CHARACTERIZATION OF THE POZZOULANIC PROPERTIES OF THE FLY ASH EVOLVED FROM TPP REPUBLIKA, BULGARIA

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1 Journal of the University L. Dimova, of Chemical I. Doykov, Technology J. Ninov, and N. Metallurgy, Petrova 46, 4, 2011, CHARACTERIZATION OF THE POZZOULANIC PROPERTIES OF THE FLY ASH EVOLVED FROM TPP REPUBLIKA, BULGARIA L. Dimova 1, I. Doykov 2, J. Ninov 3, N. Petrova 1 1 Institute of Mineralogy and Crystallography, BAS, Acad. G. Bonchev Str., bl.107, 1113 Sofia, Bulgaria 2 University of Architecture, Construction and Geodesy, Received 20 July 2011 Accepted 22 September Christo Smirnenski Ave., 1164 Sofia, Bulgaria 3 Sofia University St. Kliment Ohridski, 1 James Bouchier Ave., 1164 Sofia, Bulgaria ABSTRACT New data on the composition, physical chemical and pozzoulanic properties of the fly ash (FA), generated at TPP Republika, are obtained after the basic reconstruction during The quantities of the crystalline phases and amorphous (glassy) phase are determined using Rietveld refinement. The activity index is determined in accordance with the BDS EN requirements. The complex analyses carried out show that the fly ash improves pozzoulanic properties and this material is appropriate for effective utilization in the manufacture of different kind building products, without preliminary mechanical treatment. Keywords: fly ash, pozzoulanic properties, Rietveld refinement, mineral (phase) composition, DTA-TG analysis, specific surface area. INTRODUCTION The combustion of coal for electricity and heat generation produces a large amount of wastes the fly ash being the biggest one with its share of about 90 % of the solid residue. The total quantity of FA in Bulgaria amounts to about 6-7 million tons per year, but only a small part (under 5 %) of it is used in the construction. The remaining part is disposed in on-site ponds and landfills causing several ecological problems. Yearly, nearly 300 ktons FA is due to the coal-fired TPP Republika, which operates with low-grade subbituminous coal from the Pernik mine region. In some previous works [1-4] the FA from TPP Republika is defined as coal fly ash with low calcium content, coarser with sand-size particles predominated, with low content of glassy (amorphous) phase and relative rich in unburned carbon. Therefore, this solid residue was disposed as wastes of low quality in two landfills near the town of Pernik. The V -th steam generator (148 MW) was modernized in 2007/2008 and equipped with new air-mills and a new four-chamber electrostatic precipitator (). Due to the finer grinding and pulverization of the coal fuel and the better combustion in the boiler, the characteristics of the falling FA and of the flue gases exhaust to the atmosphere were changed. The aim of the present work is to provide new data on the composition, physical chemical and pozzoulanic properties of the FA, generated at TPP Republika after the reconstruction of its equipment. The obtained results in the study would provide possibilities to utilize the FA in several areas. The study does not concern the bottom ash, which quantity is lower than 5 % from the total waste residue. EXPERIMENTAL Materials and methods The FA samples are collected from modernized fuel installation of TPP Republika at different periods of 2010 during its processing in stable combustion parameters. Dry FA is taken from the hoppers of the mechanical collector (MC) and from all the cham- 369

2 Journal of the University of Chemical Technology and Metallurgy, 46, 4, 2011 bers of the duster system. The study of the FA comprises the determination of its composition chemical granulometric and mineral (phase), and its reactivity. For this purpose appropriate chemical and physical methods are used, as well different instrumental techniques: (i) The chemical analysis is performed using standard procedures given in BDS-EN [5] while the pozzoulanic properties are characterized using BDS EN standardization method [6]; (ii) The particle size distribution is accomplished by dispersion analysis performed on Shimadzu apparatus; (iii) The mineral (phase) composition is studied by XRD method. The quantities of the crystal phases are determined using Rietfeld refinement analysis [7-11]. The data are collected on the D8 Advance Bruker aparatus, in CuKá radiation (30 kv, 10 ma), in the range from 5 to 70 2È with 0.02 step size, the counting time per step being of 4 s; (iv) The DTA-TG analysis is proved on the STA Stanton Redcroft 780 apparatus under dynamic air conditions in the temperature interval from ambient to 1000 C, at a rate of heating 10 C/min, and using á- Al 2 as a reference material; (v) The specific surface area (S a ) is determined by BET method while the pozzoulanic activity (PA) of the FA is measured more often by the Chapelle test [12], and the standardized in BDS EN activity index (AI) method [6]; RESULTS AND DISCUSSION Chemical composition Two samples are analyzed in order to determine chemical composition: the first one is representative for the FA falling in the MC and the second is mixture from FA samples collected from the first three chambers of the, in proportion they are separated. The material from the fourth chamber, coming to about 2-3 mass % of the total FA quantity is found to be visually different in its appearance, and therefore is not included to the mixture. The experimental data obtained for the chemical composition of the both samples are close in values (Table 1). Some differences are observed only in the contents of S, the reactive SiO 2 and Al 2, which probably is a result from the influence of the granulometry on the high temperature reactions. Comparing these data with those given in [1-4], one can conclude that there are no significant differences in the chemical composition of the FA before and after the reconstruction of the equipment. We suppose that this is consequence of the relative constancy of the type and the quality of the coal fuel used in the TPP for the considered time period. The pozzoulanic properties of the low calcium FA meant for admixture to cement and concrete are determined in accordance to the standard. The chemical composition of this class FA have to meet the following requirements (in mass %): SiO 2 + Al 2 + Få 2 70; active SiO 2 25; MgO 4; alkali expressed as Na 2 O equivalent 5; loss of ignition (LOI) 5. It is seen from Table 1 that the FA samples from the both MC and of TPP Republika implement these restrictions. The content of active SiO 2, which is a major component of the amorphous phase, exceeds the standard demand, as well. The values of LOI are low, : 2.5 mass %, which indicates providing of optimal combustion regime in the boiler. Particle size distribution The particle size distribution of the samples received from the MC and from all four chambers of the is accomplished in the present work as the experimental results for the basic fractions are given in Table 2. The BDS EN 450-1[6] requires the rest of the FA retaining on the sieve with 45 ìm opens to be 40 mass%. From Table 2 one can see that this demand is fulfill for all the samples studied except that from the MC, which exceeds the restriction with about 20 %. The systematic operating data for the duster system of TPS Republika show that the quantity of FA discharged from the MC is relatively small, in order of 5-8 %. The rest to 100 % is divided between I, II, III and IV chambers in proportions, respectively: %; %; 7-10 %; and 2-3 %. With respect to these results, and the experimental data given in Table 2, one could conclude that the FA from TPP Republika is a by-product proper for direct addition in cement mortars and concretes, without mechanical treatment. Mineral (phase) composition XRD patterns of the samples originating from the MC and from the first three chambers of the are presented on Fig. 1. The patterns show that the samples consist of crystalline and amorphous parts. The 370

3 L. Dimova, I. Doykov, J. Ninov, N. Petrova Table 1. Chemical composition of the fly ash from TPP Republika in mass %. No Components Before reconstruction After reconstruction 1980 [1] 1986 [2] 2002 [3] 2008 [4] MC Chambers ², ²² and ²²² of the 1 SiO Al TiO Fe CaO MgO S K 2 O Na 2 O Reactive SiO Acid soluble Al 2 12 Acid soluble Fe 2 13 Moisture LOI phases of quartz, hematite and magnetite are identified as crystalline constituents of the samples. The quantity of the crystalline SiO 2 is found to vary from 20 to 24 %, and that of hematite from 2 to 4 %. The magnetite content in the most samples is less than 1 % and it is incorrect to be defined in this way. The presence of other phases, such as corundum, mullite and anhidrite, established by optical microscopy method in [4], is not confirmed now, probably owing to their low content. The amorphous (glassy) phase is represented on Fig. 1 as width hallo, disposed in the interval from 15 to 30 2È. The reactive SiO 2, Al 2 and Fe 2 are included in this phase. The glassy phase content is determined with the help of the program Topas 4, an individual part of the packet Diffrac Plus Evaluation (Bruker Advanced X-ray solutions), which allows to be found the quantitative correlation between amorphous and crystal constituent parts. The contents of the amorphous phase in the samples imposed on Fig. 1 are given in Table 3. One can see, they are close in value, varying in rang from 72 to 75 mass %. These values exceeded the data reported formerly in [4] for the FA from TPP Republika with about %. Thermal behavior of the FA from the fourth chamber of the Two distinguished features have the FA from the IV chamber of the : the value of LOI is above 10 %, Fig. 1. XRD patterns of FA samples from: MC (FA-B0); chambers: I (FA-B1), II (FA-B2) and III (FA-B3), Legend: Q quartz; H hematite; M - magnetite. Fig. 2. DTA-TG analysis of the sample from chamber IV of the. 371

4 Journal of the University of Chemical Technology and Metallurgy, 46, 4, 2011 Table 2. Fineness of the FA from TPP Republika Fraction, µm MC Chamber I, Percent composition with accumulation Chamber II, Chamber III, Chamber IV, > > > > > > > > > > > > > and the glassy phase content is too low, in order of 3-5 mass %. It is clear that this part of the solid waste does not comply with the standard request to the fly ash pozzoulans. The DTA-TG analysis of the sample is applied to elucidate the property of this material. The DTA-TG curves are displayed in Fig. 2. The DTA curve exhibits several endothermic effects which correspond to mass losses on the TG curve. The strong endothermic effect at 77.5 C relates to the release of the hygroscopic water. The other weak endothermic effects at temperatures near 130, 277 and 491 C are probably sequence of the decomposition of different hydrated alkaline and alkaline earth aluminates, silicate and sulfate phases. The last effect at about 679 C corresponds to alkaline earth carbonate decomposition. Mineral phases of this kind could be formed by secondary reactions and/or by condensation process at cooling of the combustion gases, and subsequently their deposition in the last chamber of the. Reactivity of the Fly Ash The pozzoulanic activity (PA) is an indicative of the lime-pozzoulan reaction, and it is applied for evalu- Table 3. Contents of crystal and amorphous constituents of the FA, mass %. Phase MC Chamber I Chamber ²² Chamber ²²² of the of the of the Crystalline SiO Hematite Amorphous phase Table 4. Activity index of FA mortars. N AI Control mix, Test mix, MPa AI, % MPa 1. Ccompressive strength (7 days) Compressive strength (28 days) Bending strength (7 days) Bending strength (28 days)

5 L. Dimova, I. Doykov, J. Ninov, N. Petrova ation the quality of the pozzoulans and to specify them for using. The most important physical chemical property affecting PA are the fineness, the glassy phase content (respectively, the active silica content) and the specific surface area (S a ). The average value of the S a obtained for the mixure collected from the is found to be 13 m 2 g -1 ; for the sample received from the MC the S a is 17 m 2 g -1. These results are high and they are in good coincidence with the data reported from other authors for the same type of FA [3]. The PA measured by the Chapelle test [12] is defined as consumption of Ca(OH) 2 per gram of FA during the pozzoulanic reaction. The method is convenient for routine control, but it is accompanied with considerable dispersion of the experimental data, probably as consequence of the complicate physical and mineral characteristics. The maximal pozzoulanic activity of 0.67 g Ca(OH) 2 /g FA is found in the case of the mixture from the first three chambers of the ; and for the sample from the MC, PA is relatively low, 0.32g Ca(OH) 2 /g FA. The other approach, based on the determination of the AI [6], is accepted as a more reliable method for evaluation the PA of the FA. The AI is defined as ratio of test to control compressive strength of the mortar prisms, prepared as per BDS EN 196-1[13], stored in water for 7 and 28 days. The control prisms are prepared with cement CEM I 42 R, and in the test prisms 25 % from the cement is replaced with FA. The requirement of BDS EN for the FA mortars are the compressive strength ratio to reach 75 % at 28 days storage, the strength values being average of three samples. The results obtained for the AI are given in Table 4. As it is seen form the table, the AI for 7 days storage exceeded the limit required in BDS EN for the 28 days storage. For 28 days storage the AI is found to be 34 % higher, in comparison with the standard limit. The bending strength changes in a similar manner like the compressive strength. Some sources refer that such high values for the AI of FA containing mortars are reached barely at 2-3 months storage [14, 15]. CONCLUSIONS Complex analyses are carried out in order to determine the qualitative characteristics of the fly ash discharged from the modernized up-to-date equipment of the 148MN steam generator of TPP Republika. From the obtained results the following conclusions can be point out: (i) It is established that the TA exhibits improved pozzoulanic properties: high fineness, proper for direct addition in mortars and concretes, without mechanical treatment; low value of LOI, high specific surface and pozzoulanic activity, the activity index being exceeded the standard requirement with 34 % for 28 day storage. These properties correlate with the high content of amorphous phase (72-75 mass %). (ii) The increasing in the pozzoulanic properties of the fly ash is related to the improved preparation and combustion of the coal, as well as to the more strongly control of the process parameters. (iii) The fly ash from TPP Republika is artificial pozzoulan of high quality. Thus, as well its low price, is prerequisite for its effective utilization in the manufacture of different kind building products, and for other purposes. REFERENCES 1. G. Babachev, Complex utilization of Fly Ashes and Slags from TPP s, Technika, Sofia, 1980, (in Bulgarian). 2. Catalog of the Quantities and the Qualities of the Fly ashes and Slags from TPP s, Energoproect, Sofia, 1986, (in Bulgarian). 3. A. Shumkova, Ph.D. Thesis, High gradient magnetic separation of Fly Ash from TPP s, University of Chemical Technology and Metallurgy, Sofia, I. Donchev, A. Lenchev, J. Ninov, L. Gigova, Study of Some Mineral and Technological Properties of Fly-Ashes from TPS Republika and Bobovdol, Compt. Rend.de l Acad. Bulgare des sciences, (Sofia), 54, 11, 2001, BDS EN 196-2/2005, Methods of Testing Cement. Part 2: Chemical Analysis of Cement. 6. BDS EN 450-1/2005, Fly Ash for Concrete. 7. H. Rietveld, A profile refinement method for nuclear and magnetic structures, J.Appl. Crystallography, 2, 1969, D. L. Bish, S. J. Howard, Quantitative phase analysis using the Rietveld method, J. Appl. Crystallography, 21, 1988,

6 Journal of the University of Chemical Technology and Metallurgy, 46, 4, D. L. Bish, J. E. Post, Quantitative mineralogical analysis using the Rietvetd full-pattern fitting method, American Mineralogist, 78, 1993, R. J. Hill, Quantitative phase analysis from neutron powder diffraction data using the Rietveld method, J. Appl. Crystallography, 20, 1991, I. C. Madsen, R. J. Hill, Rietveld Analysis Using Para- Focusing and Debye Scherrer Geometry Data Collected with a Bragg-Bentano Diffractometer, Zeitschrift fur Kristallographie, 196, 1-4, 1991, G. Largent, Estimation de l activite pouzzolanique Bull. Liaison Lab., Pont Chausees, 93, 1978, BDS-EN-196-1/2005, Methods of Testing Cement. Part 1: Determination of Strength. 14. B. Sabir, S. Wild, J. Bai, Metakaolin and Calcined Clays as Pozzolans for Concrete: A review, Cem. Concr. Composites, 23, 2001, J. Bai, B. Sabir, S. Wild, J. Kinuthia, Strength development in concrete incorporating PFA and MK, Mag. Concr. Res., 52, 3, 2000,