CHAPTER 1

Background

The objectives of this research were (1) to propose recommendations for revising the AASHTO M 295 Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete to allow use of unconventional coal ashes while not compromising the desired properties of the fresh and hardened concrete, and (2) to provide guidelines for using coal ash not meeting the recommended revised specification. The guidelines are in Appendix B. These objectives are accomplished by a comprehensive testing plan at the powder, paste, mortar, and concrete scales. Specific objectives are summarized as follows:

• Develop and investigate new test methods to characterize coal ash reactivity and adsorption and compare these results to traditional test methods.
• Understand the influence of chemical, mineralogical, and physical variations in marginal and unconventional coal ash sources compared to traditional “fresh” coal ash sources and their impact on fresh and hardened paste, mortar, and concrete properties.
• Understand the extent of property variation within a single source of unconventional coal ash.
• Identify coal ash properties most closely linked with performance of coal ash-concrete mixtures and establish meaningful specification limits that will ensure production of intended concrete properties.

To accomplish these objectives, extensive characterization was performed on a large set of coal ash samples, covering the range of coal ash properties likely to be encountered in practice. Characteristics of the coal ashes were linked with performance results from comprehensive testing of paste, mortar, and concrete properties. Both traditional testing techniques and more innovative, recently developed test methods were employed to determine the optimal testing scheme to propose for use in the revised test specification. Round-robin testing between the project team labs was carried out for foam index testing, but no other new tests were developed to an extent that warranted round-robin testing. The proposed specification changes better differentiate quality and problematic coal ashes for use in concrete. Finally, guidelines are presented in Appendix B for using off-specification coal ash for highway concrete.

Literature Review

In recent years, supplies of high-quality, freshly produced coal ash have declined due to increasing environmental controls and shutdowns of coal-fired powerplants in the United States and elsewhere (DeCarolis and LaRose 2023). While supply has decreased, demand for coal ash for concrete construction is the same or greater, as coal ash is a crucial component for producing sustainable and durable concrete. This disparity between supply and demand is exaggerated by the cyclical seasonal highs and lows of energy generation resulting from the nature of energy usage. Seasonal energy-industry highs and lows typically do not align with construction seasons

when concrete is needed. This leads to production of large excesses of coal ash in winter that power plants often have to “store” in ponds or landfills, and shortfalls in coal ash availability during periods of highest demand. As a result, there is an increasing demand to find uses for such stored coal ash in concrete.

Despite the need for increased coal ash supplies, current specifications, including AASHTO M 295, are restrictive in terms of allowing unconventional types of coal ash. Preliminary research suggests many unconventional ash sources may perform adequately in concrete mixtures even when the material properties are outside current specification limits (McCarthy et al. 2018). However, due to the perceived variability of the properties of marginal and unconventional coal ashes, testing to verify performance and property development of concrete produced using such coal ashes is crucial. Such testing needs to be performed on many ashes to establish general relationships between the physical and chemical properties of these ashes and their impact on the properties of concrete made with them, and to determine the levels of variability within sources. Based on these findings, a specification for coal ash can be developed dictating coal ash characteristic limits linked closely to ash performance, while not excluding alternative ash sources that do not meet current specifications but may also perform acceptably. Based on test results, modifications to AASHTO M 295 can be proposed.

Since the start of this project, several changes in coal ash specifications, including in ASTM C618 and AASHTO M 295, have been made that expand on the types of specified ashes. ASTM C61823e1 now allows use of harvested ash, including bottom ash and comingled bottom plus fly ash. It is expected that AASHTO M 295 will adopt similar changes. Along with others, publications by this team have actively driven these changes (Hooton and Thomas 2023). This section (1) identifies issues with the current AASHTO M 295-21 specification, (2) identifies coal ash properties most closely linked with performance of coal ash-concrete mixtures, and (3) establishes meaningful novel specification tests that can be routinely performed.

Standard Coal Ash

Coal ash, produced from burning pulverized coal in boilers, is very fine noncombustible particulate matter carried in the flue gases and typically collected using electrostatic precipitators, baghouses, cyclones, or other mechanical devices. Coal ash generally consists of spherical particles, either hollow or filled, and ranging in size from 0.1 to 100 µm, with a bulk density of 0.54–0.86 g/cm³ and a specific surface area typically between 0.3 and 1.0 m²/g (Durdziński, Dunant, et al. 2015, Yao et al. 2015).

The bulk oxide composition of coal ash can vary significantly depending on the composition of the noncarbon constituents of the coal deposit. Typically, in the United States, two categories are recognized: Class F coal ash, generally classified as such based on its composition of ≤ 18.0% CaO. Class C coal ash consists of > 18.0% CaO (ASTM C618–23, AASHTO M 295). Outside ASTM and AASHTO, classifications vary depending on the specifying agency; as an example, Canadian specifications recognize three different coal ash classes—Type F, Type CI, and Type CH—based on the calcium oxide content. A strong linear relationship between CaO content and the sum of silica, alumina, and iron oxide has been shown for 110 North American coal ashes, which suggests the two specification schemes are broadly similar (Thomas 2007). Class F fly ashes are pozzolanically reactive, that is, they react with calcium hydroxide at high pH. Class C fly ashes also show hydraulic reactivity: they can react with water without the presence of calcium hydroxide. Calcium oxide content in the ashes is known to significantly affect reactivity and other properties, with higher-calcium Class C ashes more likely to be composed of rapidly reacting calcium aluminate crystalline and amorphous phases, which display hydraulic reactivity. Fly ash reactivity is thus the extent of the pozzolanic and/or hydraulic reactivity, typically measured using indirect tests such as heat release, calcium hydroxide consumption,

and bound water. While high-calcium-content ashes tend to be more reactive, they are also known to have reduced ability to mitigate SA and alkali silica reactivity (ASR) compared to lower-CaO-content coal ashes (Mehta 1985, Thomas et al. 2017, Sutter et al. 2013, Hemalatha and Ramaswamy 2017).

While the bulk oxide composition of coal ash is important and chemical composition limits are in place in existing coal ash specifications, the amorphous (i.e., glass) content and the amounts of amorphous phases composed of calcium oxide, silica, and alumina likely have a much stronger link to performance (Kucharczyk et al. 2019). The crystalline portion of the coal ash is generally inert and does not significantly contribute to the coal ash performance. Unfortunately, the common way amorphous content and amorphous oxide contents are determined is through quantitative x-ray diffraction, a complex process for coal ash. Detailed information about amorphous contents and compositions of coal ash would be useful, but this information is generally not considered in specifications. The amorphous phase content in North American coal ash is typically in the range of 50–80%, but can vary considerably depending on coal properties, beneficiation, combustion methods, and other factors (Chancey et al. 2010, Hower et al. 1999, Mardon and Hower 2004, Mastalerz et al. 2004). Average amorphous contents reported in Asia and Australia seem to be somewhat lower, around 40–70% (Sanalkumar et al. 2019, Ward and French 2006). Advanced testing using electron microscopy has revealed different types of glass groups in the coal ash (Aughenbaugh et al. 2013, Durdziński, Dunant, et al. 2015, Kim et al. 2018, Hu et al. 2014): silicates, calcium-silicates, aluminosilicates, and calcium-rich aluminosilicate glasses. The dissolution and reactivity of these glass phases have been studied, and strong links between glass composition, behavior, and reactivity of the glasses in cement have been established (Durdziński, Dunant, et al. 2015, Durdziński, Snellings, et al. 2015, Kucharczyk et al. 2019, Kim et al. 2020, Kang et al. 2020, Kang, Ley, et al. 2021). A significant amount of research is ongoing on glass reactivity, and it is hoped that such research can help provide solutions to enhancing coal ash reactivity. Fly ashes, especially Class F fly ashes, show lower reactivity at early ages compared to materials such as slag cement, which leads to low early strength at high replacements (Aughenbaugh et al. 2013, Durdziński, Dunant, et al. 2015, Durdziński, Snellings, et al. 2015, Kim et al. 2018, Snellings 2013, Snellings et al. 2014, Glosser et al. 2021, Song et al. 2021, Liu et al. 2023, Ruiz Pestana et al. 2023).

Apart from amorphous contents, the fineness has a strong effect on the coal ash reactivity. Mechanical activation of coal ash to increase its fineness (reduce particle size) significantly increases its reactivity (Giergiczny 2019, Moghaddam 2019, Obla et al. 2003, Innocenti et al. 2021), although this is obviously accompanied by increased cost and energy-related emissions.

Marginal and Unconventional-Source Coal Ashes

In some cases, coal ash quality has suffered due to changes in processes for environmental reasons, such as dry sorbent injection, selective catalytic reduction, activated carbon injection, and changes in firing conditions, resulting in the production of marginal coal ashes (J. Wang et al. 2020, Xing et al. 2019, J. Yang et al. 2018). For example, injection of activated carbon into the flue gas for mercury capture could result in a product with more highly adsorptive carbon in the ash, which may adsorb air-entraining admixtures (AEA), resulting in susceptibility of concrete made with this coal ash to freezing and thawing damage and salt scaling if AEA dosages cannot be properly adjusted (Pedersen et al. 2008). Limestone injection, for control and capture of sulfate gas emissions, can decrease the resistance of concrete to ASR and SA in part due to the increased calcium and sulfate contents and reduced pozzolanicity of the coal ash (Shehata and Thomas 2000, Tikalsky and Carrasquillo 1993). Increases in alkali and trona (sodium carbonate) contents and decreases in amorphous contents, through similar processes, could also result in reduced concrete durability in addition to changes in early-age properties and setting time (Qin et al. 2019), apart from safety issues. Ultimately, such coal ashes may not meet the requirements

of AASHTO M 295 and may be classified as off-specification coal ashes. The following definitions are used for this report:

Ponding and landfilling have been the two primary storage methods for excess coal ash production since the 1950s for both standard coal ash in excess of available storage facilities and off-specification coal ashes (Al-Shmaisani et al. 2018, Diaz-Loya et al. 2019, Yao et al. 2015). In ponding, the coal ash is mixed with large amounts of water and pumped into ponds, where it settles over time (Al-Shmaisani et al. 2018, Diaz-Loya et al. 2019, Yao et al. 2015). In landfilling, the coal ash is mixed with smaller amounts of water optimized for dust control and maximum compaction and is then disposed of in large horizontal lifts (Al-Shmaisani et al. 2018, Diaz-Loya et al. 2019, Yao et al. 2015). Generally, landfilling is seen as the more modern and safer option, although it tends to be more expensive. Coal combustion products, including coal ash, have historically either been stored separately in ponds or comingled.

Standards and procedures for storage may have varied over the history of the impoundment. For these reasons, substantial variation in coal ash properties from the same storage facility is expected. These variations are compounded by changes induced in the materials throughout their storage life (Al-Shmaisani et al. 2018, Diaz-Loya et al. 2019, McCarthy et al. 2017, Robl et al. 2017, Yao et al. 2015). Finally, another complicating factor is the presence of bottom ash and sulfur products, mixed with fly ashes in the impoundments in a spatially variable manner. Many factors may affect coal ash properties during ponding operations, including long-term elemental exposure, settlement, mixing of different products, variations in ash chemistry and hydraulic reactivity, and the presence of sulfur or lime sorbents, used to help control mercury emissions from coal, in addition to the variations in properties inherent even in the virgin material (Al-Shmaisani et al. 2018, Diaz-Loya et al. 2019, Hassett and Eylands 1999, Kruger 1997, Kumar et al. 2007, Pedersen et al. 2009, Fox 2005, McCarthy et al. 2013, McCarthy et al. 2017, Robl et al. 2017, Yao et al. 2015).

Differences between coal ash from landfills and ponds and virgin coal ashes will depend on many factors (e.g., environment, original ash properties, comingling) and may be insignificant at certain reclamation locations. Nevertheless, the main differences between coal ash from landfills and ponds and virgin coal ashes thus far have been found to be (Al-Shmaisani et al. 2018, Diaz-Loya et al. 2019, McCarthy et al. 2013, Yao et al. 2015, Yeheyis et al. 2009):

1. Increased particle size and angularity of particle shape in ponded ashes
2. Reduced amorphous content in ponded ashes, especially with more hydraulic ashes

3. Greater moisture content and LOI in ponded ashes (the greater LOI is not necessarily correlated to increased carbon)
4. Potential contamination with bottom ash, coal rejects, flue gas desulfurization products (FGD), or landfill cover materials

While the bulk chemical composition (oxide contents) does not change significantly, the phase compositions may have changed due to reaction and/or weathering (Wirth et al. 2017). Particle shape changes could be a result of agglomeration, settling, or weathering in addition to the hydraulic reactivity of Class C ashes.

Recent work has provided more insight into the properties of bottom ash, although more work is needed to thoroughly understand this material and reconcile differences between the studies. Bottom ash has been shown to be both high in amorphous content—some research suggesting bottom ash to be as high as 90% amorphous material (Ankur and Singh 2021)—and lower in amorphous content than counterpart coal ashes (Wirth et al. 2019). Crystalline phases include many phases typical of coal ash, most prevalently quartz, mullite, hematite, and magnetite. Compressive strength of samples using unground bottom ash has been found to be lower than in coal ash mixtures using similar cement replacement rates (Wirth et al. 2019). However, reactivity is strongly linked to particle size, and materials with a fineness similar to or higher than (smaller particle size than) traditional coal ashes were found to provide adequate reactivity and contribute to durability similar to low-calcium coal ashes (Kasaniya et al. 2021, Ankur and Singh 2021).

Direct use of unprocessed ponded/landfilled ashes is uncommon, although some studies exist (Cheerarot and Jaturapitakkul 2004, McCarthy et al. 2017, McCarthy et al. 2018). These tests on wet coal ashes have shown that properties like those obtained when using normal coal ash can be ensured through adequate quality control, which mostly involved accounting for the presence of additional water in the coal ash. Typical treatment processes when harvesting ponded and landfilled coal ashes include drying, particle separation, and grinding. Testing of harvested ponded or landfilled coal ash after processing usually shows similar performance to standard coal ash originating from the same plant (Al-Shmaisani et al. 2018, Diaz-Loya et al. 2019, McCarthy et al. 2013, McCarthy et al. 2017, Yao et al. 2015, Yeheyis et al. 2009).

The two most common options are disposal of off-specification coal ashes in ponds and landfills and treatment of coal ashes to bring them into compliance with the specifications (Al-Shmaisani et al. 2018, Diaz-Loya et al. 2019). Treated coal ashes are considered remediated or beneficiated. Several methods to treat marginal and unconventional-source coal ashes, typically to reduce carbon and/or increase fineness (reduce particle size), have been developed (Al-Shmaisani et al. 2018, Diaz-Loya et al. 2019, Hassett and Eylands 1999, Kruger 1997, Kumar et al. 2007, Pedersen et al. 2009, Fox 2005, Robl et al. 2017). These processes are usually patented or proprietary and full details about them are not always known, so they are discussed here in generic terms:

1. Chemical passivation: reducing the carbon activity in coal ash by adding a chemical that neutralizes or encapsulates the residual or powder-activated carbon
2. Electrostatic separation: electric fields are used to charge particles, which leads to particle separation and carbon removal
3. Thermal beneficiation: combustion is used to reduce carbon content; ammonia content can also be reduced
4. Chemimechanical activation: coal ash is milled, typically at high energy and often in the presence of water, reducing crystallinity and increasing amorphous content (Innocenti et al. 2021)
5. Other processes: blending different coal ashes, coal ash segregation by fineness or carbon, and air classification to separate coal ash particles of different sizes are also commonly used

A detailed comparison of beneficiated and standard coal ashes has not been performed until this study. Some studies, based on a comparison of limited coal ashes, suggest that some

beneficiated coal ashes had angular shapes that negatively impacted cement paste rheology. Impacts on hydration kinetics and on air entrainer dosage were observed, but the coal ashes still resulted in concrete with good durability properties (Al-Shmaisani et al. 2018, Diaz-Loya et al. 2019). However, these findings are not generalizable as only four types of coal ash were tested.

The current usage statistics for off-specification coal ashes, split into categories for use in applications, disposal, and beneficiation, are unclear. The American Coal Ash Association was not aware of any document that compiles this information or that currently most coal combustion products not meeting the requirements for concrete or wallboard are used either in structural fill (base, subbase, and subgrade stabilization) or disposal units such as ponds and landfills (T. Adams, personal communication, Oct. 15, 2019; Yilmaz et al. 2019). It is uncommon to use ponded and landfilled coal ashes without processing to meet current specifications. With increasing coal ashes shortages, the attention power producers and ash suppliers are paying to beneficiation has increased. Coal ash suppliers have told the research team they treat ash reclamation like a mining operation. That is, they only seek and extract high-quality material and ignore low-quality ash. Quality is not clearly defined, but generally appears to mean whether the ash is blended with sulfur-based contaminants.

AASHTO M 295 Specifications

Coal ash is deemed suitable for use in highway concrete if it meets the requirements in AASHTO M 295-21, shown in Table 1. The definition of coal ash in AASHTO M 295-21 is “finely divided residue that results from the combustion of ground or powdered coal and that is transported by flue gasses.” This definition excludes the residues resulting from burning industrial/municipal garbage (for coal mixed with municipal waste) and from injecting lime into the boiler for sulfur removal. Prior to the most recent specification change in ASTM C618, it was not explicitly clear whether harvested ashes, bottom ashes, comingled ashes, or beneficiated/processed ashes were permissible. It is still not clear if they are allowed under AASHTO M 295, which separates Class F (pozzolanic) and Class C (pozzolanic and hydraulic) coal ashes. It should be noted that at time of writing, AASHTO M 295 is undergoing changes that are not identified in this report.

The AASHTO M 295-21 specifications include limits on the sum of silica plus alumina plus iron oxide, sulfate contents, moisture content, LOI, fineness, expansion, and uniformity, as well as a comparison of strength and water demand of cement-coal ash mixtures to a 100% portland cement control mixture. Several caveats on the usage of these tests, test limits, and their interpretation are presented. In addition, optional physical and chemical requirements and information on durability performance are specified. Class F and Class C coal ashes are differentiated based on their calcium

Table 1. AASHTO M 295-21 requirements.

Table 2. Frequency of testing from ASTM C311.

Note: ^A denotes a composite sample; ^B denotes a regular sample.

oxide content. Information about ordering, sampling and testing, storage and inspection, rejection, packaging, and package marketing are also included. Information on specifications used by other regions of the world is provided in Suraneni et al. (2021).

Testing Frequency and Sampling

The uniformity requirements of AASHTO M 295 and sampling/testing frequency guidelines of ASTM C311 establish the criteria, or limits of variability, allowed for that ash. Frequency of testing based on sample size (tonnage) and time (daily, monthly, etc.) for various tests is shown in Table 2 and the sample size and types are shown in Table 3. Additional uniformity requirements for parameters including density, fineness, and AEA mandate that the average of the 10 preceding tests—or all preceding tests if the number is less than 10—be within the limit of the current average using composite samples. Additionally, the foam index test, ASTM C1827, is frequently measured at coal powerplants and by ash suppliers, but a limit and frequency of testing have not been officially established. Further, state DOTs are generally allowed leeway to adopt the uniformity requirements of AASHTO M 295 and sampling/testing frequency guidelines of ASTM C311. They also have jurisdiction over what content is deemed appropriate in their state standard specifications reports.

Statistical analysis assessing the viability of these sampling/testing guidelines is not common in the literature for standard ashes, let alone for unconventional ashes. A literature review could not find the origins of the frequency of testing or sample sizes established in ASTM C311. These values are perhaps derived from input from the industry regarding enough testing to represent the expected variability in coal ash properties. Spencer et al. (2019) tested the variability of four standard ashes and showed that a reduction in the composite sampling from 3,200 tons to a testing frequency of 30 days (or monthly) was sufficient to identify the general properties (SiO₂ + Al₂O₃ + Fe₂O₃, SO₃, LOI, fineness, 7-day SAI, and 28-day SAI) of those ashes, and showed that a reduction in testing frequency on pertinent parameters had little impact on the mean. It was concluded that, depending on the ash and its historical data, testing at lower frequency did not increase the risk of accidentally missing an ash that failed the specification limits. However, it is important to note that

Table 3. Sample type, size, and terminology.

the same conclusions may not be reached if reduced testing frequency on daily or 400-ton grab samples were assessed.

In addition, Obla (2014) provided more details and recommendations for testing on the part of the ash producer to maintain satisfactory quality assurance for coal ash departing the power plant. Specifically, it was suggested that uniformity parameters such as moisture content, density, fineness, and AEA be tested more frequently if possible since these parameters may vary from shipment to shipment. One notable recommendation includes using both LOI and FIT to estimate the effect of a coal ash sample on air entrainment and developing correlations between these parameters for a particular coal ash source prior to significant testing. Other recommendations suggested by Obla (2014) include same-day testing of uniformity parameters, i.e., LOI, fineness, foam index, and mortar air content, immediately as the coal ash shipment leaves the powerplant, checking regularly to ensure a coal ash is up-to-date in meeting the uniformity requirements for density and fineness, developing a moving average chart for the uniformity test results for shipments arriving at the plant, and setting up appropriate guidelines on adjusting AEA dosage based on uniformity testing results (foam index, LOI, mortar air content) for new coal ash shipments. Overall, this would provide a pathway for systematic assessment of single-source variability of coal ash from shipment to shipment.

Issues with Certain AASHTO M 295 Specification Items

Some of the items required in AASHTO M 295 need to be updated because the nature of ashes and associated supply has changed. The sum of the oxides or SiO₂ + Al₂O₃ + Fe₂O₃ (%) requirement, while practical, may not necessarily link to performance. But it does establish that a minimum of 50% of the chemical content of the ash is not harmful; the amorphous component of silica and alumina is likely related to reactivity. As described earlier, the crystalline part of the coal ash is generally inert while the amorphous content and amorphous oxide composition have a much stronger link to the coal ash performance (Durdziński, Dunant, et al. 2015, Durdziński, Snellings, et al. 2015, Kucharczyk et al. 2019). The typical amorphous content in conventional North American coal ash is about 50–80%, but this amount could be lower for high-calcium and landfilled coal ashes due to reaction of the calcium with the amorphous silica and alumina in the landfill (Al-Shmaisani et al. 2018, Diaz-Loya et al. 2019, Chancey et al. 2010, Hower et al. 1999, Mardon and Hower 2004, Mastalerz et al. 2004, McCarthy et al. 2017, McCarthy et al. 2018, Yeheyis 2009). Therefore, simply specifying a limit on total SiO₂ + Al₂O₃ + Fe₂O₃ (%) may be problematic for landfilled Class C coal ashes, which could pass this requirement even though they may not be good candidates for use in concrete. However, no evidence from the literature has been found for this assertion.

Several issues with LOI have also been noted. The LOI test is used as a proxy for the quantity of unburnt carbon in a coal ash sample based on mass loss up to 750°C. However, other materials and phases present in the sample, including sulfate and hydrated compounds such as calcium hydroxide, may increase LOI quantities, leading to incorrect estimations of the unburnt carbon content (Mohebbi et al. 2015). Further, the LOI value is not deterministic for performance because it is known that the LOI value and adsorption of AEA may not correlate (Fan and Brown 2001, Külaots et al. 2003). This is due not only to variations in the types of carbon present (e.g., unburned carbon vs. powder-activated carbon additions) and degree of activation of the natural carbon, resulting in differences in sample adsorption, but also due to the presence of bound water, crystalline phases, and volatile organic compounds, which dehydrate in the LOI temperature range. A comprehensive study on 70 coal ashes suggested LOI consistently overestimated the unburned carbon content in coal ash due to the presence of volatile organic compounds (Fan and Brown 2001). Therefore, the authors stated that the LOI is not a good proxy for unburned carbon content; instead, they suggested a procedure based on thermogravimetric analysis (TGA) where mass loss due to carbon can be determined. LOI does provide a qualitative idea of the adsorption potential of the

ash, with most low-LOI ashes generally displaying low adsorption potential, while high-LOI ashes display greater adsorption potential (Fan and Brown 2001, Külaots et al. 2003, Sutter et al. 2013).

Ultimately, the LOI test is not sensitive enough to provide estimations of required admixture dosage increases in concrete, which makes its specification problematic. Several alternative tests have therefore been developed, including the foam index test and the direct adsorption isotherm (DAI) test, the use of which in lieu of LOI testing have also been suggested by previous test programs, including Sutter et al. (2013).

The SAI is an indirect measure of reactivity and significant testing has shown that it does not relate to concrete performance and can also result in falsely passing finely ground inert materials (Bentz et al. 2011, Dunstan 2017, Dunstan 2019, Kalina et al. 2019). One of the main issues with the SAI test is the adjustment of the w/cm of the coal ash test mixture to constant flow (Kalina et al. 2019). As w/cm has a significant impact on strength, this confounds the coal ash or other SCMs’ contribution to the strength. Therefore, switching to a constant w/cm by incorporating a superplasticizer could allow a better indication of a coal ash’s contribution to strength (Kalina et al. 2019). The mass replacement of cement with SCMs has also been criticized, as most SCMs have lower densities than cement, which results in variations in the volume fraction of water in the mixture (Bentz et al. 2011, Kalina et al. 2019). It has been suggested that using volumetric proportioning of SCMs as was done in older versions of AASHTO M 295 and ASTM C618 will benefit SAI results. Other issues include the 75% limit, which is often thought to be too low. Increasing this number from 75% to 85%, at least at 28 days, could be of interest (Bentz et al. 2011, Kalina et al. 2019). Finally, an increase in the testing age to a later age or use of 28 days with accelerated curing could be valuable to better show the benefit of slowly reacting materials such as Class F coal ashes. Note that AASHTO M 295 allows for 56-day testing, although ASTM C618 does not. Changes in testing ages and testing limits could help significantly in reducing false positives obtained from fine filler materials. In light of the concerns with the existing SAI test, it could be modified by using constant w/cm, volumetric replacements, changing the measurement age to 56 or 91 days, increasing the limit from 75% to 85%, using alternative methods such as the KHI test and the TE test (Dunstan 2017, Dunstan 2019, Pal et al. 2003), or replacing the test with a direct measure of pozzolanic and hydraulic reactivity (Avet et al. 2016, Li et al. 2018, Snellings and Scrivener 2016, Suraneni and Weiss 2017, Suraneni et al. 2019, Y. Wang and Suraneni 2019).

Finally, AASHTO M 295 has uniformity requirements to ensure consistency based on fineness and density. Variable coal ash composition can be detected by changes in density and variable reactivity can be detected by changes in fineness. However, it may be more beneficial to specify uniformity in coal ash properties that are known to affect concrete properties more strongly, including reactivity and air entrainer adsorption.

Recent Advances in Coal Ash Testing

In this section, innovative approaches are described that may complement conventional tests and increase the understanding of properties that dictate coal ash performance. Testing that could be specified as alternatives in AASHTO M 295 is highlighted.

One key to understanding and evaluating the quality of coal ash sources for use in concrete is the development of a rapid test method capable of quantifying pozzolanic and hydraulic reactivity. Methods to study the reactivity of SCMs include direct methods, which typically track calcium hydroxide consumption of the SCMs, and indirect methods, which typically track strength gain in SCM-based mixtures.

As an extension of the standard SAI test, the KHI test involves making an extra set of cubes using an inert filler material. It was originally used to assess the reactivity of blast furnace slags (Keil 1952,

Lea 1971) and then to evaluate the hydraulic activity of ground granulated blast furnace slag in concrete (Pal et al. 2003). Most recently, the KHI test has also been modified and applied directly to cement-coal ash mixtures at 20% and 35% replacement levels with major implications on practical ways to assess strength contribution of pozzolanic SCMs (Sutter et al. 2013). Essentially, KHI is a difference quotient of strength measurements between the test mixture (SAI), the control mixture, and an inert filler mixture. Unlike the strength activity tests, KHI allows for differentiation between pozzolanic and hydraulic effects from the filler effects. For evaluation, KHI may range from 0% for inert fillers to more than 100% for reactive materials that develop strength exceeding the control cement mixture (Sutter et al. 2013). It was suggested that this method may be a more practical measure of strength contribution of pozzolanic SCMs than complex evaluations of reactivity of SCMs alone (Hooton and Emery 1983). Evaluating the strength contribution of SCMs using KHI depends simply on comparing this value from one material to the next, with higher values denoting more strength contribution and vice versa.

The total efficiency (TE) test involves testing another set of mortar cubes with a different cementitious content in addition to the SAI test mortar cubes (Dunstan 2017, Dunstan 2019). These two sets of mortar cubes are used to determine the “efficiency” of the coal ash compared to a cement control. One notable feature of the TE test compared to the SAI test is that it allows for separate computation of chemical efficiency (CE) and physical efficiency, essentially differentiating between strength contribution due to a chemical reaction and water reduction. However, issues relating to not measuring the slow coal ash reaction at early ages and the filler effect remain.

Bulk resistivity has been used as a nondestructive method to evaluate the mortar quality, with resistivity being indirectly related to the porosity and connectivity of the pore network (Xiao and Li 2008). Resistivity has been used to track progression of hydration and pore refinement processes in the concrete and to differentiate between cementitious materials and SCMs (Nadelman and Kurtis 2014, Weiss et al. 2018). Mortar bulk resistivity has been linked to compressive strength (Wei et al. 2012), has been found to correlate with the ability of an SCM to control ASR and predict diffusivity, and can be used for mixtures including SCMs (Rios et al. 2021, Kang, Lloyd, et al. 2021, Chopperla and Ideker 2022). This method is believed to provide a more direct indication of the reactivity of SCMs compared to strength due to its link with porosity. However, it is known that pore solution chemistry significantly affects resistivity results (Spragg et al. 2013), so use of unconventional ashes with varying chemistries may impact results.

More fundamental reactivity tests known as R³ test methods have also been recently developed to provide standardized methods to quantify SCM reactivity (Avet et al. 2016, Li et al. 2018, Snellings and Scrivener 2016). The R³ test is promising as it is standardized in ASTM as ASTM C1897, although it does not yet appear in any specifications. In these tests, heat release in SCM-CH mixtures (mass ratio 1:3) in a simulated pore solution with a water-to-solids ratio of 0.9 is monitored at 40°C using an isothermal calorimeter. Bound water in these systems is measured using a muffle furnace. Gypsum and limestone are added to ensure the system closely mirrors conditions in a cement-SCM system. Round-robin testing has demonstrated that the heat release and bound water contents obtained from these tests correlate strongly with each other and with 28-day strengths (Avet et al. 2016, Li et al. 2018, Snellings and Scrivener 2016). The use of R³ methods has been explored for numerous SCMs including calcined clay, coal ash, silica fume (SF), metakaolin, and blast-furnace slag. Correlations between heat release and bound water have also been established (Li et al. 2018). Suraneni and Weiss (2017) modified the R³ test method to measure portlandite consumption and to classify SCMs based on the relationship between heat release and portlandite consumption, and were able to differentiate between pozzolanic and hydraulic behavior of SCMs (Suraneni et al. 2019). These methods will help improve understanding of coal ash reactivity as additional work refines the links between SCM properties (chemistry, crystallinity, and particle size), reactivity, and durability to enable even more rapid determination of coal ash performance. The bound water test is especially promising because it is a relatively simple test that uses inexpensive equipment available in most labs.

Understanding the Influence of Coal Ash on AEA Requirements

Potential increases or fluctuations in levels of coal ash adsorption of AEAs is one concern around use of unconventional ashes in concrete. In the past, changes in coal power processing conditions have resulted in increased levels of residual unburnt carbon in ashes, which is linked to air entrainment problems in coal ash concrete due to high propensity of the carbon to adsorb AEA polymers from the mixture, preventing them from stabilizing air in the concrete. Further complicating the situation, multiple forms of carbon (inertinite, isotropic, and anisotropic) can exist in a sample of coal ash, each having different adsorption levels. This can contribute to inaccuracy in predicting adsorption based on carbon content from the current LOI procedure (Maroto-Valer et al. 2001). The presence of additional contaminants in unconventional sources of coal ash may present similar challenges in air entrainment of concrete using these ash sources.

Interactions between coal ash, carbon, pore solution chemistry, and AEA are complex and incompletely understood, with particle size, material chemistry and crystallinity, solution chemistry and ion concentration, and surfactant chemistry all changing the nature of interaction between air entraining surfactant agents, cements, and coal ash materials. Better understanding of adsorption occurring between coal ash AEA will contribute to improvements in test methods and design of AEA and coal ash beneficiation methods that reduce overall adsorption and variability between samples.

Generally, AEA requirements are based on dosages needed to reach a state known as the critical micelle concentration (CMC). At the CMC the concentration of surfactant in solution reaches a level that makes formation of surfactant micelles and entrained air voids more energetically favorable than further adsorption of surfactant to solid surfaces (Chang et al. 2018). In mixtures of solids and solution, prior to reaching the CMC, surfactants will be adsorbed to the surface of solid particles, removing them from solution and lowering solution surfactant concentrations. Many factors affect this adsorption process and change the quantity of AEA required to overcome adsorption forces and generate adequate levels of foam to achieve entrained air conditions, including:

• Particle size and the quantity of liquid-accessible surface area. Smaller particle size and greater surface area typically result in increased total adsorption. In the cement-coal ash system, changes in both the coal ash and unburnt carbon particle size have been shown to correlate inversely with adsorption (J. Yu et al. 2000). Haustein and Kuryłowicz-Cudowska (2022) also showed that increased particle size led to increased 0–200 µm voids and a decrease in spacing factor.
• Multivalent metal ions present in pore solution, such as Ca⁺², Mg⁺², and Al³⁺, affect adsorption through multiple avenues:
  • They significantly change adsorption, reducing the CMC concentration (Demissie and Duraisamy 2016) and the foaming ability of surfactants, similar to what happens when washing with hard water (Baltrus and LaCount 2001, Külaots et al. 2003).
  • At low concentrations, multivalent cations increase foam stability, but high concentrations of ions decrease foam stability (Yekeen et al. 2017).
  • Cations are electrostatically attracted and bind to ionic surfactants, reducing surfactant solubility and resulting in precipitation of the surfactant, often as a calcium salt. Through this mechanism the presence of cations in solution can reduce the concentration of surfactant needed to induce precipitation by more than two orders of magnitude, resulting in a significant reduction in surfactant concentration in solution and requiring significant increases in AEA dosages required to reach the CMC (Baviere et al. 1983, Matheson et al. 1985, Talens-Alesson 1999, Chang et al. 2018, D. Yu et al. 2012).
  • Divalent cations and high-ionic-strength solutions also create charged substrates, supporting formation of multilayer adsorption (Tunstall et al. 2017). This changes the quantity of

• • AEA required to reach critical micelle formation by increasing the number of surfactant molecules able to adsorb to charged sites on the surface of solid materials.
  • Cations in solution also provide a stabilizing effect for surfactant micelles, reducing electrostatic repulsion between surfactant molecules and enabling formation of micelles at lower concentrations. This effect is supported by the increased calcium contents at the surface of entrained air voids in concrete (Ley et al. 2009).
• Monovalent alkali content (K⁺ and Na⁺) has been shown to:
  • Correlate with increased air content, likely due to competition with surfactants for charged surface sites, which could reduce surfactant adsorption (Gebler and Kleiger 1983, Atkin et al. 2003).
  • Reduce foam stability (Yekeen et al. 2017).
• Sulfate in solution has been found to correlate with air loss (Külaots et al. 2003).
• Solution pH can alter the surface charge of solid materials, affecting the strength of attraction of ionic solution components and density of surface sites to which molecules will adhere. Changes in surface charge vary with chemistry of the solid material and with the change in solution pH, creating a dynamic situation within the cementitious system as dissolution of cementitious compounds occurs, leading to changes in pore solution pH (Atkin et al. 2003).
• Foaming of surfactants is affected by chemistry of surfaces with which it interacts. A mixture of silica and calcium (70:30 SiO₂:CaO) was found to result in greater levels of foam than surfaces of 100% silica or a glass mixture of silica, calcium, and phosphorus (Jones and Hench 2003).
• Surfactant-solid interactions are time-dependent, with increasing contact time generally leading to increased adsorption. In addition, contact time affects the strength of adsorption and the ability of surfactant molecules to disassociate from solid surfaces and intermix back into system solution (Parker and Rutland 1993).
• Surfactant chain length alters the dosage of surfactant needed to reach critical micelle levels. Increasing chain length results in significant increases in hydrophobicity, increasing the driving force for aggregation and reducing the CMC (Atkin et al. 2003). Despite this, some AEA investigations have suggested that long-chain hydrocarbons, such as fatty acids and sulphonated hydrocarbons, may require higher dosages compared with vinsol resin-based AEAs. However, long-chain hydrocarbons were found to be less sensitive to varying coal ash properties (Gebler and Klieger 1983).

Thus, varying particle size and composition of coal ash particles, both in terms of crystallinity and glass composition, and dissolution kinetics likely also contribute to changes in interactions between the coal ash and AEAs. The coal ash characteristics theorized to most significantly impact levels of AEA adsorption are shown in Table 4.

Several methods have been proposed to evaluate changes in AEA requirements for coal ash: tracking LOI, the FIT, the iodine test, DAI, and the use of fluorescence-based methods (FBM).

Table 4. Parameters hypothesized to affect coal ash adsorption and generation of entrained air.

Specifying LOI may be especially problematic in harvested ponded coal ashes, with several studies indicating that despite low LOI, these ashes required larger dosages of AEA than their counterpart virgin ashes (Al-Shmaisani et al. 2018, Diaz-Loya et al. 2019). Harvested coal ashes may also have different particle size distributions than standard coal ashes and as adsorption is particle-size-dependent, differences in adsorption relative to standard coal ash may result (Külaots et al. 2004, J. Yu et al. 2000).

In the FIT test, a slurry is made of cement, coal ash, and a diluted solution of AEA in water. In another version of the test, primarily used by coal ash producers, only coal ash, water, and AEA are used. The slurry is agitated, introducing bubbles, which the AEA interacts with and entrains. The slurry viscosity is low compared to conventional cement pastes. Due to this reduced viscosity, the entrained air rises to the top of the test column, producing a layer of metastable foam. Following agitation, the foam is observed, and its stability is gauged. If the foam dissipates, additional AEA is added, typically one drop at a time, until a stable foam is produced. The FIT has been shown to correlate with AEA dosage required to reach adequate air quantities in concrete (Kang et al. 2023, Ley et al. 2008).

Several criticisms of the method exist. First, FIT may not determine AEA adsorption at equilibrium. AEA adsorption changes over time, and this may lead to differences in dosages between lab and field mixtures (Baltrus and LaCount 2001, J. Yu et al. 2000). Second, the test includes many layers of subjectivity, including variations in the amount of agitation energy applied to the solution (typically done manually), use of imprecise equipment, and subjective determination of foam stability (which is based on operator observation). Changes to remove test subjectivity, including using a mechanical agitator and calibrated pipettes for AEA dosing, have been suggested (Watkins et al. 2015). Acceptance of a standard test method is hoped to improve the repeatability of this test, and an ASTM method (ASTM C1827-20) was recently released, which is anticipated to reduce interlab variability. However, issues that still could be researched include the proper cement-to-SCM ratio to use in the test, inconsistency in vessels used for testing—shown previously to significantly affect results (Watkins et al. 2015, and Harris et al. 2008a and 2008b)—and how or if FIT results should be normalized relative to SCM or solids content.

Another method of measuring adsorption is the iodine test (ASTM D4607, ASTM D1510), which has been used in other industries (such as water treatment) to gauge the equilibrium adsorption of activated carbon as it provides a good indication of the microporosity of the carbon (Ahmed et al. 2014a). The test is modified to use a reduced concentration solution with coal ash, which has a much lower adsorption potential than materials usually tested in ASTM D1510. The iodine number of a coal ash sample is determined by mixing the coal ash with iodine solution, then filtering and titrating using sodium thiosulfate to determine the amount of iodine remaining in solution (and thus not adsorbed by the coal ash).

The iodine test provides an accurate assessment of coal ash adsorption at equilibrium and removes the measurement subjectivity inherent in the FIT test. The iodine solution is also sensitive to the presence of other chemicals—specifically lime and sulfur, common in coal ash and cement—which must be removed prior to testing through boiling in strong acid solutions. It is known that strong acids alter coal ash microstructure, surface area, and chemistry (Wu et al. 2012, Blanco et al. 2005), which may itself change coal ash adsorption behavior. Further, the test is time-consuming, despite recommendations for use of the single-point method in ASTM D1510 (Sutter et al. 2014). Another issue with the iodine test is that it does not incorporate AEAs or a molecule with a structure similar to AEAs, and therefore cannot account for variances in the structure and chemistry of the AEAs that can affect adsorption and foaming. As a result, changes in the iodine number may not give consistent indications of expected changes in AEA dosing required for use in coal ash concrete mixtures. The iodine test also does not use cement in the tested mixtures. Cement provides an alkaline buffer and contributes calcium and fine material (surface area) to the mixture, both of

which not only increase removal of AEA from solution (through chemisorption and precipitation or physisorption, respectively), but also are important for developing the surfactant behavior of the AEA. Based on this, the iodine test is likely only useful for quantifying adsorption of coal ash independent of its use in cementitious mixtures, but it may provide a good indication of the innate adsorption of the coal ashes.

The DAI method (similar to ASTM D3860) had been proposed to directly track both the adsorption of varying coal ashes and the amount of AEA adsorbed by the coal ash. In this test, coal ash and cement are mixed with a solution of AEA and water in an Erlenmeyer flask with a magnetic stirrer for 1 hour at 20°C. Following mixing, the slurry is filtered, the volume of solution is measured, and the chemical oxygen demand of the solution is rapidly measured using a spectrophotometer. The quantity of AEA adsorbed by the coal ash samples is measured for six equilibrations of AEA concentration and plotted to develop an adsorption curve for the coal ash and AEA sample. From this information the additional AEA needed to compensate for a particular sample of coal ash can be directly calculated (Ahmed et al. 2014b, Anzalone et al. 2019). The DAI test has low subjectivity: it directly measures the adsorption capacity of the coal ash; it tests adsorption using the specific coal ash, cement, and AEA combination that will be used in the concrete; and it provides easy determination of how much AEA should be added in concrete. However, the DAI method generates waste that may be classified in some areas as hazardous (Anzalone et al. 2019). In addition, the conclusion of Sutter et al. (2013) was that good agreement did not occur between air content predicted through the DAI based on a particular AEA dosage and the air achieved in the fresh concrete using that dosage. More work is required to accurately correlate DAI and fresh concrete results.

Fluorescence-based methods are another alternative for adsorption quantification. Work in other industries has suggested fluorescence can be used to understand the nature of surfactant adsorption interactions with other materials, such as polymers (Winnik and Regismond 1996, Zhang et al. 1996, Lianos and Zana, 1981). Similar to technologies used for quantifying proteins and other biological materials in solution, the SorbSensor developed by Boral Inc. measures adsorptivity of coal ash based on reductions in fluorescence intensity after their removal from solution (Chen et al. 2011, Pihlasalo et al. 2009). The fluorescence of a standardized nonionic surfactant molecular solution is measured before and after adding coal ash, with the change in fluorescence correlated with changes in surfactant concentration. The test has been shown to correlate well with adsorption as measured by the iodine number and DAI techniques (Anzalone et al. 2019). However, the test requires proprietary equipment, and work has not been done to evaluate interactions between specific AEA molecules, cement, and coal ash samples using the fluorescence method. Development of a bench method to quantify results without the SorbSensor machine could lead to increased adoption and more standardized use of this measurement technique.

Other techniques to study adsorption have included microwave power attenuation to determine coal ash carbon content, image analysis, and even laser-induced breakdown spectroscopy (Gang et al. 2017, Liu et al. 2010, T. Wang et al. 2018), although few of these methods have gained a foothold as part of construction-industry protocol, likely because of complicated analysis or sophisticated equipment requirements.

Literature Review Findings

• Differences between unconventional and standard ashes are primarily physical—differences in fineness, moisture content, LOI, and particle shape—rather than chemical, with some exceptions. Bulk oxide content does not appear to be greatly modified in unconventional ashes.
• After the start of this project, ASTM C618-23e1 was modified to include harvested ash, bottom ash, and comingled bottom plus fly ash. It is expected that AASHTO M 295 will adopt this and other similar changes.

• LOI and SAI are not accurate indicators of AEA adsorption and reactivity, respectively. However, their simplicity and historical use make them attractive to the industry.
• Development of bulk resistivity and R³ testing methods have improved the measurement of reactivity, including bulk resistivity and the R³ test. These and other strength-based tests will be studied for unconventional ashes and compared with the SAI test. Characterization and testing are presented, specifically with respect to those measuring reactivity and tracking AEA adsorption and the ability of alternative testing methods explored in this project to improve the performance prediction capabilities of the specification.
• Two potential approaches could be used for AASHTO M 295. The first is to refine and modify existing test methods or limit values in ways that better reflect performance. The second approach is using performance-based testing. It appears that R³ and modified R³ tests, KHI, FIT, and other tests all hold promise to replace current AASHTO M 295 specification items.