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Enhancement of compressive strength and thermal shock resistance of fly ash-based geopolymer composites
Construction and Building Materials 121 (2016) 653–658
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Enhancement of compressive strength and thermal shock resistance of fly ash-based geopolymer composites Patthamaporn Timakul, Weerada Rattanaprasit, Pavadee Aungkavattana ⇑ National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency (NSTDA), 114 Thailand Science Park, Pahonyothin Rd., Klong Luang, Pathumthani 12120, Thailand
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
TiO2 promoted hydration reactions in
geopolymers resulting more C S H phase. The strength increased due to the formation of C S H and anatase phases in matrix. Addition of 5% TiO2 resulted in increased by 44% strength.
a r t i c l e
i n f o
Article history: Received 1 March 2016 Received in revised form 10 June 2016 Accepted 13 June 2016 Available online 20 June 2016 Keywords: Geopolymer Composites Titanium dioxide Compressive strength Thermal shock resistance
a b s t r a c t This study presented the effect of 0 5 wt% TiO2 addition on compressive strength and thermal shock resistance of fly ash based geopolymer composites, which were prepared by activating high calcium fly ash reinforced with basalt fibers. The compressive strength of the composites after aging 28 days increased to 52 MPa when 5 wt% TiO2 was added. The retained compressive strength as 5 wt% TiO2 was added after 15 thermal cycles (800 °C) was 26 MPa, which was increased by 44% compared to TiO2 free composites. Such an enhancement was ascribed to the acceleration of hydration reactions in geopolymer structures resulting more C H S phases which resulted in a denser matrix with increased strength. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction For the past decade, geopolymeric materials have attracted a lot of attention due to their low cost and low energy for production ⇑ Corresponding author. E-mail address: [email protected] (P. Aungkavattana). http://dx.doi.org/10.1016/j.conbuildmat.2016.06.037 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
and environmental friendly nature [1]. Geopolymers are the inorganic polymeric materials with amorphous nature, formed by aluminosilicate and alkali-metal-silicate solution under alkaline conditions. Geopolymers can be employed as conventional building materials to replace the ordinary Portland cement due to their durability and comparable mechanical strength. Recently, various studies have reported the use of industrial wastes i.e. fly
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P. Timakul et al. / Construction and Building Materials 121 (2016) 653–658
ash and blast furnace slag as the potential sources for geopolymer synthesis [2–4]. Since interstitial water in geopolymer structures evaporates during high temperature applications such as 350–400 °C, which could cause mechanical cracking. Therefore, one of the solutions to reduce cracking and improve thermal stability is to make the geopolymer matrix homogeneous and pore free, which in turn will give rise to high mechanical strength matrix. Previous studies by Kriven et al. [5] and others [6–7] reported geopolymers could be used as a good binder with various reinforcements i.e. basalt fibers in order to significantly improve the tensile and flexural strength of the composites due to high elastic modulus and tensile strength of basalt fibers. The objective of the present study is to enhance the compressive strength and the thermal shock resistance of Class C fly ashbased geopolymer composites using basalt fibers as the reinforcing agent. Fly ash is the byproduct from Thailand Power Plant which currently generates 4 million tons per year at no cost in practice. Additionally, fly ash is rich in SiO2 (31.23%) and Al2O3 (15.76%) and is desirable as source materials, apart from kaolin, for the production of geopolymers. To improve the thermal shock resistance of the geopolymer composites targeted for refractory applications, this study aimed to investigate the effect of TiO2 (anatase phase) addition from 0 to 5 wt% to the composites. In cement-based materials for construction, TiO2 has been the most commonly used binder due to its fine particle size, high reactivity, and specific functional properties such as to generate ion exchange in geopolymer in order to incorporate photoactive TiO2 [8]. Moreover, the other advantages of TiO2 addition in geopolymer applications including purification of waste water, disinfection based on the bactericidal properties, self-cleaning coatings, and smog-abating functions [9]. There were several reports on the photocatalytic functions using nano-sized TiO2 which the basic principle could be summarized as when UV radiation exposed to TiO2, it could absorb photon energy which is approximately equal or larger than its band gap energy, and enable electrons (e ) to jump from the valence band to the conduction band. The electrons activation resulted in the holes (h+, electron vacancy) generation in the valence band. The electron-hole pairs then could recombine to generate redox reactions which could produce several radicals including OH, HO2, O2 and O when water vapour and oxygen were presented in the vicinity of activated TiO2 [10]. Recently, there has been a study on the effect of nano-TiO2 addition on the properties of fluidized bed fly ash-based geopolymer composites, and results showed that 5% nano-TiO2 could enhance the compressive strength of geopolymers at early stage and 28-day aged, and it also allowed the cementitious mixture to have better chemical stability, high catalytic activity, and affordable price [11]. Nevertheless, the effect of TiO2 on the thermal behavior of fly ash-based geopolymer composites has not been studied so far. The thermal shock resistance is one of the most important properties as most of refractory materials needed to undergo immediate temperature changes in their applications. Therefore, this present study aimed to investigate the effect of TiO2 addition on the basalt fibers reinforced fly ash-based geopolymer composites in terms of compressive strength and thermal shock behavior for our targeted applications in construction and refractory industries. Using these geopolymer composites for widely applications, it is required to consider the cost of TiO2 whether it is feasible. The TiO2 powder cost is in the range of $US 1.80 2.20/kg, which could take part for 5% of all raw materials cost in geopolymer. A study by Komnitsas et al. [12] reported the industrial wastes such as red mud from Aluminium of Greece plant in Veotia, Greece which contained 4.73 wt% of TiO2 and could be potentially an alternative source for TiO2 replacement in geopolymer production, which in turn reduce the raw materials cost for approximately 5%. Nevertheless, the red mud is composed of other oxides mainly Fe2O3, which may
affect the geopolymer properties, therefore further investigations shall be required and considered. 2. Materials and methods An alkali activator was prepared by combining sodium hydroxide (5 M, NaOH) solution and sodium silicate (Na2SiO3) solution for 2.5:1 in weight ratio. The 5 M concentration of NaOH was chosen for the good workability of the mixing paste during casting into the moulds. It was found in this study that the higher molarity of NaOH such as 8 10 M accelerated the setting times of the paste which made the workability difficult. This could be related to more leaching of silica and alumina in fly ash with high NaOH concentration. A study by Hardjito et al. [13] reported the use of high CaO content fly ash could produce faster initial setting time due to fly ash has high pH value (above 11). High Ca fly ash (Class C, ASTM C618) obtained from Thailand power plant was dry-mixed with basalt fibers (5 lm OD, and 12–60 lm in length) in the ratio 90:10 wt%. This particular ratio was chosen from the best performance on compressive strength in the previous study [4]. Basalt fibers, as the reinforcing agent had the density of 2.50 g/cm3 which was measured by a pycnometer (UPYC 1200E). The major chemical compositions of fly ash and basalt fibers were detailed in Table 1. TiO2 powder (P99% purity) from 0 to 5 wt% was added to the dry mixes. The specimens without TiO2 powder were also prepared as a reference for comparison. For all mixes, the liquid to solid ratio was kept constant at 0.4. The actual composition of geopolymer matrix as the following molar ratios, SiO2/Al2O3 = 3.8, Na2O/SiO2 = 0.07, and H2O/Na2O = 36 were determined from source materials. Details on sample preparation and aging conditions were already published [4]. The characterization was performed after the samples aged 28 days. Phase development was investigated using XRD. Morphology and microstructure of geopolymers was examined by SEM and EDS (Hitachi S-3400N) on Au-coated specimens. Type of bonding was identified by FT-IR (Perkin Elmer system 2000) in the region of 400–4000 cm 1. The cubic (50 50 50 mm) geopolymer composites were prepared for compressive strength and thermal shock resistance measurement. The strength tests were performed according to ASTM C109 [14] using ADR Auto-250, ELE International. The thermal shock resistance behavior of geopolymer composites was studied by measuring the retained compressive strength after the samples have been through heating-cooling for 5, 10, 15 cycles. In each cycle, tested samples were placed in the furnace at 800 °C with a soaking period of 10 min, and then they were rapidly taken out to room temperature (25 °C) for 10 min complying with ASTM C1171 procedure [15].
3. Results and discussion 3.1. Phase compositions of geopolymer composites Phase analysis of starting materials and geopolymer composites aged 28-day was shown in Fig. 1a. The mineralogical compositions of starting basalt fibers consisted largely of amorphous phase, whereas fly ash contained mainly amorphous with some crystalline phases including anhydrite (CaSO4), calcite (CaCO3), hatrurite (Ca3SiO6), quartz (SiO2), magnesioferrite (MgFe2O4), and calcium oxide (CaO). In addition, the phase structure of all geopolymer composites with TiO2 addition were composed of broad humps of alumino-silicate from 20° to 40° (2h) with some crystalline phases of anatase (TiO2), calcium-silicate-hydrate (C-S-H), hatrurite (Ca3SiO6), magnesioferrite (MgFe2O4) phases. C-S-H phase is the main hydration product formed during cement production [16]; therefore the formation of C-S-H phase and alumino-silicate framework in this experiment suggested that the hydration reactions and geopolymerization occurred simultaneously [17]. The XRD patterns of geopolymer composites with different TiO2 wt% and performed the thermal shock tests for 15 cycles were presented in Fig. 1b. The samples showed amorphous with some crystalline phases, the intensity of anatase and C-S-H phases increased with increasing TiO2 wt%. The increase in C-S-H phase suggested that TiO2 accelerated the hydration reactions in geopolymers which was in a good agreement with a study by Chi-sun Poon et al. [10] which reported the addition of nano-TiO2 from 5 to 10 wt% could accelerate the hydration of tricalcium aluminate (C3A) by inert or active ultrafine particles could act as potential heterogeneous nucleation sites for the hydration products and the grain boundary region was densely populated with nuclei and completely transformed early in the overall
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P. Timakul et al. / Construction and Building Materials 121 (2016) 653–658 Table 1 Chemical compositions of as-received fly ash and basalt fibers. Major oxides
SiO2
Al2O3
Fe2O3
CaO
SO3
K2O
MgO
Na2O
TiO2
LOI
Class C fly ash (mass%) Basalt fibers (mass%)
31.23 37.48
15.76 14.4
12.28 1.08
28.76 36.85
5.48 0.84
2.07 0.39
1.92 7.01
1.22 0.23
0.50 0.79
1.02 0.45
LOI = Loss on ignition at 800 °C for 1 h.
3.2. IR characteristics of geopolymer before and after thermal cycles The FT-IR spectra of geopolymer composites shown in Fig. 2 presented the spectra of 5 wt% TiO2 added samples before (0 cycles) and after thermal shock resistance tests at 800 °C up to 15 cycles. The main FT-IR absorption bands were summarized in Table 2. The bands at 456 and 620 cm 1 were due to the bending vibration of SiAOASi and OASiAO, respectively. The 510 cm 1 band attributed to OASiAO found after thermal shock tests suggested nepheline phases formation, which was in a good agreement with XRD results. Typically, the main SiAOAT (T@Si or Al) asymmetric stretching vibration bands observed around 1200–950 cm 1 had been identified as characteristics of geopolymer network formation [19–20]. In this study, a main narrow band spectrum of SiAOAT (T@Si or Al) appeared at 959 cm 1. The spectra at 1435–1415 cm 1 could be assigned to the stretching OACAO bond indicating the presence of carbonate trace [21] i.e. sodium carbonate in this case. The stretching (–OH) and bending (HAOAH) vibrations of water bands [17,19], shown at 3463–1644 cm 1 were shifted to lower frequencies at 3422–1637 cm 1 due to the loss of water after thermal cycles. 3.3. Morphological and elemental analysis In comparison, Fig. 3 showed the microstructure and EDS analysis of geopolymer composites without TiO2 (Fig. 3a and b) and with 5 wt% TiO2 addition (Fig. 3c and d) in which those samples were subjected to 15 thermal cycles. The morphology of the composites consisted of fly ash and basalt fibers embedded in the matrix (Fig. 3a and c). The cracks detected in the microstructure were suspected to be formed during thermal cycles and the compressive strength measurement. The analysis at fiber-matrix interface at the fiber pull-out area revealed the major elements were identified as Si, Ca, Al, O for TiO2 free composites. In addition to those elements
Fig. 1. XRD patterns of (a) as-received Class C fly ash, basalt fibers and geopolymer composites with 0–5 wt% TiO2 addition (b) geopolymer composites with 0–5 wt% TiO2 addition after thermal shock resistance tests at 800 °C for 15 cycles.
process of hydration. Previous study by Lee and Kurtis [18] also reported that TiO2 nanoparticles influenced the early stage hydration reaction of tricalcium silicate (C3S) as adding TiO2 powder at 5–15 wt% could provide more nucleation sites that accelerated the early hydration and formed the foundation for photocatalytic containing cements. The phases of nepheline (NaAlSiO4) at 21.2° and 23.1° (2h) and belite (Ca2SiO4) at 32.6° and 33.2° (2h) were developed after the samples passed the thermal shock cycles. Belite or dicalcium silicate was generally formed when CaCO3 dissociated to free CaO at temperature ranged 750–850 °C, and subsequently reacted with free SiO2 [16]. In this study, free silica and CaO could be obtained from both fly ash and basalt fibers, thus belite phase was formed after the thermal shock resistance tests at 800 °C.
Fig. 2. FT-IR spectra of geopolymer composites with 5 wt% TiO2 addition before and after thermal shock resistance tests at 800 °C for 5, 10, and 15 cycles.
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Table 2 Characteristic FT-IR bonds of synthesized geopolymer composites. Wavenumber (cm 1)
Attribution
3463–3422 1644–1637 1435–1415 1200–950 620 510 456–465
Stretching vibration (–OH, HAOAH) Bending vibration (HAOAH) Stretching vibration (OACAO) Asymmetric stretching vibration (TAOASi; T@Si or Al) Bending vibration (OASiAO) Bending vibration (OASiAO) Bending vibration (SiAOASi or OASiAO)
found, Ti was detected in the 5 wt% TiO2 added samples as anticipated. It was evident from EDS analysis (Fig. 3b and d) that the calcium silicate based-compound was formed at the fibermatrix interface. There were also a few studies on the strength development of Portland cement which was significantly relevant to Ca/Si ratio that reflected C-S-H formation in the structure [22–24]. In this present work, it was found that the tendency of Ca/Si ratio in geopolymer composites increased with TiO2 addition as it was calculated that Ca/Si ratio at the fiber-matrix interface of TiO2 free sample was 0.67, and 5% TiO2 added sample was 0.90, respectively. 3.4. Compressive strength and thermal shock resistance The compressive strength of geopolymer composites was evaluated after aged 28 days. Fig. 4 showed the strength of the
composites with respect to TiO2 addition and Ca/Si ratio in geopolymer matrix before and after thermal cycles (15 cycles). The Ca/Si ratio of the composites for each wt% of TiO2 addition was determined from the elemental traces examined by EDS analysis. The resulting ratios were plotted against the compressive strength and TiO2 addition. It could be interestingly observed from the plots that the 28-day aged compressive strength (before thermal cycle, Fig. 4a) slightly increased with TiO2 addition. The 5 wt% TiO2 added samples have gained 7.8% compressive strength higher than TiO2 free ones. With comparison to the study by Zhou et al. [11], it was reported that 22% higher compressive strength was found with 5 wt% nano-TiO2 added to geopolymers aged 28-day due to TiO2 promoted the geopolymer formation resulted in denser microstructure with less cracks. It is believed that the nano particle size of TiO2 and the microwave radiation during aging process may be able to improve the strength of the geopolymer composites. In the present study, it could be summarized from the plots in Fig. 4 that the compressive strength and Ca/Si ratio of the geopolymer composites increased with TiO2 addition applied to both before and after thermal cycles. The compressive strength of geopolymer composites as a function of thermal cycles (0–15 cycles) has been illustrated in Fig. 5. The thermal shock resistance behavior of geopolymer composites was measured by placing the samples at 800 °C with a soaking period of 10 min, and then rapidly taken out to room temperature for 10 min. The retained compressive strength thereafter the heating-cooling for 5, 10, 15 cycles was measured. The sharp decline initially in compressive strength (Fig. 5) for all samples
(a)
(b)
interface
Ca/Si=0.67
matrix
(c)
(d)
2.5 µm
Ca/Si=0.90
25 µm
Fig. 3. SEM micrographs of geopolymer composites which performed thermal shock tests for 15 cycles and EDS spectra for fiber imprint showing fiber–matrix interface (a, b) with free TiO2 and (c, d) with 5 wt% TiO2.
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P. Timakul et al. / Construction and Building Materials 121 (2016) 653–658
0%
1% 3% 5% TiO2
Fig. 5. Compressive strength of 0–5 wt% TiO2 added geopolymer composites before and after thermal shock resistance tests at 800 °C; 0%T, 1%T, N3%T, .5%T, the insert was the compressive strength as a function of density and TiO2 addition.
4. Conclusions
Fig. 4. Compressive strength of geopolymer composites (a) before and (b) after thermal shock resistance tests with respect to TiO2 addition and Ca/Si ratio in geopolymer matrix.
In this work, the effect of TiO2 addition on the compressive strength and thermal shock resistance of fly ash-based geopolymer composites was studied. It was revealed that TiO2 promoted the hydration reactions in geopolymer structures as C-S-H phases formation increased, which further improved the compressive strength. The results suggested that the addition of TiO2 not only improved the compressive strength of geopolymers but also enhanced the thermal shock resistance at 800 °C which was shown by the higher retained compressive strength in every interval inspected cycles. The compressive strength of the composites having 5 wt% TiO2 after 15 thermal cycles was 44% higher (26 MPa) than that of TiO2 free samples (18 MPa). It was indicated that TiO2 played a key role for improving the compressive strength and thermal shock resistance in fly ash-based geopolymer composites. Acknowledgements
was found after the thermal cycles, which was due to the loss of water content in geopolymer structure. The strength of 5 wt% TiO2 added samples gradually decreased from 29.2 to 25.7 MPa (12% less) as they passed the thermal tests from 5 to 15 cycles. On the other hand, the sharp decrease in compressive strength from 25.3–18.0 MPa (29% less) was found in TiO2 free samples. It could be drawn from the experiment that the addition of TiO2 not only improved the 28-day aged compressive strength of geopolymer composites but also enhanced the thermal shock resistance which was evident by the higher retained compressive strength after thermal cycles. The insert in Fig. 5 showed strength-density relation of geopolymer composites with different TiO2 addition. It was observed that the density and compressive strength increased with TiO2 which could be supported by the XRD results (Fig. 2). The samples with TiO2 addition possessed high intensity of C-S-H and anatase phases resulted in high retained compressive strength after thermal shock measurements.
This work was financial supported by the National Metal and Materials Technology Center, National Science and Technology Development Agency, Ministry of Science and Technology, Kingdom of Thailand through the research fund no. P1550118. References [1] V.F.F. Barbosa, K.J.D. MacKenzie, Synthesis and thermal behavior of potassium sialate geopolymers, Mater. Lett. 57 (2003) 1477–1482. [2] J. Temuujin, W. Rickard, A. van Riessen, Characterization of various fly ashes for preparation of geopolymers with advanced applications, Adv. Powder Technol. 24 (2013) 495–498. [3] K. Komnitsas, D. Zaharaki, G. Bartzas, Effect of sulphate and nitrate anions on heavy metal immobilisation in ferronickel slag geopolymers, Appl. Clay Sci. 73 (2013) 103–109. [4] P. Timakul, W. Rattanaprasit, P. Aungkavattana, Improving compressive strength of fly ash-based geopolymer composites by basalt fibers addition, Ceram. Int. 42 (2016). 6288-6259. [5] W.M. Kriven, J.L. Bell, M. Gordon, Microstructure and microchemistry of fullyreacted geopolymers and geopolymer matrix composites, Adv. Ceram. Matrix Comp. IX 153 (2003) 227–250.
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[6] S.S. Musil, G.P. Kutyla, W.M. Kriven, The effect of basalt chopped fiber reinforcement on the mechanical properties of potassium based geopolymer, in: Developments in Strategic Materials and Computational Design, vol. II, John-Wiley & Sons Inc., New Jersey, 2013, pp. 31–42. [7] E. Rill, D.R. Lowry, W.M. Kriven, Properties of basalt fiber reinforced geopolymer composites, in: Strategic Materials and Computational Design, John-Wiley & Sons Inc., New Jersey, 2010, pp. 57–67. [8] J.R. Gasca-Tirado, A. Manzano-Ramirez, P.A. Vazquez-Landaverde, E.I. HerreraDiaz, M.E. Rodriguez-Ugarte, J.C. Rubio-Avalos, V. Amigo-Borras, M. ChavezPaez, Ion-exchanged geopolymer for photocatalytic degradation of a volatile organic compound, Mater. Lett. 134 (2014) 222–224. [9] U. Diebold, The surface science of titanium oxide, Surf. Sci. Rep. 48 (2003) 53– 229. [10] J. Chen, S.C. Kou, C.S. Poon, Hydration and properties of nano-TiO2 blended cement composites, Cem. Concr. Comp. 34 (2012) 642–649. [11] P. Duan, C. Yan, W. Luo, W. Zhou, Effects of adding nano-TiO2 on compressive strength, drying shrinkage, carbonation and microstructure of fluidized bed fly ash based geopolymer paste, Constr. Build. Mater. 106 (2016) 115–125. [12] A. Kritikaki, D. Zaharaki, K. Komnitsas, Valorization of industrial wastes for the production of glass-ceramics, Waste Biomass Valorization 7 (2016) 1–14, http://dx.doi.org/10.1007/s12649-016-9480-x. [13] Antoni, S.W. Wijaya, D. Hardjito, Factors affecting the setting times of fly ashbased geopolymer, Mater. Sci. Forum 841 (2015) 90–97. [14] ASTM C109/C109M-12, Standard Test Method for Compressive Strength of Hydraulic Cement Mortars Cube Specimen, Annual book of ASTM standard, 2012.
[15] ASTM C1171, Standard Test Method for Quantitatively Measuring the Effect of Thermal Shock and Thermal Cycling on Refractories, Annual book of ASTM standard, 2011. [16] K. Pimraksa, S. Hanjitsuwan, P. Chindaprasirt, Synthesis of belite cement from lignite fly ash, Ceram. Int. 35 (2009) 2415–2425. [17] X. Guo, H. Shi, W.A. Dick, Compressive strength and microstructural characteristics of class C fly ash geopolymer, Cem. Concr. Comp. 32 (2010) 142–147. [18] B.Y. Lee, K.E. Kurtis, Influence of TiO2 nanoparticles on early C3S hydration, J. Am. Ceram. Soc. 93 (2010) 3399–3405. [19] K.A. Komnitsas, Petential of geopolymer technology towards green buildings and sustainable cities, Proc. Eng. 21 (2011) 1023–1032. [20] A. Palomo, M.W. Grutzeck, M.T. Blanco, Alkali-activated fly ashes A cement for the future, Cem. Concr. Res. 29 (1999) 1323–1329. [21] V.F.F. Barbosa, K.J.D. MacKenzie, C. Thaumaturgo, Synthesis and characterization of materials based on inorganic polymers of alumina and silica: sodium polysialate polymers, Int. J. Inorg. Mater. 2 (2000) 309–317. [22] R. Danzer, T. Lube, P. Supancic, R. Damani, Fracture of ceramics, Adv. Eng. Mater. 4 (2008) 275–298. [23] C.K. Yip, G.C. Lukey, J.S.J. van Deventer, The coexistence of geopolymeric gel and calcium silicate hydrate at the early stage of alkaline activation, Cem. Concr. Res. 35 (2005) 1688–1697. [24] J. Temuujin, A. van Riessen, R. Williams, Influence of calcium compounds on the mechanical properties of fly ash geopolymer pastes, J. Hazard. Mater. 167 (2009) 82–88.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Enhancement of compressive strength and thermal shock resistance of fly ash-based geopolymer composites Patthamaporn Timakul, Weerada Rattanaprasit, Pavadee Aungkavattana ⇑ National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency (NSTDA), 114 Thailand Science Park, Pahonyothin Rd., Klong Luang, Pathumthani 12120, Thailand
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
TiO2 promoted hydration reactions in
geopolymers resulting more C S H phase. The strength increased due to the formation of C S H and anatase phases in matrix. Addition of 5% TiO2 resulted in increased by 44% strength.
a r t i c l e
i n f o
Article history: Received 1 March 2016 Received in revised form 10 June 2016 Accepted 13 June 2016 Available online 20 June 2016 Keywords: Geopolymer Composites Titanium dioxide Compressive strength Thermal shock resistance
a b s t r a c t This study presented the effect of 0 5 wt% TiO2 addition on compressive strength and thermal shock resistance of fly ash based geopolymer composites, which were prepared by activating high calcium fly ash reinforced with basalt fibers. The compressive strength of the composites after aging 28 days increased to 52 MPa when 5 wt% TiO2 was added. The retained compressive strength as 5 wt% TiO2 was added after 15 thermal cycles (800 °C) was 26 MPa, which was increased by 44% compared to TiO2 free composites. Such an enhancement was ascribed to the acceleration of hydration reactions in geopolymer structures resulting more C H S phases which resulted in a denser matrix with increased strength. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction For the past decade, geopolymeric materials have attracted a lot of attention due to their low cost and low energy for production ⇑ Corresponding author. E-mail address: [email protected] (P. Aungkavattana). http://dx.doi.org/10.1016/j.conbuildmat.2016.06.037 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
and environmental friendly nature [1]. Geopolymers are the inorganic polymeric materials with amorphous nature, formed by aluminosilicate and alkali-metal-silicate solution under alkaline conditions. Geopolymers can be employed as conventional building materials to replace the ordinary Portland cement due to their durability and comparable mechanical strength. Recently, various studies have reported the use of industrial wastes i.e. fly
654
P. Timakul et al. / Construction and Building Materials 121 (2016) 653–658
ash and blast furnace slag as the potential sources for geopolymer synthesis [2–4]. Since interstitial water in geopolymer structures evaporates during high temperature applications such as 350–400 °C, which could cause mechanical cracking. Therefore, one of the solutions to reduce cracking and improve thermal stability is to make the geopolymer matrix homogeneous and pore free, which in turn will give rise to high mechanical strength matrix. Previous studies by Kriven et al. [5] and others [6–7] reported geopolymers could be used as a good binder with various reinforcements i.e. basalt fibers in order to significantly improve the tensile and flexural strength of the composites due to high elastic modulus and tensile strength of basalt fibers. The objective of the present study is to enhance the compressive strength and the thermal shock resistance of Class C fly ashbased geopolymer composites using basalt fibers as the reinforcing agent. Fly ash is the byproduct from Thailand Power Plant which currently generates 4 million tons per year at no cost in practice. Additionally, fly ash is rich in SiO2 (31.23%) and Al2O3 (15.76%) and is desirable as source materials, apart from kaolin, for the production of geopolymers. To improve the thermal shock resistance of the geopolymer composites targeted for refractory applications, this study aimed to investigate the effect of TiO2 (anatase phase) addition from 0 to 5 wt% to the composites. In cement-based materials for construction, TiO2 has been the most commonly used binder due to its fine particle size, high reactivity, and specific functional properties such as to generate ion exchange in geopolymer in order to incorporate photoactive TiO2 [8]. Moreover, the other advantages of TiO2 addition in geopolymer applications including purification of waste water, disinfection based on the bactericidal properties, self-cleaning coatings, and smog-abating functions [9]. There were several reports on the photocatalytic functions using nano-sized TiO2 which the basic principle could be summarized as when UV radiation exposed to TiO2, it could absorb photon energy which is approximately equal or larger than its band gap energy, and enable electrons (e ) to jump from the valence band to the conduction band. The electrons activation resulted in the holes (h+, electron vacancy) generation in the valence band. The electron-hole pairs then could recombine to generate redox reactions which could produce several radicals including OH, HO2, O2 and O when water vapour and oxygen were presented in the vicinity of activated TiO2 [10]. Recently, there has been a study on the effect of nano-TiO2 addition on the properties of fluidized bed fly ash-based geopolymer composites, and results showed that 5% nano-TiO2 could enhance the compressive strength of geopolymers at early stage and 28-day aged, and it also allowed the cementitious mixture to have better chemical stability, high catalytic activity, and affordable price [11]. Nevertheless, the effect of TiO2 on the thermal behavior of fly ash-based geopolymer composites has not been studied so far. The thermal shock resistance is one of the most important properties as most of refractory materials needed to undergo immediate temperature changes in their applications. Therefore, this present study aimed to investigate the effect of TiO2 addition on the basalt fibers reinforced fly ash-based geopolymer composites in terms of compressive strength and thermal shock behavior for our targeted applications in construction and refractory industries. Using these geopolymer composites for widely applications, it is required to consider the cost of TiO2 whether it is feasible. The TiO2 powder cost is in the range of $US 1.80 2.20/kg, which could take part for 5% of all raw materials cost in geopolymer. A study by Komnitsas et al. [12] reported the industrial wastes such as red mud from Aluminium of Greece plant in Veotia, Greece which contained 4.73 wt% of TiO2 and could be potentially an alternative source for TiO2 replacement in geopolymer production, which in turn reduce the raw materials cost for approximately 5%. Nevertheless, the red mud is composed of other oxides mainly Fe2O3, which may
affect the geopolymer properties, therefore further investigations shall be required and considered. 2. Materials and methods An alkali activator was prepared by combining sodium hydroxide (5 M, NaOH) solution and sodium silicate (Na2SiO3) solution for 2.5:1 in weight ratio. The 5 M concentration of NaOH was chosen for the good workability of the mixing paste during casting into the moulds. It was found in this study that the higher molarity of NaOH such as 8 10 M accelerated the setting times of the paste which made the workability difficult. This could be related to more leaching of silica and alumina in fly ash with high NaOH concentration. A study by Hardjito et al. [13] reported the use of high CaO content fly ash could produce faster initial setting time due to fly ash has high pH value (above 11). High Ca fly ash (Class C, ASTM C618) obtained from Thailand power plant was dry-mixed with basalt fibers (5 lm OD, and 12–60 lm in length) in the ratio 90:10 wt%. This particular ratio was chosen from the best performance on compressive strength in the previous study [4]. Basalt fibers, as the reinforcing agent had the density of 2.50 g/cm3 which was measured by a pycnometer (UPYC 1200E). The major chemical compositions of fly ash and basalt fibers were detailed in Table 1. TiO2 powder (P99% purity) from 0 to 5 wt% was added to the dry mixes. The specimens without TiO2 powder were also prepared as a reference for comparison. For all mixes, the liquid to solid ratio was kept constant at 0.4. The actual composition of geopolymer matrix as the following molar ratios, SiO2/Al2O3 = 3.8, Na2O/SiO2 = 0.07, and H2O/Na2O = 36 were determined from source materials. Details on sample preparation and aging conditions were already published [4]. The characterization was performed after the samples aged 28 days. Phase development was investigated using XRD. Morphology and microstructure of geopolymers was examined by SEM and EDS (Hitachi S-3400N) on Au-coated specimens. Type of bonding was identified by FT-IR (Perkin Elmer system 2000) in the region of 400–4000 cm 1. The cubic (50 50 50 mm) geopolymer composites were prepared for compressive strength and thermal shock resistance measurement. The strength tests were performed according to ASTM C109 [14] using ADR Auto-250, ELE International. The thermal shock resistance behavior of geopolymer composites was studied by measuring the retained compressive strength after the samples have been through heating-cooling for 5, 10, 15 cycles. In each cycle, tested samples were placed in the furnace at 800 °C with a soaking period of 10 min, and then they were rapidly taken out to room temperature (25 °C) for 10 min complying with ASTM C1171 procedure [15].
3. Results and discussion 3.1. Phase compositions of geopolymer composites Phase analysis of starting materials and geopolymer composites aged 28-day was shown in Fig. 1a. The mineralogical compositions of starting basalt fibers consisted largely of amorphous phase, whereas fly ash contained mainly amorphous with some crystalline phases including anhydrite (CaSO4), calcite (CaCO3), hatrurite (Ca3SiO6), quartz (SiO2), magnesioferrite (MgFe2O4), and calcium oxide (CaO). In addition, the phase structure of all geopolymer composites with TiO2 addition were composed of broad humps of alumino-silicate from 20° to 40° (2h) with some crystalline phases of anatase (TiO2), calcium-silicate-hydrate (C-S-H), hatrurite (Ca3SiO6), magnesioferrite (MgFe2O4) phases. C-S-H phase is the main hydration product formed during cement production [16]; therefore the formation of C-S-H phase and alumino-silicate framework in this experiment suggested that the hydration reactions and geopolymerization occurred simultaneously [17]. The XRD patterns of geopolymer composites with different TiO2 wt% and performed the thermal shock tests for 15 cycles were presented in Fig. 1b. The samples showed amorphous with some crystalline phases, the intensity of anatase and C-S-H phases increased with increasing TiO2 wt%. The increase in C-S-H phase suggested that TiO2 accelerated the hydration reactions in geopolymers which was in a good agreement with a study by Chi-sun Poon et al. [10] which reported the addition of nano-TiO2 from 5 to 10 wt% could accelerate the hydration of tricalcium aluminate (C3A) by inert or active ultrafine particles could act as potential heterogeneous nucleation sites for the hydration products and the grain boundary region was densely populated with nuclei and completely transformed early in the overall
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P. Timakul et al. / Construction and Building Materials 121 (2016) 653–658 Table 1 Chemical compositions of as-received fly ash and basalt fibers. Major oxides
SiO2
Al2O3
Fe2O3
CaO
SO3
K2O
MgO
Na2O
TiO2
LOI
Class C fly ash (mass%) Basalt fibers (mass%)
31.23 37.48
15.76 14.4
12.28 1.08
28.76 36.85
5.48 0.84
2.07 0.39
1.92 7.01
1.22 0.23
0.50 0.79
1.02 0.45
LOI = Loss on ignition at 800 °C for 1 h.
3.2. IR characteristics of geopolymer before and after thermal cycles The FT-IR spectra of geopolymer composites shown in Fig. 2 presented the spectra of 5 wt% TiO2 added samples before (0 cycles) and after thermal shock resistance tests at 800 °C up to 15 cycles. The main FT-IR absorption bands were summarized in Table 2. The bands at 456 and 620 cm 1 were due to the bending vibration of SiAOASi and OASiAO, respectively. The 510 cm 1 band attributed to OASiAO found after thermal shock tests suggested nepheline phases formation, which was in a good agreement with XRD results. Typically, the main SiAOAT (T@Si or Al) asymmetric stretching vibration bands observed around 1200–950 cm 1 had been identified as characteristics of geopolymer network formation [19–20]. In this study, a main narrow band spectrum of SiAOAT (T@Si or Al) appeared at 959 cm 1. The spectra at 1435–1415 cm 1 could be assigned to the stretching OACAO bond indicating the presence of carbonate trace [21] i.e. sodium carbonate in this case. The stretching (–OH) and bending (HAOAH) vibrations of water bands [17,19], shown at 3463–1644 cm 1 were shifted to lower frequencies at 3422–1637 cm 1 due to the loss of water after thermal cycles. 3.3. Morphological and elemental analysis In comparison, Fig. 3 showed the microstructure and EDS analysis of geopolymer composites without TiO2 (Fig. 3a and b) and with 5 wt% TiO2 addition (Fig. 3c and d) in which those samples were subjected to 15 thermal cycles. The morphology of the composites consisted of fly ash and basalt fibers embedded in the matrix (Fig. 3a and c). The cracks detected in the microstructure were suspected to be formed during thermal cycles and the compressive strength measurement. The analysis at fiber-matrix interface at the fiber pull-out area revealed the major elements were identified as Si, Ca, Al, O for TiO2 free composites. In addition to those elements
Fig. 1. XRD patterns of (a) as-received Class C fly ash, basalt fibers and geopolymer composites with 0–5 wt% TiO2 addition (b) geopolymer composites with 0–5 wt% TiO2 addition after thermal shock resistance tests at 800 °C for 15 cycles.
process of hydration. Previous study by Lee and Kurtis [18] also reported that TiO2 nanoparticles influenced the early stage hydration reaction of tricalcium silicate (C3S) as adding TiO2 powder at 5–15 wt% could provide more nucleation sites that accelerated the early hydration and formed the foundation for photocatalytic containing cements. The phases of nepheline (NaAlSiO4) at 21.2° and 23.1° (2h) and belite (Ca2SiO4) at 32.6° and 33.2° (2h) were developed after the samples passed the thermal shock cycles. Belite or dicalcium silicate was generally formed when CaCO3 dissociated to free CaO at temperature ranged 750–850 °C, and subsequently reacted with free SiO2 [16]. In this study, free silica and CaO could be obtained from both fly ash and basalt fibers, thus belite phase was formed after the thermal shock resistance tests at 800 °C.
Fig. 2. FT-IR spectra of geopolymer composites with 5 wt% TiO2 addition before and after thermal shock resistance tests at 800 °C for 5, 10, and 15 cycles.
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Table 2 Characteristic FT-IR bonds of synthesized geopolymer composites. Wavenumber (cm 1)
Attribution
3463–3422 1644–1637 1435–1415 1200–950 620 510 456–465
Stretching vibration (–OH, HAOAH) Bending vibration (HAOAH) Stretching vibration (OACAO) Asymmetric stretching vibration (TAOASi; T@Si or Al) Bending vibration (OASiAO) Bending vibration (OASiAO) Bending vibration (SiAOASi or OASiAO)
found, Ti was detected in the 5 wt% TiO2 added samples as anticipated. It was evident from EDS analysis (Fig. 3b and d) that the calcium silicate based-compound was formed at the fibermatrix interface. There were also a few studies on the strength development of Portland cement which was significantly relevant to Ca/Si ratio that reflected C-S-H formation in the structure [22–24]. In this present work, it was found that the tendency of Ca/Si ratio in geopolymer composites increased with TiO2 addition as it was calculated that Ca/Si ratio at the fiber-matrix interface of TiO2 free sample was 0.67, and 5% TiO2 added sample was 0.90, respectively. 3.4. Compressive strength and thermal shock resistance The compressive strength of geopolymer composites was evaluated after aged 28 days. Fig. 4 showed the strength of the
composites with respect to TiO2 addition and Ca/Si ratio in geopolymer matrix before and after thermal cycles (15 cycles). The Ca/Si ratio of the composites for each wt% of TiO2 addition was determined from the elemental traces examined by EDS analysis. The resulting ratios were plotted against the compressive strength and TiO2 addition. It could be interestingly observed from the plots that the 28-day aged compressive strength (before thermal cycle, Fig. 4a) slightly increased with TiO2 addition. The 5 wt% TiO2 added samples have gained 7.8% compressive strength higher than TiO2 free ones. With comparison to the study by Zhou et al. [11], it was reported that 22% higher compressive strength was found with 5 wt% nano-TiO2 added to geopolymers aged 28-day due to TiO2 promoted the geopolymer formation resulted in denser microstructure with less cracks. It is believed that the nano particle size of TiO2 and the microwave radiation during aging process may be able to improve the strength of the geopolymer composites. In the present study, it could be summarized from the plots in Fig. 4 that the compressive strength and Ca/Si ratio of the geopolymer composites increased with TiO2 addition applied to both before and after thermal cycles. The compressive strength of geopolymer composites as a function of thermal cycles (0–15 cycles) has been illustrated in Fig. 5. The thermal shock resistance behavior of geopolymer composites was measured by placing the samples at 800 °C with a soaking period of 10 min, and then rapidly taken out to room temperature for 10 min. The retained compressive strength thereafter the heating-cooling for 5, 10, 15 cycles was measured. The sharp decline initially in compressive strength (Fig. 5) for all samples
(a)
(b)
interface
Ca/Si=0.67
matrix
(c)
(d)
2.5 µm
Ca/Si=0.90
25 µm
Fig. 3. SEM micrographs of geopolymer composites which performed thermal shock tests for 15 cycles and EDS spectra for fiber imprint showing fiber–matrix interface (a, b) with free TiO2 and (c, d) with 5 wt% TiO2.
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0%
1% 3% 5% TiO2
Fig. 5. Compressive strength of 0–5 wt% TiO2 added geopolymer composites before and after thermal shock resistance tests at 800 °C; 0%T, 1%T, N3%T, .5%T, the insert was the compressive strength as a function of density and TiO2 addition.
4. Conclusions
Fig. 4. Compressive strength of geopolymer composites (a) before and (b) after thermal shock resistance tests with respect to TiO2 addition and Ca/Si ratio in geopolymer matrix.
In this work, the effect of TiO2 addition on the compressive strength and thermal shock resistance of fly ash-based geopolymer composites was studied. It was revealed that TiO2 promoted the hydration reactions in geopolymer structures as C-S-H phases formation increased, which further improved the compressive strength. The results suggested that the addition of TiO2 not only improved the compressive strength of geopolymers but also enhanced the thermal shock resistance at 800 °C which was shown by the higher retained compressive strength in every interval inspected cycles. The compressive strength of the composites having 5 wt% TiO2 after 15 thermal cycles was 44% higher (26 MPa) than that of TiO2 free samples (18 MPa). It was indicated that TiO2 played a key role for improving the compressive strength and thermal shock resistance in fly ash-based geopolymer composites. Acknowledgements
was found after the thermal cycles, which was due to the loss of water content in geopolymer structure. The strength of 5 wt% TiO2 added samples gradually decreased from 29.2 to 25.7 MPa (12% less) as they passed the thermal tests from 5 to 15 cycles. On the other hand, the sharp decrease in compressive strength from 25.3–18.0 MPa (29% less) was found in TiO2 free samples. It could be drawn from the experiment that the addition of TiO2 not only improved the 28-day aged compressive strength of geopolymer composites but also enhanced the thermal shock resistance which was evident by the higher retained compressive strength after thermal cycles. The insert in Fig. 5 showed strength-density relation of geopolymer composites with different TiO2 addition. It was observed that the density and compressive strength increased with TiO2 which could be supported by the XRD results (Fig. 2). The samples with TiO2 addition possessed high intensity of C-S-H and anatase phases resulted in high retained compressive strength after thermal shock measurements.
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