Performance of fly ash based geopolymer composites at elevated temperature.
Concrete structures when subjected to elevated temperature results in considerable damage or even catastrophic failures. At elevated temperature OPC concrete degenerates very fast due to physical and chemical changes. In recent years, geopolymer composites synthesized from alkali-activation of alumino-silicate materials such as metakaolin, fly ash, GGBS etc. have emerged as a new building material with better physical and mechanical properties  and significant fire resistance compared to OPC concrete.  These materials are found suitable for a wide range of engineering applications including heat resistant infrastructucture material. According to Davidovits , the physical and mechanical properties of geopolymer vary greatly depending on the Si:Al ratio of the resultant aluminosilicate gel. The geopolymer mix with Si:Al ratio of 1, 2, or 3 initiates a very rigid 3D-Network structure suitable for heat resistant composites up to the temperature of 1200[degrees]C.
Significant research work on geopolymer concrete manufactured from fly ash in combination with sodium silicate and sodium hydroxide solution has been carried out by Rangan B.V. et.al [4,5]. The authors have reported higher mechanical strength and better durability of these concrete than Portland cement concrete. J. S. J. van Deventer et al.  have reported that mechanical properties and microstructure of Na-based metakaolin geopolymer are highly dependent on Si:Al of the aluminosilicate gel. Bakharev T. studied thermal stability of fly ash based geopolymer and observed high shrinkage as well as large changes in compressive strength with increasing fired temperature in the range of 800-1200[degrees]C. Rahier et al.  broadly investigated thermal shrinkage of a geopolymer of non-prescribed composition synthesised from metakaolin and sodium silicate solution. The specimen exhibited approximately 6% shrinkage during dehydration, without significant densification at 600[degrees]C. Peter Duxson et al.  studied thermal characteristics of Na-geopolymer with a Si:Al of approximately 2.0 and found that the thermal shrinkage was associated with dehydration below 600[degrees]C and the specimen was also observed to densify at approximately 800[degrees]C. More recently Sanjayan et al.  have reported that shrinkage and densification of geopolymer at elevated temperature is dependent on calcination temperature of kaolin, activator to metakaolin ratio, curing temperature and alkali cation of geopolymer. Since geopolymer composites are reported as high performance binding materials with intrinsic fire resistance and suitable for high temperature applications, their properties at elevated temperature are of interest.
The present experimental work was aimed at studying the effect of Si:A1 and Na:A1 ratio of mix on the physico-mechanical and microstructure properties of fly ash based geopolymer paste and mortar specimen before and after exposure to elevated temperatures. The specific objective of the research was to investigate the weight loss, changes in compressive strength and microstructure after exposing them to elevated temperature at 300[degrees]C, 600[degrees]C and 900[degrees]C. Microstructure of thermally treated geopolymer gel was characterized using Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) Analysis was performed to study mineralogical changes.
Materials and experimental methods Materials
Fly ash used in the present investigation was obtained from Kolaghat Thermal Power Station located near Kolkata India. The chemical and mineral composition of fly ash is shown in Table-1 and Figure-1, respectively.
[FIGURE 1 OMITTED]
The calcium oxide content of the fly ash is less than 10%. Hence, as per ASTM standard C6128-03, it can be classified as class F fly ash (or siliceous pulverized fuel ash conforming to IS 3812(Part-I)-2003 specifications). About 90% of fly ash particles were finer than 45 micron. The Blaine specific surface was 410 [m.sup.2]/kg. According to the XRD diffractogram, the major crystalline constituents of fly ash are quartz, mullite and magnetite. The alkaline activator liquid was a combination of sodium silicate solution and sodium hydroxide. Laboratory grade Sodium hydroxide in pellet form (98 percent purity) and Sodium Silicate solution ([Na.sub.2]O = 8.5%, Si[O.sub.2] =28% and 63.5% water) with silicate modulus ~3.3 and a bulk density of 1410kg/[m.sup.3] were used to adjust desired composition of geopolymer mix. To avoid effects of unknown contaminants in laboratory tap water, distilled water was used for preparing the activating solutions. The solution was prepared at least one day prior to its use in preparing geopolymer mix. The fine aggregate was river sand obtained from local source. The specific gravity of sand was 2.54 and fineness modulus of the sand was 2.65. As per IS 383-1976, the particle size distribution of sand shows that it is in zone II. To avoid water absorption from activator solution, the sand was made saturated surface dry (SSD) before using in geopolymer mix.
In the present study, two series of Geopolymer paste and mortar specimens were prepared by varying Si:Al and Na:Al ratio of mix from 1.66 to 2.16 and 0.24 to 0.52 respectively. Water to binder ratio was kept constant to 0.32 for all mixes. The compositional changes of geopolymer mix was obtained by varying quantity of sodium hydroxide (NaOH), sodium silicate (Na2SiO3) solution and water in the activating solution. Mortar specimens were prepared by keeping sand to fly ash ratio as 1.0. Table-2 shows details of the geopolymer mix compositions used in present work.
For making geopolymer mortar specimens, the fly ash and alkaline activating solution were first mixed together in desired proportion in Hobart mixer for five minutes, sand was then slowly added and mixed for another five minutes. Geopolymer paste specimens were prepared in similar manner, without adding sand, with same chemical composition of respective mortar specimens. After mixing, fresh geopolymer mix was filled in 50mmx50mmx50mm steel moulds and vibrated for two minutes on vibration table to remove entrapped air. Cylindrical steel moulds of size [PHI]25x50 mm were used to cast paste specimen for SEM, XRD and TGA. Two sets of three specimens were prepared for each parameter studied at unexposed and temperature exposed condition. All specimens, both paste and mortar, were left undisturbed to room temperature for 60 minutes before curing in an oven at 85[degrees]C for 24 hours. The specimens were removed and demoulded after cooling down to room temperature in oven.
Elevated temperature exposure regimes
The paste and mortar specimens were subjected to elevated temperatures at 300[degrees]C, 600[degrees]C and 900[degrees]C in a muffle furnace at an incremental rate of 8 to 10[degrees]C per minute starting from room temperature. Once the desired temperature was attained in furnace, it was maintained for another two hours before the specimens were allowed to cool to room temperature naturally in furnace. The unexposed counterparts were left undisturbed at room temperature until testing.
Results and discussion
Figure-2 and Figure-3 shows geopolymer mortar and paste specimen before and after exposing to elevated temperature. Significant change in colour was observed for both paste and mortar specimens. The specimen colour was changed gradually from gray to dark brown with increasing temperature. Macro-cracks in the order of 0.1 to 0.2 mm were noticed on the surface of paste specimens after 600[degrees]C. Crack width and depth further increased up to 0.3 to 0.5mm at 900[degrees]C. The geopolymer mortar samples did not reveal any cracks on the surface at 300[degrees]C. However, few micro-cracks of shallow depth appeared on the surface at 600[degrees]C which augmented in number as temperature increased to 900[degrees]C.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
The paste and mortar specimens were tested for compressive strength using 50mmx50mmx50mm cubic specimens before and after exposing to elevated temperature. The compressive strength of the geopolymer cube was measured using 20ton capacity digital compressive testing machine with a loading rate of 20MPa/min. The average strength of three specimens was considered as the compressive strength. Figure-4 shows compressive strength evolution of geopolymer mortars for Si:Al ratio of 1.66, 1.79, 1.91, 2,02 and 2.16 with elevated temperature exposure. Compressive strength of unexposed specimen increased from 42.2MPa to 54.82Mpa with increase in Si:Al ratio from 1.66 to 2.16. Duxon Peter et al. , have reported that increasing Si:Al ratio of the mix, more Si-O-Si bonds are formed, those are stronger in comparison with Al-O-Al bonds result in more compressive strength. After exposing paste and mortar to elevated temperature compressive strength decreased with increase in elevated temperature. However, rapid reduction in compressive strength of mortar specimen was observed above 300[degrees]C due to disruptive phase change. Dilatometry test result reported by Sanjayan et al.  have shown that with increase in temperature sand particles expands and geopolymer gel shrink causes failure of gel aggregate interface which result in reduction of compressive strength. Figure-5 shows comparison of the compressive strength of paste and mortar specimen of same chemical composition with Si:Al=1.91 and Na:Al=0.43 respectively at various temperatures. Continuous deterioration in strength was observed in mortar specimen heated beyond 300[degrees]C. However, in paste specimen compressive strength was reduced up to 600[degrees]C but further heating showed some improvement in strength. This may be attributed to viscous sintering of unreacted fly ash particles in the geopolymer matrix at elevated temperature, as reported by earlier researchers. [6,8,9]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
Simultaneous DTA and TGA measurements were performed using a Perkin--Elmer Diamond DTA/TGA instrument in alumina crucibles to accurately measure mass loss while the specimens were gradually exposed to elevated temperatures. Powdered specimens were used in the tests to ensure uniform heating of the samples during transient heating. Experiments were performed between 28[degrees]C and 1000[degrees]C at constant heating rate of 10[degrees]C per minute in a inert media with nitrogen purge rate of 200 ml per minute. Figure-6 presents typical TG- DTA curve for geopolymer paste specimen M33 (Si:Al=1.91 and Na:Al=0.43) from 35[degrees]C to 1000[degrees]C. TGA curve shows 3.5% of total 13% weight loss of specimen was occurred before about 100[degrees]C. The 6.5% weight was lost from 100[degrees]C to 600[degrees]C and about 3% weight was lost between 600[degrees]C and 1000[degrees]C. The DTA curve for geopolymer show single endothermic peak at about 100[degrees]C due to dehydration of free water in the sample. Figure-7 shows thermogravimetric curves (TGA) for fly ash based geopolymer specimen M31, M33 and M35 with Si:Al ratio of 1.66, 1.91,2.16 and Na:Al ratio of 0.43 indicated that weight loss increased with increase in Si:Al ratio of the geopolymer mix. Average weight reduction of 9.7%, 14.4% and 16.4% was observed in specimen M31, M33 and M35 after 900[degrees]C. Weight reduction in specimens is mainly due to dehydration of free and chemically bonded water and changes in chemical structure after exposure to elevated temperature. Rapid reduction in weight up to 300[degrees]C was mainly due to dehydration of free evaporable water in the specimen. The remaining water is chemically bonded and less able to diffuse to surface which continues to evolve gradually up to 600[degrees]C. Weight loss at 900[degrees]C is mainly due to chemical changes in the geopolymer specimen as reported in XRD analysis.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Scanning Electron Microscopy (SEM)
Microstructure images of geopolymer paste and mortar samples were obtained for pieces cut from the specimen using scanning electron microscope (SEM), before and after exposure to elevated temperature. The samples were vacuum-dried overnight prior to SEM and sputter-coated with gold-palladium alloy to ensure that there will be not be arching or image instability during micrograph collection. Imaging was conducted using a JEOL JSM 5400 scanning electron microscope. SEM Micrograph of unexposed specimen was viewed as geopolymer matrix comprising of gel phase and partially reacted spherical fly ash particles and nano pores in the matrix. The extent of unreacted fly ash particles in geopolymer matrix influenced the mechanical and thermal properties of the material. Figure-8 shows a SEM micrograph of geopolymer paste specimen M35 (Si:Al= 2.16 and Na:Al=0.43) before and after exposure to 900[degrees]C. It can be observed that unreacted fly ash particles got dissolved at 900[degrees]C. Figure-9 shows SEM image of geopolymer paste and mortar specimen M31, before temperature exposure. Microstructure of paste specimen M35 is more homogeneous compared to specimen M31 (Si:Al=1.66 and Na:Al=0.43). The earlier results reported elsewhere indicated that compressive strength and bulk density is more and apparent porosity was less for specimen M35 than specimen M31 due to higher Si:Al ratio of mix resulted in more dense microstructure. The image of mortar specimen before heating shows very fine micro cracks near sand particles. Further widening of these crack after elevated temperature exposure causes complete rupture of gel aggregate interface resulted in reduction of compressive strength.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
X-ray diffraction (XRD) analysis
X-Ray diffraction analysis of powdered geopolymer specimens was performed using a Rigaku Geigerflex D-max II automated diffractometer (Rigaku, Japan) with Cu-K[alpha] radiation with the following conditions: 40 kV, 22.5 mA. The XRD patterns were obtained by scanning at 1[degrees] (2[theta]) per min and in steps of 0.5[degrees] (2[theta]). The slow scanning rate was used to improve resolution of peaks. Figure-10 and Figure-11 represents typical XRD traces for the geopolymer paste specimen M35 (Si:Al=2.16 and Na:Al=0.43) before and after exposure to temperature at 300[degrees]C, 600[degrees]C and 900[degrees]C.
For geopolymer paste specimen M35 (with Si:Al=2.16 and Na:Al=0.43), traces of hydroxysodalite and hematite were found in addition to quartz, mulite and magnetite as observed in XRD diffractogram of original fly ash, before exposing to higher temperature. After exposing the specimen to 900[degrees]C, peaks for hydroxysodalite disappeared and new crystalline phases like nepheline (NaAlSi[O.sub.4]) and few traces of albite (NaAl[Si.sub.3][O.sub.8]) was detected. At 300[degrees]C phase composition of geopolymer did not change apparently and it was similar to unexposed specimen. After exposure to 600[degrees]C, some peaks of peaks of hydroxysodalite disappeared, however no new phase formation was detected. As reported by earlier researchers, the new additional phases like nepheline and albite are formed in geopolymer specimen because of chemical changes occurred due to viscous sintering of unreacted fly ash particles with zeolites in aluminosilicate gel at higher temperature exposure.
[FIGURE 10 OMITTED]
[FIGURE 11 OMITTED]
Thermal behavior of geopolymer paste and mortar specimens prepared by varying Si:Al and Na:Al ratio of mix have been investigated. Based on experimental study, following conclusions are drawn.
* Si:Al ratio of the mix is the most critical parameter for elevated temperature performance of geopolymer composites.
* Compressive strength of geopolymer paste and mortar specimen depended on Si:Al ratio of mix. Maximum compressive strength 54.82Mpa was obtained for M35 specimen with Si:Al ratio of 2.16 and Na:Al ratio of 0.43.
* Mortar specimen exhibited rapid reduction in compressive strength with exposure temperature above 300[degrees]C, due to disruptive phase changes at gel aggregate interface.
* Microstructure of paste specimen was found to be more homogeneous for higher mix with Si:Al ratio. At 900[degrees]C temperature, microstructure of geopolymer showed dissolution of unreacted fly ash particles with decreasing porosity and dense microstructure.
* XRD analysis revealed formation of new mineral phases like nepheline and albite after exposing geopolymer specimen to 900[degrees]C, as a result of chemical changes occurred due to viscous sintering.
* The experimental results indicated that fly ash based geopolymer composites are highly resistant to elevated temperature exposure up to 900[degrees]C.
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Biographical Details of Authors
Ravindra N. Thakur, M.E (Civil), is working as Lecturer in Government Polytechnic Thane. He is presently pursuing Ph.D. in the field of Geopolymer composites at Jadavpur University, Kolkata-700032, under QIP program. Mr. Thakur has over 20years of teaching and industrial experience. He is life member of ISTE, IEI, ISSE, ACI (I.C.), IWWA, and ASCE (I.S.) Mobile:09883028270, firstname.lastname@example.org
Dr. Somnath Ghosh, presently serving as Professor and Head, Department of Civil Engineering Department, Jadavpur University, Kolkata-700032. His research areas include High performance concrete, Self compacting concrete, Geopolymer concrete etc. Dr. Ghosh has over 30 years of Teaching, Research and Industrial experience. Mob: 09831025676, email@example.com
Ravindra N. Thakur (1) and Somnath Ghosh (1)
(1) Department of Civil Engineering, Jadavpur University, Kolkata-700032, India E-mail: firstname.lastname@example.org
Table 1: Chemical composition of the fly ash. Oxide Mass (%) Si[O.sub.2] 56.01 [Al.sub.2][O.sub.3] 29.80 [Fe.sub.2][O.sub.3] 3.58 Ti[O.sub.2] 1.75 CaO 2.36 MgO 0.30 [K.sub.2]O 0.73 [Na.sub.2]O 0.61 S[O.sub.3] Nil [P.sub.2][O.sub.5] 0.44 LOI * 0.40 * Loss on ignition Table 2 : Geopolymer mix composition. Series-IMix compwith sodhydroxidNaOH)+sosilicatesolution ([Na.sub.2]Si[O.sub.3]) activator % [Na.sub.2]O %Si[O.sub.2] Si[O.sub.2]/ Water MIX ID (a) (b) [Na.sub.2]O /Binder M31 8.5 0.00 0.00 0.32 M32 8.5 4.25 0.50 0.32 M33 8.5 8.50 1.00 0.32 M34 8.5 12.75 1.50 0.32 M35 8.5 17.00 2.00 0.32 [Na.sub.2]O/ Si[O.sub.2]/ [Na.sub.2]O/ MIX ID Si/Al Na/Al Si[O.sub.2] [Al.sub.2] [Al.sub.2] [O.sub.3] [O.sub.3] M31 0.158 3.19 0.50 1.66 0.43 M32 0.146 3.43 0.50 1.79 0.43 M33 0.137 3.67 0.50 1.91 0.43 M34 0.129 3.92 0.50 2.02 0.43 M35 0.121 4.16 0.50 2.16 0.43 Series-II : Mix compwith sodhydroxid (NaOH) activator. % [Na.sub.2]O %Si[O.sub.2] Si[O.sub.2]/ Water MIX ID (a) (b) [Na.sub.2]O /Binder M11 4.5 0 0 0.32 M21 6.5 0 0 0.32 M31 8.5 0 0 0.32 M41 10.5 0 0 0.32 [Na.sub.2]O/ Si[O.sub.2]/ [Na.sub.2]O/ MIX ID Si/Al Na/Al Si[O.sub.2] [Al.sub.2] [Al.sub.2] [O.sub.3] [O.sub.3] M11 0.088 3.19 0.28 1.66 0.24 M21 0.123 3.19 0.39 1.66 0.33 M31 0.158 3.19 0.5 1.66 0.43 M41 0.192 3.19 0.61 1.66 0.52 (a) & (b) : % of mass added with respect to total mass of fly ash
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|Author:||Thakur, Ravindra N.; Ghosh, Somnath|
|Publication:||International Journal of Applied Engineering Research|
|Date:||Apr 1, 2009|
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