Air-Entraining Admixture Partitioning and Adsorption by Fly Ash in

Feb 23, 2014 - The adsorption of air-entraining admixtures (AEAs) on coal fly ash was .... selected to represent five different chemical classes or ca...
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Air-Entraining Admixture Partitioning and Adsorption by Fly Ash in Concrete Zeyad T. Ahmed,*,† David W. Hand,† Melanie K. Watkins,† and Lawrence L. Sutter‡ †

Department of Civil & Environmental Engineering, ‡Michigan Tech Transportation Institute, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931 United States ABSTRACT: The adsorption of air-entraining admixtures (AEAs) on coal fly ash was evaluated and quantified using adsorption isotherm experiments. The interaction between AEAs and aggregate, cement, and fly ash were studied, and the physical and chemical processes were identified and quantified. As a result, a method of fly ash adsorption capacity determination was developed to measure the residual AEA concentration in concrete mixtures. Fly ash adsorption capacity can be used for the purpose of characterization and specification of fly ash and to assess the suitability of fly ash for the use in concrete and other applications. In addition, AEA dosage adjustments can be made accordingly to compensate for the adsorbed AEA and to attain the AEA concentration required to produce the desired air void system. The procedure reported in this paper provides a solution to the AEA adsorption problem that hinders increased use of fly ash in concrete.

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INTRODUCTION In 2011, about 60 megatons of fly ash was produced as a byproduct of coal combustion in the United States. Less than 39% of this fly ash was beneficially used while the remaining 61% was handled as a solid waste which incurred environmental impact and financial disposal costs.1 The largest beneficial use of fly ash is for partial replacement of cement in the manufacture of concrete.1 However, the unburned portion of the coal or the carbon content of the fly ash limits this use of fly ash to a small fraction of the produced fly ash. The carbon content of the fly ash adsorbs air-entraining admixtures (AEAs) which leads to inconsistent levels of entrained air in the concrete.2,3 This problem is amplified by the inability to measure the adsorption capacity of fly ash; therefore, industry lacks such testing tools and procedures. Indirect measurements of adsorption capacity of AEAs such as loss on ignition (LOI) and fly ash foam index test have been used with varying degrees of success.2 These methods can be time-consuming, inaccurate, and do not actually measure the adsorption capacity.4,5 A method of direct measurement of AEA adsorption capacity of fly ash will provide a better understanding of the fly ash quality, a quantitative measurement of adsorption capacity, and possibly increased use of coal fly ash as a partial replacement of portland cement. Adsorption capacity indicators such as the various procedures of the foam index test6−8 and the loss on ignition test9 and its modifications4,10,11 provide a relative measurement of the adsorption capacity of fly ash, but both tests do not measure the actual adsorption capacity of fly ash. LOI can produce results with errors of up to 75% between the actual carbon content and the weight loss due to ignition.4 In addition, LOI measures only the amount of carbon and not the adsorption capacity of the carbon. Foam index tests are dynamic, simple field tests that measure the quantity of AEA required to establish a temporarily stable foam at the surface of the testing container. These tests do not address the kinetics of AEA adsorption nor do they quantify the actual AEA adsorption capacity at or near equilibrium, and the results are not directly related to the © 2014 American Chemical Society

AEA concentration, which is a crucial variable in adsorption and determining the proper AEA dosage required for a particular air void concrete specification. While one foam index test addresses the kinetics of adsorption,8 it has not been related to the actual AEA adsorption capacity of fly ash. The fly ash iodine number offers a precise measurement of the adsorption capacity of coal fly ash measured as iodine adsorption.12,13 This is extremely useful as a standard quality assessment tool for the characterization and specification of fly ash. However, this test has not been related to the actual AEA adsorption capacity simply because the direct measurement of AEA adsorption capacity of fly ash has not been quantified, a goal this study is trying to achieve. Through this study, it was discovered that when AEA is added to a concrete mixture containing fly ash, AEA chemisorption occurs on the cement while physical adsorption and chemisorption occurs on the fly ash. The remaining AEA compounds will stay in solution to participate in the formation of the air void system. The objectives of this work are to use adsorption isotherms to separate the variables of the aqueous AEA−fly ash−cement system and, while excluding insignificant variables such as adsorption by sand, characterize the chemisorption of AEA compounds onto cement, characterize the chemisorption and physical adsorption of AEA compounds on fly ash, and determine the remaining AEA concentration in the water. The ultimate goal is to determine the AEA dosage adjustment needed to compensate for the carbon adsorption so the remaining AEA concentration in the water can achieve the AEA concentration of a standard (fly ash free) concrete mix. To achieve these objectives, adsorption isotherm tests were developed to characterize the chemisorption and physical Received: Revised: Accepted: Published: 4239

June 11, 2013 February 20, 2014 February 22, 2014 February 23, 2014 dx.doi.org/10.1021/ie4018594 | Ind. Eng. Chem. Res. 2014, 53, 4239−4246

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Table 1. Fly Ash Properties (Weight Percent) fly ash

SiO2

Al2O3

Fe2O3

total: SiO2, Al2O3, Fe2O3

CaO

SO3

MgO

LOI

low-carbon high-carbon

44.81 39.6

23.08 20

9.51 12.7

77.4 72.3

13.58 9.1

0.96 1.1

2.97 2.28

0.39 10.49

adsorption processes using several cement and fly ash samples and various AEAs. An adsorption isotherm consists of several individual Erlenmeyer flasks containing various masses of adsorbent (fly ash) and various concentrations of adsorbate (AEA) equilibrated at a constant temperature for 1 h. The results of an isotherm provide a correlation between the adsorbent capacity (mass adsorbate/mass adsorbent) and the adsorbate aqueous phase concentration. In this case, an adsorption isotherm characterizes the mass of AEA adsorbed per mass of fly ash as a function of the AEA equilibrium liquid phase concentration. This procedure can be used on any fly ash/AEA combination to determine the AEA adsorption capacity of the fly ash. The AEA adsorption capacity of the fly ash multiplied by the mass of fly ash (per volume) in the mix produces the volume of AEA adsorbed by that fly ash (per volume). This volume of AEA is considered to be the dosage adjustment required to compensate for fly ash adsorption. In the past, researchers have reported that it is not possible to perform isotherms on fresh concrete mixes because AEAs are usually made of various unknown organic materials and concrete mixtures are semisolid systems that have a very limited amount of water.14 It is important to emphasize that the adsorption tests developed below are not designed to mimic the conditions of a concrete mixture, but instead they were designed to determine the relationship between the AEA adsorbed onto the carbon portion of the fly ash as a function of the AEA liquid phase concentration. The resulting adsorption isotherms were fit with the Freundlich isotherm equation that allows the determination of the fly ash adsorption capacity for any concentration of the AEA.

identifies and quantifies the portion of any AEA that remains in solution before and after adsorption and simply determines the volume of the AEA adsorbed by fly ash from that. Although the mechanism of action differs from one AEA to another, the way fly ash affects AEA’s availability in a fly-ash-containing mixture is similar in all cases. The carbon of the fly ash adsorbs the AEA remaining in solution (aqueous phase AEA); fly ash carbon will not affect the chemisorbed portion of the AEA because chemisorption is much stronger than adsorption and it is irreversible. Measurement of AEA Concentration. The direct concentration measurement of each individual AEA compound in the AEA mixture is problematic because AEAs are made from a mixture of complex organics. Spectroscopic methods were used by many researchers to describe the concentration of AEAs; however, the results are always characterized with low accuracy due to the dilution of sample and the phase instability of the AEA compounds in water. Total organic carbon (TOC) measured by the UV/chemical (persulfate) oxidation has also been previously considered for the measurement of AEA concentration, but numerous attempts failed to measure AEA concentrations.12 Results of many test samples for various dilutions showed that because UV/chemical (persulfate) is a relatively weak oxidant, it failed to fully oxidize the complex organic polymers in the AEA mixtures. The chemical oxygen demand (COD) test is a standard method for the indirect measurement of the organic compounds content in water. The COD test uses extreme oxidation conditions: a strong oxidizing agent (potassium dichromate K2Cr2O7), strong acid (sulfuric acid H2SO4), and high temperature (150 °C). Nearly all organic compounds are oxidized to CO2 and measured as milligrams of oxygen consumed per liter of solution. In this study, serial dilutions of three types of AEAs were made and tested with the HACH COD kit (TNT821 and TNT822) and HACH DR 5000 UV− vis spectrophotometer. The results of the correlation between AEA concentrations and COD are shown in Figure 1. The COD test was able to represent the concentration of the various AEAs. Therefore, it was used in this study for the measurement of AEA concentrations. At least three different

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MATERIALS AND METHODS Fly Ash and AEAs. Two types of coal fly ash were used in this study; low-carbon fly ash with LOI of 0.39% and highcarbon fly ash with LOI of 10.49%. The properties of these ash materials are listed in Table 1. Six AEAs from various types and manufacturers were utilized in this study and are presented with their chemical classification in Table 2. These AEAs are selected Table 2. AEAs Used in This Study no.

type of admixture

AEA1 AEA2 AEA3 AEA4 AEA5 AEA6

alpha olefin sulfonate benzene sulfonate resin/rosin and fatty acids resin/rosin and fatty acids vinsol resin admixture combination admixtures

to represent five different chemical classes or categories of AEAs;15 the AEAs chosen for this study are widely used by the concrete industry, and they are preapproved for use in many states across the United States.16 All tested AEAs functioned similarly, in different proportions, in terms of partitioning between the liquid phase (mix water) and the solid phase (cement or fly ash minerals). This test

Figure 1. Correlation between AEA concentration (percent volume) and COD (milligrams per liter). 4240

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give hydrophobic character to the cement particles and make them adhere to the air bubbles. The second condition is that the cement mixture must maintain enough AEA liquid phase concentration to entrain air during mixing. Another important observation of stable foam formation by AEAs was that in a cement solution, even though AEAs visually precipitate, the AEA retained its ability to form stable foam. However, after filtration the filtrate lost this ability to form stable foam, which means that the filtered portion of AEA still actively contributes to the foam formation.17,19 AEA Interaction with Aggregate. Gravel and sand were equilibrated for 1 h with each 0.4% vol AEA5 and 0.4% vol AEA1. No significant changes in AEA aqueous phase concentration were detected in either test, which indicates that there is no observable interaction between AEAs and aggregate materials. This is an important finding, and it means that the aggregate does not change the concentration of AEAs because the aggregate may be electrochemically neutral and carbon free and therefore can be excluded from the isotherm testing. AEA Interaction with Cement. AEAs interact strongly with cement by binding to the cement particles while a portion remains in solution. This type of sorption is called chemisorption because it is characterized by chemical bonding (ionic or covalent). Chemisorption is stronger than physical adsorption (driven by van der Waals forces), and it is irreversible under normal temperatures. Figure 3 shows the

concentrations were tested for each AEA used in this research; all AEA testing results showed 100% sample recoveries and linear correlations between COD and AEA concentrations. Isotherm Setup. All isotherms were equilibrated for 1 h in 250 mL Erlenmeyer flasks at 20 °C. The flasks were covered during equilibration to minimize the possibility of AEA volatilization. The AEA solution volume was measured using a 200 mL volumetric flask prior to its addition to the solid materials. A magnetic stirrer was used to keep the contents of the isotherm flask mixed for the entire time of equilibration. The equilibrated solution was filtered using grade 1, 11 μm, 90 mm diameter, cellulose, Whatman qualitative filter paper in a vacuum apparatus. The filtrate volume was measured using a 200 mL graduated cylinder. COD measurements were taken on the initial solution concentration and on the filtrate and were used to determine the AEA concentration and the capacity at that point. The COD released by the cement or fly ash was subtracted from the COD measurement to determine the net COD of the AEA. The 1 h time period was selected because initial kinetic tests showed that fly ash−AEA systems reach a near equilibrium state in less than 30 min. In addition, concrete mixing, transportation, and placing usually take no longer than 1 h.

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RESULTS AND DISCUSSIONS To understand and ultimately quantify the interaction between fly ash and AEAs in concrete, it is crucial to understand how AEAs partition among the various materials that make up concrete. AEA equilibration with gravel, sand, cement, and various types of fly ash were used to assess the interaction between these materials and AEAs. In this work, the fate of the AEAs was studied; chemisorption and adsorption were quantified to determine the net amount of AEAs adsorbed on the fly ash carbon. Any AEA contains two groups; the first is the ionic part that has a strong attraction to the solvent (water) and is therefore hydrophilic. The second part is nonionic, has little or no attraction to the water, and is therefore hydrophobic.14 When AEAs are added to the concrete mix, the hydrophilic anionic polar groups sorb strongly to the ionic cement particles leaving the hydrophobic nonpolar end of the surfactant molecule toward the solution as shown in Figure 2. When air

Figure 3. AEA partitioning between cement and water.

change of AEA concentration after equilibration with various masses of cement. The results in Figure 3 show that only a few grams of cement are needed to chemisorb most of the AEAs and attain steady levels of AEA aqueous phase concentration. After that point, the aqueous phase concentration remains constant regardless of the increment in cement mass. This indicates that the chemisorption process is fully completed and the remainder of the AEA stays in the solution to participate in air entrainment. The ratio of the pseudo-steady-state aqueous phase concentration after chemisorption to the initial concentration of the AEA is herein called the AEA partitioning coefficient. It is also clear that various AEAs are designed differently; some of them are designed to maintain almost half of their mass in aqueous phase, while others are designed to maintain less than 10% of their mass in the aqueous phase. The latter is heavily dependent on the solid phase AEA (chemisorbed to cement minerals) to entrain air, but it still needs a small portion of the AEA in aqueous phase to maintain the air bubble system.

Figure 2. Orientation of AEA molecules between air bubbles and cement particles.

bubbles are formed during agitation and mixing of the concrete mixture, the hydrophobic nonpolar ends of the surfactant molecules form micelles to stabilize these air bubbles and prevent them from coalescing into large bubbles.3,17,18 The entrained air bubbles have diameters less than 0.25 mm but not less than 10 μm.17 Bruere19 studied air entrainment in cement and silica pastes and concluded that for sufficient air entrainment two conditions must be satisfied. The first condition is that the AEA should be adsorbed on the solid particles with the nonpolar ends pointing toward the water to 4241

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Types of Cement. Two type I cements from different sources and one type II cement were equilibrated with the same concentration of AEA5 to determine the effect that different cement types have on the partitioning coefficient. Figure 5

Although the chemisorption process is very fast (less than a minute), all samples in the test were equilibrated for 1 h before measuring the AEA concentration. Factors Affecting AEA Partitioning Coefficient. The partitioning coefficient is very significant because it describes how much of the AEA is chemisorbed to the solids in the concrete mix and how much remains in the liquid phase. Both portions, the AEA chemisorbed to the solid phase and the AEA remaining in solution, actively contribute to air entrainment. However, AEA adsorption by fly ash affects only the AEA in liquid phase. Consequently, the partitioning coefficient describes the concentration of AEA left naturally in liquid phase, and this would be the concentration of AEA susceptible to adsorption by fly ash. The magnitude of the partition coefficient is dependent on the type and concentration of the AEA; the cement type has little to no effect as illustrated in the next sections. Types of AEA. Various AEAs differ in terms of their partitioning between solid and aqueous phase with cement. Generally, it is expected that an AEA that has a high partitioning coefficient, such as AEA3, is less susceptible to adsorption because a larger portion of the AEA remains in the solution and can participate in air entrainment, while only a small portion is chemisorbed on the cement. For an AEA with a low partitioning coefficient, such as AEA1, a very limited amount of the AEA remains in solution, and this amount can easily be adsorbed by fly ash, leaving very little AEA in solution to participate in the air void system formation. This is true only if various compounds that make up AEA aqueous phase have similar adsorptive properties. However, different AEAs leave different compounds in aqueous phase, and these compounds vary in their adsorption capacity. For example, AEA3 may leave 42% of its mass in solution and can have more affinity for adsorption (being more hydrophobic) than the 10% left in solution of AEA1. Therefore, fly ash adsorption isotherms are needed to observe the AEA aqueous phase properties. AEA Concentration. Another factor affecting AEA partitioning coefficients is the concentration of the AEA. Figure 4 shows AEA5 partitioning between the aqueous phase and cement phase for three different initial AEA5 concentrations. It is clear from Figure 4 that the partition coefficient is inversely proportional to the AEA concentration. Therefore, it is important to determine the partitioning coefficient for each AEA at the specific initial aqueous phase concentration used in the isotherm.

Figure 5. Effect of type of cement on the AEA partitioning coefficient.

displays the partitioning of AEA5 as a function of cement mass for the three cements. Results in Figure 5 show that the cement type has no significant effect on the AEA partitioning coefficient because all types of cement have an abundance of active chemisorption sites, much more than the amount required to chemisorb all the portions of any AEA solution that can be chemisorbed under practical AEA concentrations. The type I and type II cements used in this project had fineness values that ranged from 367 to 416 m2/kg. Different types of cement vary significantly in terms of fineness; consequently, the mass of cement utilized in the cement isotherm should be increased until a constant AEA liquid phase concentration is reached. It is also important to note that different types of cement release different amounts of COD background to the solution depending on their composition. The COD results shown in Figures 3−5 are for the net COD of the AEA as the COD of the cement was subtracted from the COD measurements of the mix. AEA Interaction with Fly Ash. AEA Interaction with Low-Carbon Fly Ash. Low-carbon fly ash exhibited a behavior very similar to that of cement. Figure 6 illustrates the results of

Figure 6. Low-carbon fly ash (0.39% LOI) interaction with three types of AEAs.

low-carbon fly ash equilibration with three AEAs. Most of the AEA was chemisorbed to the fly ash mineral particles. The AEAs partitioned between the aqueous and fly ash phases similar to the way they partitioned with cement. The difference between cement and low-carbon fly ash is that more mass of fly ash is required to achieve full chemisorption and attain a steady

Figure 4. AEA partitioning coefficient for various AEA concentrations. 4242

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must be quantified. Any reduction in AEA aqueous phase concentration beyond chemisorption is considered a loss due to adsorption. Adsorption Isotherms. As in every adsorption process, the solid−liquid phase partitioning of the adsorbate is governed by the solid phase capacity of the adsorbent and the initial aqueous phase concentration of the adsorbate. Sorption−desorption processes take place until equilibrium is attained. Adsorption isotherm standard test (ASTM D 3860-98)20 provides a procedure for determining the equilibrium solid and liquid phase concentrations for various masses of adsorbent. The results of the test describe the two-phase equilibrium relation and provide the solute adsorbed phase concentration for any liquid phase concentration and vice versa. This standard procedure cannot be applied directly in cases of AEA adsorption by fly ash because chemisorption and adsorption are taking place at the same time. Chemical compositions as well as physical properties of fly ash and cement are inconsistent even for the same production line. Therefore, there is no standard cement or standard fly ash; instead, fly ash and cement are classified based on a predefined range for each of their material compositions. This represents a challenge for the task of developing a procedure for the standardization and specification of fly ash. Because of this inconsistency, it is important to develop a procedure that can be performed onsite or in a simple laboratory using the exact material that will be used in the concrete. A standard procedure will provide a product-specific result and eliminate the variation due to the inconsistency of material properties. An adsorption isotherm for a fly ash and given AEA provides the equilibrium relation between the solid and liquid phase concentration of the AEA. Solid phase concentration represents the capacity (or the mass of AEA adsorbed per mass of fly ash), while liquid phase concentration stands for the mass of AEA left per volume of water. The adsorption isotherm test involved performing separate isotherms: (1) a cement isotherm to define the limit of chemisorption and (2) a fly ash isotherm to measure adsorption. The cement isotherm is used to determine the chemisorption partitioning coefficient for a given AEA concentration, while the fly ash isotherm is used to quantify the adsorption of the AEA onto the fly ash. However, it was proven as discussed earlier that chemisorption (defined by the partitioning coefficient of AEA) is insensitive to the type of cement. This can be attributed to the abundance of minerals in all types of cement. In other words, all types of cement have enough charged sites at the mineral particle surfaces to chemisorb all of the portion of the AEA compounds that can be chemisorbed. Therefore, the isotherm of any AEA with cement does need to be performed once, but it does not need to be repeated in every test. Cement Isotherms and AEA Aqueous Phase Determination. The test first step is to perform a cement isotherm with the AEA of interest. Separate measured masses of cement are equilibrated using a magnetic stirrer with 200 mL of water with a known concentration of AEA. After 1 h of equilibration, the mix is filtered and COD measurements are taken from the filtrate. This produces an isotherm point for each mass of cement. The COD of cement should be subtracted from the COD measurement of the filtrate to determine the COD of AEA for each point. The masses of cement plotted against the corresponding AEA concentration produce a graph similar to Figure 3. The COD decreases with increasing mass of cement

level of AEA aqueous phase concentration. This indicates that the chemisorption activity of cement particles is greater than that of fly ash particles. The results of cement and low-carbon fly ash suggest that a system of low-carbon fly ash, cement, and AEA will exhibit behavior similar to that of a system of cement and AEA in terms of AEA aqueous phase concentration. AEA Interaction with High-Carbon Fly Ash. The carbon in high-carbon fly ash adsorbs AEAs in the aqueous phase, reducing the low yet essential aqueous phase AEA concentration. Figure 7 shows the results of six AEA isotherms

Figure 7. Interaction between high-carbon fly ash and six different AEAs. AEA concentrations expressed as milligrams per liter of COD. The AEA concentrations are 0.4% vol for all AEAs except AEA2, for which 0.8% vol was used.

equilibrated with high-carbon fly ash (10.45% LOI). For all AEAs, the observed AEA aqueous phase concentrations decreased with increasing mass of fly ash and a steady-state level of AEA aqueous concentration is never reached. For small amounts of fly ash, large portions of the AEAs were sorbed onto the fly ash, but unlike cement, the loss in the AEA aqueous phase concentration continued with the addition of more fly ash. This indicates that the adsorption process is taking place, and as more fly ash is added to the system, more adsorbent (carbon) is added causing more AEA adsorption to occur. Eventually, if enough high-carbon fly ash is added, all of the aqueous phase AEAs will be absorbed into the carbon. Another very important observation is that not all AEAs behaved the same way with high-carbon fly ash. The aqueous phase of AEA6 maintained liquid phase concentration higher than that of the other AEAs, indicating that it is less susceptible to adsorption or has an adsorption potential less than that of the other AEAs. Conversely, AEA1 was the first to lose all of its aqueous phase concentration with only about 16 g of fly ash. This is extremely significant because it indicates that some AEAs perform better than others in this system because their aqueous phase concentration is less susceptible to adsorption; therefore, they may be more favorable for use with high-carbon fly ash materials. The results presented above show that a preferred AEA for use with high-carbon fly ash should have a high partitioning coefficient, while its aqueous phase portion should have low absorbability. The 1 h equilibration results (isotherms) with cement and fly ash can be used to quantify the amount of AEA adsorbed by fly ash. The goal of the adsorption isotherm is to isolate and measure the reduction in AEA aqueous phase concentration due to adsorption by carbon in fly ash. To achieve that goal, the full extent of chemisorption by cement and minerals of fly ash 4243

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The final AEA concentration after equilibration with fly ash can be determined using eq 2:

until it reaches a constant COD. That constant concentration provides the initial AEA concentration (in terms of COD) that is available for adsorption to occur. The constant COD level from the graph divided by the initial AEA concentration used in the isotherm yields the partitioning coefficient at that given AEA initial concentration. It was shown in a previous section that a certain AEA behaves similarly with all types of cement. Consequently, if a sufficient mass of cement is used, the partitioning coefficient for each individual AEA is a function only of the initial concentration of that AEA. The need to run a cement isotherm could be eliminated if the partitioning coefficient of the AEA of interest is known at the AEA concentration of interest. Therefore, Figure 8 can be used to

CODf,aq = CODfiltrate − CODfly ash − CODcement

(2)

where CODf,aq is the equilibrated AEA aqueous phase concentration obtained from the filtrate (mg/L), CODfiltrate the equilibrated filtrate COD (mg/L), CODfly ash the soluble COD released from the mass of fly ash in the isotherm point (mg/L), and CODcement the soluble COD released from the mass of cement in the isotherm point (mg/L). The soluble COD released from the fly ash or cement can be determined from equilibrating the fly ash or cement mass with 200 mL of distilled water and determining the soluble COD released. A mass balance is performed on each isotherm point, and the amount of AEA adsorbed can be determined from eq 3: MAEAad = Vo × CODo,aq − Vf × CODf,aq

(3)

where MAEA,ad is the mass of AEA adsorbed (mg COD), VO the initial volume of AEA solution for the isotherm point (L), and Vf the volume of the filtrate (L). The mass of AEA adsorbed is then divided by the mass of fly ash used in the isotherm point to determine the solid phase concentration. qfly ash = MAEA,ad /M fly ash

obtain the cement partitioning coefficient without performing a cement isotherm. For example, for an initial AEA5 concentration of 0.8%, the partitioning coefficient is 0.26; therefore, 26% of the COD value of the 0.8% AEA5 will remain in solution and can be considered the initial concentration for adsorption in all isotherms performed with all desired fly ash samples Fly Ash Isotherm. Once the partitioning coefficient is known, various masses of fly ash are equilibrated with the same AEA initial concentration to measure the adsorption isotherm. Each mass represents an isotherm point and is equilibrated with the AEA solution for 1 h. After equilibration, the solution is filtered and COD measurements are taken for the filtrate. All COD values that exceed the AEA aqueous phase concentration that was determined from the partitioning coefficient should be ignored because it indicates incomplete chemisorption; the amount of fly ash added was insufficient to achieve complete chemisorption in these isotherm points. Points with COD values less than 80% of the initial AEA aqueous phase concentration determined from the cement isotherms are preliminarily considered acceptable isotherm points and can be used for the isotherm calculations. In other words, these are the isotherm points where the chemisorption has most likely been satisfied. Isotherm Result Calculations. The initial AEA aqueous phase concentration available for adsorption can be determined from the COD of the AEA solution and the partitioning coefficient: CODo,aq = CODi AEA × part. coeff.

(4)

where qfly ash is the capacity of fly ash or fly ash phase AEA concentration (mg COD/g fly ash) and Mfly ash is the mass of fly ash. Solid phase concentrations (qfly ash) for all isotherm points are plotted versus the corresponding aqueous phase concentrations of AEA (CODf,aq) for each point on a log−log scale. A power line data fit produces a Freundlich equation (eq 5) that describes the equilibrium partitioning between the AEA solid phase concentration (capacity of fly ash) and the AEA aqueous phase concentration.21

Figure 8. AEA cement partitioning coefficients for six AEAs at various concentrations equilibrated with 20 g of portland cement.

q = K × C1/ n

(5)

where K is the Freundlich capacity parameter ((mg/g)(L/ mg)1/n) and 1/n is the unitless Freundlich intensity parameter. Figure 9 displays the results of two sets of isotherms for the high-carbon fly ash with 0.8% vol AEA5. The curvature of the

Figure 9. High-carbon fly ash isotherm with 0.8% AEA5.

(1)

data at high COD values indicates an error caused by incomplete chemisorption. At the curved region of the graph, both adsorption and chemisorption are taking place at the same time. The straight line part of the correlation can be considered

where CODo,aq is the initial AEA aqueous phase concentration available for adsorption (mg/L) and CODi AEA is the AEA solution concentration measured as COD (mg/L). 4244

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the adsorption isotherm and can be used to determine K and 1/ n. Figure 10 illustrates the results of adsorption isotherms of the six AEAs with the high-carbon fly ash as obtained from COD

Figure 11. High-carbon fly ash isotherm results with six AEAs. Capacity is expressed as milliliters AEA per gram of fly ash and concentration as percent volume in water. Figure 10. High-carbon fly ash isotherm results with six AEAs. Capacity and concentration are expressed as COD.

procedure can be used on any fly ash−AEA combination to determine the AEA adsorption capacity of the fly ash. Users can determine the fly ash adsorption capacity at the specific concentration of the AEA required for achieving the desired air void system in the fly ash free mix. Consequently, the required dosage adjustment can be determined by multiplying the capacity obtained by the mass of fly ash used in the mix. This dosage adjustment is the volume of AEA required to compensate for the adsorption capacity of the fly ash. This test does not predict performance or study the effect of adsorption on air-entraining mechanisms by any means. This test cannot predict how much air void will result in the concrete mix. This test simply quantifies the amount of AEA adsorbed in order to add that amount and bring the AEA aqueous solution to the preadsorption levels and eliminate the effect of fly ash adsorption.

tests and the mass balance equation. However, it is difficult to directly relate these values to the concrete mix. For these isotherms to be usable in the concrete industry it is important to express the capacity and concentration of AEAs in more practical units. To convert the capacity from milligrams COD adsorbed/ grams FA to milliliters AEA adsorbed/grams FA, the capacity has to be divided by the values of COD per milliliter of AEA. This value can be obtained from the COD measurement of the initial AEA solution. ⎛ mL ⎞ ⎛ mg ⎞ CODiAEA qFA ⎜⎜ AEA ⎟⎟ = qFA⎜⎜ COD ⎟⎟ /part. coeff./ g g AEA conc. ⎝ FA ⎠ ⎝ FA ⎠

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(6)

where part. coeff. is the paritioning coefficient and AEA conc. is the initial concentration of the AEA (mL/L). To convert the AEA aqueous phase COD to percent volume of AEA in water, the concentration should be divided by the partitioning coefficient and by the COD value of 1% vol AEA of the AEA solution.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration and was conducted as part of the National Cooperative Highway Research Program Project NCHRP 18-13. The authors thank NCHRP for their support and cooperation; the authors also thank Ms. Cara Shonsey and Mrs. Evelyn Johnson for their help and dedication.

⎡ CODiAEA ⎤ AEA conc., %vol = CODf,aq /⎢part. coeff. × ⎥ ⎣ AEA conc. ⎦ (7)

where part. coeff. is the partitioning coefficient and AEA conc. is the the initial concentration of the AEA (% vol). The values of high-carbon fly ash capacity and AEA concentration units were converted using eq 6 and eq 7, and the resulting isotherms are illustrated in Figure 11. Figure 11 is presented in the final form of the isotherm and can be used by the concrete mixers to determine the amount of AEA adsorbed by the specific fly ash under the required AEA concentration. This capacity multiplied by the mass of fly ash used in the mix provides the AEA dose adjustment required to compensate for the lost AEA due to the adsorption capacity of fly ash.

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REFERENCES

(1) American Coal Ash Association. ACAA 2011 CCP Report. http://www.acaa-usa.org/associations/8003/files/ Final2011CCPSurvey.pdf (accessed April 18, 2013). (2) Külaots, I.; Hsu, A.; Hurt, R. H.; Suuberg, E. M. Adsorption of Surfactants on Unburned Carbon in Fly Ash and Development of a Standardized Foam Index Test. Cem. Concr. Res. 2003, 33, 2091−2099. (3) Perdersen, K. H.; Jensen, A. D.; Skjøth-Rasmussen, M. S.; DamJohansen, K. A. Review of the Interference of Carbon Containing Fly Ash with Air Entrainment in Concrete. Prog. Energy Combust. Sci. 2008, 34, 135−154. (4) Brown, R. C.; Dykstra, J. Systematic Errors in the Use of Loss-onIgnition to Measure Unburned Carbon in Fly Ash. Fuel 1995, 74, 570−574.

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CONCLUSIONS Fly ash adsorption isotherms provide an accurate and much needed tool for the direct measurement of the adsorption capacity of fly ash and the amount of AEAs adsorbed by fly ash. Fly ash adsorption isotherms provide a correlation between the fly ash capacity and the AEA concentration in water. This 4245

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