CHAPTER 1

Introduction

Over the past four decades, the use in concrete of supplementary cementitious materials (SCMs), such as fly ash, slag cement, and silica fume, has improved the properties of fresh concrete, such as workability and finishability, and has reduced heat development. SCM use has also significantly improved the properties of hardened concrete strength and durability at later ages, and in some ternary mixtures, even at early ages.

The use of SCMs in well-proportioned concrete mixtures not only contributes to prolonging the service life of highway infrastructure but also helps promote a better environment. With longer-lasting infrastructure that experiences fewer repairs and less rehabilitation, the supply of quality aggregates will be preserved, which will be especially helpful in regions with a low supply of quality aggregate. Also, by replacing and lowering the Portland cement content in concrete mixtures, SCMs contribute to a better environment by reducing the emissions of CO₂ and other harmful gases during the manufacturing process.

However, since 2010, the availability of coal ash has been in continuous decline, prompted primarily by the conversion of power-generating plants from coal to natural gas. In response, many state departments of transportation (DOTs) have been addressing this challenge by modifying their specifications and allowing the use of natural pozzolans (NPs) and beneficiated harvested ash, as specified in ASTM C618-23. Some agencies are also supporting research and demonstration projects to evaluate alternative SCMs (ASCMs) for use in concrete.

Synthesis Objective and Needed Information

The objective of this synthesis is to document state DOT practices for specifying and using SCMs in concrete that is used in transportation infrastructure applications. Information gathered as part of this project includes:

• The types of SCMs allowed by state DOTs;
• Primary reasons, use, and application types of different SCMs;
• Use of NPs and harvested ash;
• Percent replacement of Portland cement by different SCMs in binary and ternary mixtures;
• Challenges associated with the use of SCMs;
• Practices for monitoring the quality of SCMs;
• Availability of fly ash and how state DOTs are responding to the shortage of fly ash;
• Use and availability of ASCMs;
• Types of demonstration projects for ASCMs; and
• Use of ASCMs in conventional DOT concrete specifications.

Background

The use of SCMs improves the fresh and hardened properties of concrete, provides sustainability benefits associated with mitigating negative impacts on the environment, and provides economic and social benefits. In fresh concrete mixtures, SCMs may enhance mixture workability, reduce water bleeding to provide better concrete placement and finishing quality, and reduce the rate of heat development from the cement hydration and, thus, control the rise in concrete temperature in mass structures. In hardened concrete, SCMs improve its ultimate strength and reduce permeability to enhance the ability of concrete to resist alkali-silica reactions (ASRs) and adverse chemical reactions from sulfates and corrosion-inducing chlorides from seawater and deicing salts. Environmental benefits are realized when SCMs are used to replace a percentage of the cement content in concrete mixtures. By removing a portion of the cement from concrete mixtures, greenhouse gas (GHG) emissions (specifically CO₂) from the manufacturing of Portland cement are reduced.

Types and Benefits of SCMs

During the past several decades, conventional SCMs, such as fly ash, slag cement, and silica fume, have been thoroughly researched and specified for use by state DOTs in concrete mixtures for paving and structural projects. Coal ash, typically specified as Class F or C fly ash, is an airborne residue from the combustion process of the pulverization of coal in power-generating plants. Slag cement is a by-product of the manufacturing of iron, and silica fume is a very fine particulate produced and collected during the manufacturing of silicon or alloys containing silicon. These SCMs possess pozzolanic characteristics that, when added to a concrete mixture, engage in pozzolanic reaction with Ca(OH)₂ or calcium hydroxide (CH), which is an alkali by-product of the cement–water hydration process. The result of this pozzolanic reaction is the depletion of CH and other alkalis in concrete and the production of additional useful calcium–silicate–hydrate (C-S-H) hydration binder gel that fills spaces in the paste capillary pores to reduce the permeability of concrete. This densification of the paste increases the concrete’s strength and durability to resist ASR, as well as its resistance to sulfate attack and the ingress of corrosion-inducing chlorides. The pozzolanic characteristics of SCMs are primarily due to their high amorphous silica content, the presence of other oxides supporting their reactivity, and their finely ground particles.

Availability of Fly Ash

In recent years, the availability of fly ash, which is the most commonly used SCM, has been in steady decline. This is mainly due to the conversion of power-generating plants from coal to natural gas. The decline in the availability of fly ash has presented challenges for many state DOTs and has prompted them to take actions to compensate for the shortage. These actions include modifying DOT specifications, importing fly ash, increasing the use of other conventional SCMs and natural pozzolans, and harvesting coal ash/bottom ash from landfills and then reprocessing it to meet current fly ash specifications (ASTM C618-23/AASHTO M 295). Another action has been to explore, experiment with, and, in some states, specify the use of ASCMs.

Beneficiated Harvested Coal Ash

In response to the reduction in the availability of coal fly ash, efforts have been underway, with support from state DOTs, to beneficiate harvested coal ash and bottom ash, previously discarded in landfills and ponds, to activate their pozzolanic properties to meet ASTM C618-23, Coal Ash

and Raw or Calcined Natural Pozzolan for Use in Concrete, and facilitate their use in concrete projects. The process begins with qualifying the landfill site/pond for ash harvesting. The subsequent rehabilitation process involves drying, filtering harmful materials, calcinating to reduce the carbon content and agents, and finely grinding the material to 45 μm or less to activate its pozzolanic characteristics.

Natural Pozzolans

NPs, such as clay-kaolin, shale, slate, and pumice, are earthen materials that may exist naturally and may have resulted from volcanic activity producing volcanic ash. The majority of NPs require processing to activate their pozzolanic characteristics. The processes required to develop the pozzolanic characteristics in NPs generally are drying, filtering, and calcination at high temperatures (above 1,000^oF), followed by fine grinding. NPs must conform to the requirements of ASTM C618-23. The pozzolanic reaction of NPs in concrete and the benefits of NPs are generally similar to those of Class F (high-silica) fly ash. The performance of NPs in concrete has been similar to the performance of fly ash in binary and ternary mixtures. With such qualities, NPs are becoming more desirable for use because of shortages of fly ash.

Binary, Ternary, and Quaternary SCM Mixtures

Depending on the concrete application and the extent of benefit required, one, two, or even three SCMs may be combined with Portland cement to form a binary, ternary, or quaternary mixture, respectively. Binary mixtures of either fly ash, slag cement, or silica fume are intended to alter or produce additional attributes in properties of concrete in its fresh or hardened phases. These changes in properties include improved workability, reduced heat of hydration, and achieving or exceeding the required strength at later ages, and in some mixtures even at an early age. In ternary (two SCMs) and quaternary (three SCMs) mixtures, the SCMs work together to improve the properties of concrete and, in some cases, offset the shortcomings of each others’ properties. Ternary and quaternary mixtures allow replacement of a fairly large percentage of Portland cement. These mixtures are used to control heat development from cement hydration in mass concrete and to reduce concrete permeability to enhance its durability.

Ternary mixtures increase later-age concrete strength, and in some SCM combinations, will also improve early-age strength as well. For example, when a mixture includes two SCMs, such as silica fume and coal ash or slag, the delay in early-age strength development due to the presence of coal ash or slag is compensated for by the presence of silica fume, which increases the strength gain and reduces the permeability of the concrete at early ages. Additionally, the coal ash and slag can counteract silica fume’s negative impacts on fresh concrete by improving the flowability and bleeding characteristics of the concrete mixture while achieving or exceeding the required later-age strength and durability. Quaternary blends are also allowed by some state DOTs to further optimize quantities of cementitious materials, including Portland cement and SCMs, to obtain the desired fresh and hardened concrete properties and performance.

Alternative SCMs

Another response to the shortage of fly ash has been the emergence and evolution of ASCMs for use in concrete applications. The sources of emerging ASCMs are often industrial by-products and recycled municipal, construction, industrial, and mine waste materials that are processed and beneficiated to support their use as pozzolanic materials. Also, new products are being manufactured to embody characteristics that mimic those of coal ash. While most states have not yet considered their use, some ASCMs with very high amorphous silica content, such

as ground-glass pozzolan (GGP), have demonstrated value as pozzolanic materials that improve concrete properties and are being included in the specifications of some state DOTs.

Report Organization

The information in this report is presented in five chapters and two appendices:

Chapter 1 presents a short introduction to and background on SCMs, including important issues related to their nature, effectiveness, availability challenges, and current uses.

Chapter 2 summarizes information gathered from a review of the literature. It discusses guides and practices of the American Concrete Institute (ACI), FHWA reports, technical briefs, TRB papers, NCHRP reports, university publications, industry technical information, ASTM standards, and publications prepared outside the United States.

Chapter 3 presents the survey results of state DOTs. A 25-question survey was transmitted electronically to the 50 state DOTs and those of the District of Columbia and Puerto Rico. Forty-three responses were received and analyzed.

Chapter 4 contains case examples from five state DOTs: those of California, Colorado, Louisiana, Minnesota, and Utah. Each case example was prepared based on interviews with a state DOT representative, and the final drafts of the case examples were later reviewed by these representatives.

Chapter 5 summarizes the findings of Chapters 2, 3, and 4 and identifies gaps in the information on SCMs, as well as aspects of SCMs and ASCMs that require research and further investigation.

Following the chapters are the references, Appendix A, which includes a copy of the 25-question survey that was sent to the state DOTs, and Appendix B, which presents the responses of the state DOTs to each of the questions.