Addressing Fly Ash Shortage with Limestone Calcined Clay Cement

About the report

Virginia and many other states are experiencing increasing challenges related to the availability of traditional supplementary cementitious materials (SCMs), particularly fly ash. These shortages—with both seasonal and regional variability—threaten the consistency, cost, and performance of concrete used in the Virginia Department of Transportation’s (VDOT) infrastructure. At the same time, an increasing emphasis is on reducing the environmental footprint of cement and concrete production, which necessitates the use of SCMs to reduce the global warming potential associated with clinker manufacturing. Calcined clays have emerged as promising alternative SCMs that may address both material supply constraints and sustainability objectives. However, their suitability for VDOT concrete—including effects on fresh properties, strength, durability, and environmental performance—had not been evaluated. This study was initiated to generate the data and guidance needed for informed decision making regarding the adoption of calcined clays in VDOT applications.

This study evaluated limestone calcined clay cement, or LC3—a ternary blended cement typically composed of 50% clinker, 30% calcined clay, 15% limestone, and 5% gypsum—and LC2, a cementitious system similar to LC3 produced by replacing 30% of Portland limestone cement (Type IL) with calcined clay. The fresh, mechanical, durability, and shrinkage characteristics of concrete incorporating LC3 and LC2 were assessed and compared with mixtures containing Class F coal ash and slag cement commonly used in VDOT concretes. In addition, the physical, chemical, and reactivity properties of SCMs were characterized, and global warming potential analyses were conducted to quantify the potential environmental benefits associated with incorporating calcined clay. Together, these evaluations provide a comprehensive framework for understanding the suitability of calcined clays for VDOT concrete.

Results showed that calcined clays are substantially more reactive than Class F coal ash and more comparable with slag cement. However, their porous micromorphology and high surface area significantly reduce workability, making admixture selection and dosage critical for field placement. At a 30% dosage, a calcined clay meeting ASTM C618 requirements for Class N pozzolans produced concrete with performance comparable with or better than mixtures containing 30% Class F coal ash, and air entrainment behavior was similar to that of the Type IL control mixture—an advantage over fly ash, which often requires higher air entraining admixture dosages. These findings demonstrate that calcined clays are a viable SCM option that can help alleviate fly ash shortages while maintaining or improving concrete performance. Collectively, this study provides data driven guidance that can improve long term material resilience, reduce the environmental footprint of VDOT concrete, and prepare the agency for the adoption of next generation SCMs and blended cements, including those with calcined clays, which are expected to become more readily available in Virginia in the near future.

_{Disclaimer Statement:The contents of this report reflect the views of the author(s), who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Virginia Department of Transportation, the Commonwealth Transportation Board, or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Any inclusion of manufacturer names, trade names, or trademarks is for identification purposes only and is not to be considered an endorsement.}

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