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Alkali activation processes for incinerator residues management
Waste Management 33 (2013) 1740–1749
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Alkali activation processes for incinerator residues management Isabella Lancellotti ⇑, Chiara Ponzoni, Luisa Barbieri, Cristina Leonelli Department of Engineering ‘‘Enzo Ferrari’’, University of Modena and Reggio Emilia, Via Vignolese 905/A, 41125 Modena, Italy
a r t i c l e
i n f o
Article history: Received 25 October 2012 Accepted 30 April 2013 Available online 4 June 2013 Keywords: Alkali activation Incinerator bottom ash Recycling Reactive phase
a b s t r a c t Incinerator bottom ash (BA) is produced in large amount worldwide and in Italy, where 5.1 millions tons of municipal solid residues have been incinerated in 2010, corresponding to 1.2–1.5 millions tons of produced bottom ash. This residue has been used in the present study for producing dense geopolymers containing high percentage (50–70 wt%) of ash. The amount of potentially reactive aluminosilicate fraction in the ash has been determined by means of test in NaOH. The final properties of geopolymers prepared with or without taking into account this reactive fraction have been compared. The results showed that due to the presence of both amorphous and crystalline fractions with a different degree of reactivity, the incinerator BA geopolymers exhibit significant differences in terms of Si/Al ratio and microstructure when reactive fraction is considered. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The generation of municipal solid waste (MSW) in Italy in 2010 was about 32.5 millions tons with an increase of 1.1% with respect to 2009. From this amount, 15.5 millions tons has been landfilled without pre-treatment, with a decrement of 3.4%, corresponding to 523.000 tons, with respect to 2009; additionally 5.1 millions tons have been incinerated (MSWI), with an increase of 13.3% passing from 2009 to 2010 (ISPRA, 2012). Since incineration represents the total oxidation process of the combustible materials present in the waste, its final effect is the reduction in volume of approximately 95–96%, depending on waste composition and on the degree of pre-treatment (Ramboll, 2006). Further, incineration process contribute to a sensible mass and toxicity reduction of the waste (about 70 wt% of the incoming waste is transformed into gases) as well as recovery of the energy produced during combustion. Despite these advantages, during the incineration some solid residues are generated accounting for roughly 300 kg per ton of waste divided in 250–300 kg/t as bottom ash and 20–30 kg/t as fly ash for landfilling or reuse (Gutmann, 1996). In 2010, being incinerated in Italy about 5.1 millions tons, the production of bottom ash can be estimated approximately 1.2–1.5 millions tons (ISPRA, 2012). Many efforts have been made to improve the environmental quality of these solid residues both in-process and by means of post-treatment techniques/technologies. In-process measures aim to change the incineration parameters in order to improve burnout or to shift the metals distribution over the various residues. Post-treatment
⇑ Corresponding author. Tel.: +39 059 2056251; fax: +39 059 2056243. E-mail address: [email protected] (I. Lancellotti). 0956-053X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.wasman.2013.04.013
techniques include: ageing, mechanical treatment, washing, additional post combustion thermal treatment and stabilization (European Commission, 2006). The fly ash constitutes a potential hazard for health because it often contains high concentrations of heavy metals such as lead, cadmium, copper and zinc (Bayuseno and Schmahl, 2011; Wan et al., 2006). The bottom ash is mainly constituted of Si, Al, Ca and Na oxides, and heavy metals in quantity to classify this waste as ‘‘not hazardous’’ (Barbieri et al., 2008). While fly ash is always classified as special hazardous waste and must be deposited in special landfills equipped with careful control of the effluents, bottom ash is generally considered safe for regular landfill, after a certain level of testing defined by the local legislation (Monteiro et al., 2008). Recently in Italy some companies became active and specialized in posttreatment technologies of bottom ash. The objective is minimizing waste production by transforming it in a reusable material, the so called ‘‘end of waste-EOW’’ material. The treatment of incineration bottom ash starts from a complex process of selection to be completed with physical/mechanical treatments (ageing, sieving and washing). After the process, an inert material with silica-based matrix, rich in iron, calcium and aluminum oxides is obtained. This material can be successfully added in the formulation of cement or ceramic materials as substitute for minerals resources (Ferraris et al., 2009; Rambaldi et al., 2010; Schabbach et al., 2011a,b, 2012a,b). Even though the authors of the present paper posses a dated experience on sintering, melting and preparation of glass–ceramics from waste incinerator residues (Saccani et al., 2001; Schabbach et al., 2011a,b, 2012a,b; Andreola et al., 2008) they are now approaching the ‘‘cold’’ inertization technique of the alkali activation being aware that this offers the most efficient reuse procedure (Van Deventer et al., 2012).
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Alkali activation of aluminosilicate inorganic powders generates solid materials with a strong three dimensional structures (Provis et al., 2005a,b). When the starting powdered materials are incinerator residues, industrial waste and/or residues from treatments of natural raw materials, then geopolymerization can represent an emerging solution since differently from other industrial processes (e.g. sol–gel, clinkerization, sintering processes), alkali activation (geopolymerization) does not require high temperature thermal treatments, nor the use of carbonate-based raw materials, with corresponding CO2 emissions, or expensive chemical reagents. The versatility of the process, which occurs at room temperature or at temperature lower than 100 °C, and the possibility of its application to unconventional materials unemployed so far or disposed to landfill, involve several benefits: (i) saving not renewable resources and, consequently, their safeguard; (ii) reducing CO2 emissions and, consequently, a good action on climate change; (iii) producing materials with excellent mechanical and thermal properties. Following European and Italian directives aimed at decreasing waste production, creating a better waste management and increasing recycling procedures, the proposed research leads a fundamental contribute for: – a better knowledge of chemical–physical–mechanical performances of the investigated geopolymers, obtained by unconventional precursors, namely bottom ash; – the development of new products ready to be introduced into the market because produced with a low cost existing technology. The proposed research is an example of development of a new sustainable technology for waste reduction and recycling. In addition, the production of geopolymers will contribute to reduce high industrial pollution potentially replacing some of the most popular building materials characterized by a high environmental impact (e.g. cement, ceramics, etc.). The commitment to recycle is driven by the need to conserve natural resources, reduce importation of aluminosilicate raw materials, save landfill space and reduce pollution. A correct waste management and urban sustainability policy cannot avoid to consider waste as a resource and an opportunity of investment and earning. In particular, the aim of this paper is to formulate chemically activated materials by using incinerator bottom ash (BA) mixed with metakaolin (MK) and to monitor the geopolymerization process by means of several techniques (FT-IR, SEM, XRD, pH, conductivity, etc.). Few case studies on the use of incinerator bottom ash as main aluminosilicate constituent for geopolymers have been found in literature to date (Onori et al., 2011). Several bottom ash, different from incinerator one, but rich in silica and alumina, have been successfully used in the geopolymers formulation such as bottom ash from thermal power plant (Chindaprasirt et al., 2009) and circulating fluidized bed combustion bottom ash (Xu et al., 2010; Topçu and Toprak, 2011; Chindaprasirt and Rattanasak, 2010). Incinerator fly ashes, conversely, are used in geopolymers only for their inertization and not as raw material having not suitable chemical composition (Luna Galiano et al., 2011; Lancellotti et al., 2010; Zheng et al., 2010, 2011).
2. Experimental procedures 2.1. Bottom ash (BA) characterization Incineration bottom ash (BA), commercially available as ‘‘end of waste material’’ with grain size between 200 and 1000 lm, were ground for 30 min in a ball mill and sieved below 75 lm to reach
Table 1a Chemical analysis of bottom ash (BA). Element
Concentration (wt%)
Si Ca Al Na Fe Mg K P Ti S Zn Ba Pb Cu Mn Cr Ni
33.26 21.27 3.96 3.21 2.46 2.71 1.03 0.31 1.22 0.33 0.53 0.45 0.45 0.36 0.13 0.04 0.03 13.21
CO3 2 C H N S (total) LOI
2.57 0.63 0.00 1.25 7.00 2.37
SO4 2 Cl
1.29
Std. dev. ± 1 wt%.
Table 1b Chemical analysis of metakaolin calcinated 4 h (MK). Elements
Concentration (wt%)
Si Al Fe K
53.69 40.78 1.39 4.14
Std. dev. ± 1 wt%.
grain size similar to that of metakaolin (MK) powder used as basic constituent for geopolymer formulation. Chemical (X-ray fluorescence, Philips PW 2004) and elemental analysis and loss of ignition at 1000 °C for 2 h were performed and reported in Table 1a. Chlorides and sulfates were analysed in eluates of sequential washing procedure and carbonate content was measured by using a Dietrich-Fruhling calcimeter on 1 g samples which were contacted with HCl and the quantity of CO2 released from the reaction of the carbonate with HCl was used to determine the percentage of CaCO3. Mineralogical analysis was performed in order to evaluate the amorphous or crystalline nature of the ash and to identify the crystalline phases present. X-ray diffraction analysis was carried out by a powder diffractometer (PW3830, Philips, NL) with Ni-filtered Cu Ka radiation in the 5–70° 2h range on powdered samples (30 lm particle size). In order to evaluate the reactive fraction of bottom ash a basic attack was performed according to the test reported in literature (Ruiz-Santaquiteria et al., 2011). One gram of bottom ash was placed in 100 mL of 8 M NaOH solution and stirred constantly for 5 h in a flask bathed at 80 ± 2 °C. The resulting solution was then filtered, the residue was rinsed with distilled water until a neutral pH was attained and the amount of insoluble material was quantified. Al and Si contents in the leachate after the immersion were quantified with ICP/AES (Philips Varian Liberty 200), using k = 251,611 nm for Si and 396,152 nm for Al; with these values the reactive Si/Al mass ratio was calculated.
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Concentration (wt%)
Si Na Ratio Si/Na (wt/wt)
12.34 3.39 3.64
Table 2 Geopolymers formulations used in this work from a mixture of metakaolin (MK) and bottom ash (BA) at 50, 60, 70, 80 wt%. Geopolymer formulations Sample
Bottom ash (BA) (g)
H2O Si/Al Metakaolin NaOH Na silicate (ml) (MK) (g) 8M (ml) (ml)
Na/ Al
50_50 40_60 30_70 20_80
25 30 35 40
25 20 15 10
1.09 1.10 1.09 1.5
MK_BA MK_BA MK_BA MK_BA
12 12 7 10
15 8 10 5
– 3 3 7
2.5 2.63 3.26 3.8
Table 3 Heavy metals and soluble anions content (ppm) in the as-received bottom ash eluate after leaching test (EN 12457). Heavy metals and soluble anions (ppm)
Bottom ash
Law limit (ppm)
Ba Zn Pb Cu Cr Ni Cl
0.11 0.09 0 0.35 0 0 283.6 208
10 5.0 1.0 5.0 1.0 1.0 1500 2000
SO24
To evaluate the hazardousness of the bottom ash, leaching test in distilled water for 24 h was performed according to European norm EN 12457 (UNI web site,
Fig. 1. XRD pattern of as-received bottom ash: Q- a-quartz (JCPDF file 33-1161); Ccalcite (JCPDF file 5-586); A-albite (JCPDF file 10-393); G-gehelenite (JCPDF file 35755).
The procedure for the samples preparation was carried out according to the following steps: – Mixing of aluminosilicate powders (bottom ash and metacaoline both previously ground and sieved) in a beaker in order to obtain a homogeneous mixture. – Addition of sodium hydroxide 8 M solution and sodium silicate solution with a mass ratio Si/Na = 3.64 (Table 1c); the amount of each solution was tailored depending on the Na and Si content present in the powder mixture. – Possible addition of a few milliliters of water in order to obtain a castable mixture; the water must be added only to what is strictly necessary otherwise compromise the subsequent step of hardening. – Intensive/thorough stirring until a homogeneous and fluid paste is formed; the paste is poured into plastic moulds (setting phase). – Setting stage maintaining the cast at room temperature; curing stage at room temperature 15 or 30 days (curing phase). – Being pH a relevant factor during the dissolution of the starting binder and considering that NaOH solution with high molarity (8 M) was used, the pH measured in all cases was near 13, almost identical for all samples. The formulations with different amounts of bottom ash have been designed in order not to cause significant shift neither in the Na/Al mass ratio nor in the Si/Al mass ratio. The optimized formulations are then summarized in Table 2.
2.3. Chemical stability tests All the compositions undergone an immersion test in distilled water for 48 h followed by a drying process at 110 °C in conventional oven for 24 h. The weight differences before and after the test permitted to evaluate the materials resistance to dissolution and consequently the efficiency of the geopolymerization process.
2.4. Materials characterization Geopolymers characterization was performed after 15 and 30 days of curing and the procedure to stop hydratation of the pastes at the different curing ages, applied to all the samples, was the immersion in acetone in order to remove residual water
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Fig. 2. Geopolymeric samples cured 15 days after test of immersion in water for 48 h, (a) 50-50 MK_BA, (b) 40_60 MK_BA, (c) 30_70 MK_BA, and (d) 20_80 MK_BA.
Fig. 3. SEM micrograph of dry polished surface of (a) 50-50 MK_BA, (b) 40_60 MK_BA and (c) 30_70 MK_BA compositions (150) after 15 days of curing.
Fig. 4. SEM micrograph of dry polished section of 50-50 MK_BA sample after 15 (a) and 30 (b) days of curing , and EDS spectrum (c) of unreacted BA particle.
present in the pores and other interstices, in agreement with Provis et al. (2012). Mineralogical analysis was carried out in order to evaluate if crystalline phases have been developed during geopolymerization
in all the samples. X-ray diffraction analysis was carried out on powdered samples as indicated above. All the geopolymers (Table 2) were broken and sieved below 2 mm, then each sample has been tested in bi-distilled water under
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Table 4 Chemical analysis (wt%) of geopolymeric gel in Fig. 4 at different distance from bottom ash particle.
Gel far from ash particle Gel near ash particle
Si
Al
Na
Ca
Si/Al
Na/Al
24.6 21.86
16.13 13.53
7.96 7.84
1.18 2.46
1.52 1.61
0.49 0.57
Shortly before SEM analysis the samples were dry polished to remove the resin layer and then coated with Au–Pd sputtered layer. FT-IR spectra were recorded on a Avatar 330 FT-IR ThermoNicolet. 32 scans between 4000 and 700 cm 1 were averaged for each spectrum at intervals of 1 cm 1. The FT-IR spectra have been collected at different bottom ash content, before and after alkali activation, in order to assess the obtained matrices stability.
3. Results and discussion 3.1. Bottom ash characterization
Fig. 5. Si/Al trend in the geopolymer gel as a function of the curing time for all the compositions (data from EDS characterization of five measurement for each sample).
stirring for 1 day in TeflonÒ bottles. pH and electrical conductivity were measured (Radiometer Analytical – Meterlab CDM210) after 1, 15, 30, 60, 120, 1440 min. The experiments had been carried out during the day 15th or the day 30th for all the samples investigated. Microstructure observations were conducted by ESEM (ESEM – Quanta200 – FEI) equipped with EDS to evaluate the role played by the bottom ash in the formation of geopolymeric amorphous phase on the hardened samples. Then the samples were encapsulated into an epoxy resin in order to isolate them from the air humidity.
The chemical composition of as received bottom ash (Table 1a) is not adequate to obtain a geopolymer with a Si/Al mass ratio below the value of 3, characteristic of materials with a 3D rigid network, suitable for a concrete, cement or waste encapsulating medium (Davidovits, 1999). Also Fletcher et al. noted that the high-alumina samples (Si/Al < 2) do not exhibit typical geopolymer characteristics, while the higher silica samples (Si/Al up to 300) appear to be geopolymers, but for high Si/Al ratios (>24) materials become rubbery in texture (Fletcher et al., 2005). Mineralogical analysis was performed in order to evaluate the amorphous or crystalline nature of the ash and to identify the crystalline phases present (Fig. 1). The XRD pattern is that of a typical amorphous/ crystalline material where the crystalline phases are only minor components. The main phase is a-quartz (a-SiO2, JCPDF file 331161) followed by calcite (CaCO3 JCPDF file 5-586), and aluminosilicates as albite (NaAlSi3O8 JCPDF file 10-393) and ghelenite (Ca2Al(Al,Si)O7 JCPDF file 35-755), reflecting the chemical analysis of a typical ash rich in calcium and sodium. Due to the low but significant concentration of heavy metals such as Ba, Pb, Cu, and Mn. in the ash, the leaching test was necessary to verify its possibly hazardous nature. The amount of heavy metals found in the eluate was compared to the limit values prescribed to dispose in landfill for not dangerous waste (Table 3). Being all the values below the regulation limits, bottom ash, a commercially available product as ‘‘end of waste material’’ does not need an inertization process and can be considered as a suitable raw material to prepare blended geopolymers.
Fig. 6. pH values of all compositions after 15 and 30 day of curing.
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Fig. 7. Conductivity values of all compositions after 15 and 30 day of curing.
3.2. Geopolymers characterization All the different samples of geopolymers, containing from 50 to 80 wt% of bottom ash mixed with metakaolin, have been cured for 15 and 30 days prior characterization. Firstly, samples (15 days of curing), after contact with water for 48 h, were observed to qualitatively evaluate the consolidation/ geopolymerization process (Fig. 2). All the compositions did not alter their aspect with exception of geopolymer containing 80 wt% of bottom ash: it results powdered as a dry sludge and not geopolymerized and was not further investigated.
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Then the first 3 geopolymers of Table 2 were investigated by means of scanning electron microscopy coupled with EDS (SEM/ EDS) in order to evaluate the homogeneity of the samples, the reaction degree of the ash and the chemical composition of geopolymeric gel in terms of Si/Al and Na/Al mass ratios compared to the theoretical ones, tailored during the compositions formulation. All the sample are studied after 15 and 30 day of curing. For the same curing time of 15 days (Fig. 3), samples show an homogeneous geopolymeric gel with dispersed particles of not completely reacted bottom ash whose composition is reported in Fig. 4c. By increasing the ash content (passing from 50-50 MK_BA to 30-70 MK_BA wt%) samples remain homogeneous and the number of not-reacted particles does not increases, confirming the suitability of this ash to be geopolimerized. The effect of curing time on the geopolimerization process is well evident comparing Fig. 3 with Fig. 4. After 30 days the matrix is more dense, with a significant decrease of the not reacted particles or a decrease of the particles’ dimensions. From the chemical analysis of the not reacted particles it appears evident that they are bottom ash particle and not metakaolin particles due to the high content of calcium. The reactivity of the ash is also demonstrated by the chemical analysis of the geopolymeric gel: near the ash particles gel is richer in Ca and lower in Al (Table 4), reflecting the pristine chemical analysis of the as-received ash reported in Table 1a. In Fig. 5 the trend of the Si/Al mass ratio present in the geopolymeric gel, determined as average value of five measurements from five different areas of each sample, are reported for all the three geopolymers as a function of curing time. SEM/EDS gives a semiquantitative analysis, but can be used to obtain an indicative value of Si/Al ratio (Williams et al., 2011), to correlate with the occurring geopolymerization. A general increase of the ratio with the curing time can be observed, particularly pronounced for 30-70 MK_BA composition when comparing the 15 days results with the 30 days ones, due to the necessity of time to complete ash reaction. For this reason longer times can favor further reactivity towards geopolimerization of bottom ash. The Si/Al mass values ranges between 1.5 and 2.2 corresponding to the value accepted in literature for structural materials useful to encapsulate wastes (Zheng et al., 2010; Aly et al., 2008), but they are far from the theoretical range of 2.5–3.26. Probably metakaolin, rich in Al, is the raw material mainly geopolymerized maintaining low Si/Al ratio, but the increase of Si/Al ratio by increasing bottom ash content suggests that BA in taking part in the geopolymerization reaction as well. In order to better evaluate the three dimensional reticulation/ geopolymerization of the different samples their chemical stability was tested in water after 15 and 30 days of curing and pH and conductivity measurement were collected for different times (1, 15, 30, 60, 120, 1440 min) (Figs. 6 and 7). For all the three geopolymers there is a significant increase of pH after few minutes in contact with water, then stabilization is reached at a pH value near 12 as observed in previous work (Aly et al., 2008; Luna Galiano et al., 2011). For longer times pH stabilizes, notwithstanding the stirring of solution. Sample 30_70 MK_BA shows the higher values of pH because the high amount of ash in this composition needs longer time then 15 days to geopolymerize as indicated by the pH decrease which already appears after 30 day of curing. Conductivity values increase in a more gradually way. Both Na+ and OH ions, which possess a particularly high equivalent conductivity, give a significant effect on the overall solution conductibility. 50-50 MK_BA composition shows lower values with respect to compositions with higher amount of bottom ash. For all the three formulations an improvement in chemical stability, hence a decreasing in conductivity, with the curing time is evident, supporting the hypothesis that the geopolymerization process is
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Fig. 8. FT-IR spectra of samples containing (a) 50, (b) 60 and (c) 70 wt% of bottom ash before and after geopolymerization.
occurring. In order to deep the structural characterization of the prepared samples, IR spectroscopy (Fig. 8) was carried out on both anhydrous mixtures and on geopolymers. In all the samples is observable the presence of the strong Si–O–T (T = Si, Al) asymmetrical stretching peak at about 1035– 1002 cm 1. The shift of the main peak from 1035 to 1025 cm 1 of untreated bottom ash to 1008–1002 cm 1 of geopolymerized ash is an indication of the degree of geopolimerization associated to the inclusion of tetrahedrally-coordinated Al in the Si–O–Si skeletal structure as observed by other authors (Prud’homme et al., 2013; Rees et al., 2007; Zhang et al., 2008; Onori et al., 2011). This peak shift, which occurred for all the percentages of bottom ash, increases by increasing BA content in the formulation demonstrating the reactivity of this raw material in the alkali activation. All the spectra show a band at 1460 cm 1, typical of carbonate species, probably sodium carbonate, derived by the reaction of Na+ ions with atmospheric carbon dioxide. Further, a less intense band at 875 cm 1 is evident mainly in untreated bottom ash. This peak has been already attributed to CaCO3 content in fly ash by other authors (Zhang et al., 2008), therefore this signal can be related to the presence of 13.21 wt% of carbonates in the bottom ash. All the identified peaks were already found both in metakaolin and metakaolin/bottom ash geopolymers (Lancellotti et al., 2013; Onori et al., 2011). X-ray analysis has been performed on all the compositions in order to evaluate if the geopolymerization phenomenon had generated new crystalline phases or degradated those already present in the pristine raw materials. The crystalline phases of bottom ash (quartz, calcite, gehlenite, anorthite) are present also in geopolymers, notwithstanding the reactivity of ash grains with alkali activator. Fig. 9a shows the comparison of metakaolin, bottom ash and geopolymers containing different amount of bottom ash. Spectra of all the geopolymers are quite similar, but 50_50 MK_BA composition results more amor-
phous with respect to 40_60 MK_BA and 30_70 MK_BA due both to the higher amorphous fraction of metakaolin and to the high crystallinity fraction of bottom ash (Fig. 9a). By comparing metakaolin and geopolymers appears evident the shift of the amorphous hump from 22° to 28° 2h value indicative of geopolymerization process as observed by Provis et al. (2005a,b). The effect of increasing curing time from 15 to 30 days is evident in a slight increase of the hump corresponding to the geopolymeric gel supporting the hypothesis that geopolymerization proceeds for longer curing time (Fig. 9b). For compositions with high bottom ash content (60 and 70 wt%) and after 30 days of curing, a newly formed phase has been identified, corresponding to calcium aluminum silicate hydrate, i.e. gismondine CaAl2Si2O84H2O (JCPDF file 20-452). The formation of this phase is related to the reactivity of calcium present in the ash, as found by Guo et al. (2010) and it confirms the ash progressive reactivity. In order to evaluate the reactive fraction of bottom ash a basic attack was performed according to the test reported in literature (Ruiz-Santaquiteria et al., 2011). The analysis of liquid fraction after the test in NaOH shows amount of Si and Al dissolved in an environment similar to that of alkaline activation conditions. Results are Si 1.53 and Al 0.5 (wt%) for a ratio of Si/Al = 3.06. On the basis of this result the three geopolymeric matrices, previously characterized have been reformulated and the compositions are reported in Table 5. From microstructure observation of sample 50-50 MK_BA and after its reformulation, taking into account the reactive fraction of bottom ash (Fig. 10), the higher homogeneity of the geopolymeric gel formed results evident during SEM observation. The modification of gel, induced by reformulation, is also confirmed by the increase of Si/Al wt% ratio values, measured by EDS and reported in Fig. 11, very close to the theoretical values (mass Si/Al = 2.5).
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Fig. 11. Comparison between average Si/Al ratio in geopolymeric matrices before and after reformulation (data from EDS characterization of five measurement for each samples).
Fig. 9. (a) XRD pattern of metakaolin, bottom ash and geopolymers, (b) 30-70 MK_BA composition after 15 and 30 days of curing (Q- a-quartz (JCPDF file 331161); C-calcite (JCPDF file 5-586); G-gehelenite (JCPDF file 35-755), M-Muscovite (JCPDF file 7-25); Gs-Gismondine (JCPDF file 20-452).
This increase supports the hypothesis that bottom ash are not completely constituted of a reactive fraction and taking into account the real active fraction, gel is more dense and compact with ratios nearer to optimum values. X-ray diffraction shows that for short curing time (15 days) the matrix is more reactive with respect to the not reformulated composition, in fact, gismondine, related to Ca reactivity, forms after only 15 day of curing, while for the not reformulated composition 30 days were necessary for its detectability. This result confirms the higher reactivity of the reformulated composition and the tendency of bottom ash to cause the formation of hydrate phases containing calcium (see Fig. 12). XRD patterns of reformulated samples show also the presence of crystalline phases deriving from metakaolin, for this reason a further reformulation taking into account the reactive fraction of metakaolin (obtained by NaOH test) can help to obtain more stable geopolymers as future work.
Table 5 Compositions of reformulated matrices. Sample
Bottom ash (BA) (g)
Metakaolin (MK) (g)
NaOH 8 M (ml)
Na-silicate (ml)
H2O (ml)
Si/Al
Na/Al
Si/AlR
Na/AlR
50_50 MK_BAR 40_60 MK_BAR 30_70 MK_BAR
25 30 35
25 20 15
8 5 5
20 18 12
– 1 3
2.5 2.63 3.26
1.09 1.10 1.09
2.03 2.14 2.06
1.11 1.13 1.24
Fig. 10. SEM micrograph of (a) 50-50 MK_BA composition and (b) 50-50 MK_BA composition after reformulation.
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Fig. 12. XRD pattern of 30-70 MK_BA composition without reformulation (below) and with reformulation (above) after 15 days of curing (Q- a-quartz (JCPDF file 331161); C-calcite (JCPDF file 5-586); G-gehelenite (JCPDF file 35-755), M-Muscovite (JCPDF file 7-25); Gs-Gismondine (JCPDF file 20-452).
4. Conclusions Incinerator bottom ash is produced in large amount worldwide and especially in Europe, therefore finding a proper way of valorization is important and could be applicable to a wide numbers of countries. This residue has been demonstrated a suitable source materials for producing metakaolin blended geopolymers by using high percentage (50–70 wt%). The amount of potentially reactive phase is of significant importance when assessing the possible use of a raw material in the alkaline activation process, so the reactive Si/Al ratio is an important parameter to take into account for a proper geopolymers formulation. For bottom ash, significant differences are observed in the Si/Al ratios and microstructure with or without reformulation by considering reactive Si/Al due to the presence of both amorphous and crystalline fractions with a different degree of reactivity. In view of sustainability where cities can be considered as urban mining, the results reported in this paper show as a waste, or an end of waste material, can be a valuable resource for obtaining new cement-like materials by saving conventional raw materials and energy. Acknowledgements Authors are grateful to Dr. Mirko Braga and Dr. Pasquale Pansini, from R.S.A. Laboratory, INGESSIL S.r.l., Montorio (Verona, Italy) for supplying sodium silicate solutions and would like to thank Dr. Maria Cannio and Dr. Martina Arletti for the experimental support. References Aly, Z., Vance, E.R., Perera, D.S., Hanna, J.V., Griffith, C.S., Davis, J., Durce, D., 2008. Aqueous leachability of metakaolin-based geopolymers with molar ratios of Si/ Al = 1.5–4. Journal of Nuclear Materials 378 (2), 172–179 (31). Andreola, F., Barbieri, L., Hreglich, S., Lancellotti, I., Morselli, L., Passarini, F., Vassura, I., 2008. Reuse of incinerator bottom and fly ashes to obtain glassy materials. Journal of Hazardous Materials 153, 1270–1274. Barbieri, L., Karamanov, A., Corradi, A., Lancellotti, I., Pelino, M., Rincòn, J.M., 2008. Structure, chemical durability and crystallisation behavior of incinerator based glassy systems. Journal of Non-Crystalline Solids 354, 521–528. Bayuseno, A.P., Schmahl, W.W., 2011. Characterization of MSWI fly ash through mineralogy and water extraction. Resources, Conservation and Recycling 55 (5), 524–534.
Chindaprasirt, P., Rattanasak, U., 2010. Utilization of blended fluidized bed combustion (FBC) ash and pulverized coal combustion (PCC) fly ash in geopolymer. Waste Management 30 (4), 667–672. Chindaprasirt, P., Jaturapitakkul, C., Chalee, W., Rattanasak, U., 2009. Comparative study on the characteristics of fly ash and bottom ash geopolymers. Waste Management 29 (2), 539–543. Davidovits, J., 1991. Geopolymers: inorganic polymeric new materials. Journal of Thermal Analysis 37, 1633–1656. Davidovits, J., 1999. Chemistry of geopolymeric systems. In: Davidovits, J., Davidovits, R., James, C. (Eds.), Terminology, Geopolymere ’99, Geopolymer International Conference, Proceedings, 30 June–2 July, 1999. Institute Geopolymere, Saint Quentin, France, pp. 9–39. European Commission, 2006. Integrated Pollution Prevention and Control, Reference Document on the Best Available Techniques for Waste Incineration. Ferraris, M., Salvo, M., Ventrella, A., Buzzi, L., Veglia, M., 2009. Use of vitrified MSWI bottom ashes for concrete production. Waste Management 29, 1041–1047. Fletcher, R.A., MacKenzie, K.J.D., Nicholson, C.L., Shimada, S., 2005. The composition range of aluminosilicate geopolymers. Journal of the European Ceramic Society 25 (9), 1471–1477. Guo, X., Shi, H., Dick, W.A., 2010. Compressive strength and microstructural characteristics of class C fly ash geopolymer. Cement and Concrete Composites 32 (2), 142–147. Gutmann, R., 1996. Thermal technologies to convert solid waste residuals into technical glass products. Glastechnische Berichte-Glass Science and Technology 69 (9), 285–299. ISPRA, 2012. Istituto Superiore per la Protezione e la Ricerca Ambientale. Waste Report . Lancellotti, I., Kamseu, E., Michelazzi, M., Barbieri, L., Corradi, A., Leonelli, C., 2010. Chemical stability of geopolymers containing municipal solid waste incinerator fly ash. Waste Management 30 (4), 673–679. Lancellotti, I., Catauro, M., Ponzoni, C., Bollino, F., Leonelli, C., 2013. Inorganic polymers from alkali activation of metakaolin: effect of setting and curing on structure. Journal of Solid State Chemistry 200, 341–348. Luna Galiano, Y., Pereira, C.F., Vale, J., 2011. Stabilization/solidification of a municipal solid waste incineration residue using fly ash-based geopolymers. Journal of Hazardous Materials 185 (1), 373–381 (15). Monteiro, R.C.C., Figueiredo, C.F., Alendouro, M.S., Ferro, M.C., Davim, E.J.R., Fernandes, M.H.V., 2008. Characterization of MSWI bottom ashes towards utilization as glass raw material. Waste Management 28 (7), 1119–1125. Murayama, N., Yamamoto, H., Shibata, J., 2002. Mechanism of zeolite synthesis from coal fly ash by alkali hydrothermal reaction. International Journal of Mineral Processing 64, 1–17. Onori, R., Will, J., Hoppe, A., Polettini, A., Pomi, R., Boccaccini, A.R., 2011. Bottom ashbased geopolymer materials: mechanical and environmental properties. In: Ceram. Eng. Sci. Proc., pp. 71–82. Provis, J.L., Duxson, P., Van Deventer, J.S.J., Lukey, G.C., 2005a. The role of mathematical modelling and gel chemistry in advancing geopolymer technology. Chemical Engineering Research and Design 83 (7), 853–860. Provis, J.L., Lukey, G.C., van Deventer, J.S.J., 2005b. Do geopolymers actually contain nanocrystalline zeolites? A reexamination of existing results. Chemistry of Materials 17, 3075–3085. Provis, J.L., Myers, R.J., White, C.E., Rose, V., van Deventer, J.S.J., 2012. X-ray microtomography shows pore structure and tortuosity in alkali-activated binders. Cement and Concrete Research 42 (6), 855–864. Prud’homme, E., Autef, A., Essaidi, N., Michaud, P., Samet, B., Joussein, E., Rossignol, S., 2013. Defining existence domains in geopolymers through their physicochemical properties. Applied Clay Science 73, 26–34. Rambaldi, E., Esposito, L., Andreola, F., Barbieri, L., Lancellotti, I., Vassura, I., 2010. The recycling of MSWI bottom ash in silicate based ceramic. Ceramics International 36, 2469–2476. Ramboll, 2006. Report: Waste to Energy in Denmark. Rees, C., Provis, J.L., Lukey, G.C., van Deventer, J.S.J., 2007. Attenuated total reflectance fourier transform infrared analysis of fly ash geopolymer gel aging. Langmuir 23, 8170–8179. Ruiz-Santaquiteria, C., Fernández-Jiménez, A., Palomo, A., 2011. Quantitative determination of reactive SiO2 and Al2O3 in aluminosilicate materials. In: Proceedings of XIII International Congress on the Chemistry of Cement, Madrid, Spain. Saccani, A., Sandrolini, F., Barbieri, L., Corradi, A., Lancellotti, I., 2001. Structural studies and electrical properties of recycled glasses from glass and incinerator wastes. Journal of Materials Science 36, 2173–2177. Schabbach, L.M., Andreola, F., Lancellotti, I., Barbieri, L., 2011a. Minimization of Pb content in a ceramic glaze by reformulation the composition with secondary raw materials. Ceramics International 37, 1367–1375. Schabbach, L.M., Andreola, F., Karamanova, E., Lancellotti, I., Karamanov, A., Barbieri, L., 2011b. Integrated approach to establish the sinter-crystallisation ability of glasses from secondary raw material. Journal of Non-Crystalline Solids 357, 10– 17. Schabbach, L., Bolelli, G., Andreola, F., Barbieri, L., Lancellotti, I., 2012a. Valorization of MSWI bottom ash through ceramic glazing process: a new technology. Journal of Cleaner Production 23 (1), 147–157. Schabbach, L.M., Andreola, F., Barbieri, L., Lancellotti, I., Karamanova, E., Ranguelov, B., Karamanov, A., 2012b. Post-treated incinerator bottom ash as alternative raw material for ceramic manufacturing. Journal of the European Ceramic Society 32 (11), 2843–2852.
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electron microscopy, and energy-dispersive spaectroscopy. Journal of the American Ceramic Society 94 (8), 2663–2670. Xu, H., Li, Q., Shen, L., Wang, W., Zhai, J., 2010. Synthesis of thermostable geopolymer from circulating fluidized bed combustion (CFBC) bottom ashes. Journal of Hazardous Materials 175 (1–3), 198–204. Zhang, Jianguo, Provis, John L., Feng, Dingwu, van Deventer, Jannie S.J., 2008. Geopolymers for immobilization of Cr6+, Cd2+, and Pb2+. Journal of Hazardous Materials 157 (2–3), 587–598. Zheng, L., Wang, W., Shi, Y., 2010. The effects of alkaline dosage and Si/Al ratio on the immobilization of heavy metals in municipal solid waste incineration fly ash-based geopolymer. Chemosphere 79 (6), 665–671. Zheng, L., Wang, C., Wang, W., Shi, Y., Gao, X., 2011. Immobilization of MSWI fly ash through geopolymerization: effects of water-wash. Waste Management 31 (2), 311–317.
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Alkali activation processes for incinerator residues management Isabella Lancellotti ⇑, Chiara Ponzoni, Luisa Barbieri, Cristina Leonelli Department of Engineering ‘‘Enzo Ferrari’’, University of Modena and Reggio Emilia, Via Vignolese 905/A, 41125 Modena, Italy
a r t i c l e
i n f o
Article history: Received 25 October 2012 Accepted 30 April 2013 Available online 4 June 2013 Keywords: Alkali activation Incinerator bottom ash Recycling Reactive phase
a b s t r a c t Incinerator bottom ash (BA) is produced in large amount worldwide and in Italy, where 5.1 millions tons of municipal solid residues have been incinerated in 2010, corresponding to 1.2–1.5 millions tons of produced bottom ash. This residue has been used in the present study for producing dense geopolymers containing high percentage (50–70 wt%) of ash. The amount of potentially reactive aluminosilicate fraction in the ash has been determined by means of test in NaOH. The final properties of geopolymers prepared with or without taking into account this reactive fraction have been compared. The results showed that due to the presence of both amorphous and crystalline fractions with a different degree of reactivity, the incinerator BA geopolymers exhibit significant differences in terms of Si/Al ratio and microstructure when reactive fraction is considered. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The generation of municipal solid waste (MSW) in Italy in 2010 was about 32.5 millions tons with an increase of 1.1% with respect to 2009. From this amount, 15.5 millions tons has been landfilled without pre-treatment, with a decrement of 3.4%, corresponding to 523.000 tons, with respect to 2009; additionally 5.1 millions tons have been incinerated (MSWI), with an increase of 13.3% passing from 2009 to 2010 (ISPRA, 2012). Since incineration represents the total oxidation process of the combustible materials present in the waste, its final effect is the reduction in volume of approximately 95–96%, depending on waste composition and on the degree of pre-treatment (Ramboll, 2006). Further, incineration process contribute to a sensible mass and toxicity reduction of the waste (about 70 wt% of the incoming waste is transformed into gases) as well as recovery of the energy produced during combustion. Despite these advantages, during the incineration some solid residues are generated accounting for roughly 300 kg per ton of waste divided in 250–300 kg/t as bottom ash and 20–30 kg/t as fly ash for landfilling or reuse (Gutmann, 1996). In 2010, being incinerated in Italy about 5.1 millions tons, the production of bottom ash can be estimated approximately 1.2–1.5 millions tons (ISPRA, 2012). Many efforts have been made to improve the environmental quality of these solid residues both in-process and by means of post-treatment techniques/technologies. In-process measures aim to change the incineration parameters in order to improve burnout or to shift the metals distribution over the various residues. Post-treatment
⇑ Corresponding author. Tel.: +39 059 2056251; fax: +39 059 2056243. E-mail address: [email protected] (I. Lancellotti). 0956-053X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.wasman.2013.04.013
techniques include: ageing, mechanical treatment, washing, additional post combustion thermal treatment and stabilization (European Commission, 2006). The fly ash constitutes a potential hazard for health because it often contains high concentrations of heavy metals such as lead, cadmium, copper and zinc (Bayuseno and Schmahl, 2011; Wan et al., 2006). The bottom ash is mainly constituted of Si, Al, Ca and Na oxides, and heavy metals in quantity to classify this waste as ‘‘not hazardous’’ (Barbieri et al., 2008). While fly ash is always classified as special hazardous waste and must be deposited in special landfills equipped with careful control of the effluents, bottom ash is generally considered safe for regular landfill, after a certain level of testing defined by the local legislation (Monteiro et al., 2008). Recently in Italy some companies became active and specialized in posttreatment technologies of bottom ash. The objective is minimizing waste production by transforming it in a reusable material, the so called ‘‘end of waste-EOW’’ material. The treatment of incineration bottom ash starts from a complex process of selection to be completed with physical/mechanical treatments (ageing, sieving and washing). After the process, an inert material with silica-based matrix, rich in iron, calcium and aluminum oxides is obtained. This material can be successfully added in the formulation of cement or ceramic materials as substitute for minerals resources (Ferraris et al., 2009; Rambaldi et al., 2010; Schabbach et al., 2011a,b, 2012a,b). Even though the authors of the present paper posses a dated experience on sintering, melting and preparation of glass–ceramics from waste incinerator residues (Saccani et al., 2001; Schabbach et al., 2011a,b, 2012a,b; Andreola et al., 2008) they are now approaching the ‘‘cold’’ inertization technique of the alkali activation being aware that this offers the most efficient reuse procedure (Van Deventer et al., 2012).
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Alkali activation of aluminosilicate inorganic powders generates solid materials with a strong three dimensional structures (Provis et al., 2005a,b). When the starting powdered materials are incinerator residues, industrial waste and/or residues from treatments of natural raw materials, then geopolymerization can represent an emerging solution since differently from other industrial processes (e.g. sol–gel, clinkerization, sintering processes), alkali activation (geopolymerization) does not require high temperature thermal treatments, nor the use of carbonate-based raw materials, with corresponding CO2 emissions, or expensive chemical reagents. The versatility of the process, which occurs at room temperature or at temperature lower than 100 °C, and the possibility of its application to unconventional materials unemployed so far or disposed to landfill, involve several benefits: (i) saving not renewable resources and, consequently, their safeguard; (ii) reducing CO2 emissions and, consequently, a good action on climate change; (iii) producing materials with excellent mechanical and thermal properties. Following European and Italian directives aimed at decreasing waste production, creating a better waste management and increasing recycling procedures, the proposed research leads a fundamental contribute for: – a better knowledge of chemical–physical–mechanical performances of the investigated geopolymers, obtained by unconventional precursors, namely bottom ash; – the development of new products ready to be introduced into the market because produced with a low cost existing technology. The proposed research is an example of development of a new sustainable technology for waste reduction and recycling. In addition, the production of geopolymers will contribute to reduce high industrial pollution potentially replacing some of the most popular building materials characterized by a high environmental impact (e.g. cement, ceramics, etc.). The commitment to recycle is driven by the need to conserve natural resources, reduce importation of aluminosilicate raw materials, save landfill space and reduce pollution. A correct waste management and urban sustainability policy cannot avoid to consider waste as a resource and an opportunity of investment and earning. In particular, the aim of this paper is to formulate chemically activated materials by using incinerator bottom ash (BA) mixed with metakaolin (MK) and to monitor the geopolymerization process by means of several techniques (FT-IR, SEM, XRD, pH, conductivity, etc.). Few case studies on the use of incinerator bottom ash as main aluminosilicate constituent for geopolymers have been found in literature to date (Onori et al., 2011). Several bottom ash, different from incinerator one, but rich in silica and alumina, have been successfully used in the geopolymers formulation such as bottom ash from thermal power plant (Chindaprasirt et al., 2009) and circulating fluidized bed combustion bottom ash (Xu et al., 2010; Topçu and Toprak, 2011; Chindaprasirt and Rattanasak, 2010). Incinerator fly ashes, conversely, are used in geopolymers only for their inertization and not as raw material having not suitable chemical composition (Luna Galiano et al., 2011; Lancellotti et al., 2010; Zheng et al., 2010, 2011).
2. Experimental procedures 2.1. Bottom ash (BA) characterization Incineration bottom ash (BA), commercially available as ‘‘end of waste material’’ with grain size between 200 and 1000 lm, were ground for 30 min in a ball mill and sieved below 75 lm to reach
Table 1a Chemical analysis of bottom ash (BA). Element
Concentration (wt%)
Si Ca Al Na Fe Mg K P Ti S Zn Ba Pb Cu Mn Cr Ni
33.26 21.27 3.96 3.21 2.46 2.71 1.03 0.31 1.22 0.33 0.53 0.45 0.45 0.36 0.13 0.04 0.03 13.21
CO3 2 C H N S (total) LOI
2.57 0.63 0.00 1.25 7.00 2.37
SO4 2 Cl
1.29
Std. dev. ± 1 wt%.
Table 1b Chemical analysis of metakaolin calcinated 4 h (MK). Elements
Concentration (wt%)
Si Al Fe K
53.69 40.78 1.39 4.14
Std. dev. ± 1 wt%.
grain size similar to that of metakaolin (MK) powder used as basic constituent for geopolymer formulation. Chemical (X-ray fluorescence, Philips PW 2004) and elemental analysis and loss of ignition at 1000 °C for 2 h were performed and reported in Table 1a. Chlorides and sulfates were analysed in eluates of sequential washing procedure and carbonate content was measured by using a Dietrich-Fruhling calcimeter on 1 g samples which were contacted with HCl and the quantity of CO2 released from the reaction of the carbonate with HCl was used to determine the percentage of CaCO3. Mineralogical analysis was performed in order to evaluate the amorphous or crystalline nature of the ash and to identify the crystalline phases present. X-ray diffraction analysis was carried out by a powder diffractometer (PW3830, Philips, NL) with Ni-filtered Cu Ka radiation in the 5–70° 2h range on powdered samples (30 lm particle size). In order to evaluate the reactive fraction of bottom ash a basic attack was performed according to the test reported in literature (Ruiz-Santaquiteria et al., 2011). One gram of bottom ash was placed in 100 mL of 8 M NaOH solution and stirred constantly for 5 h in a flask bathed at 80 ± 2 °C. The resulting solution was then filtered, the residue was rinsed with distilled water until a neutral pH was attained and the amount of insoluble material was quantified. Al and Si contents in the leachate after the immersion were quantified with ICP/AES (Philips Varian Liberty 200), using k = 251,611 nm for Si and 396,152 nm for Al; with these values the reactive Si/Al mass ratio was calculated.
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Concentration (wt%)
Si Na Ratio Si/Na (wt/wt)
12.34 3.39 3.64
Table 2 Geopolymers formulations used in this work from a mixture of metakaolin (MK) and bottom ash (BA) at 50, 60, 70, 80 wt%. Geopolymer formulations Sample
Bottom ash (BA) (g)
H2O Si/Al Metakaolin NaOH Na silicate (ml) (MK) (g) 8M (ml) (ml)
Na/ Al
50_50 40_60 30_70 20_80
25 30 35 40
25 20 15 10
1.09 1.10 1.09 1.5
MK_BA MK_BA MK_BA MK_BA
12 12 7 10
15 8 10 5
– 3 3 7
2.5 2.63 3.26 3.8
Table 3 Heavy metals and soluble anions content (ppm) in the as-received bottom ash eluate after leaching test (EN 12457). Heavy metals and soluble anions (ppm)
Bottom ash
Law limit (ppm)
Ba Zn Pb Cu Cr Ni Cl
0.11 0.09 0 0.35 0 0 283.6 208
10 5.0 1.0 5.0 1.0 1.0 1500 2000
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To evaluate the hazardousness of the bottom ash, leaching test in distilled water for 24 h was performed according to European norm EN 12457 (UNI web site,
Fig. 1. XRD pattern of as-received bottom ash: Q- a-quartz (JCPDF file 33-1161); Ccalcite (JCPDF file 5-586); A-albite (JCPDF file 10-393); G-gehelenite (JCPDF file 35755).
The procedure for the samples preparation was carried out according to the following steps: – Mixing of aluminosilicate powders (bottom ash and metacaoline both previously ground and sieved) in a beaker in order to obtain a homogeneous mixture. – Addition of sodium hydroxide 8 M solution and sodium silicate solution with a mass ratio Si/Na = 3.64 (Table 1c); the amount of each solution was tailored depending on the Na and Si content present in the powder mixture. – Possible addition of a few milliliters of water in order to obtain a castable mixture; the water must be added only to what is strictly necessary otherwise compromise the subsequent step of hardening. – Intensive/thorough stirring until a homogeneous and fluid paste is formed; the paste is poured into plastic moulds (setting phase). – Setting stage maintaining the cast at room temperature; curing stage at room temperature 15 or 30 days (curing phase). – Being pH a relevant factor during the dissolution of the starting binder and considering that NaOH solution with high molarity (8 M) was used, the pH measured in all cases was near 13, almost identical for all samples. The formulations with different amounts of bottom ash have been designed in order not to cause significant shift neither in the Na/Al mass ratio nor in the Si/Al mass ratio. The optimized formulations are then summarized in Table 2.
2.3. Chemical stability tests All the compositions undergone an immersion test in distilled water for 48 h followed by a drying process at 110 °C in conventional oven for 24 h. The weight differences before and after the test permitted to evaluate the materials resistance to dissolution and consequently the efficiency of the geopolymerization process.
2.4. Materials characterization Geopolymers characterization was performed after 15 and 30 days of curing and the procedure to stop hydratation of the pastes at the different curing ages, applied to all the samples, was the immersion in acetone in order to remove residual water
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Fig. 2. Geopolymeric samples cured 15 days after test of immersion in water for 48 h, (a) 50-50 MK_BA, (b) 40_60 MK_BA, (c) 30_70 MK_BA, and (d) 20_80 MK_BA.
Fig. 3. SEM micrograph of dry polished surface of (a) 50-50 MK_BA, (b) 40_60 MK_BA and (c) 30_70 MK_BA compositions (150) after 15 days of curing.
Fig. 4. SEM micrograph of dry polished section of 50-50 MK_BA sample after 15 (a) and 30 (b) days of curing , and EDS spectrum (c) of unreacted BA particle.
present in the pores and other interstices, in agreement with Provis et al. (2012). Mineralogical analysis was carried out in order to evaluate if crystalline phases have been developed during geopolymerization
in all the samples. X-ray diffraction analysis was carried out on powdered samples as indicated above. All the geopolymers (Table 2) were broken and sieved below 2 mm, then each sample has been tested in bi-distilled water under
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Table 4 Chemical analysis (wt%) of geopolymeric gel in Fig. 4 at different distance from bottom ash particle.
Gel far from ash particle Gel near ash particle
Si
Al
Na
Ca
Si/Al
Na/Al
24.6 21.86
16.13 13.53
7.96 7.84
1.18 2.46
1.52 1.61
0.49 0.57
Shortly before SEM analysis the samples were dry polished to remove the resin layer and then coated with Au–Pd sputtered layer. FT-IR spectra were recorded on a Avatar 330 FT-IR ThermoNicolet. 32 scans between 4000 and 700 cm 1 were averaged for each spectrum at intervals of 1 cm 1. The FT-IR spectra have been collected at different bottom ash content, before and after alkali activation, in order to assess the obtained matrices stability.
3. Results and discussion 3.1. Bottom ash characterization
Fig. 5. Si/Al trend in the geopolymer gel as a function of the curing time for all the compositions (data from EDS characterization of five measurement for each sample).
stirring for 1 day in TeflonÒ bottles. pH and electrical conductivity were measured (Radiometer Analytical – Meterlab CDM210) after 1, 15, 30, 60, 120, 1440 min. The experiments had been carried out during the day 15th or the day 30th for all the samples investigated. Microstructure observations were conducted by ESEM (ESEM – Quanta200 – FEI) equipped with EDS to evaluate the role played by the bottom ash in the formation of geopolymeric amorphous phase on the hardened samples. Then the samples were encapsulated into an epoxy resin in order to isolate them from the air humidity.
The chemical composition of as received bottom ash (Table 1a) is not adequate to obtain a geopolymer with a Si/Al mass ratio below the value of 3, characteristic of materials with a 3D rigid network, suitable for a concrete, cement or waste encapsulating medium (Davidovits, 1999). Also Fletcher et al. noted that the high-alumina samples (Si/Al < 2) do not exhibit typical geopolymer characteristics, while the higher silica samples (Si/Al up to 300) appear to be geopolymers, but for high Si/Al ratios (>24) materials become rubbery in texture (Fletcher et al., 2005). Mineralogical analysis was performed in order to evaluate the amorphous or crystalline nature of the ash and to identify the crystalline phases present (Fig. 1). The XRD pattern is that of a typical amorphous/ crystalline material where the crystalline phases are only minor components. The main phase is a-quartz (a-SiO2, JCPDF file 331161) followed by calcite (CaCO3 JCPDF file 5-586), and aluminosilicates as albite (NaAlSi3O8 JCPDF file 10-393) and ghelenite (Ca2Al(Al,Si)O7 JCPDF file 35-755), reflecting the chemical analysis of a typical ash rich in calcium and sodium. Due to the low but significant concentration of heavy metals such as Ba, Pb, Cu, and Mn. in the ash, the leaching test was necessary to verify its possibly hazardous nature. The amount of heavy metals found in the eluate was compared to the limit values prescribed to dispose in landfill for not dangerous waste (Table 3). Being all the values below the regulation limits, bottom ash, a commercially available product as ‘‘end of waste material’’ does not need an inertization process and can be considered as a suitable raw material to prepare blended geopolymers.
Fig. 6. pH values of all compositions after 15 and 30 day of curing.
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Fig. 7. Conductivity values of all compositions after 15 and 30 day of curing.
3.2. Geopolymers characterization All the different samples of geopolymers, containing from 50 to 80 wt% of bottom ash mixed with metakaolin, have been cured for 15 and 30 days prior characterization. Firstly, samples (15 days of curing), after contact with water for 48 h, were observed to qualitatively evaluate the consolidation/ geopolymerization process (Fig. 2). All the compositions did not alter their aspect with exception of geopolymer containing 80 wt% of bottom ash: it results powdered as a dry sludge and not geopolymerized and was not further investigated.
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Then the first 3 geopolymers of Table 2 were investigated by means of scanning electron microscopy coupled with EDS (SEM/ EDS) in order to evaluate the homogeneity of the samples, the reaction degree of the ash and the chemical composition of geopolymeric gel in terms of Si/Al and Na/Al mass ratios compared to the theoretical ones, tailored during the compositions formulation. All the sample are studied after 15 and 30 day of curing. For the same curing time of 15 days (Fig. 3), samples show an homogeneous geopolymeric gel with dispersed particles of not completely reacted bottom ash whose composition is reported in Fig. 4c. By increasing the ash content (passing from 50-50 MK_BA to 30-70 MK_BA wt%) samples remain homogeneous and the number of not-reacted particles does not increases, confirming the suitability of this ash to be geopolimerized. The effect of curing time on the geopolimerization process is well evident comparing Fig. 3 with Fig. 4. After 30 days the matrix is more dense, with a significant decrease of the not reacted particles or a decrease of the particles’ dimensions. From the chemical analysis of the not reacted particles it appears evident that they are bottom ash particle and not metakaolin particles due to the high content of calcium. The reactivity of the ash is also demonstrated by the chemical analysis of the geopolymeric gel: near the ash particles gel is richer in Ca and lower in Al (Table 4), reflecting the pristine chemical analysis of the as-received ash reported in Table 1a. In Fig. 5 the trend of the Si/Al mass ratio present in the geopolymeric gel, determined as average value of five measurements from five different areas of each sample, are reported for all the three geopolymers as a function of curing time. SEM/EDS gives a semiquantitative analysis, but can be used to obtain an indicative value of Si/Al ratio (Williams et al., 2011), to correlate with the occurring geopolymerization. A general increase of the ratio with the curing time can be observed, particularly pronounced for 30-70 MK_BA composition when comparing the 15 days results with the 30 days ones, due to the necessity of time to complete ash reaction. For this reason longer times can favor further reactivity towards geopolimerization of bottom ash. The Si/Al mass values ranges between 1.5 and 2.2 corresponding to the value accepted in literature for structural materials useful to encapsulate wastes (Zheng et al., 2010; Aly et al., 2008), but they are far from the theoretical range of 2.5–3.26. Probably metakaolin, rich in Al, is the raw material mainly geopolymerized maintaining low Si/Al ratio, but the increase of Si/Al ratio by increasing bottom ash content suggests that BA in taking part in the geopolymerization reaction as well. In order to better evaluate the three dimensional reticulation/ geopolymerization of the different samples their chemical stability was tested in water after 15 and 30 days of curing and pH and conductivity measurement were collected for different times (1, 15, 30, 60, 120, 1440 min) (Figs. 6 and 7). For all the three geopolymers there is a significant increase of pH after few minutes in contact with water, then stabilization is reached at a pH value near 12 as observed in previous work (Aly et al., 2008; Luna Galiano et al., 2011). For longer times pH stabilizes, notwithstanding the stirring of solution. Sample 30_70 MK_BA shows the higher values of pH because the high amount of ash in this composition needs longer time then 15 days to geopolymerize as indicated by the pH decrease which already appears after 30 day of curing. Conductivity values increase in a more gradually way. Both Na+ and OH ions, which possess a particularly high equivalent conductivity, give a significant effect on the overall solution conductibility. 50-50 MK_BA composition shows lower values with respect to compositions with higher amount of bottom ash. For all the three formulations an improvement in chemical stability, hence a decreasing in conductivity, with the curing time is evident, supporting the hypothesis that the geopolymerization process is
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Fig. 8. FT-IR spectra of samples containing (a) 50, (b) 60 and (c) 70 wt% of bottom ash before and after geopolymerization.
occurring. In order to deep the structural characterization of the prepared samples, IR spectroscopy (Fig. 8) was carried out on both anhydrous mixtures and on geopolymers. In all the samples is observable the presence of the strong Si–O–T (T = Si, Al) asymmetrical stretching peak at about 1035– 1002 cm 1. The shift of the main peak from 1035 to 1025 cm 1 of untreated bottom ash to 1008–1002 cm 1 of geopolymerized ash is an indication of the degree of geopolimerization associated to the inclusion of tetrahedrally-coordinated Al in the Si–O–Si skeletal structure as observed by other authors (Prud’homme et al., 2013; Rees et al., 2007; Zhang et al., 2008; Onori et al., 2011). This peak shift, which occurred for all the percentages of bottom ash, increases by increasing BA content in the formulation demonstrating the reactivity of this raw material in the alkali activation. All the spectra show a band at 1460 cm 1, typical of carbonate species, probably sodium carbonate, derived by the reaction of Na+ ions with atmospheric carbon dioxide. Further, a less intense band at 875 cm 1 is evident mainly in untreated bottom ash. This peak has been already attributed to CaCO3 content in fly ash by other authors (Zhang et al., 2008), therefore this signal can be related to the presence of 13.21 wt% of carbonates in the bottom ash. All the identified peaks were already found both in metakaolin and metakaolin/bottom ash geopolymers (Lancellotti et al., 2013; Onori et al., 2011). X-ray analysis has been performed on all the compositions in order to evaluate if the geopolymerization phenomenon had generated new crystalline phases or degradated those already present in the pristine raw materials. The crystalline phases of bottom ash (quartz, calcite, gehlenite, anorthite) are present also in geopolymers, notwithstanding the reactivity of ash grains with alkali activator. Fig. 9a shows the comparison of metakaolin, bottom ash and geopolymers containing different amount of bottom ash. Spectra of all the geopolymers are quite similar, but 50_50 MK_BA composition results more amor-
phous with respect to 40_60 MK_BA and 30_70 MK_BA due both to the higher amorphous fraction of metakaolin and to the high crystallinity fraction of bottom ash (Fig. 9a). By comparing metakaolin and geopolymers appears evident the shift of the amorphous hump from 22° to 28° 2h value indicative of geopolymerization process as observed by Provis et al. (2005a,b). The effect of increasing curing time from 15 to 30 days is evident in a slight increase of the hump corresponding to the geopolymeric gel supporting the hypothesis that geopolymerization proceeds for longer curing time (Fig. 9b). For compositions with high bottom ash content (60 and 70 wt%) and after 30 days of curing, a newly formed phase has been identified, corresponding to calcium aluminum silicate hydrate, i.e. gismondine CaAl2Si2O84H2O (JCPDF file 20-452). The formation of this phase is related to the reactivity of calcium present in the ash, as found by Guo et al. (2010) and it confirms the ash progressive reactivity. In order to evaluate the reactive fraction of bottom ash a basic attack was performed according to the test reported in literature (Ruiz-Santaquiteria et al., 2011). The analysis of liquid fraction after the test in NaOH shows amount of Si and Al dissolved in an environment similar to that of alkaline activation conditions. Results are Si 1.53 and Al 0.5 (wt%) for a ratio of Si/Al = 3.06. On the basis of this result the three geopolymeric matrices, previously characterized have been reformulated and the compositions are reported in Table 5. From microstructure observation of sample 50-50 MK_BA and after its reformulation, taking into account the reactive fraction of bottom ash (Fig. 10), the higher homogeneity of the geopolymeric gel formed results evident during SEM observation. The modification of gel, induced by reformulation, is also confirmed by the increase of Si/Al wt% ratio values, measured by EDS and reported in Fig. 11, very close to the theoretical values (mass Si/Al = 2.5).
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Fig. 11. Comparison between average Si/Al ratio in geopolymeric matrices before and after reformulation (data from EDS characterization of five measurement for each samples).
Fig. 9. (a) XRD pattern of metakaolin, bottom ash and geopolymers, (b) 30-70 MK_BA composition after 15 and 30 days of curing (Q- a-quartz (JCPDF file 331161); C-calcite (JCPDF file 5-586); G-gehelenite (JCPDF file 35-755), M-Muscovite (JCPDF file 7-25); Gs-Gismondine (JCPDF file 20-452).
This increase supports the hypothesis that bottom ash are not completely constituted of a reactive fraction and taking into account the real active fraction, gel is more dense and compact with ratios nearer to optimum values. X-ray diffraction shows that for short curing time (15 days) the matrix is more reactive with respect to the not reformulated composition, in fact, gismondine, related to Ca reactivity, forms after only 15 day of curing, while for the not reformulated composition 30 days were necessary for its detectability. This result confirms the higher reactivity of the reformulated composition and the tendency of bottom ash to cause the formation of hydrate phases containing calcium (see Fig. 12). XRD patterns of reformulated samples show also the presence of crystalline phases deriving from metakaolin, for this reason a further reformulation taking into account the reactive fraction of metakaolin (obtained by NaOH test) can help to obtain more stable geopolymers as future work.
Table 5 Compositions of reformulated matrices. Sample
Bottom ash (BA) (g)
Metakaolin (MK) (g)
NaOH 8 M (ml)
Na-silicate (ml)
H2O (ml)
Si/Al
Na/Al
Si/AlR
Na/AlR
50_50 MK_BAR 40_60 MK_BAR 30_70 MK_BAR
25 30 35
25 20 15
8 5 5
20 18 12
– 1 3
2.5 2.63 3.26
1.09 1.10 1.09
2.03 2.14 2.06
1.11 1.13 1.24
Fig. 10. SEM micrograph of (a) 50-50 MK_BA composition and (b) 50-50 MK_BA composition after reformulation.
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Fig. 12. XRD pattern of 30-70 MK_BA composition without reformulation (below) and with reformulation (above) after 15 days of curing (Q- a-quartz (JCPDF file 331161); C-calcite (JCPDF file 5-586); G-gehelenite (JCPDF file 35-755), M-Muscovite (JCPDF file 7-25); Gs-Gismondine (JCPDF file 20-452).
4. Conclusions Incinerator bottom ash is produced in large amount worldwide and especially in Europe, therefore finding a proper way of valorization is important and could be applicable to a wide numbers of countries. This residue has been demonstrated a suitable source materials for producing metakaolin blended geopolymers by using high percentage (50–70 wt%). The amount of potentially reactive phase is of significant importance when assessing the possible use of a raw material in the alkaline activation process, so the reactive Si/Al ratio is an important parameter to take into account for a proper geopolymers formulation. For bottom ash, significant differences are observed in the Si/Al ratios and microstructure with or without reformulation by considering reactive Si/Al due to the presence of both amorphous and crystalline fractions with a different degree of reactivity. In view of sustainability where cities can be considered as urban mining, the results reported in this paper show as a waste, or an end of waste material, can be a valuable resource for obtaining new cement-like materials by saving conventional raw materials and energy. Acknowledgements Authors are grateful to Dr. Mirko Braga and Dr. Pasquale Pansini, from R.S.A. Laboratory, INGESSIL S.r.l., Montorio (Verona, Italy) for supplying sodium silicate solutions and would like to thank Dr. Maria Cannio and Dr. Martina Arletti for the experimental support. References Aly, Z., Vance, E.R., Perera, D.S., Hanna, J.V., Griffith, C.S., Davis, J., Durce, D., 2008. Aqueous leachability of metakaolin-based geopolymers with molar ratios of Si/ Al = 1.5–4. Journal of Nuclear Materials 378 (2), 172–179 (31). Andreola, F., Barbieri, L., Hreglich, S., Lancellotti, I., Morselli, L., Passarini, F., Vassura, I., 2008. Reuse of incinerator bottom and fly ashes to obtain glassy materials. Journal of Hazardous Materials 153, 1270–1274. Barbieri, L., Karamanov, A., Corradi, A., Lancellotti, I., Pelino, M., Rincòn, J.M., 2008. Structure, chemical durability and crystallisation behavior of incinerator based glassy systems. Journal of Non-Crystalline Solids 354, 521–528. Bayuseno, A.P., Schmahl, W.W., 2011. Characterization of MSWI fly ash through mineralogy and water extraction. Resources, Conservation and Recycling 55 (5), 524–534.
Chindaprasirt, P., Rattanasak, U., 2010. Utilization of blended fluidized bed combustion (FBC) ash and pulverized coal combustion (PCC) fly ash in geopolymer. Waste Management 30 (4), 667–672. Chindaprasirt, P., Jaturapitakkul, C., Chalee, W., Rattanasak, U., 2009. Comparative study on the characteristics of fly ash and bottom ash geopolymers. Waste Management 29 (2), 539–543. Davidovits, J., 1991. Geopolymers: inorganic polymeric new materials. Journal of Thermal Analysis 37, 1633–1656. Davidovits, J., 1999. Chemistry of geopolymeric systems. In: Davidovits, J., Davidovits, R., James, C. (Eds.), Terminology, Geopolymere ’99, Geopolymer International Conference, Proceedings, 30 June–2 July, 1999. Institute Geopolymere, Saint Quentin, France, pp. 9–39. European Commission, 2006. Integrated Pollution Prevention and Control, Reference Document on the Best Available Techniques for Waste Incineration
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