UTILIZATION OF COAL FLY ASH FROM POWER PLANTS II. GEOPOLYMER OBTAINING

Transcription

1 Environmental Engineering and Management Journal May/June 2009, Vol.8, No.3, Gheorghe Asachi Technical University of Iasi, Romania UTILIZATION OF COAL FLY ASH FROM POWER PLANTS II. GEOPOLYMER OBTAINING Maria Harja 1, Marinela Bărbuţă 2, Maria Gavrilescu 1 1 Gheorghe Asachi Technical University of Iaşi, Faculty of Chemical Engineering, 71 Mangeron Blvd., , Iaşi Romania, 2 Gheorghe Asachi Technical University of Iaşi, Faculty of Civil Engineering and Building Services, 43 Mangeron Blvd., , Iaşi Romania Abstract Study of geopolymer is a developing field of research for capitalizing solid waste and by-products in the view of new materials achievement having characteristics necessary for using as cement substitute. Also, it provides a solution to many problems regarding environment, such as reduction of gaseous emissions and particles matter resulted from cement industry and raw materials economy. In this paper, the experimental conditions for obtaining new materials on the basis of fly ash from thermoelectrically stations are presented. Sixteen new materials using Class F fly ash and sodium hydroxide 2M solution were synthesized. Fourier transform infrared spectroscopy and scanning electronic microscopy were utilized in this study for characterizing the obtained geopolymer. The main product of reaction in the geopolymer materials was amorphous alkali aluminosilicate. The obtained materials were dependent on the activation treatment. Key words: activation, characterization, fly ash, geopolymer, sodium hydroxide 1. Introduction Portland cement has been used as principal component of concrete and mortars construction material in the last decades for its high mechanical performance. Several alternative materials were studied with high concern for environmental protection, economy for raw materials, the low energy consumption involved in limestone decarbonation and raw mixed materials clinkerization (temperature 1500 C, with a nominal energy demand of MJ/tons clinker) (Criado et al., 2007). Very important aspect is the emission of greenhouse gases into the atmosphere (essentially CO 2 and NO x : the manufacture of one ton of cement generates approximately one ton of CO 2 ) (Criado et al., 2007). As an alternative for replacement Portland cement, the literature reported valuable data about new materials tailored by alkali-activating of fly ash. The alkali activation of fly ash consists in mixing the fly ash with alkaline activators (alkaline solutions) and subsequently curing of the resulting slurry at a certain temperature. Coal fly ash is the most important waste of electricity production in power stations. The burnt products from power stations are estimated of about 15 millions tons/year. From each tone of coal, a quantity of tone of fly ash will result. In Romania, 15 millions tons of fly ash resulted in 1980, while 30 millions tones were obtained in In Iasi there is a thermal power station (CET II), with 610 MW, on the base of the solid coal (energetic pit coal). In the last twenty years in Iasi resulted approximately 500 millions tons of fly, from that a small part is capitalized; remainder unused is disposal on the land. On the base of actual settlement was assessed to stop hydraulic disposal of fly ash, novel potential applications have been opportune. Traditionally, fly ash is being applied in cement and ceramic industry, but the material has favorable properties for more applications, such as the conversion into zeolites and geopolymers (Harja et al., 2007). Geopolymers have been assessed as new materials having improved chemical and mechanical properties (van Jaarsveld, 2003). Name of geopolymer was proposed for describing synthetic Author to whom all correspondence should be addressed: mivanciu@ch.tuiasi.ro

2 Harja et al./environmental Engineering and Management Journal 8 (2009), 3, minerals similar to those that form in the Earth s crust (Davidson, 1988). Geopolymers are included in silicon-aluminates family as zeolites but they have a significant difference from them: they are essentially amorphous polymers (van Deventer, 2007), having also different properties. The most important properties and applications of geopolymers are as follows: good mechanical strength; durability at chemical attack without much function loss (Bakharev, 2005a; Song, 2005); easily recycled, adjustable coefficient of thermal expansion; hazardous waste disposal binder for the heavy metal fixation especially for waste solidification; fire resistant; fast setting: geopolymer obtains 70% of the final compression strength in the first 4 h of setting and it is used as construction material (Rangan, 2008a; 2008b; 2009). Geopolymers are also known as non pollution (green) material for low energy necessary and low gas emission during obtaining. Thermal processing of natural raw or by-product at relative low temperature provides suitable geopolymer material, resulting in 3/5 less energy assumption consumption than Portland cement, with less CO 2 emitted (Hardjito et al., 2004) Literature reported that certain mix compositions and reaction conditions such as Al 2 O 3 /SiO 2, alkali concentration, treatment temperature, curing time, solution/solid ratio and ph significantly influence the formation and properties of a geopolymer (Duxson et al., 2007; Fernandez Jimenez et al., 2006a; 2006b; Palomo et al., 2004). Previous research reported that heat is an important factor for the activation of fly ash (Duxson et al., 2007), the activation of fly ash by alkalis (NaOH or KOH) was studied by utilizing different temperature treatments, from ambient thought medium to elevate temperature (Baldwinet al., 1982; Wiles, 1988). The other study investigated utilization of fly ash and kaolinite clay in a geopolymer material cured at temperatures up to 70 C. Palomo et al. (1999; 2004) presented a study of alkali-activated fly ashes cured at 65 and 85 C at two liquid/solid ratios: 0.25 and 0.3, material characterization indicating formation of an amorphous alkali aluminosilicate. These mineral polymers have empirical formula: Me n [ (SiO 2 ) z AlO 2 ] n wh 2 O, where z is 1, 2 or 3; Me is an alkali cation, such as potassium or sodium, and n is the degree of polymerization, which were called polysialates (Bakharev, 2005b). This study contained the investigation of the effect of different thermal regimes, time of reaction, stirring and solid/liquid ratios on the class F fly ash activated by sodium hydroxide solution. A number of sixteen geopolymer materials were synthesized, which were analyzed by FTIR spectroscopy, density and Scan Electron Microscopy (SEM). Infrared spectroscopy (which provides information on the vibrations generated by the chemical bonds in a material was chosen as the analytical technique, inasmuch as this procedure yielded very valuable information in prior studies of the material. The objectives of the research were: to identify the reaction products of fly ash activation and to study the gel nanostructure. 2. Experimental program On the base of researches presented in the literature, in this study the following conditions were chosen: the NaOH solution concentration of 2 M (the interval being between 1 5 M); the ratio solid (fly ash)/liquid: 1/2, 1/3 and 1/4; temperature and contact time were varied in the interval: ambient temperature (22 C) during 7-21 days, C with or without stirring up during 7-8 hours and 600 C during 1-2 hours. After the thermal treatment all samples were filtered under vacuum, washed on paper till the removal of the alkalinity (ph 6-7) and seared in the oven at 90 C up to constant mass. The working conditions are presented in Table 1. Table 1. Experimental conditions Sample Materials s/l ratios Temperature Curing time 1 4g fly ash+ 8mL sol NaOH 1/2 Ambient, 22 C 7 day 2 4g fly ash + 8mL sol NaOH 1/2 Ambient, 22 C 21 day 3 4g fly ash + 12 ml NaOH 1/3 Ambient, 22 C 7 day 4 4g fly ash + 12 ml NaOH 1/3 Ambient, 22 C 21 day 5 4g fly ash + 16 ml NaOH 1/4 Ambient, 22 C 7 day 6 4g fly ash + 16 ml NaOH 1/4 Ambient, 22 C 21 day 7 10g fly ash +20 ml NaOH 1/2 80 C 4 hour 8 10g fly ash +20 ml NaOH 1/2 80 C 8 hour 9 10g fly ash + 30 ml NaOH 1/3 80 C 4 hour 10 10g fly ash + 30 ml NaOH 1/3 80 C 8 hour 11 10g fly ash + 20 ml NaOH 1/2 50 C (stirred) 4 hour 12 10g fly ash + 30 ml NaOH 1/3 50 C (stirred) 8 hour 13 5g fly ash + 5 g NaOH 1/1 600 C 1 hour 14 5g fly ash + 10 g NaOH 1/2 600 C 1 hour 15 5g fly ash + 5 g NaOH 1/1 600 C 2 hour 16 5g fly ash + 10 g NaOH 1/2 600 C 2 hour 514

3 Utilization of coal fly ash from power plants II. Geopolymer obtaining The samples 1-6 were prepared at the laboratory temperature (22 C) without stirring; the samples 7-10 were prepared at 80 C in the Teflon autoclave, cooled during 20 hours at the room temperature T=20ºC, filtered, washed and dried; the samples were obtained at a temperature of 50ºC in reactors with magnetic stirring, 500 rot/min. The samples 13, 14, 15, 16 were treated at 600ºC in electric furnace by mixing the fly ash with NaOH solid in platinum ball, cooled during 11 hours, diluted with 150 cm 3 distillate water and boiled for crystallization 3 hours at T=100ºC. determine the future hydraulic properties of geopolymers. 3. Results and discussion 3.1. Fly ash characterization The fly ash collected from CET Holboca Iasi was analyzed chemically, physically, mineralogical and technologically 20. A fly ash class F with spherical particle (Harja et al., 2008; 2009) was used, in accord with literature (Kutchko and Kim, 2006; Sarbak et al., 2004). On the base of characterization it can be observed that the particles have diameters in a large spectrum; because of that the particle size was analyzed with diffraction particle size analyzer SALD-7001 equipped with violet laser. The experimental results are presented in Fig. 1, the medium diameter having values less than 40 m. Approximately 95-99% from the fly ash is constituted of oxides of Al, Si, Fe, Ca. Chemical composition is important because the results of reactions which take place among the oxides with acidic (SiO 2, Al 2 O 3, Fe 2 O 3 ) or basic character Fig. 1. Particle size distribution For the studied fly ash the following chemical composition was obtained: SiO %; Al 2 O %; Fe 2 O %; CaO %; MgO %; SO %; et all., in accord with literature (Mishra et al., 2008). The IR analysis was performed using a DIGILAB FTS 2000 spectrometer; the results obtained for fly ash are presented in Fig. 2. In spectrum from Fig. 2 (Harja et al., 2008), the most relevant band is observed in the cm -1 range, corresponding intense T-O band (where T is Si or Al) %Transmittance Wavenumber (cm-1) Fig. 2. IR spectrum for fly ash 515

4 Harja et al./environmental Engineering and Management Journal 8 (2009), 3, In the initial ash, this band is around 1091 cm -1 attributed to asymmetric stretching of O-Si-O and O- Al-O, the band at 557 cm -1 is attributed to the bending Al-O-Si and Si-O-Si, the band 457 cm -1 is attributed on bending O-Si-O bonds, the bands 692 and 794 cm - 1 are attributed for Si-O bend, 1600 and 3450 cm -1 stretching vibration OH and H-O-H. Assigned strong and very strong absorption bands 1091 cm -1, 794 cm -1 are attributed to the presence quartz, the absorption band at 557 cm -1 is attributed to the mullite phase. Absorption bands at 1091, 692, 557 cm -1 are attributed to the presence hematite phase in the sample, 795 cm -1 is attributed to quartz/kaolinite phase. In conclusion the fly ash contains: quartz, hematite, mullite, kaolinite, montmorilonite, calcite, in accord with data reported in the literature (Harja et al., 2008; Vassilev et al., 2003) Characterization of synthesized materials The sixteen geopolymer materials obtained were analyzed by FTIR spectroscopy, density and Scan Electron Microscopy (SEM). Infrared spectroscopy (which provides information on the vibrations generated by the chemical bonds in a material) was chosen as the analytical technique, inasmuch as this procedure yielded very valuable information in prior studies of the material. The IR spectra obtained for synthesized samples are presented in Figs For interpretation, the same spectra were represented in a single figure. %Transmittance %Transmittance Sample Sample 10 Sample Wavenumber (cm-1) Fig. 3. IR spectra for samples 7, 8 and 10, geopolymer materials Wavenumber (cm-1) Fig. 4. IR spectrum for sample 12 geopolymer materials Fig. 5. IR spectra for synthesized materials 516

5 Utilization of coal fly ash from power plants II. Geopolymer obtaining Table 2. The IR assignments of phases IR cm -1, minimum IR cm -1, maximum Chemical phase Assignment Kaolin O Si O bend Kaolin/hematite O Si O/Fe O bend Kaolin/hematite Al O Si/Fe O bend Feldspar Al O Si str Quartz/ CO 3 Si O bend/c O bend Feldspar/CO 3 Al O Si str/c O bend Illite Al O Si bend Quartz Si O Si str Quartz/Kaolinite Si-O bend Illite/montmorilonite Al O H str Kaolin Al O H str Kaolin Si O Si str Amorph Al-Si (amorphous aluminosilicates) Al Si Al O Si str Water H O H bend Water H O str Kaolin/illite/Montmor O H str Kaolin O H str IR absorption spectroscopy seems to be a suitable tool to characterize geopolymer material. IR spectra were performed on the base of data from literature, from which reported data (Legodi and de Waal, 2007) for IR bands are summarized in Table 2. In accord with data from Table 2, the characteristic IR vibration bands of the geopolymer materials are presented: (s) cm -1 - stretching vibration ( OH); (s) cm -1 - stretching vibration ( OH, HOH); cm -1 - bending vibration (HOH); (s) cm -1 - asymmetric stretching (Si O Si and Al O Si), 1100 cm -1 (sh) - asymmetric stretching (Si O Si), 850 cm -1 (sh) - Si O stretching, OH bending (Si OH); 795 cm - 1 (m) - symmetric stretching (Si O Si); 688 cm -1 (sh) - symmetric stretching (Si O Si and Al O Si); cm -1 (m) - double ring vibration and 424 cm -1 (s) - bending (Si O Si and O Si O), where s is strong; w is weak; m is medium and sh is shoulder (Bakharev, 2005; Zhang et al., 2008). Analyzing the spectra for samples 7 and 8 it can be observed that few differences occur, the principal band is moving from 1084 to 1078, that means for the same conditions of temperature and ratio s/l the increase of attack time from 4 hours to 8 hours does not bring major modifications in the structure of synthesized material. Maintaining 8 hours the attack time, but increasing the ratio solid/liquid from 1/ 2 to 1/3 does not determine the occurrence of new bands, the only modification is the fact that the band corresponding to the bond OH and H-O-H is more pronounced. When the ratio s/1 is maintained 1/3 and the attack time is 8 hours, but the sample is obtained by magnetic stirring it was observed the missing of pick corresponding to band 1630 cm -1. Significant differences from initial fly ash occur in the case of thermal activation at high temperature (600 C). From Fig. 5 one may observe that the most relevant band is moving in the direction of wavenumber decreasing from cm -1 to cm -1. Also, it can be observed the occurrence of two new bands at 1411 and 1480 cm -1 respectively. This data were compared with recent data on the IR band parameters of Na 2 O-SiO 2, K 2 O-CaO- SiO 2 and glasses phase. Bands around , cm -1 in the spectra of these glasses are concluded to be envelopes, each of the envelopes covering a hydroxyl-related band and a band or two due to certain combination modes or overtones of glass matrices (Efimova and Pogareva, 2006). Also in literature bands corresponding range cm -1 were reported for gaylussite and pirssonite (double carbonated of Ca and Na hydrated). In our experimental conditions this compounds can appear because in fly ash exists CaO free, NaO is introducing with alkaline solution, carbonation took place with CO 2 from the atmosphere and the sample was hydrated. The main feature of IR spectra was the central peak between 993 cm 1 and 1001 cm 1 which is attributed to the Si O Si or Al O Si asymmetric stretching mode. IR spectrum is characterized by the shift of the bending band of the Si O bond (1050 cm 1 ) from metakaolin to lower frequency (990 cm -1 ) and a decrease of the 800 cm 1 band, with the formation of a new band at about 700 cm 1 which was reported as characteristic of the polymer formed. IR spectra of alkaline alumino-silicate had a band associated to (SiO) at 997 cm 1. Other characteristic bands of the inorganic polymer were placed at 695 and cm 1 for asymmetric stretching of Si O of glassy silica shifts to lower frequencies, when substitution of Si by Al takes place (Criado et al., 2007; Khale and Chaudhary, 2007). Broad absorption band in the area cm -1 indicated the silicate and alumosilicate glasses phase. The SEM images for synthesized materials are presented in Fig

6 Harja et al./environmental Engineering and Management Journal 8 (2009), 3, Sample 1 Sample 8 Sample 12 Sample 14 Fig. 6. SEM images for synthesized materials From the images presented in Fig. 6 it can be observed the followings: in the case of materials synthesized at room temperature the transformation degree is smaller than in the case of high temperature. Also by increasing the curing time, a more advanced destroy of fly ash particles is realized. That is an observation resulting from IR spectrum analysis. 4. Conclusions Geopolymer materials can be used as replacement of Portland cement; these are environmentally friendly and need only moderate energy for being obtained. In comparison with Portland cement CO2 emission is reduced of about 80%. 518 To decrease the cost effectiveness of geopolymer binders the aluminum and kaolin source has to be replaced by some cost economic materials such as slag, builder s waste or fly ash. It can be concluded from the study that geopolymers are Green material through small emission for obtaining and on the other hand they can be immobilized of the toxic metals within the matrices. In this study, the fly ash (Holboca Iasi) (class F) is recommended for obtaining the geopolymer material. On the base of literature researches the conditions for obtaining the geopolymeric materials were proposed. Sixteen news materials using class F fly ash and sodium hydroxide 2M solution were synthesized. IR spectroscopy and SEM were utilized in this study for characterizing the obtained geopolymer.

7 Utilization of coal fly ash from power plants II. Geopolymer obtaining On the base of IR spectra analyze it was concluded that at the same attack temperature, the increase of curing time does not bring significant modifications. The most important modification is brought by the temperature, and on the base of obtained data is recommended geopolymerazion at temperatures over 80 C. Significant differences from initial fly ash occur in the case of thermal activation at high temperatures (600 C), case in which two new bands occur at 1411, 1480 cm -1 respectively. These data were compared with recent data on the IR band parameters of Na 2 O-SiO 2, K 2 O-CaO- SiO 2. The main product of reaction in the geopolymer materials was amorphous alkali aluminosilicate. The type of material was dependent on the activation history. For obtaining better results in the geopolymer synthesis, the contact time must be increased for ensuring an advanced destroy of fly ash particles, treatment of samples in autoclaves, favoring in this way a higher modification degree and at least the increase of temperature, so accelerating the chemical reactions by modifying the crystalline network. The next studies aim is to establish the optimum conditions of geopolymer materials synthesis for obtaining cement replacement materials which result in concretes with mechanical performances compared with that of ordinary Portland concrete. Acknowledgement Part of this paper has been done with the support of the project IDEI - ID_595, Contract 132/2007, within PNCDI II, National Research Authority, Romania. 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