Chemosphere 103 (2014) 140–147
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pH-dependent leaching of constituents of potential concern from concrete materials containing coal combustion fly ash David S. Kosson a,⇑, Andrew C. Garrabrants 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. pH-dependent leaching from concrete/microconcrete with and without fly ash. Cement chemistry controls pH-dependent Sb, As, B, Cr, and Se concentrations. Microconcretes (no coarse aggregate) can be conservative concrete surrogates. Screening indicates insignificant impact for up to 45% fly ash replacement.
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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: Equilibrium-based leaching Coal combustion fly ash Concrete Microconcrete Leaching environmental assessment framework
a b s t r a c t Current concerns about the environmental safety of coal combustion fly ash have motivated this evaluation of the impact of fly ash use as a cement replacement in concrete materials on the leaching of constituents of potential concern. The chemical effects of fly ash on leaching were determined through characterization of liquid–solid partitioning using EPA Method 1313 for four fly ash materials as well as concrete and microconcrete materials containing 0% (control materials), 25% and 45% replacement of portland cement with the fly ash source. All source materials, concrete formulations and replacement levels are representative of US concrete industry practices. Eluate concentrations as a function of pH were compared to a broader range of available testing results for international concretes and mortars for which the leaching characteristics of the component fly ashes were unknown. The chemistry of the hydrated cement fraction was found to dominate the liquid–solid partitioning resulting in reduced leaching concentrations of most trace metals compared to concentrations from fly ash materials alone. Compared to controls, eluate concentrations of Sb, As, B, Cr, Mo, Se, Tl and V from concrete products containing fly ash were essentially the same as the eluate concentrations from control materials produced without fly ash replacement indicating little to no significant impact on aqueous partitioning. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In 2011, approximately 12 million short tons of coal combustion fly ash (20% of the total fly ash production) were used in the United States (US) cement industry as supplemental materials in concrete products and grouts (ACAA, 2011). Replacing a portion of the portland cement fraction with fly ash is beneficial in that it improves the handling and performance of concrete materials while simultaneously reducing the need for production of virgin cement. In terms of blended hydraulic cements, the EN-197 product specification (CEN, 2011) allows for fly ash replacement of 21– 35% under the European CEM II/B-V (portland-fly ash) designation while ASTM standard C-595 (ASTM, 2012) specifies replacement ⇑ 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.2013.11.049
levels of up to 40% in Type IP (portland–pozzolan) cements. The fly ash replacement levels for the US ready mix concrete industry generally range between 15% and 25% of the total cement fraction (ACI, 1993); however, higher rates may be used in formulations for high-volume concrete applications to minimize the effect of cracking, sulfate attack or alkali–silicate reactions (ACI, 2003). The proposed US Environmental Protection Agency (EPA) alternatives for disposal of fly ash (Federal Register, 2010) have led to concerns regarding how regulation, when finalized, may impact the perception of fly ash now beneficially used in the concrete industry (Ward, 2013). The proposed alternatives have also fueled the ongoing concerns of environmental advocates regarding the potential for increased release of hazardous constituents from concrete materials containing fly ash. The relevant constituents of potential concern (COPCs) for fly ash identified by US EPA, based on risk evaluations of fly ash impoundments and landfills (US EPA,
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2006), include antimony (Sb), arsenic (As), boron (B), cadmium (Cd), chromium (Cr), cobalt (Co), lead (Pb), mercury (Hg), molybdenum (Mo), selenium (Se), thallium (Tl), and vanadium (V). Unfortunately, little scientific evidence supports or assuages the concerns that fly ash use in concrete poses a threat to the environment through leaching of COPCs. In a recent US EPA review of available leaching data from concretes and mortars of primarily European origin (van der Sloot et al., 2012), the impact of fly ash addition to concretes and mortars was shown to minimally increase release of some COPCs. However, the review identified several information gaps including (i) gaps in the available data regarding facility configurations of fly ash sources, (ii) lack of comparative characterization of fly ash source materials, (iii) a limited range of fly ash replacement rates, consistent with EN 197-1 but not representative of US concrete applications, and (iv) standard mortar mixes that are not representative of US concrete formulations. This paper focuses on the chemical aspects of leaching through evaluation of pH-dependent release of COPCs in residential concrete and commercial concrete formulations created using fly ash from four US sources. This study is part of an ongoing research project, supported by the Electric Power Research Institute (EPRI) with partial support from the US EPA, to address the gaps in the US EPA review by evaluating the potential for environmental impacts when fly ash is used as a cement replacement in concrete materials typical of those in US production. Therefore, the project includes source materials (e.g., fly ash, cement, aggregates) currently in use within the US ready mix concrete industry, characterization of source materials and cement-based materials made from those sources, and fly ash replacement levels of 20% and 45% using concrete formulations consistent with residential and commercial ready mix applications. Data distributions from US EPA characterization of electric utility fly ash materials (Kosson et al., 2009; Thorneloe et al., 2010) and from the US EPA review of cementitious materials (van der Sloot et al., 2012) are used as a comparative basis for discussion of leaching results. 2. Environmental assessment of concrete and fly ash Many of the US studies on the COPC release from constructiongrade concrete have been based on results of single-point leaching tests, such as the Toxicity Characteristic Leaching Procedure (TCLP; Method 1311; US EPA, 2013) or the Synthetic Precipitation Leaching Procedure (SPLP; Method 1312; US EPA, 2013). These procedures provide an extract produced under a single set of test conditions designed to simulate leaching in a particular release scenario. For example, TCLP is intended to identify hazardous materials under RCRA for disposal purposes by simulating the mismanagement scenario of disposal of wastes in a municipal landfill. However, the US EPA Science Advisory Board (SAB, 1991, 1999) and others (Kosson et al., 2002; Thorneloe et al., 2010) have noted that single-point methods are insufficient for characterizing leaching over the range of plausible environmental conditions expected in many specific use or disposal scenarios or over the lifetime of materials subject to varying environmental conditions. 2.1. Leaching environmental assessment framework (LEAF) In response to the over-broad application of single-extraction leaching tests, the US EPA, in collaboration with Vanderbilt University and European partners, has developed the Leaching Environmental Assessment Framework (LEAF) as a flexible, broad-based alternative for leaching assessment (Kosson et al., 2002). The four LEAF leaching methods have been well-documented (Garrabrants et al., 2010, 2012a,b) and have been incorporated into the EPA compendium of leaching tests, SW-846 (US EPA, 2013). These
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leaching methods characterize (i) the liquid–solid partitioning (LSP) of COPCs as a function of eluate pH (EPA Method 1313), (ii) the LSP of COPCs as a function of liquid-to-solid ratio (L/S) using either an up-flow percolation column (EPA Method 1314) or parallel batch extractions (EPA Method 1316), and (iii) the rate of COPC mass transport from monolithic or compacted granular materials (EPA Method 1315). The LEAF methods are analogous to leaching procedures currently being finalized in Europe by the Comité Européen de Normalisation (CEN, 2013) and correlation between like methods was demonstrated during LEAF method validation studies (Garrabrants et al., 2012a,b). Selection of appropriate LEAF methods to characterize a specific material for an identified use or disposal scenario is based on the physical form of the material (i.e., granular or monolithic) and the anticipated environmental leaching conditions. For cementbased materials which typically form relatively low-permeability monoliths upon curing, the most appropriate leaching methods include Method 1313 data to approximate local chemical equilibrium and to provide information on the thermodynamic driving force for mass transport and Method 1315 to account for the physical nature of the solid material by providing information on the intrinsic rate of leaching due to mass transport from the monolith. In addition, LSP results from Method 1313 can be used as a screening basis for evaluating leaching because these results can be considered a limit case for water in contact with monolithic materials for long duration.
2.2. LSP characterization of fly ash for concrete replacement The LEAF methods have been used in previous US EPA characterizations of fly ash materials from a variety of coal combustion solids (Sanchez et al., 2006, 2008; Kosson et al., 2009). Differences in Method 1313 concentrations were shown to be dependent on the coal type combusted, air pollution control configuration, and facility operating conditions. When compared over the pH range 7 6 pH 6 13, eluate concentrations varied by several orders of magnitude. The pH range between 7 and 13 was considered applicable and relevant to the service life of cementitious materials, encompassing the relevant pH for porewater of fresh materials (e.g., dissolution portlandite at pH12.5) and aged materials (e.g., carbonate equilibrium at pH8.2). Recognizing that not all fly ash materials are appropriate for use in concrete, the evaluation of potential impacts of fly ash concrete should focus on only those fly ashes sources representative of the materials actually used in concrete applications.
2.3. EPA review of LSP in cementitious materials The recent US EPA review of available leaching data (van der Sloot et al., 2012) included comparison of the results of LEAF testing or LEAF-analogous tests from 15 cement mortars and concretes made with CEM II/B-V fly ash-blended cements and 17 CEM I portland cements. For each material grouping, the COPC leaching test results indicated a span of two or more orders of magnitude in the pH-dependent leaching (e.g., LSP as a function of pH) at specific pH values. Statistical distributions of the measured LSP response for CEM I and CEM II/B-V materials were developed using the median LSP concentration over the pH range bounded by the 5th and 95th percentile of eluate concentrations. Distribution comparisons showed considerable overlap between CEM I and CEM II/B-V, indicating no systematic difference in LSP response for many cases. However, this review can be considered only indicative of generalized behavior due to the limitations associated with the available leaching dataset as described above.
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3. Materials and methods Source materials for formulation of concrete and microconcrete materials were collected from a commercial concrete vendor or obtained directly from the coal combustion facility. All batch formulations, materials sampling and preparation, and chemical analyses were carried out according to a quality assurance project plan reviewed by US EPA prior to testing. Quality assurance for chemical analysis included evaluation of instrument calibration, blanks, analytical spikes and replication as discussed in the Method 1313 validation report (Garrabrants et al., 2012a).
3.1. Fly ash, cement and aggregates Fly ash samples from four coal combustion sources currently marketed and used for commercial concrete applications were tested. Comparison of fly ash source facility configurations (Table 1) indicates that two facilities burned bituminous coal (fly ash source codes FA02 and FaFA) while the other two facilities burned subbituminous coal (fly ash source codes FA18 and FA39). ASTM type I portland cement (analogous to CEM I designation in Europe), river sand and coarse limestone aggregate were obtained from a commercial concrete vendor and used for all cement-based materials. All bulk materials were homogenized prior to mixing and subsamples were collected for microwave-assisted digestion conducted by ARCADIS-US, Inc. (Durham, NC) following SW-846 Method 3052 and Method 3051A (US EPA, 2013).
3.2. Concrete and microconcrete formulation and preparation Concrete formulations in Table 2 were designed to be typical of residential and commercial concretes with either relatively high porosity or high fly ash replacement levels. Replacement levels of 20% and 45% of the portland cement fraction were used for residential and commercial formulations, respectively. In addition, a microconcrete recipe at the 45% replacement level was derived from the commercial concrete formulation by replacing the coarse aggregate fraction with fine aggregate at amounts necessary to maintain the aggregate-paste interfacial area of the concrete. Similar microconcrete materials have been employed to provide a mortar-like material with the rheological properties of concrete (Schwartzentruber and Catherine, 2000). In the current study, leaching from microconcrete materials was characterized to evaluate potential for use of microconcretes as surrogates for concrete materials in future leaching evaluations. The rationale for this evaluation was that microconcretes are easier to handle than concretes
during sample preparation for equilibrium-based testing (e.g., particle size reduction) due to the absence of course aggregate. From each of the three formulations in Table 2, four materials were blended and cured: a control material without fly ash replacement (control) and three fly ash replacement materials with the FA02, FA18 and FA39 fly ash sources. In addition, a fourth fly ash amended microconcrete material was created using the FaFA fly ash source. The water-to-binder (i.e., cement and fly ash) ratio was fixed at 0.45 for the residential concrete and 0.58 for the commercial concrete and microconcrete formulations. The application rate of a high-range water reducer was customized for each concrete material to maintain the design slump of 10 cm (400 ) in the residential concrete and 12 cm (500 ) in the commercial concrete. All materials were prepared in 0.33 cubic yard batches and cast into 36 cylindrical molds of 10-cm diameter 20-cm length. After an initial 24-hour set, specimens were released from the molds, wrapped in moist paper towels and cured for one week in a fog room at 20 °C. For the remainder of the curing time, specimens were stored at high relative humidity in sealed 5-gallon plastic buckets. Specimens were divided into curing times of 28 days, 3, 6, and 12 months for ongoing experiments; however, materials used for the leach testing shown here were prepared immediately following a 3-month cure.
3.3. pH-dependent leaching test Method 1313 (US EPA, 2012b) is a parallel batch extract test consisting of nine parallel batch extractions at L/S 10 mL/g-dry and targeted end-point pH values, using dilute nitric acid or potassium hydroxide to adjust pH. A tenth extraction at L/S 10 mL/g-dry with no acid or base addition provides the LSP at the ‘‘natural pH’’ of the solid material. Method 1313 may be used to estimate COPC concentrations at conditions approaching chemical equilibrium, although it is unlikely that complete chemical equilibrium is obtained within the duration of the leaching test for all COPCs and test conditions. The observed LSP from Method 1313, therefore, represents a pragmatic approach to approximate chemical equilibrium over a broad range of test conditions which is sufficient for the environmental assessment purposes. In addition, the broad pH domain of Method 1313 between pH 2 and 13 allows for estimation of the available fraction of a COPC for leaching through consideration of dissolution of sorptive iron phases (pH2) and increased solubility for readily leachable COPCs either at the acidic pH for many metals, near neutral pH for many oxyanions, and alkaline pH for some amphoteric elements. In the current study, Method 1313 was conducted in duplicate on fly ash source materials and in triplicate for concrete and
Table 1 Facility configurations for US coal combustion fly ash sources. Sample code:
FA02
FA18
FA39
FaFA
Source/Reference Fly Ash Class Total Calcium (wt%) Annual production Boiler type Unit size Coal type Particulate capture Hg sorbent injection Targeted market
EPRI F 4 50–200 K tons Pulverized coal 667 MW Bituminous Baghouse filter None Concrete
EPRI C/F 17 >200 K tons Pulverized coal 1376 MW Sub-bituminous Hot-side ESPa Activated carbon Mostly concrete
EPRI C 23 50–200 K tons Pulverized coal 550 MW Sub-bituminous Cold-side ESP None Concrete/geotechnical
EPAb F 12 50–200 K tons Pulverized coal 600 MW Bituminous Cold-side ESPb None Concrete
See (Kosson et al., 2009) for general facility configurations and fly ash characteristics and sample code EaFA in (Garrabrants et al., 2012a). Sample code FaFA is from the same source as EaFA but after carbon removal. a ESP = electrostatic precipitator. b To prepare fly ash for concrete use, residual carbon is removed from the produced fly ash (15% carbon) via an electrostatic/physical removal process after ESP collection to yield fly ash sample FaFA (2.5% carbon).
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D.S. Kosson et al. / Chemosphere 103 (2014) 140–147 Table 2 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
10.3 – 6.1 35.3 48.3
8.2 2.1 6.1 35.9 47.7
12.2 – 5.5 35.5 46.8
6.7 5.5 5.5 35.5 46.8
22.2 – 9.9 67.9 –
12.2 10.0 10.1 67.7 –
Fly ash used (sample codes)
None
FA02 FA18
None
FA02 FA18 FA39
None
FA02 FA18 FA39 FaFA
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
M-45-02 M-45-18 M-45-39 M-45-FaFA
microconcrete materials. Concrete and microconcrete materials were particle size reduced to 85 wt% less than 2 mm by crushing while fly ash materials were tested as received. The extraction interval for all tests was maintained at the 48-hour interval associated with the particle size of the concrete and microconcrete materials (Garrabrants et al., 2010). The eluate concentration results from test replicates were interpolated to target pH values as discussed in Method 1313 validation report (Garrabrants et al., 2012a) and the log-mean of replicates used in figures to provide clarity of presentation. 3.4. Chemical analyses Chemical analyses of digested solid materials and leach testing eluates were conducted by ICP-MS (Method 6020A) for As, Sb, Cd, Cr, Pb, Mo, Se, and Tl and ICP-OES (Method 6010C) for concentrations of B, Ba, and V. The total Hg content in the fly ash source materials and representative fly ash leaching test eluates were below relevant levels; therefore, Hg analysis was not included in subsequent analyses. Although only a select list of COPCs is presented here; leaching evaluation was based on analysis of a full suite of major (e.g., Ca, Si), minor (e.g., Mn, Zn) and trace constituents. 4. Results and discussion In Figs. 1 and 2, the log-mean of Method 1313 test replicate eluate concentrations for Sb, As, B, Cr, and Se are plotted as a function of target pH for (i) each fly ash source (left) and (ii) concrete and microconcrete materials with 45% replacement of portland cement with fly ash (right). Similar comparisons for other COPCs and a comparison of measured replicate results to calculated log-means may be found in Supplementary materials, Figs. S3 and S4 and Fig. S5, respectively. Overlain on the Method 1313 results graphs are the distributions of LSP results from US EPA fly ash characterization (Kosson et al., 2009; Thorneloe et al., 2010) and the US EPA review of cement-based materials (van der Sloot et al., 2012) to illustrate how leaching data from newly characterized materials relate to the broader range of leaching from previously characterized materials. In each graph, the analytical method limit (ML) and the analytical method detection limit (MDL) for the COPC are displayed as data quality indicators (US EPA, 2004). Also shown on each graph is a horizontal dot-dash line representing the maximum contaminant level (MCL) or drinking water equivalent level (DWEL) as outlined in US EPA guidance (US EPA,
2012a). These lines are used as reference for the pH-dependent leaching concentrations. However, comparison of leaching test results to drinking water standards should be regarded only as an initial or screening assessment, recognizing COPC concentrations in the field will undergo dilution and attenuation during transport to the point of compliance, or may be up to a factor of approximately thirty greater in pore water within the concrete for highly soluble species because of the lower liquid to solid ratio at pore water conditions. A more thorough evaluation can be achieved when appropriate by integrated use of Method 1313 and Method 1315 (mass transport rate) testing results. Furthermore, results of Method 1313 and Method 1315 can be used in conjunction with chemical speciation based mass transport modeling to provide estimates of release under conditions not readily amenable to laboratory testing. Thus, leaching concentrations less than the relevant MCL or DWEL can be viewed as an indicator of environmental safety, while concentrations above the MCL or DWEL are not necessarily indicative of adverse environmental impacts.
4.1. Fly ash versus concrete Comparison of leaching concentrations from fly ash sources to cement-based materials (i.e., comparing data from left to right, respectively, in Figs. 1 and 2) show that the band of eluate concentrations for concrete materials as a whole is typically lower and narrower than the band of concentrations for fly ash sources. The most striking examples are where the eluate concentration of a specific COPC at pH > 7 is substantially reduced in one or more of the concrete/microconcrete data series relative to the profile for the corresponding fly ash material (e.g., Sb in microconcrete M45-FaFA versus fly ash FaFA, As and V in M-45-FaFA versus FaFA and M-45-02 versus FA02, and B in all cases). The results clearly indicate that simple dilution of fly ash leaching by incorporation of fly ash into the cement matrix is not a dominant factor controlling leaching. If fly ash dilution was controlling COPC leaching, the resultant pH-dependent LSP would be a simple translocation of the eluent concentrations at each pH point to that of only 2%, 5.5% and 10% of the fly ash alone concentrations which is not the case. Furthermore, eluate concentrations at low pH from the cementitious materials are nearly the same as those in fly ash alone. As a whole, these observations indicate that the cement chemistry of the hydrated cement paste or cement/fly ash paste in each concrete or
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Fig. 1. Method 1313 leaching test results for fly ash samples (left) and concrete and microconcrete samples after 3 months of curing (right) compared to shaded regions representing 5th and 95th percentiles with medians of test results from previous studies (Kosson et al., 2009; Thorneloe et al., 2010; van der Sloot et al., 2010). The legend is common for Figs. 1 and 2.
microconcrete material controls the leaching concentration of most COPCs at alkaline conditions. For Sb, As, Ba, B, Cr, Pb, Mo, Se and V, distinct changes in the LSP trendline as a function of pH from fly ash to the cement material are reflective of changes in the solid phase speciation, likely from phenomena such as sorption to or co-precipitation with calcium silicate hydrate, portlandite, ettringite and monosulfate cement phases (van der Sloot, 2000, 2002; Halim et al., 2004; Bonhoure et al., 2006; Evans, 2008). For example, As is known to be associated with calcium-bearing minerals, e.g., Ca3(AsO4)2 and CaHAsO3 (Dutré, 1997), which precipitate at the natural pH of concrete materials. Such comparisons indicate that, although they may leach from a fly ash at high concentration, COPCs are relatively well-retained within the cement-based material at cement replacement levels of up to 45% of the portland cement fraction. While results of pH-dependent leaching tests are consistent with these phenomena, further investigation is required to confirm the degree impact of these mechanisms on COPC retention. 4.2. Representativeness of LSP behavior Comparing LSP data from four selected fly ash sources specifically selected for this study (i.e., data points in left-hand panels of Figs. 1 and 2) to the distribution of results from US EPA fly ash characterization (i.e., the shaded areas in the same panels) indicates that the selected fly ashes are a good representation of
the broader domain of fly ash leaching behavior for Sb, As, Ba, B, Cr, and V. However, the fly ash sources under-represent the domain of fly ash materials for Cd, Mo, Se, and Tl with the ranges of eluate concentrations from the selected fly ash sources. For example, the LSP curves of Se from the four fly ash sources are approximately equal to or less than the median pH-dependent leaching values reported in the US EPA fly ash studies inferring that other individual fly ash sources may exhibit greater leaching of Se over the entire pH domain. However, since not all fly ash sources have the physical and chemical properties appropriate for use in concrete, fly ash sources with greater leaching behavior than the evaluated fly ash sources might not be used in concrete production. In addition, Method 1313 eluate concentrations on fly ash source that approach the MCL do not take into consideration (i) incorporation of the COPC into the concrete matrix, (ii) the considerable dilution and attenuation between the concrete material (i.e., the source term) and the receptor, and (iii) the relatively low mobility of many of these COPCs in the environment. However, such comparisons show that the range of LSP behavior of a select number of fly ash sources used in the US concrete industry is consistent with the results observed for the broader range of fly ash sources previously characterized by the US EPA. For concrete materials, the range of LSP data from concrete and microconcrete samples (i.e., data points on the right-hand panel of Figs. 1 and 2) corresponded with the range of LSP behavior of CEM I and CEM II/B-V concretes and mortars from the US EPA review (i.e.,
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Fig. 2. Method 1313 leaching test results for fly ash samples (left) and concrete and microconcrete samples after 3 months of curing (right) compared to shaded regions representing 5th and 9th percentiles with medians of test results from previous studies (Kosson et al., 2009; Thorneloe et al., 2010; van der Sloot et al., 2010). The legend is common for Figs. 1 and 2.
shaded areas in the same panels). In the pH range above 7, measured eluate concentrations of As, B, Cd, Cr, Se and V were consistent with either the full range of the LSP distribution or the median to upper bound of the distributions. The LSP data for Sb, B, and Ba were between the median and lower bound of the earlier reported LSP distributions. Components of concrete other than the fly ash also may be a source of some of these constituents.
for portland cement in concrete materials and mortar-like materials up to a 45% replacement level are unlikely to significantly increase the liquid–solid partitioning of COPCs above that of material not incorporating fly ash. This result significantly expands the demonstrated range of fly ash replacement for cement without adverse impact to leaching of COPCs and validates that applicability to a wide range of fly ashes produced and marketed in the US.
4.3. Fly ash incorporation into concrete and microconcrete materials
4.4. Release estimates using LSP data
When LSP results are compared within a concrete or microconcrete formulation (e.g., material with fly ash replacement compared to the associated control), only minor differences in LSP are observed. Eluate concentrations of Sb, As, Cd, Cr, Pb, Se and V from concrete materials with fly ash replacement were bounded by or only slightly different than the eluate concentrations control materials made without fly ash. Also, for the most part, the results for the tested concrete and microconcrete samples were consistent with the mortar results reported by US EPA (van der Sloot et al., 2012). Thus, the use of coal combustion fly ash as a replacement
A first-order screening estimate of release from these concrete and microconcrete materials may be made using the LSP data presented here. Release values (mg of COPC/kg of material) were calculated by multiplying eluate concentrations (mg of COPC/L of eluate) by the extract-specific L/S of the leaching test (L of eluate /kg of material). Release for the maximum eluate concentration (i) across the entire Method 1313 domain (2 6 pH 6 13), (ii) across the applicable pH domain for cementitious materials (7 6 pH 6 13), and (iii) at the natural pH test position are shown in Table 3 for select COPCs leached from the microconcrete
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samples. Also shown in Table 3 are the results of total content analysis for these COPCs and materials. Complete results for all materials and COPCs are provided in the Supplementary materials, Tables S1 through S3. The values in each table represent four estimates of COPC release for comparison between like materials (e.g., 45% replacement concretes or microconcretes). In the absence of leaching data, the most conservative estimate of environmental release would be the total content which assumes that all of the COPC is available for leaching and is soluble under the conditions of the field scenario. The available content, considered the maximum cumulative amount of a constituent that is likely to leach under a range of environmental conditions, may be estimated by multiplying the maximum eluate concentration measured over the Method 1313 test pH domain by the test-specific L/S of 10 L/kg. Comparison of total content and availability date in Table 3 supports the conclusions of previous research (van der Sloot, 2000; Schiopu et al., 2007; Kosson et al., 2009; Thorneloe et al., 2010) that total content and availability cannot be correlated for most COPCs in fly ash and cement materials. Relative to total content, the availability ranged from a high of 42% for As in M-45-FaFA to a low of 6.5% for V in M-45-00, but was variable between materials and COPCs. Therefore, for most scenarios, environmental decisions based on total content are not appropriate. Screening-level release values may be calculated using the maximum concentration over the pH domain considered applicable for the material lifetime (i.e., for concrete materials an applicable pH range between 7 and 13. Comparing the estimated maximum release over the applicable pH domain to the availability indicates that LSP is controlled by either solubility constraints or sorption equilibria at the natural pH. The corresponding concentration at the maximum release over the specified pH domain also may be
used as the maximum local equilibrium concentration for assessment purposes. The reported results from Method 1313 testing relate to the LSP as a function of end-point pH from the extraction of size-reduced materials at test conditions approaching chemical equilibrium, the results presented here focus on the inherent chemistry of leaching and do not consider the further effects of low liquid-to-solid ratio in the porewater of the material coupled with the monolithic physical form of the material which can further limit the leaching of COPCs. Less conservative estimates of COPC release for intact concrete materials can be made using Method 1313 data in conjunction with mass transport test leaching data (Garrabrants et al., 2013). In addition, estimation of COPC release over the life span of concrete materials should consider the effects of long-term exposure to the environment such as by carbonation which may alter LSP behavior and mass transport. Alkaline materials, such as concrete, are particularly susceptible to carbonation which is a natural aging process where atmospheric carbon dioxide partitions into the pore solution and reacts with calcium and other divalent cations. The result of carbonation is neutralization of porewater, formation of calcite and a loss of some mineral phases such as ettringite that may provide adsorption sites for or incorporate COPCs. While pH dependent equilibrium testing (such as by Method 1313) has provided a good estimate for LSP of many constituents for carbonated materials when the change in natural pH is considered, previously published literature on cement stabilized materials indicates that carbonation can lead to increased leaching of As through changes in speciation and sorption over the applicable pH domain (Sanchez et al., 2002; Garrabrants et al., 2004). However, the data present here does not address the chemical effects of carbonation on
Table 3 Release estimates (mg/kg) for microconcrete materials based on total content,a available content,b maximum concentration over applicable pH range,c and concentration at natural pH.d
a
Sb
As
B
Cr
Mo
Se
Tl
V
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
M-45-00 Total content Available Content Max at 7 6 pH 6 13 At Natural pH of 12.2
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Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
pH-dependent leaching of constituents of potential concern from concrete materials containing coal combustion fly ash David S. Kosson a,⇑, Andrew C. Garrabrants 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. pH-dependent leaching from concrete/microconcrete with and without fly ash. Cement chemistry controls pH-dependent Sb, As, B, Cr, and Se concentrations. Microconcretes (no coarse aggregate) can be conservative concrete surrogates. Screening indicates insignificant impact for up to 45% fly ash replacement.
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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: Equilibrium-based leaching Coal combustion fly ash Concrete Microconcrete Leaching environmental assessment framework
a b s t r a c t Current concerns about the environmental safety of coal combustion fly ash have motivated this evaluation of the impact of fly ash use as a cement replacement in concrete materials on the leaching of constituents of potential concern. The chemical effects of fly ash on leaching were determined through characterization of liquid–solid partitioning using EPA Method 1313 for four fly ash materials as well as concrete and microconcrete materials containing 0% (control materials), 25% and 45% replacement of portland cement with the fly ash source. All source materials, concrete formulations and replacement levels are representative of US concrete industry practices. Eluate concentrations as a function of pH were compared to a broader range of available testing results for international concretes and mortars for which the leaching characteristics of the component fly ashes were unknown. The chemistry of the hydrated cement fraction was found to dominate the liquid–solid partitioning resulting in reduced leaching concentrations of most trace metals compared to concentrations from fly ash materials alone. Compared to controls, eluate concentrations of Sb, As, B, Cr, Mo, Se, Tl and V from concrete products containing fly ash were essentially the same as the eluate concentrations from control materials produced without fly ash replacement indicating little to no significant impact on aqueous partitioning. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In 2011, approximately 12 million short tons of coal combustion fly ash (20% of the total fly ash production) were used in the United States (US) cement industry as supplemental materials in concrete products and grouts (ACAA, 2011). Replacing a portion of the portland cement fraction with fly ash is beneficial in that it improves the handling and performance of concrete materials while simultaneously reducing the need for production of virgin cement. In terms of blended hydraulic cements, the EN-197 product specification (CEN, 2011) allows for fly ash replacement of 21– 35% under the European CEM II/B-V (portland-fly ash) designation while ASTM standard C-595 (ASTM, 2012) specifies replacement ⇑ 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.2013.11.049
levels of up to 40% in Type IP (portland–pozzolan) cements. The fly ash replacement levels for the US ready mix concrete industry generally range between 15% and 25% of the total cement fraction (ACI, 1993); however, higher rates may be used in formulations for high-volume concrete applications to minimize the effect of cracking, sulfate attack or alkali–silicate reactions (ACI, 2003). The proposed US Environmental Protection Agency (EPA) alternatives for disposal of fly ash (Federal Register, 2010) have led to concerns regarding how regulation, when finalized, may impact the perception of fly ash now beneficially used in the concrete industry (Ward, 2013). The proposed alternatives have also fueled the ongoing concerns of environmental advocates regarding the potential for increased release of hazardous constituents from concrete materials containing fly ash. The relevant constituents of potential concern (COPCs) for fly ash identified by US EPA, based on risk evaluations of fly ash impoundments and landfills (US EPA,
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2006), include antimony (Sb), arsenic (As), boron (B), cadmium (Cd), chromium (Cr), cobalt (Co), lead (Pb), mercury (Hg), molybdenum (Mo), selenium (Se), thallium (Tl), and vanadium (V). Unfortunately, little scientific evidence supports or assuages the concerns that fly ash use in concrete poses a threat to the environment through leaching of COPCs. In a recent US EPA review of available leaching data from concretes and mortars of primarily European origin (van der Sloot et al., 2012), the impact of fly ash addition to concretes and mortars was shown to minimally increase release of some COPCs. However, the review identified several information gaps including (i) gaps in the available data regarding facility configurations of fly ash sources, (ii) lack of comparative characterization of fly ash source materials, (iii) a limited range of fly ash replacement rates, consistent with EN 197-1 but not representative of US concrete applications, and (iv) standard mortar mixes that are not representative of US concrete formulations. This paper focuses on the chemical aspects of leaching through evaluation of pH-dependent release of COPCs in residential concrete and commercial concrete formulations created using fly ash from four US sources. This study is part of an ongoing research project, supported by the Electric Power Research Institute (EPRI) with partial support from the US EPA, to address the gaps in the US EPA review by evaluating the potential for environmental impacts when fly ash is used as a cement replacement in concrete materials typical of those in US production. Therefore, the project includes source materials (e.g., fly ash, cement, aggregates) currently in use within the US ready mix concrete industry, characterization of source materials and cement-based materials made from those sources, and fly ash replacement levels of 20% and 45% using concrete formulations consistent with residential and commercial ready mix applications. Data distributions from US EPA characterization of electric utility fly ash materials (Kosson et al., 2009; Thorneloe et al., 2010) and from the US EPA review of cementitious materials (van der Sloot et al., 2012) are used as a comparative basis for discussion of leaching results. 2. Environmental assessment of concrete and fly ash Many of the US studies on the COPC release from constructiongrade concrete have been based on results of single-point leaching tests, such as the Toxicity Characteristic Leaching Procedure (TCLP; Method 1311; US EPA, 2013) or the Synthetic Precipitation Leaching Procedure (SPLP; Method 1312; US EPA, 2013). These procedures provide an extract produced under a single set of test conditions designed to simulate leaching in a particular release scenario. For example, TCLP is intended to identify hazardous materials under RCRA for disposal purposes by simulating the mismanagement scenario of disposal of wastes in a municipal landfill. However, the US EPA Science Advisory Board (SAB, 1991, 1999) and others (Kosson et al., 2002; Thorneloe et al., 2010) have noted that single-point methods are insufficient for characterizing leaching over the range of plausible environmental conditions expected in many specific use or disposal scenarios or over the lifetime of materials subject to varying environmental conditions. 2.1. Leaching environmental assessment framework (LEAF) In response to the over-broad application of single-extraction leaching tests, the US EPA, in collaboration with Vanderbilt University and European partners, has developed the Leaching Environmental Assessment Framework (LEAF) as a flexible, broad-based alternative for leaching assessment (Kosson et al., 2002). The four LEAF leaching methods have been well-documented (Garrabrants et al., 2010, 2012a,b) and have been incorporated into the EPA compendium of leaching tests, SW-846 (US EPA, 2013). These
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leaching methods characterize (i) the liquid–solid partitioning (LSP) of COPCs as a function of eluate pH (EPA Method 1313), (ii) the LSP of COPCs as a function of liquid-to-solid ratio (L/S) using either an up-flow percolation column (EPA Method 1314) or parallel batch extractions (EPA Method 1316), and (iii) the rate of COPC mass transport from monolithic or compacted granular materials (EPA Method 1315). The LEAF methods are analogous to leaching procedures currently being finalized in Europe by the Comité Européen de Normalisation (CEN, 2013) and correlation between like methods was demonstrated during LEAF method validation studies (Garrabrants et al., 2012a,b). Selection of appropriate LEAF methods to characterize a specific material for an identified use or disposal scenario is based on the physical form of the material (i.e., granular or monolithic) and the anticipated environmental leaching conditions. For cementbased materials which typically form relatively low-permeability monoliths upon curing, the most appropriate leaching methods include Method 1313 data to approximate local chemical equilibrium and to provide information on the thermodynamic driving force for mass transport and Method 1315 to account for the physical nature of the solid material by providing information on the intrinsic rate of leaching due to mass transport from the monolith. In addition, LSP results from Method 1313 can be used as a screening basis for evaluating leaching because these results can be considered a limit case for water in contact with monolithic materials for long duration.
2.2. LSP characterization of fly ash for concrete replacement The LEAF methods have been used in previous US EPA characterizations of fly ash materials from a variety of coal combustion solids (Sanchez et al., 2006, 2008; Kosson et al., 2009). Differences in Method 1313 concentrations were shown to be dependent on the coal type combusted, air pollution control configuration, and facility operating conditions. When compared over the pH range 7 6 pH 6 13, eluate concentrations varied by several orders of magnitude. The pH range between 7 and 13 was considered applicable and relevant to the service life of cementitious materials, encompassing the relevant pH for porewater of fresh materials (e.g., dissolution portlandite at pH12.5) and aged materials (e.g., carbonate equilibrium at pH8.2). Recognizing that not all fly ash materials are appropriate for use in concrete, the evaluation of potential impacts of fly ash concrete should focus on only those fly ashes sources representative of the materials actually used in concrete applications.
2.3. EPA review of LSP in cementitious materials The recent US EPA review of available leaching data (van der Sloot et al., 2012) included comparison of the results of LEAF testing or LEAF-analogous tests from 15 cement mortars and concretes made with CEM II/B-V fly ash-blended cements and 17 CEM I portland cements. For each material grouping, the COPC leaching test results indicated a span of two or more orders of magnitude in the pH-dependent leaching (e.g., LSP as a function of pH) at specific pH values. Statistical distributions of the measured LSP response for CEM I and CEM II/B-V materials were developed using the median LSP concentration over the pH range bounded by the 5th and 95th percentile of eluate concentrations. Distribution comparisons showed considerable overlap between CEM I and CEM II/B-V, indicating no systematic difference in LSP response for many cases. However, this review can be considered only indicative of generalized behavior due to the limitations associated with the available leaching dataset as described above.
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3. Materials and methods Source materials for formulation of concrete and microconcrete materials were collected from a commercial concrete vendor or obtained directly from the coal combustion facility. All batch formulations, materials sampling and preparation, and chemical analyses were carried out according to a quality assurance project plan reviewed by US EPA prior to testing. Quality assurance for chemical analysis included evaluation of instrument calibration, blanks, analytical spikes and replication as discussed in the Method 1313 validation report (Garrabrants et al., 2012a).
3.1. Fly ash, cement and aggregates Fly ash samples from four coal combustion sources currently marketed and used for commercial concrete applications were tested. Comparison of fly ash source facility configurations (Table 1) indicates that two facilities burned bituminous coal (fly ash source codes FA02 and FaFA) while the other two facilities burned subbituminous coal (fly ash source codes FA18 and FA39). ASTM type I portland cement (analogous to CEM I designation in Europe), river sand and coarse limestone aggregate were obtained from a commercial concrete vendor and used for all cement-based materials. All bulk materials were homogenized prior to mixing and subsamples were collected for microwave-assisted digestion conducted by ARCADIS-US, Inc. (Durham, NC) following SW-846 Method 3052 and Method 3051A (US EPA, 2013).
3.2. Concrete and microconcrete formulation and preparation Concrete formulations in Table 2 were designed to be typical of residential and commercial concretes with either relatively high porosity or high fly ash replacement levels. Replacement levels of 20% and 45% of the portland cement fraction were used for residential and commercial formulations, respectively. In addition, a microconcrete recipe at the 45% replacement level was derived from the commercial concrete formulation by replacing the coarse aggregate fraction with fine aggregate at amounts necessary to maintain the aggregate-paste interfacial area of the concrete. Similar microconcrete materials have been employed to provide a mortar-like material with the rheological properties of concrete (Schwartzentruber and Catherine, 2000). In the current study, leaching from microconcrete materials was characterized to evaluate potential for use of microconcretes as surrogates for concrete materials in future leaching evaluations. The rationale for this evaluation was that microconcretes are easier to handle than concretes
during sample preparation for equilibrium-based testing (e.g., particle size reduction) due to the absence of course aggregate. From each of the three formulations in Table 2, four materials were blended and cured: a control material without fly ash replacement (control) and three fly ash replacement materials with the FA02, FA18 and FA39 fly ash sources. In addition, a fourth fly ash amended microconcrete material was created using the FaFA fly ash source. The water-to-binder (i.e., cement and fly ash) ratio was fixed at 0.45 for the residential concrete and 0.58 for the commercial concrete and microconcrete formulations. The application rate of a high-range water reducer was customized for each concrete material to maintain the design slump of 10 cm (400 ) in the residential concrete and 12 cm (500 ) in the commercial concrete. All materials were prepared in 0.33 cubic yard batches and cast into 36 cylindrical molds of 10-cm diameter 20-cm length. After an initial 24-hour set, specimens were released from the molds, wrapped in moist paper towels and cured for one week in a fog room at 20 °C. For the remainder of the curing time, specimens were stored at high relative humidity in sealed 5-gallon plastic buckets. Specimens were divided into curing times of 28 days, 3, 6, and 12 months for ongoing experiments; however, materials used for the leach testing shown here were prepared immediately following a 3-month cure.
3.3. pH-dependent leaching test Method 1313 (US EPA, 2012b) is a parallel batch extract test consisting of nine parallel batch extractions at L/S 10 mL/g-dry and targeted end-point pH values, using dilute nitric acid or potassium hydroxide to adjust pH. A tenth extraction at L/S 10 mL/g-dry with no acid or base addition provides the LSP at the ‘‘natural pH’’ of the solid material. Method 1313 may be used to estimate COPC concentrations at conditions approaching chemical equilibrium, although it is unlikely that complete chemical equilibrium is obtained within the duration of the leaching test for all COPCs and test conditions. The observed LSP from Method 1313, therefore, represents a pragmatic approach to approximate chemical equilibrium over a broad range of test conditions which is sufficient for the environmental assessment purposes. In addition, the broad pH domain of Method 1313 between pH 2 and 13 allows for estimation of the available fraction of a COPC for leaching through consideration of dissolution of sorptive iron phases (pH2) and increased solubility for readily leachable COPCs either at the acidic pH for many metals, near neutral pH for many oxyanions, and alkaline pH for some amphoteric elements. In the current study, Method 1313 was conducted in duplicate on fly ash source materials and in triplicate for concrete and
Table 1 Facility configurations for US coal combustion fly ash sources. Sample code:
FA02
FA18
FA39
FaFA
Source/Reference Fly Ash Class Total Calcium (wt%) Annual production Boiler type Unit size Coal type Particulate capture Hg sorbent injection Targeted market
EPRI F 4 50–200 K tons Pulverized coal 667 MW Bituminous Baghouse filter None Concrete
EPRI C/F 17 >200 K tons Pulverized coal 1376 MW Sub-bituminous Hot-side ESPa Activated carbon Mostly concrete
EPRI C 23 50–200 K tons Pulverized coal 550 MW Sub-bituminous Cold-side ESP None Concrete/geotechnical
EPAb F 12 50–200 K tons Pulverized coal 600 MW Bituminous Cold-side ESPb None Concrete
See (Kosson et al., 2009) for general facility configurations and fly ash characteristics and sample code EaFA in (Garrabrants et al., 2012a). Sample code FaFA is from the same source as EaFA but after carbon removal. a ESP = electrostatic precipitator. b To prepare fly ash for concrete use, residual carbon is removed from the produced fly ash (15% carbon) via an electrostatic/physical removal process after ESP collection to yield fly ash sample FaFA (2.5% carbon).
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D.S. Kosson et al. / Chemosphere 103 (2014) 140–147 Table 2 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
10.3 – 6.1 35.3 48.3
8.2 2.1 6.1 35.9 47.7
12.2 – 5.5 35.5 46.8
6.7 5.5 5.5 35.5 46.8
22.2 – 9.9 67.9 –
12.2 10.0 10.1 67.7 –
Fly ash used (sample codes)
None
FA02 FA18
None
FA02 FA18 FA39
None
FA02 FA18 FA39 FaFA
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
M-45-02 M-45-18 M-45-39 M-45-FaFA
microconcrete materials. Concrete and microconcrete materials were particle size reduced to 85 wt% less than 2 mm by crushing while fly ash materials were tested as received. The extraction interval for all tests was maintained at the 48-hour interval associated with the particle size of the concrete and microconcrete materials (Garrabrants et al., 2010). The eluate concentration results from test replicates were interpolated to target pH values as discussed in Method 1313 validation report (Garrabrants et al., 2012a) and the log-mean of replicates used in figures to provide clarity of presentation. 3.4. Chemical analyses Chemical analyses of digested solid materials and leach testing eluates were conducted by ICP-MS (Method 6020A) for As, Sb, Cd, Cr, Pb, Mo, Se, and Tl and ICP-OES (Method 6010C) for concentrations of B, Ba, and V. The total Hg content in the fly ash source materials and representative fly ash leaching test eluates were below relevant levels; therefore, Hg analysis was not included in subsequent analyses. Although only a select list of COPCs is presented here; leaching evaluation was based on analysis of a full suite of major (e.g., Ca, Si), minor (e.g., Mn, Zn) and trace constituents. 4. Results and discussion In Figs. 1 and 2, the log-mean of Method 1313 test replicate eluate concentrations for Sb, As, B, Cr, and Se are plotted as a function of target pH for (i) each fly ash source (left) and (ii) concrete and microconcrete materials with 45% replacement of portland cement with fly ash (right). Similar comparisons for other COPCs and a comparison of measured replicate results to calculated log-means may be found in Supplementary materials, Figs. S3 and S4 and Fig. S5, respectively. Overlain on the Method 1313 results graphs are the distributions of LSP results from US EPA fly ash characterization (Kosson et al., 2009; Thorneloe et al., 2010) and the US EPA review of cement-based materials (van der Sloot et al., 2012) to illustrate how leaching data from newly characterized materials relate to the broader range of leaching from previously characterized materials. In each graph, the analytical method limit (ML) and the analytical method detection limit (MDL) for the COPC are displayed as data quality indicators (US EPA, 2004). Also shown on each graph is a horizontal dot-dash line representing the maximum contaminant level (MCL) or drinking water equivalent level (DWEL) as outlined in US EPA guidance (US EPA,
2012a). These lines are used as reference for the pH-dependent leaching concentrations. However, comparison of leaching test results to drinking water standards should be regarded only as an initial or screening assessment, recognizing COPC concentrations in the field will undergo dilution and attenuation during transport to the point of compliance, or may be up to a factor of approximately thirty greater in pore water within the concrete for highly soluble species because of the lower liquid to solid ratio at pore water conditions. A more thorough evaluation can be achieved when appropriate by integrated use of Method 1313 and Method 1315 (mass transport rate) testing results. Furthermore, results of Method 1313 and Method 1315 can be used in conjunction with chemical speciation based mass transport modeling to provide estimates of release under conditions not readily amenable to laboratory testing. Thus, leaching concentrations less than the relevant MCL or DWEL can be viewed as an indicator of environmental safety, while concentrations above the MCL or DWEL are not necessarily indicative of adverse environmental impacts.
4.1. Fly ash versus concrete Comparison of leaching concentrations from fly ash sources to cement-based materials (i.e., comparing data from left to right, respectively, in Figs. 1 and 2) show that the band of eluate concentrations for concrete materials as a whole is typically lower and narrower than the band of concentrations for fly ash sources. The most striking examples are where the eluate concentration of a specific COPC at pH > 7 is substantially reduced in one or more of the concrete/microconcrete data series relative to the profile for the corresponding fly ash material (e.g., Sb in microconcrete M45-FaFA versus fly ash FaFA, As and V in M-45-FaFA versus FaFA and M-45-02 versus FA02, and B in all cases). The results clearly indicate that simple dilution of fly ash leaching by incorporation of fly ash into the cement matrix is not a dominant factor controlling leaching. If fly ash dilution was controlling COPC leaching, the resultant pH-dependent LSP would be a simple translocation of the eluent concentrations at each pH point to that of only 2%, 5.5% and 10% of the fly ash alone concentrations which is not the case. Furthermore, eluate concentrations at low pH from the cementitious materials are nearly the same as those in fly ash alone. As a whole, these observations indicate that the cement chemistry of the hydrated cement paste or cement/fly ash paste in each concrete or
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Fig. 1. Method 1313 leaching test results for fly ash samples (left) and concrete and microconcrete samples after 3 months of curing (right) compared to shaded regions representing 5th and 95th percentiles with medians of test results from previous studies (Kosson et al., 2009; Thorneloe et al., 2010; van der Sloot et al., 2010). The legend is common for Figs. 1 and 2.
microconcrete material controls the leaching concentration of most COPCs at alkaline conditions. For Sb, As, Ba, B, Cr, Pb, Mo, Se and V, distinct changes in the LSP trendline as a function of pH from fly ash to the cement material are reflective of changes in the solid phase speciation, likely from phenomena such as sorption to or co-precipitation with calcium silicate hydrate, portlandite, ettringite and monosulfate cement phases (van der Sloot, 2000, 2002; Halim et al., 2004; Bonhoure et al., 2006; Evans, 2008). For example, As is known to be associated with calcium-bearing minerals, e.g., Ca3(AsO4)2 and CaHAsO3 (Dutré, 1997), which precipitate at the natural pH of concrete materials. Such comparisons indicate that, although they may leach from a fly ash at high concentration, COPCs are relatively well-retained within the cement-based material at cement replacement levels of up to 45% of the portland cement fraction. While results of pH-dependent leaching tests are consistent with these phenomena, further investigation is required to confirm the degree impact of these mechanisms on COPC retention. 4.2. Representativeness of LSP behavior Comparing LSP data from four selected fly ash sources specifically selected for this study (i.e., data points in left-hand panels of Figs. 1 and 2) to the distribution of results from US EPA fly ash characterization (i.e., the shaded areas in the same panels) indicates that the selected fly ashes are a good representation of
the broader domain of fly ash leaching behavior for Sb, As, Ba, B, Cr, and V. However, the fly ash sources under-represent the domain of fly ash materials for Cd, Mo, Se, and Tl with the ranges of eluate concentrations from the selected fly ash sources. For example, the LSP curves of Se from the four fly ash sources are approximately equal to or less than the median pH-dependent leaching values reported in the US EPA fly ash studies inferring that other individual fly ash sources may exhibit greater leaching of Se over the entire pH domain. However, since not all fly ash sources have the physical and chemical properties appropriate for use in concrete, fly ash sources with greater leaching behavior than the evaluated fly ash sources might not be used in concrete production. In addition, Method 1313 eluate concentrations on fly ash source that approach the MCL do not take into consideration (i) incorporation of the COPC into the concrete matrix, (ii) the considerable dilution and attenuation between the concrete material (i.e., the source term) and the receptor, and (iii) the relatively low mobility of many of these COPCs in the environment. However, such comparisons show that the range of LSP behavior of a select number of fly ash sources used in the US concrete industry is consistent with the results observed for the broader range of fly ash sources previously characterized by the US EPA. For concrete materials, the range of LSP data from concrete and microconcrete samples (i.e., data points on the right-hand panel of Figs. 1 and 2) corresponded with the range of LSP behavior of CEM I and CEM II/B-V concretes and mortars from the US EPA review (i.e.,
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Fig. 2. Method 1313 leaching test results for fly ash samples (left) and concrete and microconcrete samples after 3 months of curing (right) compared to shaded regions representing 5th and 9th percentiles with medians of test results from previous studies (Kosson et al., 2009; Thorneloe et al., 2010; van der Sloot et al., 2010). The legend is common for Figs. 1 and 2.
shaded areas in the same panels). In the pH range above 7, measured eluate concentrations of As, B, Cd, Cr, Se and V were consistent with either the full range of the LSP distribution or the median to upper bound of the distributions. The LSP data for Sb, B, and Ba were between the median and lower bound of the earlier reported LSP distributions. Components of concrete other than the fly ash also may be a source of some of these constituents.
for portland cement in concrete materials and mortar-like materials up to a 45% replacement level are unlikely to significantly increase the liquid–solid partitioning of COPCs above that of material not incorporating fly ash. This result significantly expands the demonstrated range of fly ash replacement for cement without adverse impact to leaching of COPCs and validates that applicability to a wide range of fly ashes produced and marketed in the US.
4.3. Fly ash incorporation into concrete and microconcrete materials
4.4. Release estimates using LSP data
When LSP results are compared within a concrete or microconcrete formulation (e.g., material with fly ash replacement compared to the associated control), only minor differences in LSP are observed. Eluate concentrations of Sb, As, Cd, Cr, Pb, Se and V from concrete materials with fly ash replacement were bounded by or only slightly different than the eluate concentrations control materials made without fly ash. Also, for the most part, the results for the tested concrete and microconcrete samples were consistent with the mortar results reported by US EPA (van der Sloot et al., 2012). Thus, the use of coal combustion fly ash as a replacement
A first-order screening estimate of release from these concrete and microconcrete materials may be made using the LSP data presented here. Release values (mg of COPC/kg of material) were calculated by multiplying eluate concentrations (mg of COPC/L of eluate) by the extract-specific L/S of the leaching test (L of eluate /kg of material). Release for the maximum eluate concentration (i) across the entire Method 1313 domain (2 6 pH 6 13), (ii) across the applicable pH domain for cementitious materials (7 6 pH 6 13), and (iii) at the natural pH test position are shown in Table 3 for select COPCs leached from the microconcrete
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samples. Also shown in Table 3 are the results of total content analysis for these COPCs and materials. Complete results for all materials and COPCs are provided in the Supplementary materials, Tables S1 through S3. The values in each table represent four estimates of COPC release for comparison between like materials (e.g., 45% replacement concretes or microconcretes). In the absence of leaching data, the most conservative estimate of environmental release would be the total content which assumes that all of the COPC is available for leaching and is soluble under the conditions of the field scenario. The available content, considered the maximum cumulative amount of a constituent that is likely to leach under a range of environmental conditions, may be estimated by multiplying the maximum eluate concentration measured over the Method 1313 test pH domain by the test-specific L/S of 10 L/kg. Comparison of total content and availability date in Table 3 supports the conclusions of previous research (van der Sloot, 2000; Schiopu et al., 2007; Kosson et al., 2009; Thorneloe et al., 2010) that total content and availability cannot be correlated for most COPCs in fly ash and cement materials. Relative to total content, the availability ranged from a high of 42% for As in M-45-FaFA to a low of 6.5% for V in M-45-00, but was variable between materials and COPCs. Therefore, for most scenarios, environmental decisions based on total content are not appropriate. Screening-level release values may be calculated using the maximum concentration over the pH domain considered applicable for the material lifetime (i.e., for concrete materials an applicable pH range between 7 and 13. Comparing the estimated maximum release over the applicable pH domain to the availability indicates that LSP is controlled by either solubility constraints or sorption equilibria at the natural pH. The corresponding concentration at the maximum release over the specified pH domain also may be
used as the maximum local equilibrium concentration for assessment purposes. The reported results from Method 1313 testing relate to the LSP as a function of end-point pH from the extraction of size-reduced materials at test conditions approaching chemical equilibrium, the results presented here focus on the inherent chemistry of leaching and do not consider the further effects of low liquid-to-solid ratio in the porewater of the material coupled with the monolithic physical form of the material which can further limit the leaching of COPCs. Less conservative estimates of COPC release for intact concrete materials can be made using Method 1313 data in conjunction with mass transport test leaching data (Garrabrants et al., 2013). In addition, estimation of COPC release over the life span of concrete materials should consider the effects of long-term exposure to the environment such as by carbonation which may alter LSP behavior and mass transport. Alkaline materials, such as concrete, are particularly susceptible to carbonation which is a natural aging process where atmospheric carbon dioxide partitions into the pore solution and reacts with calcium and other divalent cations. The result of carbonation is neutralization of porewater, formation of calcite and a loss of some mineral phases such as ettringite that may provide adsorption sites for or incorporate COPCs. While pH dependent equilibrium testing (such as by Method 1313) has provided a good estimate for LSP of many constituents for carbonated materials when the change in natural pH is considered, previously published literature on cement stabilized materials indicates that carbonation can lead to increased leaching of As through changes in speciation and sorption over the applicable pH domain (Sanchez et al., 2002; Garrabrants et al., 2004). However, the data present here does not address the chemical effects of carbonation on
Table 3 Release estimates (mg/kg) for microconcrete materials based on total content,a available content,b maximum concentration over applicable pH range,c and concentration at natural pH.d
a
Sb
As
B
Cr
Mo
Se
Tl
V
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
mg/kg
M-45-00 Total content Available Content Max at 7 6 pH 6 13 At Natural pH of 12.2
