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Fly-Ash_Sutter_BCC_Handout-_12-2-20.pdf

Ioannis Giannakopoulos
Ioannis Giannakopoulos

This document summarizes a presentation on the future of fly ash usage in concrete. It discusses how fly ash production is decreasing as coal power plants are retired, which could impact concrete supply. However, fly ash has benefits like improving concrete workability and durability when used as a supplementary cementitious material. The document outlines fly ash specifications, properties, effects on concrete, and testing used to ensure it mitigates alkali-silica reaction. While fly ash supplies may be challenged, the future impact on concrete usage depends on developing alternative materials and adjusting mix designs.

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12/3/2020 The Future of Fly Ash: Dystopia or Hysteria? Larry Sutter Ph.D., P.E., F.ASTM, F.ACI Materials Science & Engineering Michigan Technological University Background We expect one key property from concrete: Longevity Service demands have increased Use of aggressive deicing chemicals Increased expectations for reduced environmental impact and lower initial and lifecycle costs SCMs assist meeting these goals Definitions cementitious material, supplementary, (SCM) - an inorganic material that contributes to the properties of a cementitious mixture through hydraulic or pozzolanic activity, or both DISCUSSIONSome examples of supplementary cementitious materials are fly ash, silica fume, slag cement, rice husk ash, and natural pozzolans. In practice, these materials are used in combination with portland cement. (ASTM C125) cementitious material (hydraulic) - an inorganic material or a mixture of inorganic materials that sets and develops strength by chemical reaction with water by formation of hydrates and is capable of doing so under water (ASTM C125) Hydration Reaction Reaction of hydraulic cementitious materials with water results in production of calcium silicate hydrates (C-S-H) and calcium hydroxide (CH), also ettringite and other hydrated aluminate phases (C-A-H) Examples: portland cement, slag cement, Class C fly ash Hydraulic Reaction: Hydraulic Cement + Water C-S-H + CH C-S-H provides strength desirable product CH provides little strength and is soluble, also is a reactant in many MRD mechanisms undesirable product
12/3/2020 SCMs consume CH through the pozzolanic reaction Improves strength Increases paste density Reduces alkali (ASR mitigation) Reduces rate of heat evolution due to hydration reaction Slower strength development Hydration Reaction: Cement + Water C-S-H + CH Pozzolanic Reaction: Pozzolan + CH + Water C-S-H Pozzolanic Reaction Effects of SCMs on Properly Cured Hardened Concrete Reduced No/Little Effect Fly ash Slag Silica fume Natural Pozzolan Increase Varies Strength Gain Abrasion Resistance Freeze-Thaw and Deicer-Scaling Resistance Drying Shrinkage and Creep Permeability Alkali-Silica Reactivity Chemical Resistance Carbonation Concrete Color Effects of SCMs on Properly Cured Hardened Concrete Reduced No/Little Effect Fly ash Slag Silica fume Natural Pozzolan Increase Varies Strength Gain Abrasion Resistance Freeze-Thaw and Deicer-Scaling Resistance Drying Shrinkage and Creep Permeability Alkali-Silica Reactivity Chemical Resistance Carbonation Concrete Color Coal Fly Ash Slag Cement Silica Fume General Characteristics - Composition Increasing silica Low calcium oxide Pozzolanic Increasing calcium oxide Moderate Silica Hydraulic
12/3/2020 General Characteristics Particle Size & Shape 2 0 mic ro ns 2 0 mic ro ns 2 0 mic ro ns 2 0 mic ro ns Portland Cement Slag Cement Fly Ash Silica Fume The finely divided residue that results from the process of combustion of ground or powdered coal and that is transported by flue gasses (ASTM 2015) Produced from pulverized coal fuel Fuel stream may have other components such as limestone, trona, other additives for pollution control Coal Fly Ash Coal Fly Ash Production Airborne residue from coal combustion processes collected from the flue gases by a variety of means Electrostatic precipitators Fabric filters (baghouse) Coal Fly Ash Production Quality and consistency depends in part on burning conditions and fuel sources An important characteristic of coal combustion fly ash is the presence of residual carbon intermixed with the fly ash Natural product of combustion more prevalent in Class F ash Powder activated carbon (PAC) added to achieve pollution control goals Not all ash produced is acceptable for use in concrete Non-spec ash may be useful for other construction applications CLSM (flowable fill) Subgrade stabilization
12/3/2020 Fly Ash Specification Fly ash is specified under ASTM C618 (AASHTO M 295) Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete Chemical Requirements Classified based on the sum of the oxides (SUM) RECENT CHANGE SUM (wt.%) = % SiO2 + % Al2O3 + % Fe2O3 Class F and Class C SUM 50% Class F CaO 18% (low calcium oxide) Class C CaO > 18% (high calcium oxide) Class N SUM 70% (natural pozzolan source only) Coal Fly Ash Specification Class C Class F Increasing Hydraulic Activity Increasing Pozzolonic Activity Coal Fly Ash Specification Shehata & Thomas, 2002 Shashiprakash and Thomas 2001 McCarthy et al 1990 Coal Fly Ash Specification Key Physical Requirements Fineness amount retained on 325 mesh sieve - Limit of 34% all classes Strength Activity Index (SAI) relative strength of a mortar with 80% portland, 20% fly ash compared to control (100% portland cement) - Limit of 75% of control, all classes at 7 or 28 day
12/3/2020 Strength Activity Index 75% Specification Limit Strength Activity Index Strength Activity Index is questioned as it allows inert materials to pass Experiments performed with non-pozzolanic quartz filler at 20% replacement they all pass the SAI Need a new test to measure SCM reactivity Strength Activity Index Coal Fly Ash Characteristics Benefits Improved workability Decreased heat of hydration Reduced cost Potential increased sulfate resistance and alkali-silica reaction (ASR) mitigation Increased late strength, and decreased shrinkage and permeability Concerns Air-entraining admixture adsorption by residual carbon in the fly ash Slow initial strength gain (Class F) Fly ash variability How reactive is it?
12/3/2020 Fly Ash Carbon Affect on Air Entrainment Air entraining admixtures (AEAs) organic compounds used to entrain a controlled amount of air AEAs typically contain ionic and non-ionic surfactants made of natural sources such as wood resins, tall oil, or synthetic chemicals Schematic view of AEA molecule Head Ionic portion (has a charge) Strong attraction to water (hydrophilic) Tail Non-ionic (has no charge) Little or no attraction to water (hydrophobic) Hydrophilic, anionic polar groups (i.e. head) sorb strongly to the ionic cement particles Hydrophobic, non-polar end of the surfactants (i.e. tail) orient towards the solution Stabilize (entrain) air bubbles, prevent coalescing into larger bubbles Fly Ash Carbon Affect on Air Entrainment Carbon in fly ash adsorbs AEA from the concrete mix water Reduces the amount of AEA remaining in the water to a point where the AEA is no longer able to stabilize the required volume of air bubbles Fly Ash Carbon Affect on Air Entrainment Carbon content in fly ash is estimated by the loss on ignition (LOI) test Determines the total volatile materials, not just carbon Test does not characterize the adsorption capacity of the carbon - most important Two ashes can have the same LOI content but affect air entrainment very differently Newly developed tests, such as the foam index test, iodine number test, and direct adsorption isotherm test, provide different approaches to measuring ash adsorption (NCHRP 749) Fly Ash Carbon Affect on Air Entrainment
12/3/2020 An emerging issue is the use of powdered-activated carbon (PAC) as an additive in the coal combustion process to adsorb mercury from flue gases PAC is highly adsorptive A small amount may not significantly affect the LOI value but can drastically affect the ash adsorption properties As PAC is more commonly included in coal fly ash, the need for adsorption-based tests and specifications will increase Fly Ash Carbon Affect on Air Entrainment ASR Mitigation with Fly Ash Class F ash (pozzolanic) best at ASR mitigation Pozzolanic materials consume CH, reducing hydroxyl ions in pore water, leads to ASR mitigation Because of the variability in ash properties, it is important to verify an ashs mitigation potential Testing Fly Ash Mitigation all tests are empirical, which means they are based on experience and observation An empirical test only means something if you do it the same way when testing, as you did when you made the observation and created the test ASR Mitigation with Fly Ash ASTM C1293 Concrete Prism Test Currently the most reliable test available not infallible Not quick one year minimum two years when validating SCM replacement Known drawbacks include alkali leaching that can lead to errors in estimating the alkali threshold need for ASR to occur
12/3/2020 ASR Mitigation with Fly Ash ASTM C1567 Accelerated Mortar Bar Test Based on ASTM C1260 Cannot be used unless there is a reasonable correlation between C1260 and C1293 for the aggregate in question Alkali-Aggregate Reactivity (AAR) Facts Book. Thomas, M.D.A., Fournier, B., Folliard, K.J. ASTM C1293 Data 0.04% at 2 years Specification Limit ASTM C1567 Data 14 day (standard) 0.10% at 14 days Specification Limit ASTM C1567 Data 28 day (non- standard) 0.10% at 14 days Specification Limit
12/3/2020 So whats the problem? The Problem Fly ash supplies are challenged by plant closures and conversions to natural gas Fly ash spot shortages have been reported in many U.S. markets Concerns center on the fact that no other material is available with the reserves that fly ash historically has provided Coal-fired Power Plants are Being Retired Navajo Generating Station 2250 megawatt net coal- fired powerplant Largest coal fired electrical generating station west of the Mississippi Produces approximately 500,000 tons a year of Class F fly ash Closed 2020
12/3/2020 Coal-fired Power Plants are Being Retired Source: U.S. Energy Information Administration, 2019 2020 2035 Ash Production is Dropping So Whats Up With Fly Ash? Domestic fly ash production (new production) will be gradually decreasing over the next 20 years and beyond Domestic production is predicted to stabilize (next 5 years) reductions in coalfired power will plateau (EIA 2019) Fewer plants, running at a higher percentage of capacity Suppliers believe that although total reserves may decrease, the volume of quality ash as a percentage of total production will increase due to dry handling no more ponding Harvested ash from landfills/ponds will become a significant fraction of the total reserves
12/3/2020 So What Else is Up With Fly Ash? Other Challenges Pollution control measures will affect fresh ash Powdered Activated Carbon Trona Competing with other markets for the material Lower supply consider ash once rejected? Harvested Ash A New Frontier Options What will replace fly ash if needed? * Slag cement (existing solution) * Harvested fly ash (emerging solution) * Ash Imports (emerging solution) Natural pozzolans (existing solution) Lower quality fly ash (last resort) New Materials (colloidal silica, ground glass) Straight cement Are existing tests and specifications adequate? Slag Cement Produced from blast-furnace slag (reduction of iron ore) in a blast furnace Predominately glassy structure with a composition very similar to OPC. Slag cement is hydraulic and produces calcium silicate hydrate (CSH) as a hydration product hot slag water Slag is changed to glassy sand like substance known as granulated blast furnace slag GBFS then ground Graphics used by permissionof the Slag Cement Association Slag Cement - Hydration Slag cement is hydraulic and produces calcium silicate hydrate (C-S-H) as a hydration product Slag cement reacts slower than portland cement Hydration of portland cement produces C-S-H and CH CH reacts with the slag cement, breaking down the glass phases and causing the material to react with water and form C-S-H Slag cement is not pozzolanic It does consume CH by binding alkalis in its hydration products Provides the benefits of a pozzolan
12/3/2020 Slag Cement - Specification ASTM C989 (AASHTO M 302) Standard Specification for Slag Cement for Use in Concrete and Mortars Classifies the material under three categories: Grade 80, Grade 100, and Grade120 The grade classification refers to the relative strength of mortar cubes using the SAI test with a 50% replacement of OPC Uses standard reference cement 75% of the Control 28-day strength = Grade 80 95% of the Control 28-day strength = Grade 100 115% of the Control 28-day strength = Grade 120 Slag Cement Because slag cement reacts slow: Setting time can be increased significantly compared to OPC concrete Has lower heat evolution making slag cement ideal for mass concrete placement where control of internal temperatures is critical - up to 80% replacement of OPC with slag cement is used for mass concrete Curing is essential for all concrete; it is even more critical with slag-cement-based concrete The slower reaction rate, especially at lower temperatures, is often overlooked, and this can lead to scaling when not properly cured Slag cement is effective at mitigating ASR Requires higher replacement rates than Class F ash (e.g., > 50%) Harvested Ash Significant volumes of high- quality fly ash have been disposed Approximately 2000 million short tons produced 1974 - 2013 Approximately 650 million short tons used 1974 2013 ~33% utilization 1350 million short tons disposed Not all is recoverable, but a large fraction is Production and Use of Coal Combustion Products in the U.S. ARTBA 2015 20.1 tons used 36.2 tons produced Harvested Ash With diminishing production, ash marketers are turning to land fills & ash ponds to recover fly ash Most harvested sources are Class F ash Limited research to date on performance of harvested ash All harvested sources will require processing Drying Sizing Blending Could lead to more uniformity - or less - depending upon source and degree of processing
12/3/2020 Harvested Ash Concerns Uniformity ash in ponds will stratify based on density and strata in land fills/ponds will represent different coal sources and burning conditions Weathering Does storage alter the chemical or physical nature of the ash? Adulteration many land fills/ponds hold bottom ash, scrubber residue, and other wastes in addition to ash Infiltration clays and other materials may infiltrate and co-deposit Testing do current specifications provide tests & limits that will adequately screen harvested ash? Harvested Ash Concerns (continued) Current federal and state regulations require near-term closure of disposal ponds, leaving insufficient time to recover and use all available ash Power producers have little to no incentive to use ash beneficially, closure (cap-in-place) is the lowest cost option. Benefits of landfilled ash Well over a billion tons of ash in disposal Proper processing could provide a more uniform product Significant reserves could help limit cost increases although processing will add costs Imported Ash
12/3/2020 Coal-fired Power Plants are Being Retired? Source: CarbonBrief Coal-fired Power Plants are Being Retired? Source: CarbonBrief Imports Certainly in the near term, and potentially long term, imports will become a significant source Imports are already a significant contributor in some markets China is COMMITTED to keeping shipping costs low, making imports cost effective (i.e., producing a large number of ocean-going cargo ships at a fraction of the cost of western countries) For imports. issues of quality must be considered - TESTING
12/3/2020 Natural Pozzolans With issues of availability for other SCMs, natural pozzolans and ASCMs are attracting interest within the industry Examples of natural pozzolans include Some diatomaceous earths Opaline cherts and shale Tuffs Volcanic ashes Pumicite Various calcined clays and shales Some natural pozzolans can be used as mined Most require processing such as drying, calcining, or grinding - TESTING Lower Quality - Increased Need for Testing So called off-spec ash is being considered Note: Existing ash specifications do not address performance (i.e., meeting the specification does not guarantee performance) If performance of a material can be demonstrated use it Common off-spec issues LOI Fineness Materials that are not coal fly ash are not off-spec; they are simply not fly ash but they may work Verify reserves New Materials Ground Glass Total Production (~ 11 million tons/year in U.S.) Container Glass (~ 3 million tons/year in U.S.) E-Glass (100,000 lbs/year in U.S.) Recycling capacity exceeds generation (U.S. EPA) Primary Processing Grinding -325 mesh Composition is uniform
12/3/2020 Nominal Glass Composition Soda Lime Glass E-Glass Bottle Glass Plate Glass Display Glass SiO2 71 71 63 60 Al2O3 1.8 0.4 18 12.5 Fe2O3 0.6 0.4 0.0 0.4 B2O3 0.01 0.02 2.0 0.0 MgO 0.90 3.9 2.5 2.9 CaO 11 9.3 0.1 21 Na2O 13 13 13 0.75 K2O 0.5 0.05 0.0 0.06 Bottom Ash ASTM is discussing a Class B for bottom ash Mimics the properties of the fly ash from the same coal but attributes are subdued, relative to the fly ash Contributes to concrete properties Mitigates ASR Angular increased water demand Commonly comingled with fly ash in harvested materials Green, B. ACI Materials Journal, SP-254-8, 121132, 2008. Kudyba-Jansen, A., Hintzen, H., Metselaar, R. Materials Research Bulletin, 36, 1215 1230, 2001. Class F Fly Ash Colloidal Silica Colloidal Silica After J. Belkowitz, Intelligent Concrete LLC
12/3/2020 Alternative SCMs Inorganic materials that react, as a pozzolan or hydraulic cement, and beneficially contribute to the strength, durability, workability, or other characteristics of concrete, and do not meet ASTM specifications C618, C989, and C1240 Examples include some slags or fly ash from co-combustion processes such as coal with biomass Used in limited applications in some markets ASTM C1709 Standard Guide for Evaluation of Alternative Supplementary Cementitious Materials (ASCM) for Use in Concrete was developed to provide a clear methodology for evaluating these materials Ternary Mixtures Concrete mixtures that contain OPC and two other materials in the binder fraction The binder materials may be combined at the batch plant, or obtained as a pre-blended product In general, ternary mixtures perform in a manner that can be predicted by knowing the characteristics of the individual ingredients One benefit of ternary mixtures is that negative properties of a one SCM can be offset by positive properties of another Straight Cement? 3:5:6 Once 3:5:6 doesnt apply (e.g., 6:6:6) the cement replacement advantage is diminished Sustainability goals are important only if incentivized A higher cement content (low alkali loading) is not out of reality IF the mixture meets performance ASR mitigation Sulfate attack prevention Physical properties ASR Risk Mitigation - AASHTO
12/3/2020 What about tests and specifications? Existing tests and specifications provide little information on performance As harvested materials and other sources become more common, new tests and specifications are required that relate to performance (i.e., pozzolanic activity, hydraulic activity, particle size, adsorption) Need to let go of historic limits/tests established in a completely different concrete world that mean little now (e.g., SAI test, LOI) Specifications need to include blending SCMs Need to get more materials in the market while improving performance and quality Trends in Specifications Concerns with consistent performance & use of harvested ash have caused ASTM & AASHTO to re-evaluate specifications Measure reactivity (done) R3 tests (rapid, reliable, reproducible) measure heat released by isothermal calorimetry or else measure bound water - both for SCM exposed to CH solution Lime Pozzolanic Activity Test Particle size need a better test Consider modifications to SAI Measure efficiency Trends in Specifications Recently removed the Effectiveness in Controlling ASR test & limits Adsorption potential just passed the foam index test at ASTM Use adsorption based tests rather than LOI Remove Autoclave soundness - nothing fails (pending) Remove available alkali test - Not required to assess ASR mitigation (pending) New natural pozzolan specification (pending) New performance-based specification (pending)
12/3/2020 Summary SCMs are essential to concrete durability Key materials Fly Ash Slag cement Silica fume Emerging Materials Natural pozzolans Alternative SCMs Summary All SCMs are expected to favorably affect the following but each does so in varying degrees Strength Permeability Heat of hydration ASR and Sulfate attack mitigation SCMs may or may not favorably affect the following Early strength Rate of strength gain Cost Summary Availability and use of SCMs is changing fly ash is in short supply in some markets Traditional material supplies will be challenged Trends will be towards more ternary mixtures where blends of SCMs will be used New materials will enter the market place Testing of all materials and verification of performance in concrete will become more important moving forward Summary Near term solutions Other SCMs (e.g., slag, ground glass, natural pozzolans) Imports Harvested Ash Straight cement possible durability may suffer if not approached carefully
12/3/2020 Questions? llsutter@mtu.edu

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Fly-Ash_Sutter_BCC_Handout-_12-2-20.pdf

  • 1. 12/3/2020 The Future of Fly Ash: Dystopia or Hysteria? Larry Sutter Ph.D., P.E., F.ASTM, F.ACI Materials Science & Engineering Michigan Technological University Background We expect one key property from concrete: Longevity Service demands have increased Use of aggressive deicing chemicals Increased expectations for reduced environmental impact and lower initial and lifecycle costs SCMs assist meeting these goals Definitions cementitious material, supplementary, (SCM) - an inorganic material that contributes to the properties of a cementitious mixture through hydraulic or pozzolanic activity, or both DISCUSSIONSome examples of supplementary cementitious materials are fly ash, silica fume, slag cement, rice husk ash, and natural pozzolans. In practice, these materials are used in combination with portland cement. (ASTM C125) cementitious material (hydraulic) - an inorganic material or a mixture of inorganic materials that sets and develops strength by chemical reaction with water by formation of hydrates and is capable of doing so under water (ASTM C125) Hydration Reaction Reaction of hydraulic cementitious materials with water results in production of calcium silicate hydrates (C-S-H) and calcium hydroxide (CH), also ettringite and other hydrated aluminate phases (C-A-H) Examples: portland cement, slag cement, Class C fly ash Hydraulic Reaction: Hydraulic Cement + Water C-S-H + CH C-S-H provides strength desirable product CH provides little strength and is soluble, also is a reactant in many MRD mechanisms undesirable product
  • 2. 12/3/2020 SCMs consume CH through the pozzolanic reaction Improves strength Increases paste density Reduces alkali (ASR mitigation) Reduces rate of heat evolution due to hydration reaction Slower strength development Hydration Reaction: Cement + Water C-S-H + CH Pozzolanic Reaction: Pozzolan + CH + Water C-S-H Pozzolanic Reaction Effects of SCMs on Properly Cured Hardened Concrete Reduced No/Little Effect Fly ash Slag Silica fume Natural Pozzolan Increase Varies Strength Gain Abrasion Resistance Freeze-Thaw and Deicer-Scaling Resistance Drying Shrinkage and Creep Permeability Alkali-Silica Reactivity Chemical Resistance Carbonation Concrete Color Effects of SCMs on Properly Cured Hardened Concrete Reduced No/Little Effect Fly ash Slag Silica fume Natural Pozzolan Increase Varies Strength Gain Abrasion Resistance Freeze-Thaw and Deicer-Scaling Resistance Drying Shrinkage and Creep Permeability Alkali-Silica Reactivity Chemical Resistance Carbonation Concrete Color Coal Fly Ash Slag Cement Silica Fume General Characteristics - Composition Increasing silica Low calcium oxide Pozzolanic Increasing calcium oxide Moderate Silica Hydraulic
  • 3. 12/3/2020 General Characteristics Particle Size & Shape 2 0 mic ro ns 2 0 mic ro ns 2 0 mic ro ns 2 0 mic ro ns Portland Cement Slag Cement Fly Ash Silica Fume The finely divided residue that results from the process of combustion of ground or powdered coal and that is transported by flue gasses (ASTM 2015) Produced from pulverized coal fuel Fuel stream may have other components such as limestone, trona, other additives for pollution control Coal Fly Ash Coal Fly Ash Production Airborne residue from coal combustion processes collected from the flue gases by a variety of means Electrostatic precipitators Fabric filters (baghouse) Coal Fly Ash Production Quality and consistency depends in part on burning conditions and fuel sources An important characteristic of coal combustion fly ash is the presence of residual carbon intermixed with the fly ash Natural product of combustion more prevalent in Class F ash Powder activated carbon (PAC) added to achieve pollution control goals Not all ash produced is acceptable for use in concrete Non-spec ash may be useful for other construction applications CLSM (flowable fill) Subgrade stabilization
  • 4. 12/3/2020 Fly Ash Specification Fly ash is specified under ASTM C618 (AASHTO M 295) Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete Chemical Requirements Classified based on the sum of the oxides (SUM) RECENT CHANGE SUM (wt.%) = % SiO2 + % Al2O3 + % Fe2O3 Class F and Class C SUM 50% Class F CaO 18% (low calcium oxide) Class C CaO > 18% (high calcium oxide) Class N SUM 70% (natural pozzolan source only) Coal Fly Ash Specification Class C Class F Increasing Hydraulic Activity Increasing Pozzolonic Activity Coal Fly Ash Specification Shehata & Thomas, 2002 Shashiprakash and Thomas 2001 McCarthy et al 1990 Coal Fly Ash Specification Key Physical Requirements Fineness amount retained on 325 mesh sieve - Limit of 34% all classes Strength Activity Index (SAI) relative strength of a mortar with 80% portland, 20% fly ash compared to control (100% portland cement) - Limit of 75% of control, all classes at 7 or 28 day
  • 5. 12/3/2020 Strength Activity Index 75% Specification Limit Strength Activity Index Strength Activity Index is questioned as it allows inert materials to pass Experiments performed with non-pozzolanic quartz filler at 20% replacement they all pass the SAI Need a new test to measure SCM reactivity Strength Activity Index Coal Fly Ash Characteristics Benefits Improved workability Decreased heat of hydration Reduced cost Potential increased sulfate resistance and alkali-silica reaction (ASR) mitigation Increased late strength, and decreased shrinkage and permeability Concerns Air-entraining admixture adsorption by residual carbon in the fly ash Slow initial strength gain (Class F) Fly ash variability How reactive is it?
  • 6. 12/3/2020 Fly Ash Carbon Affect on Air Entrainment Air entraining admixtures (AEAs) organic compounds used to entrain a controlled amount of air AEAs typically contain ionic and non-ionic surfactants made of natural sources such as wood resins, tall oil, or synthetic chemicals Schematic view of AEA molecule Head Ionic portion (has a charge) Strong attraction to water (hydrophilic) Tail Non-ionic (has no charge) Little or no attraction to water (hydrophobic) Hydrophilic, anionic polar groups (i.e. head) sorb strongly to the ionic cement particles Hydrophobic, non-polar end of the surfactants (i.e. tail) orient towards the solution Stabilize (entrain) air bubbles, prevent coalescing into larger bubbles Fly Ash Carbon Affect on Air Entrainment Carbon in fly ash adsorbs AEA from the concrete mix water Reduces the amount of AEA remaining in the water to a point where the AEA is no longer able to stabilize the required volume of air bubbles Fly Ash Carbon Affect on Air Entrainment Carbon content in fly ash is estimated by the loss on ignition (LOI) test Determines the total volatile materials, not just carbon Test does not characterize the adsorption capacity of the carbon - most important Two ashes can have the same LOI content but affect air entrainment very differently Newly developed tests, such as the foam index test, iodine number test, and direct adsorption isotherm test, provide different approaches to measuring ash adsorption (NCHRP 749) Fly Ash Carbon Affect on Air Entrainment
  • 7. 12/3/2020 An emerging issue is the use of powdered-activated carbon (PAC) as an additive in the coal combustion process to adsorb mercury from flue gases PAC is highly adsorptive A small amount may not significantly affect the LOI value but can drastically affect the ash adsorption properties As PAC is more commonly included in coal fly ash, the need for adsorption-based tests and specifications will increase Fly Ash Carbon Affect on Air Entrainment ASR Mitigation with Fly Ash Class F ash (pozzolanic) best at ASR mitigation Pozzolanic materials consume CH, reducing hydroxyl ions in pore water, leads to ASR mitigation Because of the variability in ash properties, it is important to verify an ashs mitigation potential Testing Fly Ash Mitigation all tests are empirical, which means they are based on experience and observation An empirical test only means something if you do it the same way when testing, as you did when you made the observation and created the test ASR Mitigation with Fly Ash ASTM C1293 Concrete Prism Test Currently the most reliable test available not infallible Not quick one year minimum two years when validating SCM replacement Known drawbacks include alkali leaching that can lead to errors in estimating the alkali threshold need for ASR to occur
  • 8. 12/3/2020 ASR Mitigation with Fly Ash ASTM C1567 Accelerated Mortar Bar Test Based on ASTM C1260 Cannot be used unless there is a reasonable correlation between C1260 and C1293 for the aggregate in question Alkali-Aggregate Reactivity (AAR) Facts Book. Thomas, M.D.A., Fournier, B., Folliard, K.J. ASTM C1293 Data 0.04% at 2 years Specification Limit ASTM C1567 Data 14 day (standard) 0.10% at 14 days Specification Limit ASTM C1567 Data 28 day (non- standard) 0.10% at 14 days Specification Limit
  • 9. 12/3/2020 So whats the problem? The Problem Fly ash supplies are challenged by plant closures and conversions to natural gas Fly ash spot shortages have been reported in many U.S. markets Concerns center on the fact that no other material is available with the reserves that fly ash historically has provided Coal-fired Power Plants are Being Retired Navajo Generating Station 2250 megawatt net coal- fired powerplant Largest coal fired electrical generating station west of the Mississippi Produces approximately 500,000 tons a year of Class F fly ash Closed 2020
  • 10. 12/3/2020 Coal-fired Power Plants are Being Retired Source: U.S. Energy Information Administration, 2019 2020 2035 Ash Production is Dropping So Whats Up With Fly Ash? Domestic fly ash production (new production) will be gradually decreasing over the next 20 years and beyond Domestic production is predicted to stabilize (next 5 years) reductions in coalfired power will plateau (EIA 2019) Fewer plants, running at a higher percentage of capacity Suppliers believe that although total reserves may decrease, the volume of quality ash as a percentage of total production will increase due to dry handling no more ponding Harvested ash from landfills/ponds will become a significant fraction of the total reserves
  • 11. 12/3/2020 So What Else is Up With Fly Ash? Other Challenges Pollution control measures will affect fresh ash Powdered Activated Carbon Trona Competing with other markets for the material Lower supply consider ash once rejected? Harvested Ash A New Frontier Options What will replace fly ash if needed? * Slag cement (existing solution) * Harvested fly ash (emerging solution) * Ash Imports (emerging solution) Natural pozzolans (existing solution) Lower quality fly ash (last resort) New Materials (colloidal silica, ground glass) Straight cement Are existing tests and specifications adequate? Slag Cement Produced from blast-furnace slag (reduction of iron ore) in a blast furnace Predominately glassy structure with a composition very similar to OPC. Slag cement is hydraulic and produces calcium silicate hydrate (CSH) as a hydration product hot slag water Slag is changed to glassy sand like substance known as granulated blast furnace slag GBFS then ground Graphics used by permissionof the Slag Cement Association Slag Cement - Hydration Slag cement is hydraulic and produces calcium silicate hydrate (C-S-H) as a hydration product Slag cement reacts slower than portland cement Hydration of portland cement produces C-S-H and CH CH reacts with the slag cement, breaking down the glass phases and causing the material to react with water and form C-S-H Slag cement is not pozzolanic It does consume CH by binding alkalis in its hydration products Provides the benefits of a pozzolan
  • 12. 12/3/2020 Slag Cement - Specification ASTM C989 (AASHTO M 302) Standard Specification for Slag Cement for Use in Concrete and Mortars Classifies the material under three categories: Grade 80, Grade 100, and Grade120 The grade classification refers to the relative strength of mortar cubes using the SAI test with a 50% replacement of OPC Uses standard reference cement 75% of the Control 28-day strength = Grade 80 95% of the Control 28-day strength = Grade 100 115% of the Control 28-day strength = Grade 120 Slag Cement Because slag cement reacts slow: Setting time can be increased significantly compared to OPC concrete Has lower heat evolution making slag cement ideal for mass concrete placement where control of internal temperatures is critical - up to 80% replacement of OPC with slag cement is used for mass concrete Curing is essential for all concrete; it is even more critical with slag-cement-based concrete The slower reaction rate, especially at lower temperatures, is often overlooked, and this can lead to scaling when not properly cured Slag cement is effective at mitigating ASR Requires higher replacement rates than Class F ash (e.g., > 50%) Harvested Ash Significant volumes of high- quality fly ash have been disposed Approximately 2000 million short tons produced 1974 - 2013 Approximately 650 million short tons used 1974 2013 ~33% utilization 1350 million short tons disposed Not all is recoverable, but a large fraction is Production and Use of Coal Combustion Products in the U.S. ARTBA 2015 20.1 tons used 36.2 tons produced Harvested Ash With diminishing production, ash marketers are turning to land fills & ash ponds to recover fly ash Most harvested sources are Class F ash Limited research to date on performance of harvested ash All harvested sources will require processing Drying Sizing Blending Could lead to more uniformity - or less - depending upon source and degree of processing
  • 13. 12/3/2020 Harvested Ash Concerns Uniformity ash in ponds will stratify based on density and strata in land fills/ponds will represent different coal sources and burning conditions Weathering Does storage alter the chemical or physical nature of the ash? Adulteration many land fills/ponds hold bottom ash, scrubber residue, and other wastes in addition to ash Infiltration clays and other materials may infiltrate and co-deposit Testing do current specifications provide tests & limits that will adequately screen harvested ash? Harvested Ash Concerns (continued) Current federal and state regulations require near-term closure of disposal ponds, leaving insufficient time to recover and use all available ash Power producers have little to no incentive to use ash beneficially, closure (cap-in-place) is the lowest cost option. Benefits of landfilled ash Well over a billion tons of ash in disposal Proper processing could provide a more uniform product Significant reserves could help limit cost increases although processing will add costs Imported Ash
  • 14. 12/3/2020 Coal-fired Power Plants are Being Retired? Source: CarbonBrief Coal-fired Power Plants are Being Retired? Source: CarbonBrief Imports Certainly in the near term, and potentially long term, imports will become a significant source Imports are already a significant contributor in some markets China is COMMITTED to keeping shipping costs low, making imports cost effective (i.e., producing a large number of ocean-going cargo ships at a fraction of the cost of western countries) For imports. issues of quality must be considered - TESTING
  • 15. 12/3/2020 Natural Pozzolans With issues of availability for other SCMs, natural pozzolans and ASCMs are attracting interest within the industry Examples of natural pozzolans include Some diatomaceous earths Opaline cherts and shale Tuffs Volcanic ashes Pumicite Various calcined clays and shales Some natural pozzolans can be used as mined Most require processing such as drying, calcining, or grinding - TESTING Lower Quality - Increased Need for Testing So called off-spec ash is being considered Note: Existing ash specifications do not address performance (i.e., meeting the specification does not guarantee performance) If performance of a material can be demonstrated use it Common off-spec issues LOI Fineness Materials that are not coal fly ash are not off-spec; they are simply not fly ash but they may work Verify reserves New Materials Ground Glass Total Production (~ 11 million tons/year in U.S.) Container Glass (~ 3 million tons/year in U.S.) E-Glass (100,000 lbs/year in U.S.) Recycling capacity exceeds generation (U.S. EPA) Primary Processing Grinding -325 mesh Composition is uniform
  • 16. 12/3/2020 Nominal Glass Composition Soda Lime Glass E-Glass Bottle Glass Plate Glass Display Glass SiO2 71 71 63 60 Al2O3 1.8 0.4 18 12.5 Fe2O3 0.6 0.4 0.0 0.4 B2O3 0.01 0.02 2.0 0.0 MgO 0.90 3.9 2.5 2.9 CaO 11 9.3 0.1 21 Na2O 13 13 13 0.75 K2O 0.5 0.05 0.0 0.06 Bottom Ash ASTM is discussing a Class B for bottom ash Mimics the properties of the fly ash from the same coal but attributes are subdued, relative to the fly ash Contributes to concrete properties Mitigates ASR Angular increased water demand Commonly comingled with fly ash in harvested materials Green, B. ACI Materials Journal, SP-254-8, 121132, 2008. Kudyba-Jansen, A., Hintzen, H., Metselaar, R. Materials Research Bulletin, 36, 1215 1230, 2001. Class F Fly Ash Colloidal Silica Colloidal Silica After J. Belkowitz, Intelligent Concrete LLC
  • 17. 12/3/2020 Alternative SCMs Inorganic materials that react, as a pozzolan or hydraulic cement, and beneficially contribute to the strength, durability, workability, or other characteristics of concrete, and do not meet ASTM specifications C618, C989, and C1240 Examples include some slags or fly ash from co-combustion processes such as coal with biomass Used in limited applications in some markets ASTM C1709 Standard Guide for Evaluation of Alternative Supplementary Cementitious Materials (ASCM) for Use in Concrete was developed to provide a clear methodology for evaluating these materials Ternary Mixtures Concrete mixtures that contain OPC and two other materials in the binder fraction The binder materials may be combined at the batch plant, or obtained as a pre-blended product In general, ternary mixtures perform in a manner that can be predicted by knowing the characteristics of the individual ingredients One benefit of ternary mixtures is that negative properties of a one SCM can be offset by positive properties of another Straight Cement? 3:5:6 Once 3:5:6 doesnt apply (e.g., 6:6:6) the cement replacement advantage is diminished Sustainability goals are important only if incentivized A higher cement content (low alkali loading) is not out of reality IF the mixture meets performance ASR mitigation Sulfate attack prevention Physical properties ASR Risk Mitigation - AASHTO
  • 18. 12/3/2020 What about tests and specifications? Existing tests and specifications provide little information on performance As harvested materials and other sources become more common, new tests and specifications are required that relate to performance (i.e., pozzolanic activity, hydraulic activity, particle size, adsorption) Need to let go of historic limits/tests established in a completely different concrete world that mean little now (e.g., SAI test, LOI) Specifications need to include blending SCMs Need to get more materials in the market while improving performance and quality Trends in Specifications Concerns with consistent performance & use of harvested ash have caused ASTM & AASHTO to re-evaluate specifications Measure reactivity (done) R3 tests (rapid, reliable, reproducible) measure heat released by isothermal calorimetry or else measure bound water - both for SCM exposed to CH solution Lime Pozzolanic Activity Test Particle size need a better test Consider modifications to SAI Measure efficiency Trends in Specifications Recently removed the Effectiveness in Controlling ASR test & limits Adsorption potential just passed the foam index test at ASTM Use adsorption based tests rather than LOI Remove Autoclave soundness - nothing fails (pending) Remove available alkali test - Not required to assess ASR mitigation (pending) New natural pozzolan specification (pending) New performance-based specification (pending)
  • 19. 12/3/2020 Summary SCMs are essential to concrete durability Key materials Fly Ash Slag cement Silica fume Emerging Materials Natural pozzolans Alternative SCMs Summary All SCMs are expected to favorably affect the following but each does so in varying degrees Strength Permeability Heat of hydration ASR and Sulfate attack mitigation SCMs may or may not favorably affect the following Early strength Rate of strength gain Cost Summary Availability and use of SCMs is changing fly ash is in short supply in some markets Traditional material supplies will be challenged Trends will be towards more ternary mixtures where blends of SCMs will be used New materials will enter the market place Testing of all materials and verification of performance in concrete will become more important moving forward Summary Near term solutions Other SCMs (e.g., slag, ground glass, natural pozzolans) Imports Harvested Ash Straight cement possible durability may suffer if not approached carefully