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Recycling municipal incinerator fly- and scrubber-ash into fused slag for the substantial replacement of cement in cement-mortars
Waste Management 29 (2009) 1952–1959
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Recycling municipal incinerator fly- and scrubber-ash into fused slag for the substantial replacement of cement in cement-mortars Tzen-Chin Lee *, Ming-Kang Rao Department of Civil and Disaster Prevention Engineering, National United University, Miao-Li, 360, Taiwan, ROC
a r t i c l e
i n f o
Article history: Accepted 6 January 2009 Available online 11 February 2009
a b s t r a c t Fly- and scrubber-ash (weight ratio of approximately 1:3) from municipal solid waste incinerators (MSWI) are a major land-fill disposal problem due to their leaching of heavy metals. We uniformly mixed both types of ash with optimal amounts of waste glass frit, which was then melted into a glassy slag. The glassy slag was then pulverized to a particle size smaller than 38 lm for use as a cement substitute (20– 40% of total cement) and blended with sand and cement to produce slag-blended cement-mortar (SCM) specimens. The toxicity characteristics of the leaching procedure tests on the pulverized slag samples revealed that the amount of leached heavy metals was far below regulatory thresholds. The compressive strength of the 28-day cured SCM specimens was comparable to that of ordinary Portland cement mortars, while the compressive strength of specimens cured for 60 or 90 days were 3–11% greater. The observed enhanced strength is achieved by Pozzolanic reaction. Preliminary evaluation shows that the combination of MSWI fly- and scrubber-ash with waste glass yields a cost effective and environmentally friendly cement replacement in cement-mortars. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Municipal solid waste in Taiwan and in many other industrialized countries is normally burned in incinerators. The fly- and scrubber-ash wastes produced from municipal solid waste incinerators (MSWI) usually contain many hazardous heavy metals. The ash is usually placed in a landfill after solidification with 20 wt.% cement and the addition of chelating agents; however, toxic components can leach from landfill solids and pollute groundwater (Van Herck et al., 2000; Svensson et al., 2007), emit unpleasant odors, and contaminate soil. Consequently, the potential hazards of long term landfill leaching are a persistent problem. In addition, a shortage of landfill sites is a serious problem in populated regions (as is the case in Taiwan). Therefore, recycling MSWI fly- and scrubber-ash as construction resources could be a potential way of alleviating the problem. MSWI fly-ash has been studied for use in many applications. For instance, MSWI fly-ash (after a washing treatment) can be used as a secondary raw material in road construction (Mulder, 1996). Water-permeable pavement blocks can be formed from the molten slag of MSWI fly-ash (Nishigaki, 2000). The effectiveness of different ratios of MSWI fly-ash to sand or cement in cement mortars has also been investigated (Al-Rawas et al., 2005). The experimental results of these studies indicate that MSWI fly-ash blended with
* Corresponding author. Tel.: +886 37 381668; fax: +886 37 326567. E-mail address: [email protected] (T.-C. Lee). 0956-053X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2009.01.002
cement mortar reduces slump values when it is used as a replacement for sand; in contrast, it has an opposite trend when used as a replacement for cement. The 28-day compressive strength of the latter is similar to or slightly higher than that of cement mix (0% MSWI fly-ash); however, the direct blending of MWSI fly-ash into cement mortars exhibits a poor stabilization of some harmful elements, such as antimony and chromium (Aubert et al., 2007). The output from an MSWI is usually 25% fly-ash and 75% scrubber-ash. Scrubber-ash is collected from a ventilation bag filter and contains very fine solid ash, chlorides, and some organic compounds. Due to its extreme acidity, the scrubber-ash must be neutralized by the addition of substantial amounts of calcium hydroxide and activated carbon. The scrubber-ash is thus very difficult to vitrify (melt into a glassy phase) for further application, unlike its counterpart fly-ash. The complete simultaneous recycling of both MSWI fly-ash and scrubber-ash from a single incinerator is difficult, and has not yet to be reported. Fly-ash can be vitrified (made glassy) by melting, and heavy metals in the fly-ash can be immobilized in the amorphous Si– O–Si network to form highly stable slag; hence, these toxic metals do not easily leach out (Sreenivasarao et al., 1997; Lin et al., 2003, 2004). The impact of cement replacement with slag (prepared by melting MSWI bottom-ash) on the Pozzolanic reactivity of slagblended cement-paste has been investigated (Lin and Lin, 2006). Heavy metals are immobilized in the vitrified product. In early curing stages, the strength of slag-blended cement-pastes is lower than that of ordinary cement-paste; however, at later ones, their strength is comparable to that of ordinary cement-paste.
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To the best of our knowledge, the treatment of scrubber-ash by vitrification into a glassy slag has not been proposed. The weight ratio of fly-ash to scrubber-ash produced by a typical MSWI is 1:3, which is the daily output ratio of the MSWI in the city of Hsinchu, Taiwan. Earlier trials by the authors demonstrated that the vitrification of fly-ash by simple melting in a crucible at 1200 °C is not problematic, but becomes more difficult when scrubberash is added to the fly-ash at weight ratios of 1:3 to 1:1. In order to overcome this difficulty, this study proposes the addition of pulverized waste glass bottles (glass-frit) to enhance vitrification. Earlier trials have examined the melting characteristics of the ash-mix after adding different amounts of glass-frit (Lee et al., 2008). The intent of this study was to identify the maximum ratio of optimally vitrified ash-mix slag at which the cement-mortar maintains an acceptable compressive strength after prolonged curing.
value (Lee et al., 2008). Ideally, the ratio of glass frit to fill in the ash-mix should be one-half. In this study, this mixture is referred to as ‘‘modified ash-mix”. The modified ash-mix was then vitrified at 1400 °C in an electrical oven for at least half an hour until it reached a molten state. The mix was then quenched in water to form the slag. The slag was pulverized and used as a cement replacement (20–40% by weight) in cement-mortar. The slag-cement mortar specimens were tested for compressive strength after curing for 1 to 90 days. 2.2. Cement mortar specimen: molding and curing
Pozzolanic materials typically have a high silica content, large surface area, and are amorphous to X-ray diffraction. According to ASTM C618, materials with granule sizes larger than 150 lm cannot be qualified as Pozzolanic, while those with granule sizes on the order of 75 lm (sieve #200) are marginal, depending on the proportion of granules smaller than 45 lm (Shao et al., 2000). Thus, in this study, ash-mix was combined with glass frit, melted to slag, and then ground to a powder with a granule size smaller than 38 lm (sieve #400). The materials and methods were as follows.
The mortar specimens (50 50 50 mm) were molded according to the Taiwan CNS 1010 testing specifications to test compressive strength. A water-to-cement weight ratio (W/C) of 0.485 and sand-to-cement weight ratio of 2.75 were used. Reference specimens consisting of ordinary type I Portland cement, reference sand, and water were designated ‘‘OCM” (ordinary cement mortar) and used as a standard for comparing compressive strength to slag-blended cement mortar with 20%, 30%, and 40% substitutions of cement, which were designated as SCM(20%), SCM(30%), and SCM(40%), respectively. Before testing for compressive strength, all specimens were placed in a chamber with a programmable temperature and humidity for one day, and then de-molded and cured in a saturated calcium hydroxide solution chamber maintained at 23.0 ± 1.7 °C for 1 to 90 days. Table 2 summarizes the constituent contents of the OCM and SCM specimens.
2.1. Materials
2.3. Characterization methods of the slag-cement mortar specimens
The fly-ash used in this investigation was collected from cyclones of the Hsinchu MSWI in northwest Taiwan. The ash is yellowish-light gray, and has a specific gravity of 2.91. The scrubber-ash collected from the bag filters of the Hsinchu MSWI is grayish-white, and has a specific gravity of 2.62. The cement used in this research was ordinary Portland cement (OPC), Type I, manufactured by the Taiwan Cement Company. This cement has a specific gravity of 3.15, while its physical and chemical properties meet ASTM C150 specifications. The reference sand was Ottawa-type sand that adhered to ASTM C778 specifications, and had a specific gravity of 2.63. The fine glass-frit, which had a specific gravity of 2.40, was ground from discarded dark green beer bottles. Fly- and scrubber-ash from the Hisnchu city MSWI were mixed according to the production weight ratio of 1:3. This mixture is referred to as ‘‘ash-mix” in this study. Table 1 summarizes the CaO and SiO2 contents in the ash-mix and glass-frit. The ash-mix was further modified by adding different amounts of glass-frit. The amount of added glass-frit was based on the analyzed compositions in Table 1. The CaO to SiO2 ratio was 1.1, which is the optimal
No pretreatment, other than drying, was performed before the scanning electron microscope (SEM) was used to study the flyash/scrubber-ash and slag powder. The cured slag-cement mortar specimens were cut and oven-dried at 100 °C for 24 h, and then cooled in a desiccator. The sample was then fixed on a sample holder and examined with an SEM (JEOL, JSM-5600, Japan) after a gold coating was applied. An energy-dispersive X-ray spectrometer (EDS, Oxford, 6587, UK) attached to the SEM was used to analyze the chemical composition of the cement, fly-ash, scrubber-ash, ash-mix, glass-frit, and slag. Powder X-ray diffraction (XRD) analysis was performed using an X-ray diffractometer (Rigaku, D/Max-2200, Japan) with Cu Ka1 radiation and 2h scanning ranging from 10° to 70°. The XRD scans were run at 0.05° per step with a counting time of one second. The leaching of ash-mix and slag was then analyzed by TCLP testing according to ‘‘Toxicity Characteristic Leaching Procedures (TCLP): SW846-1311 (USA).” The solid waste samples were crushed and leached by acetic acid. The extraction procedure required a preliminary pH analysis of the sample to determine the proper extraction fluid for the experiment. After testing, the
2. Materials and methods
Table 1 Chemical composition of cement, MSWI fly-ash, scrubber-ash, ash-mix, glass frit and modified slag as analyzed by EDS. Chemical composition (wt.%)
Cement
Fly-ash
Scrubber-ash
Ash-mix
Glass-frit
Modified slag
Na (as Na2O) Mg (as MgO) Al (as Al2O3) Si (as SiO2) SO3 Cl K (as K2O) Ca (as CaO) Fe (as Fe2O3) Total sum
– 2.8 5.7 22.3 – – – 62.1 3.4 96.3
5.9 1.7 – 12.8 17.6 5.4 2.7 40.0 1.0 87.1
2.9 1.3 6.5 2.5 4.1 29.0 2.4 40.4 0.5 89.6
3.1 0.9 5.8 6.1 4.5 26.6 3.6 40.5 0.5 91.6
10.7 1.2 3.4 71.8 – – 1.1 10.4 – 98.6
1.7 1.6 16.1 35.1 – 0.1 – 38.7 2.4 95.7
Remarks: minor constituents Cu (as CuO) 0–4.0, Pb (as PbO) 0–1.8, and Zn (as ZnO) 0–0.1 in different analyses.
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Table 2 The constituent contents of ordinary cement mortar (OCM) and slag-blended cement mortar (SCM) specimens (proportion weights per nine specimens). Mix design
Cement (g) Sand (g) Modified slag (g)a Water (g)
Notation of mortar specimens OCMb
SCMc (20%)
SCM(30%)
SCM(40%)
740 2035 0 359
592 2035 148 359
518 2035 222 359
444 2035 296 359
a
Particle size of slag powder <#400 (38 lm). Ordinary Portland cement mortar. Slag-cement mortar, wherein the percentage in the bracket is the cement replacement ratio. b
c
extraction fluid selected for TCLP analysis (pH 2.88 ± 0.05) was prepared by adding 5.7 mL of glacier acetic acid to 500 mL of double distilled water, and then diluted to a volume of 1 L. The 25 g samples were placed in 1 L Erlenmeyer flasks, wherein 500 mL of extraction fluid was added to each flask. The flasks were then agitated for 18 h with an electric vibrator. The slurry was filtered using ‘‘Millipore filter paper” with a 0.6 to 0.8 lm pore size. The leachates were preserved in 2% HNO3 (Wang et al., 2003; Lin and Chang, 2006). Heavy metal concentrations from the leachates of the ash-mix and slag samples were confirmed by inductivelycoupled plasma atomic-emission spectroscopy (ICP-AES). The Pozzolanic activity index of the slag-cement mortar specimens was analyzed according to ASTM C311-98a. The setting time was tested according to ASTM C187 to determine the optimal water quantity for the cement. The Vicat needle method was then used to determine the setting time for the cement paste, according to ASTM C191, to obtain the initial and final setting times of the cement paste. The fluidity test was performed according to ASTM C230. The OCM and SCM mortar-cubes were prepared according to ASTM C109 and were cured for 1 to 90 days. At each curing age, the mortar cubes were tested for compressive strength. The samples were soaked in methyl alcohol (methanol) after the compressive strength tests. Hydration reactions were terminated due to the expelling of water out from the specimens. Thus, the degree of hydration due to different curing times could be preserved. The samples were then analyzed by mercury intrusion porosimetry (MIP), XRD, Fourier transformed infrared spectroscopy (FT-IR), and differential thermal analysis (DTA). The hydration-terminated slag-blended cement mortar specimens (as preserved in methanol) were removed, oven-dried at 100 °C for 24 h, and then cooled in a desiccator. An MIP facility (PMI, Automated porosimeter, Model: 60 K-A-2, USA) was then used to analyze the samples for porosity and pore size distribution. The chemical bonding characteristics of SCM were analyzed by FT-IR (Jasco FT/IR-300E, Japan). For this test, samples were removed from the methanol and dried in a vacuum oven at 100 °C for 24 h, and then cooled in a desiccator. The samples were then ground and uniformly mixed with potassium bromide (KBr) at a KBr/specimen weight ratio of 100/1. A 0.2 g quantity of each mixture was pressed into a 13 mm diameter disk for analysis. The measurement range was between 500 and 4000 cm1. The thermal behavior of SCM specimens was analyzed using differential thermal analysis (DTA, Perkin Elmer DTA7 Analyzer, USA). Each specimen was ground to a particle size smaller than 44 lm (
3. Results and discussion In order to maintain the CaO and SiO2 weight ratio was 1.1, the amount of 49.3 g of glass-frit mixed with 100 g ash-mix was added to ensure it melting (vitrifying) into glassy slag (Lee et al., 2008). The modified ash-mix readily melted at 1400 °C to form a homogeneous melt, which was quenched in water. The slag was green, irregularly shaped, and exhibited a specific gravity of 2.81. The slag samples were then ground into a powder that could pass through a No. 400 sieve (<38 lm, specific surface 6400 cm2/g) to meet the fineness requirements of Pozzolanic materials. 3.1. Analyses of raw materials Table 1 summarizes the composition of the cement, fly-ash, scrubber-ash, ash-mix, glass-frit, and slag, according to the EDS analysis. In comparison to the fly-ash, the scrubber-ash contained less silica, but much more chlorine. The glass-frit is essentially a typical soda-lime glass. Fig. 1 depicts the X-ray diffractogram of the ash-mix. The identified phases, in decreasing intensity, were CaSO4, CaCl2, Ca(OH)2, SiO2, NaCl, KCl, CaClOH, PbO, and ZnS. According to this diffractogram, the large amount of post-added Ca(OH)2 into the filter-collected ash reacted to form various calcium baring inorganic compounds. 3.2. Characterization of the slag 3.2.1. Chemical compositions A small quantity of fly-ash, scrubber-ash, ash-mix, glass-frit, and slag powder was prepared for SEM analysis. The fly-ash, scrubberash, and ash-mix consisted of agglomerates of much smaller particles, while the slag consisted of fused granules smaller than 38 lm. Table 1 summarizes the chemical composition of the slag according to EDS analysis. The slag consisted of the non-CaO constituents SiO2, Al2O3, and Fe2O3, in percentages of 35.1%, 16.1%, and 2.4% by weight, respectively. The sum of these three compositions equaled 53.6%, which meets the requirements of grade C fly-ash according to ASTM C618. Non-calcium oxides were observed to be the primary constituents that are responsible for the Pozzolanic reaction. One concern of the vitrification process is that, at a melting temperature of 1400 °C, some elements may evaporate (in particular, Cd and Zn, whose boiling point is lower than 1400 °C). Assuming there is some elemental mass loss due to evaporation, a significant concern involves the environmental benefit of the
1. KCl; 2. CaSO4; 3. CaCl2; 4. NaCl; 5. CaClOH; 6. Ca(OH)2; 7. PbO; 8. ZnS; 9. SiO2
slag 2
Intensity (a. u.)
1954
2,3
1,7 3,8
2
4 5,6 9
6
1,2
ash-mix
20
25
30
35
40
45 2θ
50
55
60
Fig. 1. X-ray diffractogram of slag and ash-mix.
65
70
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entire vitrification process. In fact, glass melting is complicated due to the thermodynamics of a mixed-oxides-melt, in addition to the 4 bonding effect arising from the silicate unit, ðSiO4 Þn , in a glassy network, even in a molten state. The silicate unit in a long-chain glassy structure provides traps, and, in addition, tightly bonds with cations (monovalent, divalent, and trivalent). The extent of vaporization is thus significantly minimized. This phenomenon is why soda-lime (window) glasses can be manufactured after melting for a long time at 1600 °C, despite that Na2O boils at 400 °C. We believe that a small amount of evaporation does occur, but the gaseous species can be captured by a simple scrubbing system and recycled as useful resources, such as ZnO. This is certainly much safer than the uncontrolled dissolution hazard of heavy metals to underground water in the case of landfills filled with ash-mix, as is the current disposal method. According to Table 1, the analyzed alumina content of the slag, 16.1%, is much higher than that in the ash-mix 5.8%. This arise content from alumina dissolution of the alumina crucible was used. Since alumina content is crucial to satisfying the requirement of grade C fly-ash in ASTM C618, the addition of extra alumina is required as an extra component if the melting process does not pick up alumina from the crucible, such as in field-scale production. The most cost effective (free) supply of alumina can be obtained from the industrial waste of water treatment or/and aluminum industries. 3.2.2. TCLP analysis All of the investigated fly-ash, scrubber-ash, and ash-mix contained the hazardous heavy metals Pb, Zn, Cu, Cd, and Cr, which can easily leach out to pollute groundwater. As Table 3 shows, the TCLP leaching concentrations of heavy metals were far below regulatory thresholds. A glass structure is composed of two major constituents: A network-former and a modifier. The network for4 mer is ðSiO4 Þn , while the modifiers are the oxides of monovalent, divalent, or trivalent cations. The chemical bonding between the network former and modifiers is usually quite strong, depending on the valence state of the modifiers. As the valence increase, the bond strength also increases. Heavy metals, which usually have two to three valence states, can therefore bond strongly to the glass network and become immobilized (Sreenivasarao et al., 1997; Lin et al., 2003; Lin and Chang, 2006). The XRD analysis of slag (Fig. 1) revealed that it is an amorphous glassy substance without diffraction peaks, which was consistent with an earlier study (Lin et al., 2003). 3.3. Analysis of slag-cement paste and mortar 3.3.1. Pozzolanic activity index In SCM(20%), the slag-cement mortar with 20% of the cement replaced by slag, the slag was composed of SiO2 (35.1 wt.%), CaO (38.7 wt.%), and Al2O3 (16.1 wt.%), such that the Pozzolanic activity index was 99.3. This Pozzolanic activity index elucidates the suitability of the slag Pozzolanic reaction for partial cement replacement. 3.3.2. Tests of initial and final setting Table 4 depicts the setting test results. The initial setting time for ordinary cement mortar is 135 min, while the final setting time
is 216 min. In this study, the initial and final setting times of the slag cement mortars were 9–20 min and 10–17 min longer than plain cement paste, respectively. As more cement was replaced with slag powder the longer the initial and final setting times, although the increase in time was relatively small (increases of 15% and 8%, respectively). The observed increase in the initial and final settings times is due to the fact that water-quenched glassy slag itself is not a binding material, and instead has inert characteristics. The rate at which a coagulant structural network forms from residual cement due to hydration slows as the amount of replacement cement increases, which increases setting times. The difference in setting times between the plain cement paste and slag-cement paste is minimal, and is unlikely to affect construction requirements. 3.3.3. Fluidity tests The fluidity value of ordinary cement mortar is 63.2%, while those of slag-cement mortar with 20%, 30%, and 40% cement replacement are 56.4%, 51.3%, and 47.8%, respectively. The fluidity of slag-cement mortar is, therefore, lower than that of ordinary cement mortar, implying a slightly inferior workability in construction. 3.3.4. Compressive strength tests The compressive strength test results for the slag-cement and reference ordinary cement mortar specimens depicted in Fig. 2 and Table 5, respectively, are consistent with earlier reports (Lee et al., 2008). The early strengths (1 day and 3 day) of the slag-cement mortar specimens were lower (64–88%) than those of the reference specimen (OCM). As the ratio of substituted cement increased, the early strength decreased. In comparison to OCM, SCM strength increased by 78–87%, 91–103%, 105–111%, and 103–110% at 7, 28, 60, and 90 days, respectively. The 28 day strengths of all slag-cement mortars were similar to those of OCM. Further, the 60 day and 90 day strengths of all slag-cement mortar specimens, including the SCM(40%) specimen, were slightly stronger than the reference specimen. The SCM(30%) specimen exhibited the highest compressive strength, that is, approximately 10% greater than that of the reference specimen. The increased compressive strength of the slag-cement mortar at the later curing stage was related to the Pozzolanic reaction. Although a 40% substitution still results in adequate 60 to 90 day strengths, the early strengths are too low. Therefore, the optimal substitution is 30%. 3.3.5. Pore size distribution Fig. 3 depicts the pore size distributions of cement mortars revealed by the MIP test. In all samples, porosity was inversely related to hydration time due to the gradual filling of large pores by the hydration products of cement materials (Odler and Robler, 1985). The increased compressive strength of cement mortars as
Table 4 Setting times (minutes) of the OPC and SBC pastes incorporating MSWI ash-mix slag. Setting time
OCM
SCM (20%)
SCM (30%)
SCM (40%)
Initial Final
135 ± 10 216 ± 5
144 ± 10 226 ± 5
149 ± 10 230 ± 5
155 ± 10 233 ± 5
Table 3 TCLP leaching concentrations from the ash-mix and slag (mg/l). TCLP (mg/l)
Zn
Cr
Cu
Pb
Cd
Ash-mix Slag Regulatory limits
17.6 ± 0.30 0.6 ± 0.10 –
0.6 ± 0.02 0.4 ± 0.01 5.0
0.7 ± 0.02 0.1 ± 0.01 15.0
0.05 ± 0.01 ND 5.0
0.1 ± 0.01 ND 1.0
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OCM SCM(20%) SCM(30%) SCM(40%)
50
Compressive strength (MPa)
40
30
20
10
0 0
20
40
60
80
100
Curing times (days) Fig. 2. Evolution of compressive strength in slag-cement mortars.
Table 5 Compressive strengths of OCM and slag-cement mortar specimens (50 50 50 mm) after curing for the day shown. Notation of specimens
Compressive strength (Mpa) 1-day
3-day
7-day
14-day
28-day
60-day
90-day
OCM
15.99 (100%)
26.98 (100%)
34.43 (100%)
37.38 (100%)
39.04 (100%)
41.10 (100%)
42.47 (100%)
SCM(20%)
14.13 (88%)a
22.86 (85%)
30.12 (87%)
32.77 (88%)
38.85 (99%)
44.24 (108%)
45.42 (107%)
SCM(30%)
10.99 (69%)
23.54 (87%)
29.63 (86%)
34.73 (93%)
40.22 (103%)
45.81 (111%)
46.55 (110%)
SCM(40%)
10.20 (64%)
19.82 (73%)
26.88 (78%)
31.69 (85%)
35.61 (91%)
43.07 (105%)
43.75 (103%)
a
The number in parentheses denotes the percentage versus the value of OCM (100%) as a reference.
function of longer curing time (Fig. 2 and Table 5) was also observed to correspond well with the decrease in the total number of pores or capillary pores with longer curing time. 3.3.6. FT-IR analysis Fig. 4 depicts the FT-IR spectra of the reference and slag-cement mortar specimens cured for 7 days (a) and 28 days (b), respectively. Table 6 displays the corresponding bands. The absorption peak at 1070 cm1 is due to an asymmetric Si–O–Si stretching vibration. The peaks at 975 cm1 and 1207 cm1 are due to a colloidal C–S– H vibrational band, while the band at 3645 cm1 is due to calcium hydroxide bond absorption. The peak intensity of the FT-IR spectra roughly reflects the amount of the corresponding species in the ashsubstituted cement. The small peak intensities at 3645 cm1 in Fig. 4b are due to a less (as in SCM(20%)) or almost nonexistent Ca(OH)2 content (as in SCM(40%)). The consumption of Ca(OH)2 to form C– S–H and C–A–S–H gels is a typical Pozzolanic reaction (Chatterji et al., 1986; Lin et al., 2008). A comparison of evolving compressive strengths indicates that compressive strength after curing for 7 days was 78–87% of that of OCM. The 28 day strength of SCM was 91–103% of that of OCM. These findings correspond well with the observation of varying Ca(OH)2 intensity in FT-IR spectra. Therefore, when more slag is added, more Ca(OH)2 is consumed, which in turn, enhances the formation of hydrated C–A–S–H gels (at 690 cm1) arising from C–A–S–H vibration. Furthermore, the
absorption peak of the slag-cement mortar becomes more obvious than that of the referenced OCM specimen. Thus, in the SCM, more void-filling C–A–S–H gels are formed, which enhances the compressive strength of the mortar specimens. 3.3.7. DTA analysis Fig. 5 depicts DTA curves of the OCM (a) and SCM (b). The positions of the endothermic peaks for OCM and SCM are not significantly different, which means that these two hydration products are approximately the same. The endothermic peak of Ca(OH)2 in the slag-cement mortar diminishes in later stages (28 to 90 day), particularly in SCM(40%). This shows that the reaction between slag and Ca(OH)2 consumes Ca(OH)2 to form calcium aluminum oxide silicate hydrate (C–A–S–H) gels, which is a typical Pozzolanic reaction. Fig. 4 shows the similar observation noted in the FT-IR analysis. 3.4. Preliminary feasibility study of the slag-melting process This study evaluated the economic and environmental feasibility of the proposed process for vitrifying fly- and scrubber-ash with waste glass bottles. The process was compared with conventional cement production, since the vitrified slag is to be used to replace cement in the cement mortar. We assume that facility and area costs of the
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a
a
OCM
Transmittance (a.u)
Pore volume (ml/g)
7-day
1. C-A-S-H 2. C-S-H 3. Si-O-Si 4. Ca(OH)2
OCM
SCM(20%)
SCM(40%)
1
4
2 2 3
500
Curing Time (days)
1000
1500
2000
2500
3000
3500
4000
wavenumber (cm -1)
b SCM (40%)
b
1. C-A-S-H 2. C-S-H 3. Si-O-Si 4. Ca(OH)2
28-day
Transmittance (a.u)
Pore volume (ml/g)
OCM SCM(20%)
SCM(40%)
1
500
Curing Time (days) Total Pore
>50mm
2 3
1000
4
2
1500
<50mm
Fig. 3. Pore size distribution in cured ordinary cement and slag-cement mortars: (a) ordinary cement mortar (OCM); (b) SCM (40%).
production plants for cement and slag production is nearly equal. In fact, cement production is much more expensive than slag production due to the required huge plant area and extremely long rotating kiln. The energy required to heat soda-lime glasses to 1500–1600 °C (melting temperature) is reported to be 6050–6500 MJ/ton (Beerkens, 2004; Nemec, 1995). The lower limit of 6050 MJ/ton was used in this analysis, since melting the slag requires a temperature of 1400 °C, but does not require further refining. The energy required to produce cement is approximately 5400 MJ/ton (Jacott and Comunes, 2003). Thus, the energy required for slag production is 12% higher than that of cement production. Energy usually comprises 15–20% of the total cost of glass and cement production, while other costs include materials, handling, and labor. The proposed slag melting process has virtually no handling costs for mining and mineral dressing, wherein the only material cost is the glass-frit. The required glass-frit is best derived from waste glass, which has been estimated to be around 460 kilotons per year in Taiwan (Hou, 2003). The amount of glass-frit required in our process is around 130 kilotons per year (half of the ash-mix), or approximately 28% of the total waste glass. Thus, the amount of available waste glass supply would be more than adequate for our process. In comparison with cement price (about US$91 per ton), the cost of waste glass is roughly 10%, which is minor compared to the costs of mineral and mineral dressing in cement production. The much lower material cost compensates for the increased energy consumption. From a commercial perspective, slag melting to produce cement replacement should be as
2000
2500
3000
3500
4000
wavenumber (cm-1) Fig. 4. FT-IR spectra of cured plain cement and slag-cement mortar specimens: (a) cured for 7 days; (b) cured for 28 days.
Table 6 Fourier transformed infrared spectra data. Band assignments
Asymmetric Stretching vibration (cm1)
Si–O–Si C–S–H Ca, OH C–A–S–H CO2 3 CO2
1070a
a b c d
Adopted Adopted Adopted Adopted
Stretching Vibration (cm1) 975b, 1172–1207c 3645d
690c 1400–1500 2400–2500 from from from from
(Miyata et al., 2002). (Mollah et al., 1998). (Puertas et al., 2000). (Mollah et al., 1995).
profitable as cement production. In fact, in order to immobilize and treat fly- and scrubber-ash for a landfill, city governments in Taiwan pay approximately US$120 per ton of ash-mix. If city governments subsidized the slag melting industry, the potential profit could be huge. As a whole, we can say that slag production is cost effective, and potentially quite profitable if government subsidized. Cement production requires the excavation of limestone and other minerals from open pits, which has a significant environmental impact. Air pollution from the rotary kilns that produce cement is an additional problem. In the vitrification of modified ash-mix, there are no mining processes, while there is no air pollution if the ventilation of the scrubbing process is taken into consideration. By replacing approximately 30% of cement with vitrified
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a
7-day
OCM
14-day 28-day
ΔT/mg
60-day 90-day
Ca(OH)2
0
200
400
600
800
1000
o
Temperature ( C)
b
7-day 14-day
SCM(40%)
28-day 60-day
ΔT/mg
90-day
cured for 1 to 90 days. The compositions, structures, properties, and compressive strengths of OCM and SCM as a function of time were carefully studied. The developed slag-cement mortar (SCM) exhibits a slightly less fluidity compared to ordinary cement mortar (OCM). The initial and final setting times of the slag-cement paste were 9–20 min and 10–17 min, respectively, longer than those of plain cement paste. All slag powder passed the TCLP test in accordance with EPA regulations. The early stage compressive strengths of SCM were clearly 64– 88% lower than that of OCM. The 28 day strengths approximated those of the OCM; however, the 60 to 90 day strengths exceeded those of OCM, which demonstrates the feasibility of substantial cement substitution with slag powder. The MIP test revealed that the increased compressive strength of SCM as a function of longer curing time corresponded well with the decreased total number of pores as a function longer curing time. This preliminary feasibility study demonstrates that the mass production of slag from ash-mix and waste glass-frit is cost effective, mass-producible, and has a minimal environmental impact. This research reveals new possibilities for the safe and complete treatment, as well as utilization, of MSWI fly- and scrubber-ash that would otherwise be disposed of in groundwater-contaminating landfills. Acknowledgments
Ca(OH)2
The authors would like to thank the National Science Council of the Republic of China, Taiwan for supporting this research under Contract No. NSC-97-2815-C-239-009-E. 0
200
400
600
800
1000
o
Temperature ( C)
References
Fig. 5. DTA curve of the (a) OCM and (b) SCM cured for 7–90 days.
slag, air pollution and the other environmental impacts of cement production can be dramatically reduced. In addition, if all of the available ash-mix was used as a construction resource instead of being disposed in landfills, its hazard to ground water would be avoided. For these reasons, replacing cement with vitrified ashmix is environmentally and economically plausible. There is one trial pilot plant that is currently capable of vitrifying fly-ash at a rate of 1 ton of slag per hour (Haugsten and Gustavson, 2000). Given the approximate 260 kilotons (e.g., 2007) of fly- and scrubber-ash produced annually in Taiwan, 30 such pilot plants would be sufficient to consume this available ash waste. The number of plants can be reduced as the technology matures and output increases. Restated, mass treatment of all fly- and scrubber-ash is technically feasible. We preliminarily conclude that the vitrification process is cost effective and technologically feasible, as well as environmentally and economically sound. 4. Conclusions This study attempted to fully utilize the fly- and scrubber-ash produced from an MSWI as a useful resource. Fly- and scrubberash were mixed at a weight ratio of 1:3, their normal production ratio, to form an ‘‘ash-mix”. The ash-mix was blended with waste glass-frit at a weight ratio of around 1:0.5, and then melted at 1400 °C. The melt was quenched in water to form slag, which was then pulverized into a powder consisting of granules smaller than 38 lm. Cement mortar samples were prepared by replacing 20–40% of the cement content with slag powder, which were then
Al-Rawas, A.A., Hago, A.W., Taha, R., Al-Kharousi, K., 2005. Use of incinerator ash as replacement for cement and sand in cement mortars. Building and Environment 40, 1261–1266. Aubert, J.E., Husson, B., Sarramone, N., 2007. Utilization of municipal solid waste incineration (MSWI) fly-ash in blended cement Part 2. Mechanical strength of mortars and environmental impact. Journal of Hazardous Materials 146, 12– 19. Beerkens, R.G.C., 2004. Modular melting part 1. Industrial glassmelting process analysis. American Ceramic Society Bulletin 83 (4), 28–32. Chatterji, S., Jensen, A.D., Thaulow, N., Christemsens, p., 1986. Studies of Alkali-Silica reaction part 3 mechanisms by which NaCl and Ca(OH)2 affect the reaction. Cement and Concrete Research 16, 246–254. Haugsten, Kjell E., Gustavson, B., 2000. Environmental properties of vitrified fly ash from hazardous and municipal waste incineration. Waste Management 20, 167–176. Hou, P.C., 2003. Reuse of waste glass powder for substitution of fine aggregate in the recycling asphalt concrete. Master Thesis, National Yunlin University of Science and Technology, Taiwan, Republic of China. Jacott, M., Comunes, F., 2003. Energy use in the cement industry in North America: Emissions, Waste Generation and Pollution Control, 1990–2001’ Second North American Symposium on Assessing the Environmental Effects of Trade, May 30. Lee, T.C., Wang, W.J., Shih, P.Y., 2008. Slag-cement mortar made with cement and slag vitrified from MSWI fly-ash/scrubber-ash and glass frit. Construction and Building Materials 22, 1914–1921. Lin, K.L., Wang, K.S., Tzeng, B.Y., Lin, C.Y., 2003. The reuse of municipal solid waste incinerator fly-ash slag as a cement substitute. Resources Conservation and Recycling 39, 315–324. Lin, K.L., Wang, K.S., Lin, C.Y., Lin, C.H., 2004. The hydration properties of pastes containing municipal solid waste incinerator fly-ash slag. Journal of Hazardous Materials B109, 173–181. Lin, K.L., Chang, C.T., 2006. Leaching characteristics of slag from the melting treatment of municipal solid waste incinerator ash. Journal of Hazardous Materials B135, 296–302. Lin, K.L., Lin, D.F., 2006. Hydration characteristics of municipal solid waste incinerator bottom ash slag as a pozzolanic material for use in cement. Cement and Concrete Composites 28, 817–823. Lin, K.L., Chang, W.C., Lin, D.F., 2008. Pozzolanic characteristics of pulverized incinerator bottom ash slag. Construction and Building Materials 22, 324–329. Miyata, N., Fuke, K.-i., Chen, Qi., Kawashita, M., Kokubo, T., Nakamura, T., 2002. Apatite-forming ability and mechanical properties of PTMO-modified CaO–SiO2
T.-C. Lee, M.-K. Rao / Waste Management 29 (2009) 1952–1959 hybrids prepared by sol–gel processing: effect of CaO and PTMO contents. Biomaterials 23, 3033–3040. Mollah, M.Y.A., Lu, F., Cocke, D.L., 1998. An X-ray diffraction (XRD) and Fourier transform infrared spectroscopic (FT-IR) characterization of the speciation of arsenic (V) in Portland cement type-V. The Science of the Total Environment 224, 57–68. Mollah, M.Y.A., Palta, P., Hess, T.R., Vempati, R.K., Cocke, D.L., 1995. Chemical and physical effects of sodium lignosulfonate superplasticizer on the hydration of Portland cement and soldifcation/stabilization consequences. Cement and Concrete Research 25 (3), 671–682. Mulder, E., 1996. Pre-treament of MSWI fly-ash for useful application. Waste Management 16 (1–3), 181–184. Nemec, L., 1995. Energy consumption in the glass melting process. II: Results of calculations. Glass Science and Technology 68, 39–50. Nishigaki, M., 2000. Producing permeable blocks and pavement bricks from molten slag. Waste Management 20, 185–192. Odler, I., Robler, M., 1985. Investigations on the relationship between porosity, structure and strength of hydrated Portland cement paste (II). Effect of pore structure and of degree of hydration. Cement and Concrete Research 15, 401– 410.
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Puertas, F., Martinez-Ramirez, S., Alonso, S., Vazquez, T., 2000. Alkali-activated flyash/slag-cement strength behavior and hydration products. Cement and Concrete Research 30, 1625–1632. Shao, Y., Lefort, T., Moras, S., Rodriguez, D., 2000. Studies on concrete containing ground waste glass. Cement and Concrete Research 30, 91–100. Sreenivasarao, K., Warren, G.W., Mckinley, M.D., Gao, G., 1997. Hydometallurgical treatment of municipal solid waste fly-ash for simultaneous detoxification and metal recovery. Journal of Environment Science Health A 32 (4), 1225– 1245. Svensson, M., Berg, M., Ifwer, K., Sjoblom, R., Ecke, H., 2007. The effect of isosaccharinic acid (ISA) on the mobilization of metals in municipal solid waste incineration (MSWI) dry scrubber residue. Journal of Hazardous Materials 144, 477–484. Van Herck, P., Van der Bruggen, B., Vogels, G., Vandecasteele, C., 2000. Application of computer modelling to predict the leaching behavior of heavy metals from MSWI fly-ash and comparison with a sequential extraction method. Waste Management 20, 203–210. Wang, K.S., Lin, K.L., Tzeng, B.Y., 2003. Latent hydraulic reactivity of blended cement incorporating slag made from municipal solid waste incinerator fly-ash. Journal of Air and Waste Management Association 53, 1340–1346.
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Recycling municipal incinerator fly- and scrubber-ash into fused slag for the substantial replacement of cement in cement-mortars Tzen-Chin Lee *, Ming-Kang Rao Department of Civil and Disaster Prevention Engineering, National United University, Miao-Li, 360, Taiwan, ROC
a r t i c l e
i n f o
Article history: Accepted 6 January 2009 Available online 11 February 2009
a b s t r a c t Fly- and scrubber-ash (weight ratio of approximately 1:3) from municipal solid waste incinerators (MSWI) are a major land-fill disposal problem due to their leaching of heavy metals. We uniformly mixed both types of ash with optimal amounts of waste glass frit, which was then melted into a glassy slag. The glassy slag was then pulverized to a particle size smaller than 38 lm for use as a cement substitute (20– 40% of total cement) and blended with sand and cement to produce slag-blended cement-mortar (SCM) specimens. The toxicity characteristics of the leaching procedure tests on the pulverized slag samples revealed that the amount of leached heavy metals was far below regulatory thresholds. The compressive strength of the 28-day cured SCM specimens was comparable to that of ordinary Portland cement mortars, while the compressive strength of specimens cured for 60 or 90 days were 3–11% greater. The observed enhanced strength is achieved by Pozzolanic reaction. Preliminary evaluation shows that the combination of MSWI fly- and scrubber-ash with waste glass yields a cost effective and environmentally friendly cement replacement in cement-mortars. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Municipal solid waste in Taiwan and in many other industrialized countries is normally burned in incinerators. The fly- and scrubber-ash wastes produced from municipal solid waste incinerators (MSWI) usually contain many hazardous heavy metals. The ash is usually placed in a landfill after solidification with 20 wt.% cement and the addition of chelating agents; however, toxic components can leach from landfill solids and pollute groundwater (Van Herck et al., 2000; Svensson et al., 2007), emit unpleasant odors, and contaminate soil. Consequently, the potential hazards of long term landfill leaching are a persistent problem. In addition, a shortage of landfill sites is a serious problem in populated regions (as is the case in Taiwan). Therefore, recycling MSWI fly- and scrubber-ash as construction resources could be a potential way of alleviating the problem. MSWI fly-ash has been studied for use in many applications. For instance, MSWI fly-ash (after a washing treatment) can be used as a secondary raw material in road construction (Mulder, 1996). Water-permeable pavement blocks can be formed from the molten slag of MSWI fly-ash (Nishigaki, 2000). The effectiveness of different ratios of MSWI fly-ash to sand or cement in cement mortars has also been investigated (Al-Rawas et al., 2005). The experimental results of these studies indicate that MSWI fly-ash blended with
* Corresponding author. Tel.: +886 37 381668; fax: +886 37 326567. E-mail address: [email protected] (T.-C. Lee). 0956-053X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2009.01.002
cement mortar reduces slump values when it is used as a replacement for sand; in contrast, it has an opposite trend when used as a replacement for cement. The 28-day compressive strength of the latter is similar to or slightly higher than that of cement mix (0% MSWI fly-ash); however, the direct blending of MWSI fly-ash into cement mortars exhibits a poor stabilization of some harmful elements, such as antimony and chromium (Aubert et al., 2007). The output from an MSWI is usually 25% fly-ash and 75% scrubber-ash. Scrubber-ash is collected from a ventilation bag filter and contains very fine solid ash, chlorides, and some organic compounds. Due to its extreme acidity, the scrubber-ash must be neutralized by the addition of substantial amounts of calcium hydroxide and activated carbon. The scrubber-ash is thus very difficult to vitrify (melt into a glassy phase) for further application, unlike its counterpart fly-ash. The complete simultaneous recycling of both MSWI fly-ash and scrubber-ash from a single incinerator is difficult, and has not yet to be reported. Fly-ash can be vitrified (made glassy) by melting, and heavy metals in the fly-ash can be immobilized in the amorphous Si– O–Si network to form highly stable slag; hence, these toxic metals do not easily leach out (Sreenivasarao et al., 1997; Lin et al., 2003, 2004). The impact of cement replacement with slag (prepared by melting MSWI bottom-ash) on the Pozzolanic reactivity of slagblended cement-paste has been investigated (Lin and Lin, 2006). Heavy metals are immobilized in the vitrified product. In early curing stages, the strength of slag-blended cement-pastes is lower than that of ordinary cement-paste; however, at later ones, their strength is comparable to that of ordinary cement-paste.
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To the best of our knowledge, the treatment of scrubber-ash by vitrification into a glassy slag has not been proposed. The weight ratio of fly-ash to scrubber-ash produced by a typical MSWI is 1:3, which is the daily output ratio of the MSWI in the city of Hsinchu, Taiwan. Earlier trials by the authors demonstrated that the vitrification of fly-ash by simple melting in a crucible at 1200 °C is not problematic, but becomes more difficult when scrubberash is added to the fly-ash at weight ratios of 1:3 to 1:1. In order to overcome this difficulty, this study proposes the addition of pulverized waste glass bottles (glass-frit) to enhance vitrification. Earlier trials have examined the melting characteristics of the ash-mix after adding different amounts of glass-frit (Lee et al., 2008). The intent of this study was to identify the maximum ratio of optimally vitrified ash-mix slag at which the cement-mortar maintains an acceptable compressive strength after prolonged curing.
value (Lee et al., 2008). Ideally, the ratio of glass frit to fill in the ash-mix should be one-half. In this study, this mixture is referred to as ‘‘modified ash-mix”. The modified ash-mix was then vitrified at 1400 °C in an electrical oven for at least half an hour until it reached a molten state. The mix was then quenched in water to form the slag. The slag was pulverized and used as a cement replacement (20–40% by weight) in cement-mortar. The slag-cement mortar specimens were tested for compressive strength after curing for 1 to 90 days. 2.2. Cement mortar specimen: molding and curing
Pozzolanic materials typically have a high silica content, large surface area, and are amorphous to X-ray diffraction. According to ASTM C618, materials with granule sizes larger than 150 lm cannot be qualified as Pozzolanic, while those with granule sizes on the order of 75 lm (sieve #200) are marginal, depending on the proportion of granules smaller than 45 lm (Shao et al., 2000). Thus, in this study, ash-mix was combined with glass frit, melted to slag, and then ground to a powder with a granule size smaller than 38 lm (sieve #400). The materials and methods were as follows.
The mortar specimens (50 50 50 mm) were molded according to the Taiwan CNS 1010 testing specifications to test compressive strength. A water-to-cement weight ratio (W/C) of 0.485 and sand-to-cement weight ratio of 2.75 were used. Reference specimens consisting of ordinary type I Portland cement, reference sand, and water were designated ‘‘OCM” (ordinary cement mortar) and used as a standard for comparing compressive strength to slag-blended cement mortar with 20%, 30%, and 40% substitutions of cement, which were designated as SCM(20%), SCM(30%), and SCM(40%), respectively. Before testing for compressive strength, all specimens were placed in a chamber with a programmable temperature and humidity for one day, and then de-molded and cured in a saturated calcium hydroxide solution chamber maintained at 23.0 ± 1.7 °C for 1 to 90 days. Table 2 summarizes the constituent contents of the OCM and SCM specimens.
2.1. Materials
2.3. Characterization methods of the slag-cement mortar specimens
The fly-ash used in this investigation was collected from cyclones of the Hsinchu MSWI in northwest Taiwan. The ash is yellowish-light gray, and has a specific gravity of 2.91. The scrubber-ash collected from the bag filters of the Hsinchu MSWI is grayish-white, and has a specific gravity of 2.62. The cement used in this research was ordinary Portland cement (OPC), Type I, manufactured by the Taiwan Cement Company. This cement has a specific gravity of 3.15, while its physical and chemical properties meet ASTM C150 specifications. The reference sand was Ottawa-type sand that adhered to ASTM C778 specifications, and had a specific gravity of 2.63. The fine glass-frit, which had a specific gravity of 2.40, was ground from discarded dark green beer bottles. Fly- and scrubber-ash from the Hisnchu city MSWI were mixed according to the production weight ratio of 1:3. This mixture is referred to as ‘‘ash-mix” in this study. Table 1 summarizes the CaO and SiO2 contents in the ash-mix and glass-frit. The ash-mix was further modified by adding different amounts of glass-frit. The amount of added glass-frit was based on the analyzed compositions in Table 1. The CaO to SiO2 ratio was 1.1, which is the optimal
No pretreatment, other than drying, was performed before the scanning electron microscope (SEM) was used to study the flyash/scrubber-ash and slag powder. The cured slag-cement mortar specimens were cut and oven-dried at 100 °C for 24 h, and then cooled in a desiccator. The sample was then fixed on a sample holder and examined with an SEM (JEOL, JSM-5600, Japan) after a gold coating was applied. An energy-dispersive X-ray spectrometer (EDS, Oxford, 6587, UK) attached to the SEM was used to analyze the chemical composition of the cement, fly-ash, scrubber-ash, ash-mix, glass-frit, and slag. Powder X-ray diffraction (XRD) analysis was performed using an X-ray diffractometer (Rigaku, D/Max-2200, Japan) with Cu Ka1 radiation and 2h scanning ranging from 10° to 70°. The XRD scans were run at 0.05° per step with a counting time of one second. The leaching of ash-mix and slag was then analyzed by TCLP testing according to ‘‘Toxicity Characteristic Leaching Procedures (TCLP): SW846-1311 (USA).” The solid waste samples were crushed and leached by acetic acid. The extraction procedure required a preliminary pH analysis of the sample to determine the proper extraction fluid for the experiment. After testing, the
2. Materials and methods
Table 1 Chemical composition of cement, MSWI fly-ash, scrubber-ash, ash-mix, glass frit and modified slag as analyzed by EDS. Chemical composition (wt.%)
Cement
Fly-ash
Scrubber-ash
Ash-mix
Glass-frit
Modified slag
Na (as Na2O) Mg (as MgO) Al (as Al2O3) Si (as SiO2) SO3 Cl K (as K2O) Ca (as CaO) Fe (as Fe2O3) Total sum
– 2.8 5.7 22.3 – – – 62.1 3.4 96.3
5.9 1.7 – 12.8 17.6 5.4 2.7 40.0 1.0 87.1
2.9 1.3 6.5 2.5 4.1 29.0 2.4 40.4 0.5 89.6
3.1 0.9 5.8 6.1 4.5 26.6 3.6 40.5 0.5 91.6
10.7 1.2 3.4 71.8 – – 1.1 10.4 – 98.6
1.7 1.6 16.1 35.1 – 0.1 – 38.7 2.4 95.7
Remarks: minor constituents Cu (as CuO) 0–4.0, Pb (as PbO) 0–1.8, and Zn (as ZnO) 0–0.1 in different analyses.
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Table 2 The constituent contents of ordinary cement mortar (OCM) and slag-blended cement mortar (SCM) specimens (proportion weights per nine specimens). Mix design
Cement (g) Sand (g) Modified slag (g)a Water (g)
Notation of mortar specimens OCMb
SCMc (20%)
SCM(30%)
SCM(40%)
740 2035 0 359
592 2035 148 359
518 2035 222 359
444 2035 296 359
a
Particle size of slag powder <#400 (38 lm). Ordinary Portland cement mortar. Slag-cement mortar, wherein the percentage in the bracket is the cement replacement ratio. b
c
extraction fluid selected for TCLP analysis (pH 2.88 ± 0.05) was prepared by adding 5.7 mL of glacier acetic acid to 500 mL of double distilled water, and then diluted to a volume of 1 L. The 25 g samples were placed in 1 L Erlenmeyer flasks, wherein 500 mL of extraction fluid was added to each flask. The flasks were then agitated for 18 h with an electric vibrator. The slurry was filtered using ‘‘Millipore filter paper” with a 0.6 to 0.8 lm pore size. The leachates were preserved in 2% HNO3 (Wang et al., 2003; Lin and Chang, 2006). Heavy metal concentrations from the leachates of the ash-mix and slag samples were confirmed by inductivelycoupled plasma atomic-emission spectroscopy (ICP-AES). The Pozzolanic activity index of the slag-cement mortar specimens was analyzed according to ASTM C311-98a. The setting time was tested according to ASTM C187 to determine the optimal water quantity for the cement. The Vicat needle method was then used to determine the setting time for the cement paste, according to ASTM C191, to obtain the initial and final setting times of the cement paste. The fluidity test was performed according to ASTM C230. The OCM and SCM mortar-cubes were prepared according to ASTM C109 and were cured for 1 to 90 days. At each curing age, the mortar cubes were tested for compressive strength. The samples were soaked in methyl alcohol (methanol) after the compressive strength tests. Hydration reactions were terminated due to the expelling of water out from the specimens. Thus, the degree of hydration due to different curing times could be preserved. The samples were then analyzed by mercury intrusion porosimetry (MIP), XRD, Fourier transformed infrared spectroscopy (FT-IR), and differential thermal analysis (DTA). The hydration-terminated slag-blended cement mortar specimens (as preserved in methanol) were removed, oven-dried at 100 °C for 24 h, and then cooled in a desiccator. An MIP facility (PMI, Automated porosimeter, Model: 60 K-A-2, USA) was then used to analyze the samples for porosity and pore size distribution. The chemical bonding characteristics of SCM were analyzed by FT-IR (Jasco FT/IR-300E, Japan). For this test, samples were removed from the methanol and dried in a vacuum oven at 100 °C for 24 h, and then cooled in a desiccator. The samples were then ground and uniformly mixed with potassium bromide (KBr) at a KBr/specimen weight ratio of 100/1. A 0.2 g quantity of each mixture was pressed into a 13 mm diameter disk for analysis. The measurement range was between 500 and 4000 cm1. The thermal behavior of SCM specimens was analyzed using differential thermal analysis (DTA, Perkin Elmer DTA7 Analyzer, USA). Each specimen was ground to a particle size smaller than 44 lm (
3. Results and discussion In order to maintain the CaO and SiO2 weight ratio was 1.1, the amount of 49.3 g of glass-frit mixed with 100 g ash-mix was added to ensure it melting (vitrifying) into glassy slag (Lee et al., 2008). The modified ash-mix readily melted at 1400 °C to form a homogeneous melt, which was quenched in water. The slag was green, irregularly shaped, and exhibited a specific gravity of 2.81. The slag samples were then ground into a powder that could pass through a No. 400 sieve (<38 lm, specific surface 6400 cm2/g) to meet the fineness requirements of Pozzolanic materials. 3.1. Analyses of raw materials Table 1 summarizes the composition of the cement, fly-ash, scrubber-ash, ash-mix, glass-frit, and slag, according to the EDS analysis. In comparison to the fly-ash, the scrubber-ash contained less silica, but much more chlorine. The glass-frit is essentially a typical soda-lime glass. Fig. 1 depicts the X-ray diffractogram of the ash-mix. The identified phases, in decreasing intensity, were CaSO4, CaCl2, Ca(OH)2, SiO2, NaCl, KCl, CaClOH, PbO, and ZnS. According to this diffractogram, the large amount of post-added Ca(OH)2 into the filter-collected ash reacted to form various calcium baring inorganic compounds. 3.2. Characterization of the slag 3.2.1. Chemical compositions A small quantity of fly-ash, scrubber-ash, ash-mix, glass-frit, and slag powder was prepared for SEM analysis. The fly-ash, scrubberash, and ash-mix consisted of agglomerates of much smaller particles, while the slag consisted of fused granules smaller than 38 lm. Table 1 summarizes the chemical composition of the slag according to EDS analysis. The slag consisted of the non-CaO constituents SiO2, Al2O3, and Fe2O3, in percentages of 35.1%, 16.1%, and 2.4% by weight, respectively. The sum of these three compositions equaled 53.6%, which meets the requirements of grade C fly-ash according to ASTM C618. Non-calcium oxides were observed to be the primary constituents that are responsible for the Pozzolanic reaction. One concern of the vitrification process is that, at a melting temperature of 1400 °C, some elements may evaporate (in particular, Cd and Zn, whose boiling point is lower than 1400 °C). Assuming there is some elemental mass loss due to evaporation, a significant concern involves the environmental benefit of the
1. KCl; 2. CaSO4; 3. CaCl2; 4. NaCl; 5. CaClOH; 6. Ca(OH)2; 7. PbO; 8. ZnS; 9. SiO2
slag 2
Intensity (a. u.)
1954
2,3
1,7 3,8
2
4 5,6 9
6
1,2
ash-mix
20
25
30
35
40
45 2θ
50
55
60
Fig. 1. X-ray diffractogram of slag and ash-mix.
65
70
1955
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entire vitrification process. In fact, glass melting is complicated due to the thermodynamics of a mixed-oxides-melt, in addition to the 4 bonding effect arising from the silicate unit, ðSiO4 Þn , in a glassy network, even in a molten state. The silicate unit in a long-chain glassy structure provides traps, and, in addition, tightly bonds with cations (monovalent, divalent, and trivalent). The extent of vaporization is thus significantly minimized. This phenomenon is why soda-lime (window) glasses can be manufactured after melting for a long time at 1600 °C, despite that Na2O boils at 400 °C. We believe that a small amount of evaporation does occur, but the gaseous species can be captured by a simple scrubbing system and recycled as useful resources, such as ZnO. This is certainly much safer than the uncontrolled dissolution hazard of heavy metals to underground water in the case of landfills filled with ash-mix, as is the current disposal method. According to Table 1, the analyzed alumina content of the slag, 16.1%, is much higher than that in the ash-mix 5.8%. This arise content from alumina dissolution of the alumina crucible was used. Since alumina content is crucial to satisfying the requirement of grade C fly-ash in ASTM C618, the addition of extra alumina is required as an extra component if the melting process does not pick up alumina from the crucible, such as in field-scale production. The most cost effective (free) supply of alumina can be obtained from the industrial waste of water treatment or/and aluminum industries. 3.2.2. TCLP analysis All of the investigated fly-ash, scrubber-ash, and ash-mix contained the hazardous heavy metals Pb, Zn, Cu, Cd, and Cr, which can easily leach out to pollute groundwater. As Table 3 shows, the TCLP leaching concentrations of heavy metals were far below regulatory thresholds. A glass structure is composed of two major constituents: A network-former and a modifier. The network for4 mer is ðSiO4 Þn , while the modifiers are the oxides of monovalent, divalent, or trivalent cations. The chemical bonding between the network former and modifiers is usually quite strong, depending on the valence state of the modifiers. As the valence increase, the bond strength also increases. Heavy metals, which usually have two to three valence states, can therefore bond strongly to the glass network and become immobilized (Sreenivasarao et al., 1997; Lin et al., 2003; Lin and Chang, 2006). The XRD analysis of slag (Fig. 1) revealed that it is an amorphous glassy substance without diffraction peaks, which was consistent with an earlier study (Lin et al., 2003). 3.3. Analysis of slag-cement paste and mortar 3.3.1. Pozzolanic activity index In SCM(20%), the slag-cement mortar with 20% of the cement replaced by slag, the slag was composed of SiO2 (35.1 wt.%), CaO (38.7 wt.%), and Al2O3 (16.1 wt.%), such that the Pozzolanic activity index was 99.3. This Pozzolanic activity index elucidates the suitability of the slag Pozzolanic reaction for partial cement replacement. 3.3.2. Tests of initial and final setting Table 4 depicts the setting test results. The initial setting time for ordinary cement mortar is 135 min, while the final setting time
is 216 min. In this study, the initial and final setting times of the slag cement mortars were 9–20 min and 10–17 min longer than plain cement paste, respectively. As more cement was replaced with slag powder the longer the initial and final setting times, although the increase in time was relatively small (increases of 15% and 8%, respectively). The observed increase in the initial and final settings times is due to the fact that water-quenched glassy slag itself is not a binding material, and instead has inert characteristics. The rate at which a coagulant structural network forms from residual cement due to hydration slows as the amount of replacement cement increases, which increases setting times. The difference in setting times between the plain cement paste and slag-cement paste is minimal, and is unlikely to affect construction requirements. 3.3.3. Fluidity tests The fluidity value of ordinary cement mortar is 63.2%, while those of slag-cement mortar with 20%, 30%, and 40% cement replacement are 56.4%, 51.3%, and 47.8%, respectively. The fluidity of slag-cement mortar is, therefore, lower than that of ordinary cement mortar, implying a slightly inferior workability in construction. 3.3.4. Compressive strength tests The compressive strength test results for the slag-cement and reference ordinary cement mortar specimens depicted in Fig. 2 and Table 5, respectively, are consistent with earlier reports (Lee et al., 2008). The early strengths (1 day and 3 day) of the slag-cement mortar specimens were lower (64–88%) than those of the reference specimen (OCM). As the ratio of substituted cement increased, the early strength decreased. In comparison to OCM, SCM strength increased by 78–87%, 91–103%, 105–111%, and 103–110% at 7, 28, 60, and 90 days, respectively. The 28 day strengths of all slag-cement mortars were similar to those of OCM. Further, the 60 day and 90 day strengths of all slag-cement mortar specimens, including the SCM(40%) specimen, were slightly stronger than the reference specimen. The SCM(30%) specimen exhibited the highest compressive strength, that is, approximately 10% greater than that of the reference specimen. The increased compressive strength of the slag-cement mortar at the later curing stage was related to the Pozzolanic reaction. Although a 40% substitution still results in adequate 60 to 90 day strengths, the early strengths are too low. Therefore, the optimal substitution is 30%. 3.3.5. Pore size distribution Fig. 3 depicts the pore size distributions of cement mortars revealed by the MIP test. In all samples, porosity was inversely related to hydration time due to the gradual filling of large pores by the hydration products of cement materials (Odler and Robler, 1985). The increased compressive strength of cement mortars as
Table 4 Setting times (minutes) of the OPC and SBC pastes incorporating MSWI ash-mix slag. Setting time
OCM
SCM (20%)
SCM (30%)
SCM (40%)
Initial Final
135 ± 10 216 ± 5
144 ± 10 226 ± 5
149 ± 10 230 ± 5
155 ± 10 233 ± 5
Table 3 TCLP leaching concentrations from the ash-mix and slag (mg/l). TCLP (mg/l)
Zn
Cr
Cu
Pb
Cd
Ash-mix Slag Regulatory limits
17.6 ± 0.30 0.6 ± 0.10 –
0.6 ± 0.02 0.4 ± 0.01 5.0
0.7 ± 0.02 0.1 ± 0.01 15.0
0.05 ± 0.01 ND 5.0
0.1 ± 0.01 ND 1.0
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OCM SCM(20%) SCM(30%) SCM(40%)
50
Compressive strength (MPa)
40
30
20
10
0 0
20
40
60
80
100
Curing times (days) Fig. 2. Evolution of compressive strength in slag-cement mortars.
Table 5 Compressive strengths of OCM and slag-cement mortar specimens (50 50 50 mm) after curing for the day shown. Notation of specimens
Compressive strength (Mpa) 1-day
3-day
7-day
14-day
28-day
60-day
90-day
OCM
15.99 (100%)
26.98 (100%)
34.43 (100%)
37.38 (100%)
39.04 (100%)
41.10 (100%)
42.47 (100%)
SCM(20%)
14.13 (88%)a
22.86 (85%)
30.12 (87%)
32.77 (88%)
38.85 (99%)
44.24 (108%)
45.42 (107%)
SCM(30%)
10.99 (69%)
23.54 (87%)
29.63 (86%)
34.73 (93%)
40.22 (103%)
45.81 (111%)
46.55 (110%)
SCM(40%)
10.20 (64%)
19.82 (73%)
26.88 (78%)
31.69 (85%)
35.61 (91%)
43.07 (105%)
43.75 (103%)
a
The number in parentheses denotes the percentage versus the value of OCM (100%) as a reference.
function of longer curing time (Fig. 2 and Table 5) was also observed to correspond well with the decrease in the total number of pores or capillary pores with longer curing time. 3.3.6. FT-IR analysis Fig. 4 depicts the FT-IR spectra of the reference and slag-cement mortar specimens cured for 7 days (a) and 28 days (b), respectively. Table 6 displays the corresponding bands. The absorption peak at 1070 cm1 is due to an asymmetric Si–O–Si stretching vibration. The peaks at 975 cm1 and 1207 cm1 are due to a colloidal C–S– H vibrational band, while the band at 3645 cm1 is due to calcium hydroxide bond absorption. The peak intensity of the FT-IR spectra roughly reflects the amount of the corresponding species in the ashsubstituted cement. The small peak intensities at 3645 cm1 in Fig. 4b are due to a less (as in SCM(20%)) or almost nonexistent Ca(OH)2 content (as in SCM(40%)). The consumption of Ca(OH)2 to form C– S–H and C–A–S–H gels is a typical Pozzolanic reaction (Chatterji et al., 1986; Lin et al., 2008). A comparison of evolving compressive strengths indicates that compressive strength after curing for 7 days was 78–87% of that of OCM. The 28 day strength of SCM was 91–103% of that of OCM. These findings correspond well with the observation of varying Ca(OH)2 intensity in FT-IR spectra. Therefore, when more slag is added, more Ca(OH)2 is consumed, which in turn, enhances the formation of hydrated C–A–S–H gels (at 690 cm1) arising from C–A–S–H vibration. Furthermore, the
absorption peak of the slag-cement mortar becomes more obvious than that of the referenced OCM specimen. Thus, in the SCM, more void-filling C–A–S–H gels are formed, which enhances the compressive strength of the mortar specimens. 3.3.7. DTA analysis Fig. 5 depicts DTA curves of the OCM (a) and SCM (b). The positions of the endothermic peaks for OCM and SCM are not significantly different, which means that these two hydration products are approximately the same. The endothermic peak of Ca(OH)2 in the slag-cement mortar diminishes in later stages (28 to 90 day), particularly in SCM(40%). This shows that the reaction between slag and Ca(OH)2 consumes Ca(OH)2 to form calcium aluminum oxide silicate hydrate (C–A–S–H) gels, which is a typical Pozzolanic reaction. Fig. 4 shows the similar observation noted in the FT-IR analysis. 3.4. Preliminary feasibility study of the slag-melting process This study evaluated the economic and environmental feasibility of the proposed process for vitrifying fly- and scrubber-ash with waste glass bottles. The process was compared with conventional cement production, since the vitrified slag is to be used to replace cement in the cement mortar. We assume that facility and area costs of the
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a
a
OCM
Transmittance (a.u)
Pore volume (ml/g)
7-day
1. C-A-S-H 2. C-S-H 3. Si-O-Si 4. Ca(OH)2
OCM
SCM(20%)
SCM(40%)
1
4
2 2 3
500
Curing Time (days)
1000
1500
2000
2500
3000
3500
4000
wavenumber (cm -1)
b SCM (40%)
b
1. C-A-S-H 2. C-S-H 3. Si-O-Si 4. Ca(OH)2
28-day
Transmittance (a.u)
Pore volume (ml/g)
OCM SCM(20%)
SCM(40%)
1
500
Curing Time (days) Total Pore
>50mm
2 3
1000
4
2
1500
<50mm
Fig. 3. Pore size distribution in cured ordinary cement and slag-cement mortars: (a) ordinary cement mortar (OCM); (b) SCM (40%).
production plants for cement and slag production is nearly equal. In fact, cement production is much more expensive than slag production due to the required huge plant area and extremely long rotating kiln. The energy required to heat soda-lime glasses to 1500–1600 °C (melting temperature) is reported to be 6050–6500 MJ/ton (Beerkens, 2004; Nemec, 1995). The lower limit of 6050 MJ/ton was used in this analysis, since melting the slag requires a temperature of 1400 °C, but does not require further refining. The energy required to produce cement is approximately 5400 MJ/ton (Jacott and Comunes, 2003). Thus, the energy required for slag production is 12% higher than that of cement production. Energy usually comprises 15–20% of the total cost of glass and cement production, while other costs include materials, handling, and labor. The proposed slag melting process has virtually no handling costs for mining and mineral dressing, wherein the only material cost is the glass-frit. The required glass-frit is best derived from waste glass, which has been estimated to be around 460 kilotons per year in Taiwan (Hou, 2003). The amount of glass-frit required in our process is around 130 kilotons per year (half of the ash-mix), or approximately 28% of the total waste glass. Thus, the amount of available waste glass supply would be more than adequate for our process. In comparison with cement price (about US$91 per ton), the cost of waste glass is roughly 10%, which is minor compared to the costs of mineral and mineral dressing in cement production. The much lower material cost compensates for the increased energy consumption. From a commercial perspective, slag melting to produce cement replacement should be as
2000
2500
3000
3500
4000
wavenumber (cm-1) Fig. 4. FT-IR spectra of cured plain cement and slag-cement mortar specimens: (a) cured for 7 days; (b) cured for 28 days.
Table 6 Fourier transformed infrared spectra data. Band assignments
Asymmetric Stretching vibration (cm1)
Si–O–Si C–S–H Ca, OH C–A–S–H CO2 3 CO2
1070a
a b c d
Adopted Adopted Adopted Adopted
Stretching Vibration (cm1) 975b, 1172–1207c 3645d
690c 1400–1500 2400–2500 from from from from
(Miyata et al., 2002). (Mollah et al., 1998). (Puertas et al., 2000). (Mollah et al., 1995).
profitable as cement production. In fact, in order to immobilize and treat fly- and scrubber-ash for a landfill, city governments in Taiwan pay approximately US$120 per ton of ash-mix. If city governments subsidized the slag melting industry, the potential profit could be huge. As a whole, we can say that slag production is cost effective, and potentially quite profitable if government subsidized. Cement production requires the excavation of limestone and other minerals from open pits, which has a significant environmental impact. Air pollution from the rotary kilns that produce cement is an additional problem. In the vitrification of modified ash-mix, there are no mining processes, while there is no air pollution if the ventilation of the scrubbing process is taken into consideration. By replacing approximately 30% of cement with vitrified
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a
7-day
OCM
14-day 28-day
ΔT/mg
60-day 90-day
Ca(OH)2
0
200
400
600
800
1000
o
Temperature ( C)
b
7-day 14-day
SCM(40%)
28-day 60-day
ΔT/mg
90-day
cured for 1 to 90 days. The compositions, structures, properties, and compressive strengths of OCM and SCM as a function of time were carefully studied. The developed slag-cement mortar (SCM) exhibits a slightly less fluidity compared to ordinary cement mortar (OCM). The initial and final setting times of the slag-cement paste were 9–20 min and 10–17 min, respectively, longer than those of plain cement paste. All slag powder passed the TCLP test in accordance with EPA regulations. The early stage compressive strengths of SCM were clearly 64– 88% lower than that of OCM. The 28 day strengths approximated those of the OCM; however, the 60 to 90 day strengths exceeded those of OCM, which demonstrates the feasibility of substantial cement substitution with slag powder. The MIP test revealed that the increased compressive strength of SCM as a function of longer curing time corresponded well with the decreased total number of pores as a function longer curing time. This preliminary feasibility study demonstrates that the mass production of slag from ash-mix and waste glass-frit is cost effective, mass-producible, and has a minimal environmental impact. This research reveals new possibilities for the safe and complete treatment, as well as utilization, of MSWI fly- and scrubber-ash that would otherwise be disposed of in groundwater-contaminating landfills. Acknowledgments
Ca(OH)2
The authors would like to thank the National Science Council of the Republic of China, Taiwan for supporting this research under Contract No. NSC-97-2815-C-239-009-E. 0
200
400
600
800
1000
o
Temperature ( C)
References
Fig. 5. DTA curve of the (a) OCM and (b) SCM cured for 7–90 days.
slag, air pollution and the other environmental impacts of cement production can be dramatically reduced. In addition, if all of the available ash-mix was used as a construction resource instead of being disposed in landfills, its hazard to ground water would be avoided. For these reasons, replacing cement with vitrified ashmix is environmentally and economically plausible. There is one trial pilot plant that is currently capable of vitrifying fly-ash at a rate of 1 ton of slag per hour (Haugsten and Gustavson, 2000). Given the approximate 260 kilotons (e.g., 2007) of fly- and scrubber-ash produced annually in Taiwan, 30 such pilot plants would be sufficient to consume this available ash waste. The number of plants can be reduced as the technology matures and output increases. Restated, mass treatment of all fly- and scrubber-ash is technically feasible. We preliminarily conclude that the vitrification process is cost effective and technologically feasible, as well as environmentally and economically sound. 4. Conclusions This study attempted to fully utilize the fly- and scrubber-ash produced from an MSWI as a useful resource. Fly- and scrubberash were mixed at a weight ratio of 1:3, their normal production ratio, to form an ‘‘ash-mix”. The ash-mix was blended with waste glass-frit at a weight ratio of around 1:0.5, and then melted at 1400 °C. The melt was quenched in water to form slag, which was then pulverized into a powder consisting of granules smaller than 38 lm. Cement mortar samples were prepared by replacing 20–40% of the cement content with slag powder, which were then
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