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Mechanical and leaching properties of cement solidified hospital solid waste incinerator fly ash
WASTE MANAGEMENT Waste Management 18 (1998) 99±106
Mechanical and leaching properties of cement solidi®ed hospital solid waste incinerator ¯y ash F. Lombardi a, T. Mangialardi b, L. Piga c,*, P. Sirini b a
A.M.A., Municipal Environment Firm, Via Calderon de la Barca, 87-00142 Rome, Italy b University of Rome ``La Sapienza'', Via Eudossiana, 18-00184 Rome, Italy c CNR, Mineral Processing Institute, Via Bolognola, 7-00138 Rome, Italy Received 27 January 1997; accepted 27 January 1998
Abstract A ¯y ash coming from a hospital solid wastes incineration plant was solidi®ed/stabilized in cementitious matrices. Owing to the high chloride, sulphate and alkali content and the low Si, AI and Fe values this ¯y ash cannot be used in the formulation of blended cement. The objectives of solidi®cation stabilization treatment were therefore to reduce the leachability of the heavy metals present in this material so as to permit its disposal in a sanitary land®ll requiring only a low degree of environmental protection. The mechanical properties and leaching behaviour of solidi®ed products were investigated. Fly ash and Portland Cement mixtures in ratios varying between 0.25 and 1.5 were tested for uncon®ned compressive strength after curing in tap water at 20 C. Leaching tests were performed both on ¯y ash and solidi®ed/stabilized products using an acetic acid standard leaching test and a modi®ed version thereof (dynamic leaching test). # 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction The primary purpose of burning municipal solid wastes (MSW) and hospital solid wastes (HSW) is to reduce their volume, because of the great diculties encountered in acquiring sites for controlled and uncontrolled land®ll waste disposal operations. Incineration of such materials reduces the original volume by 65±70% [1,2], and generates bottom ash and ¯y ash (FA). The former collects in the bottom of the boiler and can be used for building purposes whenever the ash is considered safe on the basis of standard leaching tests. FA is generally trapped by electrostatic precipitators located downstream of the burner, before the gas and the very ®ne particles are released into the atmosphere. FA acts as an adsorber for the hazardous heavy metals that vaporize during the burning of the HSW. Hence it is not a suitable material for building purposes, at least in its original state. Moreover, owing to the ®neness and the high concentration of hazardous heavy metals, FA must be disposed of in appropriate
* Corresponding author. Tel.: 00 39 6 880 4361; fax: 00 39 6 880 4463; e-mail: [email protected].
sanitary land®lls where it is possible to avoid seepage of the soluble and harmful elements into the subsoil. The maintenance of such facilities is very expensive, thus increasing the cost of the whole burning process. A great amount of research is, therefore, being undertaken to ®nd the safest and most convenient way of inertizing this material so that it can be disposed of in land®ll sites whose characteristics are less severe than those needed for the untreated FA. At the present time, the cement-based solidi®cation/ stabilization (S/S) process seems to oer one of the best options [3,4] for the safe disposal of wastes containing heavy metals, though sight must not be lost of the possibilities oered by the emerging technology of vitri®cation whose operating costs are, however, still too high even for some industrialized countries. Solidifying matrices based on lime/FA mixtures have been proposed [5,6], as an alternative to Portland Cement (PC) or blended cement mixes. Stabilization of the hazardous heavy metals in cementitious matrices is achieved by physical and chemical means. The physical aspect concerns the low permeability of the hardened product and the chemical one the high alkalinity of the pore solution inside the cementitious matrix which allows transformation of the heavy
0956-053X/98/$Ðsee front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S0956 -0 53X(98)00006-3
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F. Lombardi et al./Waste Management 18 (1998) 99±106
during a 3-day sampling period which was adopted so as to compensate for possible alterations in incinerator feed composition. Operating conditions were kept as constant as possible during sampling. The material was quartered to obtain homogeneous samples for characterization and for preparing samples for mechanical and leaching tests.
metals into insoluble compounds. The risk of short-term leaching contamination is thus decreased. However, the long-term stability of these precipitates could be altered by aggressive environments involving such features as soil acids, acid rains and aggressive CO2-laden waters. These may transform insoluble metal hydroxides to more soluble carbonates or even hydrocarbonates, especially in the outer zones of the solidifying matrix. As far as the quality of the S/S products is concerned, the addition of 0.5% of heavy-metal compounds (hydroxides, oxides or sulphides) to PC does not lower the hardening properties of the cement. However, the presence of more than 5% of compounds containing As, Zn, Pb and Cu delays early hardening. The 28-day compressive strength of the cement is 50% reduced by As-compounds; this parameter does not seem to be aected, however, by the presence of Cr, Cd and Hgcompounds [7]. Inhibition of PC hydration reactions occurs with the addition of 3% metal plating waste, corresponding to a heavy metal (primarily zinc) content of about 1.2% [8]. Inhibition of PC hydration at even lower heavy metal additions (about 0.2% by mass of cement) takes place in the case of a high concentration of water-soluble sulphates resulting from the addition of a waste sludge deriving from conventional neutralization of acid and alkaline heavy metal wastes [9]. Most published research deals with FA from MSW. There is little information available on FA from HSW which generally contain more sulphates and chlorides than MSW. Research on HSW-¯y-ash is aimed at the inertization of these toxic materials. This paper reports the results of a S/S process for HSW-¯y ash added in various proportions to PC, in order to ascertain the best conditions for inertization. The quality of the S/S products has been evaluated by mechanical and leaching tests.
2.2. Characterization After alkaline fusion with puri®ed lithium metaborate and subsequent acid dissolution, chemical analysis of FA was carried out with an atomic absorption spectrometer (AAS) equipped with a graphite furnace. The sulphate and chloride contents were determined gravimetrically using, respectively, BaCl2 and AgNO3 as precipitating reagents [10]. Table 1 gives the elemental composition and particle size distribution of FA. The chloride and sulphate contents are very high (11.2 and 2.1% as Cl and SO3 respectively), hence the impossibility of using this material for partial replacement of Portland Cement. Chlorides can, in fact, corrode steel reinforcing bars and sulphates can form a considerable amount of Ettringite, during hydration of PC, with an accompanying increase in volume which damages the structure of the ensuing concrete. Zinc is the predominant heavy metal in the ¯y ash (3200 mg/kg), followed by lead (964 mg/kg). Suitability for land®ll is determined by assessing the leachability of these and other heavy metals. 2.3. Mixes The FA was immobilized in cement mixes made with a high-strength Portland Cement (PC)Ðwhose chemical and physical characteristics are given in Table 2Ðand distilled water (W). The FA/PC weight ratio was varied between 0.25 (Sample 25FA) and 1.50 (Sample 150FA). The highest value of the FA/PC weight ratio was 1.50 due to the inability of mixes containing higher weight ratios to solidify. The water/cement plus ¯y-ash weight ratio (W/(PC+FA)) was varied in order to achieve pastes of consistent workability, subjectively evaluated during preparation. Thus, the W/(PC+FA) ratio ranged from 0.35 to 0.68. The whole set of samples is shown in Table 3.
2. Materials and methods 2.1. Sampling The FA used in this study comes from the hospital solid wastes incinerator serving the Rome area. The material was collected from the electrostatic precipitators
Table 1 Chemical and particle size analysis of ¯y ash coming from hospital solid waste incineration Element %
Ca 60.0
Cl 11.2
Si 2.2
S 0.82
Na 2.1
K 0.8
Mg 0.4
Fe 0.4
Zn 0.3
Al 0.2
Element mg/kg
Pb 964
Cu 173
Cr 109
Mn 105
Cd 85
Co 45
Ni 45
Sn 18
As 10
Sb 6
ÿ1.1 6.12
ÿ1.8 11.79
ÿ3.1 20.69
ÿ5.0 32.88
ÿ9.0 52.46
ÿ15.0 66.87
ÿ25.0 82.47
ÿ36.0 91.62
ÿ73.0 99.73
Size (mm) % Passing
F. Lombardi et al./Waste Management 18 (1998) 99±106
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Table 2 Physical and chemical characteristics of Portland Cement used in the mixes Oxide %
CaO 65.9
SiO2 22.1
Oxide %
P2O3 0.11
Na2O 0.08
TiO2 0.08
Fe2O3 4.8
Al2O3 3.6
SO3 1.5
MgO 0.57
Mn2O3 0.04
SrO 0.04
Density kg/m3 3160
Blaine surface area, m2/kg 306
2.4. Compressive strength Nine 404040 mm samples were cast for each FA± PC mixture. The samples were demoulded after 24 h and immersed in tap water at 20 C. At established curing times (3, 7 and 28 days), they were analysed for uncon®ned compressive strength (Rc) as the average of three replicates of each mixture. However, samples 25FA (W/ (PC+FA)=0.40) and 150FA (W/(PC+FA)=0.44), to be analysed after 7 days, were tested on two replicates only, the third being stored at 20 C and relative humidity (RH) >95% for seven days, and after hardening, assessed for heavy metal leachability. The high RH favours the hydration of the cement without the risk of leaching phenomena. 2.5. Leaching tests The heavy-metal leachability was evaluated both by the acetic acid standard leaching test [11], and a modi®ed version thereof These tests are used to evaluate the behaviour of the S/S products during their disposal in a land®ll where they are subjected to acid rains and CO2aggressive waters, which may dissolve the immobilized hazardous metals then causing seepage of harmful substances into the subsoil. In the standard leaching test Table 3 Concrete mix proportions Sample
Free CaO 0.14
K2O 0.49
about 60 g of the prismatic specimens of S/S products, or the as received FA, were immersed in 960 cc of 0.5 N acetic acid to obtain a liquid-solid ratio of 16, for 24 h at 20 C and under continuous stirring. During the test it is permitted to add limited amounts of acetic acid, up to a liquid solid ratio of 20, in order to maintain the pH below 5. At the end of the test, the solution was passed through a 0.45 mm membrane ®lter and then, if there was necessity, diluted with distilled water in order to have a liquid±solid ratio of 20, with respect to the initial solid, and ®nally analysed to ascertain the heavy metal concentration. The modi®ed version of the standard leaching test consists of a dynamic test which provides information on long-term leaching behaviour. In this case about 60 g of the prismatic specimens of S/S products were subjected to a number of leachant renewals. The leaching solution was 0.1 N acetic acid, the liquid±solid weight ratio 20 and the test duration was 30 days with a 24-h leaching interval. The specimen was extracted from the solution after each interval and immersed in a fresh solution. For each leaching step, the same procedure as the previous test was followed. 2.6. X-ray analysis In some cases the broken samples were analysed for the main crystalline phases by X-ray analysis after treatment with acetone and ethyl ether. The XRD patterns were obtained by means of Ni ®ltered CuKa radiation.
FA/PC
W/(PC+FA)
W/PC
25FA
0.25
0.40 0.44 0.48
0.50 0.55 0.60
3. Results and discussion
50FA
0.50
0.40 0.47 0.53
0.60 0.70 0.80
3.1. Compressive strength of S/S products and eect of ¯y ash on cement hydration
0.37 0.46 0.54 0.63
0.65 0.80 0.95 1.10
0.35 0.45 0.55 0.65
0.70 0.90 1.10 1.30
0.44 0.56 0.68
1.10 1.40 1.70
75FA
0.75
100FA
1.00
150FA
1.50
Fig. 1 shows the results of uniaxial compressive tests performed on S/S products after 3, 7 and 28-day curing. In all cases the strength of the specimens containing ¯y ash is very much lower than that of the reference specimens (Portland Cement mixes) and decreases with the increase in the amount of FA in the mix. The reduction in the 7-day values compared to the samples with only PC is about 36, 66, 75 and 80% for FA/PC ratios of 0.25, 0.50, 0.75 and 1.0 at the W/(PC+FA) ratio of 0.40, while in the case of the 28-day curing the corresponding
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F. Lombardi et al./Waste Management 18 (1998) 99±106
values are reduced by a somewhat lesser amount, namely by about 33, 60, 69 and 78%. Nevertheless the minimum compressive strength value (3.9 MPa) measured on the 150FA, 3-day curing, sample is about twentyfold higher than that needed (about 0.2 MPa) to support the dead-load of the dump, in the case of a density of 2000 kg/m3 and a 10 mm) high column of S/S products. This suggests that higher FA/PC values could be used in the mixes, provided that the Portland Cement hydration is not inhibited by the increased amount in FA.
Fig. 2 also shows that the compressive strength of the S/S products decreases as the W/(PC+FA) ratio increases and that the addition of FA to Portland Cement increases the water demand of the mixes. Due to the dierent water demand of the mixes with dierent FA/PC ratios, it was not possible to investigate the eect of the ¯y ash content on the compressive strength at a ®xed W/(PC+FA) ratio. Consequently, the low compressive strengths of S/S products must be regarded as the result of the combined eect of both the lower Portland Cement content and the much higher water
Fig. 1. Development of compressive strength at dierent FA/PC ratio.
F. Lombardi et al./Waste Management 18 (1998) 99±106
content of the mixes. These considerations lead to the conclusion that the addition of FA greatly reduces the compressive strength but exerts no inhibiting eect on the cement hydration process. 3.1.1. X-ray analysis evaluation on S/S products Fig. 3 reports the X-ray traces of three S/S products: B, C and D. The FA/PC weight ratio of D is 0.50 and the curing time is 7 days, while that of B and C is 1.50 with curing times of 28 and 7 days respectively. The
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XRD pattern of PC after 7 days of hydration, A, is shown in the same Figure as a reference. X-ray analysis reveals the presence of Calcite (CaCO3) both in PC and in FA (spectrum not shown). The presence of this phase in PC is due to its carbonation during the hydration stage, while, in FA, is due to the large amount of calcium hydroxide used for scrubbing the acid gas during the burning process. Gypsum (CaSO4 2H2O), is barely detectable in samples B and C due to the small amount of Portland Cement in these mixes. The hydration of PC is
Fig. 2. Eect of water/total solid ratio on compressive strength of the S/S products.
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F. Lombardi et al./Waste Management 18 (1998) 99±106 Table 4 Results of acetic acid standard leaching test on untreated FA and S/S products; curing time: 7 days Heavy metal
Cd Cu Ni Pb Zn
Conc. limits (mg/l)
Fly ash as received (mg/l)
0.02 0.1 2.0 0.2 0.5
1.1 0.2 0.4 9.8 4.2
S/S products Sample 25FA mg/l
Sample 150FA mg/l
<0.1 1.9 <0.1 5.0 38
0.1 2.8 <0.1 6.7 87
3.2. Leaching behavior of the S/S products
Fig. 3. Powder XRD pattern of some S/S products.
evidenced by the formation of peaks of Portlandite (Ca(OH)2) and Ettringite, (Ca6(Fe,Al)2(SO4)3(OH)12 nH2O), that are hydrated phases of PC, while Larnite, (Ca2SiO4), is one of the anhydrous phases. The spectra of mixes A and D are quite similar, but there is a slight appearance of Portlandite in the latter. The peak of Larnite at 2 34:1 in sample D seems to be higher than the corresponding one in sample A, but this is because the peak is reinforced by that of Portlandite. When the FA/PC weight ratio increases from 0.5 to 1.50 (C), at the same curing time of 7 days, the intensity of Ettringite and mainly of Portlandite peaks increases, while the anhydrous phase peaks (Larnite) are not more detectable. These results could be ascribed to an accelerating eect of FA on the PC hydration. A higher intensity is shown by the Portlandite and Ettringite peaks when the FA/PC weight ratio is 1.50 and the curing time increases from 7 to 28 days, as a result of increased hydration of PC.
3.2.1. Standard leaching test The results of the standard leaching test are presented in Table 4 along with the limit values established by the Italian legislation. The results obtained on the as received FA show that the concentrations of cadmium (1.1 mg/l), zinc (4.2 mg/l) and lead (9.8 mg/l), exceed the law-limits, so it is necessary to dispose of the FA in a land®ll with a high level of environmental protection. The results of the test on the S/S specimens, prepared with FA/PC ratios of 0.25 and 1.50, indicate that the concentrations of all the heavy metals are well below the limits established by Italian legislation. This proves that the stabilization process is eective, even after a relatively short curing time (7 days). The ®nal pH value of the leaching solution was very high (about 12) because of the concomitant release of calcium hydroxide and alkalies from the cementitious matrix and the impossibility to add more acetic acid during the test period. However, the metal concentrations measured in all cases were very much lower than the solubility of the respective hydroxides at pH=12, thus indicating that there is chemical entrapment of heavy metals in the hydrated cement phases where they are strongly immobilized. 3.2.2. Dynamic leaching test Figs. 4 and 5 show the results of the dynamic leaching test in terms of the cumulative heavy metal content leached from the specimens 25FA and 150FA. The shape of the curves is similar for all the metals. The rate of leaching decreases with time and the total amount of metal leached increases with the increase in the FA/C ratio except for Cu and Ni that exhibit maximum release in the case of specimen 25FA. At the longest time covered by the test adopted here (30 days) the fractions of heavy metals leached from specimens 25FA and 150FA are respectively 10 and 15% for Cd, 39 and 23% for Cu, 64 and 23% for Ni, 15 and 17% for Pb and 42 and 52% for Zn.
F. Lombardi et al./Waste Management 18 (1998) 99±106
3.2.3. Diusion model and diusion coecients In order to evaluate the mechanism controlling the leaching process, an attempt was made to interpret the experimental data obtained from the dynamic leaching test with the semi-in®nite medium diusion model proposed by Godbee and Joy [12], for the diusion of radioactive isotopes from waste solids. The model was applied for a leaching time of up to 10 days. At this time the cumulative fraction leached out was negligible compared to the initial amount of metals, matching the condition of a semi-in®nite medium. The condition of zero surface concentration was satis®ed by the choice of the leaching interval that minimizes the concentration of metal in the solution which still permits analytical detection. With this model the apparent diusion coef®cient (De, (cm2/s)) of the metals can be calculated if there is a linear relationship between the cumulative leached amount of metal (an , (mg)) and the square root of the leaching time (tn, (s)): 1=2 an V De 2 t1=2 n Ao S
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The De values of each metal was calculated from the slopes (m) of the curves obtained by plotting an versus the square root of time (Figs. 6 and 7) and resolving the following equation: De
mV 2 2Ao S
2
Most of the experimental data ®t the linear relationship well over the whole time interval considered, the only exception being those for Cd at an FA/PC ratio of 0.25 where agreement with the diusion model is limited to six days. However an initial resistance to diusion is recognizable in the case of all metals and is probably due to an initial washing period [13]. This indicates that heavy metals are stabilized by chemical interaction with the hydration products of the cement and the induction period may be the time
1
where AO is the initial amount of contaminant present in the specimen (mg), V the volume (cm3) and S the surface area (cm2) of the specimen. The diusion coecient of Eq. (1) is de®ned as ``eective'' because diusion occurs in the liquid ®lling the pores of the material.
Fig. 6. Evaluation of data of Fig. 5 according to the diusion model of Eq. (1).
Fig. 4. Long-term leaching test for sample 25FA.
Fig. 5. Long-term leaching test for sample 150FA.
Fig. 7. Evaluation of data of Fig. 6 according to the diusion model of Eq. (1).
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F. Lombardi et al./Waste Management 18 (1998) 99±106
Acknowledgements
Table 5 Diusivity (De) of heavy metals calculated with Eq. (2)
25FA 150FA
Cd
Cu
Ni
Pb
Zn
8.77 8.56
7.73 8.30
7.25 8.17
8.72 8.45
7.59 9.40
necessary for the formation of the soluble species of the metals that can diuse out of the cementitious matrix. The values of the apparent diusion coecient De are reported in Table 5 as pDe ÿ log De. The mobility of a metal increases, of course, with the decrease of pDe. 4. Conclusions The Portland Cement-based solidi®cation-stabilization (S/S) process is an eective treatment for inertizing ¯y-ash resulting from the incineration of hospital solid wastes, at least in the FA/PC weight ratio range investigated (0.25±1.50). Compressive strength of the S/S products is markedly reduced as the FA/PC ratio increases. The reduction amounts to about 80% at the FA/PC ratio of 1.50 when the specimens are cured in water at 20 C for 7 days. It is considered that this marked decline in mechanical properties is also attributable to the fact that mixes prepared with ¯y ash require so much more water. However, the compressive strength of S/S products is always higher than the minimum value required for their disposal in land®ll. Furthermore, longer curing times exert a bene®cial eect on the mechanical characteristics of the S/S products; a fact that is more evident for the higher FA/PC ratios examined. As evidenced by the results of the standard acetic leaching test performed on the S/S products, the heavy metals investigated (Cd, Cu, Ni, Pb and Zn) are strongly immobilized by the cementitious matrix. The S/ S products can thus be disposed of in land®ll with a lower level of environmental protection than is needed for the untreated waste. The results obtained from dynamic leaching tests con®rm that the leaching behaviour can be described by a molecular diusion-control model.
The authors want to thank Ms A. Polettini of La Sapienza University for the experimental work and Mr G. Marrusso and Mrs S. Quaresima of the CNR's Mineral Processing Institute who helped in elaborating the experimental results of the work reported here with the greatest care. References [1] Harmernik JD, Gregory CF. Strength of concrete containing municipal solid waste ¯y ash. ACI Materials Journal 1991; 88(5):508±517. [2] Tay JH, Cheong HK. Use of ash derived from refuse incineration as a partial replacement of cement. Cem. Conc. Comp. 1991;13:171±175. [3] Corner JR. Chemical ®xation and solidi®cation of hazardous wastes. Van Nostrand Reinhold, New York, pp. 1±692 (1990). [4] Tarnas FD, Csetenyi L, Tritthart J. Eect of adsorbents on the leachability of cement bonded electroplating wastes. Cem. Conc. Res. 1992;22:339±397. [5] Ghosh MM. Management of hazardous wastes in the process industries, Kolaczkowski ST, Crittenden BD, editors. Elsevier Appl. Sci., London, pp. 1±322 (1987). [6] Roy A, Eaton HC. Solidi®cation stabilization of a synthetic electroplating waste in lime ¯y ash binder. Cem. Conc. Res. 1992;22:589±596. [7] Tashiro CD, Takahashi H, Kanaya M, Hirakida I, Yoshida R. Hardening property of cement mortar adding heavy metal compound and solubility of heavy metal from hardened mortar. Cem. Conc. Res. 1977;7:283±290. [8] Hills CD, Koe L, Sollars CJ, Perry R. Early heat of hydration during the solidi®cation of a metal plating sludge. Cem. Conc. Res. 1992;22:822±832. [9] Hills CD, Sollars CJ, Perry R. Ordinary portland cement based solidi®cation of toxic wastes: the role of OPC reviewed. Cem Conc. Res. 1993;23:196±212. [10] APHA-AWWA-WPCF, Standard methods for examination of water and wastewater, 18th ed., Washington, DC pp. 1±57 (1989). [11] Federal Register, US EPA Hazardous Waste Proposed Guidelines and Regulation and Proposal on Identi®cation and Listing, 43: (1978). [12] Godbee HW, Joy DS. Assessment of the loss of radiactive isotopes from waste solids to the environment Part 1: Background and Theory. TM-4333, Oak Ridge National Laboratory, Oak Ridge, TN (1974). [13] CoÃte PL, Denis I. Application of a Dynamic leaching Test to Solidi®ed Hazardous Wastes. In: Jackson LP, Rahlik AR, Conway RA, editors. Proceedings of 3rd Symposium ASTM STP on Hazardous and Industrial Waste. American Society for Testing and Materials, Philadelphia, 1984:48±60.
Mechanical and leaching properties of cement solidi®ed hospital solid waste incinerator ¯y ash F. Lombardi a, T. Mangialardi b, L. Piga c,*, P. Sirini b a
A.M.A., Municipal Environment Firm, Via Calderon de la Barca, 87-00142 Rome, Italy b University of Rome ``La Sapienza'', Via Eudossiana, 18-00184 Rome, Italy c CNR, Mineral Processing Institute, Via Bolognola, 7-00138 Rome, Italy Received 27 January 1997; accepted 27 January 1998
Abstract A ¯y ash coming from a hospital solid wastes incineration plant was solidi®ed/stabilized in cementitious matrices. Owing to the high chloride, sulphate and alkali content and the low Si, AI and Fe values this ¯y ash cannot be used in the formulation of blended cement. The objectives of solidi®cation stabilization treatment were therefore to reduce the leachability of the heavy metals present in this material so as to permit its disposal in a sanitary land®ll requiring only a low degree of environmental protection. The mechanical properties and leaching behaviour of solidi®ed products were investigated. Fly ash and Portland Cement mixtures in ratios varying between 0.25 and 1.5 were tested for uncon®ned compressive strength after curing in tap water at 20 C. Leaching tests were performed both on ¯y ash and solidi®ed/stabilized products using an acetic acid standard leaching test and a modi®ed version thereof (dynamic leaching test). # 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction The primary purpose of burning municipal solid wastes (MSW) and hospital solid wastes (HSW) is to reduce their volume, because of the great diculties encountered in acquiring sites for controlled and uncontrolled land®ll waste disposal operations. Incineration of such materials reduces the original volume by 65±70% [1,2], and generates bottom ash and ¯y ash (FA). The former collects in the bottom of the boiler and can be used for building purposes whenever the ash is considered safe on the basis of standard leaching tests. FA is generally trapped by electrostatic precipitators located downstream of the burner, before the gas and the very ®ne particles are released into the atmosphere. FA acts as an adsorber for the hazardous heavy metals that vaporize during the burning of the HSW. Hence it is not a suitable material for building purposes, at least in its original state. Moreover, owing to the ®neness and the high concentration of hazardous heavy metals, FA must be disposed of in appropriate
* Corresponding author. Tel.: 00 39 6 880 4361; fax: 00 39 6 880 4463; e-mail: [email protected].
sanitary land®lls where it is possible to avoid seepage of the soluble and harmful elements into the subsoil. The maintenance of such facilities is very expensive, thus increasing the cost of the whole burning process. A great amount of research is, therefore, being undertaken to ®nd the safest and most convenient way of inertizing this material so that it can be disposed of in land®ll sites whose characteristics are less severe than those needed for the untreated FA. At the present time, the cement-based solidi®cation/ stabilization (S/S) process seems to oer one of the best options [3,4] for the safe disposal of wastes containing heavy metals, though sight must not be lost of the possibilities oered by the emerging technology of vitri®cation whose operating costs are, however, still too high even for some industrialized countries. Solidifying matrices based on lime/FA mixtures have been proposed [5,6], as an alternative to Portland Cement (PC) or blended cement mixes. Stabilization of the hazardous heavy metals in cementitious matrices is achieved by physical and chemical means. The physical aspect concerns the low permeability of the hardened product and the chemical one the high alkalinity of the pore solution inside the cementitious matrix which allows transformation of the heavy
0956-053X/98/$Ðsee front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S0956 -0 53X(98)00006-3
100
F. Lombardi et al./Waste Management 18 (1998) 99±106
during a 3-day sampling period which was adopted so as to compensate for possible alterations in incinerator feed composition. Operating conditions were kept as constant as possible during sampling. The material was quartered to obtain homogeneous samples for characterization and for preparing samples for mechanical and leaching tests.
metals into insoluble compounds. The risk of short-term leaching contamination is thus decreased. However, the long-term stability of these precipitates could be altered by aggressive environments involving such features as soil acids, acid rains and aggressive CO2-laden waters. These may transform insoluble metal hydroxides to more soluble carbonates or even hydrocarbonates, especially in the outer zones of the solidifying matrix. As far as the quality of the S/S products is concerned, the addition of 0.5% of heavy-metal compounds (hydroxides, oxides or sulphides) to PC does not lower the hardening properties of the cement. However, the presence of more than 5% of compounds containing As, Zn, Pb and Cu delays early hardening. The 28-day compressive strength of the cement is 50% reduced by As-compounds; this parameter does not seem to be aected, however, by the presence of Cr, Cd and Hgcompounds [7]. Inhibition of PC hydration reactions occurs with the addition of 3% metal plating waste, corresponding to a heavy metal (primarily zinc) content of about 1.2% [8]. Inhibition of PC hydration at even lower heavy metal additions (about 0.2% by mass of cement) takes place in the case of a high concentration of water-soluble sulphates resulting from the addition of a waste sludge deriving from conventional neutralization of acid and alkaline heavy metal wastes [9]. Most published research deals with FA from MSW. There is little information available on FA from HSW which generally contain more sulphates and chlorides than MSW. Research on HSW-¯y-ash is aimed at the inertization of these toxic materials. This paper reports the results of a S/S process for HSW-¯y ash added in various proportions to PC, in order to ascertain the best conditions for inertization. The quality of the S/S products has been evaluated by mechanical and leaching tests.
2.2. Characterization After alkaline fusion with puri®ed lithium metaborate and subsequent acid dissolution, chemical analysis of FA was carried out with an atomic absorption spectrometer (AAS) equipped with a graphite furnace. The sulphate and chloride contents were determined gravimetrically using, respectively, BaCl2 and AgNO3 as precipitating reagents [10]. Table 1 gives the elemental composition and particle size distribution of FA. The chloride and sulphate contents are very high (11.2 and 2.1% as Cl and SO3 respectively), hence the impossibility of using this material for partial replacement of Portland Cement. Chlorides can, in fact, corrode steel reinforcing bars and sulphates can form a considerable amount of Ettringite, during hydration of PC, with an accompanying increase in volume which damages the structure of the ensuing concrete. Zinc is the predominant heavy metal in the ¯y ash (3200 mg/kg), followed by lead (964 mg/kg). Suitability for land®ll is determined by assessing the leachability of these and other heavy metals. 2.3. Mixes The FA was immobilized in cement mixes made with a high-strength Portland Cement (PC)Ðwhose chemical and physical characteristics are given in Table 2Ðand distilled water (W). The FA/PC weight ratio was varied between 0.25 (Sample 25FA) and 1.50 (Sample 150FA). The highest value of the FA/PC weight ratio was 1.50 due to the inability of mixes containing higher weight ratios to solidify. The water/cement plus ¯y-ash weight ratio (W/(PC+FA)) was varied in order to achieve pastes of consistent workability, subjectively evaluated during preparation. Thus, the W/(PC+FA) ratio ranged from 0.35 to 0.68. The whole set of samples is shown in Table 3.
2. Materials and methods 2.1. Sampling The FA used in this study comes from the hospital solid wastes incinerator serving the Rome area. The material was collected from the electrostatic precipitators
Table 1 Chemical and particle size analysis of ¯y ash coming from hospital solid waste incineration Element %
Ca 60.0
Cl 11.2
Si 2.2
S 0.82
Na 2.1
K 0.8
Mg 0.4
Fe 0.4
Zn 0.3
Al 0.2
Element mg/kg
Pb 964
Cu 173
Cr 109
Mn 105
Cd 85
Co 45
Ni 45
Sn 18
As 10
Sb 6
ÿ1.1 6.12
ÿ1.8 11.79
ÿ3.1 20.69
ÿ5.0 32.88
ÿ9.0 52.46
ÿ15.0 66.87
ÿ25.0 82.47
ÿ36.0 91.62
ÿ73.0 99.73
Size (mm) % Passing
F. Lombardi et al./Waste Management 18 (1998) 99±106
101
Table 2 Physical and chemical characteristics of Portland Cement used in the mixes Oxide %
CaO 65.9
SiO2 22.1
Oxide %
P2O3 0.11
Na2O 0.08
TiO2 0.08
Fe2O3 4.8
Al2O3 3.6
SO3 1.5
MgO 0.57
Mn2O3 0.04
SrO 0.04
Density kg/m3 3160
Blaine surface area, m2/kg 306
2.4. Compressive strength Nine 404040 mm samples were cast for each FA± PC mixture. The samples were demoulded after 24 h and immersed in tap water at 20 C. At established curing times (3, 7 and 28 days), they were analysed for uncon®ned compressive strength (Rc) as the average of three replicates of each mixture. However, samples 25FA (W/ (PC+FA)=0.40) and 150FA (W/(PC+FA)=0.44), to be analysed after 7 days, were tested on two replicates only, the third being stored at 20 C and relative humidity (RH) >95% for seven days, and after hardening, assessed for heavy metal leachability. The high RH favours the hydration of the cement without the risk of leaching phenomena. 2.5. Leaching tests The heavy-metal leachability was evaluated both by the acetic acid standard leaching test [11], and a modi®ed version thereof These tests are used to evaluate the behaviour of the S/S products during their disposal in a land®ll where they are subjected to acid rains and CO2aggressive waters, which may dissolve the immobilized hazardous metals then causing seepage of harmful substances into the subsoil. In the standard leaching test Table 3 Concrete mix proportions Sample
Free CaO 0.14
K2O 0.49
about 60 g of the prismatic specimens of S/S products, or the as received FA, were immersed in 960 cc of 0.5 N acetic acid to obtain a liquid-solid ratio of 16, for 24 h at 20 C and under continuous stirring. During the test it is permitted to add limited amounts of acetic acid, up to a liquid solid ratio of 20, in order to maintain the pH below 5. At the end of the test, the solution was passed through a 0.45 mm membrane ®lter and then, if there was necessity, diluted with distilled water in order to have a liquid±solid ratio of 20, with respect to the initial solid, and ®nally analysed to ascertain the heavy metal concentration. The modi®ed version of the standard leaching test consists of a dynamic test which provides information on long-term leaching behaviour. In this case about 60 g of the prismatic specimens of S/S products were subjected to a number of leachant renewals. The leaching solution was 0.1 N acetic acid, the liquid±solid weight ratio 20 and the test duration was 30 days with a 24-h leaching interval. The specimen was extracted from the solution after each interval and immersed in a fresh solution. For each leaching step, the same procedure as the previous test was followed. 2.6. X-ray analysis In some cases the broken samples were analysed for the main crystalline phases by X-ray analysis after treatment with acetone and ethyl ether. The XRD patterns were obtained by means of Ni ®ltered CuKa radiation.
FA/PC
W/(PC+FA)
W/PC
25FA
0.25
0.40 0.44 0.48
0.50 0.55 0.60
3. Results and discussion
50FA
0.50
0.40 0.47 0.53
0.60 0.70 0.80
3.1. Compressive strength of S/S products and eect of ¯y ash on cement hydration
0.37 0.46 0.54 0.63
0.65 0.80 0.95 1.10
0.35 0.45 0.55 0.65
0.70 0.90 1.10 1.30
0.44 0.56 0.68
1.10 1.40 1.70
75FA
0.75
100FA
1.00
150FA
1.50
Fig. 1 shows the results of uniaxial compressive tests performed on S/S products after 3, 7 and 28-day curing. In all cases the strength of the specimens containing ¯y ash is very much lower than that of the reference specimens (Portland Cement mixes) and decreases with the increase in the amount of FA in the mix. The reduction in the 7-day values compared to the samples with only PC is about 36, 66, 75 and 80% for FA/PC ratios of 0.25, 0.50, 0.75 and 1.0 at the W/(PC+FA) ratio of 0.40, while in the case of the 28-day curing the corresponding
102
F. Lombardi et al./Waste Management 18 (1998) 99±106
values are reduced by a somewhat lesser amount, namely by about 33, 60, 69 and 78%. Nevertheless the minimum compressive strength value (3.9 MPa) measured on the 150FA, 3-day curing, sample is about twentyfold higher than that needed (about 0.2 MPa) to support the dead-load of the dump, in the case of a density of 2000 kg/m3 and a 10 mm) high column of S/S products. This suggests that higher FA/PC values could be used in the mixes, provided that the Portland Cement hydration is not inhibited by the increased amount in FA.
Fig. 2 also shows that the compressive strength of the S/S products decreases as the W/(PC+FA) ratio increases and that the addition of FA to Portland Cement increases the water demand of the mixes. Due to the dierent water demand of the mixes with dierent FA/PC ratios, it was not possible to investigate the eect of the ¯y ash content on the compressive strength at a ®xed W/(PC+FA) ratio. Consequently, the low compressive strengths of S/S products must be regarded as the result of the combined eect of both the lower Portland Cement content and the much higher water
Fig. 1. Development of compressive strength at dierent FA/PC ratio.
F. Lombardi et al./Waste Management 18 (1998) 99±106
content of the mixes. These considerations lead to the conclusion that the addition of FA greatly reduces the compressive strength but exerts no inhibiting eect on the cement hydration process. 3.1.1. X-ray analysis evaluation on S/S products Fig. 3 reports the X-ray traces of three S/S products: B, C and D. The FA/PC weight ratio of D is 0.50 and the curing time is 7 days, while that of B and C is 1.50 with curing times of 28 and 7 days respectively. The
103
XRD pattern of PC after 7 days of hydration, A, is shown in the same Figure as a reference. X-ray analysis reveals the presence of Calcite (CaCO3) both in PC and in FA (spectrum not shown). The presence of this phase in PC is due to its carbonation during the hydration stage, while, in FA, is due to the large amount of calcium hydroxide used for scrubbing the acid gas during the burning process. Gypsum (CaSO4 2H2O), is barely detectable in samples B and C due to the small amount of Portland Cement in these mixes. The hydration of PC is
Fig. 2. Eect of water/total solid ratio on compressive strength of the S/S products.
104
F. Lombardi et al./Waste Management 18 (1998) 99±106 Table 4 Results of acetic acid standard leaching test on untreated FA and S/S products; curing time: 7 days Heavy metal
Cd Cu Ni Pb Zn
Conc. limits (mg/l)
Fly ash as received (mg/l)
0.02 0.1 2.0 0.2 0.5
1.1 0.2 0.4 9.8 4.2
S/S products Sample 25FA mg/l
Sample 150FA mg/l
<0.1 1.9 <0.1 5.0 38
0.1 2.8 <0.1 6.7 87
3.2. Leaching behavior of the S/S products
Fig. 3. Powder XRD pattern of some S/S products.
evidenced by the formation of peaks of Portlandite (Ca(OH)2) and Ettringite, (Ca6(Fe,Al)2(SO4)3(OH)12 nH2O), that are hydrated phases of PC, while Larnite, (Ca2SiO4), is one of the anhydrous phases. The spectra of mixes A and D are quite similar, but there is a slight appearance of Portlandite in the latter. The peak of Larnite at 2 34:1 in sample D seems to be higher than the corresponding one in sample A, but this is because the peak is reinforced by that of Portlandite. When the FA/PC weight ratio increases from 0.5 to 1.50 (C), at the same curing time of 7 days, the intensity of Ettringite and mainly of Portlandite peaks increases, while the anhydrous phase peaks (Larnite) are not more detectable. These results could be ascribed to an accelerating eect of FA on the PC hydration. A higher intensity is shown by the Portlandite and Ettringite peaks when the FA/PC weight ratio is 1.50 and the curing time increases from 7 to 28 days, as a result of increased hydration of PC.
3.2.1. Standard leaching test The results of the standard leaching test are presented in Table 4 along with the limit values established by the Italian legislation. The results obtained on the as received FA show that the concentrations of cadmium (1.1 mg/l), zinc (4.2 mg/l) and lead (9.8 mg/l), exceed the law-limits, so it is necessary to dispose of the FA in a land®ll with a high level of environmental protection. The results of the test on the S/S specimens, prepared with FA/PC ratios of 0.25 and 1.50, indicate that the concentrations of all the heavy metals are well below the limits established by Italian legislation. This proves that the stabilization process is eective, even after a relatively short curing time (7 days). The ®nal pH value of the leaching solution was very high (about 12) because of the concomitant release of calcium hydroxide and alkalies from the cementitious matrix and the impossibility to add more acetic acid during the test period. However, the metal concentrations measured in all cases were very much lower than the solubility of the respective hydroxides at pH=12, thus indicating that there is chemical entrapment of heavy metals in the hydrated cement phases where they are strongly immobilized. 3.2.2. Dynamic leaching test Figs. 4 and 5 show the results of the dynamic leaching test in terms of the cumulative heavy metal content leached from the specimens 25FA and 150FA. The shape of the curves is similar for all the metals. The rate of leaching decreases with time and the total amount of metal leached increases with the increase in the FA/C ratio except for Cu and Ni that exhibit maximum release in the case of specimen 25FA. At the longest time covered by the test adopted here (30 days) the fractions of heavy metals leached from specimens 25FA and 150FA are respectively 10 and 15% for Cd, 39 and 23% for Cu, 64 and 23% for Ni, 15 and 17% for Pb and 42 and 52% for Zn.
F. Lombardi et al./Waste Management 18 (1998) 99±106
3.2.3. Diusion model and diusion coecients In order to evaluate the mechanism controlling the leaching process, an attempt was made to interpret the experimental data obtained from the dynamic leaching test with the semi-in®nite medium diusion model proposed by Godbee and Joy [12], for the diusion of radioactive isotopes from waste solids. The model was applied for a leaching time of up to 10 days. At this time the cumulative fraction leached out was negligible compared to the initial amount of metals, matching the condition of a semi-in®nite medium. The condition of zero surface concentration was satis®ed by the choice of the leaching interval that minimizes the concentration of metal in the solution which still permits analytical detection. With this model the apparent diusion coef®cient (De, (cm2/s)) of the metals can be calculated if there is a linear relationship between the cumulative leached amount of metal (an , (mg)) and the square root of the leaching time (tn, (s)): 1=2 an V De 2 t1=2 n Ao S
105
The De values of each metal was calculated from the slopes (m) of the curves obtained by plotting an versus the square root of time (Figs. 6 and 7) and resolving the following equation: De
mV 2 2Ao S
2
Most of the experimental data ®t the linear relationship well over the whole time interval considered, the only exception being those for Cd at an FA/PC ratio of 0.25 where agreement with the diusion model is limited to six days. However an initial resistance to diusion is recognizable in the case of all metals and is probably due to an initial washing period [13]. This indicates that heavy metals are stabilized by chemical interaction with the hydration products of the cement and the induction period may be the time
1
where AO is the initial amount of contaminant present in the specimen (mg), V the volume (cm3) and S the surface area (cm2) of the specimen. The diusion coecient of Eq. (1) is de®ned as ``eective'' because diusion occurs in the liquid ®lling the pores of the material.
Fig. 6. Evaluation of data of Fig. 5 according to the diusion model of Eq. (1).
Fig. 4. Long-term leaching test for sample 25FA.
Fig. 5. Long-term leaching test for sample 150FA.
Fig. 7. Evaluation of data of Fig. 6 according to the diusion model of Eq. (1).
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F. Lombardi et al./Waste Management 18 (1998) 99±106
Acknowledgements
Table 5 Diusivity (De) of heavy metals calculated with Eq. (2)
25FA 150FA
Cd
Cu
Ni
Pb
Zn
8.77 8.56
7.73 8.30
7.25 8.17
8.72 8.45
7.59 9.40
necessary for the formation of the soluble species of the metals that can diuse out of the cementitious matrix. The values of the apparent diusion coecient De are reported in Table 5 as pDe ÿ log De. The mobility of a metal increases, of course, with the decrease of pDe. 4. Conclusions The Portland Cement-based solidi®cation-stabilization (S/S) process is an eective treatment for inertizing ¯y-ash resulting from the incineration of hospital solid wastes, at least in the FA/PC weight ratio range investigated (0.25±1.50). Compressive strength of the S/S products is markedly reduced as the FA/PC ratio increases. The reduction amounts to about 80% at the FA/PC ratio of 1.50 when the specimens are cured in water at 20 C for 7 days. It is considered that this marked decline in mechanical properties is also attributable to the fact that mixes prepared with ¯y ash require so much more water. However, the compressive strength of S/S products is always higher than the minimum value required for their disposal in land®ll. Furthermore, longer curing times exert a bene®cial eect on the mechanical characteristics of the S/S products; a fact that is more evident for the higher FA/PC ratios examined. As evidenced by the results of the standard acetic leaching test performed on the S/S products, the heavy metals investigated (Cd, Cu, Ni, Pb and Zn) are strongly immobilized by the cementitious matrix. The S/ S products can thus be disposed of in land®ll with a lower level of environmental protection than is needed for the untreated waste. The results obtained from dynamic leaching tests con®rm that the leaching behaviour can be described by a molecular diusion-control model.
The authors want to thank Ms A. Polettini of La Sapienza University for the experimental work and Mr G. Marrusso and Mrs S. Quaresima of the CNR's Mineral Processing Institute who helped in elaborating the experimental results of the work reported here with the greatest care. References [1] Harmernik JD, Gregory CF. Strength of concrete containing municipal solid waste ¯y ash. ACI Materials Journal 1991; 88(5):508±517. [2] Tay JH, Cheong HK. Use of ash derived from refuse incineration as a partial replacement of cement. Cem. Conc. Comp. 1991;13:171±175. [3] Corner JR. Chemical ®xation and solidi®cation of hazardous wastes. Van Nostrand Reinhold, New York, pp. 1±692 (1990). [4] Tarnas FD, Csetenyi L, Tritthart J. Eect of adsorbents on the leachability of cement bonded electroplating wastes. Cem. Conc. Res. 1992;22:339±397. [5] Ghosh MM. Management of hazardous wastes in the process industries, Kolaczkowski ST, Crittenden BD, editors. Elsevier Appl. Sci., London, pp. 1±322 (1987). [6] Roy A, Eaton HC. Solidi®cation stabilization of a synthetic electroplating waste in lime ¯y ash binder. Cem. Conc. Res. 1992;22:589±596. [7] Tashiro CD, Takahashi H, Kanaya M, Hirakida I, Yoshida R. Hardening property of cement mortar adding heavy metal compound and solubility of heavy metal from hardened mortar. Cem. Conc. Res. 1977;7:283±290. [8] Hills CD, Koe L, Sollars CJ, Perry R. Early heat of hydration during the solidi®cation of a metal plating sludge. Cem. Conc. Res. 1992;22:822±832. [9] Hills CD, Sollars CJ, Perry R. Ordinary portland cement based solidi®cation of toxic wastes: the role of OPC reviewed. Cem Conc. Res. 1993;23:196±212. [10] APHA-AWWA-WPCF, Standard methods for examination of water and wastewater, 18th ed., Washington, DC pp. 1±57 (1989). [11] Federal Register, US EPA Hazardous Waste Proposed Guidelines and Regulation and Proposal on Identi®cation and Listing, 43: (1978). [12] Godbee HW, Joy DS. Assessment of the loss of radiactive isotopes from waste solids to the environment Part 1: Background and Theory. TM-4333, Oak Ridge National Laboratory, Oak Ridge, TN (1974). [13] CoÃte PL, Denis I. Application of a Dynamic leaching Test to Solidi®ed Hazardous Wastes. In: Jackson LP, Rahlik AR, Conway RA, editors. Proceedings of 3rd Symposium ASTM STP on Hazardous and Industrial Waste. American Society for Testing and Materials, Philadelphia, 1984:48±60.