Environ. Sci. Technol. 2004, 38, 2946-2952
Radioactively Contaminated Electric Arc Furnace Dust as an Addition to the Immobilization Mortar in Lowand Medium-Activity Repositories M A R T A C A S T E L L O T E , * ,† E S P E R A N Z A M E N EÄ N D E Z , † CARMEN ANDRADE,† PABLO ZULOAGA,‡ MARIANO NAVARRO,‡ AND MANUEL ORDO Ä N ˜ EZ‡ Institute of Construction Science “Eduardo Torroja” (IETcc), CSIC, Serrano Galvache s/n, 28033 Madrid, Spain, and ENRESA (Spanish Agency for Storage of Radioactive Wastes), 7 Emilio Vargas, 28043 Madrid, Spain.
Electric arc furnace dust (EAFD), generated by the steelmaking industry, is in itself an intrinsic hazardous waste; however, the case may also be that scrap used in the process is accidentally contaminated by radioactive elements such as cesium. In this case the resulting EAFD is to be handled as radioactive waste, being duly confined in lowand medium-activity repositories (LMAR). What this paper studies is the reliability of using this radioactive EAFD as an addition in the immobilization mortar of the containers of the LMAR, that is, from the point of view of the durability. Different mixes of mortar containing different percentages of EAFD have been subjected to flexural and compressive strength, initial and final setting time, XRD study, total porosity and pore size distribution, determination of the chloride diffusion coefficient, dimensional stability tests, hydration heat, workability of the fresh mix, and leaching behavior. What is deduced from the results is that for the conditions used in this research, (cement + sand) can be replaced by EAFD up to a ratio [EAFD/ (cement + EAFD)] of 46% in the immobilization mortar of LMAR, apparently without any loss in the long-term durability properties of the mortar.
Introduction The steel industry generates a substantial amount of electric arc furnace dust (EAFD). During melting, the volatilized elements are collected as dust in filters. The chemical composition of this dust varies from one factory to another, and even for the same plant the specific composition depends on the scrap material that is used in the process, although in all the cases this dust contains hazardous metals such as Pb, Cd, Cr, Cu, or Zn. Therefore, in recent years (1, 2) disposal of this waste has become a serious problem, and several technologies have been developed in order to undertake it: On one hand several processes for separation and recovery of selected metals have been developed (3-7); on the other hand, the intention is to add EAFD to the manufacturing of cement clinkers (8-10), finally applying stabilization tech* Corresponding author e-mail:
[email protected]; telephone: +34 91 3020440; fax: +34 91 3020700. † IETcc. ‡ ENRESA. 2946
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TABLE 1. Typical Composition (Ranges of Quantity) of Elements of Interest of EAFD That Are Used in This Study element
range (%)
C Si Mn Sn Ni Cu Cr
0.2-0.4 2-4 1.8-4.5 0.03-0.3 2-3 0.2-0.6 5-15
element
range (%)
element
range (%)
P S Mo Fe Al Co
0.02-0.1 0.1-0.65 0.1-0.4 18-30 0.2-0.6 0.1-0.3
Ti Ca Pb Zn Mg V
0.1-0.2 5-10 0.2-2 5-15 2-3.5 0.01-0.05
TABLE 2. Proportion of Mixes of the Different Dosages under Study (% wt) % weight
reference
D1
D2
D3
D4
EAFD cement IV-B-32.5-SR/BC sand water additive (superplasticizer) w/c ratio EAFD/(cement + EAFD) (%)
0 31.83 55.69 11.87 0.61 0.37 0
20.96 10.87 55.69 11.87 0.61 1.09 66
20.96 31.83 34.73 11.87 0.61 0.37 40
20.96 24.21 42.38 11.85 0.6 0.49 46
14.25 25.89 45.32 14.25 0.3 0.55 35.5
niques to minimize the environmental risk associated to these wastes. Among these techniques, it is possible to distinguish between vitrification techniques (11-17); geopolymerization (18, 19); addition of pozzolanic materials, mainly lime, fly ashes, and slags (20-29); and in a lower proportion as substitution of cement as an addition to concrete (30, 31). In addition to the intrinsic hazardous nature of these wastes, accidentally what could happen is that the scrap used in the process is contaminated by radioactive elements; in this case, the resulting EAFD is to be handled as a radioactive waste; therefore, it has to be confined in LMARs, in which cement-based systems are used for immobilization and storage of low- and medium-activity wastes. In refs 30 and 31, percentages of substitution of cement of up to 5% and 10% were respectively tested, with both cases resulting in retardation of the setting process and compressive strength values in the same order of magnitude or even higher than the material of reference without any addition. However, provided that these cementitious matrixes of the repositories are to have a duration of at least 300 yr, performing a comprehensive study on the influence of the EAFD in the resulting durability and properties of the mortar cast with different amounts of said addition is necessary, which is the aim of the present paper.
Experimental Section Materials. Given that the composition of the EAFD can vary form one batch to another depending on the scrap used in the process, in Table 1 the typical composition of the powder that is used along with the estimated range of each of the elements of interest are provided. Several mixes were studied in order to determine the optimum dosage to stabilize the EAFD in a mortar matrix, the composition of which is provided in Table 2. As can be observed in Table 2, in addition to a reference mix without EAFD, four different proportions were tested. In the D1 mix, all the EAFD that was incorporated was replacing the cement. In the D2 mix, the same amount of EAFD was substituting the sand, although the same percentage of cement in the mix was maintained. D3 is a mix in which the same weight of (cement + sand), in the same proportion as these individual 10.1021/es034518p CCC: $27.50
2004 American Chemical Society Published on Web 04/08/2004
components are dosed in the reference mix, is replaced by EAFD. The D4 mix reduces the amount of EAFD by increasing the amount of water. The maximum amount of EAFD (mixes D1-D3) has been calculated by taking into consideration the fact that the contaminated EAFD is stored in 1-m3 volume bags. For the sake of simplicity, one of these bags has to be included in the filling mortar of one concrete container (in accordance to the specifications of the Spanish Low and Medium Radioactivity Waste Repository). Methods. The flexural and compressive strength at 7 and 28 d was determined (EN 196-1) for every mix outlined in Table 2. According to this standard, the test was carried out on triplicate prismatic specimens of 40 mm × 40 mm × 160 mm, the same held under water at 20 ( 1 °C until the moment of testing. According to the results that were obtained, the durability properties of the selected mixes (D3 and D4) were studied by performing the following tests: Initial and final setting time (EN 196-3): According to this standard, the mixes were tested with the Vicat’s apparatus, which introduces a needle into the mix (approximately 500 g) at regular intervals, with initial and final setting times being those that correspond to the specified penetrations. XRD study: Using a Cu KR Phillips PW1820 powder diffractometer. Total porosity and pore size distribution: Using the mercury intrusion porosimetry. Determination of effective chloride diffusion coefficient, D: Chloride transport resistance has been measured due to the fact that the matrix of the drum containers in the LMAR contains chloride ions originating from hospital waste. Steady-state migration tests were carried out using a doublecompartment cell to perform these measurements. Cylindrical mortar specimens (75 mm φ × 12 mm thick) were placed between the two chambers of the cell in which the cathode and the anode were located. Distilled water and 1 M NaCl were respectively used as anolyte and catholyte solutions. The voltage that was applied stood at 12 V DC on black steel electrodes. The effective voltage drop through the sides of the concrete disk was periodically monitored by introducing two calomel reference electrodes in both chambers. D was calculated in accordance to the procedure outlined in refs 32 and 33. Dimensional stability (ASTM C-227-81 and UNE 146508EX): In ordinary cementitious matrixes, two main causes may produce undesirable expansion: sulfate attack and alkali silica reaction. In the specific case in which EAFD is added, expansion could also be due to the reactions of some of the oxides contained in the dust. Two different standardized tests have been carried out in order to cover all the possibilities: ASTM C-227-81 and UNE 146508-EX. According to the ASTM C-227-81 standard, the specimens were stored for 7 d in a humid chamber at 96% HR, with data corresponding to this age representing the reference length to which expansion percentages refer (12 d for D3 mix). After this measurement was taken, the specimens were kept at 38 ( 2° C and 96% HR for 90 d, periodically measuring the same during said period. According to the standard, the mix is potentially reactive if expansion measured after 3 months is higher than 0.05%. According to the UNE 146508-EX standard, and in like manner to the previous case, specimens were stored for 7 d (12 d for D3 mix) in a humid chamber at 96% HR. They were then subsequently submerged in 80 °C water for 24 h, with these data representing the reference length to which the expansion percentages refer. After this measurement, the specimens were maintained submerged in a 1 N NaOH solution at 80 °C. In accordance with the standard, if the
measured expansion is lower than 0.1% after 14 d have elapsed in the alkaline solution, aggregates can then be considered as nonreactive. If the expansion is higher than 0.2%, the aggregates can be considered as potentially reactive. If the expansion stands in the range of 0.1-0.2%, the standard recommends continuing up to 28 d. If after this period expansion continues within the same range, then complementary measurements will have to be performed. During this research, specimens were monitored in all cases up to 28 d. For the following tests and provided that mix D3 includes higher amounts of EAFD than mix D4, it has been considered that if results were satisfactory enough for D3 they would be even better for D4; therefore, they have only been performed for the D3 mix. Heat of hydration (UNE 80-118-86): The method used according to the UNE 80-118-86 standard measures the hydration heat by semi-adiabatic calorimetry, likewise called the Langavant method. This method consists of the introduction of 1575 ( 1 g of fresh mortar in a Dewar vase in order to determine the amount of heat that is released from the temperature evolution. Workability of the fresh mix: This determination has been obtained by measuring the time that it takes the sample to pass through a Marsh-type 1850 cm3 cone. Leaching behavior (ANSI/ANS-16.1-1986): The ANSI/ANS16.1-1986 standard, “Measurement of the leachability of solidified low-level radioactive waste by way of a short term procedure”, is used in the field of radioactive waste to determine the release rate of a radionuclide mixed with a cementitious matrix under controlled conditions. The test consists of the immersion of the concrete specimen in water, which is then sampled and replaced at predetermined intervals duly analyzing the concentration of the species of interest in the leachate. The procedure enables the production of sufficient data in a reasonably short period of time for quality assessment purposes. In the test, the leachate is sampled and replaced after cumulative leaching times of 2, 7, 24, 48, 72, 96, 120, 456, 1128, and 2160 h as of the moment of initiation of the test. Therefore, the duration of the standard test is 90 d. Pertaining to the confinement of radioactive waste, the species of interest that is to be analyzed in this test is the cesium ion. However, the equivalent conductance and diffusion coefficient in water of chloride ions is very close to that of cesium ions (34), with chloride being somewhat more mobile than cesium ions. On the other hand, some previous work has been performed for the calculation of diffusion coefficients of Cs+ and Cl- in cementitious materials (35), obtaining very similar results. Therefore, for the sake of simplicity in the protocol and taking into account that the results obtained are conservative, the species that was to be analyzed in this research was the chloride ion.
Results and Discussion Flexural and Compressive Strength. The average results from triplicate specimens on flexural and compressive strength at 7 and 28 d for the different mixes are outlined in Figure 1. Obtaining a result for mix D1 has not been possible as the specimens were disaggregated when attempting to de-mould them at the age of 12 d. Therefore, it can be concluded that enough cement was not included in the D1 mix proportion; therefore, it is not a suitable mix for stabilization of EAFD in a radioactive repository. In that pertaining to the other mixes, from Figure 1 it can be deduced that mix D2, even with the same amount of cement, presents an average 50% increase in compressive strength with respect to the reference mortar. The first data for D3 corresponds to the 12th day as until that age the setting period is not completely finished. According to this retardaVOL. 38, NO. 10, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Flexural and compressive strength at 7 and 28 d for the different mixes.
TABLE 3. Initial and Final Setting Times for Reference, D3, and D4 Mixes initial setting time (h) final setting time (h)
reference
D3
D4
9.5 15.3
96 288
34 144
tion in the setting time, the strength at 12 d is very low. However, after 28 d the values of the D3 mortar are similar to those of the D4 mix, which shows steadily increasing values until those of the reference mortar are reached. Therefore, considering that mixes D3 and D4 do not imply a reduction in the strength values that are reached at 28 d and that they correspond to lower cement conditioned waste volumes, mixes D3 and D4 were selected to perform the durability program, assuming that a lower substitution of (cement + sand) by EAFD would lead to an improved product.
Initial and Final Setting Time (EN 196-3). The results for the setting time test according to the EN 196-3 are given in Table 3. As was expected, there is a delay in the setting time when EAFD is added, which is attributed to the fact that calcium ions that are dissolved from the clinker minerals are bonded by zinc ions to form calcium hydroxozincate hydrate (36). It is only after all the zinc is fixed that additional dissolved calcium ions initiate the ordinary setting process. Thus the delay in the setting time is higher in the case of mix D3, in which a higher percentage of EAFD has been added. XRD Results. Mortar samples that correspond to reference, D3, and D4 mixes have been analyzed by X-ray diffraction, using a Cu KR Phillips PW1820 powder diffractometer to make a comparative study of the crystal phases in the different mixes. The diffraction patterns that were obtained are outlined in Figure 2. From Figure 2, it can be deduced that there has been an important change in relation to the amount of Portlandite. No Portlandite is present in mix D3, and a significant reduction takes place for mix D4. This is attributed to the same mechanism as in the delay of the setting time, as calcium ions are dissolved from the clinker minerals, then bonding with zinc ions that are present in the EAFD to form calcium hydroxyzincate hydrate according to eq 1. It is only after all the zinc is fixed that additional dissolved calcium ions initiate the ordinary setting process:
2ZnO + Ca(OH)2 + 4H2O f CaZn2(OH)6‚2H2O (1) Development of this reaction has been corroborated by the presence of calcium hydroxyzincate hydrate in the diffraction patterns for D3 and D4 mixes, with the D3 mix presenting a higher concentration. However, despite the reduction of the alkaline reserve of the resulting matrix, when phenolphthalein is applied to the specimens of the D3 mix, the indicator turns pink, which indicates that the resulting matrix is sufficiently alkaline with a pH rate that surpasses 10 (see Figure 3). In the rest of species, as was expected, a decrease took place in the amount of anhydrous cement as well as in the crystal phases of the calcium silicate hydrates, which is
FIGURE 2. Diffraction patterns for the reference, D3, and D4 mixes: E, ettringite; F, iron oxide; P, Portlandite; Q, quartz, Z, calcium hydroxy zincate hydrate; C, calcite; A, anhydrous cement; H, calcium silicate hydrate. 2948
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TABLE 5. Effective Chloride Diffusion Coefficients, by Migration Tests, through Different Mixes Cl- Deff (cm2/s) mix reference mortar D3 D4
FIGURE 3. D3 mix proportion after applying the phenolphthalein indicator.
TABLE 4. MIP Microstructure Parameters of the Samples mix
total porosity (% vol)
mean pore size (µm)
density (g/cm3)
ref D3 D4
12.58 16.25 17.74
0.0539 0.0153 0.0470
2.129 2.151 2.063
higher in the case of the D3 mix. Likewise, a reduction takes place in the amount of calcite when the EAFD is added, although there is no defined trend for ettringite. Total Porosity and Pore Size Distribution by Mercury Intrusion Porosimetry. The results obtained from mercury intrusion porosimetry for the reference mortar and for the D3 and D4 mixes are outlined in Table 4, in which the total porosity (percent in volume), the mean pore diameter, and the density of the matrix are given. In Table 4, it can be observed that the addition of EAFD induces an increase in the total porosity. The largest increase is found in the D4 mix proportion; however, it should be observed that the mean pore diameter is lower in D4 and even lower in D3 in comparison with the reference, with a clear tendency in the density of the different matrixes not being encountered. A more detailed analysis can be performed by observing the accumulated and differential pore size distribution for the different samples, as outlined in Figure 4a,b. In Figure 4, it can be observed that the trends obtained for the D3 and D4 mixes are completely different. Despite
sample 1 2 1 2 3 1 2
individual value 10-8
6.27 × 4.51 × 10-8 1.94 × 10-8 1.82 × 10-8 1.87 × 10-8 2.68 × 10-8 3.97 × 10-8
mean value 5.39 × 10-8 1.88 × 10-8 3.33 × 10-8
the increase in the total porosity with regards to the reference mortar, in the case of D3 the main change corresponds to the pore size shifting toward smaller diameters, thus changing distribution of the pore size. The reference mortar presents its maximum porosity at around 0.4-0.5 µm, which corresponds to high capillary pores, with pores being found in the entire meso-porosity range. Addition of EAFD leads to distribution at two maximums: a small maximum at about 0.2 µm and the main one at about 0.01 µm. That is to say, the D3 mix proportion leads to refinement of the microstructure that can be attributed to packing and filling of the voids with the powder with a higher porosity in the range of gel pores. In the case of the D4 mix, the trend of porosity is closer to that of the reference mortar with maximum pore size and a shift leading to higher values that has not been attributed to the EAFD effect but to the increase in the water/ cement ratio. Determination of the Effective Chloride Diffusion Coefficient. The results that have been obtained are given in Table 5, in which it can be deduced that the addition of EAFD reduces the diffusion of chloride ions throughout the resulting matrix, which is mainly in the case of the D3 mix and in accordance with the MIP results. Dimensional Stability. As has been previously outlined, according to the ASTM C-227-81 standard the mix is potentially reactive if expansion measured after 3 months is higher than 0.05%. The results that were obtained are depicted in Figure 5, in which it can be observed that according to the tested standard the mixes are not potentially expansive, even though expansion is higher than in the reference mortar. In the UNE 146508-EX standard, if expansion is higher than 0.2%, then aggregates must be considered as potentially reactive. If expansion stands in the range of 0.1-0.2%, then the standard recommends continuing up to 28 d. Complementary measurements will have to be performed if after this period has elapsed expansion continues within the same range. In our case, specimens were monitored during 28 d.
FIGURE 4. (a) Cumulative and (b) differential pore size distribution of the samples. VOL. 38, NO. 10, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 6. Results of Heat of Hydration for Reference and D3 Mixes According to UNE 80-118-86 Standard time mix
calorimeter
reference D3
average average
unit
6h
12 h
41 h
72 h
120 h
kJ/kg 89.3 227.98 243.23 244.91 kJ/kg 9.32 10.34 12.07 15.72 80.99
TABLE 7. Results from Leaching Test for Chloride Ions According to ANSI/ANS-16.1-1986 Standard mix
sample
Dns (cm2/s)
Li
99.9% CI
r
D3
1 2
2.97 × 10-8 2.02 ×10-8
7.8 7.9
6.9-8.7 7.1-8.7
0.53 0.54
The results that were obtained are outlined in Figure 6 in which, in agreement with the other test that was performed, a higher amount of EAFD and higher expansion are observed, although always within the limit of mixes that are not potentially expansive. Therefore, according to the two different types of standards, it can be concluded that the mixes cannot be considered as potentially expansive. Heat of Hydration. The heat of hydration has been carried out on the reference mortar and on the D3 mix, in duplicate specimens and according to the UNE 80-118-86 standard. The results obtained are outlined in Table 6, in which the averaged values are presented for 6, 12, 41, 72, and 120 h.
FIGURE 5. Expansion of the mixes according to the ASTM C-227-81 standard. The results in Table 6 imply that the heat of hydration diminishes when EADF is added, with the value obtained for the D3 mix being very low. According to the standard that is used, cement can be classified as a low heat of hydration material when the value that is obtained is lower than 272 kJ/kg once 120 h have elapsed. This low value has been attributed to the retarded setting that allows dissipation of the heat that is produced.
TABLE 8. Qualitative Summary of the Influence of EAFD on Properties of the Mortar
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A qualitative summary of the influence of the EAFD on the properties of the mortar is outlined in Table 8, in which the symbols vand V respectively represent positive and negative influences, and T means that there is no significant influence. This table is to be considered as a qualitative assessment pertaining to the direction of the variables, with no further pretensions.
Acknowledgments The authors are grateful to ENRESA (Spanish Agency for Storage of Radioactive Waste) for the funding that was provided for development of this work.
Literature Cited
FIGURE 6. Expansion of the mixes according to the UNE 146508-EX standard. Workability of the Fresh Mix. Determination of the workability of the fresh mix has been performed by measuring the time used by the sample to pass through a Marsh-type 1850 cm3 cone. The result corresponding to the reference mortar stood at 31 s. In the D3 mix the mortar started to pass through the cone, although at a certain point it stopped and no more passed, even though the mix had sufficient workability and compaction when molding and after mixing it. Therefore, it can be deduced that the addition of EAFD reduces the workability of the sample. Thus, maintaining the w/c ratio at 0.49 (in like manner to the D3 mix), the dosage of the mix was changed in order to find the mix proportion that would provide the same time as the reference. The composition that was obtained incorporated 15.6% of EAFD and 12.7% of water, providing a ratio (EAFD/(cement + EAFD)] of 37.63%, which is quite high and even higher than the one that corresponds to the D4 mix. Leaching Behavior (ANSI/ANS-16.1-1986). The ANSI/ ANS-16.1-1986 standard “Measurement of the leachability of solidified low-level radioactive waste by way of a shortterm procedure” was applied on duplicate D3 specimens that contained 0.14% Cl- on mortar weight, the same cured during a period of 90 d in a humid chamber at 95% HR and 38 ( 2 °C. In this test the leachate was sampled and replaced after cumulative leaching times of 2, 7, 24, 48, 72, 96, 120, 456, 1128, and 2160 h after initiation of the test. Therefore, duration of the test stands at 90 d. The results obtained with this procedure are the leachability index (Li) and the effective diffusivity (Dns) which are given in Table 7. The higher value of Li, the higher confining ability. According to the standard that was used, the 99.9% confidence interval (CI) must be reported and likewise the correlation coefficient between Li and time (r). The value of this coefficient varies from -1 to +1, and the sign indicates whether L has a tendency to increase (+) or decrease (-) as time increases. The results in Table 7 indicates that the D3 mix presents sufficient confining ability for chloride ions, which could be extensive to Cs+ ions. As has been explained the positive “r” sign means that evolution of parameter Li tends toward higher values, which means higher confining ability as time increases. Therefore, from the results that are outlined above, it can be deduced that radioactive contaminated EAFD can be used as an addition in the immobilizing mortar for LMARs. The maximum addition tested in the present research, [EADF/ (cement + EAFD)] ) 46%, with the type of cement and mix proportion that is hereby studied does not seem to have a significant negative influence on the durability of the resulting matrix.
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Received for review May 23, 2003. Revised manuscript received January 27, 2004. Accepted March 2, 2004. ES034518P
