Construction materials with increased electrical conductivity could be possibly used in health monitoring of structures (stress, deformation, damages), their maintenance or traffic monitoring. The aim of this study was the application of functional filler and its influence on the electrical properties of the alkali-activated fly ash matrix. The graphite powder was added to the reference material in the amount of 2–10 %. Besides the assessment of the critical amount of filler necessary to achieve a percolation threshold in the structure of the composite, the effect on the electrical properties of the matrix (resistance, capacitance, conductivity) was determined. The optimal amount of filler was also determined with respect to the changes in microstructure of the binder and its mechanical properties

Kusák, Ivo

Mizerová, Celílie

Rovnaník, Pavel

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

doi:10.14311/APP.2019.22.0072Acta Polytechnica CTU Proceedings 22:72–76, 2019 © Czech Technical University in Prague, 2019available online at http://ojs.cvut.cz/ojs/index.php/appELECTRICAL PROPERTIES OF FLY ASH GEOPOLYMERCOMPOSITES WITH GRAPHITE CONDUCTIVE ADMIXTURESCecílie Mizerová∗, Ivo Kusák, Pavel RovnaníkBrno University of Technology, Faculty of Civil Engineering, Žižkova 17, 602 00 Brno, Czech Republic∗ corresponding author: mizerova.c@fce.vutbr.czAbstract. Construction materials with increased electrical conductivity could be possibly used inhealth monitoring of structures (stress, deformation, damages), their maintenance or traffic monitoring.The aim of this study was the application of functional filler and its influence on the electrical propertiesof the alkali-activated fly ash matrix. The graphite powder was added to the reference material inthe amount of 2–10%. Besides the assessment of the critical amount of filler necessary to achieve apercolation threshold in the structure of the composite, the effect on the electrical properties of thematrix (resistance, capacitance, conductivity) was determined. The optimal amount of filler was alsodetermined with respect to the changes in microstructure of the binder and its mechanical properties.Keywords: Fly ash, geopolymer, graphite, electrical properties, mechanical properties, microstructure.1. IntroductionRecently, one of the field in materials and structuresresearch is the concept of advanced smart (or self-sensing) material. Its composition design is basedon the content of conductive fillers that enhance itselectrical properties. For example, smart compositewith higher electrical conductivity can be used for theassessment of structural health in terms of stress mon-itoring, localization of cracks and damages, humidityand temperature changes etc. [1]. There is a widerange of different functional fillers to use: steel fibres,carbon black, graphite powder, carbon fibres (CF),carbon nanotubes (CNF), graphene, nickel powder.So far, this approach has been studied predomi-nantly in traditional cement concretes and compositestechnology. Alkali-activated materials are binders de-rived from the reactions of raw materials with a highcontent of amorphous aluminosilicate phase with alka-line activator solution. Alkaline activator (most oftenin form of alkali hydroxides and/or alkali silicates)is required for the appropriate conditions for the dis-solution rate and the formation of new phases [2].The first theoretical background of alkaline activationwas developed by Glukhovsky in 1959 [3]. The term“geopolymer” was firstly used by prof. Davidovitsand refers to a material formed by almost exclusivelyhighly coordinated units of aluminosilicate structuresforming regular polymeric three-dimensional struc-ture [4].Compared to clay-based geopolymers, fly ash pro-vides more favourable rheology of the fresh binder[5], strength of the hardened binder can be enhancedby thermal treatment. Properties of alkali-activatedbinders are comparable or superior to Portland ce-ment binders [6]. Fly ash geopolymers are generallymore durable because of higher chemical and thermalresistance [7]. Moreover, the geopolymer productionusing mostly secondary raw materials do not requirean exploitation of natural resources and high energyconsumption that significantly contribute to loweremissions of CO2 compared to Portland cement [8].According to Hanjitsuvan et al. [9, 10], the electricalconductivity of fly ash geopolymer matrix is affectedby the NaOH concentration in activator solution, thefrequency spectrum and liquid activator/ash ratio(L/A). Payakaniti [11] studied properties of geopoly-mer incorporated with CF stating the percolationthreshold was reached by the geopolymer with 0.5%CF content, this value was increasing with growingL/A. This CF content was also responsible for the bestmechanical performance. Vaidya et al. [12] tested flyash geopolymer concrete beams and cylinders with0.4% CF while exposed to bending and compressivestress to assess the changes of electrical resistance.Regarding the application of CNT in geopolymer con-crete, Saafi et al. [13] states that the optimal dosage is0.5% which ensures the improvement of both electricaland mechanical performance, higher percentage leadsto less effective dispersion and thus cluster formationwithin the matrix. In another study, the author usedgraphene as a conductive filler [14]. Only 0.35% ad-dition of graphene resulted in significant increase ofstrength and modulus of elasticity accompanied bylower porosity and 209% increase of conductivity.This paper is focused on the application of graphitepowder as a conductive admixture in fly ash geopoly-mer mortars considering its impact on selected elec-trical properties, strength and changes in binder mi-crostructure.2. Materials and methodsThe geopolymer binder was produced by mixing offly ash (FA) with commercial sodium silicate solutionwith SiO2/Na2O = 1.6. The fly ash is a product ofblack coal combustion and its chemical compositionis given in Table 1. Quartz sand with a maximum72vol. 22/2019 Electrical properties of fly ash geopolymer composites. . .SiO2 Al2O3 Fe2O3 CaO MgO Stotal Na2O K2O(%) 49.82 24.67 7.5 3.91 2.68 0.91 0.7 2.78Table 1. Chemical composition of fly ash.REF G2 G4 G5 G6 G8 G10Fly ash (g) 350 350 350 350 350 350 350Sodium silicate (g) 280 280 280 280 280 280 280Graphite powder (g) - 7 14 17.5 21 28 35Sand (g) 1050 1050 1050 1050 1050 1050 10502% Triton X-100 (g) - 7 14 17.5 21 28 351% Lukosan (g) - 3 6 7.5 7.5 7.5 7.5Water (g) 35 30 30 32 32 32 32Table 2. Mix composition of FA geopolymer mortars with graphite powder.grain size of 2.5mm was used as aggregate. Graphitepowder PMM 11 was used as a conductive filler in 2,4, 5, 6, 8 and 10% wt. of the fly ash and its dispersionwithin the matrix was supported by the addition ofdispersing agent (2% Triton X-100). Defoaming agent(1% Lukosan) was introduced to reduce the air contentin fresh and hardened binder.The raw mix composition is presented in Table 2.Each of the mixes was produced following these steps:at first, the fly ash was mixed with water glass. Then,the graphite powder and dispersing agent were addedand homogenized with the binder and a small amountof water, if needed. Thereafter, each fraction of ag-gregate (from fine to coarse) was added and mixedproperly. The defoaming agent and remaining amoutof water were added at the very end of the mixingprocedure.The mortars were cast into prismatic moulds (40×40 × 160mm) and covered with a plastic sealant toavoid moisture loss. After 2 hours in the ambientconditions, the specimens were heated at 40 °C for 24 h.After demoulding, the hardened specimens were storedin the laboratory conditions (22 ± 2 °C, ϕ = 45 ± 5%)till the age of testing.The electrical and mechanical properties (compres-sive and flexural strength) were tested at the age of56 days. The prepared prismatic samples were char-acterized by impedance spectroscopy in the range of40 Hz to 1 MHz using an Agilent 33220A sinusoidal sig-nal generator and an Agilent 54645A dual-channel os-cilloscope. The output voltage of the signal generatorwas 5.5V. The input values for the electrical capacityand the resistance of the oscilloscope were 13 pF and1MΩ, respectively. In order to perform impedanceanalysis, the prismatic specimens were placed betweenparallel brass electrodes (30 × 100mm) so that a dis-tance between electrodes was 40mm. Conductivitymeasurements were performed in the range of 10 to3510MHz. The microstructure and porosity of thegeopolymers were evaluated using Micromeritics Pore-sizer 9310 and scanning electron microscope (SEM)Tescan MIRA3 XMU.3. Results and discussionElectrical resistance of all samples decreased withhigher frequency applied, as can be seen from Fig. 1.The lowest resistance was observed in case of 10%graphite content (G10), at all frequencies. The dif-ferences in resistance curves are clearly visible up to10 kHz. Above this frequency, the resistance of allsamples fluctuate and the curves overlap. At 100Hz,the resistance of reference sample was 159.80MΩ. Theresistance of G5 dropped to 43.78MΩ and 0.85MΩonly at G10.Changes of resistance and conductivity at chosenfrequency are shown in Fig. 2. While the resistancedecreased continuously, we registered an exceptionalincrease in conductivity of specimen G10 from 0.38 to6.31mS·m−1. These results suggest that geopolymerwith 10% of graphite may have reached the percolationthreshold.The capacitance of geopolymers with variousgraphite content as a function of frequency is shownin Fig. 3. Up to 6% of graphite, the changes in capac-itance are comparable, but the capacitance of G8 andG10 shows the most notable increase over all frequen-cies. Capacitance of all samples except G10 decreasedwith higher frequency. The capacitance of G10 firstdropped from 222 to 122 pF (40–180Hz), within thefrequency 180–10 000Hz it was about 125 pF and athigher frequency applied it was decreasing again.The changes of mechanical performance depend-ing on the graphite content are displayed in Fig. 4.The flexural strength was not negatively influencedby the graphite addition and fluctuated around5MPa at all graphite concentrations. On the con-trary, the compressive strength gradually decreased.While the reference sample without graphite reached23.5MPa, the samples with 2% graphite content (G2)reached 17.7MPa and 10% graphite content (G10)73C. Mizerová, I. Kusák, P. Rovnaník Acta Polytechnica CTU ProceedingsFigure 1. Variation in electrical resistance of FA geopolymers with different graphite content.Figure 2. Resistance (at 100Hz) and conductivity (at 100MHz) of FA geopolymers with different graphite content.Figure 3. Variation in capacitance of FA geopolymers with different graphite content.74vol. 22/2019 Electrical properties of fly ash geopolymer composites. . .Figure 4. Mechanical properties of FA geopolymers with different graphite content.Figure 5. Cumulative intruded volume of FA geopolymers with different graphite content.(a) . REF. (b) . G2. (c) . G5. (d) . G10.Figure 6. SEM images of REF, G2, G5 and G10 geopolymers.75C. Mizerová, I. Kusák, P. Rovnaník Acta Polytechnica CTU Proceedingsonly 7.8MPa, which is equal to 25% and 65% drop ofstrength, respectively.Regarding the microstructure of the geopolymerbinder, even 2% of graphite caused an increase intotal porosity of the samples (Fig. 5). While thegeopolymers with a graphite content of 2–8% did notexhibit considerable differences, we observed a furtherincrease in pore volume of G10 which is more than60% higher than the reference sample.In Fig. 6 we can see the nature of chosen bindersin SEM images. Reference sample is characterizedby homogeneous amorphous binder phase whereasnumber of pores and inhomogeneities increased withthe amount of conductive filler.4. ConclusionsThe results mentioned above confirm that graphitepowder can be applied to fly ash geopolymer matrix toenhance its electrical properties. Higher graphite con-tent resulted in reduced electrical resistance, especiallyin frequency up to 1 000Hz. Likewise, the sampleswith high graphite content exhibited an increase in ca-pacitance. The best performance in terms of electricalproperties was observed in case of geopolymer with10% graphite content providing significant increase inconductivity, which may refer to achieving the perco-lation threshold. However, the presence of conductivefiller in such concentration deteriorated the mechan-ical performance in compression by more than 65%;flexural strength of all samples remained constant re-gardless of the amount of graphite powder. Despitethe use of the defoaming agent, the microstructureof all geopolymers with graphite was characterizedby higher pore volume which is in a good accordancewith reduced compressive strength. We can concludethat graphite powder in fly ash geopolymer allowsto develop a conductive network within the matrixbut also causes generation of additional pores duringmixing. The negative impact of graphite powder onthe compressive strength is apparently attributed toa weak bond between the matrix and graphite parti-cles because of nonpolar character of graphite surface.Low toughness of graphite itself can further contributeto the strength loss at high concentrations. Possibleways to reduce these impacts include more efficientdefoaming or improved adhesion of graphite particlesto the matrix by partial modification of its structurewith hydrophilic substituents.AcknowledgementsThis study has been financially supported by the BUT spe-cific research project FAST-J-18-5484 and by Ministry ofEducation, Youth and Sports under the ‘National Sustain-ability Programme I’ – the project No. LO1408 AdMaSUP.References[1] B. Han, L. Zhang, J. Ou. Smart and MultifunctionalConcrete Toward Sustainable Infrastructures, chap.Self-Sensing Concrete, pp. 81–116. Springer, Singapore,2017. doi:10.1007/978-981-10-4349-9_6.[2] C. Shi, D. Roy, P. Krivenko. Alkali-Activated Cementsand Concretes. CRC press, 2003.doi:10.1201/9781482266900.[3] V. D. Glukhovsky. Soil silicates (Gruntosilikaty).Budivelnik Publisher, Kiev, 1959.[4] J. Davidovits. Properties of geopolymer cements. InFirst international conference on alkaline cements andconcretes, pp. 131–149. 1994.[5] L. Provis, J. S. van Deventer. Alkali Activated Materials.Springer, 2014. doi:10.1007/978-94-007-7672-2.[6] P. Duxson, A. Fernández-Jiménez, J. L. Provis, et al.Geopolymer technology: the current state of the art.Journal of materials science 42(9):2917–2933, 2007.doi:10.1007/s10853-006-0637-z.[7] A. Fernández-Jiménez, I. García-Lodeiro, A. Palomo.Durability of alkali-activated fly ash cementitiousmaterials. Journal of Materials Science 42(09):3055–3065, 2007. doi:10.1007/s10853-006-0584-8.[8] K. P. Mehta. Reducing the environmental impact ofconcrete. Concrete international 23(10):61–66, 2001.[9] S. Hanjitsuwan, S. Hunpratub, P. Thongbai, et al.Effects of NaOH concentrations on physical andelectrical properties of high calcium fly ash geopolymerpaste. Cement and Concrete Composites 45:9–14, 2014.doi:10.1016/j.cemconcomp.2013.09.012.[10] S. Hanjitsuwan, P. Chindaprasirt, K. Pimraksa.Electrical conductivity and dielectric property of fly ashgeopolymer pastes. International Journal of Minerals,Metallurgy, and Materials 18(1):94–99, 2011.doi:10.1007/s12613-011-0406-0.[11] P. Payakaniti, S. Pinitsoontorn, P. Thongbai, et al.Electrical conductivity and compressive strength ofcarbon fiber reinforced fly ash geopolymeric composites.Construction and Building Materials 135:164–176, 2017.doi:10.1016/j.conbuildmat.2016.12.198.[12] S. Vaidya, E. N. Allouche. Strain sensing of carbonfiber reinforced geopolymer concrete. Materials andstructures 44(8):1467–1475, 2011.doi:10.1617/s11527-011-9711-3.[13] M. Saafi, K. Andrew, P. L. Tang, et al. Multifunctionalproperties of carbon nanotube/fly ash geopolymericnanocomposites. Construction and Building Materials49:46–55, 2013. doi:10.1016/j.conbuildmat.2013.08.007.[14] M. Saafi, L. Tang, J. Fung, et al. Graphene/fly ashgeopolymeric composites as self-sensing structuralmaterials. Smart materials and structures 23(6):065006,2014. doi:10.1088/0964-1726/23/6/065006.76

ELECTRICAL PROPERTIES OF FLY ASH GEOPOLYMER COMPOSITES WITH GRAPHITE CONDUCTIVE ADMIXTURES

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ELECTRICAL PROPERTIES OF FLY ASH GEOPOLYMER COMPOSITES WITH GRAPHITE CONDUCTIVE ADMIXTURES

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