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Geopolymer synthetized from bottom coal ash and calcined paper sludge
Journal of Cleaner Production xxx (2013) 1e6
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Geopolymer synthetized from bottom coal ash and calcined paper sludge Rozineide A. Antunes Boca Santa a, Adriano Michael Bernardin b, *, Humberto Gracher Riella a, Nivaldo Cabral Kuhnen a a b
Chemical Engineering Department, Santa Catarina Federal University, P.O. Box 476, Trindade, Florianópolis, SC 88.040-900, Brazil Ceramic Materials Group, Santa Catarina Extreme South University, Av. Universitária 1105, Criciúma, SC 88.806-000, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 October 2012 Received in revised form 4 May 2013 Accepted 14 May 2013 Available online xxx
This study deals with the development of geopolymers synthetized from industrial waste containing aluminosilicates. Geopolymers are inorganic polymers formed by the activation of amorphous aluminosilicates (Al2O3.SiO2), which react in a strongly alkaline medium. Bottom ash (SiO2/Al2O3 ¼ 3.3e4.5) was used as source of aluminosilicate and sodium hydroxide (NaOH ¼ 5, 10 and 15 M) and sodium silicate (Na2SiO3, SiO2/Na2O ¼ 1.58) were used as alkaline medium. Calcined paper sludge was used to increase the reactivity of the partially crystallized bottom ash. The solid waste was characterized by XRF and XRD and the geopolymer samples were characterized by XRF, XRD, SEM, FTIR and compressive strength tests. The best results were obtained with a solution of 15 M NaOH and sodium silicate and a mixture of 2:1 bottom ash and calcined paper sludge. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Bottom ash Calcined paper sludge Alkaline activation Aluminosilicates Geopolymer
1. Introduction The increasing industrial development induced by a constant demand for new products has resulted in an exponential growth of waste generated at the end of the manufacturing processes as well as post-consumer disposal. Therefore, the best way to solve the problem of waste generation, besides the consumption reduction, is the use of those as raw materials for the production of high quality and valuable products. In addition, there is the saving of natural resources because the environmental aspect must be considered and evaluated when thinking about social and economic development, in other words, sustainability (McLellan et al., 2011; Castro-Gomes et al., 2012). In this study, bottom ash from coal and calcined paper sludge from paper sludge were used as raw materials for the synthesis of geopolymer, an inorganic polymeric material formed by the activation of aluminosilicates in strongly alkaline medium formed by the activation of aluminosilicates in strongly alkaline medium, resulting in poly(sialates) with the structure of phyllosilicates. The term “geopolymer” was created by J. Davidovits in 1979 to describe the chemical properties of inorganic polymers based on
* Corresponding author. Tel./fax: þ55 48 34312669. E-mail addresses: [email protected], [email protected] (A.M. Bernardin).
aluminosilicates. Geopolymers present cementitious properties and, therefore, great potential for use in the construction industry. They can be formed using natural raw materials or industrial wastes from several sources, provided that the wastes are rich in amorphous or semi-crystalline aluminosilicates or have passed through heat treatment, making them more reactive and suitable to alkaline activation. The alkaline activation is a chemical process that transforms amorphous, partially amorphous or metastable vitreous structures into a compacted cementitious material (Palomo et al., 1999). The aluminosilicates can dissolve in contact with an alkaline solution because leaching of the Al3þ and Si4þ species occurs. The concentration of the activator should be high (Rattanask and Chindaprasirt, 2009). When mixing an aluminosilicate with an alkaline solution a reaction is triggered and in few seconds there is the dissolution of the particles, reorganization of the structure and gelation, resulting in a three-dimensional aluminosilicate network. The Si and Al species released on the surface are responsible for the geopolymerization reaction. The dissolution of the amorphous aluminosilicate is rapid at high pH resulting in a supersaturated solution of Si and Al. Concentrated solutions result in the formation of a gel with increasing oligomers. The aqueous gel forms large networks by condensation (Duxson et al., 2007). The Si4þ and Al3þ in the solution are coordinated by oxygen bridges. The negative charge of the AlO4 is responsible for balancing the alkaline cations.
0959-6526/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jclepro.2013.05.017
Please cite this article in press as: Antunes Boca Santa, R.A., et al., Geopolymer synthetized from bottom coal ash and calcined paper sludge, Journal of Cleaner Production (2013), http://dx.doi.org/10.1016/j.jclepro.2013.05.017
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R.A. Antunes Boca Santa et al. / Journal of Cleaner Production xxx (2013) 1e6
Geopolymers do not show stoichiometric composition and they are analogs of zeolites. The chemistry between both materials is quite similar, although their structures and compositions are different. Geopolymers comprise amorphous to semicrystalline structures and their empirical formula can be described by: Mn[e (SiO2)zeAlO2]n$wH2O, where z is 1, 2 or 3, M is the cation of alkali metal and n is the degree of polycondensation. From the findings about geopolymers, the interest in unraveling the mechanisms involving these materials and develop new formulations have expanded (Davidovits et al., 1991; Davidovits, 2011; Turgut, 2012; Habert et al., 2011). The geopolymeric technology results in a product that can be easily handled, stored and monitored, because its production is done with simple technology. 0.180 ton of CO2 is emitted to produce one ton of geopolymeric cement from kaolin. To produce one ton of Portland cement one ton of CO2 is emitted because the clinker is calcined at about 1500 C. Thus, the production of geopolymer emits approximately six times less CO2 (Davidovits, 2002). If produced from fly ash the emission is even smaller, because it does not require heat treatment. The wide variety of potential applications of geopolymeric materials includes: fire-resistant building materials, insulation materials, ceramic tiles, refractories resistant to thermal shock, casting materials, composite materials for aircraft and car interiors, high technology resin systems, containment barriers for toxic and radioactive waste, among others (Davidovits, 2011). A recent study suggests that geopolymers can be used in oral drug delivery due to its combination of good mechanical strength and chemical stability. It was demonstrated that its mechanical strength does not allow accidental breakage, with the possibility of adjusting the porosity, allowing drug release as needed (Forsgren et al., 2011). Geopolymers are amorphous materials, difficult to characterize, but the geopolymerization process can be identified by some characteristics. The factors that define the microstructure of the matrix and chemical, physical and mechanical properties of the geopolymers are the particle size distribution of the raw materials, the dissolution ratio of the gel phase, the amorphous nature of the solid raw materials, the degree of crystallinity, the amount of calcium in the raw material, the relationship between Si/Al, the concentration of activator, among others (Komnitsas and Zaharaki, 2007). Regarding the durability of Portland cements currently used, some studies address that the modern cement is strongly affected when exposed to aggressive climatic conditions, and its time duration is approximately 50 years. However, mortars like those found in Rome are still unchanged and were built more than 2000 years ago. The conventional Portland cements depend on the use of lime and can be dissolved by acidic solutions, reaching 30e60% mass loss (Davidovits, 2002). There are many reports regarding the early deterioration in structures built with modern Portland cement (Torgal and Jalali, 2010). Among the motivations for researchers to seek viable alternatives in the production of Portland cement is the durability of ancient cements compared to modern cements. Conventional Portland cements undergo rapid degradation, thus requiring constant demolitions and renovations of buildings with high energy consumption, emission of toxic gases into the atmosphere, and greater consumption of natural raw materials. The subject is increasingly being grounded and the diversity of raw materials used favors the possibility of industrial production, since geopolymers not require raw materials with high purity and uniformity. Within this panorama, the economy of the southern region of Santa Catarina State, Brazil, is based on mining and industrialization of minerals such as coal, clay and kaolin and, therefore, the region is considered one of the most polluted areas in Brazil. All those waste are formed by aluminosilicates, justifying their use as
raw materials for geopolymers. Furthermore, the global demand for coal will grow by 2030, reaching a doubling in relation to the current demand. With the growing consumption of coal for power generation there will be an increased production of ash as a byproduct of coal combustion (Gavronski, 2007; McLellan et al., 2011; Castro-Gomes et al., 2012). The cementitious materials produced by alkaline activation of aluminosilicates have been studied in the search for materials with binding properties more resistant than current Portland cements as well as materials that can be produced using raw materials of low cost, with little energy expenditure and especially with low emission of toxic gases in the atmosphere. Those researches present great possibilities for worldwide deployment and these materials can be produced on a large scale, suppressing the demand for cement in a market that grows every year. The synthesis of new products from industrial byproducts requires a full characterization to know the properties and characteristics of the raw material because each waste, inhere treated as a byproduct, is the result of the processing technique. From the results of the characterization new products can be obtained. In this work, the main feature is the use of calcined paper sludge obtained from the processing of paper waste. Pulp and paper industries produce millions of tons each year to meet the global demand for paper. According to the Brazilian Association of Pulp and Paper (2013) in 2011 were produced worldwide 183.8$106 tons of pulp and 398.9$106 tons of paper. Brazil have produced 10.2$106 tons of paper in 2011 (Brazilian Association of Pulp and Paper, 2013) and, therefore, a large amount of waste generated every day. In this work, waste of paper industry was chosen to be a raw material to the geopolymeric systems because it has in its structure kaolin that becomes metakaolin after heat treatment. However, the waste paper is also rich in CaCO3, which may be harmful in large amounts to produce geopolymers. Thus, the wastepaper has undergone purification with hydrochloric acid (HCl), in order to remove the CaCO3 through chemical reaction. Therefore, the objective of this study was the synthesis of geopolymers using bottom ash from coal combustion and calcined paper sludge by alkaline activation. As alkaline activators, sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) were used. 2. Experimental Bottom ash from coal burning and calcined paper sludge were used as raw materials. Both wastes were collected from a control lot. Sodium hydroxide (Vetec, 97% purity) and sodium silicate (Manchester Química do Brasil S.A., SiO2/Na2O ¼ 3.3) were used as alkaline activators. The bottom ash was dried in a laboratory stove for 24 h at 100 C and, in sequence, ground in a laboratory ball mill (high alumina grinding elements and lining) for 8 h. The resulting material was analyzed by the laser diffraction method to evaluate the particle size distribution. As the paper sludge is composed by cellulose, calcium carbonate and kaolin, a purification step was required for elimination of cellulose and calcium carbonate in the composition of the residue. The paper sludge was treated with hydrochloric acid (HCl) in order to extract the calcium carbonate according to: 2HCl þ CaCO3/CaCl2þH2O þ CO2. The mass balance between calcium oxide (CaO) and calcium carbonate (CaCO3) was used to evaluate the concentration of the hydrochloric acid (HCl), and the amount of CaO present in the residue before treatment, as obtained from XRF data, was used as a reference (Table 3). Therefore, for each 150 g of residue 1 L of HCl solution 1.8 M was necessary to remove the CaCO3 present in the residue. Small amounts of CaO appear to be beneficial for geopolymers and therefore the HCl solution was prepared with 1.5 M. After purification with HCl all material was
Please cite this article in press as: Antunes Boca Santa, R.A., et al., Geopolymer synthetized from bottom coal ash and calcined paper sludge, Journal of Cleaner Production (2013), http://dx.doi.org/10.1016/j.jclepro.2013.05.017
R.A. Antunes Boca Santa et al. / Journal of Cleaner Production xxx (2013) 1e6 Table 1 Bottom ash/calcined paper sludge mass ratios for the synthesis of geopolymeric materials. Solids
Ratios
Bottom ash (BA) Calcined paper sludge (CPS) Bottom ash/Calcined paper sludge
100% 100% 2:1
Mass (g)
Oxides (g)
100 100 133.3:66.7
SiO2
Al2O3
59.8 49.57 112.77
22.6 38.87 56.04
Molar ratios 4.5 2.16 3.45
3
Table 3 Bottom ash (BA) and calcined paper sludge (CPS), chemical composition in (%) mass. Oxide
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
TiO2
LoI
BA CPS CPS (after HCl treatment)
59.8 7.69 49.5
25.2 6.08 38.8
4.5 0.17 1.55
1.3 32.3 5.56
0.5 0.62 1.43
2.3 0.05 0.24
1.1 0.15 1.38
6.6 52.6 0.80
obtain the morphological and chemical characterization of the materials. washed with deionized water and filtered to remove the residue of calcium chloride resulting from the reaction. In sequence, the material is dried in a stove at 100 C for 24 h and calcined at 850 C for 2 h to cause the dehydroxylation of kaolin. The resulting material is highly reactive calcined paper sludge with approximately 5.56% CaO content (Table 3). After heat treatment, the as formed calcined paper sludge was grinded (high alumina laboratory ball mill, 8 h grinding) to adjust the particle size distribution. The raw materials were characterized by X-ray fluorescence (XRF) analysis (Philips PW 2400 WDXRF). Table 1 shows the bottom ash/calcined paper sludge mass ratios for the synthesis of geopolymeric materials. Table 2 shows the sodium hydroxide/sodium silicate ratios. The bottom ash and the calcined paper sludge were weighed and mixed and the sodium hydroxide and sodium silicate were added using a mechanical stirrer until complete homogenization of the mixture for about 5 min. The geopolymeric paste was poured into plastic containers (62.83 cm3) and cured at room temperature. The container must be chosen so as not to contaminate the silica (Rattanasak and Chindaprasirt, 2009). After 24 h the samples were demolded and tested for compressive strength. The tests were made according to the NBR 7215/1996 standard from the Brazilian Association of Technical Standards (1996). The standard was developed to be applied in cement mortars; however, samples which were subjected to testing of compressive strength for this study were molded only with the geopolymer paste, without the use of sand or other aggregates. The same procedure was performed at 7, 28 and 90 days after demolding. The compressive strength tests were performed in a universal testing machine (Emic DL20000, 1 mm/min load speed). The samples were characterized by X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The XRD analysis of the geopolymers and residues was made with accelerating voltage of 40 kV and 30 mA, Cu Ka (l ¼ 1.5418 A) radiation ranging from 5 to 25 and 2q scan rate of 0.05 /s (PanAnalytical X’Pert PRO Multi Purpose). The FTIR analysis (Shimadzu IR Prestige 21) was used to determine the functional groups. The samples were prepared for IR spectroscopy with KBr (Vetec). Finally, geopolymeric samples were submitted to scanning electron microscopy (SEM, Phillips XL30) in order to
3. Results and discussion The XRF results (Table 3) revealed that the bottom ash contains as major components SiO2 and Al2O3. The way the SiO2 and Al2O3 molecules are arranged largely depends on the raw material from which they are derived therefore defining the matrix that will form the inorganic polymer, its microstructure and chemical, physical and mechanical properties (Duxson et al., 2007). The bottom ash (BA) chemical analysis shows that the SiO2/Al2O3 ratio is 2:1, suitable for use in the geopolymeric reaction. The calcined paper sludge (CPS) chemical analysis before and after purification in HCl determined by XRF (Table 3) shows that the amount of CaCO3 was reduced, therefore showing that the purification step was effective to reduce the cellulose and calcium carbonate amounts. The particle size distribution of the raw materials resulted in an average diameter (D50) of 31.8 mm for the bottom ash with 90% of the particles below 39.6 mm and an average diameter (D50) of 24.8 mm for the paper waste with 90% of the particles below 9.3 mm. In their study on geopolymerization, Fernández-Jiménes et al. (2005) have used ligands with ash with 90% of the particles smaller than 45 mm and 50% smaller than 10 mm. Vargas et al. (2007) have used particles with an average size of 29.2 mm to perform their work. Therefore, the particle size distribution of the materials used in this work after grinding are within the size needed to cause a good dissolution and therefore a good activation to produce geopolymeric materials. The geopolymers synthesized in this study were set at room temperature and without aggregates (such as sand and stone), only the geopolymer matrix. The setting time and temperature affect the compressive strength of the product. A higher setting temperature results in greater mechanical strength of the geopolymers (Hardjito and Rangan, 2006). The results of compressive strength for setting intervals of 24 h, 7 days, 28 days and 90 days are shown in Fig. 1. The mechanical strength provides an overview of the product quality
Table 2 Sodium hydroxide/sodium silicate ratios for the synthesis of geopolymeric materials. Activators
Ratios (mol/l)
Oxides (g/l) SiO2
Na2O
Sodium hydroxide
5 10 15 e 1.65:1e5 M 3.3:1e10 M 4.9:1e15 M
e e e 275.7 454.9 909.81 1350.93
155 310 465 85.5 296.07 592.15 883.95
Sodium silicate Na2SiO3/NaOH Na2SiO3/NaOH Na2SiO3/NaOH
Molar ratios
H2O/Na2O molar ratio
2.5 5.0 7.5 3.28 1.58 1.58 1.58
18.8 7.77 3.96 25,71 22.07 16.27 14.28
Fig. 1. Compressive strength results for geopolymeric samples synthetized with a 2:1 ash/calcined paper sludge ratio, 5, 10 and 15 mol/l alkaline addition and 24 h, 7, 28 and 90 days setting time at room temperature.
Please cite this article in press as: Antunes Boca Santa, R.A., et al., Geopolymer synthetized from bottom coal ash and calcined paper sludge, Journal of Cleaner Production (2013), http://dx.doi.org/10.1016/j.jclepro.2013.05.017
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and can be used as a parameter for assessing the degree of polymerization, since the more the structure is polymerized, the greater the resistance. The compressive strength for samples synthetized with 5 and 10 mol/l sodium hydroxide present similar resistances, only samples with 15 mol/l NaOH had presented little resistance. Another characteristic of geopolymers is the progressive increase of resistance over time. The compressive strength results refer to the geopolymer paste and therefore the results cannot be compared with mortars and concretes that are formed by a cement matrix and aggregates. The compressive strength of a geopolymeric paste is significantly lower than the compressive strength of geopolymeric mortar regardless the type of aggregate (Mazza, 2010). In Fig. 1 it is also possible to observe a significantly lower compressive strength for geopolymers synthesized with an ash/calcined paper sludge ratio of 1:0 and cured at 30, 60 and 90 days. The compressive strength of geopolymers may have varying results, since they depend on the processing conditions and raw material used. Chindaprasirt et al. (2009) have shown that geopolymers synthesized with fly ash and 15 M NaOH cured at 65 C for 48 h resulted in compressive strength of 35 MPa. Under the same processing conditions, but using bottom ash as raw material the geopolymers have shown a compressive strength of 18 MPa. The XRD analysis can define the degree of reaction that takes place in the raw materials to form a new structure, being possible to evaluate the degree of amorphicity as the area bellow the spectrum pattern formed in each raw material and final products (Zaharaki and Komnitsas, 2009). When comparing the XRD patterns of the raw materials with the diffractograms of the geopolymers it is possible to observe that the crystal phases remain the same, because the geopolymerization process do not dissolve the crystals, but the peaks appear lower and more scattered, Fig. 2. After alkaline activation followed by the dissolution of the aluminosilicates,
polymer reorganization, gel phase formation and hardening, the geopolymeric system forms a new amorphous phase. By the pattern of the diffraction spectra it is possible to identify the amorphous phase and the changing in the peaks corresponding to the raw materials, as indicated by arrows in the spectra of Fig. 2. The changes in the spectrum of the geopolymeric materials were promoted by alkali metal cations which dissolved the material and subsequently destroyed the aluminosilicate structure of the original raw materials transforming them into sodium aluminosilicate gels (Miranda et al., 2005). The results obtained for the geopolymer materials synthesized in this study are consistent with the XRD results of other authors (Panias and Giannopoulou, 2006; Fernández-Jiménez and Palomo, 2005) obtained with fly ash and the main phases identified in this study were alpha-quartz and Ale Si zeolites. The authors were able to identify a change in the spectra between 2q ¼ 20 e30 , which appear slightly shifted toward higher values 2q ¼ 25 e35 in the diffractograms of their materials. The FTIR analysis was performed in order to determine the functional groups and atomic arrangements in the aluminosilicates that form the geopolymeric material. According to Pinto (2004), generally the vibrational bands of AleOH are in the range of 915e 920 cm1, the SieO bonds are among the ranges of 693e710, 752e 760 and 1010e1110 cm1 and the vibrational bands of the hydroxyl groups are between 3500 and 4000 cm1 (Pinto, 2004). The SieOe Al band at 810 cm1 is due to large perturbations in the Al environment and vibrational bands appear weaker in the range 600e 800 cm1. Comparing the peak at 800 cm1, characteristic of geopolymers, with the peaks in this same range of the raw materials, the intensity is lower, indicating that the raw material reacted. The presence of bands at about 1460 cm1 is related to sodium carbonate and this feature may appear on samples which were not well set because the flow of water through the bulk drags the unreacted sodium to the surface, and on contact with atmospheric
Fig. 2. Diffraction spectra of: a) ash/calcined paper sludge before activation, b) geopolymer treated with 15 mol/l NaOH, c) geopolymer treated with 10 mol/l NaOH, d) geopolymer treated with 5 mol/l NaOH (Q ¼ a quartz, Z ¼ Al and Si zeolite).
Fig. 3. FTIR spectra of: a) bottom ash/calcined paper sludge before activation, b) geopolymer treated with 15 mol/l NaOH, c) geopolymer treated with 10 mol/l NaOH, d) geopolymer treated with 5 mol/l NaOH.
Please cite this article in press as: Antunes Boca Santa, R.A., et al., Geopolymer synthetized from bottom coal ash and calcined paper sludge, Journal of Cleaner Production (2013), http://dx.doi.org/10.1016/j.jclepro.2013.05.017
R.A. Antunes Boca Santa et al. / Journal of Cleaner Production xxx (2013) 1e6
CO2, carbonation occurs (Barbosa et al., 1999). These features (Fig. 3) can be observed with the occurrence of vibrational bands in the regions suggested by the authors. The FTIR analyses of geopolymeric samples show that the more consistent vibrations appear in the range of 1090 cm1, where the peaks appear smaller than the vibrational peaks of the raw materials, indicating that dissolution have occurred due to the high concentration of alkali. As a result, the structure of the material is changed and subsequently rearranged with the incorporation of the Al ion in the SiO4 tetrahedra forming the SieOeAl network. Also, at this moment SieOeNaþ linkages can be formed, allowing the reduction of the molecular vibration strength, causing the displacement of the peak and therefore resulting in the asymmetric stretching associated with the SieOeSi bond, favoring the formation of lower waves. Some new peaks are formed at 700 cm1 related to the development of geopolymeric rings (Rees et al., 2004). In this study, the FTIR results of the geopolymeric matrices obtained from bottom ash and calcined paper sludge at seven days setting (Fig. 3) are in agreement with the geopolymeric characteristics obtained from literature and indicate that geopolymerization have occurred in the structure. The spectra show a change in the
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size of the peak in the range of 950e1090 cm1 as compared to the FTIR analysis of the mixture made from bottom ash and calcined paper sludge. At all molarities used, 5, 10 or 15 mol/l of sodium hydroxide (NaOH), there was a change in the geopolymer structure in greater or lesser degree, being greater at 10 mol/l. At 5 mol/l concentration the vibrational disorder is stronger at 1090 cm1, but the peak appears to be much smaller due to the greater degree of polymerization of the geopolymeric matrix synthetized with 5 mol/ l. Changes in the spectra related to the presence of water have occurred in all matrices and are assigned to the bands between 1650 and 3450 cm1, related to the OeH bond. The spectra of the geopolymeric matrices also show a small shoulder at 700 cm1, assigned to the formation of polymeric rings, which is in agreement with the literature. Fig. 4 shows the micrographs of the surface and fracture of the geopolymers synthetized with 5, 10 and 15 mol/l. It can be observed that at 5 mol/l concentration the sample shows a higher degree of cracking and unreacted material. The sample synthesized with 10 mol/l shows the same appearance of the sample treated with 5 mol/l NaOH, but with a small degree of cracking. At higher concentration, 15 mol/l NaOH, the raw material is more reacted and the matrix is denser, showing higher degree of geopolymerization.
Fig. 4. SEM micrographs after 7 days setting: a) 5 M, c) 10 M and e) 15 M of the surface; b) 5 M, d) 10 M f) 15 M of the fracture.
Please cite this article in press as: Antunes Boca Santa, R.A., et al., Geopolymer synthetized from bottom coal ash and calcined paper sludge, Journal of Cleaner Production (2013), http://dx.doi.org/10.1016/j.jclepro.2013.05.017
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The pores that appear in the samples are typical of geopolymers synthesized with ash, because the ash have some hollow particles in its microstructure which react when they are partially activated, therefore forming pores in the geopolymeric matrix. The pores must be controlled because through them water penetrates into the structure and the entry of water by capillary action, absorption or sorptivity is deleterious to the durability of the material (Olivia et al., 2008). 4. Conclusions The use of coal bottom ash and calcined paper sludge synthesized from paper waste showed satisfactory results in the synthesis of geopolymer. The alkaline activation solution consisting of sodium hydroxide and sodium silicate reacted quickly with the powdered materials and the geopolymer characteristics are consistent with parameters found in literature. The resulting ash is partially crystallized, leaving only a portion of amorphous phase and therefore calcined paper sludge was used as a supplement of amorphous aluminosilicate. With an increasing amount of amorphous material available to react the matrix became more polymerized and denser, and consequently more resistant. However, the best results were achieved in samples prepared with an alkaline mixture of 15 mol/l because with this solution the reaction was faster. The compressive strength results showed that, for all geopolymeric samples, the higher the setting time, the greater the strength. The compressive strength results were about 10e25 MPa. The XRD results of the geopolymers have shown a marked offset of the peaks in the diffractograms in comparison with the peaks of the raw materials, characterizing a change in the amorphous state of the material. The FTIR analysis showed vibrations that are characteristic of geopolymeric materials. Finally, the SEM observation revealed that, in the samples synthesized with 15 mol/l concentration the matrix became denser, with fewer cracks and imperfections. Acknowledgments The authors wish to thank CNPq (302246/2009-6 PQ 2009 and 552455/2011-3 PROCAD 2011), CAPES, FAPESC and UFSC for the support to this research. References Barbosa, V.F.F., Mackenzie, K.J.D., Taumaturgo, C., 1999. Synthesis and characterization of sodium polysialate inorganic polymer based on alumina and silica. In: Second International Conference Geopolymére’99. Institut Géopolymère, SaintQuentin, France, pp. 65e78. Brazilian Association of Pulp and Paper, 2013. http://www.bracelpa.org.br/bra2/ index.php. Castro-Gomes, J.P., Silva, A.P., Cano, R.P., Durán Suarez, J., Albuquerque, A., 2012. Potential for reuse of tungsten mining waste-rock in technical-artistic value added products. J. Clean. Prod. 25, 34e41.
Chindaprasirt, P., Jaturapitakkul, C., Rattanasak, U., Chalee, W., 2009. Comparative study on the characteristics of fly ash and bottom ash geopolymers. Waste Manage. 29, 539e543. Davidovits, J., 2002. 30 years of successes and failures in geopolymer applications: market trends and potential breakthroughs. In: Geopolymer 2002 Conference, October 28e29, Melbourne, Australia. Davidovits, J., 2011. Geopolymer Chemistry and Applications, third ed. Institut Géopolymère, Saint-Quentin, France. Davidovits, J., Davidovits, M., Davidovits, N., 1991. Geopolymer, ultra hightemperature tooling material for the manufacture of advanced composites. Int. SAMPE Symp. Exhibit. 36, 1939e1949. Duxson, P., Jimenez, A.F., Provis, J.L., Lukey, G.C., Palomo, A., van Deventer, J.S.J., 2007. Geopolymer technology: the current state of the art. J. Mater. Sci. 42, 2917e2933. Fernández-Jiménez, A., Palomo, A., 2005. Composition and microstructure of alkali activated fly ash binder: effect of the activator. Cement Concrete Res. 35, 1984e1992. Fernández-Jiménes, A., Palomo, A., Criado, M., 2005. Microstructure development of alkali-activated fly ash cement: a descriptive model. Cement Concrete Res. 35, 1204e1209. Forsgren, J., Pedersen, C., Strømme, M., Engqvist, H., 2011. Synthetic geopolymers for controlled delivery of oxycodone: adjustable and nanostructured porosity enables tunable and sustained drug release. PLoS ONE 6, e17759. Gavronski, J.D., 2007. Mineral Coal and Renewable Energies in Brazil. Federal University of Rio Grande do Sul, Porto Alegre, Brazil (in Portuguese). Habert, G., d’Espinose de Lacaillerie, J.B., Roussel, N., 2011. An environmental evaluation of geopolymer based concrete production: reviewing current research trends. J. Clean. Prod. 19, 1229e1238. Hardjito, D., Rangan, B.V., 2006. Development of fly ash-based geopolymer concrete: progress and research needs. In: 2nd Asian Concrete Federation Conference, Bali, Indonesia. Komnitsas, K., Zaharaki, D., 2007. Geopolymerisation: a review and prospects for the minerals industry. Miner. Eng. 20, 1261e1277. Mazza, S.C., 2010. Mechanical properties and adhesion of a mortar repair system using geopolymer cement/concrete substrate containing Portland cement. Environmental Engineering, Polytechnic School, Federal University of Bahia, Salvador, Brazil (in Portuguese). McLellan, B.C., Williams, R.P., Lay, J., van Riessen, A., Corder, G.D., 2011. Costs and carbon emissions for geopolymer pastes in comparison to ordinary Portland cement. J. Clean. Prod. 19, 1080e1090. Miranda, J.M., Fernández-Jiménez, A., González, J.A., Palomo, A., 2005. Corrosion resistance in activated fly ash mortars. Cement Concrete Res. 35, 1210e1217. NBR 7215, 1996. Portland Cement: Determination of Compressive Strength. Brazilian Association of Technical Standards, ABNT, Rio de Janeiro, Brazil. Olivia, M., Sarker, P., Nikraz, H., 2008. Water penetrability of low calcium fly ash geopolymer concrete. In: Conference on Construction and Building Technol International, Kuala Lumpur, Malaysia, pp. 517e530. Palomo, A., Grutzeck, M.W., Blanco, M.T., 1999. Alkali-activated fly ashes: a cement for the future. Cement Concrete Res. 29, 1323e1329. Panias, D., Giannopoulou, I.P., 2006. Development of inorganic polymeric materials based on fired coal fly ash. Acta Metall. Slovaca 12, 321e327. Pinto, A.T., 2004. New Binder Systems Obtained by Alkaline Activation. University of Minho, Minho, Portugal. Rattanasak, U., Chindaprasirt, P., 2009. Influence of NaOH solution on the synthesis of fly ash geopolymer. Miner. Eng. 22, 1073e1078. Rees, C., Lukey, G.C., van Deventer, J.S.J., 2004. The role of solid silicates on the formation of geopolymers derived from coal ash. In: International Symposium of Research Students on Material Science and Engineering, Chennai, India, pp. 20e22. Torgal, F.P., Jalali, S., 2010. Cimento Portland normal versus ligantes geopoliméricos: Durabilidade e desempenho ambiental. Teoria e Prática na Engenharia Civil 15, 1e9. Turgut, P., 2012. Manufacturing of building bricks without Portland cement. J. Clean. Prod. 37, 361e367. Vargas, A.S., Dalmolin, D.C.C., Vilela, A.C.F., Jalali, S., Gomes, J.P.C., 2007. Fly ash alkali-activated with NaOH and Ca(OH)2 solution. Matéria 12 (3), 462e469. Zaharaki, D., Komnitsas, K., 2009. Role of alkali metals on the synthesis of low Ca ferronickel slag-based inorganic polymers. In: 3rd AMIREG International Conference: Assessing the Footprint of Resource Utilization and Hazardous Waste Management, Athens, Greece, pp. 1e6.
Please cite this article in press as: Antunes Boca Santa, R.A., et al., Geopolymer synthetized from bottom coal ash and calcined paper sludge, Journal of Cleaner Production (2013), http://dx.doi.org/10.1016/j.jclepro.2013.05.017
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Geopolymer synthetized from bottom coal ash and calcined paper sludge Rozineide A. Antunes Boca Santa a, Adriano Michael Bernardin b, *, Humberto Gracher Riella a, Nivaldo Cabral Kuhnen a a b
Chemical Engineering Department, Santa Catarina Federal University, P.O. Box 476, Trindade, Florianópolis, SC 88.040-900, Brazil Ceramic Materials Group, Santa Catarina Extreme South University, Av. Universitária 1105, Criciúma, SC 88.806-000, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 October 2012 Received in revised form 4 May 2013 Accepted 14 May 2013 Available online xxx
This study deals with the development of geopolymers synthetized from industrial waste containing aluminosilicates. Geopolymers are inorganic polymers formed by the activation of amorphous aluminosilicates (Al2O3.SiO2), which react in a strongly alkaline medium. Bottom ash (SiO2/Al2O3 ¼ 3.3e4.5) was used as source of aluminosilicate and sodium hydroxide (NaOH ¼ 5, 10 and 15 M) and sodium silicate (Na2SiO3, SiO2/Na2O ¼ 1.58) were used as alkaline medium. Calcined paper sludge was used to increase the reactivity of the partially crystallized bottom ash. The solid waste was characterized by XRF and XRD and the geopolymer samples were characterized by XRF, XRD, SEM, FTIR and compressive strength tests. The best results were obtained with a solution of 15 M NaOH and sodium silicate and a mixture of 2:1 bottom ash and calcined paper sludge. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Bottom ash Calcined paper sludge Alkaline activation Aluminosilicates Geopolymer
1. Introduction The increasing industrial development induced by a constant demand for new products has resulted in an exponential growth of waste generated at the end of the manufacturing processes as well as post-consumer disposal. Therefore, the best way to solve the problem of waste generation, besides the consumption reduction, is the use of those as raw materials for the production of high quality and valuable products. In addition, there is the saving of natural resources because the environmental aspect must be considered and evaluated when thinking about social and economic development, in other words, sustainability (McLellan et al., 2011; Castro-Gomes et al., 2012). In this study, bottom ash from coal and calcined paper sludge from paper sludge were used as raw materials for the synthesis of geopolymer, an inorganic polymeric material formed by the activation of aluminosilicates in strongly alkaline medium formed by the activation of aluminosilicates in strongly alkaline medium, resulting in poly(sialates) with the structure of phyllosilicates. The term “geopolymer” was created by J. Davidovits in 1979 to describe the chemical properties of inorganic polymers based on
* Corresponding author. Tel./fax: þ55 48 34312669. E-mail addresses: [email protected], [email protected] (A.M. Bernardin).
aluminosilicates. Geopolymers present cementitious properties and, therefore, great potential for use in the construction industry. They can be formed using natural raw materials or industrial wastes from several sources, provided that the wastes are rich in amorphous or semi-crystalline aluminosilicates or have passed through heat treatment, making them more reactive and suitable to alkaline activation. The alkaline activation is a chemical process that transforms amorphous, partially amorphous or metastable vitreous structures into a compacted cementitious material (Palomo et al., 1999). The aluminosilicates can dissolve in contact with an alkaline solution because leaching of the Al3þ and Si4þ species occurs. The concentration of the activator should be high (Rattanask and Chindaprasirt, 2009). When mixing an aluminosilicate with an alkaline solution a reaction is triggered and in few seconds there is the dissolution of the particles, reorganization of the structure and gelation, resulting in a three-dimensional aluminosilicate network. The Si and Al species released on the surface are responsible for the geopolymerization reaction. The dissolution of the amorphous aluminosilicate is rapid at high pH resulting in a supersaturated solution of Si and Al. Concentrated solutions result in the formation of a gel with increasing oligomers. The aqueous gel forms large networks by condensation (Duxson et al., 2007). The Si4þ and Al3þ in the solution are coordinated by oxygen bridges. The negative charge of the AlO4 is responsible for balancing the alkaline cations.
0959-6526/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jclepro.2013.05.017
Please cite this article in press as: Antunes Boca Santa, R.A., et al., Geopolymer synthetized from bottom coal ash and calcined paper sludge, Journal of Cleaner Production (2013), http://dx.doi.org/10.1016/j.jclepro.2013.05.017
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Geopolymers do not show stoichiometric composition and they are analogs of zeolites. The chemistry between both materials is quite similar, although their structures and compositions are different. Geopolymers comprise amorphous to semicrystalline structures and their empirical formula can be described by: Mn[e (SiO2)zeAlO2]n$wH2O, where z is 1, 2 or 3, M is the cation of alkali metal and n is the degree of polycondensation. From the findings about geopolymers, the interest in unraveling the mechanisms involving these materials and develop new formulations have expanded (Davidovits et al., 1991; Davidovits, 2011; Turgut, 2012; Habert et al., 2011). The geopolymeric technology results in a product that can be easily handled, stored and monitored, because its production is done with simple technology. 0.180 ton of CO2 is emitted to produce one ton of geopolymeric cement from kaolin. To produce one ton of Portland cement one ton of CO2 is emitted because the clinker is calcined at about 1500 C. Thus, the production of geopolymer emits approximately six times less CO2 (Davidovits, 2002). If produced from fly ash the emission is even smaller, because it does not require heat treatment. The wide variety of potential applications of geopolymeric materials includes: fire-resistant building materials, insulation materials, ceramic tiles, refractories resistant to thermal shock, casting materials, composite materials for aircraft and car interiors, high technology resin systems, containment barriers for toxic and radioactive waste, among others (Davidovits, 2011). A recent study suggests that geopolymers can be used in oral drug delivery due to its combination of good mechanical strength and chemical stability. It was demonstrated that its mechanical strength does not allow accidental breakage, with the possibility of adjusting the porosity, allowing drug release as needed (Forsgren et al., 2011). Geopolymers are amorphous materials, difficult to characterize, but the geopolymerization process can be identified by some characteristics. The factors that define the microstructure of the matrix and chemical, physical and mechanical properties of the geopolymers are the particle size distribution of the raw materials, the dissolution ratio of the gel phase, the amorphous nature of the solid raw materials, the degree of crystallinity, the amount of calcium in the raw material, the relationship between Si/Al, the concentration of activator, among others (Komnitsas and Zaharaki, 2007). Regarding the durability of Portland cements currently used, some studies address that the modern cement is strongly affected when exposed to aggressive climatic conditions, and its time duration is approximately 50 years. However, mortars like those found in Rome are still unchanged and were built more than 2000 years ago. The conventional Portland cements depend on the use of lime and can be dissolved by acidic solutions, reaching 30e60% mass loss (Davidovits, 2002). There are many reports regarding the early deterioration in structures built with modern Portland cement (Torgal and Jalali, 2010). Among the motivations for researchers to seek viable alternatives in the production of Portland cement is the durability of ancient cements compared to modern cements. Conventional Portland cements undergo rapid degradation, thus requiring constant demolitions and renovations of buildings with high energy consumption, emission of toxic gases into the atmosphere, and greater consumption of natural raw materials. The subject is increasingly being grounded and the diversity of raw materials used favors the possibility of industrial production, since geopolymers not require raw materials with high purity and uniformity. Within this panorama, the economy of the southern region of Santa Catarina State, Brazil, is based on mining and industrialization of minerals such as coal, clay and kaolin and, therefore, the region is considered one of the most polluted areas in Brazil. All those waste are formed by aluminosilicates, justifying their use as
raw materials for geopolymers. Furthermore, the global demand for coal will grow by 2030, reaching a doubling in relation to the current demand. With the growing consumption of coal for power generation there will be an increased production of ash as a byproduct of coal combustion (Gavronski, 2007; McLellan et al., 2011; Castro-Gomes et al., 2012). The cementitious materials produced by alkaline activation of aluminosilicates have been studied in the search for materials with binding properties more resistant than current Portland cements as well as materials that can be produced using raw materials of low cost, with little energy expenditure and especially with low emission of toxic gases in the atmosphere. Those researches present great possibilities for worldwide deployment and these materials can be produced on a large scale, suppressing the demand for cement in a market that grows every year. The synthesis of new products from industrial byproducts requires a full characterization to know the properties and characteristics of the raw material because each waste, inhere treated as a byproduct, is the result of the processing technique. From the results of the characterization new products can be obtained. In this work, the main feature is the use of calcined paper sludge obtained from the processing of paper waste. Pulp and paper industries produce millions of tons each year to meet the global demand for paper. According to the Brazilian Association of Pulp and Paper (2013) in 2011 were produced worldwide 183.8$106 tons of pulp and 398.9$106 tons of paper. Brazil have produced 10.2$106 tons of paper in 2011 (Brazilian Association of Pulp and Paper, 2013) and, therefore, a large amount of waste generated every day. In this work, waste of paper industry was chosen to be a raw material to the geopolymeric systems because it has in its structure kaolin that becomes metakaolin after heat treatment. However, the waste paper is also rich in CaCO3, which may be harmful in large amounts to produce geopolymers. Thus, the wastepaper has undergone purification with hydrochloric acid (HCl), in order to remove the CaCO3 through chemical reaction. Therefore, the objective of this study was the synthesis of geopolymers using bottom ash from coal combustion and calcined paper sludge by alkaline activation. As alkaline activators, sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) were used. 2. Experimental Bottom ash from coal burning and calcined paper sludge were used as raw materials. Both wastes were collected from a control lot. Sodium hydroxide (Vetec, 97% purity) and sodium silicate (Manchester Química do Brasil S.A., SiO2/Na2O ¼ 3.3) were used as alkaline activators. The bottom ash was dried in a laboratory stove for 24 h at 100 C and, in sequence, ground in a laboratory ball mill (high alumina grinding elements and lining) for 8 h. The resulting material was analyzed by the laser diffraction method to evaluate the particle size distribution. As the paper sludge is composed by cellulose, calcium carbonate and kaolin, a purification step was required for elimination of cellulose and calcium carbonate in the composition of the residue. The paper sludge was treated with hydrochloric acid (HCl) in order to extract the calcium carbonate according to: 2HCl þ CaCO3/CaCl2þH2O þ CO2. The mass balance between calcium oxide (CaO) and calcium carbonate (CaCO3) was used to evaluate the concentration of the hydrochloric acid (HCl), and the amount of CaO present in the residue before treatment, as obtained from XRF data, was used as a reference (Table 3). Therefore, for each 150 g of residue 1 L of HCl solution 1.8 M was necessary to remove the CaCO3 present in the residue. Small amounts of CaO appear to be beneficial for geopolymers and therefore the HCl solution was prepared with 1.5 M. After purification with HCl all material was
Please cite this article in press as: Antunes Boca Santa, R.A., et al., Geopolymer synthetized from bottom coal ash and calcined paper sludge, Journal of Cleaner Production (2013), http://dx.doi.org/10.1016/j.jclepro.2013.05.017
R.A. Antunes Boca Santa et al. / Journal of Cleaner Production xxx (2013) 1e6 Table 1 Bottom ash/calcined paper sludge mass ratios for the synthesis of geopolymeric materials. Solids
Ratios
Bottom ash (BA) Calcined paper sludge (CPS) Bottom ash/Calcined paper sludge
100% 100% 2:1
Mass (g)
Oxides (g)
100 100 133.3:66.7
SiO2
Al2O3
59.8 49.57 112.77
22.6 38.87 56.04
Molar ratios 4.5 2.16 3.45
3
Table 3 Bottom ash (BA) and calcined paper sludge (CPS), chemical composition in (%) mass. Oxide
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
TiO2
LoI
BA CPS CPS (after HCl treatment)
59.8 7.69 49.5
25.2 6.08 38.8
4.5 0.17 1.55
1.3 32.3 5.56
0.5 0.62 1.43
2.3 0.05 0.24
1.1 0.15 1.38
6.6 52.6 0.80
obtain the morphological and chemical characterization of the materials. washed with deionized water and filtered to remove the residue of calcium chloride resulting from the reaction. In sequence, the material is dried in a stove at 100 C for 24 h and calcined at 850 C for 2 h to cause the dehydroxylation of kaolin. The resulting material is highly reactive calcined paper sludge with approximately 5.56% CaO content (Table 3). After heat treatment, the as formed calcined paper sludge was grinded (high alumina laboratory ball mill, 8 h grinding) to adjust the particle size distribution. The raw materials were characterized by X-ray fluorescence (XRF) analysis (Philips PW 2400 WDXRF). Table 1 shows the bottom ash/calcined paper sludge mass ratios for the synthesis of geopolymeric materials. Table 2 shows the sodium hydroxide/sodium silicate ratios. The bottom ash and the calcined paper sludge were weighed and mixed and the sodium hydroxide and sodium silicate were added using a mechanical stirrer until complete homogenization of the mixture for about 5 min. The geopolymeric paste was poured into plastic containers (62.83 cm3) and cured at room temperature. The container must be chosen so as not to contaminate the silica (Rattanasak and Chindaprasirt, 2009). After 24 h the samples were demolded and tested for compressive strength. The tests were made according to the NBR 7215/1996 standard from the Brazilian Association of Technical Standards (1996). The standard was developed to be applied in cement mortars; however, samples which were subjected to testing of compressive strength for this study were molded only with the geopolymer paste, without the use of sand or other aggregates. The same procedure was performed at 7, 28 and 90 days after demolding. The compressive strength tests were performed in a universal testing machine (Emic DL20000, 1 mm/min load speed). The samples were characterized by X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The XRD analysis of the geopolymers and residues was made with accelerating voltage of 40 kV and 30 mA, Cu Ka (l ¼ 1.5418 A) radiation ranging from 5 to 25 and 2q scan rate of 0.05 /s (PanAnalytical X’Pert PRO Multi Purpose). The FTIR analysis (Shimadzu IR Prestige 21) was used to determine the functional groups. The samples were prepared for IR spectroscopy with KBr (Vetec). Finally, geopolymeric samples were submitted to scanning electron microscopy (SEM, Phillips XL30) in order to
3. Results and discussion The XRF results (Table 3) revealed that the bottom ash contains as major components SiO2 and Al2O3. The way the SiO2 and Al2O3 molecules are arranged largely depends on the raw material from which they are derived therefore defining the matrix that will form the inorganic polymer, its microstructure and chemical, physical and mechanical properties (Duxson et al., 2007). The bottom ash (BA) chemical analysis shows that the SiO2/Al2O3 ratio is 2:1, suitable for use in the geopolymeric reaction. The calcined paper sludge (CPS) chemical analysis before and after purification in HCl determined by XRF (Table 3) shows that the amount of CaCO3 was reduced, therefore showing that the purification step was effective to reduce the cellulose and calcium carbonate amounts. The particle size distribution of the raw materials resulted in an average diameter (D50) of 31.8 mm for the bottom ash with 90% of the particles below 39.6 mm and an average diameter (D50) of 24.8 mm for the paper waste with 90% of the particles below 9.3 mm. In their study on geopolymerization, Fernández-Jiménes et al. (2005) have used ligands with ash with 90% of the particles smaller than 45 mm and 50% smaller than 10 mm. Vargas et al. (2007) have used particles with an average size of 29.2 mm to perform their work. Therefore, the particle size distribution of the materials used in this work after grinding are within the size needed to cause a good dissolution and therefore a good activation to produce geopolymeric materials. The geopolymers synthesized in this study were set at room temperature and without aggregates (such as sand and stone), only the geopolymer matrix. The setting time and temperature affect the compressive strength of the product. A higher setting temperature results in greater mechanical strength of the geopolymers (Hardjito and Rangan, 2006). The results of compressive strength for setting intervals of 24 h, 7 days, 28 days and 90 days are shown in Fig. 1. The mechanical strength provides an overview of the product quality
Table 2 Sodium hydroxide/sodium silicate ratios for the synthesis of geopolymeric materials. Activators
Ratios (mol/l)
Oxides (g/l) SiO2
Na2O
Sodium hydroxide
5 10 15 e 1.65:1e5 M 3.3:1e10 M 4.9:1e15 M
e e e 275.7 454.9 909.81 1350.93
155 310 465 85.5 296.07 592.15 883.95
Sodium silicate Na2SiO3/NaOH Na2SiO3/NaOH Na2SiO3/NaOH
Molar ratios
H2O/Na2O molar ratio
2.5 5.0 7.5 3.28 1.58 1.58 1.58
18.8 7.77 3.96 25,71 22.07 16.27 14.28
Fig. 1. Compressive strength results for geopolymeric samples synthetized with a 2:1 ash/calcined paper sludge ratio, 5, 10 and 15 mol/l alkaline addition and 24 h, 7, 28 and 90 days setting time at room temperature.
Please cite this article in press as: Antunes Boca Santa, R.A., et al., Geopolymer synthetized from bottom coal ash and calcined paper sludge, Journal of Cleaner Production (2013), http://dx.doi.org/10.1016/j.jclepro.2013.05.017
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and can be used as a parameter for assessing the degree of polymerization, since the more the structure is polymerized, the greater the resistance. The compressive strength for samples synthetized with 5 and 10 mol/l sodium hydroxide present similar resistances, only samples with 15 mol/l NaOH had presented little resistance. Another characteristic of geopolymers is the progressive increase of resistance over time. The compressive strength results refer to the geopolymer paste and therefore the results cannot be compared with mortars and concretes that are formed by a cement matrix and aggregates. The compressive strength of a geopolymeric paste is significantly lower than the compressive strength of geopolymeric mortar regardless the type of aggregate (Mazza, 2010). In Fig. 1 it is also possible to observe a significantly lower compressive strength for geopolymers synthesized with an ash/calcined paper sludge ratio of 1:0 and cured at 30, 60 and 90 days. The compressive strength of geopolymers may have varying results, since they depend on the processing conditions and raw material used. Chindaprasirt et al. (2009) have shown that geopolymers synthesized with fly ash and 15 M NaOH cured at 65 C for 48 h resulted in compressive strength of 35 MPa. Under the same processing conditions, but using bottom ash as raw material the geopolymers have shown a compressive strength of 18 MPa. The XRD analysis can define the degree of reaction that takes place in the raw materials to form a new structure, being possible to evaluate the degree of amorphicity as the area bellow the spectrum pattern formed in each raw material and final products (Zaharaki and Komnitsas, 2009). When comparing the XRD patterns of the raw materials with the diffractograms of the geopolymers it is possible to observe that the crystal phases remain the same, because the geopolymerization process do not dissolve the crystals, but the peaks appear lower and more scattered, Fig. 2. After alkaline activation followed by the dissolution of the aluminosilicates,
polymer reorganization, gel phase formation and hardening, the geopolymeric system forms a new amorphous phase. By the pattern of the diffraction spectra it is possible to identify the amorphous phase and the changing in the peaks corresponding to the raw materials, as indicated by arrows in the spectra of Fig. 2. The changes in the spectrum of the geopolymeric materials were promoted by alkali metal cations which dissolved the material and subsequently destroyed the aluminosilicate structure of the original raw materials transforming them into sodium aluminosilicate gels (Miranda et al., 2005). The results obtained for the geopolymer materials synthesized in this study are consistent with the XRD results of other authors (Panias and Giannopoulou, 2006; Fernández-Jiménez and Palomo, 2005) obtained with fly ash and the main phases identified in this study were alpha-quartz and Ale Si zeolites. The authors were able to identify a change in the spectra between 2q ¼ 20 e30 , which appear slightly shifted toward higher values 2q ¼ 25 e35 in the diffractograms of their materials. The FTIR analysis was performed in order to determine the functional groups and atomic arrangements in the aluminosilicates that form the geopolymeric material. According to Pinto (2004), generally the vibrational bands of AleOH are in the range of 915e 920 cm1, the SieO bonds are among the ranges of 693e710, 752e 760 and 1010e1110 cm1 and the vibrational bands of the hydroxyl groups are between 3500 and 4000 cm1 (Pinto, 2004). The SieOe Al band at 810 cm1 is due to large perturbations in the Al environment and vibrational bands appear weaker in the range 600e 800 cm1. Comparing the peak at 800 cm1, characteristic of geopolymers, with the peaks in this same range of the raw materials, the intensity is lower, indicating that the raw material reacted. The presence of bands at about 1460 cm1 is related to sodium carbonate and this feature may appear on samples which were not well set because the flow of water through the bulk drags the unreacted sodium to the surface, and on contact with atmospheric
Fig. 2. Diffraction spectra of: a) ash/calcined paper sludge before activation, b) geopolymer treated with 15 mol/l NaOH, c) geopolymer treated with 10 mol/l NaOH, d) geopolymer treated with 5 mol/l NaOH (Q ¼ a quartz, Z ¼ Al and Si zeolite).
Fig. 3. FTIR spectra of: a) bottom ash/calcined paper sludge before activation, b) geopolymer treated with 15 mol/l NaOH, c) geopolymer treated with 10 mol/l NaOH, d) geopolymer treated with 5 mol/l NaOH.
Please cite this article in press as: Antunes Boca Santa, R.A., et al., Geopolymer synthetized from bottom coal ash and calcined paper sludge, Journal of Cleaner Production (2013), http://dx.doi.org/10.1016/j.jclepro.2013.05.017
R.A. Antunes Boca Santa et al. / Journal of Cleaner Production xxx (2013) 1e6
CO2, carbonation occurs (Barbosa et al., 1999). These features (Fig. 3) can be observed with the occurrence of vibrational bands in the regions suggested by the authors. The FTIR analyses of geopolymeric samples show that the more consistent vibrations appear in the range of 1090 cm1, where the peaks appear smaller than the vibrational peaks of the raw materials, indicating that dissolution have occurred due to the high concentration of alkali. As a result, the structure of the material is changed and subsequently rearranged with the incorporation of the Al ion in the SiO4 tetrahedra forming the SieOeAl network. Also, at this moment SieOeNaþ linkages can be formed, allowing the reduction of the molecular vibration strength, causing the displacement of the peak and therefore resulting in the asymmetric stretching associated with the SieOeSi bond, favoring the formation of lower waves. Some new peaks are formed at 700 cm1 related to the development of geopolymeric rings (Rees et al., 2004). In this study, the FTIR results of the geopolymeric matrices obtained from bottom ash and calcined paper sludge at seven days setting (Fig. 3) are in agreement with the geopolymeric characteristics obtained from literature and indicate that geopolymerization have occurred in the structure. The spectra show a change in the
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size of the peak in the range of 950e1090 cm1 as compared to the FTIR analysis of the mixture made from bottom ash and calcined paper sludge. At all molarities used, 5, 10 or 15 mol/l of sodium hydroxide (NaOH), there was a change in the geopolymer structure in greater or lesser degree, being greater at 10 mol/l. At 5 mol/l concentration the vibrational disorder is stronger at 1090 cm1, but the peak appears to be much smaller due to the greater degree of polymerization of the geopolymeric matrix synthetized with 5 mol/ l. Changes in the spectra related to the presence of water have occurred in all matrices and are assigned to the bands between 1650 and 3450 cm1, related to the OeH bond. The spectra of the geopolymeric matrices also show a small shoulder at 700 cm1, assigned to the formation of polymeric rings, which is in agreement with the literature. Fig. 4 shows the micrographs of the surface and fracture of the geopolymers synthetized with 5, 10 and 15 mol/l. It can be observed that at 5 mol/l concentration the sample shows a higher degree of cracking and unreacted material. The sample synthesized with 10 mol/l shows the same appearance of the sample treated with 5 mol/l NaOH, but with a small degree of cracking. At higher concentration, 15 mol/l NaOH, the raw material is more reacted and the matrix is denser, showing higher degree of geopolymerization.
Fig. 4. SEM micrographs after 7 days setting: a) 5 M, c) 10 M and e) 15 M of the surface; b) 5 M, d) 10 M f) 15 M of the fracture.
Please cite this article in press as: Antunes Boca Santa, R.A., et al., Geopolymer synthetized from bottom coal ash and calcined paper sludge, Journal of Cleaner Production (2013), http://dx.doi.org/10.1016/j.jclepro.2013.05.017
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R.A. Antunes Boca Santa et al. / Journal of Cleaner Production xxx (2013) 1e6
The pores that appear in the samples are typical of geopolymers synthesized with ash, because the ash have some hollow particles in its microstructure which react when they are partially activated, therefore forming pores in the geopolymeric matrix. The pores must be controlled because through them water penetrates into the structure and the entry of water by capillary action, absorption or sorptivity is deleterious to the durability of the material (Olivia et al., 2008). 4. Conclusions The use of coal bottom ash and calcined paper sludge synthesized from paper waste showed satisfactory results in the synthesis of geopolymer. The alkaline activation solution consisting of sodium hydroxide and sodium silicate reacted quickly with the powdered materials and the geopolymer characteristics are consistent with parameters found in literature. The resulting ash is partially crystallized, leaving only a portion of amorphous phase and therefore calcined paper sludge was used as a supplement of amorphous aluminosilicate. With an increasing amount of amorphous material available to react the matrix became more polymerized and denser, and consequently more resistant. However, the best results were achieved in samples prepared with an alkaline mixture of 15 mol/l because with this solution the reaction was faster. The compressive strength results showed that, for all geopolymeric samples, the higher the setting time, the greater the strength. The compressive strength results were about 10e25 MPa. The XRD results of the geopolymers have shown a marked offset of the peaks in the diffractograms in comparison with the peaks of the raw materials, characterizing a change in the amorphous state of the material. The FTIR analysis showed vibrations that are characteristic of geopolymeric materials. Finally, the SEM observation revealed that, in the samples synthesized with 15 mol/l concentration the matrix became denser, with fewer cracks and imperfections. Acknowledgments The authors wish to thank CNPq (302246/2009-6 PQ 2009 and 552455/2011-3 PROCAD 2011), CAPES, FAPESC and UFSC for the support to this research. References Barbosa, V.F.F., Mackenzie, K.J.D., Taumaturgo, C., 1999. Synthesis and characterization of sodium polysialate inorganic polymer based on alumina and silica. In: Second International Conference Geopolymére’99. Institut Géopolymère, SaintQuentin, France, pp. 65e78. Brazilian Association of Pulp and Paper, 2013. http://www.bracelpa.org.br/bra2/ index.php. Castro-Gomes, J.P., Silva, A.P., Cano, R.P., Durán Suarez, J., Albuquerque, A., 2012. Potential for reuse of tungsten mining waste-rock in technical-artistic value added products. J. Clean. Prod. 25, 34e41.
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Please cite this article in press as: Antunes Boca Santa, R.A., et al., Geopolymer synthetized from bottom coal ash and calcined paper sludge, Journal of Cleaner Production (2013), http://dx.doi.org/10.1016/j.jclepro.2013.05.017