50 °C
FA-8-0-650 °C
FA-8-90-50 °C
S
2.52
1.91
1.39
0.88
Note: S is seebeck coefficient (lV/°C).
This work was financially supported by Open Foundation of Key Laboratory of Concrete and Prestressed Concrete Structure of Ministry of Education CPCSME2018-06, National Natural Science Foundation of China under 51908117.
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J. Cai et al. / Construction and Building Materials 237 (2020) 117757
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