Chemosphere 103 (2014) 131–139
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Effect of coal combustion fly ash use in concrete on the mass transport release of constituents of potential concern Andrew C. Garrabrants a, David S. Kosson a,⇑, Rossane DeLapp a, Hans A. van der Sloot b a b
Department of Civil and Environmental Engineering, Vanderbilt University, VU Station B351831, Nashville, TN 37215, United States Hans van der Sloot Consultancy, Dorpsstraat 216, 1721 BV Langedijk, The Netherlands
h i g h l i g h t s First LEAF leaching study on US sources of concrete materials containing fly ash. Mass transport leaching from concrete and microconcrete with and without fly ash. Cumulative release dependent on liquid–solid partitioning concentration. Microconcretes (no coarse aggregate) can be concrete surrogates for leaching. Fly ash replacement causes minimal to no increases in leaching from monoliths.
a r t i c l e
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Article history: Received 27 July 2013 Received in revised form 12 November 2013 Accepted 21 November 2013 Available online 19 December 2013 Keywords: Mass transport Coal combustion fly ash Concrete Microconcrete Leaching environmental assessment framework
a b s t r a c t Concerns about the environmental safety of coal combustion fly ash use as a supplemental cementitious material have necessitated comprehensive evaluation of the potential for leaching concrete materials containing fly ash used as a cement replacement. Using concrete formulations representative of US residential and commercial applications, test monoliths were made without fly ash replacement (i.e., controls) and with 20% or 45% of the portland cement fraction replaced by fly ash from four coal combustion sources. In addition, microconcrete materials were created with 45% fly ash replacement based on the commercial concrete formulation but with no coarse aggregate and an increased fine aggregate fraction to maintain aggregate-paste interfacial area. All materials were cured for 3 months prior to mass transport-based leach testing of constituents of potential concern (i.e., Sb, As, B, Ba, Cd, Cr, Mo, Pb, Se, Tl and V) according to EPA Method 1315. The cumulative release results were consistent with previously tested samples of concretes and mortars from international sources. Of the 11 constituents tested, only Sb, Ba, B, Cr and V were measured in quantifiable amounts. Microconcretes without coarse aggregate were determined to be conservative surrogates for concrete in leaching assessment since cumulative release from microconcretes were only slightly greater than the associated concrete materials. Relative to control materials without fly ash, concretes and microconcretes with fly ash replacement of cement had increased 28-d and 63-d cumulative release for a limited number 10 comparison cases: 2 cases for Sb, 7 cases for Ba and 1 case for Cr. The overall results suggest minimal leaching impact from fly ash use as a replacement for up to 45% of the cement fraction in typical US concrete formulations; however, scenario-specific assessment based on this leaching evaluation should be used to determine if potential environmental impacts exist. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction On an annual basis, the United States (US) produces approximately 180 million cubic meters of ready mix concrete (PCA, 2013) with about 50% utilizing coal combustion fly ash as a supplemental cementitious material (Obla, 2008). Concrete materials incorporating fly ash exhibit improved handling properties as well
⇑ Corresponding author. Tel.: +1 615 322 1064; fax: +1 615 322 3365. E-mail address: [email protected] (D.S. Kosson). 0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.11.048
as higher long-term strength and durability than concretes made with portland cement alone (Liu et al., 2011; Obla, 2008; Poon et al., 2000; Duran-Herrera et al., 2011). Fly ash may replace 15– 40% of the portland cement fraction in Type IP cements used in read mix formulations, with higher replacement levels designed for specific applications (ACI, 1993, 2003; Poon et al., 2000). The fly ash disposal alternatives proposed by US Environmental Protection Agency (US EPA; Federal Register, 2010) are expected to have an impact on the beneficial uses of fly ash in commercial applications. Regulatory uncertainty surrounding the disposal rule
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and the perceived effects of proposed regulation on beneficial use of fly has been attributed as a cause of the steady, but not increasing, percentage of annual fly ash used in the cement industry (Ward, 2013). The potential for impact to the beneficial use market has increased the urgency to evaluate the potential environmental impacts of fly ash used in the concrete industry. Central to this evaluation, is the understanding of the leaching behavior of those constituents of potential concern (COPCs) in fly ash identified in disposal risk evaluations and through characterization of fly ash from a wide range of sources (US EPA, 2006; Kosson et al., 2009): antimony (Sb), arsenic (As), cadmium (Cd), chromium (Cr), lead (Pb), molybdenum (Mo), selenium (Se), thallium (Tl) and vanadium (V). Environmental assessment of concrete and cement-based materials has previously relied on the results of the Toxicity Characteristic Leaching Procedure (TCLP; EPA Method 1311) as the basis for leaching prediction (Cheng et al., 2008; Eckert and Guo, 1998; Kanare and West, 1993; PCA, 1992; Zhang et al., 2001a,b). While these studies indicate concrete and other cement-based materials are not classified as hazardous based on leaching criteria (i.e., leaching concentrations of RCRA metals are below regulatory levels), the impacts of the detectable concentrations of potentially hazardous COPCs from use of coal fly ash have been not been explored. More recently, the US EPA conducted a review available leaching data foQuantitation and detection limit values are carried r cementitious materials with and without fly ash replacement from primarily European sources (van der Sloot et al., 2012) and noted minimal impact for most species during use of fly ash-blended cements containing up to 35% coal combustion fly ash. The report cautioned that these observations were considered only indicative due to the identification of several gaps in the assembled data and underlying studies. The Electric Power Research Institute (EPRI), with partial support from the US EPA, has initiated a research project specifically to address the gaps identified in the US EPA review. The EPRI program is the first study to date that provides comprehensive testing of the equilibrium- and mass transport-based leaching from cementitious materials representative of actual US concrete formulations containing fly ash from a range of commercial sources. The program includes leaching characterization according to new US EPA testing methods with direct comparison testing results between material component sources (e.g., fly ash, cement, aggregates), control materials made without fly ash, and fly ash blended concrete materials. In Kosson et al. (2013), the pH-dependent leaching of concrete and microconcrete materials utilizing up to 45% replacement of cement with fly ash from four US coal combustion facilities showed that the liquid–solid partitioning (LSP) as a function of pH was controlled by the hydrated cement chemistry such that only limited differences in pH-dependent leaching between concrete and microconcrete materials made with and without fly ash were observed. However, the study focused on the chemical effects of leaching through aqueous partitioning of COPCs from concrete using size reduced material and approaching liquid–solid chemical equilibrium and did not address the rate of COPC release based on the physical nature of the concrete material. The current study investigates the impact of fly ash replacement for cement in the same materials through characterization of COPC transport from monolithic test specimens using EPA Method 1315. The objectives of this study are to (i) evaluate the impact of the use of coal combustion fly ash as a partial cement replacement for portland cement in concrete on the rate of COPC leaching from monolithic samples, (ii) compare the results of mass transport-based testing of fly ash concrete from US formulations and sources with the results of the primarily European material in the US EPA review (van der Sloot et al., 2012), and (iii) evaluate the suitability of using microconcrete as a surrogate for concrete in mass transport rate leaching testing.
2. Mass transport-based leaching and environmental assessment approaches An overview of leaching processes and environmental assessment methodology has been provided in Kosson et al. (2002, 2013). Mass transport, the combined result of diffusion through a tortuous pore network with aqueous partitioning at the solid– liquid interface, is the primary mechanism of constituent leaching from monolithic materials (e.g., concrete and compacted soils). The driving force for mass transport is the gradient in concentration (or thermodynamic activity) between the bulk contacting solution and the pore solution at the core of the monolith. Within the pore structure, local aqueous concentrations are controlled by the same interfacial and chemical mechanisms that dominate at equilibrium (e.g., dissolution/precipitation, adsorption/desorption, complexation, interaction with dissolved organic carbon). For the purposes of leaching evaluation, these mechanisms may be approximated by the LSP as functions of porewater pH (as presented for the current project in Kosson et al., 2013) and liquid–solid ratio L/S.
2.1. Toxicity characteristic leaching procedure Several studies have used TCLP as the basis, or a partial basis, for determining the leaching of COPCs from concretes (Cheng et al., 2008; Eckert and Guo, 1998; Kanare and West, 1993; PCA, 1992; Zhang et al., 2001a,b). TCLP is a single-batch leaching test intended to provide a leachate representative of leaching under the conditions co-disposal in a municipal solid waste landfill. However, the US EPA Science Advisory Board (SAB, 1991, 1999) and others (Eckert and Guo, 1998; Kosson et al., 2002; Thorneloe et al., 2010) have cautioned that TCLP (i) provides little relevant information for concrete assessment because the test conditions are not applicable to highly alkaline monolithic materials, (ii) the municipal solid waste landfill scenario simulated by the TCLP test condition is not indicative of actual use conditions, and (iii) single-batch tests performed on size-reduced materials do not account for the monolithic nature of concrete materials. Thus, evaluating the impacts of the fly ash source and usage rate on COPC leaching should be based on leaching approaches that provide a more fundamental understanding of the release mechanisms dominant when concrete materials are used.
2.2. Leaching environmental assessment framework The Leaching Environmental Assessment Framework (LEAF) was developed to provide a robust approach to environmental assessment through characterization of leaching behavior of a solid material (e.g., soils, concretes, process wastes, etc.) while considering a range of potential use and disposal scenarios. The leaching methods within LEAF have been thoroughly documented (Garrabrants et al., 2010, 2012a,b; Kosson et al., 2002) and recently included in SW-846, the US EPA compendium of laboratory methods (US EPA, 2013). These procedures characterize a suite of fundamental leaching properties including (i) LSP as a function of eluate pH (EPA Method 1313), (ii) LSP as a function of L/S using an upflow percolation column (EPA Method 1314) or parallel batch extractions (EPA Method 1316), and (iii) mass transport rates from monolithic and compacted granular materials (EPA Method 1315). The LEAF methods are appropriate for beneficial use evaluations in that method selection is based on material properties, fundamental leaching mechanisms, and the conditions of the anticipated utilization or disposal scenario. For concrete materials, the most applicable LEAF tests include Method 1313 to measure partitioning between solid and liquid phases and Method 1315 to provide the
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rate of COPC mass transport from the monolithic materials (i.e., the form in which the concrete materials are used). 2.3. EPA review of mass transport-based leaching from cementitious materials The US EPA reviewed the results of leaching studies conducted on cement-based materials made with CEM I portland cement or CEM II/B-V blended portland cement/fly ash from primarily European sources (van der Sloot et al., 2012). Test methods used the assembled data included the new EPA procedures as well as LEAF-analogous methods developed by the Comité Européen de Normalisation (CEN) for which correlation to the LEAF test results was demonstrated during validation of the LEAF methods (Garrabrants et al., 2012a,b). The US EPA review included 27 cement mortars and concretes made with cements containing 21–35% coal fly ash (CEM II/B-V) and 21 cement mortars and concretes made with portland cement alone (CEM I) meeting the specifications of EN 197-1 (CEN, 2011). Distributions of COPCs cumulative release after 30 d of leaching were compared between material groups and indicated somewhat greater upper bound (95th percentile) release for Sb, Be, B, Pb, and Mo from samples containing fly ash. However, the review noted several gaps in the assembled dataset including that the leaching characteristics of the fly ash source materials used in the CEM II/B-V cements were unknown. This gap infers that representativeness of the US EPA review data to the boarder range of fly ashes used in the US concrete industry is questionable. Addition gaps identified in the US EPA dataset included (i) a more limited range of fly ash replacement than frequently used in commercial applications, (ii) use of standard mortar recipes which do not represent commercial concrete formulations, and (iii) samples cured for 28 d rather than longer curing times which are indicative of commercial applications.
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formulations, respectively. Fly ash replacement for the microconcrete formulation also included a fourth fly ash source (i.e., sample code FaFA) at the 45% replacement level. For all formulations, control materials were made without fly ash. Mixed materials were cast in 10-cm (400 ) diameter 20-cm (800 ) long cylindrical molds. Samples were unmolded after an initial 24-h set and cured in a fog room at 20 °C for 28 d wrapped in moistened paper towels. For this report, all samples were cured for an additional 2 months (3 months total) in sealed plastic buckets at high relative humidity. The extended cure time was designed to allow for sufficient development of the material physicalchemical structure. Prior to testing, cured materials were dry cut perpendicular to the major axis, creating cylindrical test specimens approximately 8.9 cm (3.5") tall. 3.2. Mass transport-based leaching procedure Method 1315 (US EPA, 2013) is a semi-dynamic tank leaching procedure that involves interval leaching of monolithic or compacted granular specimens in deionized water at a leachant volume-to-specimen surface area (L/A) equal to 9 ± 1 mL cm2. The leaching solution is refreshed at specified cumulative test times (i.e., 2, 25, 48 h, 7, 14, 28, 42, 49, and 63 d) in or to maintains the driving force for mass transport between the specimen porewater and tank. Eluate concentrations of COPCs and the geometries of the tested sample are used to calculate the mean flux of COPCs released in each interval (i.e., mass released per surface area per unit time of the leaching interval) and the cumulative release of COPCs from the start of the test (total mass released per unit surface area as a function of time). In the current study, Method 1315 testing was completed in duplicate for monolithic specimens of material controls (i.e., without fly ash), concrete materials at the 20% fly ash replacement level, and both concrete and microconcrete materials at the 45% fly ash replacement level.
3. Materials and methods 3.3. Chemical analysis of eluates The collection of source materials, characteristics of the fly ash source materials, formulation of concrete and microconcrete materials, and preparation methods for the tested materials have been previously described (Kosson et al., 2013). 3.1. Concrete and microconcrete formulation and preparation Study materials were designed to represent US residential and commercial ready mix formulations and all component materials (e.g., cement, aggregates and fly ash sources) are those marketed for use in the US concrete industry. Concrete formulations (Table 1) were designed to be representative of typical US residential and commercial applications, having either relatively high porosity or relative high fly ash replacement level. In addition, a microconcrete material (Schwartzentruber and Catherine, 2000), was formulated based on the commercial mix design with substitution of fine aggregate for coarse aggregate in order to maintain the aggregate-paste interfacial surface area of the commercial formulation. Resulting microconcrete materials were monolithic in physical form and subjected to the same sample preparation and testing as the concrete formulations. Testing of microconcretes, which are easier to process for leaching tests due to absence of coarse aggregate, was intended to evaluate the potential for use of microconcretes as surrogates for concrete in leaching characterization through comparison of results to those of associated concrete formulations. Fly ash from three sources (i.e., sample codes FA02, FA18 and FA39) was used to replace a percentage of the design fraction of type I portland cement at 20% and 45% for residential (concrete) and commercial (concrete and microconcrete)
All chemical analysis were conducted by ICP-MS according to Method 6020A for As, Sb, Cd, Cr, Pb, Mo, Se, and Tl and by ICPOES following Method 6010C for concentrations of B, Ba, and V. Based on insufficient total content and low concentrations in equilibrium-based testing eluate (Kosson et al., 2013), mercury was not selected as a COPC in mass transport testing. Although this paper focuses on a select list of COPCs, leaching characterizations were based on chemical analysis of a suite of major (e.g., Ca, Si), minor (e.g., Mn, S) and trace constituents. Chemical analysis included instrument calibration, blanks, analytical spikes and replication requirements as documented prior to testing in a quality assurance project plan reviewed and concurred with by the US EPA. 4. Results and discussion 4.1. Presentation of Method 1315 data The typical presentation of Method 1315 data for a COPC includes the following (see example antimony in Fig. 1; clockwise from upper left): concentrations of COPC in the eluates, final eluate pH, COPC cumulative release (mg m2), and COPC interval flux (mg m2 s1). For Method 1315 results, all data are displayed as a function of cumulative leaching time. The cumulative release is a running total of the mass passing through the external surface area of the test specimen as a function of cumulative leaching time (e.g., the mass at the second cumulative time value is the sum of the mass released in both the first and second leaching intervals). Mean interval flux shows the mean rate of COPC mass transport
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Table 1 Mix designs for concrete and microconcrete samples. Residential concrete
Commercial concrete
Microconcrete
Control
Blend
Control
Blend
Control
Blend
Nominal mix (lb/cy) Fly ash replacement (%)
420 –
420 20
500 –
500 45
866 –
866 45
Composition (wt%) Portland cement Fly ash Water Fine aggregate Coarse aggregate Fly ash used (sample codes)
10.3 – 6.1 35.3 48.3 None
8.2 2.1 6.1 35.9 47.7 FA02 FA18 FA39
12.2 – 5.5 35.5 46.8 None
6.7 5.5 5.5 35.5 46.8 FA02 FA18 FA39
22.2 – 9.9 67.9 – None
Test material ID (sample codes)
C-20-00
C-20-02 C-20-18 C-20-39
C-45-00
C-45-02 C-45-18 C-45-39
M-45-00
12.2 10.0 10.1 67.7 – FA02 FA18 FA39 FaFA M-45-02 M-45-18 M-45-39 M-45-FaFA
Fig. 1. Leaching results for antimony from Method 1315 testing of concrete and microconcrete materials: eluate concentrations, eluate pH, cumulative mass release, and mean interval flux (clockwise from upper left).
through the exposed surface area of the test specimen. Note that flux in a diffusion-control process is proportional to the inverse square root of time while cumulative release is proportional to the square root of time. Therefore, mean interval flux decreases with cumulative leaching time and shows an inverse response in comparison to cumulative release. Each COPC-specific graph includes data quality indicators based on the analytical method
limits (ML), otherwise known as the quantitation limit, and the analytical method detection limits (MDL) determined following US EPA guidance (US EPA, 2004). Quantitation and detection limit values are carried through the data reduction process in the same manner as eluate concentrations to produce method limits and P P method detection limits for cumulative release ( ML, MDL) and flux (JML, JMDL).
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4.2. Comparison of cumulative release from Method 1315 The cumulative release for Ba, B, Cr and V are shown in Fig. 2 while similar graphs for the remaining COPCs may be found in the Supplementary Materials. Of the eleven COPCs identified for this study (i.e., Sb, As, Ba, B, Cd, Cr, Pb, Mo, Se, Tl and V) only five (i.e., Sb, Ba, B, Cr, and V) showed eluate concentrations above detection limits. The cumulative release after 63 d of leaching P was less than the cumulative quantitation limit ( ML) for all COPCs except Sb and B in M-45-FaFA and Ba for all materials other than C-45-02. In general, release concentrations were very low, which is consistent with other concrete mass transport studies (Hillier et al., 1999; Marion et al., 2005; Müllauer et al., 2012). Barium showed the greatest divergence in cumulative release graphs between all cases evaluated and most clearly displays differences between fly ash sources and fly ash replacement levels. The observed difference in leaching behavior for Ba from concrete and microconcrete materials made with fly ash is due to changes in the leachable Ba, as determined by the maximum concentration over the pH range 7 6 pH 6 13, when fly ash is incorporated into concrete. For example, the leachable Ba content for C-45-18 and C-45-39 are respectively 3 and 6 greater than in the control material (see Table S-3 in Kosson et al., 2013). The observed higher leaching concentrations for Ba in microconcrete are most likely due to the absence of limestone coarse aggregate used in the concrete. In the concrete materials, formation of barium carbonates would result in lower pore water concentrations of barium. In contrast, most other COPCs have nominal incremental additions to leachable content as a result of fly ash replacement and minimal changes in
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liquid–solid partitioning. Due to the fact that there are clear differences in mass transport results, the remaining data interpretation will focus on barium for illustrative cases. Fig. 3 shows as comparison of Ba leaching from Method 1313 (Kosson et al., 2013) to Method 1315 cumulative release. Note that the materials with the highest cumulative release in Method 1315 testing also had the highest pH-dependent data at the test condition where no acid or base was added (i.e., natural pH) which indicates that higher porewater concentrations (Method 1313) result in higher cumulative release (Method 1315) for closely related materials. This observation is consistent with the expected mass transport release mechanism where the chemical activity gradient between the porewater and the bulk leaching fluid serves as the driving force for mass transport. The correlation also is important because it demonstrates consistency in results from Methods 1313 and 1315, further indicating the utility of using Methods 1313 and 1315 as a screening evaluation tools. 4.3. Comparison to distributions of cumulative release from US EPA review The cumulative release from CEM I and CEM II/B-V concretes from the US EPA review (van der Sloot et al., 2012) were used to create a distribution of cumulative release values at 30 d of leaching based on the median value bounded by the 5th and 95th percentiles. Because these materials were tested using a variety of LEAF analogous methods and not Method 1315, some tests were of short duration such that the 30 d cumulative release data was the most statistically robust. Method 1315 28-d cumulative release
P Fig. 2. Cumulative release for Ba, B, Cr and V from Method 1315 testing of concrete and microconcrete materials. Cumulative release based on method limit ( ML) and P method detection limit ( MDL) are indicated using red dashed and red continuous lines, respectively, while a dot-dash line shows the slope of cumulative release as a 1/2 power or leaching time. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. Correlation between Method 1315 cumulative release for Ba from monolithic specimens (left) and Method 1313 concentrations (Kosson et al., 2013) at natural pH (right).
values fall within the distributions of prior results from the US EPA review of CEM I and CEM II/b-V concretes and mortars (Fig. 4). Control samples without fly ash replacement are either less than or within the 5–95th percentile range reported in the US EPA review. The cumulative release from specimens with fly ash replacement generally was less than the median values reported in the US EPA review with the exception of Ba, Cd and V. Significant factors that may be responsible for the lower cumulative release results from the current study include (i) the longer curing interval (i.e., 3 months rather than 28 d) which results in continued hydration and greater densification of cement paste within concrete and (ii) the fact that the test materials were formulated to represent actual products rather than a standard mortar as considered in the US EPA review. The formulations between US EPA reviewed materials and current study materials differ in the fly ash sources, fly ash replacement levels, and the composition and size of the aggregate materials which would influence pore structure (e.g., porosity and pore connectivity) and, hence, mass transport. Except for vanadium, none of the results from the current study was greater than the 95th percentile of the US EPA results. P For Cd and Tl, Method 1315 results were less than MDL but still greater than the majority of the CEM I and CEM II results. Discrepancies were attributed to the lower L/S in the majority of the LEAF-analogous tests and instrument, rather than method detection limit values, reported as data quality limits. The combined differences reduce the cumulative release detection limit results for data in the US EPA review by approximately a factor of 100. 4.4. Microconcrete as a leaching surrogate for concrete When pairwise comparisons are made between microconcrete materials and associated concrete specimens (Fig. 5), the cumulative release of Ba from the microconcrete is very similar to, but
slightly higher than, cumulative release for the corresponding concrete case in all cases including the pair with no fly ash replacement. Calculation of the ratio of cumulative release for microconcrete to the corresponding case for concrete indicates that the release from microconcrete is either the same as (within testing reproducibility of 23% relative standard deviation) or greater than that from the concrete in all cases for COPCs with detectable leaching (Table 2). The most significant factors in the table are approximately 2 shown for Ba when comparing C-45-02 to M-45-02. This result correlates with the approximately 2 greater maximum leaching concentration over the applicable pH domain observed for C-4502 to M-45-02. This effect may be the result of use of limestone coarse aggregate in the concrete along with quartz sand fine aggregate but only quartz sand in the microconcrete. Previous work has indicated that addition of carbonate (such as may have resulted from limestone fines) results in decreased leaching of group II elements of the periodic table. Given the uncertainty in environmental conditions external to release from the source material (e.g., wetting factors, dilution and attenuation factors), a 2 deviation in cumulative mass release is rather insignificant and can be considered based on leaching equilibrium concentration. Overall, these results indicate that microconcrete can be considered a conservative surrogate for concrete when evaluating mass-transport based leaching. 4.5. Fly ash replacement in concrete and microconcrete The impact of fly ash replacement on mass transport leaching of COPCs can be best illustrated by identifying significant differences between the cumulative release of controls (i.e., without fly ash) and blended materials with fly ash replacement at a particular point of the cumulative release curve. The cumulative release for Sb, Ba, Cr, and V after 28 and 63 d of leaching are presented in
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Fig. 4. Comparison of cumulative release at 28 d from the current study with 30 d cumulative release distributions presented as bold italic plots indicating the median, 5th P and 95th percentiles and outriggers showing minimum and maximum the data (van der Sloot et al., 2012). Cumulative release based on method limit ( ML) and method P detection limit ( MDL) for the current study are indicated using red dashed and red continuous lines, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Barium cumulative release for 45% replacement concrete and microconcrete.
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Table 2 Ratio of cumulative release of COPCs from microconcrete to the corresponding concrete after 28 d and 63 d of leaching. Microconcrete/concrete
Ratio at 28-d cumulative release
M-45-00-3m/C-45-00-3m M-45-02-3m/C-45-02-3m M-45-18-3m/C-45-18-3m M-45-39-3m/C-45-39-3m a b
Ratio at 63-d cumulative release
Sb
Ba
Cr
V
Sb
Ba
Cr
V
1.3a 1.9a 1.3a 1.2a
1.4 2.1b 1.5 1.4
1.5 1.1a 1.4 1.2
1.1 1.2 1.0 0.8
1.2 1.6 1.6a 1.2a
1.3 1.9b 1.3 1.2
1.5 1.1a 1.3 1.2
1.1 1.2 1.1 0.9
P P Cumulative release for concrete samples (denominator) is < MDL; therefore, the MDL value was used to calculate the ratio. Ratio values in bold text indicate microconcrete cumulative release outside the reproducibility range of the control material.
Table 3 Cumulative release of COPCs after 28 d and 63 d of leaching.c 28-D cumulative release (mg/m2)
C-20-00-3m C-20-02-3m C-20-18-3m C-20-39-3m C-45-00-3m C-45-02-3m C-45-18-3m C-45-39-3m M-45-00-3m M-45-02-3m M-45-18-3m M-45-39-3m M-45-FaFA-3m P MDL
Sb P < MDL P < MDL P < MDL P < MDL P < MDL P < MDL P < MDL P < MDL 0.055 0.086 0.049 P < MDL 0.16 0.048
63-d cumulative release (mg/m2)
Ba
Cr
V
Sb
Ba
Cr
V
19 8.9a 43b 66 10 4.1 44 83 15 8.5 64 120 27 0.60
0.48 0.34 0.56 0.82 0.55 P < MDL 0.58 0.51 0.85 0.31 0.81 0.60 0.96 0.30
1.9 1.9 1.7 1.9 2.3 2.2 1.8 2.4 2.6 2.5 1.8 2.0 2.1 0.90
0.077 0.079 P < MDL P < MDL 0.082 0.090 P < MDL P < MDL 0.099 0.14 0.084 0.080 0.24 0.072
27 12 61 92 15 5.5 65 130 19 11 85 160 30 0.90
0.59 P < MDL 0.72 0.96 0.70 P < MDL 0.70 0.64 1.0 P < MDL 0.95 0.77 1.1 0.45
2.7 2.6 2.3 2.6 3.2 2.9 2.5 3.2 3.6 3.4 2.6 2.8 2.9 1.4
As (0.38, 0.58) in all materials; Cd (0.10, 0.15) in all materials; Pb (0.14, 0.21) in all materials; Mo (0.46, 0.68) in all materials; Se (0.31, 0.47) in all materials; Tl (0.30, 0.46) in all materials; B (0.6, 0.9) in all materials but M-45-FaFA with measured release of 1.2 at 28 d and 1.6 at 63 d. a Bold values indicate release from fly ash materials outside the control material repeatability range. b Bold italic values indicate release from fly ash materials outside the control material repeatability range and greater than the upper bound (notable effect). P P c Cumulative release values below the cumulative MDL (mg m2): element ( MDL28d, MDL63d).
Table 3 for all materials. Repeatability limit intervals about the control material for 28- and 63-d cumulative release were developed using the overall Method 1315 repeatability of 8% derived from test method validation (Garrabrants et al., 2012b) according to the approach described in the Supplementary Materials. If fly ash replacement has no effect on leaching of COPCs, the cumulative release from materials made with fly ash would be within the repeatability limit interval of the control materials. Fly ash concrete cumulative release outside the repeatability limit interval (bold text in Table 3) would indicate notable effects of fly ash replacement on leaching; however, only values above the repeatability limit would indicate the potential for increased release in comparison to analogous materials without fly ash (bold italic values in Table 3). This analysis indicates only the potential for increased release to the environment due to the inclusion of fly ash in each concrete formulation but does not evaluate the importance of the increased release to the environment (i.e., the potential for detrimental impacts). Cumulative release of V was equal to or less than the corresponding control case for all cases tested with fly ash replacement. The cumulative release of Cr was greater than the control case only for the case of 20% replacement of portland cement with FA-39, and, notably, not for the corresponding 45% replacement concrete and microconcrete cases, C-45-39-3m, M-45-39-3m. For Sb, cumulative release was greater than the control case for two fly ashes substituted at 45% in microconcrete (M-45-02-3m, M-45-FaFA-3m), but not for the corresponding case of concrete (C-45-02-3m; for FaFA, only microconcrete was tested). Ba cumulative release was greater than the corresponding control cased
for materials with FA-18 and FA-39 at both 20% and 45% replacement levels. Overall, the variability of the cases in Table 3 where COPC cumulative release from concrete materials with fly ash replacement are greater than the corresponding control materials without fly ash does not imply systematic adverse environmental impacts. The mass transport leaching data shown here do not account for dilution and attenuation between the source material and a point of compliance. Rather, increased release in comparison to control cases may point to the need for an environmental impact screening assessment using these data as input to a source-term to determine if the increased leaching is potentially of significance to human health or the environment.
5. Conclusions Mass transport-based leach testing was conducted on residential and commercial formulations of concrete and microconcrete created with and without replacement of a portion of the portland cement fraction replace with coal combustion fly ash. The sources of fly ash and material formulations were representative of those currently in use in the US ready-mix concrete industry. For all materials, the cumulative release from EPA Method 1315 testing for eleven COPCs (i.e., Sb, As, Ba, B, Cd, Cr, Pb, Mo, Se, Tl, and V) were determined as a function of leaching time. The results indicated that the cumulative release of COPCs from commercial and residential concrete materials with fly ash replacement is very low with only Ba and V showing detectable
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cumulative release in all materials after both 28 and 63 d of leaching. Differences between materials for the cumulative release of Ba were consistent with Ba concentrations at natural pH determined in Kosson et al., 2013. Microconcretes, preferred over concrete materials for equilibrium-based tests due to lack of coarse aggregate, were shown to conservative surrogates for concretes with similar COPC cumulative release values. Based on the results of this study, the use of fly ash as a cement replacement in residential and commercial formulations of concrete material may lead to increased potential for source term release of a few COPCs; however, whether or not the increased release has significant impact on the environment needs to be assessed in further investigations. Acknowledgements This study was supported by the Electric Power Research Institute (Ken Ladwig, project manager) and the US EPA (Susan Thorneloe, project officer) through research contracts to Vanderbilt University. The authors gratefully acknowledge the contributions of Peter Kariher ARCADIS-US, Inc. (Durham, NC) for conducting total content digestions and the US EPA for reviewing the quality assurance project plan. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere.2013. 11.048. References ACI, 1993. Guide for selecting proportions for high-strength concrete with portland cement and fly ash. ACI 226.4R. ACI Committee 211 Report, American Concrete Institute, Farmington Hills, MI. ACI, 2003. Use of fly ash in concrete. ACI 232.2R-03. ACI Committee 232 Report. Cheng, C.-M., Taerakul, P., Tu, W., Zand, B., Butalia, T., Wolfe, W., Walker, H., 2008. Surface runoff from full-scale coal combustion product pavements during accelerated loading. J. Environ. Eng. 134 (8), 591–599. Duran-Herrera, A., Juarez, C.A., Valdez, P., Bentz, D.P., 2011. Evaluation of sustainable high-volume fly ash concretes. Cem. Concr. Compos. 33, 39–45. Eckert Jr., J.O., Guo, Q., 1998. Heavy metals in cement and cement kiln dust from kiln co-fired with hazardous waste-derived fuel: application of EPA leaching and acid-digestion procedures. J. Hazard Mater. 59, 55–93. Federal Register, 2010. Disposal of coal combustion residuals from electric utilities; 75 Federal Register 118 (June 21, 2010), 35127-35264. Garrabrants, A.C., Kosson, D.S., van der Sloot, H.A., Sanchez, F., Hjelmar, O., 2010. Background information for the leaching environmental assessment framework (LEAF) test, methods. EPA-600/R-10/170, November 2010. Garrabrants, A.C., Kosson, D.S., Stefanski, L., DeLapp, R., Seignette, P.F.A.B., van der Sloot, H.A., Kariher, P., Baldwin, M., 2012a. Interlaboratory validation of the leaching environmental assessment framework (LEAF) method 1313 and method 1316. EPA-600/R-12/623, September 2012. Garrabrants, A.C., Kosson, D.S., DeLapp, R., Kariher, P., Seignette, P.F.A.B., van der Sloot, H.A., Stefanski, L., Baldwin, M., 2012b. Interlaboratory validation of the leaching environmental assessment framework (LEAF) method 1314 and method 1315. EPA/600/R-12/624, September 2012.
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Hillier, S.R., Sangha, C.M., Plunket, B.A., Walden, P.J., 1999. Long-term leaching of toxic trace metals from portland cement concrete. Cem. Concr. Res. 29, 515– 521. Kanare, H.M., West, B.W., 1993. Leachability of select chemical elements from concrete. In: Proceedings of the Emerging Technologies Symposium on Cement and Concrete in the Global, Environment, 10–11 March 1993. Kosson, D.S., van der Sloot, H.A., Sanchez, F., Garrabrants, A.C., 2002. An integrated framework for evaluating leaching in waste management and utilization of secondary materials. Environ. Eng. Sci. 19 (3), 159–204. Kosson, D.S., Sanchez, F., Kariher, P., Turner, L.H., DeLapp, R., Seignette, P.F.A.B., 2009. Characterization of coal combustion residues from electric utilities – leaching and characterization data. EPA-600/R-09/151, December 2009. Kosson, D., Garrabrants, A.C., DeLapp, R., van der Sloot, H.A., 2013. pH-dependent leaching of constituents of potential concern from concrete containing coal combustion fly ash. Chemosphere. http://dx.doi.org/10.1016/j.chemosphere. 2013.11.049. Liu, R., Durham, S.A., Rens, K.L., 2011. Effects of post-mercury-control fly ash on fresh and hardened concrete properties. Constr. Build. Mater. 25, 3283–3290. Marion, A.-M., De Lanève, M., De Grauw, A., 2005. Study of the leaching behavior of paving concretes: Quantification of heavy metal content in leaching issued from tank test using demineralized water. Cem. Concr. Res. 35, 951–957. Müllauer, W., Beddoe, R.E., Heinz, D., 2012. Effect of carbonation, chloride and external sulphates on the leaching behavior of major and trace elements from concrete. Cem. Concr. Compos. 34, 618–626. Obla, K.H., 2008. Specifying fly ash for use in concrete. Concrete InFocus, Spring 2008, 60–66. . PCA (Portland Cement Association), 1992. An analysis of selected trace metals in cement and kiln dust. SP 109T. Portland Cement Association, Skokie, IL. PCA, 2013. Concrete technology. Portland Cement Association. . Poon, C.S., Lam, L., Wong, Y.L., 2000. A study on high strength concrete prepared with large volumes of low calcium fly ash. Cem. Concr. Res. 30, 447–455. SAB, 1991. Leachability phenomena. EPA-SAB-EEC-92-003. US EPA Science Advisory Board. SAB, 1999. Waste leachability: the need for review of current agency procedures. EPA-SAB-EEC-COM-99-002. US EPA Science Advisory Board. Schwartzentruber, A., Catherine, C., 2000. La méthode du mortier de béton equivalent (MBE) – un novel outil d’aide à la formulation des bétons adjuvantés (Method of the concrete equivalent mortar (CEM) – a new tool to designing concrete containing admixture). Mater. Struct. 33, 475–482. Thorneloe, S., Kosson, D.S., Sanchez, F., Garrabrants, A.C., Helms, G., 2010. Evaluating the fate of metals in air pollution control residues from coal-fired power plants. Environ. Sci. Technol. 44, 7351–7356. US EPA, 2004. Revised assessment of detection and quantitation approaches. EPA821-B-04-005, October 2004. US EPA, 2006. Human and ecological risk assessment of coal combustion wastes, Docket # EPA-HQ-RCRA-2006-0796. . US EPA, 2013. SW-846 On-line . van der Sloot, H.A., Kosson, D.S., Garrabrants, A.C., Arnold, J., 2012. The impact of coal combustion fly ash used as a supplemental cementitious material on the leaching of constituents from cements and concretes. EPA/600/R-12/704, October 2012. Ward, J.N., 2013 Stigma and regulatory uncertainty: proposed coal ash disposal regulation effects on US beneficial use markets and practices, In: 2013 World of Coal Ash Conference, 22-25 April 2013, Lexington, KY. Zhang, M.H., Blanchette, M.C., Malhotra, V.M., 2001a. Leachability of trace metal elements from fly ashes, and from concrete incorporating fly ashes. In: Seventh CENMET/ACI International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Malhotra, V.M. (Ed.), ACI International, SP-199, pp. 1–28. Zhang, M.H., Blanchette, M.C., Malhotra, V.M., 2001b. Leachability of trace metal elements from fly ash concrete: results from column-leaching and batchleaching tests. ACI Mater. J. 98 (2), 126–136.
Contents lists available at ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Effect of coal combustion fly ash use in concrete on the mass transport release of constituents of potential concern Andrew C. Garrabrants a, David S. Kosson a,⇑, Rossane DeLapp a, Hans A. van der Sloot b a b
Department of Civil and Environmental Engineering, Vanderbilt University, VU Station B351831, Nashville, TN 37215, United States Hans van der Sloot Consultancy, Dorpsstraat 216, 1721 BV Langedijk, The Netherlands
h i g h l i g h t s First LEAF leaching study on US sources of concrete materials containing fly ash. Mass transport leaching from concrete and microconcrete with and without fly ash. Cumulative release dependent on liquid–solid partitioning concentration. Microconcretes (no coarse aggregate) can be concrete surrogates for leaching. Fly ash replacement causes minimal to no increases in leaching from monoliths.
a r t i c l e
i n f o
Article history: Received 27 July 2013 Received in revised form 12 November 2013 Accepted 21 November 2013 Available online 19 December 2013 Keywords: Mass transport Coal combustion fly ash Concrete Microconcrete Leaching environmental assessment framework
a b s t r a c t Concerns about the environmental safety of coal combustion fly ash use as a supplemental cementitious material have necessitated comprehensive evaluation of the potential for leaching concrete materials containing fly ash used as a cement replacement. Using concrete formulations representative of US residential and commercial applications, test monoliths were made without fly ash replacement (i.e., controls) and with 20% or 45% of the portland cement fraction replaced by fly ash from four coal combustion sources. In addition, microconcrete materials were created with 45% fly ash replacement based on the commercial concrete formulation but with no coarse aggregate and an increased fine aggregate fraction to maintain aggregate-paste interfacial area. All materials were cured for 3 months prior to mass transport-based leach testing of constituents of potential concern (i.e., Sb, As, B, Ba, Cd, Cr, Mo, Pb, Se, Tl and V) according to EPA Method 1315. The cumulative release results were consistent with previously tested samples of concretes and mortars from international sources. Of the 11 constituents tested, only Sb, Ba, B, Cr and V were measured in quantifiable amounts. Microconcretes without coarse aggregate were determined to be conservative surrogates for concrete in leaching assessment since cumulative release from microconcretes were only slightly greater than the associated concrete materials. Relative to control materials without fly ash, concretes and microconcretes with fly ash replacement of cement had increased 28-d and 63-d cumulative release for a limited number 10 comparison cases: 2 cases for Sb, 7 cases for Ba and 1 case for Cr. The overall results suggest minimal leaching impact from fly ash use as a replacement for up to 45% of the cement fraction in typical US concrete formulations; however, scenario-specific assessment based on this leaching evaluation should be used to determine if potential environmental impacts exist. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction On an annual basis, the United States (US) produces approximately 180 million cubic meters of ready mix concrete (PCA, 2013) with about 50% utilizing coal combustion fly ash as a supplemental cementitious material (Obla, 2008). Concrete materials incorporating fly ash exhibit improved handling properties as well
⇑ Corresponding author. Tel.: +1 615 322 1064; fax: +1 615 322 3365. E-mail address: [email protected] (D.S. Kosson). 0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.11.048
as higher long-term strength and durability than concretes made with portland cement alone (Liu et al., 2011; Obla, 2008; Poon et al., 2000; Duran-Herrera et al., 2011). Fly ash may replace 15– 40% of the portland cement fraction in Type IP cements used in read mix formulations, with higher replacement levels designed for specific applications (ACI, 1993, 2003; Poon et al., 2000). The fly ash disposal alternatives proposed by US Environmental Protection Agency (US EPA; Federal Register, 2010) are expected to have an impact on the beneficial uses of fly ash in commercial applications. Regulatory uncertainty surrounding the disposal rule
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and the perceived effects of proposed regulation on beneficial use of fly has been attributed as a cause of the steady, but not increasing, percentage of annual fly ash used in the cement industry (Ward, 2013). The potential for impact to the beneficial use market has increased the urgency to evaluate the potential environmental impacts of fly ash used in the concrete industry. Central to this evaluation, is the understanding of the leaching behavior of those constituents of potential concern (COPCs) in fly ash identified in disposal risk evaluations and through characterization of fly ash from a wide range of sources (US EPA, 2006; Kosson et al., 2009): antimony (Sb), arsenic (As), cadmium (Cd), chromium (Cr), lead (Pb), molybdenum (Mo), selenium (Se), thallium (Tl) and vanadium (V). Environmental assessment of concrete and cement-based materials has previously relied on the results of the Toxicity Characteristic Leaching Procedure (TCLP; EPA Method 1311) as the basis for leaching prediction (Cheng et al., 2008; Eckert and Guo, 1998; Kanare and West, 1993; PCA, 1992; Zhang et al., 2001a,b). While these studies indicate concrete and other cement-based materials are not classified as hazardous based on leaching criteria (i.e., leaching concentrations of RCRA metals are below regulatory levels), the impacts of the detectable concentrations of potentially hazardous COPCs from use of coal fly ash have been not been explored. More recently, the US EPA conducted a review available leaching data foQuantitation and detection limit values are carried r cementitious materials with and without fly ash replacement from primarily European sources (van der Sloot et al., 2012) and noted minimal impact for most species during use of fly ash-blended cements containing up to 35% coal combustion fly ash. The report cautioned that these observations were considered only indicative due to the identification of several gaps in the assembled data and underlying studies. The Electric Power Research Institute (EPRI), with partial support from the US EPA, has initiated a research project specifically to address the gaps identified in the US EPA review. The EPRI program is the first study to date that provides comprehensive testing of the equilibrium- and mass transport-based leaching from cementitious materials representative of actual US concrete formulations containing fly ash from a range of commercial sources. The program includes leaching characterization according to new US EPA testing methods with direct comparison testing results between material component sources (e.g., fly ash, cement, aggregates), control materials made without fly ash, and fly ash blended concrete materials. In Kosson et al. (2013), the pH-dependent leaching of concrete and microconcrete materials utilizing up to 45% replacement of cement with fly ash from four US coal combustion facilities showed that the liquid–solid partitioning (LSP) as a function of pH was controlled by the hydrated cement chemistry such that only limited differences in pH-dependent leaching between concrete and microconcrete materials made with and without fly ash were observed. However, the study focused on the chemical effects of leaching through aqueous partitioning of COPCs from concrete using size reduced material and approaching liquid–solid chemical equilibrium and did not address the rate of COPC release based on the physical nature of the concrete material. The current study investigates the impact of fly ash replacement for cement in the same materials through characterization of COPC transport from monolithic test specimens using EPA Method 1315. The objectives of this study are to (i) evaluate the impact of the use of coal combustion fly ash as a partial cement replacement for portland cement in concrete on the rate of COPC leaching from monolithic samples, (ii) compare the results of mass transport-based testing of fly ash concrete from US formulations and sources with the results of the primarily European material in the US EPA review (van der Sloot et al., 2012), and (iii) evaluate the suitability of using microconcrete as a surrogate for concrete in mass transport rate leaching testing.
2. Mass transport-based leaching and environmental assessment approaches An overview of leaching processes and environmental assessment methodology has been provided in Kosson et al. (2002, 2013). Mass transport, the combined result of diffusion through a tortuous pore network with aqueous partitioning at the solid– liquid interface, is the primary mechanism of constituent leaching from monolithic materials (e.g., concrete and compacted soils). The driving force for mass transport is the gradient in concentration (or thermodynamic activity) between the bulk contacting solution and the pore solution at the core of the monolith. Within the pore structure, local aqueous concentrations are controlled by the same interfacial and chemical mechanisms that dominate at equilibrium (e.g., dissolution/precipitation, adsorption/desorption, complexation, interaction with dissolved organic carbon). For the purposes of leaching evaluation, these mechanisms may be approximated by the LSP as functions of porewater pH (as presented for the current project in Kosson et al., 2013) and liquid–solid ratio L/S.
2.1. Toxicity characteristic leaching procedure Several studies have used TCLP as the basis, or a partial basis, for determining the leaching of COPCs from concretes (Cheng et al., 2008; Eckert and Guo, 1998; Kanare and West, 1993; PCA, 1992; Zhang et al., 2001a,b). TCLP is a single-batch leaching test intended to provide a leachate representative of leaching under the conditions co-disposal in a municipal solid waste landfill. However, the US EPA Science Advisory Board (SAB, 1991, 1999) and others (Eckert and Guo, 1998; Kosson et al., 2002; Thorneloe et al., 2010) have cautioned that TCLP (i) provides little relevant information for concrete assessment because the test conditions are not applicable to highly alkaline monolithic materials, (ii) the municipal solid waste landfill scenario simulated by the TCLP test condition is not indicative of actual use conditions, and (iii) single-batch tests performed on size-reduced materials do not account for the monolithic nature of concrete materials. Thus, evaluating the impacts of the fly ash source and usage rate on COPC leaching should be based on leaching approaches that provide a more fundamental understanding of the release mechanisms dominant when concrete materials are used.
2.2. Leaching environmental assessment framework The Leaching Environmental Assessment Framework (LEAF) was developed to provide a robust approach to environmental assessment through characterization of leaching behavior of a solid material (e.g., soils, concretes, process wastes, etc.) while considering a range of potential use and disposal scenarios. The leaching methods within LEAF have been thoroughly documented (Garrabrants et al., 2010, 2012a,b; Kosson et al., 2002) and recently included in SW-846, the US EPA compendium of laboratory methods (US EPA, 2013). These procedures characterize a suite of fundamental leaching properties including (i) LSP as a function of eluate pH (EPA Method 1313), (ii) LSP as a function of L/S using an upflow percolation column (EPA Method 1314) or parallel batch extractions (EPA Method 1316), and (iii) mass transport rates from monolithic and compacted granular materials (EPA Method 1315). The LEAF methods are appropriate for beneficial use evaluations in that method selection is based on material properties, fundamental leaching mechanisms, and the conditions of the anticipated utilization or disposal scenario. For concrete materials, the most applicable LEAF tests include Method 1313 to measure partitioning between solid and liquid phases and Method 1315 to provide the
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rate of COPC mass transport from the monolithic materials (i.e., the form in which the concrete materials are used). 2.3. EPA review of mass transport-based leaching from cementitious materials The US EPA reviewed the results of leaching studies conducted on cement-based materials made with CEM I portland cement or CEM II/B-V blended portland cement/fly ash from primarily European sources (van der Sloot et al., 2012). Test methods used the assembled data included the new EPA procedures as well as LEAF-analogous methods developed by the Comité Européen de Normalisation (CEN) for which correlation to the LEAF test results was demonstrated during validation of the LEAF methods (Garrabrants et al., 2012a,b). The US EPA review included 27 cement mortars and concretes made with cements containing 21–35% coal fly ash (CEM II/B-V) and 21 cement mortars and concretes made with portland cement alone (CEM I) meeting the specifications of EN 197-1 (CEN, 2011). Distributions of COPCs cumulative release after 30 d of leaching were compared between material groups and indicated somewhat greater upper bound (95th percentile) release for Sb, Be, B, Pb, and Mo from samples containing fly ash. However, the review noted several gaps in the assembled dataset including that the leaching characteristics of the fly ash source materials used in the CEM II/B-V cements were unknown. This gap infers that representativeness of the US EPA review data to the boarder range of fly ashes used in the US concrete industry is questionable. Addition gaps identified in the US EPA dataset included (i) a more limited range of fly ash replacement than frequently used in commercial applications, (ii) use of standard mortar recipes which do not represent commercial concrete formulations, and (iii) samples cured for 28 d rather than longer curing times which are indicative of commercial applications.
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formulations, respectively. Fly ash replacement for the microconcrete formulation also included a fourth fly ash source (i.e., sample code FaFA) at the 45% replacement level. For all formulations, control materials were made without fly ash. Mixed materials were cast in 10-cm (400 ) diameter 20-cm (800 ) long cylindrical molds. Samples were unmolded after an initial 24-h set and cured in a fog room at 20 °C for 28 d wrapped in moistened paper towels. For this report, all samples were cured for an additional 2 months (3 months total) in sealed plastic buckets at high relative humidity. The extended cure time was designed to allow for sufficient development of the material physicalchemical structure. Prior to testing, cured materials were dry cut perpendicular to the major axis, creating cylindrical test specimens approximately 8.9 cm (3.5") tall. 3.2. Mass transport-based leaching procedure Method 1315 (US EPA, 2013) is a semi-dynamic tank leaching procedure that involves interval leaching of monolithic or compacted granular specimens in deionized water at a leachant volume-to-specimen surface area (L/A) equal to 9 ± 1 mL cm2. The leaching solution is refreshed at specified cumulative test times (i.e., 2, 25, 48 h, 7, 14, 28, 42, 49, and 63 d) in or to maintains the driving force for mass transport between the specimen porewater and tank. Eluate concentrations of COPCs and the geometries of the tested sample are used to calculate the mean flux of COPCs released in each interval (i.e., mass released per surface area per unit time of the leaching interval) and the cumulative release of COPCs from the start of the test (total mass released per unit surface area as a function of time). In the current study, Method 1315 testing was completed in duplicate for monolithic specimens of material controls (i.e., without fly ash), concrete materials at the 20% fly ash replacement level, and both concrete and microconcrete materials at the 45% fly ash replacement level.
3. Materials and methods 3.3. Chemical analysis of eluates The collection of source materials, characteristics of the fly ash source materials, formulation of concrete and microconcrete materials, and preparation methods for the tested materials have been previously described (Kosson et al., 2013). 3.1. Concrete and microconcrete formulation and preparation Study materials were designed to represent US residential and commercial ready mix formulations and all component materials (e.g., cement, aggregates and fly ash sources) are those marketed for use in the US concrete industry. Concrete formulations (Table 1) were designed to be representative of typical US residential and commercial applications, having either relatively high porosity or relative high fly ash replacement level. In addition, a microconcrete material (Schwartzentruber and Catherine, 2000), was formulated based on the commercial mix design with substitution of fine aggregate for coarse aggregate in order to maintain the aggregate-paste interfacial surface area of the commercial formulation. Resulting microconcrete materials were monolithic in physical form and subjected to the same sample preparation and testing as the concrete formulations. Testing of microconcretes, which are easier to process for leaching tests due to absence of coarse aggregate, was intended to evaluate the potential for use of microconcretes as surrogates for concrete in leaching characterization through comparison of results to those of associated concrete formulations. Fly ash from three sources (i.e., sample codes FA02, FA18 and FA39) was used to replace a percentage of the design fraction of type I portland cement at 20% and 45% for residential (concrete) and commercial (concrete and microconcrete)
All chemical analysis were conducted by ICP-MS according to Method 6020A for As, Sb, Cd, Cr, Pb, Mo, Se, and Tl and by ICPOES following Method 6010C for concentrations of B, Ba, and V. Based on insufficient total content and low concentrations in equilibrium-based testing eluate (Kosson et al., 2013), mercury was not selected as a COPC in mass transport testing. Although this paper focuses on a select list of COPCs, leaching characterizations were based on chemical analysis of a suite of major (e.g., Ca, Si), minor (e.g., Mn, S) and trace constituents. Chemical analysis included instrument calibration, blanks, analytical spikes and replication requirements as documented prior to testing in a quality assurance project plan reviewed and concurred with by the US EPA. 4. Results and discussion 4.1. Presentation of Method 1315 data The typical presentation of Method 1315 data for a COPC includes the following (see example antimony in Fig. 1; clockwise from upper left): concentrations of COPC in the eluates, final eluate pH, COPC cumulative release (mg m2), and COPC interval flux (mg m2 s1). For Method 1315 results, all data are displayed as a function of cumulative leaching time. The cumulative release is a running total of the mass passing through the external surface area of the test specimen as a function of cumulative leaching time (e.g., the mass at the second cumulative time value is the sum of the mass released in both the first and second leaching intervals). Mean interval flux shows the mean rate of COPC mass transport
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Table 1 Mix designs for concrete and microconcrete samples. Residential concrete
Commercial concrete
Microconcrete
Control
Blend
Control
Blend
Control
Blend
Nominal mix (lb/cy) Fly ash replacement (%)
420 –
420 20
500 –
500 45
866 –
866 45
Composition (wt%) Portland cement Fly ash Water Fine aggregate Coarse aggregate Fly ash used (sample codes)
10.3 – 6.1 35.3 48.3 None
8.2 2.1 6.1 35.9 47.7 FA02 FA18 FA39
12.2 – 5.5 35.5 46.8 None
6.7 5.5 5.5 35.5 46.8 FA02 FA18 FA39
22.2 – 9.9 67.9 – None
Test material ID (sample codes)
C-20-00
C-20-02 C-20-18 C-20-39
C-45-00
C-45-02 C-45-18 C-45-39
M-45-00
12.2 10.0 10.1 67.7 – FA02 FA18 FA39 FaFA M-45-02 M-45-18 M-45-39 M-45-FaFA
Fig. 1. Leaching results for antimony from Method 1315 testing of concrete and microconcrete materials: eluate concentrations, eluate pH, cumulative mass release, and mean interval flux (clockwise from upper left).
through the exposed surface area of the test specimen. Note that flux in a diffusion-control process is proportional to the inverse square root of time while cumulative release is proportional to the square root of time. Therefore, mean interval flux decreases with cumulative leaching time and shows an inverse response in comparison to cumulative release. Each COPC-specific graph includes data quality indicators based on the analytical method
limits (ML), otherwise known as the quantitation limit, and the analytical method detection limits (MDL) determined following US EPA guidance (US EPA, 2004). Quantitation and detection limit values are carried through the data reduction process in the same manner as eluate concentrations to produce method limits and P P method detection limits for cumulative release ( ML, MDL) and flux (JML, JMDL).
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4.2. Comparison of cumulative release from Method 1315 The cumulative release for Ba, B, Cr and V are shown in Fig. 2 while similar graphs for the remaining COPCs may be found in the Supplementary Materials. Of the eleven COPCs identified for this study (i.e., Sb, As, Ba, B, Cd, Cr, Pb, Mo, Se, Tl and V) only five (i.e., Sb, Ba, B, Cr, and V) showed eluate concentrations above detection limits. The cumulative release after 63 d of leaching P was less than the cumulative quantitation limit ( ML) for all COPCs except Sb and B in M-45-FaFA and Ba for all materials other than C-45-02. In general, release concentrations were very low, which is consistent with other concrete mass transport studies (Hillier et al., 1999; Marion et al., 2005; Müllauer et al., 2012). Barium showed the greatest divergence in cumulative release graphs between all cases evaluated and most clearly displays differences between fly ash sources and fly ash replacement levels. The observed difference in leaching behavior for Ba from concrete and microconcrete materials made with fly ash is due to changes in the leachable Ba, as determined by the maximum concentration over the pH range 7 6 pH 6 13, when fly ash is incorporated into concrete. For example, the leachable Ba content for C-45-18 and C-45-39 are respectively 3 and 6 greater than in the control material (see Table S-3 in Kosson et al., 2013). The observed higher leaching concentrations for Ba in microconcrete are most likely due to the absence of limestone coarse aggregate used in the concrete. In the concrete materials, formation of barium carbonates would result in lower pore water concentrations of barium. In contrast, most other COPCs have nominal incremental additions to leachable content as a result of fly ash replacement and minimal changes in
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liquid–solid partitioning. Due to the fact that there are clear differences in mass transport results, the remaining data interpretation will focus on barium for illustrative cases. Fig. 3 shows as comparison of Ba leaching from Method 1313 (Kosson et al., 2013) to Method 1315 cumulative release. Note that the materials with the highest cumulative release in Method 1315 testing also had the highest pH-dependent data at the test condition where no acid or base was added (i.e., natural pH) which indicates that higher porewater concentrations (Method 1313) result in higher cumulative release (Method 1315) for closely related materials. This observation is consistent with the expected mass transport release mechanism where the chemical activity gradient between the porewater and the bulk leaching fluid serves as the driving force for mass transport. The correlation also is important because it demonstrates consistency in results from Methods 1313 and 1315, further indicating the utility of using Methods 1313 and 1315 as a screening evaluation tools. 4.3. Comparison to distributions of cumulative release from US EPA review The cumulative release from CEM I and CEM II/B-V concretes from the US EPA review (van der Sloot et al., 2012) were used to create a distribution of cumulative release values at 30 d of leaching based on the median value bounded by the 5th and 95th percentiles. Because these materials were tested using a variety of LEAF analogous methods and not Method 1315, some tests were of short duration such that the 30 d cumulative release data was the most statistically robust. Method 1315 28-d cumulative release
P Fig. 2. Cumulative release for Ba, B, Cr and V from Method 1315 testing of concrete and microconcrete materials. Cumulative release based on method limit ( ML) and P method detection limit ( MDL) are indicated using red dashed and red continuous lines, respectively, while a dot-dash line shows the slope of cumulative release as a 1/2 power or leaching time. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. Correlation between Method 1315 cumulative release for Ba from monolithic specimens (left) and Method 1313 concentrations (Kosson et al., 2013) at natural pH (right).
values fall within the distributions of prior results from the US EPA review of CEM I and CEM II/b-V concretes and mortars (Fig. 4). Control samples without fly ash replacement are either less than or within the 5–95th percentile range reported in the US EPA review. The cumulative release from specimens with fly ash replacement generally was less than the median values reported in the US EPA review with the exception of Ba, Cd and V. Significant factors that may be responsible for the lower cumulative release results from the current study include (i) the longer curing interval (i.e., 3 months rather than 28 d) which results in continued hydration and greater densification of cement paste within concrete and (ii) the fact that the test materials were formulated to represent actual products rather than a standard mortar as considered in the US EPA review. The formulations between US EPA reviewed materials and current study materials differ in the fly ash sources, fly ash replacement levels, and the composition and size of the aggregate materials which would influence pore structure (e.g., porosity and pore connectivity) and, hence, mass transport. Except for vanadium, none of the results from the current study was greater than the 95th percentile of the US EPA results. P For Cd and Tl, Method 1315 results were less than MDL but still greater than the majority of the CEM I and CEM II results. Discrepancies were attributed to the lower L/S in the majority of the LEAF-analogous tests and instrument, rather than method detection limit values, reported as data quality limits. The combined differences reduce the cumulative release detection limit results for data in the US EPA review by approximately a factor of 100. 4.4. Microconcrete as a leaching surrogate for concrete When pairwise comparisons are made between microconcrete materials and associated concrete specimens (Fig. 5), the cumulative release of Ba from the microconcrete is very similar to, but
slightly higher than, cumulative release for the corresponding concrete case in all cases including the pair with no fly ash replacement. Calculation of the ratio of cumulative release for microconcrete to the corresponding case for concrete indicates that the release from microconcrete is either the same as (within testing reproducibility of 23% relative standard deviation) or greater than that from the concrete in all cases for COPCs with detectable leaching (Table 2). The most significant factors in the table are approximately 2 shown for Ba when comparing C-45-02 to M-45-02. This result correlates with the approximately 2 greater maximum leaching concentration over the applicable pH domain observed for C-4502 to M-45-02. This effect may be the result of use of limestone coarse aggregate in the concrete along with quartz sand fine aggregate but only quartz sand in the microconcrete. Previous work has indicated that addition of carbonate (such as may have resulted from limestone fines) results in decreased leaching of group II elements of the periodic table. Given the uncertainty in environmental conditions external to release from the source material (e.g., wetting factors, dilution and attenuation factors), a 2 deviation in cumulative mass release is rather insignificant and can be considered based on leaching equilibrium concentration. Overall, these results indicate that microconcrete can be considered a conservative surrogate for concrete when evaluating mass-transport based leaching. 4.5. Fly ash replacement in concrete and microconcrete The impact of fly ash replacement on mass transport leaching of COPCs can be best illustrated by identifying significant differences between the cumulative release of controls (i.e., without fly ash) and blended materials with fly ash replacement at a particular point of the cumulative release curve. The cumulative release for Sb, Ba, Cr, and V after 28 and 63 d of leaching are presented in
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Fig. 4. Comparison of cumulative release at 28 d from the current study with 30 d cumulative release distributions presented as bold italic plots indicating the median, 5th P and 95th percentiles and outriggers showing minimum and maximum the data (van der Sloot et al., 2012). Cumulative release based on method limit ( ML) and method P detection limit ( MDL) for the current study are indicated using red dashed and red continuous lines, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Barium cumulative release for 45% replacement concrete and microconcrete.
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Table 2 Ratio of cumulative release of COPCs from microconcrete to the corresponding concrete after 28 d and 63 d of leaching. Microconcrete/concrete
Ratio at 28-d cumulative release
M-45-00-3m/C-45-00-3m M-45-02-3m/C-45-02-3m M-45-18-3m/C-45-18-3m M-45-39-3m/C-45-39-3m a b
Ratio at 63-d cumulative release
Sb
Ba
Cr
V
Sb
Ba
Cr
V
1.3a 1.9a 1.3a 1.2a
1.4 2.1b 1.5 1.4
1.5 1.1a 1.4 1.2
1.1 1.2 1.0 0.8
1.2 1.6 1.6a 1.2a
1.3 1.9b 1.3 1.2
1.5 1.1a 1.3 1.2
1.1 1.2 1.1 0.9
P P Cumulative release for concrete samples (denominator) is < MDL; therefore, the MDL value was used to calculate the ratio. Ratio values in bold text indicate microconcrete cumulative release outside the reproducibility range of the control material.
Table 3 Cumulative release of COPCs after 28 d and 63 d of leaching.c 28-D cumulative release (mg/m2)
C-20-00-3m C-20-02-3m C-20-18-3m C-20-39-3m C-45-00-3m C-45-02-3m C-45-18-3m C-45-39-3m M-45-00-3m M-45-02-3m M-45-18-3m M-45-39-3m M-45-FaFA-3m P MDL
Sb P < MDL P < MDL P < MDL P < MDL P < MDL P < MDL P < MDL P < MDL 0.055 0.086 0.049 P < MDL 0.16 0.048
63-d cumulative release (mg/m2)
Ba
Cr
V
Sb
Ba
Cr
V
19 8.9a 43b 66 10 4.1 44 83 15 8.5 64 120 27 0.60
0.48 0.34 0.56 0.82 0.55 P < MDL 0.58 0.51 0.85 0.31 0.81 0.60 0.96 0.30
1.9 1.9 1.7 1.9 2.3 2.2 1.8 2.4 2.6 2.5 1.8 2.0 2.1 0.90
0.077 0.079 P < MDL P < MDL 0.082 0.090 P < MDL P < MDL 0.099 0.14 0.084 0.080 0.24 0.072
27 12 61 92 15 5.5 65 130 19 11 85 160 30 0.90
0.59 P < MDL 0.72 0.96 0.70 P < MDL 0.70 0.64 1.0 P < MDL 0.95 0.77 1.1 0.45
2.7 2.6 2.3 2.6 3.2 2.9 2.5 3.2 3.6 3.4 2.6 2.8 2.9 1.4
As (0.38, 0.58) in all materials; Cd (0.10, 0.15) in all materials; Pb (0.14, 0.21) in all materials; Mo (0.46, 0.68) in all materials; Se (0.31, 0.47) in all materials; Tl (0.30, 0.46) in all materials; B (0.6, 0.9) in all materials but M-45-FaFA with measured release of 1.2 at 28 d and 1.6 at 63 d. a Bold values indicate release from fly ash materials outside the control material repeatability range. b Bold italic values indicate release from fly ash materials outside the control material repeatability range and greater than the upper bound (notable effect). P P c Cumulative release values below the cumulative MDL (mg m2): element ( MDL28d, MDL63d).
Table 3 for all materials. Repeatability limit intervals about the control material for 28- and 63-d cumulative release were developed using the overall Method 1315 repeatability of 8% derived from test method validation (Garrabrants et al., 2012b) according to the approach described in the Supplementary Materials. If fly ash replacement has no effect on leaching of COPCs, the cumulative release from materials made with fly ash would be within the repeatability limit interval of the control materials. Fly ash concrete cumulative release outside the repeatability limit interval (bold text in Table 3) would indicate notable effects of fly ash replacement on leaching; however, only values above the repeatability limit would indicate the potential for increased release in comparison to analogous materials without fly ash (bold italic values in Table 3). This analysis indicates only the potential for increased release to the environment due to the inclusion of fly ash in each concrete formulation but does not evaluate the importance of the increased release to the environment (i.e., the potential for detrimental impacts). Cumulative release of V was equal to or less than the corresponding control case for all cases tested with fly ash replacement. The cumulative release of Cr was greater than the control case only for the case of 20% replacement of portland cement with FA-39, and, notably, not for the corresponding 45% replacement concrete and microconcrete cases, C-45-39-3m, M-45-39-3m. For Sb, cumulative release was greater than the control case for two fly ashes substituted at 45% in microconcrete (M-45-02-3m, M-45-FaFA-3m), but not for the corresponding case of concrete (C-45-02-3m; for FaFA, only microconcrete was tested). Ba cumulative release was greater than the corresponding control cased
for materials with FA-18 and FA-39 at both 20% and 45% replacement levels. Overall, the variability of the cases in Table 3 where COPC cumulative release from concrete materials with fly ash replacement are greater than the corresponding control materials without fly ash does not imply systematic adverse environmental impacts. The mass transport leaching data shown here do not account for dilution and attenuation between the source material and a point of compliance. Rather, increased release in comparison to control cases may point to the need for an environmental impact screening assessment using these data as input to a source-term to determine if the increased leaching is potentially of significance to human health or the environment.
5. Conclusions Mass transport-based leach testing was conducted on residential and commercial formulations of concrete and microconcrete created with and without replacement of a portion of the portland cement fraction replace with coal combustion fly ash. The sources of fly ash and material formulations were representative of those currently in use in the US ready-mix concrete industry. For all materials, the cumulative release from EPA Method 1315 testing for eleven COPCs (i.e., Sb, As, Ba, B, Cd, Cr, Pb, Mo, Se, Tl, and V) were determined as a function of leaching time. The results indicated that the cumulative release of COPCs from commercial and residential concrete materials with fly ash replacement is very low with only Ba and V showing detectable
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cumulative release in all materials after both 28 and 63 d of leaching. Differences between materials for the cumulative release of Ba were consistent with Ba concentrations at natural pH determined in Kosson et al., 2013. Microconcretes, preferred over concrete materials for equilibrium-based tests due to lack of coarse aggregate, were shown to conservative surrogates for concretes with similar COPC cumulative release values. Based on the results of this study, the use of fly ash as a cement replacement in residential and commercial formulations of concrete material may lead to increased potential for source term release of a few COPCs; however, whether or not the increased release has significant impact on the environment needs to be assessed in further investigations. Acknowledgements This study was supported by the Electric Power Research Institute (Ken Ladwig, project manager) and the US EPA (Susan Thorneloe, project officer) through research contracts to Vanderbilt University. The authors gratefully acknowledge the contributions of Peter Kariher ARCADIS-US, Inc. (Durham, NC) for conducting total content digestions and the US EPA for reviewing the quality assurance project plan. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere.2013. 11.048. References ACI, 1993. Guide for selecting proportions for high-strength concrete with portland cement and fly ash. ACI 226.4R. ACI Committee 211 Report, American Concrete Institute, Farmington Hills, MI. ACI, 2003. Use of fly ash in concrete. ACI 232.2R-03. ACI Committee 232 Report. Cheng, C.-M., Taerakul, P., Tu, W., Zand, B., Butalia, T., Wolfe, W., Walker, H., 2008. Surface runoff from full-scale coal combustion product pavements during accelerated loading. J. Environ. Eng. 134 (8), 591–599. Duran-Herrera, A., Juarez, C.A., Valdez, P., Bentz, D.P., 2011. Evaluation of sustainable high-volume fly ash concretes. Cem. Concr. Compos. 33, 39–45. Eckert Jr., J.O., Guo, Q., 1998. Heavy metals in cement and cement kiln dust from kiln co-fired with hazardous waste-derived fuel: application of EPA leaching and acid-digestion procedures. J. Hazard Mater. 59, 55–93. Federal Register, 2010. Disposal of coal combustion residuals from electric utilities; 75 Federal Register 118 (June 21, 2010), 35127-35264. Garrabrants, A.C., Kosson, D.S., van der Sloot, H.A., Sanchez, F., Hjelmar, O., 2010. Background information for the leaching environmental assessment framework (LEAF) test, methods. EPA-600/R-10/170, November 2010. Garrabrants, A.C., Kosson, D.S., Stefanski, L., DeLapp, R., Seignette, P.F.A.B., van der Sloot, H.A., Kariher, P., Baldwin, M., 2012a. Interlaboratory validation of the leaching environmental assessment framework (LEAF) method 1313 and method 1316. EPA-600/R-12/623, September 2012. Garrabrants, A.C., Kosson, D.S., DeLapp, R., Kariher, P., Seignette, P.F.A.B., van der Sloot, H.A., Stefanski, L., Baldwin, M., 2012b. Interlaboratory validation of the leaching environmental assessment framework (LEAF) method 1314 and method 1315. EPA/600/R-12/624, September 2012.
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